Potassium titanyl phosphate (KTP) is a unique nonlinear optical material that is being widely used for second harmonic generation of Nd lasers emitting around 1 μm. KTP is also very attractive for various sum and difference frequency and optical parametric applications over its entire transparency range from 0.35 to 4.5 μm. Its combination of large electrooptic coefficients and low dielectric constants make KTP potentially useful for various electrooptic applications. Low loss optical waveguides can be formed in KTP and several electrooptic and nonlinear optic devices have been fabricated. Potassium titanyl arsenate (KTA), which is isostructural with KTP has recently been grown and characterized. Initial results show that KTA has nonlinear optic and electrooptic properties that are superior to KTP.

Miniature sources of coherent blue radiation can he produced by using nonlinear optical materials for frequency upconversion of the infrared radiation emitted by laser diodes. Direct upconversion of laser diode radiation is possible, but there are several advantages to using the diode laser to pump a solid-state laser which is then upconverted. In either case, the challenge is to find combinations of nonlinear materials and lasers for efficient frequency upconversion. Several examples have been demonstrated. These include intracavity frequency doubling of a diode-pumped 946-nm Nd:YAG laser, intracavity frequency mixing of a 809-nm GaAlAs laser diode with a diode-pumped 1064-nm Nd: YAG laser, and direct frequency doubling of a 994-nm strained-layer InGaAs laser diode.

Features of nonlinear crystals of BBO, LBO, KTP, KNbO₃, MgO:LiNbO₃ and NYAB are presented and compared for various applications.

The atmospheric transmission window between 3.4 and 4.2 μm is an important infrared band for many optical remote sensing applications. Not only is the atmospheric attenuation minimal, but several detectors (e.g. InAs, InSb, PbSe) have excellent sensitivity in this spectral region. Unfortunately active system development has been impeded by the lack of a convenient, efficient laser source in this spectral range. Only the DF chemical laser has capability for high power output in this region, but the complications associated with storage and handling of fuel, and the limited operating life have curtailed the application of these devices.

We report recent progress in the generation of continuously tunable picosecond pulses by three-photon interaction in KTiOPO4. The system is based on a cw mode-locked Nd:LiYF₄ laser, which synchronously pumps a dye laser and seeds a regenerative amplifier. Two different modes of operation are available simultaneously. Both operating regimes have equally large tunability : from 0.6 μm to 4.5 μm. In the high repetition rate mode, difference frequency mixing of the tunable output of the synchronously mode-locked dye laser and the Nd:LiYF₄ laser generates a 100 MHz train of picosecond tunable IR pulses with typical average power in the milliwatt range. Substantially higher energy per pulse can be obtained over the full tuning range by subsequent amplification at lower repetition rates. For this purpose we use a KTiOPO₄ parametric amplifier, pumped by the regenerative amplifier or its second harmonic. Typically, a single-pass gain of 106 is observed in this regime while maintaining the spatial, spectral and temporal characteristics of the seed beam. In the non-seeded configuration on the other hand, a parametric generator-amplifier consisting of two KTiOPO4 crystals operates in the saturation regime with energy conversion efficiencies of 10-20 % without pulse breakup and with very satisfactory pulse-to-pulse stability. However, in this configuration the spatial and spectral quality is traded for the ease of tunability.

The generation of blue, tuneable (424-449 nm) laser light is achieved via direct SHG in temperature-tuned KNb0₃, with a tuneable Ti⁺³:A1₂0₃ cw laser source. Thermal regulation of the crystal over the temperature range of 0 to 90°C is accomplished using a thermoelectric device with thermistor feedback control. The room temperature phase match wavelength of KNbO₃ crystals doped with Ta, Rb and Na has been found to shift toward the red. Room temperature SHG efficiency, crystal purity, beam characteristics and system components are discussed.

The introduction of new nonlinear optical crystals has provided researchers with a wide choice of materials for their particular applications. These materials potentially allow nonlinear interactions from the UV to the far IR. However, choosing the most appropriate material for a specific application is often difficult. For certain applications, traditional materials may have properties (other than the nonlinear coefficient) which make them very competitive with new, highly nonlinear materials. This choice is complicated further if additional crystal development is required. This paper will discuss, from the user's perspective, the material characteristics of KDP, KD*P, d-LAP, d-urea, BBO, AgGaS₂, AgGaSe₂, ZnGeP₂ and others. The physical, optical and frequency conversion properties are summarized, with particular emphasis on the interaction of these properties. To provide a guide or tutorial to the user, several application analysis techniques are described and applied to specific examples. These include the calculation of first and second order phase matching characteristics and one and two dimensional SHG modeling.

