U.S. patent application number 15/760945 was filed with the patent office on 2018-09-20 for photonic device.
The applicant listed for this patent is OXFORD UNIVERSITY INNOVATION LTD.. Invention is credited to Harish BHASKARAN, Wolfram PERNICE, Carlos RIOS, Matthias STEGMAIER.
Application Number | 20180267386 15/760945 |
Document ID | / |
Family ID | 57003530 |
Filed Date | 2018-09-20 |
United States Patent
Application |
20180267386 |
Kind Code |
A1 |
RIOS; Carlos ; et
al. |
September 20, 2018 |
PHOTONIC DEVICE
Abstract
A photonic device (100) comprising: an optical waveguide (101),
and a modulating element (102) that is evanescently coupled to the
waveguide (101); wherein the modulating element (102) modifies a
transmission, reflection or absorption characteristic of the
waveguide (101) dependant on its state, and the state of the
modulating element (102) is switchable by an optical switching
signal (125) carried by the waveguide (101), or by an electrical
signal that heats the modulating element (102).
Inventors: |
RIOS; Carlos; (Oxford,
GB) ; BHASKARAN; Harish; (Oxford, GB) ;
PERNICE; Wolfram; (Oxford, GB) ; STEGMAIER;
Matthias; (Oxford, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OXFORD UNIVERSITY INNOVATION LTD. |
Oxford |
|
GB |
|
|
Family ID: |
57003530 |
Appl. No.: |
15/760945 |
Filed: |
September 15, 2016 |
PCT Filed: |
September 15, 2016 |
PCT NO: |
PCT/GB2016/052871 |
371 Date: |
March 16, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 1/0126 20130101;
G02F 1/313 20130101; G11C 13/04 20130101; G02F 3/02 20130101 |
International
Class: |
G02F 1/313 20060101
G02F001/313; G02F 3/02 20060101 G02F003/02; G11C 13/04 20060101
G11C013/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 18, 2015 |
GB |
1516579.8 |
Apr 27, 2016 |
GB |
1607344.7 |
Claims
1. A memory cell, comprising a photonic device comprising: an
optical waveguide, and a modulating element that is evanescently
coupled to the waveguide; wherein the modulating element modifies a
transmission, reflection or absorption characteristic of the
waveguide dependant on its state, and the state of the modulating
element is switchable by an optical switching signal carried by the
waveguide; wherein the modulating element comprises a material
having more than two states, between which the material is
switchable, each state associated with a different transmission,
reflection or absorption characteristic of the waveguide, wherein
the memory cell is configured to encode multiple bits in a single
modulating element by using the said more than two states to encode
information.
2. An optical system, comprising: the memory cell of claim 1, a
light source coupled to the waveguide for providing the switching
signal; and a controller operable to modify the transmission,
reflection or absorption characteristic of the waveguide by
operating the light source to produce the switching signal.
3. The memory cell of claim 1, wherein the modulating element
comprises a phase change material such as a phase change
superlattice material.
4. (canceled)
5. (canceled)
6. The memory cell of claim 1, wherein the modulating element
comprises a plurality of stable solid states, each corresponding
with a different transmission, reflection or absorption
characteristic of the waveguide.
7. (canceled)
8. The memory cell of claim 1, wherein the modulating element
comprises a material comprising a compound or alloy of a
combination of elements selected from the following list of
combinations: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AgInSbTe, InSb,
InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe,
AuSbTe, and AlSb.
9. (canceled)
10. (canceled)
11. The memory cell of claim 1, wherein a core material of the
waveguide has an optical bandgap of at least 1 eV.
12. (canceled)
13. (canceled)
14. The memory cell of claim 1, wherein the modulating material has
a thickness of less than 100 nm.
15. The memory cell of claim 1, wherein the waveguide comprises an
optical structure configured to enhance interaction of the
switching signal with the modulating element.
16. The memory cell of claim 15, wherein the optical structure
comprises at least one of: a photonic crystal, a cavity in a core
of the waveguide, an antenna and a plasmonic antenna.
17. The memory cell of claim 1, wherein the waveguide comprises an
optical resonator, the modulating element being evanescently
coupled to the optical resonator.
18. The memory cell of claim 17, wherein the waveguide comprises a
plurality of optical resonators or cavities, each having a
different resonant frequency, the memory cell comprising a
modulating element evanescently coupled to each optical resonator,
wherein the transmission, reflection or absorption properties of
the waveguide at each of a plurality of wavelengths is
independently modified depending on the state of the respective
modulating element coupled to the resonator corresponding with the
respective wavelength, the state of each modulating element being
switchable by an optical switching signal carried by the
waveguide.
19. The memory cell of claim 1, wherein the waveguide is a coupling
waveguide, and further comprising a first and second waveguide, the
coupling waveguide optically coupling the first waveguide to the
second waveguide, the degree of coupling depending on the state of
the modulating element.
20. The memory cell of claim 17, wherein the optical resonator
comprises a ring resonator.
21. (canceled)
22. The memory cell of claim 1, comprising an electrical conductor
configured to switch the state of the modulating element using an
electrical signal that heats the modulating element.
23. The memory cell of claim 22, further comprising a first
electrode in contact with the modulating element, and a second
electrode in contact with the modulating element, so that a
conducting path is defined through the modulating element between
the first and second electrode.
24. The memory cell of claim 23, wherein the modulating element
comprises a planar layer sandwiched between the first and second
electrode, so that the conducting path is substantially normal to a
plane of the modulating element.
25. The memory cell of claim 23, wherein the first and second
electrode are arranged to define a lateral conducting path,
substantially parallel to a plane of the modulating element.
26. The memory cell of claim 21, wherein the first and/or second
electrode comprises a substantially transparent material.
27. The memory cell of claim 21, wherein the electrical conductor
comprises a resistor in thermal contact with the modulating
element, so that the modulating element is switchable by passing
the electrical signal through the resistor, and heating the
modulating element primarily by conduction.
28. (canceled)
29. (canceled)
30. (canceled)
31. (canceled)
32. (canceled)
33. (canceled)
34. (canceled)
35. (canceled)
36. (canceled)
37. (canceled)
38. (canceled)
39. (canceled)
40. (canceled)
41. (canceled)
42. (canceled)
43. (canceled)
44. A method of varying the transmission, reflection or absorption
properties of a memory cell according to claim 1, comprising
changing a state of the modulating element using an optical
switching signal carried by the waveguide thereby switching the
modulating element between at least two different states using a
plurality of optical switching signals carried by the
waveguide.
45. (canceled)
46. (canceled)
47. (canceled)
48. (canceled)
Description
[0001] The invention relates to a switchable photonic device, and
to a memory element, and an optical switch.
[0002] The advent of photonic technologies, in particular in the
area of optical signalling, coupled with advances made in
fabrication capabilities has created a growing need for practical
photonic memories. Such memories are essential to supercharge
computational performance in serial computers by speeding up the
von-Neumann bottleneck, i.e. the information traffic jam between
the processor and the memory. This bottleneck limits the speed of
almost all processors today; it has already led to the introduction
of multicore processor architectures and drives the search for
viable on-chip optical interconnects. However, shuttling
information optically from the processor to electronic memories is
presently not efficient because electrical signals have to be
converted to optical ones and vice-versa. Information transfer and
storage exclusively by optical means is highly desirable because of
the inherently large bandwidth, low residual cross-talk and high
speed of optical information transfer. On a chip it has been
challenging to achieve because practical photonic memories may need
to retain information for long periods of time and achieve
full-integration with the ancillary electronic circuitry, thus
requiring compatibility with semiconductor processing.
[0003] A further performance limitation may occur between the rapid
access volatile memory (e.g. DRAM) associated with the processor,
and the longer term, non-volatile memory of a storage device (e.g.
NAND based memory of a solid state device, or magnetic domain based
storage of a hard disk). Storage class memory has been proposed as
an intermediate class of memory that is both non-volatile, and with
faster reading than existing storage devices.
[0004] Optical gate switches have been fabricated using
Ge.sub.2Sb.sub.2Te.sub.5 (GST), for example Tanaka, Daiki, et al.
