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When light propagates through highly disordered media such as biological tissue, multiple scattering prevents light from reaching depths much larger than the transport mean free path, making the material opaque. This poses a problem for Raman spectroscopy in biological media, where in order to obtain spontaneous Raman signal they need to work in the superficial region of the material or increase the pump power, which is not always a safe option. In this work we show that wavefront shaping techniques can significantly increase light penetration through opaque media, thus allowing detection of Raman scattered light of materials deep inside an opaque medium, avoiding the increase of the pump power.
Wavefront shaping techniques are capable of manipulating the amplitude and/or phase of a light beam, which allows control over light propagation through such media. Wavefront shaping was originally proposed to focus light through a scattering material [1], and was recently shown capable of increasing light penetration in very thin (8 μm) scattering layers [2]. But efficiently delivering light at larger depths (~100 μm) in strongly scattering material, with optical densities comparable to thick biological tissues, is still an unsolved problem.
In this work we use a fast Digital Micromirror Device (DMD) to control the phase profile of the pump light incident on the sample, made of two layers of different strongly scattering materials (TiO2 and Hidroxyapatite). Using the transmitted light as feedback, an iterative algorithm adapts the phase pattern controlled by the DMD maximizing the penetration depth of the incident light. A spectrometer collecting the reflected light quantifies the depth at which the pump light is reaching by analysing the spontaneous Raman signal of the inner layer of the sample. We show an increase of 40% in the Raman signal collected from the inner material when the wavefront is optimized, equivalent to 40% deeper penetration of the pump light, given the linear characteristics of spontaneous Raman scattering.
This result shows the usefulness of wavefront shaping techniques to increase the penetration depth of light, improving the applicability of Raman spectroscopy in thicker materials. This is of enormous interest in the fields of non-invasive breast cancer diagnosis, light-activated cancer drugs or white LEDs where penetration depth of light is limited by scattering and applied power needs to be in the safe regime.
[1] I.M. Vellekoop, and A.P Mosk, Optics Letters 32, 2309 (2007).
[2] W. Choi, et al. Scientific Reports 5, 11393 (2015).
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