Automated dual-tip scanning near-field optical microscope for investigation of nanophotonic systems

The main goal of this dissertation was to realize a fully automated and robust dual-tip scanning near-field optical microscope (SNOM) and demonstrate its capabilities to characterize nanophotonic systems. The dual-tip SNOM accesses optical information not simply attained with other super-resolution microscopy techniques. This work thoroughly explained the implementation of a collision prevention scheme and the realization of the fully automated dual-tip SNOM, in which the detection tip automatically scans the entire area surrounding the excitation tip without collision. After successfully implementing the automated dual-tip SNOM, the setup was utilized to measure the near-field of different photonic materials. First, the dual-tip SNOM was used to explore the polarization characteristic of the emission from the bent fiber aperture tip through exciting surface plasmon polaritons (SPPs) on an air-gold interface. Another unique application of the dual-tip SNOM is to excite the edge or corner of a photonic system locally. The automated detection tip was used to map a complex near-field pattern due to the excited SPPs and reflected SPPs from the edges of the truncated triangular gold platelet. The most distinguished capability of the automated dual-tip SNOM shown in this thesis is spectral- and spatial-dependent near-field measurements of a nanodisk metasurface as a nanostructured sample. In spectral-dependent near-field measurements, the excitation tip illuminated the silicon metasurface at different wavelengths. In spatial-dependence near-field measurements, the fixed excitation wavelength was used to measure the position-dependent near-field intensities when the metasurface was displaced relative to the excitation tip. It was demonstrated that the integrated measured near-field intensities by the detection tip could be related to the metasurface's partial local density of optical states at the excitation tip's position

Investigation of dipole emission near a dielectric metasurface using a dual-tip scanning near-field optical microscope

A wide variety of near-field optical phenomena are described by the interaction of dipole radiation with a nanophotonic system. The electromagnetic field due to the dipole excitation is associated with the Green’s function. It is of great interest to investigate the dipole interaction with a photonic system and measure the near-field Green’s function and the quantities it describes, e.g., the local and cross density of optical states. However, measuring the near-field Green’s function requires a point-source excitation and simultaneous near-field detection below the diffraction limit. Conventional single-tip near-field optical microscope (SNOM) provides either a point source excitation or amplitude and phase detection with subwavelength spatial resolution. The automated dual-tip SNOM, composed of two tips, has overcome the experimental challenges for simultaneous near-field excitation and detection. Here, we investigate the dipole emission in the near-field of a dielectric metasurface using the automated dual-tip SNOM. We have analyzed the near-field pattern and directional mode propagation depending on the position of the dipole emission relative to the metasurface. This study is one further step toward measuring the dyadic Green’s function and related quantities such as cross density of optical states in complex nanophotonic systems for both visible and near-infrared spectra

Tailored structural disorder in optical metasurfaces

We experimentally realize various optical metasurfaces with tailored rotational and positional disorder, and demonstrate their ability to support pure circular dichroism and to tune the intensity of the transmitted light almost independently from its phase