CA2119450A1 - Optically encoded phase matched second harmonic generation, self frequency doubling laser material, and holographic optical memory using semiconductor microcrystallite doped glasses - Google Patents

Abstract

There is described a semiconductor microcrystallite doped glass that exhibits SHG, and a method of preparing, or encoding, a semiconductor microcrystallite doped glass by the simultaneous injection of fundamental and second harmonic fields, such as 1.06 µm and 532 nm. More specifically, the disclosure pertains to a structure that exhibits SHG, the structure being comprised of, by example, borosilicate glass that contains CdSxSe1-x microcrystallites. Also disclosed are embodiments of devices having an optical waveguide structure formed within a glass substrate that contains semiconductor microcrystallites. The optical waveguide structure guides and contains injected radiation and also converts a portion thereof to the second harmonic. Also disclosed are optoelectronic devices that include frequency doublers, self-doubling lasant material, bichromatic optical switches, and a volume holographic medium, all of which include a glass host having semiconductor microcrystallites embedded within.

Description

' . w0,93/08s00 2 1 1 9 4 5 0 PCT/US92/08364 OPTICALLY ENCODED PHASE MATCHED SECOND HARMONIC
GENERATION, SELF FREQUENCY DOUBLING LASER
MATERIAL j AND HOLOGRAPHIC OPTICAL ME~ORY USING
SE:MICONDUCTORMICROCRYSTALLITE DOPED GLASSES

CROSS REFERE~CE TO RELATED PA'FENT APPlICATION:

This patent application is related to U.S. Patent Application S.N.
07/722,345, filed~ June 27, 1991, entitled "Second HarIr.onic Generation and Self~ Frequency Doubling Laser Materials Comprised of: Bul~k Germanosilicate and Aluminosilicate Glasses", byNa~il M. Lawandy.

~ELD OF TH~TNVEN~ON:

This~invention~relatesi~generally to non-linear optical devices and, in =, to non-linear optical devices construc~ed from a glass BACKGROUND OF THE INVENTION:

Recently there~has :been considera~le interest in glasses do~ed with CdSxSel x semiconduetor microcrystallites. This has been due to interest:in;~the~damental:physics of low dimensional systems, as well :~as; the te~noiogically important areas associated with optical switching as :rèferred.:to in K.M. Le~g, Phys. Rev. A 33, 2461 (~986) and A. I. Ekimovet al. Solid State Comm. 69, 565 (1989). In the case of commercially~available colloidally colored filter glasses the crystallitè size::is of ~the~order of 5-10 nm, making the crystallite :larger than the~ bulh~exciton radius, and thus out of the quantum dot regime. ~: These~ materials have been the subject of several investigations usIn~g four wave mixing, interferometric methods and luminescence:~detection, as mentioned in R. K. Jain et ai., J.

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' ' ~ W093/~8~00 2 1 1 9 4 ~i O PCI/USg~/0~364 Opt. Soc. Am. 73,~46 ~1983~ and M. Tomita et al.~ J. Opt~ Soc. Am B
6,16~ (1989). From these measurem~nts X~3)(cd, ~D- ,~) values have been measured which range from 10-1l to 10-7 esu. In addition, a large spread in response times, ranging from 72 llsec to 10 psec, has : been observed along with an intensity dep~ndence~ Other effects which are indirectly associated with these observations are : ~ thermally reversible photodarkening, non-quadratic dependence of phase conjugate reflectiYity on pump intensity, Franz-Keldysh ~ oscillations, and luminescence.
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~:~ ~ It is an object of this invention to provide second harmonic ~: generation: (SHG) in ~lasses doped with semiconductor microcrystallites.
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~ It is another object of ~he invention to provide a method of preparing :
a semiconductor doped~glass material so as to exhibit SHG.

lt is a further object ~of the invention to plovide SHG in a silica-based glass that ; contains, by example~ CdSX S e 1 - x or CuCl microcrystallites.:: ~

It is one further~object of the invenhon to provide optical wave~ide struct.ures, optical switching devices, and holographic memory :devices that~are fab~icated with a silica-based glass that contains semiconductor microcrystallites.
., . ~ It:is another~ object of the invention to provide a lasant material that simultaneously !ases and frequency doubles the laser radiation.
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It is one further o~ject of the invention to provide a semiconductor ~t ~ laser diode that includes ~ frequency doubler compl ised of a ~ t ,~ semiconductor microcrysta}lite doped glass.
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~ ~ C-tDC_~S~ r~ ~u~r~r 93/O~Oo 2 1 1 9 ~ S O PCl/US~2/08364 The foregoing objects are realized by a semiconductor microcrystallite doped glass that exhibits SHG, and by a method of preparing, or encoding, a microcrystallite doped glass by the simultaneous injection of fundamental and second harmonic fields, such as 1.06 ~,lm and 532 nm. Msre specifically, the invention provides a structure that exhibits SHG, the structure being comprised of a silica-based glass that is doped with9 by example and not by lim;tation,~ Cd8xsel.x . CuCl, PbS, GaAs, InP, ZnSe, or ZnSeS
microcrystallites.
Although these composite :materials have a center of inversion on a macroscopic scale, and are therefore expacted to possess no second order ~susceptibility,~ thé inventor has determined that this symmetry ca;l be broken, ~and~that phase matching can be encoded, when the material is simultaneously exposed to optical radiation having a :first wavelength and a second wavelength, the second wavelength being: one~ half :of ~ th~e~ first wavelength. By example, the first wavelength is :1.06~ m~and-the second wavelength is 532 nm. The radiation may be~generated by a modelacked and Q-s~ntched laser.
The SHG~;ef~ect~is~permanent in some glass~microcrystallite systems and is :a strong function of the position of the microcrystallite energy bandgap.

The use of ~the~invention~ also proYides a permanent, quasi-phase matched, second: harmonic signal which is approximately 105 ti~es an initial background value. The inventor has obtained second harmonic~ signals which are visible in room lights, orresponding to~ a conversion ef~iciency of 10-6 fnr modeloc~ed, Q-switched input pulses. Experimentally obtained results are presented~ which: explain the under!ying physical mechanisms.
These include polarization dependence, OIR and sesond harmonic preparation intensity effects, thermal erasure, ar,d the application ,: ,, ., . , . ., - .

wo 93/08500 ~ 1 1 9 4 ~i O PCr/US92/08364 of external static electric fields. These results are shown to indicate that a most likely mechanism is an enc~ding of a periodic internal electIic field that results in a phase mat~hed Electric Field Induced Se~ond Harmonic generation ~EFISH~ process.
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The înventioIl also pr~vides an op~ical wa~reguide structure, and a method of fabricating same. The wave~uide structure provides SHG
fo~ optîcal radiation propagating therethrough. The waveguide structure is ~abricated through an ion exchange process in conjunction with 8 photolithographic masking process. Both planar and channel waveguides are des~ribed.
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The invention also prondes a laser medium, such as a glass laser ~: rod, or optical fiber that simultaneously lases and frequency doubles the laser radiation.
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Further embodiments of the invention provide optical switching devices and a holographic medium that frequency double an inpu beam wavelength.