"Ultra-small, self-holding, optical gate switch using
Ge.sub.2Sb.sub.2Te.sub.5 with a multi-mode Si waveguide." Optics
express 20.9 (2012): 10283-10294. This device uses an external,
free-space optical light path to provide an optical switching
signal to the GST element to change the state of the switch. This
requires precision alignment of bulk free-space optical elements to
the GST element, and is impractical for controlling multiple GST
elements, especially where the elements are densely packed (e.g. in
an array), which may be useful in a practical memory module. It is
also difficult to address a small GST element in this way (due to
diffraction limits).
[0005] It is desirable to address at least some of the above
mentioned problems.
[0006] According to a first aspect of the invention, there is
provided a photonic device comprising: an optical waveguide, and a
modulating element that is evanescently coupled to the waveguide;
wherein the modulating element modifies a transmission, reflection
or absorption characteristic of the waveguide dependant on its
state, and the state of the modulating element is switchable by an
optical switching signal carried by the waveguide.
[0007] The term optical as used herein relates to electromagnetic
wavelengths of between 100 nm and 2500 nm.
[0008] According to another aspect, there is provided a device
comprising: a waveguide; and a modulating element that is
evanescently coupled to the waveguide; wherein the modulating
element modifies a transmission, reflection or absorption
characteristic of the waveguide dependant on its state, and the
state of the modulating element is switchable by a switching signal
carried by the waveguide.
[0009] The modulating element may comprise a phase change
material.
[0010] The modulating element may comprise a phase change
superlattice material.
[0011] The modulating element may comprise a material with a
refractive index that is switchable between at least two stable
values.
[0012] The modulating element may comprise a plurality of stable
solid states, each corresponding with a different transmission,
reflection or absorption characteristic of the waveguide.
[0013] A reflection characteristic of the waveguide may be modified
by switching the state of the modulating element.
[0014] The modulating element may comprise a material comprising a
compound or alloy of a combination of elements selected from the
following list of combinations: GeSbTe, VO.sub.x, NbO.sub.x, GeTe,
GeSb, GaSb, AgInSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe,
SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb.
[0015] The material may comprise a mixture of compounds of alloys
or combinations of elements from the list.
[0016] A core material of the waveguide may comprise an
insulator.
[0017] A core material of the waveguide may have an optical bandgap
of at least 1, 1.5, 2, 2.5 or 3 eV.
[0018] A core material of the waveguide may comprise a material
selected from: silicon, silicon nitride, gallium nitride, gallium
arsenide, magnesium oxide, and diamond (single crystal or
polycrystalline).
[0019] The modulating element may comprise a material having more
than two states, between which the material is switchable, each
state associated with a different transmission, reflection or
absorption characteristic of the waveguide.
[0020] The modulating material may have a thickness of less than 40
nm or 20 nm.
[0021] The waveguide may comprises an optical structure configured
to enhance interaction of the switching signal with the modulating
element.
[0022] The optical structure may comprise at least one of: a
photonic crystal, a cavity in a core of the waveguide, an antenna
and a plasmonic antenna.
[0023] The waveguide may comprise an optical resonator, the
modulating element being evanescently coupled to the optical
resonator.
[0024] The waveguide may comprise a plurality of optical
resonators, each having a different resonant frequency, the device
comprising a modulating element evanescently coupled to each
optical resonator, wherein the transmission, reflection or
absorption properties of the waveguide at each of a plurality of
wavelengths is independently modified depending on the state of the
respective modulating element coupled to the resonator
corresponding with the respective wavelength, the state of each
modulating element being switchable by an optical switching signal
carried by the waveguide.
[0025] The waveguide may be a coupling waveguide, and further
comprising a first and second waveguide, the coupling waveguide
optically coupling the first waveguide to the second waveguide, the
degree of coupling depending on the state of the modulating
element.
[0026] The optical resonator may comprise a ring resonator, disk
resonator, race-track resonator, wheel resonator.
[0027] The device may comprise an optical cavity, and the
modulating element may be used to tune a resonant frequency of the
optical cavity. The transmission, reflection or absorption
characteristic may comprise a resonant frequency or a transmission,
reflection or absorption spectrum.
[0028] According to a second aspect, there is provided a photonic
latch, comprising a device according to the first aspect, wherein
the first waveguide comprises an input port, for receiving an input
signal, the second waveguide comprises an output port, and the
coupling waveguide comprises a control port for receiving the
switching signal, wherein the degree of optical coupling between
the input port and output port is varied by switching the
modulating element via the control port.
[0029] According to a third aspect of the invention, there is
provided a photonic device comprising: an optical waveguide, and a
modulating element that is evanescently coupled to the waveguide;
wherein the modulating element modifies a transmission, reflection
or absorption characteristic of the waveguide dependant on its
state, and the device comprises an electrical conductor configured
to switch the state of the modulating element using an electrical
signal that heats the modulating element.
[0030] The electrical conductor may comprise a first electrode in
contact with the modulating element, and a second electrode in
contact with the modulating element, so that a conducting path is
defined through the modulating element between the first and second
electrode. The state of the modulating element may thereby be
switchable by passing the electrical signal through the conducting
path.
[0031] The modulating element may comprise a layer of material. The
modulating element may be sandwiched between the first and second
electrode, so that the conducting path is substantially normal to a
plane of the modulating element. The first and second electrode may
alternatively be arranged to define a lateral conducting path
substantially parallel to a plane of the modulating element.
[0032] At least one of the first and second electrode may comprise
a substantially transparent material (e.g. at the wavelength of an
optical switching signal), such as indium tin-oxide (ITO).
[0033] The device may comprise a resistor (e.g. a conducting track)
in thermal contact with the modulating element, so that the
modulating element is switchable by passing the electrical signal
through the resistor (with the electrical signal not passing
through the modulating element).
[0034] The resistor may comprise a metal or semiconductor
element.
[0035] According to an fourth aspect of the invention, there is
provided a photonic device comprising: a conducting path, an
optical waveguide, and a modulating element that is evanescently
coupled to the waveguide and which forms part of the conducting
path; wherein the modulating element modifies the resistance of the
conducting path depending on its state, and the state of the
modulating element is switchable by an optical switching signal
carried by the waveguide.
[0036] The device may comprise a first electrode in contact with
the modulating element, and a second electrode in contact with the
modulating element, so that the conducting path is defined through
the modulating element between the first and second electrode. The
state of the modulating element may thereby be determined by the
resistance of the conducting path.
[0037] According to a fifth aspect, there is provided a
Mach-Zehnder interferometer that splits an input signal received at
an input port between a first and second optical path, and then
recombines the input signal after it has passed through the first
and second optical path, wherein at least one of the first and
second path comprise a device according to the first aspect or
third aspect and the transmission, reflection or absorption
property is an optical path length.
[0038] According to a sixth aspect, there is provided a tunable
grating comprising a device according to the first aspect or third
aspect, wherein the grating is defined by a plurality of modulating
elements disposed side-by-side on the waveguide.
[0039] The grating may be a Bragg grating.
[0040] The waveguide may be a planar waveguide, and the grating may
be configured to define an out-of-plane coupler that couples light
between the waveguide and free space in a direction at an angle to
the plane of the waveguide, wherein the transmission, reflection or
absorption property is resonant frequency of the coupler.
[0041] According to a seventh aspect, there is provided a tuneable
optical filter comprising a device according to the first aspect or
third aspect.
[0042] According to a eighth aspect, there is provided an optical
switch comprising a device according to the first aspect or third
aspect.
[0043] According to a ninth aspect, there is provided a switching
fabric comprising: a plurality of horizontal waveguides, and a
plurality of vertical waveguides, and a device according to the
first aspect or third aspect optically coupling each horizontal
waveguide to each vertical waveguide at each respective
intersection therebetween.
[0044] According to a tenth aspect, there is provided an optical
system, comprising: [0045] a device according to the first aspect,
[0046] a light source coupled to the waveguide for providing the
switching signal; and [0047] a controller operable to modify the
transmission, reflection or absorption characteristic of the
waveguide by operating the light source to produce the switching
signal.