In this paper we present IR to visible conversion effects in luminescent materials developed by scientists of Sunstone Inc. We will discuss three classes of Sunstone's luminescent materials. One class of IR to visible converting materials is characterized by high sensitivity to IR light in the wavelength range of 0.7-2.0 um. The materials require pre-excitation by visible light at any wavelength in the range, of 300-550 um. This excitation can be retained by the materials for months.

In our search of novel materials for use in solid state tunable lasers operating in the near IR region we have grown, using the laser-heated pedestal growth method, crystals of Gd₃Ga₅O₁₂ (GGG) and Gd₃Sc₂Ga₃O₁₂ (GSAG) doped with Ni and Zr, and also crystals of GGG doped with Ni, Zr and Nd. Analysis of the characterization data of these crystals indicates the presence of Ni²⁺ in both the octahedral and the tetrahedral sites. New garnet compositions are currently being investigated that would inhibit the incorporation of Ni²⁺ in the tetrahedral site since the presence of this center is undesirable for good laser performance.

Titanium-doped YA1O₃ (Ti:YA1O₃) is an attractive candidate for a visible to near-infrared tunable solid-state laser. Un-seeded crystal growth utilizing the heat exchanger method has produced Ti:YA1O₃ crystals up to 1-cm in length. Spectroscopic measurements at room temperature showed that the Ti³⁺ absorption peaked at approximately 430 nm and extended from 380 nm to 550 nm. The fluorescence emission peaked at 600 nm and extended from 540 nm to 800 nm. No detectable parasitic absorption was observed in the 600-800 nm region; the figure of merit was estimated to be in excess of 15 as compared to previously reported values of 1.5 to 3. The lifetime of the excited state was measured to be 11.4 ± 0.5 gsec. In this work lasing was not observed because of pump-induced excited-state absorption in our crystals, presumably arising from charge transfer transitions to energy levels of defects, and not because of parasitic absorption.

Organic nonlinear optical crystals offer extensive possibilities of materials engineering in many fields of solid state science. They exhibit interesting potential uses in optoelectronics. In the search for new organic nonlinear optical crystals, solution growth is suitable for rapidly preparing specimens for first assessment of properties. Examples of apparatus and procedures for determining the solublilty vs. temperature dependence, mapping the primary crystallization range in different solvents, as well as for growing crystals of several selected materials are described.

Submillimeter-length, monolithic, Fabry-Perot-cavity lasers were fabricated from the stoichiometric neodymium compound, Lithium Neodymium Tetraphosphate (LNP) and pumped via close-coupling to the output facet of a high-power, diode-laser array. TEM₀₀ output powers in excess of 80 mW near 1047 nm and 26 mW near 1317 nm were obtained at a diode laser power level of 240 mW. Greater than 15 mW of single longitudinal mode output at 1047 nm was also observed.

Ammonium oxalate (AMO) has one of the highest figures of merit of known electro-optical materials and is potentially very useful. Single crystals of AMO 2 cm X 2 cm X 4 cm were grown from aqueous solutions by the temperature lowering technique. The crystals were oriented by optical birefringence techinques used for biaxial crystals. Preliminary experiments indicate that a halfwave voltage in excess of 6 KV is required for a sample oriented such that one of its optic axes, the applied field and the light beam are collinear.

Single crystals of gadolinium scandium gallium garnet (GSGG) doped with Cr and co-doped with Ca have been grown using the laser-heated pedestal growth method. This material emits in an eyesafe region of the spectrum, making it a potential candidate for use as an active medium in solid state lasers for a number of applications. Samples with various concentrations of chromium were prepared and analyzed using several characterization techniques. It was found that optical spectroscopy alone is not sufficient to discriminate accurately between the (CrO₄)^4- or the (Cr04)3- oxyanion centers which are formed when Cr⁴⁺ or Cr⁵⁺ substitutes for Ga³⁺ in the tetrahedral sites.

