U.S. patent application number 12/891437 was filed with the patent office on 2012-03-29 for single photon source.
This patent application is currently assigned to Technische Universitat Berlin. Invention is credited to Dieter BIMBERG, Andrei SCHLIWA, Erik STOCK.
Application Number | 20120076166 12/891437 |
Document ID | / |
Family ID | 44675542 |
Filed Date | 2012-03-29 |
United States Patent
Application |
20120076166 |
Kind Code |
A1 |
SCHLIWA; Andrei ; et
al. |
March 29, 2012 |
SINGLE PHOTON SOURCE
Abstract
An embodiment of the invention relates to a single-photon source
for emitting single photons, comprising a cavity having a first
mirror and a second mirror and exhibiting a longitudinal resonance
frequency between the first and second mirror; at least one quantum
dot arranged inside said cavity, said quantum dot being
strain-dependent and configured to generate radiation at a
strain-dependent radiation frequency; a device capable of exciting
the quantum dot to generate radiation; a piezoelectric crystal
being arranged outside the cavity and mechanically coupled to the
second mirror's outer surface, said piezoelectric crystal
configured to receive a control voltage and capable of applying
either a laterally tensile and vertically compressive strain to
both the cavity and the quantum dot, or a laterally compressive and
vertically tensile strain to both the cavity and the quantum dot,
depending on the control voltage's polarity; wherein, in response
to said strain, the resonance frequency and the radiation frequency
shift in opposite directions.
Inventors: |
SCHLIWA; Andrei; (Storkow,
DE) ; STOCK; Erik; (Berlin, DE) ; BIMBERG;
Dieter; (Berlin, DE) |
Assignee: |
Technische Universitat
Berlin
|
Family ID: |
44675542 |
Appl. No.: |
12/891437 |
Filed: |
September 27, 2010 |
Current U.S.
Class: |
372/45.011 ;
977/951 |
Current CPC
Class: |
H01L 33/20 20130101;
H01L 33/06 20130101; H01L 33/30 20130101; H01L 33/465 20130101 |
Class at
Publication: |
372/45.011 ;
977/951 |
International
Class: |
H01S 5/34 20060101
H01S005/34 |
Claims
1. A single-photon source for emitting single photons, comprising a
cavity having a first mirror and a second mirror and exhibiting a
longitudinal resonance frequency between the first and second
mirror; at least one quantum dot arranged inside said cavity, said
quantum dot being strain-dependent and configured to generate
radiation at a strain-dependent radiation frequency; a device
capable exciting the quantum dot to generate radiation; a
piezoelectric crystal being arranged outside the cavity and
mechanically coupled to the second mirror's outer surface, said
piezoelectric crystal configured to receive a control voltage and
capable of applying either a laterally tensile and vertically
compressive strain to both the cavity and the quantum dot, or a
laterally compressive and vertically tensile to both the cavity and
the quantum dot, depending on the control voltage's polarity;
wherein, in response to said strain, the resonance frequency and
the radiation frequency shift in opposite directions.
2. Single-photon source of claim 1 wherein said quantum dot is
configured to increase its radiation frequency in case of laterally
compressive and vertically tensile strain, and to decrease its
radiation frequency in case of laterally tensile and vertically
compressive strain.
3. Single-photon source of claim 1 wherein the laterally tensile
and vertically compressive strain reduces the distance between the
first and second mirror and increases the cavity's resonance
frequency.
4. Single-photon source of claim 1 wherein the laterally
compressive and vertically tensile strain increases the distance
between the first and second mirror and decreases the cavity's
resonance frequency.
5. Single-photon source of claim 1 wherein the laterally tensile
and vertically compressive strain reduces the distance between the
first and second mirror, increases the cavity's resonance
frequency, and decreases the radiation frequency; and wherein the
laterally compressive and vertically tensile strain increases the
distance between the first and second mirror, decreases the
cavity's resonance frequency, and increases the radiation
frequency.
6. Single-photon source of claim 1 wherein photons are coupled out
of the cavity through the first mirror if the emission frequency
corresponds to the longitudinal resonance frequency of the
cavity.
7. Single-photon source of claim 1 wherein the strain induced by
the piezoelectric crystal inside the cavity, is biaxial.
8. Single-photon source of claim 1 wherein the first and second
mirror each comprise distributed Bragg reflectors.
