Abstract:

This thesis investigates and explores experimentally new optical devices based on a novel nanophotonic structure known as the subwavelength resonant grating, as well as its new applications. In its simplest form, a subwavelength resonant grating consists of a thin layer of subwavelength surface relief grating patterned on a thin film of waveguiding layer. It is an efficient optical filter that can completely reflect a certain wavelength and transmit all other wavelengths, with a very narrow spectral bandwidth and low loss.

In this work, subwavelength resonant gratings with operating wavelengths around 1.55 ?m are designed by rigorous coupled-wave analysis (RCWA) and fabricated by nanoimprint lithography. The grating pitch is around 1 ?m and the grating linewidth is between 400 to 500 nm. Optical measurements show that a spectral linewidth as narrow as 0.38 nm can be achieved by a resonant structure less than 500 nm in thickness. A new two-dimensional resonant grating with sub-nm spectral linewidth is demonstrated and is shown to exhibit a polarization-independent resonance as opposed to its one-dimensional counterpart. This thesis will also present the first demonstration of a cascaded resonant grating structure that shows flat bandpass spectral characteristics. By incorporating a nematic liquid crystal into the resonant structure, a tunable filter is demonstrated with over 20 nm tuning range. To facilitate liquid crystal alignment in the tunable filter, a superimposed grating structure is proposed and fabricated by nanoimprint lithography and is shown to improve the tuning range significantly.

A novel tunable external cavity laser using a subwavelength resonant grating as wavelength-selective reflector is proposed and demonstrated. It shows a sidemode suppression ratio of 36 dB and 7 nm tuning range. All-optical modulation has also been demonstrated using a resonant grating coated with the light-sensitive bacteriorhodopsin protein.

The results show that using the resonant grating as basic building block, a variety of devices and applications can be achieved by incorporating functional or active materials into the grating structure and by tailoring their optical properties through more complex grating design. Due to their inherent structural simplicity, devices based on subwavelength resonant grating have potential for low-cost fabrication, especially using nanoimprint lithography, and for integration with other optical elements.