To focus-match or not to focus-match inverse spatially offset Raman spectroscopy: a question of light penetration

TL;DR

This paper compares traditional and focus-matched inverse spatially offset Raman spectroscopy (ISORS) techniques, demonstrating how focus-matched ISORS enhances signal collection from samples while maintaining container signal suppression, through experiments and simulations.

Contribution

It introduces and evaluates focus-matched ISORS, showing its advantages over traditional ISORS in improving signal from samples without losing container suppression.

Findings

Focus-matched ISORS increases sample signal strength.

Both techniques effectively suppress container signals.

Simulations confirm the benefits of focus-matched ISORS across various conditions.

Abstract

The ability to identify the contents of a sealed container, without the need to extract a sample, is desirable in applications ranging from forensics to product quality control. One technique suited to this is inverse spatially offset Raman spectroscopy (ISORS) which illuminates a sample of interest with an annular beam of light and collects Raman scattering from the centre of the ring, thereby retrieving the chemical signature of the contents while suppressing signal from the container. Here we explore in detail the relative benefits of a recently developed variant of ISORS, called focus-matched ISORS. In this variant, the Fourier relationship between the annular beam and a tightly focused Bessel beam is exploited to focus the excitation light inside the sample and to match the focal point of excitation and collection optics to increase the signal from the contents without out compromising the suppression of the container signal. Using a flexible experimental setup which can realise both traditional and focus-matched ISORS, and Monte-Carlo simulations, we elucidate the relative advantages of the two techniques for a range of optical properties of sample and container.