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.
Highlights
Coherent Raman scattering (CRS) microscopy[1,2,3] allows label‐free identification of molecules based on their intrinsic vibrational response, which provides a fingerprint of their chemical structure
Hz), which corresponds to a 2‐ms acquisition time per spectrum; during the scan, the lock‐in time constant was set to its minimum value of 1.8 μs, showing the possibility to reduce the acquisition time by more than one order of magnitude using optimized resonant galvanometric mirrors
This can be understood by looking at the relative intensity noise (RIN) curve in Figure 1b, which shows a decrease by 6 dB when increasing the modulation frequency from 1 to 7 MHz, bringing the system close to the shot noise limit
Summary
Coherent Raman scattering (CRS) microscopy[1,2,3] allows label‐free identification of molecules based on their intrinsic vibrational response, which provides a fingerprint of their chemical structure. In single‐frequency SRS, which uses narrowband (≈10–20 cm−1) pump and Stokes pulses and detects the signal for a specific detuning Ω, high sensitivity is typically achieved by combining high‐frequency (up to tens of MHz) modulation of the pump/Stokes beam and synchronous detection of the SRG/SRL signal with a lock‐ in amplifier, leading to acquisition speeds up to the video rate.[18] its information content is limited and in many cases not sufficient to distinguish the different components within complex heterogeneous systems containing several spectrally overlapped chemical species This has motivated intense research in the development of broadband SRS techniques, which have the goal of recording, for each pixel of the image, a complete SRS spectrum.[19] They can be classified into two categories[19]: hyperspectral SRS, in which the spectrum is acquired sequentially by rapidly scanning the pump–Stokes frequency detuning; multiplex SRS, in which the SRS spectrum is acquired in parallel by an array of photodetectors
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