9. Single-photon source of claim 1 wherein the piezoelectric
crystal has a surface section that is mechanically coupled to the
second mirror's outer surface, and a given thickness, wherein the
size of the surface section and the size of the second mirror's
outer surface increase and the thickness decreases if the control
voltage applied has a first polarity; and wherein the surface size
of the surface section and the size of the second mirror's outer
surface decrease and the thickness increases if the control voltage
has an opposite second polarity.
10. Single-photon source of claim 1 wherein the piezoelectric
crystal has a surface section that is mechanically coupled to the
second mirror's outer surface; and wherein the surface section of
the piezoelectric crystal is is at least as large as the second
mirror's outer surface.
11. Single-photon source of claim 1 wherein-seen along the beam
path of the photons leaving the cavity through the first mirror,
the surface section of the piezoelectric crystal completely covers
the second mirror's outer surface.
12. Single-photon source of claim 1 wherein an insulator is
arranged between the piezoelectric crystal and the second mirror's
outer surface, the insulator being configured to electrically
insulate the piezoelectric crystal from the second mirror's outer
surface and to forward the mechanical strain of the piezoelectric
crystal to the cavity.
13. Single-photon source of claim 1 wherein the cavity is arranged
on a front surface of a substrate and the piezoelectric crystal is
arranged on the back surface of the substrate.
14. Single-photon source of claim 13 wherein an insulator is
arranged between the piezoelectric crystal and the back surface of
the substrate, the insulator being configured to electrically
insulate the piezoelectric crystal from the substrate and to
forward the mechanical strain of the piezoelectric crystal through
the substrate to the cavity.
15. Single-photon source of claim 1 having a controller and a
voltage source, which is connected to the piezoelectric crystal and
controlled by the controller, wherein the controller is adapted to
control the voltage of the voltage source such that the quantum
dot's emission frequency corresponds to the longitudinal resonance
frequency of the cavity.
16. Single-photon source of claim 1 wherein the cavity is
cylindrical and forms a cylinder, wherein the first and second
mirrors are arranged at opposite cylinder end faces of the
cylinder; wherein the piezoelectric crystal is a piezoelectric
crystal layer; and wherein the quantum dot is comprised by an
active layer which is parallel to the first and second mirror and
the piezoelectric crystal layer.
17. Single-photon source of claim 16 wherein the quantum dot is
arranged in or adjacent to a current aperture that focuses charge
carriers onto the quantum dot, the current aperture being formed by
an insulating layer having an opening, and the quantum dot being
arranged in or adjacent to the opening, and wherein the insulating
layer is arranged parallel to the first and second mirror, the
active layer, and the piezoelectric crystal layer.
18. Method of emitting single photons, particularly for use in
quantum cryptography, using a single-photon source having a cavity
and at least one quantum dot arranged therein, the method
comprising the step of: applying a voltage to a piezoelectric
crystal being arranged outside the cavity and mechanically coupled
to the cavity, wherein in response to said voltage the
piezoelectric crystal applies either a laterally tensile and
vertically compressive strain to the cavity and the quantum dot, or
a laterally compressive and vertically tensile strain, depending on
the control voltage's polarity, and thereby shifts the resonance
frequency and the radiation frequency in opposite directions.
19. Method of claim 18 wherein the laterally tensile and vertically
compressive strain reduces the distance between the first and
second mirror, increases the cavity's resonance frequency, and
decreases the strain-dependent radiation frequency; wherein the
laterally compressive and vertically tensile strain increases the
distance between the first and second mirror, decreases the
cavity's resonance frequency, and increases the strain-dependent
radiation frequency; and wherein the voltage applied to the
piezoelectric crystal is varied until the emission frequency
corresponds to the longitudinal resonance frequency of the cavity.
Description
[0001] The invention relates to single-photon sources.
Single-photon sources are photon sources which can emit single
photons, particularly with a defined or predetermined polarization,
transposed photons and cascades of correlated photons.
BACKGROUND OF THE INVENTION
[0002] Single-photon sources are the core element of quantum
cryptography. In the exchange of sensitive data such as e.g. online
business transactions, they offer absolute interception protection
based on the laws of quantum mechanics.
[0003] An ideal single-photon source (photon gun) is a component
which emits a single photon after a trigger signal, and only then
(on demand). The central element of a single-photon source is
optimally a quantized system with discrete energy levels.
