Infrared-energized Visual Displays Using Up-converting Phosphor

Abstract

A color pictorial display is produced by scanning a phosphor layer with a frequency and/or amplitude-modulated infrared beam. Visible emission results by virtue of a two-photon or high-order multiphoton process.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is concerned with phosphorescent pictorial displays such as are used in television systems.

2. Description of the Prior Art GaAs

A variety of systems have been utilized for producing pictorial phosphorescent displays. Some have been used for producing colored pictures.

The most common system, that which is used in essentially all home television receivers, depends on the secondary phosphorescent emission produced by an incident electron beam. In operation, the electron beam, ordinarily resulting from a thermal source, scans the phosphor coating level by level in sequence to produce a raster. The beam is amplitude modulated as it scans so as to produce an attendant change in secondary emission and a reproduction image. Scan rates, emission lifetimes, and the persistence of human vision are all such as to produce the illusion of motion. Required voltages, minimization of collisions with gas molecules, the nature of usual cathode materials, and other considerations give rise to the requirement that both electron gun and phosphor coating be contained in a sealed tube.

This cathode-ray tube has been adapted to the production of colored pictures. The commercial form of the color tube generally includes a triple gun producing three beams each containing picture information for one of three addition colors. In this arrangement there are three types of phosphors each of which produces secondary emission of a fundamental color. Since all of the phosphors may be excited by any of the electron beams, it is necessary to produce "islands" of the basic color phosphors and to somehow provide for selective excitation of each member of the trio by the associated beam of the trio of beams. This is generally accomplished by shadow masking in which selection depends upon beam angle. The engineering problems resulting from the registry requirement on the fine scale involved have been significant. The color cathode-ray tube remains by far the single, most expensive element in the television receiver.

Proposed alternative systems generally also make use of scanning, amplitude modulated, high-energy beams which are down-converted to produce secondary emission at visible wavelength from a phosphor coating. One type of excitation energy which has been considered is at ultraviolet wavelength, and the phosphors used are selected from the large variety of secondary emission materials such as are presently used in luminous dyestuffs lasers, etc.

Where such systems are adapted to the production of a colored image, use has generally been made of the same type of three-gun configuration as in the CRT. The screen may again be composed of separate trios of islands with a member of each trio emitting at a suitable characteristic wavelength. An alternative system depends upon depth of penetration of one or more energizing beams to successive homogeneous phosphor layers.

While there has been a widespread desire to produce a video display on a flat, solid-state panel, this desire has not been fulfilled in any as yet commercially realized from. PHosphor panels thus far proposed have generally made use of a cross-point array of electroluminescent elements. Such devices are ordinarily monochromatic and detail has, in general, been coarse.

SUMMARY OF THE INVENTION

In accordance with the invention, a pictorial phosphorescent display results by up conversion from information introduced into the phosphor coating in the form of infrared energy. Phosphorescent materials suitable for this use are all capable of emission in the visible spectrum at at least two distinct wavelengths, both of which are readily discernible by the unaided human vision.

In one form of the invention, the display is apparently monochromatic although the substantially white image actually results from simultaneous emission of two different wavelengths. Since, in such embodiment, efficiency of emission of the different wavelengths differs, "color toning" to produce the apparently substantially white image results from adjustment of amplitude of the exciting infrared energy.

Various embodiments of the invention may result in a colored image. As suggested by the preceding paragraph, amplitude modulation of the exciting energy may produce an apparent color shift. In a specific embodiment, this mechanism is accompanied by an additional one which results in a third wavelength produced from a mechanically admixed phosphor energized at a frequency which differs from the peak absorption frequency of the two-color phosphor. In this embodiment, complete apparent color selection results from a combination of amplitude and frequency modulation of the infrared excitation. In an alternative, two or three separate beams may simply be amplitude modulated.

A commercially promising form of the invention utilizes but a single infrared source, usually in the form of a columnated beam. A convenient beam source is a solid-state laser or gas laser although it is not a requirement that the energy be coherent. Elements for amplitude modulating and elements for shifting frequency are in advanced stages of development and several forms are already commercially available.

