Abstract
A comprehensive study and design of air-clad suspended ridge diamond waveguides for operation across the 2.5–16 µm spectral range is presented, specifically targeting nanocrystalline diamond (NCD) thin films directly grown on silicon substrates. Three film thicknesses of 520, 1000 and 2000 nm are considered, to cover overlapping sub-bands of 2.5–5, 4–9 and 8–16 µm, respectively. Within each sub-band, the waveguide dimensions for single mode quasi-TE operation are found and the waveguide material losses and bending losses are estimated at each design point. In addition, in each case the minimum required undercut depth and etch hole placement for optical isolation of the waveguide mode from the silicon substrate is also quantified. We also estimate the losses associated with scattering from surface roughness, which is an unavoidable byproduct of the NCD thin film growth process. Our results indicate that despite the relatively low film thickness-to-wavelength ratio, mechanically stable waveguides with good optical confinement and low material and bending losses can be realised to cover the full 2.5–16 µm range. In addition, scattering loss estimations predict a drastic drop in roughness-induced scattering losses above 6 µm, even for relatively rough films. In addition to highlighting the utility of suspended NCD as a versatile platform for mid-infrared integrated photonics, the approaches and results presented here can be used to inform the design of suspended air-clad waveguides in other material platforms.
Highlights
IntroductionOf new optical sensing devices, since many environmental and industrial gaseous compounds can be detected by their unique absorption spectra in the mid-infrared
A comprehensive study and design of air-clad suspended ridge diamond waveguides for operation across the 2.5–16 μm spectral range is presented, targeting nanocrystalline diamond (NCD) thin films directly grown on silicon substrates
Since the film thicknesses are assumed to be three discrete values and in each case the etch depth is fixed by the membrane thickness, the only parameter that can be varied freely is the width of the waveguide, so at each wavelength we find the maximum ridge width for single mode operation
Summary
Of new optical sensing devices, since many environmental and industrial gaseous compounds can be detected by their unique absorption spectra in the mid-infrared This requires suitable material platforms compatible with the spectral regions of interest. Conventional cladding materials used in other integrated photonics platforms, such as SiO2, will inhibit operation at longer wavelengths due to their high optical absorption. An effective approach to circumvent this issue is to make use of integration platforms with air claddings, in the form of suspended waveguides Such suspended waveguide platforms have been successfully demonstrated in various materials such as silicon [6,7,8] and germanium [3]. The results support our assertion that operation at longer wavelengths will be practically feasible despite the granular nature of NCD films These results provide an optimistic pathway for the development of mid-infrared integrated photonics using NCD films
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