BRIEF~ DES(~RI~TION OF THE DR~W~N~

The abo~e set fo~th:: and other features o~ the invention are made more apparent in:the ensuing Detailed Descnption of the In~rention, when read in conJunction with the attached Drawing, wherein:

Fig la depicts~an enlarged view of a glass host material having semiconductor microcrystallites contained within;
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Fig. lb~ shows optlcal apparatus for preparing a semiconductor microcrystallite doped glass for SHG;
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~ :: Fig. 2 shows ~a time dependence for SHG preparation in a 1 mm ,:~ : ~ : thick OG 5~0 filter illllminated ~rith 2 W and 50 mW of 1.06 um and ~ : : 532nm radiati:on, respectiYely;
~:: : ~:
~ ~ _ ~o g3/085~0 2 1 1 9 4 ~ O PCr/US92~08364 Fig. 3a illustrates a dependence of SHG on input radiation intensity for 1 mm thic~c samples;
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~ Fig. 3b illustrates a log-log plot showing a quadratic relationship of :~ ~ SHG to input intensity;
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Fig. 4 shows~a relationship between SHG and a lellgth of prepared . :
sample;

;~; ~ Fig. 5 dispiays relative conversion ef~ciencies of dif~erent colloidally ~ :: doped filters; :~:

; ~ : Fig. 6 illustrajtes SHG as a ~unction of input polarization relative to :: writingpolarization foran OG 550 filter;

~ Fig. 7 depicts; SHG dependence; on 1.06 ~m radiation preparation .` ~ :int~sity;

Fig.~ 8::depicts~S~IG~dependence on 532 nm radiation preparation intensity;~
. ,;~
. ~ Fig.~ 9 depicts ~a~SHG erasure effect accomplished with 1.06 um . .~ . . .
, ~ ra~la~lon; .~

` ~ Fig. 10 depicts a SHG :erasure ef~éct accomplished with 532 nm radiation; ::~
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: Fig. ~11 depicts thermal SH~ erasure as a function of time for three ~ ~ different ambient~temperatures;:

i ~ Fig. 12 depicts an energy band ~iagram that illu~trates a mech~anism~ ~ for ~SHG preparation in a semiconductor . :~ microcrystallite :doped glass;
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, wn 93/08500 2119~50 PCT/US92/08364 Fig. 13a is a plan view showing a spiral wave~ide fabncated within a surface region of a semiconductor microcrystallite doped gla~s;

Fig. 13b is a cros~ sectional view showing the waveguide of Fig. 13a;
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Fig. 14 is a cross sectional view showing an optical device that incl~des a fireq~lency doubler constructed in accordance with the invention;

Fig. 15 is a graph illustrating a change in bandgap as a function of composition of a Ge-Si alloy;

Fig. 16 shows a hoIographic medium that is constructed and operated in accordance with the invention;

Fig. 17 is a block diagram showing apparatus f~r preparing a frequency doubling glass laser rod;

Fig. 18 shows a ~block diagram of a frequency doubling laser that includes a semiconductor ~nicrocrystallite glass rod prepared in accordance wit~ Fig. 17; and `~
Fig. l9 shows a top view of a bichromatic logic switching device.

:: D~AILED l)E~iÇRlPTIQN ~E THE INVENTION

Fig. la illustrates a volume of silica-based glass host matenal 1 having a plurality of microcrystallites 2 embedded therein. The microcrystallites are comprised of a semiconductor material. The ~ invention is desmbed below primarily in the context of a borosilicate :; glass host material having CdS%Se(l x) micro~rystallites contained ~ therein. These microcrystallites are uniformly distributed ;~ throughout the glass host material and have a nominal spacing between them ~hat is a function of the concentration of the microcrystallltes. It should be realized, however, that the teaching : , .

W~ 93~08s00 2 1 1 !~ O P~/US92/08364 . 7 of the in~erltion i8 not to be construed to be limited to only this material combination or to uniform distributi~ns of l microcrystallites. For example, a glass host may include l semiconductor PbS, Cu~l~ GaAs, InP, ZnSe, or ZnSeS
microcrystallites. Furthermore, the ~oncentration of the : microc~ystallites may be other than uniform for prov~ding SHG a~
only selected portions of the glass host mater~al.

:: As empl~yed herein, a semiconductor microcrystallite or crystallite is considered to be a single crystal or a polycrystalline aggregate of a semiconductor material having an energy band structure.
Aggregates which exhibit bulk, as well as quantum dot behavior, are included ~nthin this definition.

The CdSxSe~l.x) microcrystallites 2 may be present in a I , concentratio~ of approximately 0.3 mole percent to concentrations up to 50 mole:percent or greater. The greater the mole percent ccncentration tne greater is the SHG effect~ The microcrystallites 2 are randomly oriented and have diInensions on the order of approximately 100 angstroms to approximately 200 angstroms.
Although GdSxSe~l x) is noncentrosymmetric, the random ~orientations of the crystallites 2 result in x(2) - O for the composite system. This result is exploited by the inYention to provide SHG in the manner described in detail below.

The glass host 1 may :also contain Na or K in a concentration range of approximately 5 mole percent to 20 mole percent. Nd may also be presen~ in a concentration of, for example, 1.5 percent. The in~ention also pro~ides for the construction of glass laser rods or o~tical fibers that provide a fundamental frequency and twice the fundamental frequency. This aspect of the invention is described in detail below.

Each microcrystallite 2 retains, within the glass hos~ 1, the basic properties of the bulk semiconductor. Also, the nonlinear ~: :~ :
~: : :

wo ~3/08~00 2 1 1 3 ~ ~ O P~/US92/08364 . 8 susceptibility of the microcrystallites, Xt3)mC~ is greatly enhanced for above bandgap excitation. For example, quoted values of X(3)(cl)2-2~ , 031~ -~2~ for CdS at ~ ~0.694 ~lm and ~=0.53 ~lm are 2.24 x 10-20 m2/V2 and 1.05x10-17 m2/~2, respectively. The second wavelength, which is at the band edge of CdS, result~ in a X(3)mc whi~ is 103 times ~he of~-resonance value, and spproximately 105 times larger than that of silica. Thus, if electron dynamics within :; the microcrystallites 2 are considered, internal optical rectification fields as large as 107 V/m are expected, which includes the static dielectri~ ~onstant of CdS, ~r~8.9 for above-gap excitation. Such :
large fields result in energy increases as large as several tenths of an electron-volt across the microcrystallite.