Amplitude modulation may be accomplished by electro-optic or magneto-optic interaction, see, for example, Vol. 38, Journal of Applied Physics, p.1611 (1967) and Abstract IEEE Transactions on Magnetics, Vol. Mag. 2, p.304, Sept. 1966. Frequency shifting may be accomplished parametrically and a particularly effective element operating on this principle was recently described in 12 Applied Physics Letters, p. 308 (1968).

Deflection systems may be digital or continuous, and these too have been described in the literature, see Proceedings IEEE, Vol. 54, No. 10, p. 1437, Oct. 1966. A particularly useful arrangement may utilize digital deflection in one direction (i.e., fixed position raster lines) in combination with continuous scanning in the other in the manner of the usual CRT arrangement.

While the extreme simplicity of operating but a single beam is appealing, certain embodiments may desirably use two or even three separate beams. One exemplary phosphor material requires a substantially different energy level for one of the colors. A separate beam at such level (of such frequency incidentally as to have little excitation effect on the remainder of the phosphor) may be desirable. A different arrangement may use separate beams of differing infrared wavelength to selectively excite the different colors.

Regardless of whether excitation is by single or multiple beams, the registration problem in the conventional color CRT is completely avoided since the phosphor composition is homogeneous at least on the scale required for image resolution.

While the preferred aspect of the invention is largely concerned with the arrangements briefly described above, certain of the compositions are novel, per se, and may be otherwise utilized. Accordingly, such materials are intended to form a part of this invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of one display system in accordance with the invention;

FIG. 2 is a schematic representation of another display system in accordance with the invention; and

FIG. 3 is an energy level diagram in ordinate units of wave numbers for activator ions of concern in accordance with the invention.

DETAILED DESCRIPTION

In FIG. 1, infrared radiation is produced by source 1. In this embodiment, source 1 produces infrared emission, either coherent or incoherent, such as may be produced by diode 2 biased through leads 3 and 4 connected to electrical energizing means not shown. Diodes 5 and/or 6, shown in phantom, may emit energy either or both at an amplitude and/or frequency different from that of diode 1. These optional diodes are provided with leads 7-8 and 9-10 also connected with biasing source not shown. Diodes are provided with a parabolic reflector 11 which, together with external lens 12, may serve to columnate and/or focus beam 13 on the surface of deflector mirror 14. Deflector 14 is provided with a pivot point 15 to permit scanning in at least one direction. In any event, the deflected beam, now denoted 16, is focused by lens system 17 so as to result in focused beam 18 which excites phosphor coating 19 on screen substrate 20. Also not shown but usefully incorporated in certain forms of the system represented by FIG. 1 are means for varying the amplitude of emission from diodes 2, 5 or 6. This variation, which may be continuous or digital, may take any appropriate form depending on the type of visual display desired.

It has been noted that research and development in laser technology has produced rapid advances in the infrared field in general. The arrangement of FIG. 2 is one form of inventive embodiment which makes use of some of those advances. In the arrangement shown, coherent infrared radiation is produced by laser source 25 which may consist of a single laser 26 as supplemented by either or both of lasers 27 and 28 shown in phantom. While emission from laser source 25 may be utilized directly and its output may be modulated internally by any of the various means such as have been described in the literature, present frequencies availability from the most efficiently operating of the solid-state lasers suggests use of certain auxiliary nonlinear elements. Of course, use of such solid-state lasers is to be considered as to be exemplary only and similar arrangements may make use of other coherent light sources such as gas lasers or liquid lasers. In the embodiment shown, the emission from laser source 25 is first passed through a second harmonic generator 29 which produces a first overtone of the fundamental emission of 25. Thereafter, the resulting half-wavelength beam 30 is passed through one or more parametric oscillators 31 which may be electrically tuned by means of leads shown schematically as 32 and 33 so as to produce a desired fixed or varying output frequency in a beam now depicted as 34. This beam 34 may now be passed through one or more amplitude modulators 35 which may, for example, depend on an electro-optic interaction, in which event, modulation is furnished in the form of an electric bias applied via leads 36 and 37 connected to source not shown. Exciting beam 38 which may now contain frequency and/or amplitude adjusted components is next introduced into deflector system 39 so designed as to produce at least a single axis scanning beam 40 which thereby illuminates excessive portions of phosphor coating 41 supported on screen substrate 42.