[0048] The system may further comprise a light detector coupled to
the waveguide and configured to determine the transmission,
reflection or absorption characteristic by detecting a probe signal
from the waveguide.
[0049] The controller may be configured to use the light source to
provide the probe signal and the switching signal.
[0050] A further light source may be provided coupled to the
waveguide for providing the probe signal; wherein the controller is
configured to use the further light source to provide the probe
signal.
[0051] The light source and the further light source may both be
monochromatic, and have different wavelengths. The light source and
further light source may be coherent, or incoherent.
[0052] According to another aspect, there is provided an optical
system, comprising: [0053] a device according to the third aspect,
[0054] an electrical controller coupled to the conducting path for
providing an electrical switching signal to the conducting path;
[0055] a light source coupled to the waveguide for reading the
state of the modulating element, based on the transmission,
reflection or absorption characteristic of the waveguide.
[0056] According to another aspect, there is provided an optical
system, comprising: [0057] a device according to the fourth aspect,
[0058] a light source coupled to the waveguide for providing the
switching signal; and [0059] an electrical controller coupled to
the conducting path for reading the state of the modulating
element.
[0060] According to an eleventh aspect, there is provided a memory
cell, comprising a device according to the first, third or fourth
aspect.
[0061] According to a twelfth aspect, there is provided an optical
memory module, comprising a plurality of memory cells according to
the eleventh aspect.
[0062] According to a thirteenth aspect, there is provided a
computer comprising a processor and a memory module according to
the twelfth aspect.
[0063] According to an fourteenth aspect, there is provided a
method of varying the transmission, reflection or absorption
properties of a device comprising an optical waveguide, comprising
changing a state of a modulating element that is evanescently
coupled to the waveguide using an optical switching signal carried
by the waveguide.
[0064] The device may be according to the first aspect or the
fourth aspect.
[0065] The modulating element may comprise a phase change material,
and changing the state of the modulating element may comprise
changing the degree to which the phase change material is amorphous
or crystalline.
[0066] The modulating element may comprise more than two stable
solid states, each state corresponding with a different
transmission, reflection or absorption characteristic of the
waveguide, wherein the method comprises switching the modulating
element between at least two different states using a plurality of
optical switching signals carried by the waveguide,
[0067] The device may comprise a plurality of modulating elements
coupled to the waveguide, and the method may comprise independently
addressing each modulating element using a switching signal having
a wavelength corresponding with the respective modulating
element.
[0068] Each feature (or features) of each aspect may be combined
with each feature (or features) of any other aspect.
[0069] Each and every embodiment and feature disclosed in the
priority documents is optionally disclaimed from the scope of the
present disclosure.
[0070] These and other aspects of the invention will be apparent
from, and elucidated with reference to, the embodiments described
hereinafter.
[0071] Embodiments will be described, by way of example only, with
reference to the accompanying drawings, in which:
[0072] FIG. 1 is a schematic plan view of a device according to an
embodiment;
[0073] FIG. 2 is a rendering illustrating the operation of a device
according to an embodiment;
[0074] FIG. 3 illustrates the transition between states of an
example phase change material;
[0075] FIG. 4 is a scanning electron micrograph of a device
according to an embodiment;
[0076] FIG. 5 is a simulation of a TE optical mode in a device
according to an embodiment;
[0077] FIG. 6 is a graph showing repeated switching of a
transmission characteristic of a waveguide in accordance with an
embodiment;
[0078] FIG. 7 is a graph showing the repeatability of the switching
according to an embodiment over an increased number of cycles;
[0079] FIG. 8 is a transmission electron micrograph of a cross
section through a device according to an embodiment, where the
modulating element is in a crystalline state;
[0080] FIG. 9 shows a Fourier analysis of the TEM data of FIG. 8
with the device in a crystalline state, and a Fourier analysis of a
TEM taken of a similar device with the modulating element in an
amorphous state;
[0081] FIG. 10 is a scanning electron micrograph of a device
according to an embodiment, comprising a first, second and third
ring resonator, and a first, second and third modulating element
coupled respectively coupled to each of the resonators;
[0082] FIG. 11 is a graph of a transmission characteristic of the
device of FIG. 10 with respect to wavelength;
[0083] FIG. 12 is a graph showing switching of a transmission
characteristic of the device of FIG. 11 at a first wavelength
corresponding with the first ring resonator;
[0084] FIG. 13 is a graph showing independent switching of the
transmission characteristic at a second and third wavelength
(respectively corresponding with the second and third
resonators);
[0085] FIG. 14 is set of three graphs showing independent switching
of a transmission characteristic of the device of FIG. 10 at each
of the first, second and third wavelengths;
[0086] FIG. 15 is a set of graphs showing switching between four
clearly distinguishable levels of transmission characteristic for a
device according to an embodiment for: a) successively increasing
levels; b) arbitrary ascending order of levels; c) arbitrary
descending order;
[0087] FIG. 16 is a graph showing the transmission characteristics
of a device switchable between eight different levels of
transmission characteristic;
[0088] FIG. 17 is a 3D bar chart illustrating the relationship
between the energy of the optical switching signal, the addressed
level, and the corresponding change in transmission characteristic
according to an embodiment;
[0089] FIG. 18 is a schematic plan view of a device according to an
embodiment in which a photonic structure is used in enhance
coupling between the waveguide and modulating element;
[0090] FIG. 19 is a schematic plan view of a device according to an
embodiment in which a plasmonic antenna is used in enhance coupling
between the waveguide and modulating element;
[0091] FIG. 20 is a schematic plan view of a device according to an
embodiment comprising an input port for receiving a probe signal,
an output port for monitoring a transmission characteristic of the
waveguide using the probe signal, and a control port for receiving
the switching signal;
[0092] FIG. 21 is a schematic plan view of a device according to an
embodiment comprising a first and second waveguide and a coupling
waveguide optically coupling the first waveguide to the second
waveguide, the degree of coupling depending on the state of the
modulating element;
[0093] FIG. 22 is a schematic plan view of a switching fabric
comprising plurality of horizontal waveguides and a plurality of
vertical waveguides, and a device according to an embodiment
optically coupling each horizontal waveguide to each vertical
waveguide at each respective intersection therebetween;
[0094] FIG. 23 is a schematic plan view of a photonic latch
according to an embodiment;
[0095] FIG. 24 is a schematic plan view of a Mach-Zehnder
interferometer according to an embodiment;
[0096] FIG. 25 is a schematic plan view of a tuneable Bragg filter
in according to an embodiment;
[0097] FIG. 26 is a schematic plan view of a tuneable grating
coupler according to an embodiment;
[0098] FIG. 27 is a schematic view of a device according to an
embodiment in which the state of the modulating element can be
changed by passing an electrical current through the modulating
element; and
[0099] FIG. 28 is a schematic view of a device according to an
alternative embodiment in which the state of the modulating element
can be changed by passing an electrical current through a resistive
element in thermal contact with the modulating element.
[0100] A candidate technology for all-optical memories is to use
phase-change materials (PCMs), which are already the subject of
intense research and development over the last decade, in the
context of electronic memories. A striking and functional feature
of these materials is the high contrast between the crystalline and
amorphous phase of both their electrical and optical properties. In
particular, PCMs (e.g. chalcogenide-based PCMs) have the ability to
switch between these two states in response to appropriate heat
stimuli (crystallization) or melt-quenching processes
(amorphization). These PCMs (which include tellurides and
antimonides) can be switched on a sub-nanosecond timescale with
high reproducibility which enables ultra-fast operation over
switching cycles up to 10.sup.12 times using current-generation
materials. New and improved PCM materials, such as the so-called
phase-change super-lattice materials, are expected to deliver even
better performance in the future. In addition, at normal operating
temperatures (e.g. NIST standard temperature and pressure
conditions) the states may be stable for years, which may be
appropriate for non-volatile memory.
[0101] Many PCMs show significant change in refractive index in the
visible and even larger changes in the near-infrared wavelength
regime, which is typically the spectral region of choice for
telecommunication applications. According to embodiments, PCMs are
embedded in photonic circuits, to provide fast and repeatable
all-optical, multi-level, multi-bit, non-volatile memory.