For the purpose of characterizing the SHG effect with mîcrocrystallite doped glass materials, experiments vvere performed on optical: filters of a type manufa~tured by Schott Glass;
the filters ranging~from GG 495 to RG 630 and ha~ring a Yariety of thic~esses (1 mm to; :5 cm). These filters are comprised of a glass ::host doped with ~CdSx~e(l x~microcrystallites. The filter nom~n~lature is ~such that the number, such as 495, g~ves the approximate semiconductor bandgap in nanometers.

One possible mechanism~ for the SHG encoding process is shown in Fig.: 12. The dia~am represents a bas;c energy leYel structure for a direct gap semiconductor and its relatiorl to the surrounding ~lass.
The primary optical encoding steps are believed to be: 1) optical excitation of the: electron to the conduction band (Ec); 2) motion of the electron under the influence of the intern~l optica! rectification field, establisning~ ;a wa~Jefunction pinned against one side of the crystallite; and 3) trapping in an "exterior" deep trap of energy Et.
Measurements on~photo-ionization of CdS microcrystallites in glasses have demonstrated such trapping when a~ove bandgap excitation was employed, and ind;cate that the trap site is most likely a deep electron trap in the glass matrix near the crystallite . ........ , . . . - . . -wo g3/08s00 2 1 ~ ~ 1 5 Q PCI/US9~/08364 surface. The optical en~oding, described below, i8 believed to add directionality to this basic process.

In Fig. 12 the arrow designated 4 shows a thermal SHG erasure via ionization to the conduction band, and arrow 5 indicates an optical SHG erasure mechanism via direct absorption. The arrow 6 indicates luminescence from interior 6urface trapping sites. The :thermal and optical SHG erasure mechanisms are described in further detail below.

Optical apparatus for preparing semiconductor microcrystallite doped glasses (SMDG) for SHG is shown Fig. lb, wherein P1 and P2 are polarizers; L1 and L2 are 10 cm lenses; S is a microcrystallite doped glass sample; BFP is a 532 nm bandpass interference filter;
and PMT is a photomultiplier tube.

More specifically, the apparatus includes a modelocked, Q-suntched and ~frequency doubled Nd:YAG laser 10, a KTP crystal 11, a 10cm focussing Iens 12, cross-polarizers 14 and 16, and a phase sensitive detection system 18 capable of detecting 10-14W of average power.
The laser 10 produces~pul~ses that are 120 psec and 90 psec in duration at 1.06~ and:5~2 nm, respectively, with a 76 MHz modelocking rate and:a~ Q-switched rate of 1 kHz. The pulses incident on a Sh~DG`~sample 20 are linearly polarized and are focussed to a measured: spot size 30 }lm in diameter (for 1.06 ~m radiation). The laserJcross-polarizer system delivers up to 3 Watts of average power at 1.06 ~lm, and up to 1 watt at 532 nm. The two beams are not separated in order to minimize any relative phase jitter e~fects due to dispersive thermal index e~ects in beam separation and recombination optics.

The seconà ha~nonic signals are detected using a lens 22 and up to four band pass filters (BPFs) 24. A photomultiplier tube (PMT) 26 ain is held constant throughout all expenments. The signals from prepared samples were measured using calibrated neutral density ~, wo 93/08500 2 1 ~ ~ 4 S 1~ PC~/US9~/08364 . 10 filters. In a~cordanee with the invention typical ~alues of background SHG corresponded to a sonversion efficiency of appr~ximately 10-13 to 10-12, with 1 W of i~icident ~verage power at 1.06 t It is s~oted that the app~ratu~i described above provides preparatiorl for CdSxSe1.x and employs 1.06 llm and 532 nm radiation. However, for other semiconductor micrccrystallites other wavelengths are appropriate. For example, for CuCl wavelengths of approximately 0.7 nm and approxim8tely 0.35 nm are employed. In general, the fundamental wavelength is within a range ~f approximately two micrometers to approximately 0.5 micrometers, and the second harmonic wavelength is one half of the fimdamental.
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Initial expe~ments were performed on one mm thick OG 550 filter glass in order to determine a time evollltion of the SHG process. The preparation process was interrupted periodically to read out the second harmonic power. Fig. 2 shows the time eYolution for the OG
50 filter exposed to 2W and 50 mW of a~erage power at 1.06 ,um and 532 nm? respectively,. The results show that the SHG increases by 105 and that saturation occurs on the ~me scale of a few hours.
Simllar experiments~ were performed on a non-resonant bandgap fiIter (GG 495) and resulted in the same basic time evolution.
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:~ : In order to fu~ther ~erify that a second order process was indeed : responsible for~the sign:als, a dependence of the SH& signal OIl input power was de~ermined. The results are shown in ~ig. 3a for the OG
550 and a ~G 495 filter. The GG 495 filter was ~ound to have negligible ~bsorption at 532 nrn (a- 0.03 cm^1). This material is I
: expect2d to be less sensitiYe to pumping and readout induced index ~: changes from carrier excitation and thermally e~ected shrin~cage of the energy gap~ as noted in J.I. Pankove, Optical Processes in ¦ ~ Semiconductors, Dove~ Publications, Inc., New York (1971), p. ~7.
¦~ The dependence of SHG on IR power for the GG 496 filter was ~ ~ determined from a least squares fit to a log-log plot of the data in Fig :

' ` W~ 93/085~0 21 1~ k 5 1~ PCr/US92tO8364 3a. Fig. 3b show~ the transformed data and indicates that the process is dependen~ on I(c~ g8 wîth a co~Telation greater than a.ss.
Based on these results it i~ cleas that an effectiYe second order ~usceptibility ~x(2)) is induced in these materials. The de~nations from second order beha~rior in the OG 550 filter are believed to be due to direct and indirect intensity dependent phase matching ef~ects.
In addition to the semiconductor doped glass~s, a single crystal sample of CdS was also examined. This sample exhibited a preparation induced increase in 5HG of a factor of two. The ~esult serves to demonstrate that the obse~ed increase of seYeral orders of magnitude observed in the SMDG is unique to the microcrystallite guest-glass host: system.

