William W. Ho

Tungsten bronze Sr1−xBaxNb2O6: a case history of versatility

Abstract We review the chronological development of ferroelectric tungsten bronze Sr1-x Ba x Nb2O6 (SBN) single crystals in terms of crystal growth and applications ranging from pyroelectric to surface acoustic wave (SAW) to nonlinear optics, with specific emphasis on the compositions SBN:60, SBN:75 and SBN:50. In addition to a number of bulk crystal applications, these crystals are also now being used as substrates for the growth of various lattice-matched thin films, including ferroelectric and superconducting oxide materials.

An intercomparison of measurement techniques for the determination of the dielectric properties of solids at near millimetre wavelengths

The results of a measurement intercomparison aimed at assessing the systematic errors in near-millimetre-wavelength dielectric measurements on reasonably transparent solid specimens are presented and discussed. Various monochromatic, broad band, guided wave and free space measurement methods were intercompared in the frequency region from 30 to 900 GHz using round-robin measurements of the refractive index and absorption coefficient of seven specimens. Typically, systematic errors of up to 1% were found in the refractive index results, while very much larger systematic errors were found in the absorption results. >

Nonlinear microwave susceptibility measurement of an artificial Kerr medium

A suspension of short graphite fibers was shown to provide an effective medium for nonlinear microwave interactions. Using a 18‐GHz pump beam with up to 20 W continuous power, changes in the 94‐GHz refractive index were measured by interferometry. The induced birefringence and anisotropic absorption indicate an alignment of the fibers with the electric field of the pump beam. The resulting nonlinear refractive index coefficient of n2=2.6×10−4 cm2/W is of sufficient magnitude to demonstrate microwave bistability.

Low and high frequency dielectric properties of ferroelectric tungsten bronze Sr2KNb5O15 crystals

Single crystals of Sr2KNb5O15 (SKN), approximately 5-8 mm in diameter and 10-20 mm long, have successfully been grown by the Czochralski technique. The crystal has a ferroelectric tetragonal tungsten bronze structure with a Curie temperature of about 150°C. The dielectric and electromechanical coupling constants for this crystal have been established: the polar axis dielectric constant, K33, exceeds 20,000 at the Curie temperature while the coupling constants are K33 = 0.52 and K31 = 0.19, respectively. High frequency dielectric properties of SKN samples were also determined between 90-100 GHz. These measurements indicate that the bulk of the polar axis permittivity has relaxed below 90 GHz, implying that the low frequency dielectric response is not soft-mode controlled.

Microwave dielectric properties of Sr.61Ba.3Nb2O6 single crystals at 35 and 58 GHz

Abstract Microwave measurements on Sr.61Ba.39Nb2O6 single crystal samles show a large nonlinear susceptibility at 58 GHz and a large refractive index (∼ 18 to 20 along the polar axis) at 35 GHz. For propagation along the direction of an applied electric field in a sample cut at 45° to the polar axis, the change in index with applied field is found to be at least 1.2 × 10−6 m/V, corresponding to a half-wave voltage of 2000 V.

Dielectric properties of ferroelectric tungsten bronze Ba2-xSrxK1-yNayNb5O15 crystals at RF and millimeter wave frequencies

Abstract Permittivity and dielectric loss have been measured for a number of single crystal samples of barium strontium potassium sodium niobate (BSKNN) in three frequency ranges: 0–1 MHz, 30–40 GHz, and 90–100 GHz. This work is part of an ongoing study of millimeter wave properties of ferroelectrics with potential applications in active components such as phase shifters, modulators, and switches. BSKNN has been selected as typifying one extreme in the tungsten bronze family, where all 15- and 12-fold coordinated cation sites are occupied. Measurements on these crystals show significantly lower millimeter wave absorption than is found in strontium barium niobate (SBN) crystals grown by the same techniques. The permittivities along the principal crystal axes showed a substantial decrease with increasing frequencies over the range of observations, which may be indicative of a relaxation mechanism occurring in the GHz frequency region.

Heterogeneous ozone decomposition on sulfuric acid surfaces at stratospheric temperatures

Abstract In this investigation, methods of forming sulfuric acid aerosols at stratospheric temperatures were studied, and the rate constant for the heterogeneous decomposition of ozone on sulfuric acid surfaces was determined over the temperature range 217–263 K. The ozone decomposition rate was observed to be first order in ozone concentration and frozen sulfuric acid surface area, and was found to be decreased by the presence of H 2 O. The measured ozone first order decay efficiencies were found to be on the order of 10 −8 and 10 −9 reactions per collision for pure frozen H 2 SO 4 and 0.045 mole fraction H 2 SO 4 in H 2 O respectively. The addition of A1 2 O 3 and Fe 2 O 3 to the H 2 SO 4 did not substantially increase the O 3 decomposition rate. Based on these results it is concluded that the heterogeneous decomposition of ozone on frozen stratospheric sulfuric acid aerosol particles is of little significance in establishing stratospheric ozone concentrations.

Microwave phase conjugation via degenerate four-wave mixing in an artificial Kerr medium

Collinear microwave phase conjugation was observed in an artificial Kerr medium consisting of short graphite fibers suspended in a binary liquid mixture. Using an 18 GHz pump beam with up to 20 W continuous power, characterization of the changes in the 94 GHz refractive index were made by interferometry. A nonperturbative method for describing the response of the medium was used to analyze the phase-shift measurements for the static berefringence and the time response as functions of microwave intensity.

Microwave Phase Conjugation In Artificial Kerr Media

One dimensional phase conjugation of microwaves via degenerate four-wave mixing in an artificial Kerr medium has been demonstrated. The medium consisting of a liquid suspension of elongated graphite particles is confined within a single mode waveguide. The decay of the nonlinear refractive index grating is shown to agree with previous relaxation time measurements.

Microwave power and efficiency performance of AlGaAs/GaAs self-aligned HBTs

The microwave power performance of AlGaAs/GaAs self-aligned HBTs from 10 to 35 GHz is described. A record value of 68% power added efficiency was obtained at 10 GHz. At 18 GHz, 16.3 dB associated gain was achieved with 1.83 W/mm power density and 40% efficiency. At 35 GHz, a 15 dB small signal gain was observed. The tested HBTs have 2 micron feature size. Further improvement is expected with optimization of the HBT structure.