This PDF file contains the front matter associated with SPIE Proceedings Volume 11466, including the Title Page, Copyright information, and Table of Contents.

Since III-nitride semiconductor-based ultraviolet (UV) light-emitting diodes (LEDs) are compact and efficient, they can be suggested as a substitute for conventional arc-lamps. However, reported UV LEDs focused on a narrow range of UV spectrum contrary to conventional arc-lamps. Here, we introduce GaN quantum dots (QDs) grown on different facets of hexagonal truncated pyramid structures on a conventional sapphire substrate. These structures include semipolar facets as well as a polar facet, which obtain intrinsically different piezoelectric fields and growth rates of QDs. Consequently, we demonstrated a plateau-like broadband UV emitter ranging from UV-C to UV-A from the GaN QDs.

Remarkable improvements in both structural and optical properties of wafer-scale hexagonal boron nitride (h-BN) films grown by metal-organic chemical vapor deposition (MOCVD) enabled by high-temperature post-growth annealing is presented. The enhanced crystallinity and homogeneity of the MOCVD-grown h-BN films grown at 1050 °C is attributed to the solid-state atomic rearrangement during the thermal annealing at 1600 °C. In addition, the appearance of the photoluminescence by excitonic transitions as well as enlarged optical band gap were observed for the post-annealed h-BN films as direct consequences of the microstructural improvement. The post-growth annealing is a very promising strategy to overcome limited crystallinity of h-BN films grown by typical MOCVD systems while maintaining their advantage of multiple wafer scalability for practical applications towards two-dimensional electronics and optoelectronics.

Exfoliated gallium oxide (Ga₂O₃) has been reported as an ultrathin channel material in field-effect transistors. Unlike in 2D materials with van der Waals forces between stacked layers, weak bonding in β-Ga₂O₃ along the (100) direction enables mechanical exfoliation. The thickness of these exfoliated films has been limited to tens to hundreds of nanometers. Here we summarize our latest work, which followed the process developed by Carey et al., enabling us to attain ~2 nm thick Ga₂O₃ films over a large surface area (< 1 mm²). The films are characterized using optical microscopy, AFM, XPS, Raman spectroscopy, photoluminescence (PL), and TEM. Optical microscope images showed color changes to the film upon annealing. AFM revealed the film thickness to be as thin as 2 nm over areas << 1 mm². XPS and Raman spectra revealed characteristic signatures of β-phase Ga₂O₃. Characteristic PL of Ga₂O₃ was seen in all samples with overall intensity of luminescence increasing after annealing, attributed to increased crystallinity and grain size. Changes in PL after annealing are associated with an increase in oxygen interstitials and a decrease in oxygen vacancies. Lastly, TEM analysis revealed the film as β-phase Ga₂O₃ polycrystalline. Overall, our results demonstrate that annealing of thin films obtained from oxide printing of liquid metal Ga is a non-expensive and straightforward process that can lead to β-Ga₂O₃ films that are nanometer-thin over wafer-scale areas.

UV plasmonics has drawn increased attention in recent years, holding promise in enabling label-free sensing of biomolecules such as DNA, peptides, and proteins whose intrinsic fluorescence lies in the UV range. However, these biomolecules exhibit relatively small quantum yields (QY) and extinction cross sections. In order to realize label-free detection of biomolecules, significant enhancement needs to be achieved. Several plasmonic structures have been reported to enhance native fluorescence of DNA and amino acids, with <80 x net enhancement for DNA and <15 x net enhancement for amino acids. Orders of magnitude improvement in the net enhancement factor are needed in order to achieve a detection limit comparable to commercial bioassays. In addition, quantitative fluorescence analysis that can differentiate the contribution of radiative and excitation enhancement is needed for UV studies. Here we report fluorescence enhancement of tryptophan on aluminum hole arrays. By optimizing excitation geometry, the hole size and spacings, we are able to achieve <40 x net enhancement factor, the highest ever observed for tryptophan molecules. We conducted photobleaching experiments and observed 2 x reduction in the fluorescence decay rate on the aluminum hole array compared to an aluminum thin film. The enhancement of total photon yield reaches 17 x, which indicates enhanced radiative rate. The studies we conducted will pave a way for label-free biosensing using UV plasmonics.

A key point of exciton-polaritons is the real-time potential energy controllability due to the interaction from excionic components. Although wide-bandgap semiconductors can form room-temperature polaritons, lateral localizations (disorders) of planar cavities still obstruct the establishment of ballistic extensions of polariton condensates. Here, we propose a novel room-temperature polariton platform with ultralow disorders enabling to ballistic extensions. Hexagonal GaN wires moderates disorders in both photon-perspective and exciton-perspective. This structure allow us to actively control the potential energy and its landscape of room-temperature polartion condensate for the ballistic propagation. The correlation between real- and momentum-space provides strong indication of ballistic propagations.

Ultraviolet (UV) plasmonic nanostructures hold promises in enabling label-free sensing of biomolecules using their native fluorescence. Several UV plasmonic structures have been explored to enhance native fluorescence of biomolecules, including metallic thin film, particle array, hole array using aluminum, magnesium, indium, etc. However, the enhancement factor of them is quite small, with less than 80 times for nucleic acids and less than 15 times for amino acids. In order to achieve higher enhancement factor, we study a bowtie nano-antenna (BNA) made of aluminum (Al) in the ultraviolet region. The effect of the native oxide layer on Al is also investigated. The numerical simulation has shown 1026x net enhancement with the optimal geometry.

Conventional histopathology has involved cutting, fixing, embedding, sectioning, and staining of surgical tissue specimens and has been the gold-standard for tissue diagnosis including tumor margin determination for over a century. However, this process is laborious and time-consuming and can lead to delays in diagnosis that are typically weeks after a surgery. In as many as 20-40% of solid tumor surgeries, residual tumor tissue is left behind and repeat surgeries are required. To address this critical issue we propose a new form of microscopy, photoacoustic remote sensing (PARS), that is capable of histology-like imagery that could enable histopathological inspection of tissues while a patient is still on the operating table. This would enable a surgeon to go back and remove additional tumor tissues that are left behind. Using ultraviolet 266nm light, we obtain images of cell nuclei from fixed and fresh tissue specimens with strong correlation to traditional HE pathology.