It is not considered within the necessary scope of this description to describe the ancillary modulating and deflector elements such as 29, 31, 35 and 39 in detail. Briefly, SHG element 29 may be constructed of the phase-matchable, nonlinear material Ba.sub.2 Na.sub.5 NbO.sub.15. Engineering criteria such as crystallographic direction, temperature, etc., have been adequately described in the literature, see, for example, Vol. 12, Applied Physics Letters, p. 308 (1968). Element 31 may also be constructed of Ba.sub.2 Na.sub.5 NbO.sub.15. Modulator 35 may desirably utilize a crystal of lithium tantalate, and an element utilizing this material and demonstrating frequency capability sufficient for the purposes of this arrangement has been described in Vol. 38, Journal of Applied Physics, p. 1611 (1967). Deflector system 39 may take any of several forms. It may be a mechanically rotating prism or prisms or it may be one of the electric deflector systems in which the outgoing direction of beam 40 is dependent upon refractive index which is, in turn, dependent upon the electrical bias applied by means not shown. One composition which has demonstrated the desired capability is a mixture of potassium tantalate and potassium niobate known as KTN. This material is substantially crystallographically cubic in its operating condition and, consequently, is characterized by large effective aperture for the incoming and outgoing beams. A description of a deflector utilizing this material is described in U.S. Pat. No. 3,290,619 (Dec. 6, 1966). As there described, suitable KTN compositions may be represented by the atomic formula KTa.sub.x Nb.sub.1.sub.-x O.sub.3 in which x equals from 0.2 to 0.8 .

Phosphor material such as that of which layers 19 and 41 are composed is a critical element of the invention and is discussed in some detail. It is convenient to describe this material in terms of an energy level diagram. A preferred composition for use either in a colored or black and white display makes use of the sensitizer and activator trivalent cations of ytterbium and erbium, respectively. Whether for black and white or color display purposes, it may be desired to utilize different visible emission wavelengths available from the single ion Er.sup.3.sup.+ or the pair Er.sup.3.sup.+ and Ho.sup.3.sup.+. A preferred matrix which permits readily discernible levels of different wavelength emissions is an oxychloride of a more complex stoichiometry than MOCL (in which M is any cation) in which the chlorine to oxygen ratio is greater than one. Where a color display is desired, a third wavelength of visible emission may be obtained from trivalent thulium in a mechanically admixed compound. Selection as between the Er.sup.3.sup.+ and Ho.sup.3.sup.+ wavelengths, the Ho.sup.3.sup.+ wavelength and the Im.sup.3.sup.+ wavelength is on the basis of a difference in absorption by the common sensitizer Yb.sup.3.sup.+ in different hosts.

The energy level diagram of FIG. 3 is representative of such a system. The details of the absorption and emission levels were measured spectroscopically. Excitation routes for certain of the multiphoton processes are, however, deduced from observed emission. Recognizing that excitation routes may differ somewhat from those indicated, the diagram is nevertheless sufficient for describing in general the type of mechanism which is responsible for at least a preferred phosphor in accordance with the invention.

FIG. 3 contains information on Yb.sup.3.sup.+, Er.sup.3.sup.+, Ho.sup.3.sup.+ and Tm.sup.3.sup.+. While the pairs Yb.sup.3.sup.+ --Ho.sup.3.sup.+ and Yb.sup.3.sup.+ --Tm.sup.3.sup.+ are not the most efficient for energy up conversion, the former does provide a strong green fluorescence and enables a desirable color shift and improvement in efficiency when included as an ancillary pair with Yb.sup.3.sup.+ --Er.sup.3.sup.+. Further, the Yb.sup.3.sup.+ --Tm.sup.3.sup.+ couple provides a source of blue fluorescence.

The ordinate units are in wavelengths per centimeter (cm..sup..sup.-1). These units may be converted to wavelength in angstrom units (A.) or microns (.mu.) in accordance with the relationship.