In order to further verify that a phase matched process occurs in the SMDG the scattered sidelight at 532 nm from the GG, 495 filter, prepared along five centimeters, was examined using the optical system shown in Fig. lb. Fig. 4 shows the growth of the second harmonic beam along the propagation axis of the SMDG. The filter was GG 495 prepared for ~our hours with 1 W and 1.5 mW of 1.06 ~lm and 532 nm radiation respectively. Maximum con~rersion ef~iciency corresponds to 5x10-7. Althou~h the dependence on length is not perfectly quadratic, the result serves to illustrate that phase matching occurs:. The second harmonic conversion efficiency of this SMDG materi~l, after nine hours of preparation, was found to be 5x10-7. This value, along with the input beam parameters, results in a X~2) of the order of 10-16m/V~

In addition to the two SMDG materials discussed thus far other SMDG mate2ials were evaluated to determine the role of resonance.
For GG 495 through OG 590 filters the experiments were performed on one mm thiclc filters with identical preparation and readout processes. The results in Fi~. 5 show the SHG ef~ciency of nine llters, where the OG 55û filter provides an increase of approximately~ fou~ orders of magnitude. In Fig. 5 the squares are measured values, and the solid line is a best fit based on the model ,.. ., , . , .: , .,-::

wo ~3/~5~û 2 1 1 ~ ~ 5 0 PCr/U592/08364 presented below. The G& 400 (plotted as a point at 400 nm), GG 450 and GG 475 filters were two mm thick, and all other filters were one mm thick. It appears that resonance ~trongly enhsnces the SHG
~f~ect, a~ will be described.
i In a further experiment the dependencs of the output second harmonic transverse beam structure on khe writing second harmonic transverse beam profile was examined. By adjusting the K~P csystal 11 at a steep angle~ the second harmonic generated by the 1.06 ~,lm Gaussian beam emerged as a double lobe pattern, due to the KTP crystal 11 birefringence. When this beam was used in conjunction with the uniform 1.06 llm Guassian fundamental beam to prepare the GG 495 filter, it was found that the second harmonic signal generated during readout was double lobed. When the second ha~monic was adjusted to have a uniform profilo, the readout second harmonic emerged in a solid mode as well. Tnis behavior of slaving the output SHG to the encoding beam pattern is identical to the ef~ect observed in germanosilicate optical fibers, except that in the SMDG material there are no modal constraints as there are with optical fibers.

The dependence of the SHG output on input polar~2ation, relative to encoding:polarization, is shown in Fig. 6 for the three mm OG 560 filter. Squ~res~ are measured Yalues, and so~;d diamonds are proportional to cos~(~). Points are scaled to account for optical erasure while reading:and are corrected to account for a temporal decay of:the SHG inherent in the readout process. The output power at 532 nm includes both output polarizations. SHG i5 seen to behave as cos2(~), where ~ is the angle between writing and reading polarizations. An expected cos4 (~) dependence may be masked by the summing over three tensor elements, all of which contribute to SHG for the linearly polarized input radiation.

It is noted that no self-preparation ~i. e. preparation with no second harmonic seed radiation) was obtained over the course of a twelve , . . . .
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' WQ 93~Q8500 211 ~ 4 5 D P(~US92/08364 hour period, even at inten$ities just below the dama~e threshold of the S~DG material (~500 W/llm23.

In or~ler to better understand the proce~s of induced SHG in the SMDG m~terial the intensity dependence of the pr~paration process was examined. Experiments were perfo~ned with GG 49S and OG
550 filterfi as a function bath of 1.06 ~m and 532 nm inc~dent powers.
The experiments were performed on one mm thick ~amples.
Portions of the S~G material which had never be~n exposed were prepared for a measurement and each exposure was limited to 20 minutes maximum. In order to minimize phase matc~ling changes between preparatiorl and readout, the readout IR power at each point was set to the writihg Yalue.

Fig. 7 shows the results of the twenty minute preparation as a function of the average~ IR power, with the second harmonic seed power held constant at 10 mW. The OG 550 filter was three mm thick, and points were prepared ~r 20 minutes with 20 mW of 532 nm light in addition to the indicated IR power. The G(} 495 filter was~ one mm thick,~ and points were prepared for 10 minutes with 5mW of:532 nrn light,. It~is important to note that the readout and :wri~ing powers are the same. Log-log plots of the data in the unsaturated region (<1 watt) revealed that the X( X L product s 1 jEt~)l-48 and E(~)2 l5 for the GG 495 aIld OG 550 filters, respectively. SHG values were no~nalized assuming square law dependence:on readoutintensity.

Fig. 8 shows the results of the twenty minute preparation as a function of the average second harmonic seed power with the fundamental average~ power held constant at one watt. The GG 495 filter was 3 mm thick, and points were prepared for 10 minutes with two W of 1.06 ~Lm radiation in ad~ition to the indicated power at 532 nm. The OG 5~0 filter was one mm thick, and points were prepared for 20 minutes with 2 W of 1.06 ~lm radiation. The results indicate that there is a sharp rise, a maximum, and a region of decreasing ' WO 93/08500 2 1 1 ~ g 5 0 PCI~/US9~/08364 SHG. This is indicatiY~ of an erasure mechanism, which is believed to be qualitatively similar t~ bcha~ior obse~red in 2 g~rmanosilicate.
fiber, a~ referred t~ by F. OulleSt~, K.O. Jill and D. Johnson, Optt.
Let. 13, 515 (lg88).

As was noted, the:second harmonic dependen~e in t~e preparation fitage i~dicate~ evid~nca of an erasure mechanism. GG 495 and OG
550 filters, each three mm thick, were prepared for 20 minutes with 2 W ~ and 50: mW of fundamental and second harmonic p~wers, respectively. Once prepared, it was observed that over a period of several days no: apparent decay could be obser~red when the samples were maintained: under ambient conditions (25C) in the absence of illumination. However, when the prepared samples were read out with :IR radiation only, the signal decayed ~nth time. Fig. 9 shows tne decay~of;both the OG 5~0 and GG 4g5 samples. Both filters were three mm~:thick and~were illuminated with two W of 1.06 llm radiation. The ~OG 550 exhibits a rapid decay over a period of fifteen minutes, while the OG 495 decayed by only ~ few percent over the same~:period.~;Fitting exponential decays to the data~gives a decay r ate of l.5x10~2 sec-1 for the GG 495 filter at two W of average readout power. ~The~OG~550 gives values of 4.1xlO-~ sec-1 and 7.7xlO-2:sec-l at two:W and:thr~ee~W, respectively. From the decay rates it sppears that~:the ~erasure e~ect~ has a near quadratic dependence on IR
power. ~

In~àddition~to~the~measurements using 1.06 ~lm radiation the erasure ~pro:cess:~was examined using the second harmonic (532 nm). The decay~of the second harmonic with time for three different average powers is shown in; Fig. 10. The filter was OG 515 having a thickness of three mm. Analysis of the 0.5 watt case: shows that the decay cannot~: be ~described by a single exponential. The curves xhibit a decay rate whic~. decreases with time and is of the order of 10-3 sec-l for0.5W~ofaverage powertO<t~lO min.) , i ~:
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, W0 93/085t~0 2 1 1 9 ~ PCl /US92~0836 The induced SHG ef~ect u~as found to b~ perma~nt on a time scale of several days under dark condition3 at room temp~rature. This implies that if trap states are respon~ible for the encoding process, they are deep enough to sccour t for the long lifstime under arnbient condi~ions. In order to determine the activation energy involved, a ther~nal erasure of the OG 550 fi}ter was examined. Fig. 11 shsws the decay of the SHG signal in the OG 550 filter a~ a ~unction of time for three different temperatures. All ~amples were 1 mm OG 550 filters, and were read with ~wo W at 1.06 um. The decays are well approximated by exponentials and result in an activation energy of 0.6 eV.

