The optimization of nonlinear optical devices such as frequency doublers is discussed. A nonlinear material will convert efficiently if the laser brightness (P/Q2, where P is the peak power, and Q is the number of times diffraction-limited the pulse is) exceeds a material parameter called the threshold power. The conversion efficiency is shown to be independent of the beam aperture. Thus surmounting optical damage requires only that the beam aperture be large enough. Data on the threshold powers of common nonlinear materials is presented.
An investigation is reported of the dependence of laser-damage site density on fluence and on various parameters of crystal growth and post-growth treatment. Damage site density (number of defects per unit volume) was found to increase monotonically with fluence (for single 1-ns pulses of 1.06 μm laser light). Site density (at a fluence of 10 J/cm2) was influenced by solution flow rate, seed defect removal prior to growth, quaternary ammonium cations, and thermal cycling following growth. No dependence of damage susceptibility on growth rate was found over the range 3-30 mm/d. A model is presented which postulates the electrostatic adsorption cf organic material onto the growing crystal surface and subsequent inclusion into the crystal. According to the model, damage occurs by absorption of light by inclusions of solution-wetted organic debris, resulting in pressure increase and hydraulic fracture of the surrounding crystal.
Recent developments relating to the application of the urea crystal to non-linear optics reviewed. The urea crystal has been shown to be a useful material for non-linear optics applications. Urea has been studied within the context of both frequency upconversion1 and, more recently, optical parametric oscillation (0P0).2-4 It is particularly the latter application which will be discussed.
Urea is an organic crystal within the 42m space group class, the same as the ADP ismorphs. It is optically clear from 200 nm to 1.4 μm, which is consistent with parametric oscillation in the visible and near infrared. Its birefringence is approximately twice that of ADP, which leads to an OPO producing light at shorter wavelengths than for most other non-linear crystals. The non-linear coefficient of urea is approximately 2.5 times that of ADP. While a relatively soft crystal, urea can be optically polished to a flatness of less than an optical wavelength using methods similar to those of ADP. The thermal behavior of urea is excellent; the temperature-dependence of the phase-matching angle is much smaller then ADP. Urea is a hygroscopic crystal, a fact which complicates its practical use. Typically, this problem is overcome by immersing the crystal in an index matching liquid such as hexane. The most difficult problem with regards to the use of urea has been and continues to crystal growth. However, high quality urea crystals of length greater than 20 mm in the (110) direction have been grown from solution in the laboratory. Solution growth requires precise temperature control over very long growth times (on the order of one year). Recently, crystal sizes on the order of 1 cm3 have become commercially available. The urea crystal is positive uniaxial, a characteristic which is advantageous for OPO ications. By utilizing type II (o -> o + e) phase-matching and resonating the ordinary wave, the degree of Poynting vector walk-off of the signal from the pump due to double action is significantly reduced. Furthermore, noncritical phase matching is allowed use the effective d coefficient is a maximum at 90°. The urea optical parametric oscillator has generally consisted of a very simple design. Feedback is accomplished by the use of flat dichroic mirrors, which allow for only one of the generated waves to be resonated. The pump wave is collinear with these generated waves. The pump frequency has usually been 355 nm. Frequency tuning is accomplished through the angular rotation of the crystal. The frequency range covered extends from 498 to 1.23 μm, which is obtained with a single crystal and a single set of dichroic mirrors. Never, a small gap between 640 and 790 nm exists in this range; this is due to metrical obstructions of the particular crystals used and is not thought to be intrinsic.) The mirror reflectivity at the resonated wavelength need not be high, as the total number of round trips allowed during the 7 ns pump pulse is small. The linewidth of the oscillator output was measured to be about 1.2 A near 90° phase-matching.
