Semiconductor nanostructures are ideal candidates for non- metallic plasmonic materials that operate in the near- to mid-infrared range. In contrast to metal nanostructures, semiconductor nanomaterials have the advantage of possessing tunable carrier concentrations. However, unlike metal nanoparticles which are already widely exploited in plasmonics, little is known about the shape-dependent localized surface plasmon resonances (LSPRs) and near- field electromagnetic behavior of semiconductor nanocrystals. Moreover, a major challenge in the fabrication of plasmonic semiconductor nanomaterials is the ability to control LSPRs by independently varying the size, shape, and carrier density of the nanocrystal. In this dissertation, I describe colloidal synthetic methods for fabricating shaped Cu₂-xS nanocrystals in which the morphology and stoichiometry of Cu₂-xS can be modulated. These shaped Cu₂-xS nanocrystals are used to observe the plasmon response for specific LSPR modes. Specifically, I discuss the plasmon response of Cu₂-xS nanodisks as a model nanocrystal system. I demonstrate that LSPR wavelength can be tuned by independently varying the aspect-ratio of the disk and the overall carrier density of the nanocrystal. Increased carrier density in Cu₂-xS occurs with oxidation and the formation of copper vacancies, an effect which can be suppressed by carrying out synthesis under an inert atmosphere. Using post- synthetic oxidation, Cu₂-xS nanodisks achieve a critical carrier density beyond which the nanocrystals undergo an irreversible phase change, which limits tuning capability. To circumvent this, I use a solvothermal process to generate nanodisks with different crystal phases that enable carrier densities beyond this critical limit. This dissertation also explores the differences in near-field coupling between Cu₂-xS nanodisks. These experiments were carried out on self-assembled two-dimensional nanodisk arrays. Varying nanodisk orientation produces a dramatic change in the magnitude and polarization direction of the local field generated by LSPR excitation. Moreover, plasmonic coupling is only observed for Cu₂-xS phases that possess carrier densities above a critical value. Overall, this dissertation provides new methods for tuning the plasmonic response of semiconductor nanocrystals by controlling size, shape, and carrier density. It also demonstrates new strategies for designing electromagnetic junctions or coupled plasmonic architectures that operate in the infrared using nanocrystals as building blocks

New materials that exhibit strong second-order optical nonlinearities at a desired operational frequency are of paramount importance for nonlinear optics. Giant second-order susceptibility χ ⁽²⁾ has been obtained in semiconductor quantum wells (QWs). Unfortunately, the limited confining potential in semiconductor QWs causes formidable challenges in scaling such a scheme to the visible/near-infrared (NIR) frequencies for more vital nonlinear-optic applications. Here, we introduce a metal/dielectric heterostructured platform, i.e., TiN/Al₂O₃ epitaxial multilayers, to overcome that limitation. This platform has an extremely high χ ⁽²⁾ of approximately 1500 pm/V at NIR frequencies. By combining the aforementioned heterostructure with the large electric field enhancement afforded by a nanostructured metasurface, the power efficiency of second harmonic generation (SHG) achieved 10^-4 at an incident pulse intensity of 10 GW/cm², which is an improvement of several orders of magnitude compared to that of previous demonstrations from nonlinear surfaces at similar frequencies. The proposed quantum-engineered heterostructures enable efficient wave mixing at visible/NIR frequencies into ultracompact nonlinear optical devices.

Transvaginal ultrasound is widely used for ovarian cancer screening but has a high false positive rate. Photoacoustic imaging provides additional optical contrast to supplement ultrasound and might be able to improve the accuracy of screening. Here, we report two copper sulfide (CuS) nanoparticles types (nanodisks and triangular nanoprisms) as the photoacoustic contrast agents for imaging ovarian cancer. Both CuS nanoprisms and nanodisks were ~6 nm thick and ~26 nm wide and were coated with poly(ethylene glycol) to make them colloidally stable in phosphate buffered saline (PBS) for at least 2 weeks. The CuS nanodisks and nanoprisms revealed strong localized surface plasmon resonances with peak maxima at 1145 nm and 1098 nm, respectively. Both nanoparticles types had strong and stable photoacoustic intensity with detection limits below 120 pM. The circular CuS nanodisk remained in the circulation of nude mice (n=4) and xenograft 2008 ovarian tumors (n=4) 17.9-fold and 1.8-fold more than the triangular nanoprisms, respectively. Finally, the photoacoustic intensity of the tumors from the mice (n=3) treated with CuS nanodisks was 3.0-fold higher than the baseline. The tumors treated with nanodisks had a characteristic peak at 920 nm in the spectrum to potentially differentiate the tumor from adjacent tissues.

On-chip plasmonic circuitry offers a promising route to meet the ever-increasing requirement for device density and data bandwidth in information processing. As the key building block, electrically-driven nanoscale plasmonic sources such as nanoLEDs, nanolasers, and nanojunctions have attracted intense interest in recent years. Among them, surface plasmon (SP) sources based on inelastic electron tunneling (IET) have been demonstrated as an appealing candidate owing to the ultrafast quantum-mechanical tunneling response and great tunability. However, the major barrier to the demonstrated IET-based SP sources is their low SP excitation efficiency due to the fact that elastic tunneling of electrons is much more efficient than inelastic tunneling. Here, we remove this barrier by introducing resonant inelastic electron tunneling (RIET)-follow a recent theoretical proposal-at the visible/near-infrared (NIR) frequencies and demonstrate highly-efficient electrically-driven SP sources. In our system, RIET is supported by a TiN/Al₂O₃ metallic quantum well (MQW) heterostructure, while monocrystalline silver nanorods (AgNRs) were used for the SP generation (localized surface plasmons (LSPs)). In principle, this RIET approach can push the external quantum efficiency (EQE) close to unity, opening up a new era of SP sources for not only high-performance plasmonic circuitry, but also advanced optical sensing applications.