Mercurous halides have become the subject of great interest because of their very high acousto-optic figure of merit and large transmission range. These crystals have unique acousto-optic properties that make possible a variety of high performance devices for signal processing and spectral processing and analysis, particularly in the mid- and long-wavelength infrared region. Mercurous chloride is the most developed in this class and large crystals have been grown by the physical vapor transport method. Current research is attempting to extend the efficiency of devices using the mercurous halides. The authors' research centers on purification methods, determining the factors that affect optical quality and improving the process of single crystal growth.
Fundamental optical and electronic parameters of Zn3P2 are presented. Mg-Zn3P2 structures are examined as a solar energy converter and broad-range photodetector. A distinct photodichroism, observed for junctions prepared on oriented single crystal, is applied in light polarization step indicator.
Band-edge photorefractive effect provides large sensitivities and nonlinearities in bulk and multiple quantum well semiconductors. Recent results in InP:Fe and a novel photorefractive device based on the quantum confined stark effect in semi-insulating II-VI multiple quantum wells are reported.
Potassium niobate (KNbO3) has great potential as a crystal for sum-frequency generation in the blue end of the spectrum, possessing, as it does, high nonlinear coefficients and high birefringence. However, concerns about wide-spread applications of the crystal in solid-state blue lasers persist; principally because of uncertainties surrounding the mechanical and thermal stability of the material. These instabilities result largely from the readiness with which ferroelectric domains can be formed within the crystal (at room temperature KNbO3 possesses an orthorhombic distortion of the parent perovskite crystal structure) by application of mechanical or thermal stress. In addition, fabrication of the as-grown crystal for device application requires removal of ferroelectric domains which result from cooling from the melt temperature, where the crystal is in the paraelectric cubic phase, through a tetragonal phase to the orthorhombic form. As part of a systematic investigation of the properties of KNbO3, we have characterized the nature of domain formation in as-grown crystals and have studied conditions under which single domain crystals are obtained by application of electric fields, using predominantly optical techniques. The electrical properties of the crystals are highly complex and appear to depend on a number of factors. We relate the extent of domain formation to crystal purity, size, and seed orientation.
The performance of longitudinally pumped Nd:YAG was evaluated before and after exposure to 60Co gamma radiation. For comparison, other Nd-doped materials, Cr:GSGG and YLF, were also included in this study. The cw unirradiated optical-to-optical slope efficiencies for Nd:YAG and Nd:YLF were 63% and degraded to 48% and 36%, respectively, after 600 kRads of irradiation. Nd:Cr:GSGG performed significantly worse, exhibiting a slope efficiency of 42%, but was not affected by irradiation (a result that is in agreement with previous reports). Electron paramagnetic resonance studies of the Nd:YAG samples indicated that there was no modification of the Nd3+ sites resulting from exposure to the radiation. It is concluded from the performance and spectroscopic analysis that the degradation in Nd:YAG is primarily due to an induced passive optical loss of approximately 0.02 cm-1. Furthermore, this effect was observed to saturate at exposure levels of 50 kRad. The relatively low induced loss indicates that Nd:YAG systems employing pulsed diode pumping in the longitudinal configuration, should be resistant to ambient space environment radiation damage. This point was experimentally verified with respect to the effect of gamma rays on performance.
When 001 plates of KD2PO4 (KD*P) are used in Pockels cells, strain induced refractive index variations result in beam depolarization and transmitted wavefront distortion. The depolarization is determined by the induced birefringence while the wavefront distortion is controlled by the average index shift. Here we show that the birefringence is determined by the shear stress in the xy-plane of the crystal while the average index shift depends only on the normal stresses. Furthermore, for depolarization losses of 0.1 to 1.0% and wavefront distortion of 0.1 to 1.0 (lambda) , the critical range of stress is 105 to 106 Pa. We also present measured depolarization loss and wavefront distortion profiles for 5, 16, and 27 cm, 95% deuterated, KD*P crystals. Using the analysis described above we show that the maximum internal stresses in these crystals are within the critical range, but that the area averaged stresses are substantially lower. We find that crystals from different locations along the length of a boule have similar strain birefringence and wavefront distortion profiles indicating that the growth conditions which generate the internal strain persist throughout much of the growth history of the boule. Finally, we discuss potential sources of strain in KD*P.