[0004] German Patent Application DE 10 2008 036 400 describes a
single-photon source having a cylindrical cavity. The cavity
comprises a first mirror and a second mirror and exhibits a
longitudinal resonance frequency between the first and second
mirrors. The single-photon source emits photons efficiently only if
the quantum dot's radiation frequency corresponds to the cavity's
longitudinal resonance frequency. As such, the quality factor of
the cavity needs to be limited since a large quality factor would
reduce the chance that the radiation frequency matches the cavity's
longitudinal resonance frequency. In summary, the tolerance range
of this type of single-photon source is small, and the fabrication
yield is poor.
OBJECTIVE OF THE PRESENT INVENTION
[0005] An objective of the present invention is to provide a
single-photon source which has a larger tolerance range for
fabrication than prior art single photon sources.
[0006] A further objective of the present invention is to provide a
single-photon source that allows including cavities having a higher
Q-factor than prior art single photon sources.
[0007] Furthermore, it is an objective of the present invention to
provide a single-photon source that allows efficiently compensating
temperature drifts.
BRIEF SUMMARY OF THE INVENTION
[0008] An embodiment of the present invention relates to a
single-photon source for emitting single photons, comprising a
cavity having a first mirror and a second mirror and exhibiting a
longitudinal resonance frequency between the first and second
mirror; at least one quantum dot arranged inside said cavity, said
quantum dot being strain-dependent and configured to generate
radiation at a strain-dependent radiation frequency; a device
capable of exciting the quantum dot to generate radiation; a
piezoelectric crystal being arranged outside the cavity and
mechanically coupled to the second mirror's outer surface, said
piezoelectric crystal configured to receive a control voltage and
capable of applying either a laterally tensile and vertically
compressive strain to both the cavity and the quantum dot, or a
laterally compressive and vertically tensile strain to both the
cavity and the quantum dot, depending on the control voltage's
polarity; wherein, in response to said strain, the resonance
frequency and the radiation frequency shift in opposite
directions.
[0009] The cavity is oriented along and photon emission occurs in
the vertical direction. A lateral direction is any direction that
is perpendicular to the vertical direction along which the cavity
is oriented and photons are emitted. A lateral direction is a
direction in 3-dimensional space that is perpendicular to the
vertical direction.
[0010] According to this embodiment of the invention, the resonance
frequency and the radiation frequency are shifted in opposite
directions in response to strain. This is achieved by applying the
same kind of strain (laterally tensile and vertically compressive
or laterally compressive and vertically tensile) to both the cavity
and the quantum dot. As such, by applying a voltage of the
appropriate polarity, the resonance frequency and the radiation
frequency may always be brought to a match. Thus, it is not
mandatory that the resonance frequency and the radiation frequency
match exactly after fabrication of the single-photon source since a
mismatch may later be corrected by simply applying a voltage to the
piezoelectric crystal. Further, in case the temperature changes,
any drift of the resonance frequency relative to the radiation
frequency may be easily compensated.
[0011] The device capable of exciting the quantum dot to generate
radiation may be a charge carrier injection device which is capable
of injecting charge carriers into the cavity in order to excite the
quantum dot to generate radiation. Alternatively the device may be
an optical pump source which excites the quantum dot optically.
[0012] According to a preferred embodiment, the quantum dot is
configured to increase its radiation frequency in case of laterally
compressive and vertically tensile strain, and to decrease its
radiation frequency in case of laterally tensile and vertically
compressive strain.
[0013] The laterally tensile and vertically compressive strain
preferably reduces the distance between the first and second mirror
and increases the cavity's resonance frequency. The laterally
compressive and vertically tensile strain preferably increases the
distance between the first and second mirror and decreases the
cavity's resonance frequency.
[0014] The photons are preferably coupled out of the cavity through
the first mirror if the emission frequency corresponds to the
longitudinal resonance frequency of the cavity.
[0015] The strain induced by the piezoelectric crystal inside the
cavity and inside the quantum dot, is preferably biaxial.
[0016] The first and second mirror may each comprise distributed
Bragg reflectors.
[0017] The piezoelectric crystal may have a surface section that is
mechanically coupled to the second mirror's outer surface, and a
given thickness. The size of the surface section preferably
increases and the thickness preferably decreases if the control
voltage has a first polarity, and the surface size of the surface
section preferably decreases and the thickness preferably increases
if the control voltage has an opposite second polarity.