The new crystal of NYAB (neodymium yttrium aluminum borate) is presented. The optical and mechanical properties, crystal structure and growth of this crystal is investigated. Performance of emissions at 1063 nm and 532 nm is presented for both flash-lamp-pumped (pulsed) and dye-laser and diode-laser pumped (cw). A comprehensive model for intracavity doubling including the dopant-dependent gain and absoprtion (loss) is also analyzed. In this paper, we denornstrate the first experimental results of diode-pumped NYAB with green ligth output power in the 10 μW range.

This paper reviews a series of experiments carried out at Stanford University over a several year period on the growth of β-BaB₂O₄ crystals. Experiments on the bulk growth of β-BaB₂O₄ using the top-seeded solution growth method with and without pulling, using Na₂O as a solvent, has yielded relatively large boules. Solvent inclusions, however, limit the yield of high optical quality crystals from these boules. Oriented, single crystal fibers of β-BaB₂O₄ have also been grown, from both Na₂O and B₂O₃ solutions, using the laser-heated pedestal growth technique. B₂O₃ solutions were found to be quite stable under laser heating, and yielded high optical quality single crystal fibers with minimal inclusions.

A brief introduction to quadratic nonlinear molecular crystals outlines the basic features of a molecular engineering approach which has lead to the design of such materials as MAP, POM, NPAN and NPP. Phase-matching configurations for subpicosecond second-harmonic generation or degenerated parametric mixing must be compatible with group-velocity matching requirements. We show that spectrally non-critical phase-matching where the wavelength dispersion of the phase-matching angle is stationnary meets this condition at the expense of the angular aperture. Efficient type I second-harmonic generation at 1.32 and 1.34 μm fundamental wavelength in high quality POM crystals in the subnanosecond regime is reported. The eigenpolarization angles at these wavelength happen to maximize the effective d coefficient at the full d₁₄ coefficient value (i.e. 20 10^-9 e.s.u.). Subpicosecond time-resolution of parametric emission in NPP is achieved by a two stage Parametric Amplification and Sampling Spectroscopic (PASS) set-up. The temporal profiles of idler and signal waves are clearly and simultaneously analized at significant departures from degeneracy (up to 1500 cm^-1). This confirms the broadband acceptance of NPP and its relevance towards future ultrafast signal processing operations.

Single crystals of Ti:LaMgA1₁₁O₁₉ (Ti:LMAO) up to 20 mm length have been obtained from unseeded ingots grown by the Heat Exchanger Method (HEM^TM). The ingots were grown under reducing atmosphere in order to minimize formation of Ti⁴⁺. Strong fluorescence centered at 780 nm with a half-width ranging from 700 to 880 nm has been observed. The upper state fluorescence lifetime at room temperature was between 3.7 and 7.6 μs. The main broad-band absorption due to Ti³⁺ was centered at 510 nm. In addition, parasitic absorptions at 600 nm and 959 nm have been observed which may be associated with the formation of Ti²⁺.

An overview of FIBERTEK's program to explore new Nd-doped materials better suited for diode pumping than Nd:YAG is presented. The primary drawback of Nd:YAG, the most often employed diode-pumped solid state laser material, is a very narrow absorption band at the Al_1-xGa_xAs diode emission wavelength. Stringent (i.e. expensive) requirements on diode array manufacturing and temperature control standards are required for optimum diode pumped solid state lasing. The purpose of this investigation is to study materials with much broader absorption bands resonant with the 810 nm diode emission. A wide variety of crystals have been produced via laser heated pedestal growth during this effort which include melilites, garnets, and several rare earth silicates. These materials have been characterized using absorption and fluorescence spectroscopy, as well as with fluorescence lifetime measurements.

In this article we examine the structural characteristics of several of the most frequemtly mentioned nonlinear optical crystals which are transparent in both UV and visible ranges. We have proposed a double radical structure model which combines both inorganic and organic structures responsible for high nonlinear effect. The preliminary investigation on the nonlinear organometallic complex compounds have shown quite promising result. We will discuss in detail about the material characteristics and their potential as nonlinear optical crystals.

Very recent advances in the design, synthesis, and processing of nonlinear optical polymers for second-order and third-order devices are presented. Second-order polymers are now competitive with oxide crystals, and device performance has been demonstrated. Significant advances have been made in ladder polymers for third order, nonresonant devices as a result of fundamental theory and experiment. Transparent, optical quality films are reported with broadband femtosecond response and χ⁽³⁾ approaching 10^-7 esu. Resonant third-order polymers witn n₂ comparable to AlGaAs multi-quantum well structures (MQW) have been predicted by theory, synthesized, and thin film etalons constructed and tested.