The large effect that heating has on the signal decay complicates the interpretation of the optical erasure results, since the locally irradiated region experiences a temperature increase w~th bvth IR
and green (532 nm) illumination. Measurements were made of the local temperature increase for the two filters. When one W of IR
was incident on the one mm filters, the GG 49S temperature lmm from the besm center increased by 3.1K, and the OG 6~0 by 3.0 K.

Illumination with 0.1 W of 532 nm light resulted in temperature increas~s of 0.3 and 0.5 ~ ~or the GG 495 and OG 550 filters, respectively. Thus, heating plays a roll in optical erasure, but is most like}y not the dominant mechanism.

The results discussed thus far strongly fa~or the encoding of a periodic symmetry breaking phenomenon, most probably an internal elec~ric field. This encoding is believed to find its or.igin in a nonlinear holographic process, where the spatial phase information is carried by the fundamental and second harmonic waves. Models for similar behavior in germanosilicate fibers suggest that optical rectification fields of the form:

%(3)(0;Cd,(d,-2~)E2(CIl)E (2L))ei ~kZ, Cl ll~TISl 1~ CL~T

'lO 93/08~0 2 1 1 ~ ; O PCr~US92/08~64 where ~k _ 2k(cD~-k(2~), in the bulk material i~ responsible for the eIacoding, a~ publi~ihed by R. H. St~len and H.W.K. Tom, C)pt. Lett.
12, 585 (1987). Th~ X~3) in silica i~ ve~y small ~10-~10-2~ m21V2) and resiults in apprQximately one Vfcm ~lelds in the fiber. In the prepared SMDG filters of the invention, howcver, the composite X(3~(o;~,~,-2~0) is believed to be much larger, espeGially near the microcrystallite band edge.

In order to place some lower estimate on the internal field, the e~ect of an applied external field was also investigated. Experiments were performed on OG 550 ~nd GG 495 filters between transverse ~: electrodes. 13oth samples were prepared with th~ optical field polarization parallel to the applied electnc field, and the GG 495 was also prepared with light polarized perpendicular to the applied field.
The application of fields as large as 10~ ~Im during preparation and readout resulted in no measurable change in SHG eo~version efPiciency. It is therefore concluded that if the enccding process is viewed in terms of an ef~ective optically generated d.c. field, then this field is large compared to 106 ~/m.
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The results presented above on the length ~nd readout intensity dependence give evidence that a second order phasie matched nonlinear interaction takes place in the prepared samples. The results of, the index-summed rlonlinear susceptibility tensor properties, determined by varying the readout polarization, are ` consisterlt with the presence of a symmetry breaking electric field within the material.

~- i A most likely process for increased interaction lengtll is quasi-phase matching. This mechanism requires a penodic ef~ective nonlinear susceptibility given by:

X~2) - xo~2) cos (~kz+~p), (1) .
L c~ ~ ~ C~J~r Wo 93/08~0 ~ Pc-r/US92/083~4 where ~k-2ktb~)-k(2c~)l and ~p is a c~nst~nt phase. Combining this phase matching proce~ unth the pres~nce of an internal electr~c field~ Edc(z) to break symmetly, leads to X(2~ ~ X(3)(-2~;~,~,0)Edccos(~kz~ (2) where Xt3 is the third order susceptibility tensor fcr the composite cry$tallite giass material~ and EdC i~ the amplitude of th~ internal field encoded by the writing beams. The p~lar~zation experiments equire that EdC point 810ng the d;rection established by the polarization of the writing beams.
The preceding discussion shows that ~ymmetry can be broken and quasi^phase matching can occur if the optical encodiIlg process results in the estabtishment of a permsnent periodic electric field.
The results on the response of glasses doped with varying relative concentrations of S and Se, to tune the crystallite bandgap, reveal a preparation resonance of approximately 550 nm. The bandgap can also be exoitonically tuned using quantum size effects when the part;cles are smaller: than the exciton radius. Tllis may be :cantrolled by the glass striking conditions~ The increase of' SHG as the bandgap moYes sloser into resonance from the long wavelength :side:indicates that carrier excit~tion is required. The decrease in - ~ ~ , , : ~ SHG ~ter the resonance may be a consequence of a~sorption in the writing and readout process. By examplet Fig. 15 shows the ef~ect on bandgap of a Ge-Si~alloy for changes in composition uf the Ge and Si components.
:, :~: ~ . .
Th~ invention ha~ been presented in terms of the results of a variety ~:~ : of measurements on optically encoded second harmonic gen~oration ~:~ ~ ~ in CdSxSel x doped glasses. The results indicate that effective x(2) ~: values as large as 10-l6 m/V are a~tainable from commercially :: ~ aYailable filter g~asses. This val~e, along with ~ cm of active length, : results in a 10 con-~ersion efIlciency, and the g~neration of a second ~ , wo g3/08~00 2 1 1 9 ~ ~i O PCrtU592JO~364 harm~nic signal ~at i8 visible i~ room lights. Th~ e~c~vene~s of one ~pecific ty;pe o~ glass filter sver another i8 believed t~ ba due tQ Fe impurities which provide msre electron trapping sites to lock-in the 5eld.

It ~hould be realized l;h~t the use of th~ in~ention is not re~tricted to only the com~er~i~lly available borosilicate filter glasses described thus far. That is, for a given application the gla~s host and the particular semiconductor microcryatallite and the concentration thereof may be explicitly defined and fab~cated. Also, the u~e of the invention is not restricted to the bulk, monolithic fiDrms of th~ glass host material as is typically proYided i~ a filter ~laBs material. That is, th~ glass host material containin~ semiconductor microcrystallites may be provided as a coating or layer upon a substrate. By example, a semiconductor doped glass i8 sputtered into a thin film with doping densities of, for example, 30 percent.
Such a film or Isyer may be integrated with, by example~ a conventional laser diod~ so as tu frequency convert the output thereof, after suitable preparation.
: ~
~ ~ , By example, and referring to Fig. 14, there i~ shown an optical device 20 that includes a substrate 22 and a frequency doubler 24.
The frequency doublsr 24 is comprised of a glass containing semiconductor microcrystallites of the t~pe described ~bove. Device 20 includes, by example, a semiconductor diode laser 26 positioned ~r radiating the frequency doubler 24. Laser 26 may be of conYentional: construction having an ac~ive region 2~ that is bounded by cladding layers 30a and 30b. A pair of electrodes 32a and 32b ~re provided for coupling the laser diode 26 to a sour~e of power, schematically~ shown as a battery 34. The semiconductor laser diode 26 has an output wavelength of 850 nm. In accordance with the in~ention the frequency doubler 24 is prepared as described above so as to gen~rate 425 nm radiation from the input 850 nm. Suitable semiconductor microcrystallite compositions for doubling 850 nm e~ T~ cL~rs " w093/08500 211~3lsn P~/US92/08364 include CdS or CuCl, and related alloyfi includ~ng a third element such as in CdSexSl x semiconductor~.