[0018] The surface section of the piezoelectric crystal is
preferably as large as the second mirror's outer surface or larger
than the second mirror's outer surface.
[0019] Preferably, the surface section of the piezoelectric crystal
completely covers the second mirror's outer surface--seen along the
beam path of the photons leaving the cavity through the first
mirror.
[0020] The piezoelectric crystal may be a piezoelectric crystal
layer having a surface section that is mechanically coupled to the
second mirror's outer surface, and a given layer thickness. The
size of the surface section preferably increases and the layer
thickness preferably decreases if the control voltage has a first
polarity, and the size of the surface section preferably decreases
and the thickness preferably increases if the control voltage has
an opposite second polarity.
[0021] Preferably, an insulator is arranged between the
piezoelectric crystal and the second mirror's outer surface, the
insulator being configured to electrically insulate the
piezoelectric crystal from the second mirror's outer surface and to
forward the mechanical strain of the piezoelectric crystal to the
cavity.
[0022] The cavity is preferably arranged on a front surface of a
substrate and the piezoelectric crystal is preferably arranged on a
back surface of the substrate. The substrate may be formed by an
etch stop layer. Such an etch stop layer may have been placed on a
preliminary substrate that is removed during the processing of the
device.
[0023] An insulator may be arranged between the piezoelectric
crystal and the back surface of the substrate, the insulator being
configured to electrically insulate the piezoelectric crystal from
the substrate and to forward the mechanical strain of the
piezoelectric crystal though the substrate to the cavity.
[0024] The single-photon source may have a controller and a voltage
source, which is connected to the piezoelectric crystal and
controlled by the controller. The controller is preferably adapted
to control the voltage of the voltage source such that the quantum
dot's emission frequency corresponds to the longitudinal resonance
frequency of the cavity.
[0025] The cavity preferably forms a cylinder, and the quantum dot
is preferably arranged inside the cylindrical cavity. The base
contour of the cylindrical cavity may be of any form. For instance,
the base contour may be a circle, a square, a rectangle, an
ellipse, etc. The first and second mirrors are preferably arranged
at the upper and lower cylinder end faces of the cylindrical
cavity.
[0026] The quantum dot is preferably arranged in or adjacent (below
or above) a current aperture that focuses charge carriers onto the
quantum dot. The current aperture may be formed by an insulating
layer having an opening. The quantum dot is preferably arranged in
or adjacent (below or above) this opening. The insulating layer is
preferably arranged parallel to the first and second mirror.
[0027] The quantum dot is preferably comprised by an active layer
which is parallel to the first and second mirror. In this way, the
same kind of strain (laterally tensile and vertically compressive
or laterally compressive and vertically tensile) may be easily
induced to both the cavity and the quantum dot by applying strain
to the second mirror's surface. The active layer is preferably also
parallel to the insulating layer of the current aperture.
[0028] According to a further preferred embodiment, the cavity is
cylindrical and forms a cylinder; the first and second mirrors are
arranged at opposite cylinder end faces of the cylinder; the
piezoelectric crystal is a piezoelectric crystal layer; and the
quantum dot is comprised by an active layer which is parallel to
the first and second mirror and the piezoelectric crystal layer.
The quantum dot may be arranged in or adjacent to a current
aperture that focuses charge carriers onto the quantum dot, the
current aperture being formed by an insulating layer having an
opening, and the quantum dot being arranged in or adjacent to the
opening. The insulating layer may be arranged parallel to the first
and second mirror, the active layer, and the piezoelectric crystal
layer.
[0029] The invention also relates to a method of emitting single
photons, particularly for use in quantum cryptography, using a
single-photon source having a cavity and at least one quantum dot
arranged therein, the method comprising the step of: [0030]
applying a voltage to a piezoelectric crystal being arranged
outside the cavity and mechanically coupled to the cavity, [0031]
wherein in response to said voltage the piezoelectric crystal
applies either a laterally tensile and vertically compressive
strain to the cavity and the quantum dot, or a laterally
compressive and vertically tensile strain, depending on the control
voltage's polarity, and thereby shifts the resonance frequency and
the radiation frequency in opposite directions.
