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The two major global problems are to provide health safety and to meet energy demands for ever growing population on a large scale. The study of light interaction with nanostructures has shown a promising solution in improving the fields of bio-sensor and solar energy devices which addresses above mentioned two major global problems. Nanostructures have tunable physicochemical properties such as light absorption, electrical and thermal properties unlike bulk materials, which gives an advantage in applications like bio-sensing and energy harvesting devices. The development of nanofabrication techniques along with the discovery of Surface Enhanced Raman Scattering (SERS) and Plasmon Enhanced Fluorescence (PEF), led to the development of Point of Care (POC) sensing devices. The fundamental understanding of light path in a nanostructured material led to the improvement in solar energy harvesting performance. For both of these applications, engineering nanostructures is the key to improving performance. In this work, different plasmonic nanostructures were designed, fabricated and analyzed for biosensor and light management applications. A new fabrication route, which combines nanosphere lithography with silicon-based clean-room microfabrication processes, has been developed to produce large-area long-range ordered gold nanoring array patterns in a controllable fashion. The developed nanoring structure has SERS enhancement of 2*109 and is used for miRNA detection. A novel pyramid array on gold film 3D plasmonic nanostructure is designed to convert plasmonic light scattering to confined light absorption. This structure generates a cavity mode by hybridization of fundamental modes, which creates a strong electric and magnetic field with a large mode volume. Due to its unique properties pyramids coupled film structure is used for both solar light management device and in Metal Enhanced Fluorescence (MEF). The fabricated structure is used to demonstrate plexiton (plasmon – exciton coupling) generation and is very effective in light trapping in the gap mode. In MEF, the sandwich nanostructure is used for Metal Organic Framework (MOF) fluorescence enhancement and the enhancement factor is around 5*102. With the plasmonic metal nanostructure optimization, the performance of a specific application is improved. However, the metals used for plasmonic applications are noble metals like gold and silver to support strong localized surface plasmon resonance (LSPR), which are expensive. Two-dimensional semiconductor materials have shown plasmon resonance in the visible region, having a lot of applications in sensing and photonics. Heavily doped semiconductors could replace expensive metals without compromising the performance. LSPR in metals is tuned by shape, size and refractive index of surroundings. This restricts plasmon resonance tuning over a narrow wavelength range and need to choose a different metal to exceed the rage of application. In contrast, LSPR in plasmonic semiconductors can be tuned with parameters like carrier density, annealing temperature and doping. This gives an advantage of tuning the plasmon peak over a broad range including visible, Near Infrared (NIR) and Infrared(IR) regions. This is because, for semiconductor materials, the carrier concentration can be varied over a large range. Herein, the molybdenum oxide thin films were directly deposited and nitrogen annealed which showed a tunable localized surface plasmon resonance (LSPR). A chip based 2D semiconductor material is fabricated to study the structural and size dependent plasmon resonance. This work establishes a way to fabricate chip based ordered semiconductor nanostructures, which helps in a systematic study of plasmon properties on nanostructures.

The world’s booming population projected to reach 10 billion by 2050 causes enormous stresses on environmental safety, food supply, and healthcare, which in return threatens human civilizations. One of the most promising solutions lies at innovating point-of-care (POC) sensing technologies to conduct detection of environmental hazards, monitoring of food safety, and early diagnosis of diseases in a timely and accurate manner. The discovery of surface-enhanced spectroscopy in the 1970s has significantly stimulated research on light-matter interaction which gives rise to enhanced optical phenomena such as surface-enhanced Raman scattering (SERS), plasmon-enhanced fluorescence (PEF), and particularly, they have found enormous applications in optical sensing. To fully exploit surface-enhanced spectroscopy to advance sensing technologies, it requires innovations in the sensor design as well as the plasmonic metallic nanostructures, which is exactly the focus of this dissertation. Owing to their strong capabilities of revealing molecular fingerprints and conducting single molecule analysis, both SERS and PEF have received extensive research interests. Since SERS directly correlates with the local electromagnetic (EM) field enhancement, it is featured by the simplicity in signal amplification. However, high SERS spectral resolution cannot be achieved without a tightly focused laser beam, which compromises the design of SERS-based POC sensing platforms. In contrast, the emission nature of fluorescence makes PEF easily coupled with POC readers, but optimal PEF requires a delicate control of the separation distance between the fluorophore and the nanostructure to minimize fluorescence quenching. SERS and PEF are essentially two complementary techniques and both hold great promise for POC sensing technologies. In the dissertation, in the first place, two label-free SERS sensors have been developed aiming to reduce the number of elements used in a sensor, which could potentially minimize interference, reduce the cost, and enhance the performance. In this regard, a label-free SERS sensor for mercury ions (Hg2+) detection has been developed based on functionalized gold nanoparticles, which employs a small molecule 4-mercaptobenzoic acid to capture mercury ions. A coordination bond formed only in the presence of mercury ions produces a new SERS peak at 374 cm-1, allowing unique detection of mercury ions. The other label-free SERS sensor has been developed for nitrite (NO2-) detection following the mechanism of Griess reaction based on the plasmonic coupling between gold nanostars and silver nanopyramid arrays. A newly formed azo compound produces at least three characteristic SERS peaks at 1140 cm-1, 1389 cm-1, and 1434 cm-1, which allow a highly specific detection of nitrite. While label-free SERS sensing has proved effective to enhance the performance, the need for a tightly focused laser beam hinders SERS from being easily coupled with POC readers for rapid signal readout. To address this limitation of SERS, on-chip PEF sensors have been developed, which can be inserted into POC readers for rapid signal readout. Optimizing PEF usually requires a delicate control of the separation distance between the fluorophore and the nanostructure to balance the excitation and emission enhancement which have different distance dependence. In addition to the separation distance, scattering has been found to be strongly correlated with quantum efficiency enhancement, which has been established as another tuning parameter in optimizing PEF. By making PEF work in the near-infrared (NIR) biological transparency window, the strength of PEF is further manifested by its compatibility with biological matrix featured as low background interference and high penetration depth. As a proof of concept, a NIR fluorescent biosensor has been developed for detection of traumatic brain injury biomarker in the blood plasma. The selection of a gold nanopyramid array pattern as the sensing platform not only generates intense localized EM field for the excitation enhancement, but also allows all the tests to be conducted using a POC fluorescence reader. While noble metals such as gold and silver are often used in developing sensing technologies as they support strong localized surface plasmon resonance (LSPR), it remains an open question as whether they could be replaced by alternative inexpensive metals such as copper and aluminum without compromising the performance. The discovery of a strong and sharp LSPR on copper nanoparticles when the shape is made cubic strongly suggests this possibility. By means of a numeric and theoretical study, it is found that the observed LSPR on copper nanocubes originates from the corner mode which survives damping as it is spectrally separated from the interband transitions. Compared to the dipole mode of a gold nanosphere of the same volume, a copper nanocube displays a comparable extinction coefficient but a local EM field enhancement 7.2 times larger. Furthermore, a film-coupled copper nanocube system has been designed for plasmon-enhanced NIR fluorescence. Because of the coupling between the copper nanocube and the underlying film, a plasmonic cavity mode is generated and featured as a spectrally tunable LSPR and an intense local EM field. By tailoring the resonance to the NIR wavelength region, the film-coupled copper nanocube system has been demonstrated to support a large NIR fluorescence enhancement owing to the strong excitation enhancement and the quantum efficiency enhancement.