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Physical Review Applied

Spectral-focusing coherent anti-Stokes Raman scattering driven by bright correlated twin beams

Yujiro Eto, Mutsuo Nuriya, Hideaki Kano

Published May 16, 2025• 2 citations

Abstract

An understanding of the interaction between bright correlated twin beams and matter is important for expanding the applications of optical quantum technology. In particular, it is worthwhile to clarify the advantages of twin-beam correlation in spectroscopic applications, since twin-beam sources that use parametric down-conversion have a wide range of wavelength tunability. Here, we theoretically and experimentally demonstrate that the spectral-focusing technique used in coherent anti-Stokes Raman scattering (CARS) can be implemented by twin beams without the use of ultrashort-pulse lasers. In typical spectral focusing using ultrashort pulses, spectral resolution is improved by narrowing the instantaneous frequency width due to frequency chirping. Similarly, we show theoretically that the formation of a chirp structure within the temporal correlation of twin beams produces an effect similar to typical spectral focusing. To experimentally demonstrate twin-beam spectral focusing, CARS spectroscopy of polyethylene is performed when the time delay between the chirped twin beams is set within the correlation width or it is set outside the correlation width. We observe that the spectral resolution is improved when the correlation is effective. By comparing with theoretical simulations, we confirm that the improvement is caused by frequency chirping.

DOI: 10.1103/7pvj-3dqt

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We theoretically investigate coherent anti-Stokes Raman scattering (CARS) driven by bright twin beams with sum-frequency generation (SFG) readout. Expressing the SFG output in terms of quantum-field correlation functions yields multiple correlation structures with distinct properties associated with different interaction pathways on the bra and ket sides. Among them, frequency correlations between the CARS probe and SFG gating fields enable the two fields to act collectively as an effective narrowband probe, avoiding probe-induced spectral convolution. When this behavior appears on either the bra or ket side, the other retains Raman relaxation information as if probed by an ultrashort pulse.

Abstract A chirped-pulse quantum control scheme applicable to coherent anti-Stokes Raman scattering spectroscopy, named as C-CARS, is presented aimed at maximizing the vibrational coherence in molecules. It implies chirping of three incoming pulses in the four-wave mixing process of CARS, the pump, the Stokes and the probe, to fulfill the conditions of adiabatic passage. The scheme is derived in the framework of rotating wave approximation and adiabatic elimination of excited state manifold simplifying the four-level model system into a ‘super-effective’ two level system. We demonstrate that the selectivity of excitation of vibrational degrees of freedom can be controlled by carefully choosing the spectral chirp rate of the pulses. The robustness, spectral selectivity and adiabatic nature of this method are advantageous for improving the existing methods of CARS spectroscopy for sensing, imaging and detection.

The resolution of spectroscopic measurements is fundamentally limited by the convolution between a system's intrinsic response and the instrument response function, creating an ill-posed inverse problem. While entangled photons can theoretically bypass these limits, low photon flux has hindered their experimental realization. Here, we propose quantum cross-correlation spectroscopy (QCCS), utilizing macroscopic twin beams (signal and idler beams) to physically circumvent this convolution limit in coherent anti-Stokes Raman scattering. Employing the signal beam as a broadband probe, QCCS achieves all-optical deconvolution through a correlation-enhanced sum-frequency generation channel between the convoluted anti-Stokes and idler beams. By subtracting the uncorrelated background using a 500-femtosecond temporal window, we successfully retrieve narrow Raman bands (8 cm^{-1} and 15 cm^{-1}) completely obscured by a 75 cm^{-1} probe bandwidth without numerical deconvolution or prior knowledge. Our results establish macroscopic quantum correlations as a robust resource for high-fidelity spectroscopy, paving the way for ultrashort-pulse-free operando studies under natural, incoherent excitation conditions.

A chirped-pulse quantum control scheme applicable to Coherent Anti-Stokes Raman Scattering spectroscopy, named as C-CARS, is presented aimed at maximizing the vibrational coherence in molecules. It implies chirping of three incoming pulses in the four-wave mixing process of CARS, the pump, the Stokes and the probe, to fulfil the conditions of adiabatic passage. The scheme is derived in the framework of rotating wave approximation and adiabatic elimination of excited state manifold simplifying the four-level model system into a ``super-effective'' two level system. The robustness, spectral selectivity and adiabatic nature of this method are helpful in improving the existing methods of CARS spectroscopy for sensing, imaging and detection. We also show that the selectivity of excitation of vibrational degrees of freedom can be controlled by carefully choosing the spectral chirp rate of the pulses.

We present a theoretical investigation of coherent anti-Stokes Raman scattering (CARS) that is modulated by periodically depleting the ground-state population through Rabi oscillations driven by an additional control laser. We find that such a process generates optical sidebands in the CARS spectrum and that the frequency of the sidebands depends on the intensity of the control laser light field. We show that analyzing the sideband frequency upon scanning the beams across the sample allows one to spatially resolve emitter positions where a spatial resolution of

65

nm, which is well below the diffraction limit, can be obtained.

A semiclassical theory that describes the generation of a coherent anti-Stokes Raman scattering (CARS) signal is presented that maximizes vibrational coherence in a mode predetermined by the pump, the Stokes, and the probe chirped pulse trains and takes into account the field propagation effects in a cloud of molecules. The buildup of the anti-Stokes signal, which may be used as a molecular signature in the backward CARS signal, is demonstrated numerically. The theory is based on the solution of the coupled Maxwell's and Liouville--von Neumann equations and focuses on the quantum effects induced in the target molecules by the control pulse trains. A deep convolutional neural network technique is implemented to evaluate time-dependent phase characteristics of the control fields. The effect of decoherence induced by spontaneous decay and collisional dephasing is examined.