fig1 is a schematic side view of a layered structure 10 for quasi - phase - matching according to the teachings of the present invention showing thin al x ga 1 - x as epitaxial layers 14 sandwiched between gaas epitaxial layers 12 . the layers are grown on top of a gaas substrate wafer 16 . fig2 is a schematic side view of another crystal structure 20 according to the teachings of the present invention showing the input of a pump beam 22 into crystal 20 . thin al x ga 1 - x as epitaxial layers 26 are sandwiched between gaas epitaxial layers 24 . the layers are grown on top of a gaas substrate wafer 25 . fig3 is a schematic side view of another crystal structure 30 according to the teachings of the present invention showing propagation of a pump beam 36 through wafer structure 30 and generation of daughter beams 38 . a fraction of the radiation incident at each gaas — algaas interface will be reflected and the rest transmitted . only some daughter beams 38 are shown and only the start of the propagations of daughter beams 38 are shown for clarity . fig4 shows one possible implementation of the new invention . a gaas — algaas crystal multi - layer wafer 40 is shown on its side . the gaas and algaas epitaxial layers ( not shown in this view ) are parallel to the long sides . a pump laser 44 sends a beam through an optical parametric oscillator ( opo ) 48 . a pair of beams 54 and 56 exit opo 48 , pass through a partially mirrored window for a spectrometer 46 and enter a beveled end of crystal multi - layer wafer 40 . in use , gaas crystal wafer 40 and filters 42 are mounted on a turn table ( not shown ) along an axis 52 to allow the angle between input beams 54 and 56 and gaas crystal 40 to be changed . this “ tuning ” will change the wavelength of the generated output beam 54 . for the new invention , a fresnel quasi - phase - matched structure may be fabricated by growing many epitaxial layers of gaas and al x ga 1 - x as . fig1 depicts the gaas wafer with many thin epitaxial layers 14 and 12 of gaas and al x ga 1 - x as layers . the total thickness of the epitaxial layers plus the wafer must be sufficient to allow easy input and output of the laser beams — approximately 1 mm . however , as the total thickness increases , there will be fewer bounces off the top and bottom surfaces . therefore , it may be preferable to grind and polish part ( or all ) of the starting gaas wafer . in a post - growth processing step , the wafer would be cut into rectangles . the ends of the rectangle may be beveled so that the incident beam would bounce through the crystal from the bottom of the wafer up through the epitaxial layers , bounce off the top of the wafer and continue through the crystal to the output end of crystal wafer 20 . the output end ( not shown ) may also be beveled to allow for easy extraction of the output light . the completely fabricated crystal in the new invention will have the same outward appearance as crystals used in the prior art . at each gaas — algaas interface during propagation , the pump beam is partially reflected and the rest transmitted . this is shown in fig3 ( only some of the many daughter beams are shown for clarity ). the amount reflected is dependent on the index of refraction difference between the gaas layer and the algaas layer and is an easily calculated quantity . at the top and bottom surfaces , the beams are totally reflected ( as long as the angle is below a critical value ). the efficiency of the conversion of the pump beam into the signal and idler beams is dependent on how many bounces the pump beam makes through the crystal . one can see from fig3 that the highest efficiency will be achieved if one could confine the pump beam into just one of the gaas channels , which is unfortunately not possible . however , it is easily seen that with the new invention , the pump beam and its daughters will undergo many more bounces then without the multilayer structure . the epitaxial layers of gaas and al x ga 1 - x as may be grown on standard gaas substrates by hydride vapor phase epitaxy ( hvpe ) or organometallic vapor phase epitaxy ( omvpe ). the thickness of the each gaas layer is determined by the phase matching requirements for the propagating beams . this is readily calculated by known formulas for given frequencies of the pump , probe and signal beams and the index of refraction of gaas at each optical frequency . the thickness of the algaas layer is much thinner and its primary purpose is to cause a partial reflection of the pump beam at each of the gaas — algaas interfaces ( the rest of the beam is transmitted ). the percentage of the total incident intensity reflected at each gaas / al x ga 1 - x as interface is a function of the indices of refraction and the angle of incidence . the composition of the al x ga 1 - x as layer should be chosen to maximize the reflected intensity at the gaas / al x ga 1 - x as interfaces . the alas — gaas will have the largest index difference and is preferred . however , it is known that very high alas fractions deteriorate in moist air . therefore , it may be advantageous to keep the alas fraction at about 50 %. the benefit of using al x ga 1 - x as to produce the index difference is the very small lattice parameter mismatch between alas ( a = 5 . 665 å ) and gaas ( a = 5 . 653 å ) which only produces a very small mismatch strain . the growth of thin al x ga 1 - x as layers will therefore not result in dislocation formation and the subsequent gaas growth will be unstrained . the present invention has many of the same advantages as the total reflection birefringence method of haidar et al . haidar et at claim that , in their method , fresnel dephasing at each reflection can compensate for almost any phase lag between interacting waves . this non - resonant phase - matching scenario allows both large tunability and nonlinear device design tolerance compared to the other techniques for obtaining nonlinear optical generation in gaas . one of the disadvantages of the tir - qpm scheme of haidar et at and others is that the number of phase reversals that can be reasonably produced in gaas ( 40 - 70 bounces ) is many times fewer than can be produced in the equivalent opgaas structure . a major advantage of the present invention compared to the haidar et at tir - qpm scheme is that because it undergoes