[0032] The laterally tensile and vertically compressive strain may
reduce the distance between the first and second mirror, increase
the cavity's resonance frequency, and decrease the strain-dependent
radiation frequency. The laterally compressive and vertically
tensile strain may increase the distance between the first and
second mirror, decrease the cavity's resonance frequency, and
increase the strain-dependent radiation frequency. The voltage
applied to the piezoelectric crystal may be varied until the
emission frequency corresponds to the longitudinal resonance
frequency of the cavity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] In order that the manner in which the above-recited and
other advantages of the invention are obtained will be readily
understood, a more particular description of the invention briefly
described above will be rendered by reference to specific
embodiments thereof which are illustrated in the appended figures.
Understanding that these figures depict only typical embodiments of
the invention and are therefore not to be considered to be limiting
of its scope, the invention will be described and explained with
additional specificity and detail by the use of the accompanying
drawings in which
[0034] FIG. 1 shows a first exemplary embodiment of a single photon
emitter according to the present invention;
[0035] FIG. 2 shows the single photon emitter according to FIG. 1
if an electric voltage is applied to the piezoelectric crystal;
[0036] FIG. 3 shows in an exemplary fashion the frequency
characteristic of two cavities having different Q-factors; and
[0037] FIG. 4 shows a second exemplary embodiment of a single
photon emitter according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] The preferred embodiments of the present invention will be
best understood by reference to the drawings, wherein identical or
comparable parts are designated by the same reference signs
throughout.
[0039] It will be readily understood that the present invention, as
generally described herein, could vary in a wide range. Thus, the
following more detailed description of the exemplary embodiments of
the present invention, is not intended to limit the scope of the
invention, as claimed, but is merely representative of presently
preferred embodiments of the invention.
[0040] FIG. 1 shows a first exemplary embodiment of a single-photon
source 10 according to the present invention. The single-photon
source 10 comprises a cylindrical cavity 20 having a first mirror
30 and a second mirror 40. The first mirror 30 and the second
mirror 40 may be distributed Bragg reflectors DBR as indicted in an
exemplary fashion in FIG. 1. The distributed Bragg reflectors DBR
may consist of Al(Ga)O.sub.x-material.
[0041] The cavity 20 exhibits a plurality of longitudinal resonance
frequencies between the first and second mirror 30 and 40. The
longitudinal resonance frequencies depend on the distance between
both mirrors 30 and 40.
[0042] At least one quantum dot 50 is formed in an active layer 55
and arranged inside the cavity 20. The quantum dot 50 is positioned
inside or adjacent (above or below) an opening 60 of a
non-conductive layer 70. The opening 60 is filled with conductive
semiconductor material such as AlGaAs. The non-conductive layer 70
forms a current aperture through which electrical current may flow
in vertical direction. The current aperture focuses the electrical
current towards the single quantum dot 50 in order to increase the
current efficiency and to avoid pumping of other (unused) quantum
dots, which might be positioned in the active layer 55.
[0043] The quantum dot 50 generates radiation at a specific
radiation frequency when charge carriers (e.g. electrons and holes)
are injected therein. The quantum dot 50 preferably consists of
semiconductor material such as InGaAs material, and is thus very
strain-dependent. As such, the radiation frequency of the emitted
radiation is also very strain-dependent.
[0044] A charge carrier injection device is formed by a p-doped
contact layer 90 and an n-doped contact layer 100. If a positive
voltage is applied to the contacts 110 and 120, which are connected
with both contact layers 90 and 100, charge carriers are generated
and injected into the cavity 20. These charge carriers excite the
quantum dot 50 to generate radiation in form of single photons P.
The photons P are coupled out of the cavity 20 through the first
mirror 30 if the emission frequency corresponds to the longitudinal
resonance frequency of the cavity 20.
[0045] The contacts 110 and 120 may be within the cavity or outside
the mirrors. In FIG. 1, the contacts 110 and 120 are additionally
marked as "p" and "n" since they contact the p-doped layer 90 and
the n-doped layer 100, respectively.
[0046] As can be seen in FIG. 1, the second mirror's outer surface
41 is arranged on the front surface 131 of a substrate 130. A
piezoelectric crystal 140 is arranged on the back surface 132 of
the substrate 130. The substrate 130 may consist of GaAs
semiconductor material.
