Brian A. Slovick

Directional control of infrared antenna-coupled tunnel diodes.

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.

Angular Resolution Improvement of Infrared Phased-Array Antennas

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.

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

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.

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

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.

Infrared Linear Tapered Slot Antenna

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.

Negative refractive index induced by percolation in disordered metamaterials

An effective medium model is developed for disordered metamaterials containing a spatially random distribution of dielectric spheres. Similar to effective medium models for ordered metamaterials, this model predicts resonances in the effective permeability and permittivity arising from electric- and magnetic-dipole Mie resonances in the spheres. In addition, the model predicts a redshift of the electric resonance with increasing particle loading. Interestingly, when the particle loading exceeds the percolation threshold of 33\%, the model predicts that the electric resonance overlaps with the magnetic resonance, resulting in a negative refractive index.

Tailoring diffuse reflectance of inhomogeneous films containing microplatelets

We develop an analytical model for calculating the diffuse reflectance of inhomogeneous films containing aligned microplatelets with diameters much greater than the wavelength. The scattering parameters are derived by modeling the platelets as one-dimensional thin films, and the overall diffuse reflectance of the slab is calculated using the Kubelka-Munk model. Our model predicts that reflection minima and maxima arising from coherent interference within the platelets are preserved in the diffuse reflectance of the disordered slab. Experimental validation of the model is provided by reflectance measurements (0.3–15 μm) of a solid aerosol film of aligned hexagonal boron nitride platelets.

High diffraction efficiency from one- and two-dimensional Nyquist frequency binary-phase gratings

We examine the diffraction properties of one- and two-dimensional binary-phase gratings encoded onto pixelated liquid crystal displays (LCDs). We find that the first-order diffracted intensity from these binary-phase patterns can reach 100% of the zero-order intensity when the period of the grating approaches the Nyquist limit of the LCD. Experimental results show excellent agreement with theoretical predictions. This is a surprising result that has a number of implications for the encoding of diffractive optical elements.

Wavelength-dependent diffraction patterns from a liquid crystal display

Liquid crystal displays (LCDs) are invaluable for a variety of optical applications, including the encoding of programmable diffractive optical elements. The pixel structure in these devices produces a set of diffracted orders of which the central order is the strongest. In most devices that we have examined, the intensity distribution of the diffraction pattern is independent of the wavelength of the illuminating light. Recently we have been examining the performance of LCDs having very small pixel sizes. We compare results for two devices from the same manufacturer. One of them exhibits the normal behavior. For the other, we find surprisingly strong wavelength dependence. The diffraction pattern varies from having most of the energy in the zero order for long wavelengths to having the energy distributed among 50-60 orders as the wavelength decreases. We attribute this behavior to a phase structure over each pixel. We analyze this behavior using a simple two-dimensional model that qualitatively explains the phenomenon. These results can be viewed in two ways--on the positive side this behavior might lead to optical logic or fan-out applications. On the negative side, there is less intensity available in the normally used zero order.

Thermal insulator transition induced by interface scattering

We develop an effective medium model of thermal conductivity that accounts for both percolation and interface scattering. This model accurately explains the measured increase and decrease of thermal conductivity with loading in composites dominated by percolation and interface scattering, respectively. Our model further predicts that strong interface scattering leads to a sharp decrease in thermal conductivity, or an insulator transition, at high loadings when conduction through the matrix is restricted and heat is forced to diffuse through particles with large interface resistance. The accuracy of our model and its ability to predict transitions between insulating and conducting states suggest it can be a useful tool for designing materials with low or high thermal conductivity for a variety of applications.