Abstract
Long-wave infrared (LWIR) spectra are of interest in a variety of applications, such as sensing, imaging or medical applications. Research and design in silicon photonics have been primarily focused on devices optimised for 1. 55 pm wavelength, as utilised in the area of telecommunications. The most popular waveguide platform in silicon photonics is the silicon-on-insulator (SOI) structure, which is in the form of either a strip or a rib waveguide. This material structure however is not suitable for longer wavelengths (except in the 2.9 μm-3.6 μm range) due to the absorption spectra of silicon dioxide. There have been several potential LWIR waveguide systems reported in the literature. However none of them satisfies the full criteria of low-cost, easy implementation and widely accepted wavelength. In this work, a direct-write lithography technique called proton beam writing (PBW) is employed to fabricate LWIR waveguides in a single Si substrate. The waveguide has a Si core, which is transparent in 1.1-9 μm and 24-200 μm wavelength ranges, and uses either porous silicon (PSi) or air cladding. PBW is excellent on fabricating multilevel and freestanding structures by varying the dose and proton energy. In this work, freestanding Si waveguides are demonstrated for the first time in a single etching step. The propagation loss of the waveguides is optically characterised at 1550 nm for comparison with other waveguide in the literature. For annealed waveguides, the main contribution to loss is surface scattering. Utilising atomic force microscopy (AFM), the surface roughness is quantified. By this method, the loss is minimised through optimising irradiation and electrochemical etching parameters. In addition, post processing oxidation is used to smooth the waveguide surface. After oxidation, the propagation loss is reduced to as low as 1.1 dB/cm, which is the lowest reported value of loss for waveguides produced using electrochemical etching. A theoretical model is introduced for irregular waveguide shapes to predict the propagation loss in near- and long- wavelength. This model incorporates the real surface roughness measured using AFM, and is based upon field intensities at the core/cladding interface. The calculated loss is matched closely with experimental results in the near-infrared. As the wavelength increases, it is expected that the loss decreases. Therefore, proton-beam-written waveguides have the potential to be used in longer wavelength applications.