The nonlinear optical properties of β-BaB204 (beta barium borate) are demonstrated in the generation of second through fifth harmonics of 1.06 μm neodymium laser radiation and in optical parametric oscillation pumped by 532-nm radiation. βBaB204 is particularly useful for high average power applications and nonlinear frequency generation in the ultraviolet to wavelengths as short as 200 nm. An internal energy conversion efficiency of 84% for 1064-nm to 532-nm second harmonic generation, and cascaded harmonic conversion to the 213-nm fifth harmonic with 11% overall conversion were obtained. The observed performance agrees well with that predicted by modeling.
The crystal growth and optical quality of magnesium oxide doped lithium niobate are characterized. Measured physical, optical, and nonlinear optical properties are discussed and compared with undoped lithium niobate. Frequency doubling results of 1.06 gm radiation are presented, where 53 percent conversion efficiency is obtained in lithium niobate doped with 5 mole percent magnesium oxide.
Direct measurements of the phase matching properties of small (submillimeter) crystals of some new second and third harmonic generators is discussed. Such measurements represent an intermediate level of characterization between powder tests and detailed dispersion and d coefficient determinations. Since crystals as small as 300 microns may be studied, a decision to pursue a material can be made at an early stage of crystal growth. Representative data on some new fluoride harmonic generators is presented.
Recent work on the single crystal growth of laser host materials is described. The garnets Nd:YAG and Nd,Cr:GSGG remain as the most useful for 1.06pm operation. For tunable systems based on Ti3+, we have grown several garnets, YA1O3, and MgAl204 spinel. Potentially tunable systems which utilize Ceas the activator have been attempted in many perovskites and orthosilicates. The growth of a new class of laser materials based on "-alumina is also being investigated by unique methods of preparation. The principle applications of these oxide materials appears to be in diode pumped miniature lasers, solid state tunable systems, and special medical systems. Host crystals of various fluorides are described also and are useful for laser fusion oscillators or high power Nd3+ lasers.
Large scale solid state lasers will require single crystal gain elements which, by virtue of requisite size and optical quality, represent a significant challenge to current crystal growth technology. This paper examines these requirements and relates them to crystal growth options. Crystal growth scaling considerations are reviewed and a host material figure of merit for scaling is derived.
Crystal growth of titanium doped sapphire crystals is described. Effects of growth atmosphere, titanium concentration, dopant oxidation state are described. Growth of garnet hosts gadolinium scandium gallium garnet and gadolinium scandium aluminum garnet doped with chromium is described. Effects of impurities on optical characteristics of chromium doped garnets are described, as well as approach leading to improvement of optical quality. Growth of magnesium and yttrium aluminates and their doping with titanium is described.
The adaptation of the Heat Exchanger Method for the growth of high quality laser crystals, viz., Co:MgF2 and Ti:Al203, is discussed. In case of Co:MgF2 problems associated with dopant polarization and reaction with the crucible have been solved to produce 1 wt.% CoF2 crystals. For Ti:Al201 crystals 10 cm diameter boules have been grown. This material does not exhibit the 800 nm absorption and laser rods with uniform Ti concentration can be fabricated from these boules.
A seeded vertical gradient-freeze technique has been used to grow Ti:Al203 single crystals with low residual infrared absorption from charges containing between 0.15 and 1.0 wt.% Ti203 in tungsten crucibles. In laser experiments on samples cut from these crystals, room-temperature cw operation has been achieved for rods containing between 0.024 and 0.099 wt.% Ti203.
Experimental work is described in this paper demonstrating the feasibility of growing crystals of an incongruently melting compound from its melt, provided that its composition is not far removed from a congruently melting composition. Approximately 60 crystal-growth runs were made from the melt of ammonium dihydrogen orthophosphate (ADP). Crystals of acceptable quality were grown in 10-mm and 25-mm diameters, with pulling rates of 2 mm/h or less and 1 mm/h or less, respectively. Although still rather slow, these growth rates are at least one order of magnitude faster than those obtained in industrial crystal growth from solutions of the same material.
Materials exhibiting strong Faraday rotation are of interest for numerous laser and other applications. The characteristics of diamagnetic, paramagnetic, and ferromagnetic Faraday rotator materials for use in different spectral regions are described; both crystals and glasses are included. Properties of selected materials and relative figures of merit for various applications are also presented.
