Broadband stimulated Raman scattering microscopy with wavelength‐scanning detection

Abstract

We introduce a high-sensitivity broadband stimulated Raman scattering (SRS) setup featuring wide spectral coverage (up to 500 cm −1) and high-frequency resolution (≈20 cm −1). The system combines a narrowband Stokes pulse, obtained by spectral filtering an Yb laser, with a broadband pump pulse generated by a home-built optical parametric oscillator. A single-channel lock-in amplifier connected to a single-pixel photodiode measures the stimulated Raman loss signal, whose spectrum is scanned rapidly using a galvanometric mirror after the sample. We use the in-line balanced detection approach to suppress laser fluctuations and achieve close to shot-noise-limited sensitivity. The setup is capable of measuring accurately the SRS spectra of several solvents and of obtaining hyperspectral data cubes consisting in the broadband SRS microscopy images of polymer beads test samples as well as of the distribution of different biological substances within plant cell walls. KEYWORDS coherent Raman spectroscopy, hyperspectral microscopy, nonlinear optical microscopy, optical parametric oscillators, stimulated Raman scattering 1 | INTRODUCTION Coherent Raman scattering (CRS) microscopy [1-3] allows label-free identification of molecules based on their intrinsic vibrational response, which provides a fingerprint of their chemical structure. CRS exploits the resonant third-order nonlinear optical signal generated in a sample by interaction with two light pulses, the pump (at frequency ω p) and the Stokes (at frequency ω S), when their frequency detuning ω p-ω S matches a vibrational frequency Ω of the molecule. In coherent anti-Stokes Raman scattering, [4-7] the response is read at the anti-Stokes frequency ω aS = ω p + Ω, whereas in stimulated Raman scattering (SRS), [8-10] the nonlinear signal is imprinted on the pump/Stokes pulses themselves, in the form of Stokes pulse amplification (stimulated Raman gain, SRG) or pump pulse attenuation (stimulated Raman loss, SRL). Coherent anti-Stokes Raman scattering suffers from the superposition with a frequency-independent nonresonant background, which often distorts and masks the resonant signal of interest [11,12] ; SRS, on the other hand, is almost free from nonlinear background and directly measures the resonant signal, making it the CRS microscopy technique of choice.

High-Speed and Low-Noise Multichannel System for Broadband Coherent Raman Imaging

“…With an integration time of 44µs the obtained sensitivity is lower than 10ppm. The acquired Raman spectra are in agreement with the results available in literature [6]. Finally, thanks to a piezo for the X-Y movement of the sample, a 512x512 pixels image is also acquired, practically at shot-noise limit (Input-referred estimated shot-noise: 1.15nW rms , Measured noise: 1.59nW rms ), with an integration time of 10µs (Fig.5).…”

Accessing Excited State Molecular Vibrations by Femtosecond Stimulated Raman Spectroscopy

“…FSRS exploits the combination of a narrowband Raman pulse (RP) with a femtosecond probe pulse (PP) to coherently stimulate and probe vibrational excitations, read out as Raman gain on the high directional PP field and probed over a wide spectral range. Hence, the stimulated Raman spectrum can be isolated considering the ratio between the PP spectrum measured in the presence ( I On ) and absence ( I Off ) of the RP, conventionally termed the Raman gain, providing high structural sensitivity − to the sample under investigation. The use of a broadband probe pulse provides the chance to access the entire vibrational spectrum, from low- to high-frequency Raman modes, in a single acquisition.…”

Multiplex Chemical Imaging Based on Broadband Stimulated Raman Scattering Microscopy

“…This protocol has three minor limitations. First, the detection scheme employed in this contribution consists of a nonconventional, multichannel lock-in detector designed and built in-house by Sciortino et al 26 As demonstrated previously 25 , this detector can be replaced by an off-the-shelf system complexity and lead to losses of optical power and pulse broadening.…”