The left-hand portion of the diagram is concerned with the relevant manifolds of Yb.sup.3.sup.+ in a host of the invention. Absorption in Yb.sup.3.sup.+ results in an energy increase from the ground manifold Yb.sup.2 F.sub.7/2 to the Yb.sup.2 F.sub.5/2 manifold. This absorption defines a band which includes levels at 10,200 cm..sup..sup.-1, 10,500 cm..sup. .sup.-1 and 10,700 cm..sup..sup.-1. The positions of these levels are affected by the crystal field of splitting within the structures having at least one each of two different anions or at least one anion vacancy per unit cell or formula unit. In the oxychlorides, for example, they include a broad absorption which peaks at about 0.94.mu.(10,600 cm..sup..sup.-1), there is an efficient transfer of energy from a silicon-doped GaAs diode (with its emission peak at about 0.93.mu.). This contrast with the comparatively small splitting in lanthanum fluoride and other less anisotropic hosts in which absorption peaking is at about 0.98.mu. for Yb.sup.3.sup.+.

The remainder of FIG. 3 is discussed in conjunction with the postulated excitation mechanism. All energy level values and all relaxations indicated on the figure have been experimentally verified.

POSTULATED EXCITATION MECHANISMS

Following absorption by Yb.sup.3.sup.+, of emission from the GaAs diode, a quantum is yielded to the emitting ion Er.sup.3.sup.+ (or as also discussed in conjunction with the figure, to Ho.sup.3.sup.+ or Tm.sup.3.sup.+). The first transition is denoted 11. Excitation of Er.sup.3.sup.+ to the .sup.4 I.sub.11/2 is almost exactly matched in energy (denoted by m ) to the relaxation transition of Yb.sup.3.sup.+. However, a similar transfer, resulting in excitation of Ho.sup.3.sup.+ to Ho.sup.5 I.sub.6 or Tm.sup.3.sup.+ to Tm.sup.3 H.sub.5, requires a simultaneous release of one or more phonons (+P). The manifold Er.sup.4 I.sub.11/2 has a substantial lifetime, and transfer of a second quantum from Yb.sup.3.sup.+ promotes transition 12 to the Er.sup.4 F.sub.7/2 manifold. Transfer of a second quantum to Ho.sup.3.sup.+ or Tm.sup.3.sup.+ results in excitation to Ho.sup.5 S.sub.2 or , after internal relaxation from Tm.sup.3 H.sub.5 to Tm.sup.3 H.sub.4 (by yielding energy as phonons in the matrix), excitation to Tm.sup.3 F.sub.2 with simultaneous generation of a phonon. Internal relaxation is represented on this figure by the wavy arrow. In erbium, the second photon level (Er.sup.5 F.sub.7/2) has a lifetime which is very short due to the presence of close, lower lying levels which results in rapid degradation to the Er.sup.4 S.sub.3/2 state through the generation of phonons.

The first significant emission of Er.sup.3.sup.+ is from the Er.sup.4 S.sub.3/2 state (18,200 cm..sup..sup.-1 or 0.55.mu. in the green). This emission is denoted in the figure by the broad (double-line) arrow A. The reverse of the second photon excitation, the nonradiative transfer of a quantum from Er.sup.4 F.sub.7/2 back to Yb.sup.3.sup.+ must compete with the rapid phonon relaxation to Er.sup.4 S.sub.3/2 and is not limiting. The phonon relaxation to Er.sup.2 F.sub.9/2 also competes with emission A and contributes to emission from that level. The extent to which this further relaxation is significant is composition dependent. Erbium emission B is, in part, brought about by transfer of a third quantum from Yb.sup.3.sup.+ to Er.sup.3.sup.+ which excites the ion from Er.sup.4 S.sub.3/2 to Er.sup.2 G.sub.7/2 with simultaneous generation of a phonon (transition 13). This is followed by internal relaxation to Er.sup.4 G.sub.11/2 which, in turn, permits relaxation to Er.sup.2 F.sub.9/2 by transfer of a quantum back to Yb.sup.3.sup.+ with the simultaneous generation of a phonon (transition 13'). The Er.sup.2 F.sub.9/2 level is thereby populated by at least two distinct mechanisms and indeed experimental confirmation arises from the finding that emission B is dependent on a power of the input intensity which is intermediate in character to that characteristic of a third-photon process and that characteristic of a second-photon process for the Y.sub.3 OCl.sub.7 host. The relationship of the power of the variation in output intensity and intensity of the pump for multiphoton processes is well known (see, for example, Quantum Electronics, pp. 356-360, John Wiley & Sons, 1967). Emission B, in the red, is at about 15,250 cm..sup..sup.-1 or 0.66.mu..