Potassium dihydrogen phosphate (KDP) crystals have been used as harmonic converters on the Nova laser at LLNL for more than six years. All crystals were coated with a single layer, quarterwave AR coating of porous silica with a refractive index of 1.22. This was prepared by a sol-gel process and was applied from a colloidal suspension by spin coating at room temperature. A few crystals were also coated with a methyl silicone coating prior to the application of the AR coating for environmental protection. The initial optical performance of all crystals was very good but there has been some deterioration over the years because of environmental and laser damage degradation. The deterioration in the silicone samples was, however, much less than the others. We are now in the process of replacing all 10 KDP arrays with new crystals and will apply the silicone undercoat to all samples. Recently, we have been evaluating a new perfluorinated organic polymer coating which has a refractive index of 1.29. This material is soluble in fluorinated solvents and can be applied by dip coating from solution at room temperature. We hope that this can provide environmental protection when applied to KDP and also act as an AR coating at the same time. The optical performance is not as good as our porous silica because of the higher index; about 0.3% reflection per surface is obtained.
The atomic structures of the nonlinear optical materials potassium titanyl phosphate (KTiOPO4, or KTP) and potassium titanyl arsenate (KTiOAsO4) feature one- dimensional channels through which the potassium ions are relatively free to migrate. Ion exchange results when these materials are immersed in molten salts containing alkali metal ions. Sodium, lithium, and silver all exchange readily for K+ in single crystals of both KTP and KTA to yield the exchanged derivatives Na.95K.05TiOPO4 (NaTP), Na.83K.17TiOAsO4 (NaTA), Ag.85K.15TiOPO4 (AgTP), Ag.98K.02TiOAsO4 (AgTA), Li.45K.55TiOPO4 (KLTP), and Li.46K.54TiOAsO4 (KLTA), which are all KTP isostructures. The optical nonlinearities (measured as SHG intensities) of the limiting compositions in the NaTA, KLTP, and KLTA systems are similar to that of KTP, but are much smaller in NaTP, AgTP, and AgTA. Single crystal x-ray data have revealed differences in coordination of the mobile cations to oxygen atoms linking the TiO6 groups in these compounds, and these differences correlate with changes in optical nonlinearity. The observed nonlinearities can be rationalized if they are viewed as being dependent on the degree to which delocalized charge- transfer excited state character can be mixed into ground state bonding and nonbonding orbitals in the TiO6 chains. The relative lack of association of Na+ and Li+ ions with these chains in NaTA, KLTP, and KLTA allows extensive excited state delocalization, and thus significant electronic hyperpolarizability.
Potassium titanyl phosphate (KTiOPO4 or KTP) has applications in nonlinear optics and electro-optics. It is most commonly employed in the second harmonic generation of .530 um light from 1.06 m Nd:YAG laser radiation. However, applications of KTP are limited by optical damage in the form of thin gray tracks produced by high-power, high-repetition-rate laser pulses.
It is difficult to obtain samples of KTP with laser-induced gray tracks that are suitable for quantitative measurements. The gray coloration absorbs both the fundamental and second harmonic, and continued operation after the formation of these defects may quickly lead to catastrophic failure. Another complication arises because the gray tracks characteristic of laser damage are not stable at room temperature (they decay in a matter of days). Even if gray-tracked samples were readily available, it is questionable whether the concentration of responsible defects would be sufficient to provide definitive results. These difficulties have led researchers to investigate alternative methods for producing the defects responsible for laser-induced optical damage in KTP.
The ionic conductivity and damage susceptibilities of KTP crystals are related to the defects present in the crystals, which result from the conditions of growth by the flux, high, and low temperature hydrothermal techniques. The effects of Ba impurities on the ionic conductivity and damage are also discussed.
