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Optical and Quantum Sensing and Precision Metrology II

Enhancing stimulated Raman spectroscopy using Kerr squeezing

Nikolay A. Kalinin, Kilian Scheffter, Hannah Gallop, Hanieh Fattahi, Mehdi Alizadeh, Adrian F. Pegoraro, Albert Stolow, Luis Lorenzo Sánchez-Soto, A. Andrianov, Gerd Leuchs

Published Mar 5, 2026• 0 citations
Publisher

Abstract

In many state-of-the-art stimulated Raman spectroscopy (SRS) experiments, shot noise is the dominating noise source. We apply polarization-squeezed light generated in a silica fiber via the Kerr effect for SRS. The non-classical state of light allows for lowering the noise power below the shot noise limit, which enhances the signal-to-noise ratio (SNR). In our experiment, we achieved 3.0 dB SNR improvement when compared to coherent state SRS. We additionally investigate spectral focusing of squeezed pulses to improve SRS resolution without loss in SNR.

DOI
10.1117/12.3089893

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In many state-of-the-art stimulated Raman spectroscopy (SRS) experiments, shot noise is the dominating noise source. We apply polarization-squeezed light generated in a silica fiber via the Kerr effect for SRS. The non-classical state of light allows for lowering the noise power below the shot noise limit, which enhances the signal-to-noise ratio (SNR). In our experiment, we achieved 3.0 dB SNR improvement when compared to coherent state SRS. We additionally investigate spectral focusing of squeezed pulses to improve SRS resolution without loss in SNR.
Stimulated Raman spectroscopy has become a powerful tool to study the spatiodynamics of molecular bonds with high sensitivity, resolution, and speed. However, the sensitivity and speed of state-of-the-art stimulated Raman scattering spectroscopy are currently limited by the shot-noise of the light beam probing the Raman process. Here, we demonstrate in a proof-of-principle experiment an enhancement of the sensitivity of continuous-wave stimulated Raman spectroscopy by reducing the quantum noise of the probing light below the shot-noise limit by means of amplitude squeezed states of light. Probing polymer samples with Raman shifts around 2950 c m − 1 with squeezed states, we demonstrate a quantum enhancement of the stimulated Raman signal-to-noise ratio (SNR) of 3.60 dB relative to the shot-noise limited SNR. Our proof-of-concept demonstration of quantum-enhanced continuous-wave Raman spectroscopy paves the way for more elaborate demonstrations using state-of-the-art stimulated Raman scattering microscopes, and thus constitutes the very first step towards a new generation of Raman microscopes, where weak Raman transitions can be imaged without the use of markers or an increase in the total optical power.
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.
Abstract The sensitivity of classical Raman spectroscopy methods, such as coherent anti-stokes Raman spectroscopy (CARS) or stimulated Raman spectroscopy (SRS), is ultimately limited by shot-noise from the stimulating fields. We present the complete theoretical analysis of a squeezing-enhanced version of Raman spectroscopy that overcomes the shot-noise limit of sensitivity with enhancement of the Raman signal and inherent background suppression, while remaining fully compatible with standard Raman spectroscopy methods. By incorporating the Raman sample between two phase-sensitive parametric amplifiers that squeeze the light along orthogonal quadrature axes, the typical intensity measurement of the Raman response is converted into a quantum-limited, super-sensitive estimation of phase. The resonant Raman response in the sample induces a phase shift to signal-idler frequency-pairs within the fingerprint spectrum of the molecule, resulting in amplification of the resonant Raman signal by the squeezing factor of the parametric amplifiers, whereas the non-resonant background is annihilated by destructive interference. Seeding the interferometer with classical coherent light stimulates the Raman signal further without increasing the background, effectively forming squeezing-enhanced versions of CARS and SRS, where the quantum enhancement is achieved on top of the classical stimulation.
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 spectroscopy has become a powerful tool to study the spatio-dynamics of molecular bonds with high sensitivity, resolution, and speed. However, the sensitivity and speed of stimulated Raman spectroscopy are ultimately limited by the shot-noise of the light beam probing the Raman process. Here, we demonstrate an enhancement of the sensitivity of stimulated Raman spectroscopy by reducing the noise below the shot-noise limit by means of squeezed states of light. Our demonstration constitutes the first step towards a new generation of quantum-enhanced Raman microscopes.
Quantum-enhanced stimulated Raman scattering (QE-SRS) becomes a powerful method for high-sensitivity molecular vibrational spectroscopy and imaging by breaking the shot-noise-limited sensitivity of classical SRS. However, it’s still challenging to achieve quantum-enhancement with typical pulse power (<10-mW) for SRS. Although amplitude-squeezing technique was utilized in the previous QE-SRS, it limits the available optical power by its deamplification or optical loss at asymmetric beamsplitter. To circumvent this problem, we propose and demonstrate SRS spectroscopy with quantum-enhanced balanced detection (QE-BD) scheme, where squeezed vacuum (SQV) is combined with a strong local oscillator (LO) to enhance the sensitivity of SRS. We generated SQV at the central wavelength of 844-nm, using a periodically-poled stoichiometric LiTaO3 waveguide. Picosecond LO light is modulated by a spatial light modulator to achieve spatial matching with SQV , and resultant high squeezing level. At each photodiode, LO has ~10-mW optical power, and –3.1-dB squeezing level was observed. To obtain the QE-SRS spectrum of dimethyl sulfoxide-d6 (d-DMSO), we scanned the wavelength of Stokes laser by a high-speed spectral scanner composed of a grating and galvanometric scanner . The QE-SRS spectrum in the wavenumber range of 1950-2300-cm-1 was acquired with 2.1-dB signal-to-noise ratio (SNR) improvement compared with classical shot-noise-limited SRS under balanced-detection. The proposed QE-BD SRS paves the way for QE-SRS with high-power pulses, and shows a potential to exceed the SNR of state-of-the-art classical SRS system without quantum enhancement