Aperture-Less Stimulated Raman Photothermal Microscopy in Epi-Configuration

Stimulated Raman scattering (SRS) is a widely used tool in various fields such as medicine, tumor diagnosis, and biology due to its chemical selectivity as well as nonlinear characteristics. However, the detection of the SRS process is limited by its small cross section, leading to low signal strength, as well as the noise of the used light source, reducing the achievable signal-to-noise ratio (SNR). Recently, Zhu et al. developed a technique called stimulated Raman photothermal (SRP) microscopy to circumvent these limitations [1]. In SRP, similar to SRS, a pump and a Stokes photon interact to excite a vibrational level of a molecule, leading to energy deposition within the sample. The corresponding heating of the sample causes a change in refractive index, which has a thermal lens effect [2]. A continuous-wave beam is used to probe this thermal lens as its divergence is changing by propagating through the thermal lens, leading to a change in intensity after an aperture being detected with a photodetector. In contrast to the previous work, where the transmitted beam was measured, we implemented an SRP setup with epi-detection of the signal to improve the applicability of the SRP scheme further for enabling investigations of thick and non-transmissive samples. Furthermore, the change in refractive index results in a change of the reflection coefficient of the sample, allowing for aperture-less and, therefore, easy detection of the signal.