Michael Gasper

Radio Frequency Carbon Nanotube Thin-Film Bolometer

A radio frequency bolometer was realized using a thermistor fabricated from a carbon nanotube thin-film deposited on a sapphire substrate. Power detection performance was characterized at room and liquid nitrogen temperatures. With the thermistor held at a temperature of 15 °C, the bolometer’s sensitivity of 915-MHz test signals was found to be 0.36 mV/mW, and power levels as low as −45 dBm could be detected with a 20-s integration time constant. The sensitivity increased to 2.3 mV/mW when the thermistor was cooled to −193 °C. Experiments over the temperature range of 15 °C–240 °C indicate that internanotube mechanisms dominate power detection at temperatures below a value of approximately 200 °C and that intrananotube mechanisms (primarily Joule-heating) dominate at higher temperatures.

Uncooled radio frequency bolometer based on carbon nanotube thin films

Carbon nanotube thin films deposited on sapphire substrates have been used to realize a microwave power sensor that operates at and above room temperature. The detector includes a power-sensitive resistor that has been incorporated into a voltage divider circuit. Using lock-in detection, experiments were performed with 915 MHz test signals that showed detection down to -45 dBm. A sensitivity of 0.36mV/mW was achieved with the device held at a temperature of 15°C. Additional experiments (which included static and pulsed current versus voltage measurements) indicate that the primary physical mechanism responsible for power detection near room temperature is Joule heating.

Microwave bolometers based on carbon nanotube thin films and CVD-grown graphene

We have investigated microwave power detection based from carbon nanotube (CNT) thin films and chemical vapor deposition (CVD) grown graphene. Our experiments indicate that power detection from the CNT devices is primarily due to bolometric mechanisms. While, power detection from the graphene devices is primarily due to signal rectification. Both enabling materials are relatively inexpensive and easily blanketed on a variety of substrates{enabling low-cost/disposable, surface-conformal power sensors for wideband spectrum sensing applications. However, it is significantly less challenging to pattern and integrate CNT thin films than it is to do the same with graphene. CNT thin film and graphene power detectors were realized by fabricating metallic Corbino disc test structures over these enabling materials. Such test structures are convenient for on-wafer characterization with ground-signal probes. The CNT devices were also evaluated with transient current-versus- voltage traces and microwave reflection spectroscopy to respectively measure thermal time constants and values of complex conductivity. The bolometer performance of these devices was gauged in terms of power detection sensitivity, noise equivalent power, and dynamic range. The measurements were performed with 915 MHz test signals and yielded sensitivities as high as 0.36 mV/mW at room temperature and 2.3 mV/mW when cooled with liquid nitrogen. Similarly, graphene Corbino disc test structures were characterized with 433.92 MHz test signals and yielded power detection sensitivities of 3.25 mV/mW (at room temperature) and 5.43 mV/mW (at 80 K). These devices feature gate control over the channel conductance, which contributed a frequency-limiting parasitic capacitance. Our investigations revealed that rectification, due to characteristic nonlinear current versus voltage behavior, was more prevalent in the graphene than bolometric detection, due to Joule heating.

Millimeter-wave Faraday rotation from ferromagnetic nanowires

Nickel nanowires embedded in track-etched polycarbonate membrane filters have been used to impart Faraday rotation on millimeter wave signals with frequencies of 61.25 GHz. A customized apparatus for measuring Faraday rotation was designed and built using quasi-optical components. Experiments were performed to rule out the possibility of measuring reciprocal polarization rotation due to birefringence. Experiments were also performed to ensure that the non-reciprocal polarization rotation, attributed to the Faraday effect, had direct proportionality to the strength of a static magnetic field applied in the direction of the millimeter wave propagation.

Microwave power detection with gated graphene

Gated graphene has been used to realize a microwave power detector. A Corbino disc structure with Ti/Ag contacts has been fabricated on top of graphene deposited on P-type Si substrates with SiO2 gate oxides. This device exhibits power detection with a sensitivity reaching 0.87 mV/mW at a frequency of 433.92 MHz using lock-in detection at room temperature.

Apparatus for characterizing millimeter-wave propagation through magnetoelastic multiferroic materials

This paper presents a unique apparatus for measuring the Faraday rotation imparted on millimeter waves as they propagate through magnetoelastic multiferroic materials. The instrument is designed to operate in the V-band, specifically over the frequency range of 57 GHz to 67 GHz. The instrument focuses a Gaussian beam with a spot size of approximately 12 mm onto a customized material sample holder. A V-Band dual mode polarized horn antenna with horizontal and vertical ports and an intergrated orthomode transducer is used to detect the power from the material under test (MUT) using a lock-in amplifier. Experiments were performed with and without MUT to determine the change in polarization of the millimeter wave in each case.

Microwave power detection from an anharmonic dipolar resonance

Electric-field-induced, anharmonic dipolar resonances of room-temperature, barium strontium titanate (BST) thin film varactors have been used to rectify and detect microwave signals with frequencies ranging from 2GHz to 3GHz. The resonant frequency was shown to have strong dependence on film thickness with some amount of voltage-controlled tunability. Our experiments involved lock-in detection of a 100% amplitude modulated microwave signal with power levels ranging from -20 dBm to +10 dBm. An on-resonant sensitivity of 0.6 mV/mW was observed. This power detection sensitivity was shown to have built-in bandpass filtering arising from resonant line shape.