Single crystals of the perovskite-related rare earth orthogallates have been grown and examined for laser hosts. LaGaO₃ in pure form generally contains color centers or other intrinsic defects which lead to a high absorption from the ultra-violet to about 600 nm. NdGaO₃ can be grown in large clear boules with proper care. Most orthogallates contain some twins which limit optical quality. Twins can be eliminated in growth by a judicious choice of mixed crystals involving La-Nd, La-Gd, or La-Y crystal compositions which have their lattice constants adjusted to an ideal cubic unit cell. We have grown crystals of one atomic percent Nd:LaGaO₃ for specific evaluation. The fluorescent lifetime was measured as 225 μs. We also report absorption spectra and fluorescent emission data for this crystal.

Diode pumped CW lasers operating between 2 and 3μm in the heavy rare earth activator ions are reviewed. In Ho,Tm:YAG we demonstrated high efficiency using Tm as the sensitizer ion which absorbed the pump radiation. This is followed by a cross relaxation process which allows nearly two excited Tm ions to be produced from one absorbed photon. There is rapid energy migration among the Tm ions followed by energy transfer to the Ho ion. The 2μm laser action is to a level 460 cm-1 above the ground state. In Tm,Ho:YLF we demonstrated CW cascade laser emission at 2.31 and 2.08μm. Above threshold for both transitions, two infrared photons are produced for each absorbed pump photon. The theoretical slope efficiency of this system is 72.3% for pumping at 0.791μm. In Er:YLF CW laser emission at 2.8μm with a 10% slope efficiency was demonstrated. The laser transition is normally self-terminating, but an upconversion process which depopulates the lower laser level and populates the upper laser level allows this transition to operate in a CW mode.

Potassium titanyl phosphate (KTP) has been shown to be a premier material for optical frequency second harmonic generation (SHG). Monovalent cations covering a large size range, e.g. Na⁺, Ag⁺, T1⁺, Rio⁺, and Cs⁺, have been used to partially or completely replace K⁺ in KTiOPO₄ or KTiOAsO₄, causing subtle structural variations which are frequently accompanied by marked changes in the nonlinear optical properties of these materials. The syntheses of (Na,K)TiOPO₄ and (NH4,K)TiOAsO₄, both by direct methods and by ion exchange with KTP and KTA are described here, and SHG intensities as a function of composition are presented. In the (Na,K)TiOPO₄ phase space, powder SHG intensity initially remains constant with sodium content, then drops by a factor of ten when the KTP isostructure NaTiOPO₄ is reached. In the (K,NH₄)TiOAs0₄ system, SHG intensity is reduced by a factor of 5 between 40 and 50 mole percent NH₄⁺. Single crystal X-ray data were collected on the compositions K_.5(NH4)_.5TiOAs0₄ (KNTA) and NaTiOPO₄ (NaTP). In KNTA, refinement reveals little change in the Ti coordination environment, however, unlike K.5(NH₄)_.5Ti0PO₄, no selective siting of the potassium and ammonium ions occurs. In NaTP, it is found that one of the Ti-O-Ti bond angles is reduced to 1300, and that one of the sodium atoms has the unusually low coordination number of four. KTixGa_1-x0xPO₄(F,OH)_1-x is isostructural with KTP, with ▵(Ga-0)trans bond length differences of 0.056 and 0.027Å. Preliminary studies of the compositional dependence of SHG in the system KTi_xGa_1-x0_xPO4(F,OH)_1-x show that SHG drops by 2 orders of magnitude between 50% and 60% Ga substitution.

Infrared optical systems, including ellipsometers and sensors, make frequent use of traditional waveplate technology. In some cases, the limited spectral bandwidth associated with conventional IR waveplates has limited the wavelength range over which these systems can perform. The mathematical procedures for designing broadband waveplates have long been available, but suitable materials for IR applications have been limited. Recent efforts in IR crystal growth have provided a wider range of birefringent materials which can be used for novel waveplate designs. This paper examines basic waveplate theory, including the relatively unknown effects of etaloning in the presence of finite surface reflectance, and proceeds to a discussion of both conventional and novel waveplate designs. These include tunable and achromatic waveplates covering all or part of the NIR-IR wavelength spectrum. Materials such as AgGaS₂, MgF₂, CdS and mixed CdS_xSe_1-x crystals will be discussed. Several designs specific to multiple wavelength lasers and frequency agile lasers will be covered.