[0047] The piezoelectric crystal 140, which may consist of PMN-PT
(lead magnesium niobate-lead titanate
Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3--PbTiO.sub.3) material, is
electrically connected to a controllable voltage source 150 which
is controlled by a controller 160.
[0048] The piezoelectric crystal 140 is mechanically coupled to the
second mirror's outer surface 41 through the substrate 130. As
such, the piezoelectric crystal 140 may apply either a laterally
tensile and vertically compressive strain to the cavity 20 and the
quantum dot 50 inside the cavity 20, or a laterally compressive and
vertically tensile strain. The kind of strain depends on the
control voltage's polarity. Therefore, the controller 160 may
influence and control the emission behavior of the quantum dot 50
and the resonance frequency of the cavity 20 by applying an
appropriate control voltage.
[0049] In order to efficiently transfer the mechanical stress
induced by the piezoelectric crystal 140 into the cavity 20, the
upper surface of the piezoelectric crystal 140 is preferably larger
than the second mirror's outer surface 41 and preferably interacts
with the entire second mirror's outer surface 41.
[0050] The active layer 55 is preferably arranged parallel to the
first and second mirror 30 and 40. As such, the same kind of strain
(laterally tensile and vertically compressive or laterally
compressive and vertically tensile) is induced to both the cavity
and the quantum dot when strain is induced to the second mirror's
surface 41.
[0051] As discussed above, the embodiment shown in FIG. 1 comprises
a substrate 130 which separates the piezoelectric crystal 140 from
the second mirror's outer surface. In order to provide efficient
transfer of mechanical strain from the piezoelectric crystal 140 to
the second mirror 40, the quantum dot 50 and the first mirror 30,
the thickness of substrate 130 should be as small as possible, or
the substrate 130 should be omitted, if possible.
[0052] For instance, the substrate 130 as shown in FIG. 1, may be
formed by an etch stop layer which was formerly placed on top of a
preliminary substrate (not shown in FIG. 1) during the fabrication
of the second mirror 40, the quantum dot 50 and the first mirror
30. During said fabrication, the second mirror 40, the quantum dot
50 and the first mirror 30 may be deposited on the etch stop layer
which is placed on top of the preliminary substrate. Afterwards,
the preliminary substrate is removed, for instance by etching. The
remaining structure composed of the second mirror 40, the quantum
dot 50, the first mirror 30, and the etch stop layer may then be
equipped with the piezoelectric crystal 140.
[0053] In the embodiment shown in FIG. 1, the cavity is oriented
along and photon emission occurs in the vertical direction "z". A
lateral direction is any direction (e.g. "x" and "y") that is
perpendicular to the vertical direction "z" along which the cavity
is oriented and photons are emitted. A lateral direction is a
direction in 3-dimensional space that is perpendicular to the
vertical direction "z".
[0054] FIG. 2 shows the embodiment of FIG. 1 during operation. In
the middle section of FIG. 2, the voltage V.sub.piezo is zero and
no strain is applied to the cavity 20 or the quantum dot 50. In the
following, it is assumed that the radiation frequency f.sub.QD of
the photons generated by the quantum dot 50 does not match with the
resonance frequency f.sub.cav of the cavity 20 at a given
temperature T1. As such the photon energy E.sub.QD=h*f.sub.QD does
not equal the resonance energy E.sub.cav=h*f.sub.cav either.
[0055] Depending on the cavity's quality factor, there is no
efficient emission of photons P when the emission frequency of the
radiation does not lie in the spectral transmission window of the
cavity 20. This is shown in FIGS. 3a and 3b by two examples. The
cavity's quality factor is indicated by the reflection
characteristics R of the cavity 20.
[0056] In FIG. 3a, the cavity's quality factor is small and the
radiation I is emitted even though the radiation frequency f.sub.QD
does not perfectly match the cavity dip, i.e. the resonance
frequency f.sub.cav of the cavity 20. In FIG. 3b, the cavity's
quality factor is better and the radiation I is not emitted since
the radiation frequency f.sub.QD does not match the resonance
frequency f.sub.cav of the cavity 20. In order to increase (FIG.
3a) or enable (FIG. 3b) an efficient emission of radiation the
radiation frequency f.sub.QD and the resonance frequency f.sub.cav
need to be shifted relative to each other as indicated by reference
numeral I' in FIGS. 3a and 3b.