The development of practical devices utilizing the photorefractive effect in BaTiO3 depends upon the availability of single crystals with improved properties. In order to achieve this goal, we are pursuing a program involving crystal growth, characterization, and modeling. High quality, nominally undoped BaTiO3 crystals have been prepared by the top-seeded solution growth method and have photorefractive properties similar to those of commercial crystals. Models of the photorefractive effect suggest that improvements in the response time may be achieved by heating the crystal in a reducing atmosphere to convert to electron photoconductivity. Experiments with a commercial crystal whose major transition metal impurity was Fe resulted in a factor of 10 improvement in its response time, but at the expense of increased dark conductivity. Other approaches to reduce the response time are discussed.
The development of synchrotron radiation x-ray sources has provided the means to greatly extend the capabilities of x-ray fluorescence analysis for determinations of trace element concentrations. A brief description of synchrotron radiation properties provides a background for a discussion of the improved detection limits compared to existing x-ray fluorescence techniques. Calculated detection limits for x-ray microprobes with micrometer spatial resolutions are described and compared with experimental results beginning to appear from a number of laboratories. The current activities and future plans for a dedicated x-ray microprobe beam line at the National Synchrotron Light Source (NSLS) of Brookhaven National Laboratory are presented.
The electron paramagnetic resonance (EPR) technique has been used to monitor a variety of titanium-associated centers in doped aluminum oxide crystals. Because of rapid spin-lattice-relaxation times, these EPR spectra could only be observed in the 5 to 25 K temperature range. Among the distinguishable defects are isolated Ti3+ ions, several S=1/2 centers involving Ti3+ ions with adjacent imperfections, and an S=1 center which is either a pair of Ti3+ ions or is a Ti2+ ion. Once specific defect models have been assigned to each of the spectra, the EPR method promises to be a sensitive indicator of Ti-sapphire crystal quality and may be used to help select material appropriate for laser use.
Investigations on the electrical transport properties of dielectric garnets used for laser and optical device applications are reviewed. Electrical conductivity studies of YAG (Y3A15012), GSGG (Gd3Sc2Ga3012), and CALGAR (Ca3Al2Ge3012) are interpreted in terms of possible point defecE models and correlated, where possible, with spectroscopic investigations of optically deleterious point defects. The defect chemistry and electrical properties of insulating garnets are compared with results of previous studies on highly conducting magnetic garnets. The ability of electrical conductivity experiments to assist in the understanding of the defect structure of optical and laser garnets is demonstrated, with particular focus on the behavior of color centers and anomalous absorption in garnets at high temperature and in controlled atmospheres.
The tensorial nature of the photoelastic effect in optical materials is discussed. The commonly used photoelastic constants, the piezo-optic and the elasto-optic constants, are defined and the general form of these tensors is presented. The most commonly used methods for measuring photoelastic constants, interferometry, polarimetry, the acousto-optic effect and Brillouin scattering are discussed. In addition, the effect of wavelength dispersion is examined.
Methods of measuring the thermal variation of the refractive index, an = (1/n)(dn/dT) are reviewed. A comparison is made between various prism and interferometric methods. A double interferometric method utilizing simultaneous Fizeau and Twyman-Green interferometers is discussed in detail. Data on KDP isomorphs are presented for comparison.
Approaches to characterizing the mechanical behavior of single crystal ceramics are reviewed. Consideration is given to techniques applicable to large crystals and to indentation techniques that can be used on crystals of 1 mm or less. The importance of flaws in controlling the mechanical behavior of brittle ceramics is discussed, leading to an emphasis on fracture mechanics methods. These techniques are applicable to the determination of fracture toughness and to the measurement of slow crack growth in aggresive environments. Indentation processes have been analyzed extensively and the good understanding of stress fields and micro-mechanics of indentation has led to techniques to measure hardness, toughness and elastic modulus. Measurements of hardness anistropy can be used to determine slip planes and also provide considerable information on local plastic flow in brittle crystals.