Gregory A. Vawter

Giant microwave photoresistance of two-dimensional electron gas

We measure microwave frequency (4–40 GHz) photoresistance at low magnetic field B, in high mobility two-dimensional electron gas samples, excited by signals applied to a transmission line fabricated on the sample surface. Oscillatory photoresistance vs B is observed. For excitation at the cyclotron resonance frequency, we find a giant relative photoresistance ΔR/R of up to 250%. The photoresistance is apparently proportional to the square root of applied power, and disappears as the temperature is increased.

Fabrication techniques for low loss silicon nitride waveguides

Optical waveguide propagation loss due to sidewall roughness, material impurity and inhomogeneity has been the focus of many studies in fabricating planar lightwave circuits (PLCs) In this work, experiments were carried out to identify the best fabrication process for reducing propagation loss in single mode waveguides comprised of silicon nitride core and silicon dioxide cladding material. Sidewall roughness measurements were taken during the fabrication of waveguide devices for various processing conditions. Several fabrication techniques were explored to reduce the sidewall roughness and absorption in the waveguides. Improvements in waveguide quality were established by direct measurement of waveguide propagation loss. The lowest linear waveguide loss measured in these buried channel waveguides was 0.1 dB/cm at a wavelength of 1550 nm. This low propagation loss along with the large refractive index contrast between silicon nitride and silicon dioxide enables high density integration of photonic devices and small PLCs for a variety of applications in photonic sensing and communications.

High-Q microring resonator for biochemical sensors

The large refractive index difference between silicon nitride and silicon dioxide allows silicon nitride/dioxide waveguides to have a small mode size and low radiation bending loss. Low radiation bending loss enables high quality (Q) factor microring resonators. In this paper, we will present a record high quality factor microring resonator using silicon nitride and silicon dioxide on a silicon wafer. The microring resonator was fabricated using a deep UV photolithography and etching process. The microring resonator was critically coupled to a straight waveguide. An intrinsic quality factor of 240,000 has been measured. We will also present our result of using on-chip high-Q microring resonators for liquid phase chemical sensing application.

Monolithic optoelectronic switch based on the integration of a GaAs/AlGaAs heterojunction bipolar transistor and a GaAs vertical-cavity surface-emitting laser

We describe the design and the first experimental demonstration of a monolithic integrated optoelectronic switch combining a GaAs/AlGaAs heterojunction bipolar transistor (HBT) with a vertical-cavity surface-emitting laser (VCSEL). The switch has high current gain (500-700), low power dissipation (27 and 55 mW DC at optical output levels of 0.4 and 1.2 mW, respectively), and a high optical-to-electrical conversion efficiency (150 W/A) under DC bias conditions, thus providing a high-performance electrical-to-optical interface for high-speed optical interconnections.<<ETX>>

Reconfigurable binary optical routing switches with fan-out based on the integration of GaAs/AlGaAs surface-emitting lasers and heterojunction phototransistors

The design, fabrication, and experimental demonstration of dynamically reconfigurable binary optical switches based on the integration of GaAs/AlGaAs vertical-cavity surface-emitting lasers and heterojunction phototransistors are reported. These new monolithic optical switches can perform spatial routing and optical amplification functions on input optical data with a fan-out of two, and can be programmed using simple voltage control. The 1/spl times/2 and 2/spl times/2 devices provide the basis for a high performance, two-dimensional optical switching fabric with electrical interfaces for optical switching and interconnection networks.<<ETX>>

Light coupling mechanism of quantum grid infrared photodetectors

Rigorous electromagnetic modeling based on a modal solution of the pertinent boundary-value problem was performed to study the light coupling mechanism in quantum grid infrared photodetectors with lamellar patterns. Our theory shows that vertical field component and absorption quantum efficiency η can be strongly enhanced by judiciously adjusting the width w of individual grid lines. This enhancement is further increased if the top of the grid lines is covered by metal, which behave as a collection of dipole scatterers. We have experimentally verified the dipole scattering characteristics with different w, and found that the variation of η agrees very well with the theory. We also found that, as expected, the conductivity of the metal strips affects η significantly due to internal dissipation.

High magnetic-field microwave conductivity of two-dimensional electrons in an array of antidots

We find the high magnetic field (B) microwave conductivity [Re(σ x x )] of a two-dimensional electron system containing an antidot array increases strongly with frequency (f) in the regime with Landau filling 1/3<ν<2/3. Atmicrowave f, Re(σ x x ) vs B exhibits a broad peak centered around ν = ½. On the peak, Re(σ x x ) at 10 GHz can be five times its dc-limit value. Unlike the dc-limit conductivity, the microwave conductivity is extremely temperature (T) sensitive, increasing as T decreases down to 100 mK (the lowest T measured) and disappearing above 0.5 K. The enhanced microwave conductivity is apparently due to antidot edge excitations of fractional quantum Hall effect states, and may be associated with the chiral Luttinger liquids.

Optimization of Blazed Quantum Grid Infrared Photodetectors

In a quantum grid infrared photodetector (QGIP), the active multiple quantum well material is patterned into a grid structure. The purposes of the grid are on the one hand to create additional lateral electron confinement and on the other to convert part of the incident light into parallel propagation. With these two unique functions, a QGIP allows intersubband transition to occur in all directions. In this work, we focused on improving the effectiveness of a QGIP in redirecting the propagation of light using a blazed structure. The optimization of the grid parameters in terms of the blaze angle and the periodicity was performed by numerical simulation using the modal transmission-line theory and verified by experiment. With a blazed structure, the sensitivity of a QGIP can be improved by a factor of 1.8 compared with a regular QGIP with rectangular profiles.

Fabrication and measurement of wideband achromatic waveplates for the mid-infrared region using subwavelength features

Subwavelength diffractive features etched into a substrate lead to form birefringence that can produce polarization sensitive elements such as wave plates. Using etched features allows for the development of pixelated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Form birefringence exhibits dispersion that can be advantageous to the design of wave plates with an achromatic response. Taking advantage of this dispersion, diffractive wave plates with good achromatic characteristics can be designed for the 2- to 5-µm spectral region. Previous work in this area has produced good results over a subset of this wavelength band, but designing for this extended band is particularly challenging. The fabrication processes for the subwavelength features will be discussed and fabricated devices with a measured average phase retardation of 80.6 deg and rms variation of 9.41 deg will be presented.

The RF performance of long junction active devices using TlCaBaCuO step edge structures

The authors report the fabrication of long, step-edge junctions using TlCaBaCuO with associated control lines that demonstrate strong modulation of critical current, usable current gains, large power gains, and fairly large bandwidths. The devices are operational at up to 95 K and RF measurements have been made at 77 K at frequencies up to a probe-limited 10 GHz. These four terminal devices have output impedances of 2-20 Omega , small signal current gains greater than 2.5, available power gains of over 10 dB from 0.5 to 10 GHz, and minimum noise figures of less than 1 dB.<<ETX>>