Wavelength division multiplexed (WDM) access can be achieved on a
chip comprising at telecommunications wavelengths compatible with
on-chip optical interconnects. In contrast to prior art free-space
optical implementations where PCM cells are switched with a focused
laser in the far-field, devices according to embodiments are
operated in the optical near-field (by evanescent coupling).
[0102] Waveguide integrated modulating elements are not restricted
in size by the diffraction limit of the input light and can hence
be miniaturized to nanoscale dimensions. In the detailed exemplary
embodiments described below, the well-studied alloy
Ge.sub.2Sb.sub.2Te.sub.5 (GST) is used because of its proven data
retention capabilities and high state discrimination down to
nanoscale cell sizes, which enables dense packaging and low-power
memory switching.
[0103] In embodiments, data may be stored in a modulating element
comprising a PCM material. The modulating element may be placed
directly on top of a nanophotonic waveguide core. This is a
convenient way of coupling the waveguide to the modulating element
in the nearfield. Both changing the state (writing/erasing) of the
modulating element, and read-out of the current state of the
modulating element is carried out via evanescent coupling between
the waveguide and the modulating element and is thus not subject to
the diffraction limit. The writing and readout of the modulating
element is done directly within the waveguide, and may use
nanosecond optical pulses. Embodiments therefore provide a
promising route towards fast all-optical data storage in photonic
circuits.
[0104] FIG. 1 shows a device 100 according to an embodiment,
comprising a waveguide 101 and a modulating element 102. The
modulating element 102 is evanescently (near field) coupled to the
waveguide 101, so that a light signal carried by the waveguide 101
interacts with the modulating element 102.
[0105] The modulating element 102 may comprise any material that is
switchable between different states, each different state
corresponding with different optical properties of the modulating
element. Preferably, the modulating element 102 comprises a phase
change material such as GST. The modulating element 102 may
comprise a further encapsulation layer, which may comprise ITO, for
example to protect the PCM layer from oxidation.
[0106] The waveguide 101 may be, but is not limited to, a planar
waveguide, for example a rib waveguide. The waveguide 101 comprises
a core material that is capable of carrying an optical switching
signal 125 to the modulating element 102 so as to switch the state
of the modulating element 102. In general, suitable materials for
the waveguide core may have a bandgap of at least 1 eV.
[0107] One example of a suitable material for the core of the
waveguide 101 is silicon nitride. Alternative materials include
silicon, gallium nitride, gallium arsenide, diamond
(monocrystalline or polycrystalline) and magnesium oxide, but any
material with a bandgap greater than 1 eV may be suitable. The
waveguide core may comprise an insulating material or a
semiconductor.
[0108] An air cladding may be used around the waveguide core 101.
In alternative embodiments, other materials may be used. Solid
phase cladding materials may be used to reduce a thermal time
constant of the modulating element 102.
[0109] The waveguide 101 comprises a first port 115 and a second
port 116. The transmission characteristics of the optical waveguide
101 may be inferred by applying a probe signal 105 at the first
port 115, and monitoring the resulting output probe signal 106 at
the second port 116. The probe signal 105 may be a pulsed signal or
a continuous wave signal.
[0110] The state of the modulating element 102 is switchable by an
optical switching signal 125 carried by the waveguide 101. An
optical switching signal 125 may be input to the waveguide at
either of the first or second port 115, 116. The switching signal
125 may have a higher power than the probe signal 105. The
evanescent coupling of the optical switching signal 125 may result
in the absorption of optical power by the modulating element 102.
The consequent heating of the modulating element 102 by the optical
switching signal 125 may change the state of the modulating element
102. Since the modulating element 102 is optically coupled to the
waveguide 101, changes to the optical properties of the modulating
element 102 result in changes to the transmission, reflection or
absorption characteristics of the optical waveguide 101. The state
of the modulating element 102 may be used to encode
information.
[0111] FIG. 2 illustrates the operation of a device 100 according
to an embodiment. A probe signal 105 applied to a first port 115
may be used to monitor a transmission characteristic of the
waveguide 101 by monitoring the output probe signal 106 at the
second port 116.
[0112] As illustrated in FIG. 3, the crystalline state 201 (which
may be assigned as Level 0, 208) exhibits higher optical
attenuation and thus less optical transmission than the amorphous
state (which may be assigned Level 1, 209). Therefore, stored data
may be encoded in the amount of light 106 transmitted through
(along) the waveguide 101 (i.e. exiting the second port 116 of the
waveguide) and can be read-out with a low-power probe signal 105,
which may comprise a series of optical pulses 107. The state (e.g.
degree of amorphisation/crystallinity) of the modulating element
102 influences the optical properties of the waveguide 101 and
therefore the waveguide mode profile, as illustrated for the
simulated transverse-electric (TE) mode 501 in FIG. 5.
[0113] In the crystalline state 201, the PCM may be more
absorptive, thus pulling the light towards the modulating element
102, resulting in stronger attenuation of the passing optical
signal. In the amorphous phase 202, on the other hand, the
absorption is reduced and therefore the modulating element 102 does
not attenuate the waveguide transmission to the same degree.
[0114] Changing the state of the modulating element 102 may be
achieved by inducing a phase-transition (or partial
phase-transition) with an optical switching signal 125, which may
comprise a more intense light pulse 135 than the probe pulses 107
of the probe signal 105. If the energy absorbed by the modulating
element 102 is sufficient to heat it up to a transition temperature
of the PCM, these pulses 135 can initiate either amorphization or
crystallization. Referring to FIG. 3, a transition 211 from a
crystalline state 209 to an amorphous state 208 may be thought of
as a write step, and a transition 210 from an amorphous state 208
to a crystalline state 209 may be thought of as an erase step.
[0115] For amorphization, the PCM (e.g. GST) may be melted and then
cooled down rapidly to preserve this disordered state. On the other
hand, heating the PCM above the crystallization temperature (but
below the melting temperature) for a few nanoseconds may enable
recovery of the atomic ordering and thus crystallization.
Transmission properties of the waveguide 101 may therefore be
modulated by varying the absorptive state of the modulating element
102. This is different from the phenomenon employed in conventional
optical storage (where reflectivity is instead modulated).
[0116] In an embodiment, an all photonic nonvolatile memory element
is provided. Multiple modulating elements may be addressed using
wavelength multiplexing, which may be applied to deliver multi-bit
memory access. Multiple levels of information (i.e. multiple bits)
may be encoded in a single modulating element 102 by using more
than two states to encode information. A single shot read and write
switching signal may be used to transition the modulating element
between any of the more than two states.
[0117] Although a PCM with multiple stable states is particularly
appropriate for non-volatile memory applications, in alternative
embodiments different materials may be used that do not comprise
states that are stable (e.g. at NIST STP). In some embodiments the
state of the modulating element 102 may be volatile, and a
controller may be provided to periodically refresh the state of the
modulating so as to maintain desired state thereof (e.g. maintain
information encoded by the state). In other embodiments, the device
may be temperature controlled (e.g. cooled), so that the PCM is
stable, rather than operating at typical room temperatures.
[0118] The waveguide 101 may be fabricated from 335 nm
Si.sub.3N.sub.4/3350 nm SiO.sub.2 wafers using lithography. A soft
reflow process may be employed to reduce intrinsic surface
roughness of the photosensitive resist following exposure.
Subsequently, reactive ion etching (RIE) may be used (e.g. in
CHF.sub.3/O.sub.2) to pattern the Si.sub.3N.sub.4 followed by the
complete removal of the remaining resist under O.sub.2 plasma. The
depth of the etch (and of the starting layers) may be selected
depending on the design of the device. In the present examples, an
etch depth of 165 nm was used, except for the ring devices, where
an etch depth of 330 nm was used. Example devices for which results
are described herein are designed with a waveguide width of 1.3
.mu.m or 0.9 .mu.m (for ring devices).
[0119] The modulating elements 102 for which results are shown
herein are fabricated by depositing GST and indium tin oxide (ITO)
in a lift-off process. For this, a second lithography step is
carried out using positive tone PMMA resist at 8% concentration.