Preferably, the frequency doubler 24 i8 deposited as a film or coating upon the substrate 2~ by sputt~ring ar an equiYalent technique.
Howe~er, the frequenc~r doubler 24 may be bonded to the substrate by an epoxy or ang suitable adhesive. In lil~e manne~, the substrate 22 may be a substrate that :tlle laser diode 26 is fabr~cated uponl or the las:er diode 26 may be:attached to the substrate by an epoxy or any suitable adhesive. The total length L of the ~equency doubler 24 need not be any longer~ than an amount of the bulk glass that is prepared for SHG:by the above: describ¢d method. For examplet L
may be :equal to approximately 0.~ mm. The frequency doubler 24 may be prepared, after deposition, by irrad~ating the face of the :frequency doubler ;24 that is opposite the output face of the laser. The irradiation of the frequency doubler 24 can be a~comp}ished ~nth a system as~seen in Fig. lb modified:to include a Ti-Sapphire laser tuned to 850 nm:~and~a::frequency doubler for providing the 425 nm ra~diation. If the frcquency~ doubler 24 is otherw~se attached to the substrate 22,~the;frequency:doubler24 maybe prepared as in Fig. lb, ,, ~
as modified~ above, and~ then subsequently bonded to the substrate 22.

. ` ~ T; ~be~;optical ~device~ 20: thus::includes a SHG wa~elength converter of : ~ small~size~ând ~high~ef~ciency~ for convertin~g the near IR output of : ~ th~e-~diode:26 to blue~ greèn light. One application for such a device is i n:~op~:tical data~ stor~ge ~readout systems wherein it is desirable to minimize the~optical:wavelength so`as to increase the bit packing ~: ~ ; density of the media.: ~ ~
Further in accordance with the :invention, and referring to Figs~ 13a and 13b, there is d:escribed an optical waveguide that provides SHG.

~ Spec~fically. a~waveguide: 36 is formed in a bulk glass substrate 38.
: ` ~ : ~e waveguide is~ defined by a channel region 3~a having an index of ~;~ refraction that is larger than~ he index of refraction of the . .
- ~ .
.

WO 93/08500 2 1 1 ~ ~ 5 0 PCI/US92/08364 ~0 surrounding s~ trate 38. Thi~ result~ in a guiding and confinement of iniected radiation about the waYQguide channel. In accordance with the in~ention a porti~n of the guid~d radiatian is frequency doubled by th~ SHG ef~ect re~ulting from semiconductor microcry~tallites ~mbsdded withi~l th~ substrat~ 38.
: :
; Two examples of the fabrication of the raveguide 36 are now esented.
;
Ex~rnpt~ 1 Low pass filters manufactured by Corning Glas~l numbers 3-70 ~514nm~ and 3-71 (493nm), were placed in a KN0~ melt ~t 400C for 20 hours. These filters had Na2O concentrations of 14.3 percent and 14.4 percent, respectively. Planar waveguides were fab~icated through an: ion-exchange process to have a depth of 15)1m. ~harnel waveguides were fabricated by photolithographically masking the dass surface~ with~aluminum during dîf~usion. Th¢ mask provided open diffusion channels having a width of 60~m. From an e~ective rnode size: determined by an output diffraction pattern, in ~ ~ .
conjunc~ion with ~measured inde~ of refraction changes, the waveguide depth was determined to be 15~1m.

Example 2 A filter glass ~Schott 495) wa5 employed, the glass containing a : small:amount of Na~ and 20 percent K~. Two samples were placed in a~:350C melt of RbN~3 for 22 hours and 41 hours, respectively. Planar waveguides were made with a depth of 25,um and 41~,1m, respectively. Channel wa~eguides were fabricatedJ with an aluminum mask, to have dimensions of 65,um by 30~Lm.

It was Çound that ion-exchange in these systems resulted in a smaller index of refraction diflference in spite of the higher ~m~unt of potassium.

~ CTI ~1~ CY~T
- . ~ - . . ,: , . , , :

Wo 93/08500 2 1 1 9 ~ ~i O PCr/US9t/~
~ 21 This may b~ explained in term~ of a simpl~ model which scales the index change by (a~ an amount of ;ons to be exchanged, and (b~ a change in polar~zability caused by replacem~nt of ;ons with lar~er :~ radii.

This is shown by tlae equation:

K-Na NNa[(RK) _ (R~a)~]
- - ~ 2 nRb-K NK ( )3 (~ ) (O

~ , ~
where, RNa = 1.57, R~ = 2.03, RRb = 2.16, NNa = 14.4%, NK = 20%.

The ~waveguides,~ prepared by the first and second examples d:escribed above both provided SHG, after preparation, when a 1.0~6~m beam was injected. Preferably, a grating structure 38a is provided at a terminal end of~ the spiral waveguide to enable extraction of the fundamental ~l and the secorld harmonic ~2 .

The:~use of the SHG: e~ect, in conjunctiQn wi~h an ion-exchange waveguide~ fabrication technique in the g~asæ hostJ results in an ntegration;~Qf the optical; switching capabilities of these materials :w`ith~ efficientl low cost~frequency doubling. As an example~ and referring to Fig~ l9, ~:there is shown a ~ichr~matic l~gic switching device 50 that ;ncludes two cha~el waveguides 52 and 54, fabricated .
as described previously within a surface of a semi~onduct~r microc~ystallite~ doped glass substrate 51. The waveguide 52 is not : ~
prepared to generate the second harmonic, while the wave~uide ~4 :is~:prepared, as des~ribed above, to generate ~he second harmonic.
The wa~.reguides approach one another with;n a region designated by A and aré spaced apart at a distance: of = ~ where ~ is the ` ~ `: :

Cl lDCTI~ IT~ cs~r~T
~: , , .. , . :
... , , .. .~ ; ~ , . , .~ ., WO 93t085û0 Pcr/VS92f~8364 211~

funda~ental wavelength p~opagating in waveguid~ S2, n9 i8 the index of re~ac~ion of She glas~ host, and nC i8 the index of refra~tion of the cladding. Th~ spacing b¢tween the waveguide~ is thus ~enerally o~ the or~er of the mode confinement length. ~t high intensity (I ~ Ic) radiation propagating in waveguide 52 couples into the wavegu;de 54 in a Icnown fashion, where I i~ the inten~ity of the radiation propagating in waveguide 52 and lc i~ a critical inten~ity.

