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arXiv:2002.04674 · Optica

Quantum-enhanced continuous-wave stimulated Raman scattering spectroscopy

Rayssa B. de Andrade, Hugo Kerdoncuff, Kirstine Berg‐Sørensen, Tobias Gehring, Mikael Lassen, Ulrik L. Andersen

Published Apr 13, 2020• 113 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1364/optica.386584

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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.
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.
Stimulated Raman scattering (SRS) microscopy is a powerful label-free imaging technique that probes the vibrational response of chemicals with high specificity and sensitivity. High-power, quantum-enhanced SRS microscopes have been recently demonstrated and applied to polymers and biological samples. Quantum correlations, in the form of squeezed light, enable the microscopes to operate below the shot noise limit, enhancing their performance without increasing the illumination intensity. This addresses the signal-to-noise ratio (SNR) and speed constraints introduced by photodamage in shot noise-limited microscopes. Previous microscopes have either used single-beam squeezing, but with insufficient brightness to reach the optimal ratio of pump-to-Stokes intensity for maximum SNR, or have used twin-beam squeezing and suffered a 3 dB noise penalty. Here we report a quantum-enhanced Raman microscope that uses a bright squeezed single-beam, enabling operation at the optimal efficiency of the SRS process. The increase in brightness leads to multimode effects that degrade the squeezing level, which we partially overcome using spatial filtering. We apply our quantum-enhanced SRS microscope to biological samples and demonstrate quantum-enhanced multispectral imaging of living cells. The imaging speed of 100×100 pixels in 18 seconds allows the dynamics of cell organelles to be resolved. The SNR achieved is compatible with video-rate imaging, with the quantum correlations yielding a 20% improvement in imaging speed compared to shot noise-limited operation.
Stimulated Raman scattering (SRS) microscopy has emerged as a powerful technique for probing the spatiotemporal dynamics of molecular bonds with exceptional sensitivity, resolution, and speed. However, classically, its performance remains fundamentally constrained by optical shot noise, which imposes a strict limit on detection sensitivity and speed. Here, we demonstrate a quantum-enhanced SRS microscopy platform that circumvents this barrier by harnessing amplitude-squeezed light. Specifically, we generate a Stokes beam with 5.2 dB5.2~\mathrm{dB}5.2 dB of amplitude squeezing using traveling-wave optical parametric amplification in second-order nonlinear waveguides, and combine it with a tunable coherent pump to access vibrational modes spanning from 100010001000 to 3100 cm−13100~\mathrm{cm}^{-1}3100 cm−1. Applied to quantum imaging of metabolites in biological tissue (pork muscle), our quantum-enhanced Raman microscope achieves an average noise suppression of 3.6 dB3.6~\mathrm{dB}3.6 dB and a 51%51\%51% enhancement in signal-to-noise ratio (SNR) -- to the best of our knowledge, the largest improvement reported to date in quantum-enhanced SRS microscopy of biological samples.
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.
Quantum-enhanced stimulated Raman scattering (QESRS) microscopy is expected to realize molecular vibrational imaging with sub-shot-noise sensitivity, so that weak signals buried in the laser shot noise can be uncovered. Nevertheless, the sensitivity of previous QESRS did not exceed that of state-of-the-art stimulated Raman scattering (SOA-SRS) microscopes mainly because of the low optical power (3 mW) of amplitude squeezed light [Nature594, 201 (2021)10.1038/s41586-021-03528-w]. Here, we present QESRS based on quantum-enhanced balanced detection (QE-BD). This method allows us to operate QESRS in a high-power regime (>30 mW) that is comparable to SOA-SRS microscopes, at the expense of 3 dB sensitivity drawback due to balanced detection. We demonstrate QESRS imaging with 2.89 dB noise reduction compared with classical balanced detection scheme. The present demonstration confirms that QESRS with QE-BD can work in the high-power regime, and paves the way for breaking the sensitivity of SOA-SRS microscopes.
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.