[0057] Referring again to FIG. 2, one can see that by applying a
positive voltage V.sub.piezo the piezoelectric crystal 140 at the
temperature T1, a laterally compressive and vertically tensile
strain is induced in the piezoelectric crystal 140, the cavity 20,
and the quantum dot 50. The thickness of the piezoelectric crystal
140 increases from a thickness d (V.sub.piezo=0) to a larger
thickness d' (V.sub.piezo>0), and the surface A of the
piezoelectric crystal 140 decreases from a value A (V.sub.piezo=0)
to a reduced value A' (V.sub.piezo>0). This mechanical
deformation has an impact on the cavity 20 and the quantum dot 50.
The distance H between both mirrors 30 and 40 increases, and the
resonance frequency f.sub.cav and the resonance energy E.sub.cav
decrease. Due to the deformation of the quantum dot 50, the photon
energy E.sub.QD of the quantum dot 50 increases. As such, applying
a positive voltage V.sub.piezo does not increase the radiation
efficiency since E.sub.cav and E.sub.QD further separate from each
other.
[0058] However, by applying a negative voltage V.sub.piezo to the
piezoelectric crystal 140 at the temperature T1, the radiation
efficiency will be increased. A negative voltage V.sub.piezo
induces a laterally tensile and vertically compressive strain in
the piezoelectric crystal 140, the cavity 20, and the quantum dot
50. It can be seen in FIG. 2 that the thickness of the
piezoelectric crystal 140 decreases from a thickness d
(V.sub.piezo=0) to a smaller thickness d'' (V.sub.piezo<0), and
the surface A of the piezoelectric crystal 140 increases from a
value A (V.sub.piezo=0) to a larger value A'' (V.sub.piezo<0).
Again, this mechanical deformation has an impact on the cavity 20
and the quantum dot 50. The distance H between both mirrors 30 and
40 decreases, and the resonance frequency f.sub.cav and the
resonance energy E.sub.cav increase. Due to the deformation of the
quantum dot 50, the photon energy E.sub.QD decreases. By applying
the appropriate negative voltage V.sub.piezo, E.sub.cav and
E.sub.QD may be brought to a perfect match.
[0059] If the temperature increases from T1 to T2, E.sub.cav
increases and E.sub.QD decreases, and the radiation efficiency will
drop. This negative effect can be compensated by applying a
different voltage as shown in FIG. 2. In case of a positive voltage
V.sub.piezo, E.sub.cav and E.sub.QD will be shifted together and a
perfect match may be also achieved at the temperature T2.
[0060] In order to control the voltage V.sub.piezo that is applied
by the voltage source 150, the controller 160 may be connected to a
detector which detects the photons P emitted through the first
mirror 30. For instance, a feed-back loop may be provided which
controls the voltage V.sub.piezo in order to keep E.sub.cav and
E.sub.QD matched and the photon emission at its maximum level.
[0061] FIG. 4 shows a second exemplary embodiment of a
single-photon source 10 according to the present invention. In
contrast to the first embodiment, an insulating layer 170 is
disposed between the substrate 130 and the piezoelectric crystal
140.
REFERENCE SIGNS
[0062] 10 single-photon source [0063] 20 cavity [0064] 30 first
mirror [0065] 40 second mirror [0066] 41 second mirror's outer
surface [0067] 50 quantum dot [0068] 55 active layer [0069] 60
opening [0070] 70 non-conductive layer [0071] 90 p-doped contact
layer [0072] 100 n-doped contact layer [0073] 110 contact to
p-doped layer 90 [0074] 120 contact to n-doped layer 100 [0075] 130
substrate [0076] 131 front surface [0077] 132 back surface [0078]
140 piezoelectric crystal [0079] 150 controllable voltage source
[0080] 160 controller [0081] 170 insulating layer [0082] A surface
[0083] A' surface [0084] A'' surface [0085] d thickness [0086] d'
thickness [0087] d'' thickness [0088] f.sub.cav resonance frequency
[0089] f.sub.QD radiation frequency [0090] E.sub.cav resonance
energy [0091] E.sub.QD photon energy [0092] H distance [0093] H'
distance [0094] H'' distance [0095] I radiation [0096] I' radiation
[0097] P Photon [0098] R reflection characteristic [0099] T1
temperature [0100] T2 temperature [0101] V.sub.piezo voltage
* * * * *