In accordance with the invention, when I ~ Ic a portion of the coupled radiation of the fundamental (Al) is converted to the second harmonic (A2) A filter 56 that is transmissiv~ at ~2 i~ provided at the output of the waveguide 54. ~ detector 58 i5 positioned fnr detecting the presence of the second harmon;c. If the detector 58 detects the presence of th~ second harmonic it ;s indicated that I >
lc~ As a result, it is unnecessary to spatially resolve the outp~ts of the two waveguid~ 52 and 54 so lorlg as the presence of the second harmonic is de~ected. The filter 56 may be photolithographically formed at the terrninal end of waveguîde 54, or may be provided as a separate component.

It is now shown that the provision of ~3HG in semieonductor micr~crystallite~doped glasses, as taught by the invention, further enables the ~u~e of a laser rod or optical fiber to ~enerate a fundamentalbwave!ength and to also generate a frequency doubled waYel2ngth.

By~ example, a ;common and most useful glass laser is Nd:~lass, where Nd is dop~ed at 1-5 wt% into a base glass with, for example, 66 wt % SiO2, 16 w~% Na~O, 5% BaO, 2 wt% Al2O3 and ~ wt % Sb~03.
In this regard reference is made to E. Switzer and C. G Young "Glass Lasers" in Lasers Vol. 2y A.K Lev;ne ed., Marcel DPkker ~, In., NY ~1968) p. l9l.

One recipe of interest herein includes Nd, or any other well known laser-ion such as Tm3~, Er3~, Nd3+, Yb3+, or Ho3+, in a silica-base ::
~ D ~ . ~ L~

:

wu g3/08s00 2 1 1 9 ~ 5 0 pcr/uss2lo8364 glass that includes semiconductor microcry~tallites, such as CdSxSel.x-Su~h a laserldoubler may be prepared a~ ~oll~ws.

Re~erring to Fig. 17 ther~ i$ illustrat~d a l~er rod preparationsystem 40 that incltldes a la~er cavity 42 bounded by reflective mirrors 44a and 44b. A laser rod 46 ~o be prepared for SHG is installed in the cavity 42 and is ~ptically coupled to a flashlamp 48.
An optical firequency doubling component, such as a ~P cry~tal 50, is provided within the c~vity 42. Mirror 44a ;s 100% renec~ve at the fundamental wavelength ~) and mir~or 44b is lOO~o reflective at c~
and 2c~. By example, the~ fundamental wavelen~th is 1.û6 ~lm and the harmonic is 532 nm , The laser rod 46 is pwnped by the flashlamp 48 and operated for a period of time of from seYeral minutes ~ seYeral hours urith the KTP
crystal 5~. This produces ~ large ~ field and a 2~ field and prepares f ~ the laserldoubler for SHG in a manner sim~lar to the injected 1.06 m and 532 nm used: to prepare the sample 20 of ~g. lb.
.
.
Referring to Fig. 18, :after the laser rod 46 has been prepared the output mi~or 44b:is replaced with a mirror 44c that is 100%
;:retlective at cp:and~ substantially transparent a~ 2~. The KTP
: crystal 50 is remaved, ~and the laser is operated ~ simul~aneously : produce 1.06 llm and:~ 532 nm. In that ths mirror 44c is substantially `~ : t ransparent to the:second harmonic the coherent optical output of ~ ~ : the laser is at twice: the firequency of the laser rod fundamental : ~ requency. In addition, the efiiciency is high since the intracavaty ~ ~ : fi~ld at 1.06 ,~rn is verylarge.
, ~
. :~
: It is within ~he scope of the inYention to remove the prepared rud 46 from the cavity 42 and install same within another laser cavity. It is also within the sc~pe of the invention to provide the mirror 44c such . ~:
~: : ~ OC~'I~I lT~ ~L~ r'T
: ~ ~ . , : ~
,, .

WO 93t08s00 2 1 1 9 4 5 PCI/US92~083~4 that it is partially transmis~i~e to the fundamental frequency, thereby pr~iding both 1.06 ,um and ~32 nm at ~he autput.
.

~eferring to Fig. 16 there is shown a further embodiment of the invention, specifically, a transmission holographic medium 60.
l!~edium 60 i8 cnmpnsed of a semiconductor mic~ocrystallite doped glass and has ~ypical dimen~ions of one centimeter on a side. Tke medium 60 i8 prepared and r¢corded with a preparation beam that includes ~1 and th~ second harmonic ~2. ~nd i~ resd out with a readout beam having a waYelengtl~ of ~1. The holographic medium 60 has a plurality of volum~ holograms stored w~thin that ~re stored by illuminating a region of the medium with Al, such as 1.06 ~
while ~2 is provided ~o reflect of~ of an object or pattern to be recorded befare entering the medium 60. The medium 60 is exposed to both wavelengths for a period of tiIne sufficient to provide a desired degree of preparation. As a result, simultaneous recording and prepara~ion occurs.~ Subsequently, when the readout beam is applied to a previously recorded region, an output beam, corresponding to a selected one of the volume holograms, is output at a wavelength of 7 2 . As a result, the bolographic medium 60 provides~ a frequency doubled output. That is, when illuminated with, ~or~ exarnple, 1.û6 llm radiation, She holographic medium produces a green:image.
A
The erasure mechanism described above can be beneficially employed to erase:a ~selected one or to erase all of the volume ~ ~ , ho~ograms stored within the medium 60. As a result7 the medium 60 may be written with new in~ormation. Optical erasure may be accomplished using another wavelength that is short enough to pump the bandgap of the semiconductor microcrystallites embedded within the medium 60. For exampleS the e~asure beam wavelength may be appr~ximately 40~0 Angstroms. The erasure may also be ccomplished7 depending on the glass host/microcrystallite composition, with ~ , andlor thermaily. By what ever erasure mechanism is employed, a random access read/write optical : :
:
2e~1~ IT~ l~a~Flr WO 93/08~00 2 1 1 9 4 5 0 PC~r/US92/08364 ~5 memory is provided. By directing She erasure beam to a selected re~on, o~lly the infarmation stored within that region is erased.
The medium 60 may also be partially or totally erased by blanket illiminating a selected portion or the entire volume of the medium 60.

