Uma B. Ramabadran

Electro‐optic, piezoelectric, and dielectric properties of zinc tris thiourea sulfate

Recently, it has been demonstrated that semiorganic materials have potential for very efficient nonlinear optical devices. Among many recently reported materials is zinc tris thiourea sulfate. We report measurements of the electro‐optic, piezoelectric, and dielectric constants of this new material.

Infrared corrected Sellmeier coefficients for potassium titanyl arsenate

KTA is a crystallographic isomorph of KTP that shows promise for nonlinear frequency generation throughout the 1–5-μm region. We present updated Sellmeier coefficients for KTA based on measurements of the refractive index in the 0.4–3.6-μm region. Predictions of phase-matching behavior by means of these coefficients agree well with experimental data.

Rubidium titanyl arsenate difference-frequency generation and validation of new Sellmeier coefficients

Rubidium titanyl arsenate (RTA), a crystallographic isomorph of potassium titanyl phosphate, shows promise for nonlinear-frequency generation throughout the 1–5-µm spectral region. Difference-frequency generation in an RTA crystal produced tunable output in the 3.2–4.2-µm wavelength range. A single 1.064-µm Nd:YAG laser pumped both a LiNbO3 optical parametric oscillator used to generate a tunable signal beam and the RTA crystal used for difference-frequency generation. Conversion efficiencies were limited to 4% primarily by the large beam divergence of the signal beam. Phase-matching measurements were in excellent agreement with new IR-corrected RTA Sellmeier equations based on refractive-index measurements.

Optical properties of deuterated zinc tris thiourea sulphate

We report measurements of the optical properties of deuterated zinc tris thiourea sulfate (d‐ZTS). The material is being studied for its potential value in optical frequency conversion and electro‐optic applications. The electro‐optic and piezoelectric measurements along with those for the dielectric constant indicate that the figure of merit for electro‐optic modulation is comparable to that of KDP. However, careful refractive index measurements along with an analysis of the nonlinear coefficient tensor show that d‐ZTS will not noncritically phase match for second‐harmonic generation despite the promise shown by its undeuterated analog. In addition, the acceptance angle even for critical phase matching is very small.

OPTICAL AND STRUCTURAL CHARACTERIZATION OF TRIALLYL THIOUREA CADMIUM CHLORIDE (ATCC)

We report measurements of the structural and optical properties of triallyl thiourea cadmium chloride (ATCC). X-ray diffraction determined the point group to be 3 and the space group to be R3. ATCC is one of a series of semiorganic crystals that demonstrated good optical frequency conversion. We have investigated this material for its potential value in electro-optic and piezoelectric applications.

Chemical-Mechanical Polishing and Rapid Thermal Annealing of SiC: Raman Spectroscopy and ESCA (XPS) Studies

The effects of Chem-Mechanical Polishing (CMP) and Rapid Thermal Annealing (RTA) on n-type 4H:SiC samples doped with nitrogen were investigated using Raman scattering and X-ray Phtoelectron Spectroscopy (XPS a.k.a. ESCA) measurements. A comparison of the Raman spectra from Mechanically Polished (MP) SiC annealed at 600°C and 800°C displays a frequency shift in the coupled plasmon LO-phonon mode. Since the coupled mode frequency is a direct measure of the free carrier concentration, this observation may suggest the removal of polishing induced carrier traps with increasing annealing temperature. The CMP samples did not show this frequency shift, thereby indicating that such polishing traps were not created in that process. The Si-peak observed in the XPS spectra of the unannealed CMP sample indicates primarily a Si-C bonding, while that for the MP sample is more complex, indicating other bonds beside Si-C. Drastic changes in O, C, Si surface content were observed for annealing between 1000°C and 1100°C. The peaks in the XPS spectra associated with the chemical environment for C, O, and N are complex and may be explained as silicon oxycarbide type structures near the surface or possibly around the interface of the SiC substrate with a thin surface oxide layer.

Investigation of Second Harmonic Heneration in Glutamic Acid-Metal Complexes

To design new second order nonlinear crystals, we have haracterized a series of dipeptide complexes and copper glutamate. We tested 16 materials using powder second harmonic generation. The best of these materials was copper glutamate. Results of initial nonlinear optical characterization of the copper glutamate powder determined by the Kurtz powder test are presented.

Nonlinear optical crystal development at the USAF Wright Laboratory

Wright laboratory materials directorate has been the primary DoD organization developing nonlinear optical (NLO) materials. This effort has resulted in numerous successes, especially in the development of new NLO crystals for use in laser sources. These crystals significantly enhance laser performance by making possible wavelength shifting and tunability and higher power. Technical progress has been made in developing several crystals which together cover the spectral range from middle ultraviolet to the far infrared. One crystal in particular, zinc germanium phosphide, has already demonstrated average power of 3.5 watts tunable over much of the mid-infrared atmospheric window. Significant improvements have also been made with extending the transparency range and laser-damage resistance of the KTP family of compounds.