Openings on top of the waveguides are defined, aligned to the
previously written waveguide structures. Subsequently a 10 nm layer
of GST is sputtered and capped with 10 nm of ITO to avoid
oxidation. Finally, lift-off of the GST/ITO layers is done in hot
acetone supported by soft sonication.
[0120] The features and steps of this fabrication process flow are
merely exemplary, and embodiments may be fabricated in other
ways.
[0121] The device 100 may be operated by using an optical switching
signal 125 and an optical probe signal 105. Both writing and
erasing of the modulating element 102 may be performed with
nanosecond light pulses that are generated off-chip. This is
convenient in an experimental context, but it is also envisaged
that a suitable light source may be included on-chip. An
electro-optical modulator (EOM) may be used to modulate the optical
switching signal 125. The optical switching signal 125 may be
coupled into the on-chip waveguides, for example using grating
couplers. The device 100 may be tuned for operation in the optical
telecommunications C and L band.
[0122] The read-out of the state of the modulating element 102 may
be performed using a probe signal 105 having readout pulses (e.g.
of 500 ps duration, generated with the EOM) or with a continuous
wave (CW) probe signal 105. In both cases, the probe signal 105 may
have at least one order of magnitude less power than the switching
signal 125. The probe signal 105 may be spectrally separated from
the switching signal 125. A colour selective filter may be used to
separate the probe signal 105 from the switching signal 125.
Further suppression of the switching signal 125 may be achieved by
counter-propagation of the probe signal 105 and the switching
signal 125 through the waveguide 101.
[0123] This counter propagation of the probe and switching signal
may be implemented with a set of two optical circulators (not
shown) which direct light coming from a laser source onto the
device 100 and subsequently onto the photodetectors. An optical
band-pass filter may be used to suppress the influence of
back-reflections. The filter may have a 1 dB insertion loss, a 3 dB
bandwidth of 2 nm and an off-resonance suppression exceeding 40 dB
for wavelengths further apart than 10 nm.
[0124] The switching signal 125 may be generated from a continuous
wave (CW) diode laser in combination with an electro-optical
modulator, controlled by an electrical pulse generator. Switching
signal pulses 135 may be further amplified in power by a low-noise
erbium-doped fiber amplifier (EDFA). Such an implementation may
provide accurate control over both pulse length and power, enabling
pulses as short as 300 ps, with pulse edges in the
picosecond-regime and with peak powers up to 150 mW.
[0125] The probe signal 105 may comprise CW-light from a second
laser source. After propagation through the device 100 and the
band-pass filter, the light may be is split into two beams which
are respectively detected by a fast photodetector and a low-noise
photodetector. The signal of the fast detector enables recording a
high-resolution time-trace of the response with a 6 GHz
oscilloscope while the overall device transmission may be monitored
at all times with the low-noise detector.
[0126] Crystallization of amorphous PCM may enhance optical
absorption in the modulating element 102 at telecommunication
wavelengths, for instance by one order of magnitude. This increase
in optical absorption results in an increase in the proportion of
the energy of a switching signal 125 in the waveguide 101 that is
absorbed by the modulating element 102. This renders both
amorphization and crystallization transitions possible with optical
switch signal pulses 135 of comparable length and power. Full
recrystallization with a single light pulse 135 may require
optimisation of the pulse (e.g. duration and power).
[0127] If the phase transition occurs before the end of the pulse
135, the continued optical energy supply may heat up the PCM
further, to the melting temperature, and cause immediate
reamorphization. Temperature variations across the memory cell may
mean that this cannot be prevented completely since all parts of
the PCM material may not crystallize simultaneously. To address
this issue, an erase transition 210 (making the PCM material more
crystalline) may be based on stepwise partial recrystallization
using a train of consecutive pulses 135. In order to prevent
reamorphization of already crystallized regions, the individual
pulse energies may be gradually decreased from pulse to pulse (e.g.
by approximately 5% of the initial pulse energy). The initial pulse
energy may correspond with the pulse energy used for a write
transition 211. The energy of the final pulse determines which
state is achieved, and therefore what transmission characteristic
of the waveguide 101 is achieved. The final state can be fully
crystalline or an intermediate state (i.e. partially amorphous and
partially crystalline), for instance by stopping the erase
transition 210 at the same energy required for a write transition
211 to that specific level.
[0128] High reproducibility of the transition operations may be
achieved by an initial conditioning step. A conditioning step may
comprise performing a write/erase switching cycle a few times on
the as-deposited and subsequently annealed GST. Within the first
few cycles the read-out transmissions, which initially vary
slightly from cycle to cycle, stabilize to a fixed value.
[0129] FIG. 6 shows the change in the detected output signal 106
(which indicates a transmission characteristic of the waveguide
101) for an embodiment upon repeated switching between the
crystalline (low transmission) state 209 and amorphous (high
transmission) state 210 of the PCM. Each transition 211, 210
results in a change in the state of the modulating element 102
(which may be referred to as "switching"), resulting in a clear
change in the output signal 106 between a high level 208
(corresponding with a more amorphous state) and a low level 209
(corresponding with a fully crystalline state). The results
demonstrate unequivocal binary data storage in an embodiment, with
good reversibility and high transmission contrast (around 20% in
this example). Switching over 14 cycles is shown in FIG. 6. The
write transition 211 was initiated by a single 100 ns pulse, while
the erase transition used a sequence of six consecutive 100 ns with
decreasing power. The time between each transition 210, 211 was one
minute.
[0130] FIG. 7 illustrates that the switching of the modulating
element 102 is highly reproducible over fifty cycles with a
measured confidence interval of .+-.7.1%. The first standard
deviation 281, 291 for the amorphous state 208 and crystalline
state 209 are shown, along with the respective second standard
deviations 282, 292.
[0131] The low-transmission state 209 is initially prepared from
the fully crystallized phase in such a way that reversibility of
the operation is ensured (as discussed above). On the other hand,
the absolute transmission at the high level 208 is determined by
the employed switching energy, which defines the final level of
amorphization, and the GST dimension along the waveguide 101, which
defines the modulation depth.
[0132] To ensure high transmission contrast in the results of FIGS.
6 and 7 a GST cell of 5 .mu.m length was used. A change in read-out
transmission of 21% was observed using a single 100 ns write pulse
of 533 pJ energy. Since the GST cell 102 absorbs nearly 80.7% of
the pulse in the crystalline state (derived from a measured optical
attenuation of -7.14 dB past the device), this corresponds to a
switching energy of 430 pJ. Further demonstrations of binary
operation were realized by employing devices with smaller GST
lengths and lower write pulse energy, as described in the
supplementary material. In particular, modulation depth up to 58.2%
and binary operation with pulses as short as 10 ns with switching
energies of 13.4 pJ have been achieved. While the data in FIGS. 6
and 7 demonstrates the nonvolatility of states encoded in a
modulating element 102 comprising a PCM for several minutes, in
other embodiments it has been confirmed that the phase-state may be
preserved over a much longer times, up to a period of at least
three months. Indeed, extrapolating from the well-studied data
retention properties of GST an optical memory according to an
embodiment can be expected to remain non-volatile on a timescale of
years.
[0133] FIG. 8 shows a transmission electro micrograph of a section
through a device 100. The TEM specimens were prepared by focused
ion beam (FIB). The cross sectional lamellae were cut from a device
according to an embodiment along the waveguide and thinned to a
thickness of less than 50 nm for TEM imaging. The silicon nitride
waveguide 101 is visible, and the deposited GST layer 801 and the
ITO layer 802 can also be seen. The ordered lattice structure of
GST in the crystalline state is visible. Fourier analysis of the
TEM image of FIG. 8 produces a diffractogram with clear features
corresponding to the ordered lattice structure of cubic GST. FIG. 9
shows a Fourier analysis of similar data from a PCM material of
device that has been optically switched into an amorphous state
showing the pronounced halo expected for the amorphous phase.
