Operational Characteristics of GaSe Crystals for Mid-IR and Far-IR Applications
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ABSTRACT GaSe has a number of attractive properties for nonlinear optical applications including large birefringence for ease in phase matching. Its biggest drawback is its mechanical properties. GaSe has a strong tendency to cleave along the plane which has made it difficult to grow and fabricate. We have developed a method to modify GaSe by structurally strengthening the material by doping. We have synthesized large boules of GaSe reacted mixtures and grown centimeter size single crystals by the Bridgman technique. Depending on the dopant and crystal quality, SHG measurements indicate a d0, of 51 to 76 pmN. SHG power levels were theoretically calculated and appear to be ingood agreement with the experimental data. The measured threshold are also performance of crystals for the fourth harmonic generation and laser damage reported in this paper. The damage threshold was greater than 2.8 J/cm 2 and 85 KW/cm 2 at the surface of the crystal.
BACKGROUND There is a great need for optical wavelength conversion devices for a number of applications including electro-optic countermeasures. The particular interest is in high-averagepower devices spanning the atmospheric windows of 3-5 jim (Mid IR)and 8-12 jLm (Far-IR) range; tunable devices in these wavelength ranges are also highly desirable. Since most suitable highaverage-power lasers lie outside the 2-12 i~m range, their output must be shifted to these ranges using nonlinear devices fabricated from single crystal materials. To perform effectively, these materials must possess a unique combination of optical, thermal, and mechanical properties. None of the currently available materials have all the characteristics needed for high-average-
power operation. GaSe has some interesting properties for nonlinear frequency conversion applications of infrared laser light either as second harmonic generation, sum or difference frequency generation, or optical parametric oscillation. GaSe's most outstanding feature is its broad transparency from 0.7 to 18 pim with no intermediate absorption features. It has a rather large nonlinear coefficient, moderately high thermal conductivity perpendicular to the optic axis, and an above average damage threshold. These latter properties facilitate high power operation. This makes GaSe a very useful nonlinear material for the 2-18 Rim range and is a good candidate for use between the 10 and 18 gim. GaSe has a strong tendency to cleave along the plane which makes them very difficult to grow and fabricate. We have synthesized large at batches of GaSe reacted mixtures and grown centimeter size doped single crystals by the vertical Bridgman technique. With proper growth conditions and slow cooling we have grown cm size crystals with good optical transparency and reduced cracking. These results along with demonstrated capability of growing device-sized single crystals and high second harmonic generation (SHG) "d" values for modified GaSe crystals are reported in this paper.
EXPERIMENTAL METHOD Gallium and selenium (6-9's pure) were further purifie
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