Optical properties of nano-structured materials studied by means of interferometric techniques

In this thesis work we avail ourselves of optical interferometric techniques for the study of the optical properties of nano-structured materials. As a first step, we set up a multiple heterodyne scanning probe optical microscope (MH-SPOM). This apparatus allows to detect amplitude and phase of the optical field with sub-wavelength resolution. In addition, it is provided with other two capabilities: beam labelling and polarization sensitive detection system. The former allows to simultaneously illuminate the sample under investigation with both TE and TM polarized light and to discriminate the contribution of the different polarizations to the total detected field by means of the heterodyne signals. The latter allows to measure the polarization state of the light as it is transferred by the collecting SPOM system (probe +fiber ) to the detection system. By means of such an apparatus we investigate the response of un uncoated tapered fiber probe to the vectorial components of the electric field. Our experiments demonstrate that the probe response to transverse polarized fields is linear and that the polarization conversion of the system probe+fiber can be described by means of a Jones matrix. The response to longitudinally polarized fields is also investigated. By comparing the experimental results with a simplified numerical model for the probe-field interaction, we demonstrate that the probe is sensitive to longitudinal field components with a coupling efficiency of  80% compared to the case of transverse fields. In a second experiment, we individuate selective injection techniques in order to excite HE11 and TM01 modes into microfabricated fully metal-coated cantilevered probes. Numerical simulations demonstrate that such modes provide the best probe performances in terms of field transmission and lateral optical resolution. The effectiveness of the injection technique is proven by imaging the field throughput, at a few microns from the probe apex, by means of a high resolution Mach-Zehnder microscope. For the first time, we demonstrate experimentally the excitation of a highly confined longitudinally polarized ‘hot spot’ at the probe apex when the TE01 mode is excited into the probe. In the last experiment, we use a heterodyne scanning near-field optical microscope (H-SNOM) to investigate light propagation in photonic crystal waveguides. The light source is a tuneable cw laser working in a range of wavelengths around 1.5m. By scanning the probe in the near field of the guiding channel we detect amplitude and phase distributions of the propagating modes. The simultaneous detection of amplitude and phase allows to perform a Fourier analysis of the optical patterns and determine the spatial frequencies for the propagating modes. The experimentally obtained values are compared with 3D numerical simulations of the investigated structure. The excellent agreement between measured and calculated values demonstrates the effectiveness of the scanning technique for the analysis of light propagation in photonic structure


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