[0134] Besides repeatability, speed may be an important factor for
applications in which a device 100 is used as a memory element. In
this context, the speed at which read, write and erase operations
can be achieved in a device is important. In devices according to
an embodiment, read-out may rely on photon absorption, with
information encoded in the amount of signal (e.g. signal power)
transmitted through the waveguide 101. The read-out can therefore
be performed on picosecond time scales and is not a bottleneck in
achieving high speed operation.
[0135] On the other hand, write operations 211 and erase operations
210 are linked both to amorphization and crystallization times
which are intrinsic properties of the modulating element 102. In
prior art GST cells, amorphization times in the picosecond range
are reported, and crystallisation times in the nanosecond to
sub-nanosecond and nanosecond.
[0136] In the case of a memory cell comprising a device according
to an embodiment, the writing speed (amorphization) may be
considered to be the more stringent requirement since it determines
how quickly information can be stored. As outlined above, in the
initial prototype devices it has been shown that write operations
are possible with pulses as short as 10 ns. To determine how fast a
device 100 according to an embodiment might be operated, the
phase-transition of the modulating element 102 was monitored by
performing time-resolved measurements during optical switching.
[0137] The observed transient behaviour of a device 100 according
to an embodiment showed that, besides the length of the write
pulse, the speed of the device 100 is also limited by the
post-excitation relaxation time (which may be termed dead time).
For 700 ps write pulses, an operation speed of 800 MHz was obtained
(taking pulse length and dead time into account). Writing speeds of
a few GHz can be expected using picosecond instead of nanosecond
pulses to switch the state of the modulating element 102.
[0138] FIG. 10 shows a device comprising a waveguide 101, the
waveguide 101 comprising a first, second and third ring resonator
160a-c. Each of the ring resonators 160a-c has a different
diameter, and is thereby tuned to a different resonant frequency
and corresponding wavelength. A grating coupler 151 is provided at
a first and second port of the waveguide 101, for coupling from
free space into the waveguide 101. Each of the ring resonators
160a-c comprises a modulating element 102, the location of which is
indicated by the dotted circles 170a-c. Since each of the ring
resonators 160a-c admits only a specific wavelength of light (due
to cavity internal interference preventing off-resonance
wavelengths from entering a ring), only light with a wavelength
close to resonance can be used to switch or read-out the respective
memory cell.
[0139] Each of the modulating elements is therefore individually
addressable, and the spectral transmission properties of the
waveguide 101 is adjusted by switching the state of each modulating
element. In this embodiment, there may be a plurality of probe
signals 105 at different wavelengths, each probe signal 105 having
a wavelength tuned to address a specific modulating element 102
(which may encode data). In this way the state of a plurality of
modulating elements 102 may be determined simultaneously, using a
wavelength division multiplexed (WDM) signal.
[0140] This type of embodiment may be used to implement a single
memory element or cell that can store a plurality of bits of data,
which can subsequently be read in parallel. The simplicity of
devices according to an embodiment make them fully compatible to
on-chip nanophotonic circuitry, allowing for easy integration and
exploitation of a wide range of commonly used optical signal
processing techniques such as WDM. The embodiment of FIG. 10
illustrates a wavelength-multiplexed integrated multi-bit
architecture that is suitable for ultra-fast read-out with up to 10
dB modulation depth.
[0141] This approach uses the wavelength-filtering property of the
on-chip optical cavities (in this example, ring resonators) which
enables wavelength selective addressing of individual modulating
elements 102.
[0142] FIG. 11 shows a spectral transmission characteristic of the
waveguide of FIG. 10, in which the distinct resonances 230a-c,
respectively corresponding with resonators 160a-c, are clearly
visible (along with further such resonances 235). The switching
signal for the first, second and third modulating elements (of the
first 160a, second 160b and third 160c resonators) comprise laser
pulses at 1563.35 nm, 1561.5 nm and 1560.1 nm respectively. While a
write transition is carried out with a single 10 ns pulse,
repeatability is again ensured by performing an erase transition
with a train of consecutive 50 ns pulses of decreasing energy (at
the appropriate wavelength).
[0143] In FIGS. 12 and 13 the individual changes of the three
resonances upon switching, resulting from the modified refractive
index of the GST element, are shown. FIG. 12 shows a spectral
response corresponding with the first resonance, showing a spectra
301 before a writing operation 211, a spectra 302 after a writing
operation 211, and a spectra 303 after an erase operation 210
(returning the PCM to a crystalline state). It can be seen that the
initial state is recovered after one write/erase cycle, since the
spectra 301 and 303 are substantially identical.
[0144] FIG. 13 illustrates that each modulating element can be
addressed individually.
[0145] Spectra for both the second and third resonance are shown:
[0146] 310; before a write operation (in the crystalline state);
[0147] 311; after a write operation 211 is performed to switch the
state of the second modulating element; and [0148] 312; after a
further write operation 211 to switch the state of the third
modulating element.
[0149] Switching signals applied to address a single modulating
element clearly do not affect the other modulating elements.
[0150] FIG. 14 shows that each modulating element can be read-out
individually. In this example 500 ps probe pulses 107 were used,
with a pulse energy of 0.48.+-.0.03 pJ. Here, probe signals at
three distinct wavelengths (1563.3 nm, 1561.45 nm and 1560.0 nm,
respectively) are used to probe the transmission characteristics of
the waveguide 101, and hence the state of each modulating element
(and any data encoded in the state thereof). FIG. 14 shows a time
history 321, 322, 323 of a first, second and third probe signal
respectively corresponding with each of the first, second and third
modulating elements. In order to maximise the readout contrast on
switching, probe pulses for each modulating element may be detuned
(e.g. red-detuned) from the wavelength of the corresponding
switching signal onto the slope of the cavity resonance. While a
modulation depth of 3 dB is achieved upon switching each individual
memory cell, the read-out level of the non-addressed memory
elements does not change. With this approach modulation depths
exceeding 10 dB are possible.
[0151] In some embodiments a plurality of states of the modulating
element 102 are used to encode more than one bit of information in
each modulating element 102. This may be combined with wavelength
addressable multi element cell architectures, and may further
increase the number of bits that can be encoded or stored in a
device according to an embodiment.
[0152] Embodiments of the invention may be applicable to future
high-density data storage, where it may be desirable to reduce the
overall dimensions of the device and to use multi-level access
(i.e. storing multiple bits per modulating element) in a single
cell. The smallest prototype memory element realised thus far has a
footprint of 0.25 .mu.m.sup.2, but it is expected that smaller
cells could be achieved in accordance with recent reports on
electrical PCM-based devices.
[0153] FIG. 15 is a graph illustrating switching between more than
two states of a modulating element 102. A detected probe signal 106
is shown with respect to time, and the detected probe signal 106 is
switched between four different levels (0, 1, 2, and 3) by writing
operations 211 and erase operations 210. At a) the state of the
modulating element 102 and hence the detected probe signal 106 is
switched from 0, to 1, to 2, to 3, and then directly back to 0. At
b) the state of the modulating element 102 and hence the detected
probe signal 106 is switched from 0, directly to 2, then to 3, then
directly to 0, then to 1, then to 3, and then directly back to 0.
At c) the state of the modulating element 102 and hence the
detected probe signal 106 is switched from 0, directly to 3, then
directly to 0, then to 2 and back directly to 0, the to 1 and back
to 0.
[0154] The modulating element 102 may not only have a small
footprint (smaller than can readily be addressed using free space
optics) but may also be capable of encoding multiple levels in an
element, using simple but extremely effective write/erase and read
techniques. Using optical switching pulses 135 with varying pulse
energy it is possible to move freely and reliably between more than
two states with high repeatability. This multi-level operation
relies on the freely accessible intermediate crystallographic
states of the GST, i.e. states with a mixture of crystalline and
amorphous regions. These mixed states exhibit optical transmission
properties lying between those of the level 1 and level 0 shown in
FIG. 3
[0155] The data of FIG. 15 was recorded using a 5 .mu.m long PCM
element, with each transition between levels being initiated by a
single 100 ns light pulse. Four clearly distinguishable levels are
reached with switching pulses P.sub.1 of level-specific energies in
the range 465 to 585 pJ. The energy of pulse P.sub.1 was 465.+-.13
pJ, the energy of pulse P.sub.2 was 524.+-.14 pJ, and the energy of
pulse P.sub.3 was 585.+-.14 pJ. In FIG. 15a these levels were
reached in a serial manner and subsequently the erase operation R
was carried out from level 3. Furthermore, the same bit levels were
also shown to be accessible in random order as shown in FIG. 15b.