While emissions in the green and red are predominant, there are many other emission wavelengths of which the next strongest designated C is in the blue (24,400 cm.sup..sup.-1 or 0.41.mu.). This third emission designated C originates from the Er.sup.2 H.sub.9/2 level which is, in turn, populated by two mechanisms. In the first of these, energy, is received by a phonon process from Er.sup.4 G.sub.11/2. The other mechanism is a four-photon process in accordance with which a fourth quanta is transferred from Yb.sup.3.sup.+ to Er.sup.3.sup.+ exciting Er.sup.4 G.sub.9/2 from Er.sup.4 G.sub.11/2 (transition 14). This step is followed by internal relaxation to Er.sup.2 D.sub.5/2 from which level energy can be transferred back to Yb relaxing Er to Er.sup.2 H.sub.9/2 (transition 14').

Significant emission from holmium occurs only by a two-photon process. Emission is predominantly from Ho.sup.5 S.sub.2 in the green (18,350 cm..sup..sup.-1 or 0.54.mu.). A similar process in thulium also results in emission by a three-photon process (from Tm.sup.1 G.sub.4 in the blue at about 21,000 cm..sup..sup.-1 or 0.47.mu.). The responsible mechanisms are clear from FIG. 3 and the foregoing discussion.

The detail at the bottom of FIG. 3 is an expansion of a portion of the .sup.2 F.sub.5/2 multiplet for ytterbium in two different exemplary hosts. The expansion is in the same ordinate units of wave numbers. Absorption spectra are shown for Yb.sup.3.sup.+ in an oxychloride host and also for the same trivalent sensitizer ion in a tungstate host. The oxychloride splitting results in more pronounced peaks in the portion of the spectrum shown, and one of these peaks denoted a occurs at about 10,200 cm..sup..sup.-1 or about 0.98 .mu.. By contrast, there are many more sharp absorption peaks in the spectrum for the tungstate, and, for the purpose of this discussion, an absorption in the region b is considered. In actuality blue emission from Tm.sup.3.sup.+ is relatively difficult to excite. By pumping the phosphor at about 10,200 cm..sup..sup.-1 (a) or at 10,500 cm..sup..sup.-1 at an appropriate amplitude, only Er.sup.3.sup.+ and Ho.sup.3.sup.+ emission from the oxychloride host will be discernible to the unaided vision. By varying the amplitude at either of these wavelengths, the Er.sup.3.sup.+ and Ho.sup.3.sup.+ activator emission may be varied from essentially pure green to apparently essentially pure red as discussed. By pumping the tungstate or b lattice at about 10,350 cm..sup..sup.-1 at an appropriate amplitude (for this example the required amplitude is an order of magnitude higher than for the oxychloride), discernible emission may result from the activator ion contained in the tungstate. In accordance with a particular embodiment of the invention, the tungstate lattice contains as its only activator ion the trivalent ion of thulium. Accordingly, pumping of this wavelength produces blue Tm.sup.3.sup.+ emission. The absorption for Yb.sup.3.sup.+ in the oxychloride at this wavelength is sufficiently weak so the discernible emission does not result from the Er.sup.3.sup.+ or Ho.sup.3.sup.+ activator in that lattice.

MATERIAL PREPARATION

Since the phosphors of the invention may be in powder or polycrystalline form, growth presents no particular problem. Oxychlorides, for example, may be prepared by dissolving the oxides (rare earth and yttrium oxides) in hydrochloric acid, evaporating to form the hydrated chlorides, dehydrating, usually near 100.degree. C. under vacuum, and treating with Cl.sub.2 gas at an elevated temperature (about 900.degree. C.). The resulting product can be the one or more oxychlorides, the trichloride or mixtures of these depending on the dehydrating conditions, vacuum integrity and cooling conditions. The trichloride melts at the elevated temperature and may act as a flux to crystallize the oxychlorides. The YbOCl structure is favored by high Y contents, intermediate dehydration rates and slow cooling rates while more complex chlorides such as Yb.sub.3 OC1.sub. 7 are favored by high rare earth content, slow dehydration and fast cooling. The trichloride may subsequently be removed by washing with water. Dehydration should be sufficiently slow (usually 5 minutes or more) to avoid excessive loss of chlorine.