A quantitative assessment of the bulk darkening phenomenon for flux-grown KTiOPO4 (KTP) is reported. Measurements were made for KTP use as a component for second harmonic generation of 1064 nm Nd:YAG operating in a pulsed mode. The results show that bulk darkening severity increases with fluence and exposure time. Irreversible bulk darkening has been observed at laser energies ranging from 8.8 to 18 J/cm2 depending on the shot count. Fluences exceeding 15 J/cm2 can cause catastrophic damage. Data taken at elevated temperatures indicates an improved reliability. The onset of slight bulk darkening revealed no adverse effects on conversion efficiency.
A wide range of nonlinear optical coefficients and various systems of notation have been used to describe optical second-harmonic generation (SHG). To avoid possible confusion, the techniques of optical nonlinear coefficient measurement and the elementary theory of SHG are briefly reviewed. Absolute and relative nonlinear coefficient measurements by phase-matched SHG are described. The following results were obtained: d36(KDP) equals 0.38 pm/V, d36(KD*P) equals 0.37 pm/V, d22(BaB2O4) equals 2.2 pm/V, d31(LiIO3) equals -4.1 pm/V, d31(5%MgO:LiNbO3) equals -4.7 pm/V, d15(KTP) equals 1.9 pm/V and d24(KTP) equals 3.5 pm/V. The accuracy of these measurements is estimated to be better than 10%. An example of high-repetition-rate pulsed SHG in AgGaSe2 is given to demonstrate the use of quantitative analysis of harmonic generation for evaluation of nonlinear optical material performance.
KTP waveguides are being investigated for frequency upconversion of strained-layer InGaAs lasers in the 900 - 1100 nm range. Phasematching for the lowest-order mode interaction can be obtained using the modal dispersion properties of these diffused waveguides and modeling calculations have been used to determine the waveguide parameters required for phasematching to occur at a particular wavelength. Ion-exchange in pure RbNO3 has been used to form a waveguide that permits phasematched frequency doubling at wavelengths near 1040 nm.
We discuss second harmonic generation of green and blue light in periodically-poled LiNbO3 and LiTaO3 waveguides, and difference frequency generation of infrared radiation in periodically-poled LiNbO3 waveguides.
Frequency doubling and optical parametric oscillation are investigated with potassium niobate in an external resonator. For frequency doubling of 860 nm input, 650 mW of cw blue light around 430 nm has been generated for 1.35 W of infrared input. In a cavity with reduced losses, overall conversion efficiency of up to 70% has been achieved. With regard to parametric oscillation with a 430 nm pump, 100 mW of stable cw emission from the optical parametric oscillator has been obtained for pump power 250 mW. Blue-light induced infrared absorption has been discovered to have significant deleterious effects and to be the principal reason why even higher efficiencies have not been reached.
Compact and efficient visible laser sources operating at wavelengths below 480 nm are required for a number of important commercial and scientific applications. These include high density optical storage, high definition projection video, reprographics and fluorescence-based bioanalytical instrumentation. During the past few years a number of efficient nonlinear optical techniques for the upconversion of GaAlAs diode lasers have been demonstrated. Unfortunately, each of these is currently separated from the commercial marketplace by at least one key technical problem.
One factor influencing the efficiency of KDP frequency conversion arrays on the Nova laser system at LLNL has been environmental degradation, or `fogging,' of the crystal surfaces. Decreases in array transmission by as much as 20% have been attributed to crystal fogging. The surfaces of the 27 cm square Nova array crystals are prepared by a wet diamond-turning process. The rate of surface fogging has been associated with several parameters of the diamond turning and subsequent cleaning processes. High humidity during diamond turning, storage, and use on the laser tends to accelerate the fogging. We suspect that some of the additives present in the diamond turning oil increase the fogging rate and have found a machining oil which minimizes this surface degradation. Efficient removal of the machining oils from the crystal surface also minimizes the fogging problem. Care must be taken to use cleaning solvents which do not cause additional surface degradation. The fogging rate is sensitive to the crystallographic orientation of the material as well as to surface roughness related to the diamond turning process. Accelerated fogging at diamond turning artifacts may increase crystal surface roughness causing increased beam modulation and scattering losses.