Development of a static fourier transform spectrometer and real-time substrates for surface enhanced raman scattering

Official URL: http://risc01.sabanciuniv.edu/record=b1655715 (Table of Contents)

In the first part of this thesis a novel broadband static Fourier transform spectrometer (static-FTS) con guration based on the division of the optical spectrum into multiple narrow-bands is proposed and implemented by combining a static-FTS and dispersive elements. Dividing the broadband input spectrum into narrow-band signals we have used the band-pass sampling theorem to reduce the sampling frequency (or increase the scanning step in the FTS concept). The dispersive part includes a double di raction grating structure to disperse the input spectrum in horizontal direction (to divide the input light into multiple narrow-band signals) and the static-FTS part includes a static Michelson interferometer to make di erent path length di erences (PLD) in the vertical direction. The static Michelson interferometer is composed of a beam-splitter (BS), a at mirror and a stair-case mirror. However, in actual setup a di raction grating in Littrow con guration is used to realize the stair-case mirror. Using o -shelf di raction gratings as the stair-case mirror decreases the total cost of the prototyped device. A CCD camera is used at the exit port of the static-FTS part, to record the formed interferograms. The proposed novel con guration based on band-pass sampling theorem not only decreases the spectrometer size but also allows operation in the traditional spectrometer wavelength range, namely 400 nm - 1100 nm with better resolution. This technique solves the Nyquist sampling rate issue and enables recording high resolution spectrums with regular CCDs. The proposed con guration and the method, in fact, solve the trade of between resolution and bandwidth, and also eliminate the need for nanometer step size mirrors. An algorithm is developed to process the recorded signal and calculate the Fourier transform of the recorded interferograms on the CCD camera. In the second part, we have shown the capability of copper oxide (CuO) nanoparticles formed on copper (Cu) electrodes by the electrolysis as a real time active substrate for surface enhanced Raman scattering (SERS). We have experimentally found that using just the ultra pure water as the electrolyte and the Cu electrodes, ions are extracted from the copper anode form copper oxide nanoparticles on the anode surface in matter of minutes. Average particle size on the anode reaches to 100nm in ninety seconds and grows to about 300nm in ve minutes. This anode is used in SERS experiments in real time as the nanoparticles were forming and the maximum enhancement factor (EF) of Raman signals were over ve orders of magnitude. Other metal electrodes made of brass, zinc (Zn), silver (Ag) and aluminum (Al) also tested as candidate anode materials for their potential as real-time substrates for SERS applications. Experimentally obtained enhancement factors were above ve orders of magnitude for brass electrodes like the copper but for the other metals no enhancement is observed. Electron microscope images show the cubic nanoparticle formation on copper and brass electrodes but none in the other metals studied. The standard electrode potential of the electrodes plays the key role in production of the nanoparticles. The proposed method has some key advantages over existing SERS substrates: its not only a real time SERS substrate but also is a very fast, simple and a low cost technique. Furthermore this substrate is tunable in wavelength -albeit only irreversibly and in one direction- as the particle size is increasing as a function of time which implies that plasmon resonance wavelengths are increasing as well. This technique also omits the need for an electrolyte containing the metal ions of interest for the nanoparticle production as just the deionized or distilled water is enough. This is an important point for the SERS measurements because the electrolyte being simply just the water there will not be an extra background noise added to the spectrum. This technique also allows preparation of a large e ective area for SERS enhancement, virtually unlimited area. As long as the current distribution over the anode is uniform which is a trivial arrangement, nanoparticle distribution will have quite homogeneous distribution. The homogeneous nanoparticle distribution means a uniform enhancement factor on the produced substrate.