Oxybromides and oxyiodides may be prepared by similar means using hydrobromic acid and gaseous HBr or hydroiodic acid and gaseous HI in place of hydrochloric acid and Cl.sub.2 in the process.

Lead fluorochloride and fluorobromide may be prepared simply by melting PbF.sub.2 and PbCl.sub.2 or PbBr.sub.2 together. The products can, in turn, be melted together with the oxyhalide phosphors to adjust their properties.

Sodium ytterbium tungstates containing Tm can be grown from a Na.sub.2 W.sub.2 O.sub.7 flux by slow cooling from 1,275.degree. C., and yttrium ortho-aluminates containing Yb and Ho can be similarly grown from lead oxide based fluxes and by pulling from the melt.

COMPOSITION

The essence of the invention is the use of a mixture of powders, each having a different crystal field environment for rare earth ions, each sensitized by Yb.sup.3.sup.+, and one containing Tm.sup.3.sup.+ as a sensitizer while the other(s) is sensitized by or and/or Ho all in conjunction with an infrared source whose output can be varied in frequency as well as in intensity. Examples of the phosphor matrices are rare earth oxychlorides, oxybromides, oxyiodides, the corresponding bismuth compounds (those containing BiOC1, for example), the oxychalkogenides (those containing ThOS, for example), and fluorohalides (those containing PbFCl or PbFBr, for example), rare earth fluorides, ortho-aluminates and gallium garnets, tungstates, molybdates, phosphates and vanadates. They are best employed in combinations where the broadest Yb.sup.3.sup.+ absorption lines are for the matrix containing Tm.sup.3.sup.+ and narrower absorptions are associated with those containing Er and/or Ho.

The oxychlorides, oxybromides and oxyiodides are preferred embodiments of the narrow band Yb.sup.3.sup.+ absorption type and, of these, the oxychlorides are the preferred class. The latter consist of at least two varieties although others are not to be construed as excluded. These have various structures including (a) the tetragonal D7/4h -P4/nmm structure in common with YOCl or (b) a hexagonal structure with an oxygen-to-chlorine ratio of less than one, for which a composition with the analyzed metal ratios: Y=56%, Yb=43% and Er=1%, lattice constants A.sub.o =5.607 and C.sub.o =9.260 and prominent d-spacings of 9.20, 2.33, 3.09, 4.62 and 2.83 are typical. Analysis indicates a structure (RE).sub.3 OCl.sub.7, where RE= any of the rare earths or yttrium. Of these two structures, (b) is preferred due to a greater range of fluorescent characteristics and this structure is generallized as Y.sub.3 OCL.sub.7 for simplification herein. Na.sub.0.5 Yb.sub.0.5 WO.sub.4, Na.sub.0.5 Yb.sub.0.5 Mo0.sub.4 and divalent ion-containing fluorides are preferred embodiments of the broad band Yb.sup.3.sup.+ absorption group. However, the latter need not be employed if a sufficient number of narrow-band absorption types are available.

While the structural considerations are essential the compositions must also contain the requisite ion pair Yb.sup.3.sup.+ --Er.sup.3.sup.+, Yb.sup.3.sup.+ --Ho.sup.3.sup.+, mixtures thereof, or Yb.sup.3.sup.+ --Tm.sup.3.sup.+. As described in conjunction with FIG. 3, initial transfer of energy is to Yb.sup.3.sup.+. A minimum of this ion is set at 5 percent based on total A cation content (e.g., ABO.sub.3, A.sub.3 B.sub.5 O.sub.12, (A,A')WO.sub.4) since appreciably below this level transfer is insufficient to produce an expedient output efficiency regardless of the activator content. A preferred minimum of about 10 percent on the same basis may, under appropriate conditions, result in an output intensity competitive with the best gallium phosphide diodes. The maximum ytterbium content is essentially 100 percent on the same basis, and it is an advantage of compositions of the invention that such rare earth levels may be tolerated. For ytterbium content above 80 percent, however, brightness does not increase substantially with increasing ytterbium; and this level, therefore, represents a preferred maximum.

