Jeffrey A. Bean

Performance Optimization of Antenna-Coupled

Signal-to-noise ratio (SNR) is a valuable figure of merit in determining the operating scope of infrared detectors. Antenna-couple metal-oxide-metal diodes have been shown to detect infrared radiation without cooling or applied bias, but so far have been hampered by their SNR. This paper details a comprehensive study of the fabrication parameters that control the formation of the tunneling oxide barrier to optimize the performance of these detectors. Since the tunneling barrier affects both current-voltage and infrared detection characteristics, fabrication parameters can be optimized to improve device performance. The current-voltage characteristics of the devices are detailed in this paper; resistance, nonlinearity, and curvature coefficient are parameterized on fabrication procedures. Infrared detection characteristics are detailed and SNR is studied as a function of device nonlinearity and biasing conditions.

{\rm Al}/{\rm AlO}_{x}/{\rm Pt}

Directional control of received infrared radiation is demonstrated with a phased-array antenna connected by a coplanar strip transmission line to a metal-oxide-metal (MOM) tunnel diode. We implement a MOM diode to ensure that the measured response originates from the interference of infrared antenna currents at specific locations in the array. The reception angle of the antenna is altered by shifting the diode position along the transmission line connecting the antenna elements. By fabricating the devices on a quarter wave dielectric layer above a ground plane, narrow beam widths of 35° FWHM in power and reception angles of ± 50° are achieved with minimal side lobe contributions. Measured radiation patterns at 10.6 μm are substantiated by electromagnetic simulations as well as an analytic interference model.

Tunnel Diode Infrared Detectors

Measured and simulated angular response patterns at 10.6 μm demonstrate considerable improvement in angular resolution with a four-element phased-array antenna versus that of a two-element array. Due to propagation loss in the transmission line that connects the antenna elements, further resolution improvement is minimal with a six-element phased array. Additional measurements of a two-element array with increased metal thickness indicate that further improvement in angular resolution is possible by reducing propagation loss in the transmission line. With the combination of additional antenna elements and reduced propagation loss, substantial improvement in the angular resolution of off-broadside performance is also observed. All devices use a metal-oxide-metal tunnel diode as the detector element.

Directional control of infrared antenna-coupled tunnel diodes.

The far-field angular response pattern for dipole antenna-coupled infrared detectors is investigated. These devices utilize an asymmetric metal-oxide-metal diode that is capable of rectifying infrared-frequency antenna currents without applied bias. Devices are fabricated on both planar and hemispherical lens substrates. Measurements indicate that the angular response can be tailored by the thickness of the electrical isolation standoff layer on which the detector is fabricated and/or the inclusion of a ground plane. Electromagnetic simulations and analytical expressions show excellent agreement with the measured results.

Angular Resolution Improvement of Infrared Phased-Array Antennas

An antenna-coupled detectors directional properties can be verified by measuring its angular radiation pattern. At infrared frequen- cies, this pattern can be measured by rotating the device while illuminat- ing it with a laser beam. An accurate radiation pattern can be measured only if the device is coaligned with the axis of rotation and the focus of the laser beam. In the alignment procedure presented, the device is rotated to various angles and the distance along the orthogonal axis from the current device position to the laser beam is measured by maximizing its response. Calculations based on these distances provide the new location of the device, which will coalign it with the axis of rotation and the focus of the laser beam. The successful alignment enables accurate radiation pattern measurements.

Influence of substrate configuration on the angular response pattern of infrared antennas.

A method for determining the complex anisotropic permittivity for electrically small material specimens of complex shape with biaxial dielectric anisotropy is described and representative measured results are presented. The method extracts the anisotropic tensor elements from specimen reflection measurements made with a shorted rectangular waveguide. A number of independent reflection measurements, using different specimen orientations in the waveguide equal to the number of unknown permittivity terms, are required. The specimens need not fill either dimension of the waveguide cross section and are permitted to be electrically short in the propagation direction. Measurements using WR1500 and WR1150 waveguide were made for a known isotropic low-loss dielectric specimen of complex shape. Additional measurements in WR1500 were made on two engineered anisotropic artificial dielectric specimens. Tensor permittivity elements were extracted from the measurements and were used to validate and demonstrate the accuracy and capability of the method by comparison with known values for the dielectric specimen or with explicit inclusion-binder simulation results for the engineered specimens.

Alignment procedure for radiation pattern measurements of antenna-coupled infrared detectors

For the first time, a tapered slot antenna coupled to a metal-oxide-metal (MOM) diode is designed, fabricated, and characterized at an infrared wavelength of 10.6 μm. Polarization ratio was measured to be approximately 6.7:1. The antennas radiation pattern shows beamwidth symmetry between the E-plane and the H-plane data, having full width at half-maximum beamwidths of 45 ° and 30 °, respectively.

Biaxial Permittivity Determination for Electrically Small Material Specimens of Complex Shape Using Shorted Rectangular Waveguide Measurements

In this work, we present a new X-band waveguide (WR90) measurement method that permits the broadband characterization of the complex permittivity for low dielectric loss tangent material specimens with improved accuracy. An electrically long polypropylene specimen that partially fills the cross-section is inserted into the waveguide and the transmitted scattering parameter (S21) is measured. The extraction method relies on computational electromagnetic simulations, coupled with a genetic algorithm, to match the experimental S21 measurement. The sensitivity of the technique to sample length was explored by simulating specimen lengths from 2.54 to 15.24 cm, in 2.54 cm increments. Analysis of our simulated data predicts the technique will have the sensitivity to measure loss tangent values on the order of 10(-3) for materials such as polymers with relatively low real permittivity values. The ability to accurately characterize low-loss dielectric material specimens of polypropylene is demonstrated experimentally. The method was validated by excellent agreement with a free-space focused-beam system measurement of a polypropylene sheet. This technique provides the material measurement community with the ability to accurately extract material properties of low-loss material specimen over the entire X-band range. This technique could easily be extended to other frequency bands.

Infrared Linear Tapered Slot Antenna

Compact-range measurement facilities have been used successfully for many years to characterize antenna performance as well as radar signatures. This paper investigates strategies for improving compact-range-measurement accuracy by mitigating errors associated with ground reflections inherent in most range designs. A methodology is developed for strategically modifying, or patterning, the surface between the ranges source antenna and the reflector to reduce error terms, thereby increasing measurement accuracy. Candidate patterns were evaluated using a full-wave computational Finite-Difference Time-Domain (FDTD) model at VHF/UHF frequencies to determine baseline performance, and to develop trade rules for more advanced designs. Physical Optics (PO) models were used to analyze the final design at the frequencies of interest.

An X-band waveguide measurement technique for the accurate characterization of materials with low dielectric loss permittivity

Metal-oxide-metal (MOM) tunnel diode detectors when integrated with phased-array antennas provide determination of the angle of arrival and degree of coherence of received infrared radiation. Angle-of-arrival measurements are made with a pair of dipole antennas coupled to a MOM diode through a coplanar strip transmission line. The direction of maximum angular response is altered by varying the position of the MOM diode along the transmission line connecting the antenna elements. Phased-array antennas can also be used to measure the degree of coherence of a partially coherent infrared field. With a two-element array, the degree of coherence is a measure of the correlation of electric fields received by the antennas as a function of the element separation. Antenna-coupled MOM diode devices are fabricated using electron beam lithography and thin-film deposition through a resist shadow mask. Measurements at 10.6 μm are substantiated by electromagnetic simulations and compared to analytic results.