many more bounces , it will be more efficient in the nonlinear optical generation . it may approach and even surpass the efficiency of the opgaas technique . one of the advantages of this invention relative to the opgaas growth of patterned substrates is the simplicity of the structure . it does not require preparation of a template by lithography , etching , or mbe growth that the opgaas requires . a second advantage of the new invention compared to opgaas is that it is often difficult to maintain the patterned gaas domains throughout the epitaxial growth , limiting the total thickness of the structure . the method described in this invention does not have such a limitation as the growth is on a uniform planar substrate . a third advantage compared to the opgaas method is that the opgaas method has trouble growing on thin columns . op — gaas fabrication is based on a photolithography and molecular beam epitaxy ( mbe ) process that results in a thin - film template with periodic crystal inversions . these periodic crystal inversions consist of many thin , long columns ( 5 μm - 200 μm wide , 4 - 5 mm long ). a thick film ( 0 . 5 - 1 mm ) is then grown upon this template by hydride vapor phase epitaxy to produce bulk op — gaas . the film thickness must be thick enough to allow the pump laser beam to pass through the columns from the side . one problem with the technique is that it is nearly impossible to grow thin columns high enough to be useful for nonlinear optical applications . the columns close up before useful thicknesses can be obtained . this is not true for the present invention . it is actually easier to grow thinner layers in the new invention because the growth times to produce a set number of gaas — algaas pairs will be less . a fourth advantage of the new invention compared to the opgaas technique is that it is possible to use commercially available deposition equipment ( for example mocvd systems from axitron or emcore ) that have light - based thickness monitors for the real time determination of the growth rate of the gaas and algaas layers . this allows for precise growth of the many gaas — algaas pairs needed for this application . in principle , it might be possible to do this in the opgaas case , but the many columns would interfere with the thickness monitors operation . it is not possible to calculate the efficiency of the new invention since the total number of reflections is not easily calculated . however , it is possible to calculate is the fraction of the pump beam that is diverted by the many reflections at all of the gaas — algaas interfaces . assume that there are m layers of gaas separated by thin al x ga 1 - x as ( x = 0 . 5 ) layers . that makes a total of 2m interfaces . following haidar et at , assume the angle of incidence , θ i = 25 °. for a 10 mm long crystal , haidar et al . measured 41 bounces in a gaas crystal . consequently , the pump beam will see 81m interfaces . for ir beams in the 4 - 5 μm range , it is known that the coherence length of a λ = 2 μm pump laser in gaas is about 20 μm . assuming a total of about 500 μm of epi is grown , this corresponds to about 25 layers of gaas . therefore , the total number of interfaces , m t , is m t = 2025 in this example . what is the fraction of λ = 2 μm light that is reflected at each interface ? the index of refraction of algaas is calculated according to gehrsitz et . al ., “ the refractive index of al x ga 1 - x as below the band gap : accurate determination and empirical modeling ,” j . appl . phys ., vol . 87 , pp . 7825 - 7837 ( 2000 ). at λ = 2 μm , n gaas = 3 . 34363 for gaas and n algaas = 3 . 0976 for 50 % algaas ( x = 0 . 5 ). for comparison , n gaas = 3 . 3389 for gaas ( from t . skauli et al ., “ improved dispersion relations for gaas and applications to nonlinear optics ,” j . appl . phys ., 94 , 6448 - 6455 ( 2003 )). the angle of the transmitted beam is θ t = 27 . 14 °. fresnel &# 39 ; s equations give the reflection coefficient , r p for the parallel polarization case : this means that the reflected and transmitted energy is given by : parallel polarization case : reflected energy = r p □ r { circumflex over ( r )} 0 . 0844 %. perpendicular polarization case : reflected energy = r p □ r { circumflex over ( r )} 0 . 2239 % with the above information , it is possible to calculate the fraction of the pump beam that will be diverted by m layers of gaas separated by thin al x ga 1 - x as ( x = 0 . 5 ) layers . for a 10 mm long crystal , the total number of interfaces , m t , is m t = 2025 . at each interface , the transmitted fraction is 0 . 999155 for the parallel polarization case and 0 . 99776 for the perpendicular polarization case . thus , for m t interfaces : thus , in this example multilayer structure , the pump beam polarized in the perpendicular direction loses 82 % of its energy creating daughter beams . for the parallel polarization case , the multilayer structure diverts 99 % of the pump beam . the teachings of the disclosed new and improved approach for optical frequency conversion using gaas crystal wafers comprising alternating gaas and algaas layers , while specialized , will find application in other areas where combining different optical materials can produce optical devices having improved optical performance properties . terms used in the claims are used with their ordinary meanings as understood by those with skill in their application art areas . for example , while the term “ wafer ” may technically refer to a single layer , in practice , wafers are usually thought of a multi - layered . similarly , the term “ crystal ,” which may be thought of as referring to a crystal of a single material , will be understood by those with skill in the art to refer to a multi - material “ crystal ” when required by the context . the more general term , “ structure ,” is used in the claims to broadly include any unintended limitations caused by the use of terms such as “ wafer ” and “ crystal .” various modifications to the invention as described may be made , as might occur to one with skill in the art of the invention , within the scope of the claims . therefore , all contemplated embodiments have not been shown in complete detail . other embodiments may be developed without departing from the spirit of the invention or from the scope of the claims .