The laser heated pedestal growth technique is presented as a fast and cost effective means of supplying small crystals for laser material evaluation. A simple method of preparing source rods to feed this crystal growth is described. High quality crystals of rare earth doped garnets are grown for laser induced fluorescence study. This results in a comprehensive study of energy kinetics in Tm:YAG and the determination of the microparameter CDA for cross relaxation of Tm³⁺.(³H₄).

The absorption, fluorescence decay and stimulated emission properties of several multiply doped laser crystals including (Er, Nd):YAG and (Cr, Er, Nd):GSGG are investigated. The presence of different laser ions in one crystal host results in strong ion-ion interactions and energy transfer among these ions. Consequently, both fluorescence decay and lasing characteristics of one kind of laser ions are strongly affected by the presence of the other. Simultaneous lasing of both Er⁺³ (2.94 μm) and Nd⁺³ (1.06 μm) ions has been obtained from the codoped laser crystals.

Laser action has been demonstrated in a rhodamine doped polyacrylamide sol-gel. Other organic and inorganic dopants have been successfully incorporated at room temperature. The polyacrylamide gel possesses good optical characteristics and is chemically unreactive with many organic and inorganic materials over a broad pH range. Laser characteristics, transmission spectra and material properties are discussed.

Polydiacetylenes, a class of organic polymers are important because of large nonresonant |χ⁽³⁾| I and sub-picosecond response times. The nonresonant domain of these materials covers the wavelength from ~0.8 μ to ~2.0 μ which is a desirable spectral window from the standpoint of telecommunication applications. The magnitude of χ⁽³⁾ in this wavelength range is ~10^-9 esu which is the largest nonresonant value known for any material. The absorption spectrum of these materials consists of a sharp excitonic peak at ~2 eV (α~106 cm^-1) followed by weak interband transitions at ~2.4 eV. The excitonic state has been shown to be a combination of Frenkel and charge-transfer states. Thus these excitons are unlike the excitons that are usually observed in inorganic semiconductors and seldom dissociate into free carriers (as evidenced by poor photoconductivity throughout the absorption range). Consequently the relaxation time of the excited state of these systems has been found to be as low as ~1.8 ps. Away from the resonance the response is instantaneous even at ~0.3 ps time scale. These are some of the unique characteristics that make polydiacetylenes attractive for many future applications in all-optical signal processing. In this paper we will discuss the crystal growth, our present understanding of the origin of nonlinearity and waveguiding aspects of these materials.

The effect of processing atmosphere and corona polarity has been considered when using a dc-corona to electric field pole polymer films for second harmonic generation (SHG). Positive and negative polarity coronas were used to pole doped polystyrene and poly(methyl methacrylate) films in air, nitrogen, nitrous oxide, and carbon dioxide environments. A possible correlation between the magnitude of the initial SHG intensity and the ionic mobility and the corona-onset voltage for a given atmosphere is discussed.

The performance of several Nd³⁺ doped laser materials were compared in a 1 watt diode-end-pumped configuration. The host materials included in this study were YVO₄, YAG, BEL, Cr:GSGG, and LNA. Measurements were limited to laser performance on the ⁴F_3/2-->⁴I_11/2 transition. The predictive quality of traditional modelling was examined by generating detailed theoretical performance curves for all the materials. The best theoretical fit to the data was obtained by using cross section values deter-mined by the Birnbaum laser method and quantum efficiencies (QE) greater than 94% for all the materials except BEL and Cr:GSGG. The low QE for Cr:GSGG can be traced to excited state absorption into the Cr bands. bands. Uncertainty in the fit relates to the tradeoff between QE and loss. A performance comparison between coated and uncoated YAG suggests that surface losses contribute more than expected. The data and the model indicate that all materials studied here (except for Cr:GSGG) should display approximately a 55% or greater slope efficiency (optical-to-optical) above 1 watt of pump power if fabricated with a monolithic high reflector and antireflective coating at the appropriate faces. Based on projections from the model and data from the literature LNA has the potential to be a high efficiency and energy storage laser. Utilizing a new 2 watt 20 stripe diode laser array, 1.22 watts at an optical efficiency of 61% was obtained at 1064 nm in the TEM₀₀ from a Nd:YVO₄ crystal.