It has been noted that strong activator fluorescence may vary from essentially pure green emission at about 0.54 to 0.55.mu. to a mixture of green and red, the latter at about 0.66.mu. when Er.sup.3.sup.+ or Er.sup.3.sup.+ +Ho.sup.3.sup.+ is the activator. Due to the effect of exchange coupling of Yb.sup.3.sup.+ to Er.sup.3.sup.+ on internal relaxation, red emission from erbium tends to be dominant for larger ytterbium concentration. Ytterbium concentration between about 20 and 50 percent results in mixed green and red output for (YbErY).sub.3 OCl.sub.7 while amounts in excess of about 50 percent, under most circumstances, result in output approaching pure red. A preferred range for a red emitting phosphor coating, therefore, lies between 50 and 80 percent Yb.sup.3.sup.+.

The erbium range is from about 1/16 to about 20 percent. Below the minimum, erbium output is not appreciable. Above the maximum, which is only approached for high Yb concentrations, internal radiationless processes substantially quench erbium output. A preferred range is from about 1/4 to about 2 percent. The minimum is dictated by the subjective criterion that only at this level does a coated diode with sufficient brightness for observation in a normally lighted room result. The upper limit results from the observation that further increase does not substantially increase output, for any given pump level.

Holmium, recommended as an adjunct to erbium in conjunction with ytterbium, as well as with ytterbium alone, may be included in an amount from about 1/50 to about 5 percent to obtain green emission of to strengthen the green output of erbium. Such activation may be desirable in the intermediate 20 to 50 percent Yb range alone or when erbium is present as well as at greater concentrations of Yb. Lesser amounts of holmium produce little discernible output as viewed by the eye. Amounts substantially larger than 2 percent result in no substantial increase and above about 10 percent result in substantial quenching. Thulium may also activate phosphors, and its value is premised on its blue output. Amounts of from about 1/16 to about 5 percent are effective. Limits are derived from the same considerations discussed with holmium.

Where the required cation content of the hose is not met by the total Yb+Er+Ho+Tm, "inert" cations may be included to make up the deficiency. Such cations desirably have no absorption levels below and within a small number of phonons of any of the levels relevant to the described multiphoton processes. A cation which has been found suitable is yttrium. Others are Pb.sup.2.sup.+, Gd.sup.3.sup.+ and Lu.sup.3.sup.+.

Other requirements are common to phosphor material in general. Various impurities which may produce unwanted absorption or which may otherwise "poison" the inventive systems are to be avoided. As a general premise, maintaining the compositions at a purity level resulting from use of starting ingredients which are three nines pure (99.9 percent) is adequate. Further improvement, however, results from further increase in purity at least to the five nines level.

The absorption bands of Yb.sup.3.sup.+ lie at various energies depending upon the properties of the host material containing the ion. Therefore, one of a mixture of two or more phosphors that are sensitized by Yb.sup.3.sup.+ can be excited preferentially by use of narrow band excitation in a unique absorption region for Yb.sup.3.sup.+ in a particular host. Diode arrays in which different diodes emit at a different frequency (e.g., through various indium doping levels) may be used to obtain such selectivity. Alternatively, individual components of a phosphor mixture may be excited with a single beam using a parametric oscillator that can be tuned in output wavelength to match the desired absorption regions of Yb.sup.3.sup.+ in the various matrices and may be varied in intensity to obtain the desired levels of fluorescence.

Consider the exemplary mixture (a) (Yb.sub.0.3 Er.sub.0.01 Ho.sub.0.002 Y.sub.0.6898).sub.3 OC1.sub.7 and (b) Na.sub.0.5 Yb.sub.0.49 Tm.sub.0.01 WO.sub.4. The former emits green (0.54- 5.mu.) under weak infrared excitation, appearing to shift through intermediate colors to red (0.66.mu.) as excitation amplitude increases. In this compound, Yb.sup.3.sup.+ has absorption peaks at 0.94, 0.95 and 0.98.mu. with no absorption between 0.96 and 0.97.mu.. The latter emits blue (0.47.mu.) under strong infrared excitation and has absorption peaks in the 0.96- 0.97.mu. region as well as at other frequencies. Hence, it is possible to excite blue emission from the tungstate (b) alone (through the Yb.sup.3.sup.+ absorption peaks in the 0.96-0.97.mu. region) or in the presence of the oxychloride (a). The process for blue emission from Tm.sup.3.sup.+ is relatively inefficient and requires much stronger excitation than those producing green or red emission from erbium and/or holmium in the oxychloride. Hence emission from (b) will have little affect on the hue of the overall output when both (a) and (b) are excited, say at 0.98.mu. at a level sufficient to produce erbium emission.

