New excitation method of stimulated Raman scattering achieves natural-linewidth-limit spectral lines

Stimulated Raman Scattering (SRS) has been developed as an essential quantitative contrast for chemical imaging in recent years. However, the spectral resolution of the mainstream SRS modalities is always lower than the state-of-the-art spontaneous Raman system. This problem arises from the excitation strategy: the most widely used SRS modalities are all excited in the frequency domain. They have to compromise between the detection sensitivity and the spectral resolution: as the nonlinear process benefits from pulsed excitations, the fundamental time-energy uncertainty limits the spectral resolution.

 In a new paper published in Light: Science & Applications, a team led by Dr. Hanqing Xiong from the National Biomedical Imaging Center, College of Future Technology at Peking University (Beijing, China) reported a novel method named transient stimulated Raman scattering (TSRS) (Fig.1). The team manipulated the interference of vibrational wave packets in the time domain by broadband femtosecond laser pulse trains, and finally achieved natural-linewidth-limit Raman spectra at sub-mM sensitivity in a Fourier spectroscopic way. Furthermore, TSRS hyperspectral imaging of living Hela cells has been comprehensively performed in the Raman fingerprint region, the cell-silence region, and the popular C-H stretching region (Fig.2). To show the advantage of the natural-linewidth-limit spectral resolution, The team also preliminarily constructed a set of high-density Raman probes with Raman mode intervals down to 12 cm^-1, and further demonstrated its corresponding barcode imaging (Fig.3). The paper was published under the title of "Transient Stimulated Raman Scattering Spectroscopy and Imaging".

 

Time-domain SRS techniques can find their origin in the 1980s, which is actually not new. However, previous time-domain SRS techniques cannot provide a sensitivity comparable to the widely used Frequency-domain methods. From the authors’ point of view, the difference between the TSRS technique with other existing time-domain SRS methods is the use of stimulated Raman loss (SRL) as the signal. SRL has a linear relation to the molecular concentration and the Raman cross section, and it can be detected by classical heterodyne detection method to achieve the same shot-noise-limited sensitivity as the Frequency-domain methods. In order to construct a time-domain SRL signal, the authors abandoned the popular pump-probe excitation strategy. Instead, they generated vibrational wave packet interference by two successive identical impulsive excitations with controlled time delay (Fig.1). The interference induces modulations on the SRL signal. A Fourier transform of the modulated SRL signal trace enables natural-linewidth-limit spectral lines.

 

“The spectral range of T-SRS imaging is only determined by the laser pulses bandwidths. The bandwidths of our excitation laser pulses can only support a spectral range of ~124 cm^-1. We are constructing a laser system with much shorter pulses for TSRS, which may give full-range SRS spectra similar to the state-of-the-art spontaneous Raman system.” summarized by Dr Hanqing Xiong.

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