Here the erase operation R (i.e. a return to level 0) was not only
possible from the highest transmission state, but from any
intermediate level as shown in FIG. 15c.
[0156] These results demonstrate that both write and erase
operations, to and from any level, are possible with high accuracy
allowing a reliable multi-bit memory operation. This is
particularly attractive because such arbitrary transitions are very
difficult to achieve in electronic memories employing phase change
materials, where iterative write-and-erase algorithms involving
multiple (typically 3 to 5) write/read(/re-write) cycles are needed
to achieve a pre-defined level, adversely affecting the overall
write speed and power consumption.
[0157] The number of possible levels in a device according to an
embodiment is limited by the separation (difference in
transmission) between the highest and lowest state and the required
confidence interval of an intermediate level. The former can be
increased by using either a larger modulating element or higher
pulse energies. The confidence interval, on the other hand, is
mainly limited by the minor variations in the switching and by the
signal-to-noise ratio (SNR) of the read-out measurement. Therefore,
the number of memory levels can be increased by just using a higher
read-out power ensuring a better SNR (within limits).
[0158] This is illustrated in FIG. 16, where 8 levels of state
discrimination (i.e. 3 bits per cell) are demonstrated within a
modulating element 102. Each level corresponds to a partially
crystalline state, presenting a specific change in transmission by
applying pulses with varying energies as presented in FIG. 17. The
individual levels are reached with pulses P.sub.i of level-specific
energies in the range 372 to 601 pJ (the energies in pJ being
approximately EP.sub.1=372, EP.sub.2=415, EP.sub.3=465,
EP.sub.4=524, EP.sub.5=561, EP.sub.6=585, and EP.sub.7=601). In
FIG. 16 it can also be observed that the difference between the
transmissions of any two consecutive levels is much higher than the
uncertainty marked by the a band 170 across each level. In FIG. 16
it is also demonstrated that each level can be reached from both
directions, i.e. with an amorphization as well as a crystallization
step. This implies that any level is accessible from all others,
with very accurate control of the transmission levels and
remarkable repeatability (as seen by the accurate re-writing of
levels 1 to 4 in FIG. 16), just by applying the appropriate write
or erase pulse. This provides a significant progression in
functionality and may be important for the realization of
practicable photonic memories.
[0159] Another aspect of device performance for data storage
applications is energy consumption per bit. In a memory cell
comprising a device according to an embodiment both writing and
erasing may rely on changes in state of the modulating element
material. For the example device comprising a GST modulating
element 102, the switching energy is given by the amount of energy
that is required to heat the GST above the melting (amorphization)
or glass-transition (crystallization) temperature, respectively.
Therefore, the energy consumption is directly related to the volume
of the memory element and read-out contrast. The relationship
between switching energy and read-out contrast is shown in FIG. 17.
In binary operation, a read-out contrast of 21% was demonstrated
with 430 pJ switching energy. On the other hand, switching energies
as low as 13.4 pJ for the same device are possible for a reduced
contrast of 0.7% which still enabled clear distinction of the two
levels. In addition, it is estimated that energy consumption can be
improved by up to one order of magnitude by operating the device
with sub-nanosecond instead of tens of nanosecond pulses. A
thermo-optical analysis has shown that the portion of absorbed
energy that gets lost due to thermal diffusion increases
significantly with increasing pulse length. Therefore, shorter and
more intense pulses are beneficial in terms of energy requirement
by quickly heating up the modulating element 102 to the required
transition temperature while reducing thermal diffusion losses.
[0160] In this early prototype of a device, energy consumption and
speed achieved compares well with pre-existing electrical
counterparts. For example, current commercial PCM-based electrical
memories (at the 45 nm node) typically require write pulses of
50-100 ns duration and read pulses of 10 ns (considerably longer
than the 10 ns/500 ps write/read demonstrated for an embodiment),
along with 5-10 pJ write energy (c.f. .about.13 pJ for an
embodiment). Although research-level electrical PCM devices improve
on such performance figures (e.g. 3.4 pJ write energy and 20 ns
write pulses), the performance of an embodiment can be further
improved by operating them with shorter pulses and by moving to
modulating elements with smaller footprint, as well as through the
development of new materials with faster and lower temperature
switching. Higher signal to noise ratio to improve the read-out
contrast could also be obtained with the use of optical cavities,
which would also reduce switching energies.
[0161] To reduce the device footprint, alternative architectures,
such as plasmonic antennas could be explored. Alternatively,
scaling down is plausible by using photonic circuitry operating at
shorter wavelengths (therefore, narrower waveguides) or by using
alternative phase-change materials with a higher difference in
refractive index (e.g. in the C and L-band). This way, smaller
devices may provide sufficiently good contrast. While multi-bit
access has been demonstrated with micro-ring resonators with
relatively large footprint, alternative technologies such as
ultra-compact on-chip optical multiplexer/demultiplexers can be
employed for size reduction. In addition, optical cavities with
smaller mode volume such as photonic crystal devices may be used to
localize the interaction volume of the optical mode with the memory
element further and thus lead to a smaller system size for
wavelength selective memory access.
[0162] FIGS. 18 and 19 respectively illustrate examples of a device
100 according to an embodiment, similar to that of FIG. 1, in which
photonic crystal structures 181 (FIG. 18) and plasmonic antennas
182 (FIG. 19) are used to enhance interaction with the modulating
element 102.
[0163] FIG. 20 illustrates an embodiment in which the waveguide 101
comprises a first port 115 for receiving a probe signal 105, a
second port 116 for detecting the output probe signal 106, and a
separate control port 117 for receiving the switching signal
125.
[0164] Although examples have been described with reference to
memory applications, it will be understood that embodiments are not
restricted to memory, and may also be used in optical switching.
FIG. 21 illustrates an optical switch 400 according to an
embodiment, comprising a first waveguide 701, a second waveguide
702, and a coupling waveguide 101. The coupling waveguide comprises
a resonant optical cavity 160 and a modulating element 102
evanescently coupled to the coupling waveguide 101. The state of
the modulating element 102 controls a transmission characteristic
of the coupling waveguide 101, so as to vary the degree to which
light is coupled between the first and second waveguide 720, 721.
An input light signal 720 can thereby be switched between output
signal 711 and output signal 712. The coupling may be directional,
so that light launched in the first waveguide 701 is coupled and
launched in a specific direction in the second waveguide 702.
[0165] FIG. 22 illustrates a switching fabric using the same
principles as the switch of FIG. 20. A plurality of horizontal
waveguides 431 each intersect a plurality of vertical waveguides
432. At each intersection, at least one coupling waveguide 101 is
provided, operating on the same principle as described with
reference to FIG. 21, controlling the degree to which the
intersecting vertical and horizontal waveguides 432, 431 are
coupled. Two coupling waveguides may be provided at each
intersection, for controlling coupling in each direction at each
intersection. The coupling between each intersection need not be
via a ring resonator, but may instead by via any optical
arrangement comprising a device according to an embodiment (e.g. an
optical switch).
[0166] FIG. 23 illustrates a transistor like latch 410, in which
transmission, via a coupling waveguide 101, between a first and
second waveguide 701, 702 is controlled based on the state of a
modulating element 102 that is evanescently coupled to the coupling
waveguide 101. A control port 117 is provided in the coupling
waveguide (analogous to a gate contact) for controlling
transmission between the first and second waveguides 701, 702.
