Ge-on-Si waveguides for sensing in the molecular fingerprint regime.

Ugne Griskeviciute,Joao Valente,Kevin Gallacher,Ross W Millar,Douglas J Paul

doi:10.1364/oe.382356

Abstract

Low loss, single mode, Ge-on-Si rib waveguides are used to demonstrated optical sensing in the molecular fingerprint region of the mid-infrared spectrum. Sensing is carried out using two spin-coated films, with strong absorption in the mid-infrared. These films are used to calibrate the modal overlap with an analyte, and therefore experimentally demonstrate the potential for Ge-on-Si waveguides for mid-infrared sensing applications. The results are compared to Fourier transform infrared spectroscopy measurements. The advantage of waveguide spectroscopy is demonstrated in terms of the increased optical interaction, and a new multi-path length approach is demonstrated to improve the dynamic range, which is not possible with conventional FTIR or attenuated total reflection (ATR) measurements. These results highlight the potential for Ge-on-Si as an integrated sensing platform for healthcare, pollution monitoring and defence applications.

Highlights

The mid-infrared (MIR) spectral region is of significant interest for establishing on-chip spectroscopy; in the spectral region between 6.7 and 20 μm wavelength, as the unique molecular vibration modes allow for a label free sensing platform [1]
We demonstrate a unique benefit of waveguide spectroscopy, by using a multi-path length geometry to effectively improve the dynamic range of the measurement, allowing the observation of both strong and weak absorption lines
The increased attenuation from the waveguide geometry can clearly be observed, with the signal being almost fully attenuated at ∼ 8.4 μm. This demonstrates that despite an optical overlap of < 2 %, the waveguide geometry allows for enhanced interaction with the analyte due to the increased optical path length compared to surface normal Fourier transform infrared (FTIR) measurements. This is appropriate for analysis of proteins and DNA, as surface normal FTIR measurements are dominated by water absorption in the solution, and attenuated total reflection measurements have limited interaction due to a finite number of reflections [24]

Summary

Introduction

The mid-infrared (MIR) spectral region is of significant interest for establishing on-chip spectroscopy; in the spectral region between 6.7 and 20 μm wavelength, as the unique molecular vibration modes allow for a label free sensing platform [1]. A number of on-chip optical sensing devices have been fabricated using Si, which are low loss in the near-IR (NIR) part of the spectrum, while these NIR devices can be highly sensitive, they typically rely on the refractive index shift caused by an analyte to detect its presence, in the absence of strong absorption lines. This allows for molecular identification only when the waveguide surface is treated with a surfactant to promote binding to a particular molecule, as a number of analytes have comparable refractive indices

Results

Discussion

Conclusion