The excitation source can be an array of coherent or incoherent diodes with one diode emitting in a controlled manner at each critical frequency in response to programmed signals or it can be a coherent source, followed by a parametric oscillator that can shift the output frequency over the necessary range, and a modulator to change the output intensity. In the parametric oscillator case, a frequency shift may result from the application of an electrical stress to the parametric oscillator as well as by a temperature change, and intensity modulation can be controlled by the use of a standard electro-optic modulator such as the LiTaO.sub.3 --based device described in 38 J. Appl. Phys. 1611 (1967).

The expected short term impact of the invention is in the field of pictorial representation produced by infrared energizing. This energy, generally in the form of one or more beams, is generally caused to scan a substantially homogeneous phosphor either from the front or from the back. An arrangement resulting in a substantially black and white image has been described. Systems whereby at least two-color or three-color images may be produced by use of variations in frequency and/or amplitude to match different lattice absorptions and/or to cause different multiphoton processes to predominate have been described.

While a significant advantage of the invention is considered to derive from the use of a homogeneous (albeit sometimes mechanically admixed) phosphor, certain arrangements may give rise to a desire for a patterned screen. It is expected that any such arrangement will also utilize a scanning beam although scanning may be discontinuous or quasi-continuous.

Under certain circumstances, it may be desired also to utilize certain of the described phosphors of the invention in conjunction with an array of infrared sources. Proposed arrangements include cross-point arrays of gallium arsenide diodes. While such arrangement is not preferred in accordance with these teachings, certain of the phosphors described herein are advantageously employed in such a system.

The invention is largely concerned with the representation of pictorial information. By pictorial information is ordinarily meant representative information as seen by the human eye in life situations. Such information is represented not only in varying color where color is employed but also in varying color intensity. Under certain circumstances, it is desired to represent information in terms which do not include gradations of intensity. The inventive systems and phosphors are, of course, equally suitable for such purposes.

Claims

What is claimed is:

1. A system for producing a visible video display comprising a phosphor layer containing at least one activator ion, said phosphor layer being capable of emitting at least two visible wavelengths, each of the at least two wavelengths being of an amplitude discernible to unaided human vision when said phosphor layer is energized by radiation within the infrared spectrum; a source of infrared radiation incident upon said phosphor layer; a source of a video signal; means responsive to said video signal for modulating a characteristic of said incident infrared radiations to produce selection between and amplitude modulation of the at least two visible wavelengths of radiation emitted by said phosphor layer as a function of the variation in said video signal; and means for scanning said phosphor layer with said infrared radiation.

2. System of claim 1 in which apparent selection between two said wavelengths of visible radiation is dependent upon a characteristic of said infrared radiation.

3. System of claim 2 in which a said characteristic is amplitude.

4. System of claim 3 in which the two said wavelengths of visible radiation result from different multiphoton processes associated with a single activator ion.

5. System of claim 4 in which the said ion is Er.sup.3.sup.+.

6. System of claim 2 in which a said characteristic is frequency.

7. System of claim 6 in which the said phosphor consists essentially of a mechanical admixture of at least two distinct compounds each containing at least one exclusive activator ion, and in which variation in frequency of the said infrared radiation results in selective absorption and consequent selective emission within a given member of the admixed compounds.

8. System of claim 1 in which the said infrared radiation is coherent.

9. System of claim 8 in which the radiation source is at least one laser.

10. System of claim 9 in which the laser output is controlled by at least one element interposed between the said laser and the said phosphor.

11. System of claim 10 in which one of the said elements is an amplitude modulator.

12. System of claim 11 in which the said modulator is dependent upon an electro-optic interaction.

13. System of claim 10 in which one of the said elements is a parametric oscillator.

14. System of claim 1 in which the said infrared source is incoherent.

15. System of claim 14 in which the said incoherent source is at least one gallium arsenide infrared emitting diode and in which means is provided for forward biasing said diode.

16. System of claim 15 in which said source includes at least two diodes with peak emission at different infrared wavelengths.