[0167] FIG. 24 illustrates a Mach-Zehnder interferometer 420
comprising a device 100 according to an embodiment. The
inteferometer receives an input signal 425 at an optical splitter,
which divides the input signal 425 between a first path 421 and a
second path 422 of the interferometer. Each of the first and second
path 421, 422 comprise a waveguide. The first optical path
comprises a waveguide 101, the transmission properties of which are
varied by the state of a modulating element 102 evanescently
coupled to the waveguide 101. In this example, the modulating
element 102 may adjust an optical path length of the first optical
path 421 (for instance by varying refractive index). Signals of the
first and second optical path are recombined at optical combiner
424, so as to produce a first and second output signal 426,
427.
[0168] FIG. 25 illustrates a tuneable optical filter 440 comprising
a device 100 according to an embodiment. The filter 440 comprises a
waveguide 101, to which is evanescently coupled a plurality of
modulating elements 102, so as to define a grating, such as a Bragg
grating 441. The transmission characteristics of the grating 441
are varied depending on the state of the modulating elements, so as
to vary the transmission and reflection properties of the filter
440, thereby varying the degree to which an input signal 445 is
reflected as return signal 446, or transmitted as signal 447.
[0169] FIG. 26 illustrates a grating coupler 450 comprising a
device 100 according to an embodiment. The coupler comprises a
waveguide 101, to which is evanescently coupled a plurality of
modulating elements 102, so as to define a grating. The
transmission characteristics of the grating are varied depending on
the state of the modulating elements, so as to vary the
transmission and reflection properties of the grating coupler 450,
thereby varying the degree to which an input signal 445 is
reflected as return signal 446, or transmitted as signal 447.
[0170] FIG. 27 illustrates a device 100 according to an embodiment
in which the state of the modulating element 102 may be altered by
an electrical signal. The device 100 comprises an optical waveguide
101, with a modulating element 102 evanescently coupled to the
waveguide 101. The state of the modulating element 102 modifies the
transmission, reflection or absorption characteristics of the
waveguide 101, depending on its state. A resistor 123 is provided
in thermal contact with (e.g. on top of, or adjacent to) the
modulating element 102. When an electrical current is passed
through the resistor 123, it will heat up the modulating element
102, which results in the modulating element 102 changing state (as
previously described). As previously described, an optical probe
signal may be used to determine the state of the modulating element
102.
[0171] The resistor 123 may comprise part of a conducting track,
for example a metal or semiconductor track. In some embodiments the
resistor 123 may comprise a material that is substantially
transparent at the optical probe signal wavelength. The resistor
123 may, for example comprise a resistor track patterned over the
modulating element. A dielectric layer or insulating layer may be
interposed between the resistor 123 and the modulating element
102.
[0172] FIG. 28 illustrates an embodiment in which the state of the
modulating element 102 can be read and/or written electrically
(i.e. by an electrical signal). The state of the modulating element
102 may further be read and/or written optically, by optical
signals carried by the waveguide 101 (as described above). The
device 100 comprises a waveguide 101, modulating element 102, first
electrode 121 and second electrode 122.
[0173] The first electrode 121 and second electrode 122 are both in
electrical contact with the modulating element 102, so that a
voltage difference applied between the first and second electrodes
121, 122 results in a current through the modulating element 102.
Where the modulating element 102 comprises a layer of material, it
may be convenient for the first electrode 121 to be disposed under
the layer, and the second electrode 122 to be disposed on top of
the layer.
[0174] Alternatively, a lateral arrangement of electrodes may be
used, in which the flow of current through the layer of the
modulating element 102 is substantially in the plane of the
layer.
[0175] The resistance of the modulating element 102 may be inferred
from its voltage-current characteristics via the first and second
electrodes 121, 122. The state of the modulating element 102 may
thereby be inferred from an electrical probe signal applied to via
the first and second electrodes 121, 122. Furthermore, the state of
the modulating element 102 can be varied by Joule heating the
modulating element 102 by applying a voltage difference between the
first and second electrodes 121, 122.
[0176] At least one of the first and second electrodes 121, 122 may
comprise an optically transparent material, such as ITO. The
optical reading and/or writing of the modulating element 102 may be
substantially as described above, with reference to other
embodiments.
[0177] Features of the example embodiments described with reference
to FIGS. 27 and 28 may be combined. For example, an arrangement
that includes both a resistor 123, enabling heating of the
modulating element 102 by thermal conduction (without passing
current through the modulating element 102) may be combined with
first and second electrodes 121, 122. Such a device may be
electrically written via the resistor and/or first and second
electrodes 121, 122. Furthermore, the device may be read and/or
written optically, by optical signals in the waveguide 101.
[0178] The stacked arrangement of layers depicted in the drawings
is merely schematic, and each layer may be partially embedded
within another layer (e.g. by an patterning and planarization
process), or may be conformal over the topography of other layers.
For example, the upper surface of the lower electrode in FIG. 28
may be coplanar with the upper surface of the waveguide core. Other
variations are possible.
[0179] Electrical reading and/or writing may be more
straightforward to interface with an electrical controller. Optical
reading and/or writing may offer faster speed. Depending on the
application, different combinations of electrical and optical
reading and writing may be appropriate.
[0180] A device has been described that is suitable for use as an
integrated, all-photonic, truly-nonvolatile memory that provides
multi-level (e.g. 8 level) storage in a single cell along with
multi-bit (e.g. 3 bit) wavelength division multiplexed access (via
a single waveguide). A low-dimensional (i.e. small) modulating
element (e.g. phase-change elements) may be integrated with a
suitable waveguide. The modulating elements are switched between
states by evanescent coupling to light travelling along the
waveguides and are thus not restricted in size by the diffraction
limit.
[0181] Furthermore, the ability to switch readily and directly
between more than two levels has been shown, with accurate control
of the readout signal and excellent repeatability (capabilities
that requires complex iteration based algorithms in electronic
phase change memories). The capability for fast (.about.500 ps),
low power (.about.480 fJ), single shot readout of the modulator
element state has been shown, along with repeated (.times.100)
write/erase cycling while maintaining high readout contrast.
Embodiments of the invention are fully scalable: large arrays of
all-optical memory elements can be realised in accordance with
embodiments which are conveniently addressed, using WDM techniques,
through on-chip waveguides; such attributes are applicable for the
realization of practical on-chip optical interconnects. Embodiments
of the present invention may be used for storage class memory.
[0182] Hybrid circuits exploiting modulating elements (which may
comprise PCMs) may be enabled in accordance with embodiments,
leading to new forms of non-conventional (non-von Neumann)
computation and processing.
[0183] Embodiments of the present invention may be suitable for use
in a neuromorphic or synaptic-based processor. For example, the
accumulation of phase change in the modulating element may be
exploited to operate a device as an accumulator or adder, which is
a basis computational element. A phase change switching element can
be configured to exhibit spike-timing dependant plasticity, which
relies on relative spike timings from either side of the synapse.
The gradual programming of state of a PCM modulating element using
optical (or electrical) signals may be used to emulate a synaptic
connection.
[0184] It will be appreciated that the vast majority of
applications embodiments may comprise all optical reading and
writing, but some embodiments may comprise an optical/electrical
interface, so that electrical signals are provided to the device
for reading and/or writing.
[0185] From reading the present disclosure, other variations and
modifications will be apparent to the skilled person. Such
variations and modifications may involve equivalent and other
features which are already known in the art of photonic devices,
and which may be used instead of, or in addition to, features
already described herein.
[0186] Although the appended claims are directed to particular
combinations of features, it should be understood that the scope of
the disclosure of the present invention also includes any novel
feature or any novel combination of features disclosed herein
either explicitly or implicitly or any generalisation thereof,
whether or not it relates to the same invention as presently
claimed in any claim and whether or not it mitigates any or all of
the same technical problems as does the present invention.
[0187] Features which are described in the context of separate
embodiments may also be provided in combination in a single
embodiment. Conversely, various features which are, for brevity,
described in the context of a single embodiment, may also be
provided separately or in any suitable sub-combination. The
applicant hereby gives notice that new claims may be formulated to
such features and/or combinations of such features during the
prosecution of the present application or of any further
application derived therefrom. For the sake of completeness it is
also stated that the term "comprising" does not exclude other
elements or steps, the term "a" or "an" does not exclude a
plurality, and reference signs in the claims shall not be construed
as limiting the scope of the claims.
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