UC Irvine

Ultra-low temperature photon detectors have been able to achieve high detection sensitivity but they remain a high-maintenance and expensive option. Thermal detectors, on the other hand, can be operated at room-temperature and can provide a more affordable and portable option, albeit with lower performance compared to its counterpart. As such, there remains a need for uncooled detectors with higher sensitivity, higher pixel density, and higher bandwidth for critical applications, such as on-chip optical readout of incident radiation, high-resolution medical imaging, autonomous driving at low-light conditions, and military applications. This dissertation presents a study on the design, fabrication, and characterization of radiation detectors with high sensitivity, specifically plasmo-thermomechanical and microbolometric types. Here, plasmonically enhanced thermo-mechanical and thermo-electrical properties of materials and device geometries are intelligently exploited to achieve uncooled electromagnetic radiation detectors with improved sensitivity and bandwidth of operation.

Many novel applications in plasmonics and integrated optics have emerged from the advances in micro and nanofabrication techniques. In particular, plasmonic structures have demonstrated their ability to focus and control light at a micro- and nano-scales, which leads to intense light-matter interactions - key to high sensitivity at reduced area. In addition to their electromagnetic properties, micro and nanoscale plasmonic structures may open up new opportunities for novel applications based on their thermal and mechanical properties. Along with patterned metallic nanostructures, 2D materials like graphene possess extra-ordinary thermal and plasmonic properties that are key to the enablement of novel sensing devices.

The first work described in this thesis demonstrates a true full integration of plasmo thermo mechanics and integrated optical waveguides that turn efficient optical absorption into light modulation in waveguides through the use of plasmonic fishbone nanowires. By using plasmonic absorbers, specific band of radiation energy from free space is transformed into heat, causing the nanowires suspended above a waveguide to bend. The radiation is finally detected all-optically as a fluctuation of the waveguide insertion loss. The efficiency and bandwidth of such devices are further shown to be augmented by the incorporation of a graphene layer on top of the metallic nanowires due to graphene's extraordinary plasmonic and heat-spreading properties.

The second part of the dissertation presents a microbolometric detector that incorporates a vanadium dioxide (VO2) nanobeam integrated with plasmonic absorbers to thermo-electrically detect the incident electromagnetic radiation. VO2 is a phase-transition material (PTM) that exhibits a unique property known as semiconductor to metal transition, whereby it can switch between semiconducting and metallic states at a specific temperature called the transition temperature. As such, this transition metal oxide is highly sensitive to optical and thermal signals, making their perception precise and efficient. In particular, its sharp temperature-dependent resistivity makes it a suitable material of choice for microbolometers. The incorporation of plasmonic absorber, on the other hand, provides design flexibility and tunability to detect narrowband, broadband, or even multiband electromagnetic radiation. Presented in this study is an example design of an infrared detector that is designed to sense thermal information from a human body emitting infrared (IR) radiation peaking near 10 um wavelength. Unlike other bolometric detectors, the proposed detector incorporates VO2 nanobeam (instead of a VO2 film) and is designed to operate near the VO2 phase transition temperature to achieve enhanced responsivity.

Scaling the device geometries of plasmonic absorbers has the potential to transition the example design of bolometer to operate at far infrared spectrum or THz frequency. However, simple scaling is not sufficient to achieve such operation as the electromagnetic properties of constituent materials as well as the thermodynamic and mechanical properties of bolometers are subject to substantial changes with changes in frequency and size. To demonstrate design methodology and its scalability, an uncooled high-sensitivity bolometric terahertz detector operating near 1.7 THz is studied. The design additionally facilitates simultaneous sensing of intensity and polarization of incident radiation by utilizing polarization-dependent plasmonic field enhancement. Estimated responsivity is over 5 kV/W, noise equivalent power (NEP) is below 12 pW, and normalized detectivity over 10^8 cm.Hz^{1/2}.W^{-1} which are competitive to state-of-the-art THz bolometer designs.

The final section of this work presents the fabrication steps for a proof-of-concept device that incorporates plasmonic absorbers and VO2 beams. This includes details on the deposition techniques and lithographic patterning used for the VO2 beams and plasmonic absorbers. Additionally, the work includes the results of x-ray diffraction test and temperature-dependent electronic probing of deposited VO2 films and beams, and the IR spectroscopy of fabricated plasmonic absorbers demonstrating the practicality of the proposed design concept for high-resolution bolometric radiation detectors.