In conclusion, it has been shown that centrosyr;lmetric glasses doped with CdSxSel.x microcrystallites may be optically prepared to produce a phase matched second harmonic generation process.
This invention extendable to the quantum dot regime, wh~re quantum confinemen~ results in larger nonlinearities, and to other semiconductors, than those specifically mentioned above. C3ther wavelengths may also be employed to prepare and readout the material~ The bulk glass m~jr b~ provided as a monolithic body, as a film, or as a ooatirlg applied to a substrate. Sputtcrirlg is one suitable process for fabricating the coating. In this regard the material may be i~tegrated with a laser diode device to provide a fundamen:tal and a second harmonic outpu~.
While the inventiorl h~s been particularly sho~vn and described wi~h respect to a preferred emb~diment thereof, it will be understood by those skilled in the art; that changes in ~orm and details may be m~de therein without departing from the scope and spirit of the invention.

Claims (30)

26What is claimed is:

1. A method of preparing a material so as to exhibit second harmonic generation for optical radiation incident on the material, comprising the steps of:
providing a glass host having semiconductor microcrystallites contained within; and irradiating the glass host with radiation having a first wavelength and a second wavelength that is one-half of the first wavelength, the glass host being irradiated for a period of time sufficient to obtain a desired value of conversion efficiency of the first wavelength into the second wavelength.

2. A method as set forth in Claim 1 wherein the glass host is comprised of a silica-based glass and wherein the semiconductor microcrystallites are comprised of CdSxSe1-x, wherein x has a value within a range of zero to one.

3. A method as set forth in Claim 2 wherein the silica-based glass further includes a material selected from the group consisting of Na, K, and Nd.

4. A method as set forth in Claim 1 wherein the first wavelength is within a range of approximately 2 µm to approximately 0.5 µm and wherein the second wavelength is one half of the first wavelength.

5. A method as set forth in Claim 1 wherein the semiconductor microcrystallites have a density of approximately 0.3 mole percent to approximately 50 mole percent.

6. An optical device comprising:
a source of optical radiation having a first wavelength; and means, optically coupled to an output of said source, for converting the output of said source to a second wavelength that is one-half of the first wavelength, said converting means including a glass host having semiconductor microcrystallites embedded within.

7. An optical device as set forth in Claim 6 wherein the bulk glass is comprised of a silica-based glass and wherein the semiconductor microcrystallites are comprised of CdSxSe1-x, wherein x has a value within a range of zero to one.

8. An optical device as set forth in Claim 7 wherein the boroslilicate glass further includes a material selected from the group consisting of Na, K, and Nd.

9. An optical device as set forth in Claim 6 wherein the first wavelength is within a range of approximately 2 µm to approximately 0.5 µm and wherein the second wavelength is one half of the first wavelength.

10. An optical device as set forth in Claim 6 wherein the microcrystallites are selected from the group consisting essentially of CdSSe, GaAs, InP, ZnSe, CuCl, PbS, and ZnSeS.

11. An optical device as set forth in Claim 6 wherein said source includes a semiconductor laser diode.

12. An optical device as set forth in Claim 6 wherein said source includes a Nd:YAG laser.

13. An optical device as set forth in Claim 6 wherein said converting means deposited upon a substrate.

14. An optical device comprising a laser diode means having an output for providing radiation at a wavelength .lambda.1, said optical device further including a frequency doubler means that is optically coupled to the output of said laser diode means, said frequency doubler means being comprised of a silica-based glass host having semiconductor microcrystallites embedded within.

15. An optical device for frequency doubling optical radiation, said device including a glass substrate having semiconductor microcrystallites contained within, the glass substrate having a waveguide structure formed within at least one surface thereof for guiding radiation having a first wavelength and for converting a portion thereof to a second wavelength.

16. An optical switching device comprising a glass substrate having semiconductor microcrystallites embedded within, said glass substrate including a first waveguide and a second waveguide formed within a surface thereof and spaced apart along at least a portion of a length thereof such that radiation having a wavelength of .lambda.1 propagating in said first waveguide couples into said second waveguide when the intensity of the radiation is greater than a critical intensity, said second waveguide receiving the coupled radiation and converting same to a wavelength .lambda.2, wherein .lambda.2 is one half of .lambda.1.

17. An optical switching device as set forth in Claim 16 and further including means for detecting the presence of radiation having a wavelength of .lambda.2, said detecting means being optically coupled to said second waveguide.

18. An optical switching device as set forth in Claim 17 wherein said detecting means includes a filter for passing radiation having a wavelength of .lambda.2.

19. A laser rod formed from a bulk glass comprised of a silica-based glass having semiconductor microcrystallites embedded therein, said laser rod simultaneously generating coherent radiation having a first wavelength and a second wavelength that is one half of the first wavelength.

20. A laser rod as set forth in Claim 19 wherein the laser rod is prepared for frequency doubling in accordance with a method that includes a step of irradiating the bulk glass with optical radiation having the first wavelength and the second wavelength.

21. A method of preparing a laser material so as to exhibit second harmonic generation, comprising the steps of:
providing a laser material within an optical cavity bounded by reflectors, the laser material including a bulk glass having semiconductor microcrystallites embedded therein;
providing a frequency doubling optical component within the optical cavity;
pumping the material such that the material generates coherent radiation at a fundamental wavelength;
converting with the frequency doubling optical component a portion of the fundamental wavelength to a second harmonic thereof; and continuing to pump the material such that the material is exposed to the second harmonic so as to prepare the material to generate the fundamental wavelength and the second harmonic thereof.

22. A laser having an optical cavity bounded by a first mirror and by a second mirror, the laser including within the optical cavity a lasant material that includes a bulk glass having semiconductor microcrystallites embedded therein for generating coherent radiation having a first wavelength and a second wavelength that is one half of the first wavelength.

23. A laser as set forth in Claim 22 wherein the lasant material is prepared for frequency doubling in accordance with a method that includes a step of irradiating the bulk glass with optical radiation having the first wavelength and the second wavelength.

24. A laser as set forth in Claim 22 wherein one of the mirrors is output mirror that is substantially transparent to the second wavelength.

25. A laser as set forth in Claim 22 wherein one of the mirrors is an output mirror that is at least partially transparent to both of said first and said second wavelengths.

26. A volume holographic medium comprising a bulk glass having semiconductor microcrystallites embedded within.

27. A method of operating a holographic memory, comprising the steps of:
providing a holographic medium comprising a bulk glass having semiconductor microcrystallites contained within;
recording information within the volume holographic medium by irradiating a region of the holographic medium with radiation having a first wavelength and a second wavelength, the second wavelength being one half of the first wavelength; and reading out the information by irradiating the region with the first wavelength, the information being read-out at the second wavelength.

28. A method as set forth in Claim 27 and including a further step of erasing the information.

29. A method as set forth in Claim 28 wherein the step of erasing includes a step of irradiating the region with radiation having the first wavelength, the second wavelength, or a third wavelength selected to pump the bandgap energy of the semiconductor microcrystallites.

30. A method as set forth in Claim 28 wherein the step of erasing includes a step of heating the medium.