Semiorganics are a new class of high performance nonlinear optical (NLO) materials in which a polarizable organic molecule is stoichiometrically incorporated within an inorganic host forming either an organic/ inorganic salt (e.g., DLAP1) or an organic ligand/metal ion complex (e.g., ZTS2). This design methodology offers a high degree of design flexibility and allows the linear properties, such as the refractive index and absorption characteristics, of a new material to be “engineered” to a certain extent. We have examined the second-order nonlinear optical response of over 700 such semiorganics. Two materials in this group, guanidinium tetrafluoroborate (GFB) and L-histidine tetrafluoroborate (HFB), show significant promise as a NLO crystals for the UV through the near-IR (see Fig. 1).

The properties for members of a new class of nonlinear optical (NLO) materials which stoichiometrically incorporate organic and inorganic constituents into a single crystalline lattice are reported. Recent results for our synthetic, crystal growth, and optical studies suggest that a number of these relatively transparent ”semiorganic” compounds have significant second and/or third order NLO responses and often display favorable crystal growth morphologies. The prototype material of this class, zinc tris(thiourea) sulfate, or ZTS, has a UV cutoff at about 325 nm, can be readily grown to cm3 sizes, and has been shown to be a highly efficient Type II frequency doubler for 1064 nm Nd:YAG laser radiation. ZTS also possesses a moderate third order nonlinear optical response (ca. 0.1 × CS2) which occurs on at least a picosecond time scale as determined by degenerate four-wave mixing (DFWM) experiments at 532 nm. Refractive index, second harmonic generation, and DFWM data for a number of these new compounds are presented.

It has been demonstrated in a series of recent articles that the new crystal, LiCaAlF6: Cr3+, (known as Cr:LiCAF), holds substantial promise as a practical laser material.1-3 The laser output is tunable from 720-840 nm, and the Cr3+ ion exhibits two broad absorption bands in the visible range, thereby permitting the efficient absorption of flashlamp light. In this paper we report measurements of the thermal and thermo-optical properties of Cr:LiCAF that are relevant to the laser performance. The thermal lensing of Cr:LiCAF has been directly measured and compared to that of alexandrite (BeAl2O4:Cr3+).4

Over the last few years, substantial progress has been made at the Lawrence Livermore National Laboratory in nonlinear materials for high power laser applications. Specifically, we are developing materials for frequency conversion of lasers used in laser driven, thermonuclear fusion experiments and in high average power laser systems. We have developed new experimental procedures for fully characterizing the linear and nonlinear optical properties of microcrystals. Using new theoretical results we have developed a systematic method of selecting and optimizing nonlinear crystals for high-power and high-average-power laser applications. Our molecular engineering strategy for developing new materials for the fusion application has resulted in the discovery of several new materials with more attractive parameters than KDP.

Water soluble organic salts comprise a large class of crystalline materials containing many efficient harmonic generators for producing blue and near ultraviolet light. Among the salts of simple chiral organic acids and bases, about 1 in 10 crystals has a nonlinearity greater than or equal to KDP. The distribution of noncritical wavelengths among these crystals peaks around 500 nm for Type I doubling and 700 nm for Type II. We estimate the number of chiral organic salts which must be examined in a survey in order to assure with high probability that an "optimized" crystal for a given frequency conversion process will be found.

Procedures for measuring the linear optical properties of small, transparent crystals are described. Using a spindle stage refractometer, the refractive indices for light from 0.38 to 1.10 μm can be determined. The methods are employed to study new nonlinear crystals to evaluate their potential as second and third harmonic generators of light at 1.064 μm. The measurements can be made on single crystal fragments as small as 50 μm, thus permitting preliminary evaluations to be made prior to extensive crystal growth efforts.

We report measurements of all the material constants necessary to fully characterize barium borate as a nonlinear optical material. All data was taken on crystals supplied by Professor Chuangtien Chen, Fuzhou, People’s Republic of China. We have determined the crystal structure, the optical absorption, the refractive indices from the UV to the near IR, the thermo-optic coefficients, the nonlinear optical or coefficients, the resistance to laser damage, the elastic constants, the thermal expansion, thermal conductivity and dielectric constants, and the fracture toughness. This data is used to evaluate barium borate for a variety of applications. We find that, in general, barium borate has a low acceptance angle, and that despite its higher optical nonlinearity, it is therefore not significantly more efficient than other commonly available materials, except in the UV below 250 nm. On the other hand, it has a high damage threshold, it is physically robust, it has good UV and IR transparency, and it has excellent average power capability. It permits deep UV generation, and has great potential for generating tunable visible and IR light as an optical parametric amplifier.

A new method to optical characterization of microcrystals is described and illustrated with experimental results. Techniques for characterizing the linear and nonlinear parameters of new materials at early stages of crystal growth have been developed. The linear indices of refraction and their dispersion are determined using combined methods of spindle stage microscopy and refractometry. This method has been extended to measurements at nonvisible wavelengths. An SHG apparatus of novel design is used to evaluate the overall nonlinearity. Both methods are used to study materials with single crystal domains as small as 50 ..mu..m. 14 refs., 7 figs., 2 tabs.