Quantum-enhanced stimulated Raman scattering (QE-SRS) is a promising technique for highly sensitive molecular vibrational imaging and spectroscopy surpassing the shot noise limit. However, the previous demonstrations of QE-SRS utilized rather weak optical power which hinders from competing with the sensitivity of state-of-the-art SRS microscopy and spectroscopy using relatively high-power optical pulses. Here, we demonstrate SRS spectroscopy with quantum-enhanced balanced detection (QE-BD) scheme, which works even when using high-power optical pulses. We used 4-ps pulses to generate pulsed squeezed vacuum at a wavelength of 844 nm with a squeezing level of −3.28 ± 0.12 dB generated from a periodically-poled stoichiometric LiTaO3 waveguide. The squeezed vacuum was introduced to an SRS spectrometer employing a high-speed spectral scanner to acquire QE-SRS spectrum in the wavenumber range of 2000–2280 cm-1 within 50 ms. Using SRS pump pulses with an average power of 11.3 mW, we successfully obtained QE-SRS spectrum whose SNR was better than classical SRS with balanced-detection by 2.27 dB.
Stimulated Raman scattering (SRS) microscopy is a powerful technique that allows for the visualization of molecular vibrational images for label-free imaging, metabolic imaging, and supermultiplex imaging. However, its sensitivity is mainly limited by the shot noise of laser pulses, and long acquisition times are sometimes required to detect weak SRS signals hidden by the shot noise. To overcome this limitation, quantum-enhanced (QE-) SRS microscopy has been demonstrated, while the low optical power of squeezed pulses limits the sensitivity. In this work, we present QE-SRS microscopy using quantum-enhanced balanced detection (QE-BD) scheme, where the sensitivity of balanced detection SRS microscope is enhanced by injecting squeezed vacuum, allowing for QE-SRS imaging with high power SRS pump pulses (typically several tens of milliwatt), while balanced detection causes 3 dB drawback in the signal-to-noise ratio. We experimentally demonstrate QE-SRS imaging with 2.6 dB noise reduction compared with shot-noise-limited balanced detection SRS. We also demonstrate hyperspectral QE-SRS imaging by fast wavelength tuning of Stokes pulses. These results show the potential feasibility of high-power QE-SRS whose sensitivity is beyond that of classical shot-noise-limited SRS microscopes.
Squeezed light has been applied in stimulated Raman scattering (SRS) spectroscopy [1] and microscopy [2] to break the shot-noise-limited sensitivity. The sub-shot-noise sensitivity achieved in quantum-enhanced SRS (QE-SRS) systems is advantageous for uncovering weak signals and achieving faster imaging speed, which attract the attention of biomedical scientists. To break the sensitivity of state-of-the-art SRS microscopes, the applied squeezed light power in QE-SRS systems should be high enough (>10mW). Recently, high-power picosecond-pulsed QE-SRS spectroscopy [3] based on balanced detection scheme [4] was reported, and a sensitivity that is comparable to state-of-the-art SRS microscopes was firstly demonstrated in QE-SRS microscopy based on the same scheme [5]. An important issue of QE-SRS based on balanced detection is the 3-dB additional noise, so >3-dB squeezing level is required to demonstrate an absolute sensitivity improvement compared with state-of-the-art SRS microscopes in single-photodiode detection scheme. However, it's hard to generate highly squeezed pulsed light because of the difficulty in temporal and spatial mode matching between squeezed vacuum (SQV) and local oscillator (LO). Here, we introduce QE-SRS in dual-polarization scheme, which can be implemented in high-power regime but is free from the 3-dB signal-to-noise ratio drawback.