Nanoscale Light Focusing and Imaging with Nano-Optical Devices

Energy transport analysis of micro/nano optics as well as their optimization to achieve high-throughput deep nanoscale patterning and microscopy is the goal of this study.

To understand the energy transport in nano-optical devices, the transient heating behavior of a commercial available nano-optical probe, NSOM, under pulsed laser operation is examined first. Based on the thermal analysis, it is observed that the major limitation of the NSOM probe under higher energy operation arises from the joule heating of its thin metal coating and the resulting thermal/mechanical damage during operation.

Based on the understanding, a diffraction-based micro-zone plate (MZP) optical probe suffers less from joule heating and with advantages of higher optical throughput as well as longer working distance is designed and constructed. The MZP is fabricated at the end face of an optical fiber with a micro-fabrication technique based on e-beam negative tone lift-off lithography on non-planar substrates. The fabricated MZP can achieve spot sizes ~ 0.7? at a focus distance of ~ 6? with an optical transport efficiency of 20 %, which is more than 3 orders higher than that of NSOM.

To further reduce the size of the focus spot to deep sub-wavelength scales and to eliminate metallic structures which can cause joule heating during high energy operation, two all-dielectric optical probes are then designed/constructed, namely, (a) solid immersion probe, and (b) scattering dielectric probe. Both dielectric probes focus light by combining more than one of the diffraction mechanisms associated with dielectric material (e.g., refraction, solid immersion, Mie and near-field Rayleigh scattering) to achieve deep nanoscale light focusing with minimum energy loss. The solid immersion optical probe is constructed with a ball lens and microsphere for macro to micro scale focusing with far field refraction and micro to mesocale focusing with solid immersion. The ball lens and the microsphere are stacked on an optical fiber for achieving the cascade focusing configuration. The solid immersion probe can achieve a focus spot size of ~ 0.45?/n (~ 0.3? when n = 1.5) on a target in the near field of the surface of the microsphere when the light is radially polarized. To achieve an even smaller focal spot in the near field, the scattering dielectric optical probe combines the verified solid immersion probe with a nano-scatterer at its focal spot. Due to the near-field Rayleigh scattering, the nano scatterer can induce a deep nanoscale spot with a diameter comparable to the forward radius of curvature of the scatterer under radially polarized light. It is verified with full wave electrodynamics simulations that the resulting scattering optical probe can achieve a ~10 nm spot with an intensity enhancement of ~ 10^5, which can be valuable in all kinds of bio-detection as well as nano fabrications.

At the end of the Ph.D study, mechanisms for deep sub-wavelength imaging resolutions with microspheres, which is recently demonstrated in different groups, are identified with full wave electrodynamics simulations. It is found that the high spatial imaging resolution of microlenses can be attributed to refractive index of the microsphere which is placed on the target plane (i.e. solid immersion effect), refractive index of the base material of the target and polarization of the emitters.