Waveguide Enhanced Raman Spectroscopy (WERS): principles, performance, and applications

Abstract

Integrated optics is set to revolutionise the translation of laboratory-based, bulky, and expensive spectroscopy instruments to miniaturised, low-cost, automated yet powerful analytical instruments, which will be utilised to tackle challenges faced in a multiplicity of applications from food safety, environmental monitoring, security, personal medicine, pharmacogenetics and rapid point-of-care diagnostics. Waveguides, as the most fundamental integrated optics component, were utilised to replace bulky and alignment-critical free space optics in conventional spectroscopy. Raman spectroscopy, which provides the fingerprint of the analyte, was incorporated into the waveguide platform to be waveguide-enhanced Raman spectroscopy (WERS). The optical confinement provided by waveguide gives rise of the enhancement. The goal of this project is to push the limit of conventional waveguide-enhanced Raman spectroscopy with significantly improved performance while minimising utilisation of costly materials, components and techniques.

Electromagnetic models were established to obtain optimised 2D slab waveguide design for maximising Raman excitation. The fabricated waveguide samples made of Ta2O5 on fused silica substrates were in excellent agreement with the design parameters. WERS of bulk toluene liquid and polystyrene film were successfully measured from the conventional configuration of the collection, which was at the waveguide surface. Optimised 110 nm thick Ta2O5 waveguides on fused silica substrates excited at a wavelength of 637 nm were shown experimentally to yield overall system power conversion efficiency of ~5×10-12 from the pump power in the waveguide to the collected Raman power in the 1002 cm-1 Raman line of toluene. For the first time, a power budget analysis of WERS was reported, which aided comparisons of intrinsic WERS performance between different waveguide designs and shed light on the improvements needed for collection efficiency.

The spontaneous emission characteristic of an emitted molecule on the presence of a waveguide was analysed. In particular, the spatial radiation distributions were calculated for near-field and far-field radiations. It was found that near-field radiation dominated for the 110 nm thick Ta2O5 waveguide at an excitation wavelength of 633 nm. The emission patterns of both near-field and far-field radiations showed angular characteristic that were different to that from the free-space, which explained the low collection efficiency observed at the waveguide surface, and suggested that collection configuration played a critical role in efficient Raman collection.

For the first time, Raman collection from the waveguide front edge showed experimentally 2-3 orders of enhancement in comparison with that from the waveguide surface for bulk liquid toluene and polystyrene film under TM excitation. Theoretical analysis of spatial distributions of an emitted dipole located at the waveguide surface showed features of asymmetric emission for both near-field and far-field radiations. Notably, the waveguide front edge received 71.6% more near-field emission than that at the back edge. By summing up contributions of near-field and far-field radiations, collection from the waveguide front edge compared to that from the waveguide surface had a radiative enhancement factor of 7.2 and area enhancement factor of 7.7, hence a total theoretical collection enhancement factor of 55.

For the first time, WERS measurements of monolayers of trichloro(phenyl)silane and p-tolyltrichlorosilane with multiple Raman features were successfully shown by using the simplest all-dielectric slab waveguides. The structural difference of the methyl group between PTCS and TTCS was clearly shown, which validated the measurements. Polarised Raman measurements of TTCS on WERS platform were successfully performed, which resulted in depolarisability ratio being calculated. The tensor nature of Raman polarisability was utilised to calculate the relationship between the depolarisability ratio and the tilt angle of monolayer molecules on waveguide surface. Based on that, tilt angle was predicted to be 300 with the assumptions of randomly oriented molecules in the azimuthal and rotational plane.

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Identifiers

ORCID for James Wilkinson: orcid.org/0000-0003-4712-1697

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