Theoretical, computational and experimental studies on plasmonic and optofluidic platforms

Recent years, theoretical and technological developments in the optic and photonic researches lead up to ensue new research and application areas. These developments enable to combine different disciplines and create complete new fields, and enable interdisciplinary studies. Plasmonics and optofluidics are best examples of these interdisciplinary studies. While plasmonics studies optical properties of metals, optofluidics combines microfluidic technology with optics. Each research area has specifical applications. Plasmonics is the study of the interaction between electromagnetic field and free electrons in a metal. There are some key terms including plasmon, polariton, surface plasmons (SPs), surface plasmon polaritons (SPPs), and surface plasmon resonance (SPR) to understand the plasmonics. Free electrons in the metal can be excited by the electric component of light to have collective oscillations. The quantization of these collective plasma oscillations is called plasmon. Also, surface plasmons are those plasmons that are confined to surfaces. SPs can be excited by electromagnetic waves resulting with a resonance situation that is called SPR. As a result of SPR, another quasiparticle called a plasma polariton is created. SPPs can be described as surface electromagnetic waves that propagate in a direction parallel to the metal/dielectric interface. Surface plasmon polaritons can be excited by electrons or photons. In the case of photons, it cannot be done directly, but requires a prism, or a grating. These methods are called as Kretchman configuration and grating coupling. For both method,the key fact is matching the wave vectors of the photon and SPs. In Kretchman configuration, the light is sent to the metal film through a glass prism by the angle which the resonance occurs. The evanescent wave penetrates through the metal film and excites the SPs on the metal-air interface. The excitation of surface plasmons takes place resulting in a reduced intensity of the reflected light.In the grating coupling, a coupler matches the wave vectors by increasing the parallel wave vector component by an amount related to the grating period. Simple surface gratings such as grooves, slits or corrugations can ensure the required circumstances to excite SPs. Since plasmons are the quantization of classical plasma oscillations, most of their properties can be derived directly from Maxwell's equations. Within this theses, we derived one of the most important property of plasmons called dispersion relation using Maxwell's equations. In the experimental studies, we focused on grating coupling method. First of all, we coated the glass with a thin gold film to prepare the grating structure. Differently from the present works, we ablated the thin gold film with a femtosecond laser to form nano slits. Mostly, electron beam lithography (EPL) is used for generating nano structures but EPL systems are very expensive and it takes too much time to complete a nano structure. In our work, we showed that direct fs laser ablation is also can be used for creating nanostructures for plasmonics applications. Compare to the lithographical methods, direct fs laser ablation is cost effective, precise and easy to use. After the ablation, we obtained a nano slit array and characterized the plasmonic properties of this array by applying the transmission spectrum in the range of 400 – 1100 nm. To compare the experimental results with computational ones, we used FDTD Solutions module which is be released to the market by LumericalTM company. FDTD Solutions uses the finite difference time domain (FDTD) method for analyzing the specific structures within the context of electromagnetics. In the FDTD algorithm, first, the structure to be analyzed is divided small boxes called as Yee cells. For each cell, Maxwell's equations are solved with respect to each time step. Lastly, the program gives information like transmission, reflection, by combining the results of Maxwell's equations. To simulate our structure, we modeled a nano slit array and characterized transmission spectrum. We observed that our experimental results are well-matched with simulation results. Optofluidics one of the developing area of photonics that combines optics and microfluidics. Fluids have specific properties that cannot be found in solid materials, and these properties can be used to design novel devices such as optofluidic microscope (OFM), dye lasers, optical trapping and manipulation, and chemical and biological sensors. Also, one of the most important application of optofluidic platforms is waveguides. The overwhelming majority of optical waveguides operates based on total internal reflection (TIR). Waveguides based on TIR have a high refractive index core region surrounded by low index cladding materials. By reason of liquids provide a large variety of refractive index, they are used as core medium of waveguides in a widespread manner. However, the material used as cladding, mostly glass (refractive index of 1.5), restricts types of liquids act as core region. Recently, aerogels become very important materials for optofluidic waveguides with their extremly low refractive index of 1.05. Although aerogels are very attractive materials for a wide variety of applications due to their unique properties, it is very challenging to process them with traditional methods such as milling, drilling, or cutting because of their fragile structure. Therefore, there is a need to develop alternative processes for formation of complex structures within the aerogels without damaging the material. Within this thesis, we developed a new method to form 3D optofluidic waveguides inside hydrophobic silica aerogels. We used femtosecond laser pulses which provide precise machining and good quality feature definition to ablate hydrophobic silica aerogels. During the ablation, we directed the laser beam with a Galvo mirror system and focused through a scanning lens to the surface of bulk aerogel which was placed on a translation stage. We succeeded to obtain high quality 3D linear microchannels inside aerogel monoliths with simultaneous motion of Galvo mirror and translation stage. In order to demonstrate waveguiding and measure optical attenuation of these microchannels, we filled the channels with ethylene glycol. In an attempt to confirm the existence of liquid inside the channel, we add the fluorescein which is a synthetic organic compound with peak excitation at 494 nm and peak emission at 521 nm into ethylene glycol. After that, we coupled light with 632 nm wavelength inside the microchannel with a fiber. To observe the guided light, we placed a microscope and CCD camera focused directly to the exit of the channel. Than we moved the coupling fiber through channel and measured the intensity distribution on exit of the channel for each 100 µm. When we analyzed the results, we calculated the propagation loss as 9.9 dB/cm. This result is comparable to present optofluidic waveguide's losses. In conclusion, we can say forming channels inside hydrophobic silica aerogels with femtosecond laser ablation is easier and more precise than mechanical methods. Also, our results show that the microchannels formed by femtosecond laser ablation in silica aerogels can be used as optofluidic waveguides. The results of optical attenuation show that the light guiding properties of aerogels are much better than the ones produced by mechanical methods of ablation.