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arXiv:2409.05124 · Nanophotonics

Photon antibunching in single‐molecule vibrational sum‐frequency generation

Fatemeh Moradi Kalarde, Francesco Ciccarello, Carlos Sánchez Muñoz, Johannes Feist, Christophe Galland

Published Jan 1, 2025• 11 citations
arXivHTMLPDF

Abstract

Abstract Sum‐frequency generation (SFG) enables the coherent upconversion of electromagnetic signals and plays a significant role in mid‐infrared vibrational spectroscopy for molecular analysis. Recent research indicates that plasmonic nanocavities, which confine light to extremely small volumes, can facilitate the detection of vibrational SFG signals from individual molecules by leveraging surface‐enhanced Raman scattering combined with mid‐infrared laser excitation. In this article, we compute the degree of second order coherence ( g (2) (0)) of the upconverted mid‐infrared field under realistic parameters and accounting for the anharmonic potential that characterizes vibrational modes of individual molecules. On the one hand, we delineate the regime in which the device should operate in order to preserve the second‐order coherence of the mid‐infrared source, as required in quantum applications. On the other hand, we show that an anharmonic molecular potential can lead to antibunching of the upconverted photons under coherent, Poisson‐distributed mid‐infrared and visible drives. Our results therefore open a path toward bright and tunable source of indistinguishable single photons by leveraging “vibrational blockade” in a resonantly and parametrically driven molecule, without the need for strong light‐matter coupling.

DOI
10.1515/nanoph-2024-0469

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Abstract Sum‐frequency generation (SFG) enables the coherent upconversion of electromagnetic signals and plays a significant role in mid‐infrared vibrational spectroscopy for molecular analysis. Recent research indicates that plasmonic nanocavities, which confine light to extremely small volumes, can facilitate the detection of vibrational SFG signals from individual molecules by leveraging surface‐enhanced Raman scattering combined with mid‐infrared laser excitation. In this article, we compute the degree of second order coherence ( g (2) (0)) of the upconverted mid‐infrared field under realistic parameters and accounting for the anharmonic potential that characterizes vibrational modes of individual molecules. On the one hand, we delineate the regime in which the device should operate in order to preserve the second‐order coherence of the mid‐infrared source, as required in quantum applications. On the other hand, we show that an anharmonic molecular potential can lead to antibunching of the upconverted photons under coherent, Poisson‐distributed mid‐infrared and visible drives. Our results therefore open a path toward bright and tunable source of indistinguishable single photons by leveraging “vibrational blockade” in a resonantly and parametrically driven molecule, without the need for strong light‐matter coupling.
We identify enhanced strong coherent Surface-Enhanced Raman Scattering (SERS) interactions facilitated by hybridized metallo-dielectric cavities with a resonant fluorescent molecule. By illuminating a detuned electronic transition near saturation via a narrowband hybrid plasmonic mode, we observe enhanced splittings in the Stokes and anti-Stokes spectra, revealing strong optomechanical coupling and nonlinear vibrational modifications at intensities far below irreversible SERS damage thresholds. We compute the second-order photon correlations that allows identifying the nonclassical character associated to these strong optomechanical interactions and SERS configurations that enables them to optically characterize the anharmonic character of the vibrational modes.
Controlling bond breaking is a long-standing goal in molecular physics. Infrared nanocavities are currently being developed for reaching exotic coupling regimes of cavity QED with a few molecules, but it is not well understood how chemical reactions would proceed in such systems. To address this, we study infrared laser photodissociation of a single CS2_{2}2​ molecule with a stretching mode that strongly interacts with a resonant infrared vacuum, subject to a strong laser field that either resonantly drives the molecule at its fundamental vibration frequency or injects photons at the cavity resonance. We show that the intensities required for photodissociation are significantly lower inside the cavity than in free space, with a strong dependence on the type of driving condition. By directly injecting photons into the cavity, the molecule dissociates with two orders of magnitude less laser energy than by directly driving the vibrational mode. This photodissociation enhancement is a purely quantum mechanical effect that cannot be captured semi-classically. The intracavity ladder climbing dynamics is substantially modified relative to free space due to vacuum-induced admixing a large number of vibrational quantum numbers and the cavity field acting as a surrogate molecular mode that strongly interacts with the dissociative vibrational motion. Our work provides fundamental mechanistic understanding of chemical dynamics that can be used for designing new types of nanophotonics experiments that probe single-molecule chemistry.
Abstract Interactions between a single emitter and cavity provide the archetypical system for fundamental quantum electrodynamics. Here we show that a single molecule of Atto647 aligned using DNA origami interacts coherently with a sub-wavelength plasmonic nanocavity, approaching the cooperative regime even at room temperature. Power-dependent pulsed excitation reveals Rabi oscillations, arising from the coupling of the oscillating electric field between the ground and excited states. The observed single-molecule fluorescent emission is split into two modes resulting from anti-crossing with the plasmonic mode, indicating the molecule is strongly coupled to the cavity. The second-order correlation function of the photon emission statistics is found to be pump wavelength dependent, varying from g (2) (0) = 0.4 to 1.45, highlighting the influence of vibrational relaxation on the Jaynes-Cummings ladder. Our results show that cavity quantum electrodynamic effects can be observed in molecular systems at ambient conditions, opening significant potential for device applications.
Molecular cavity optomechanical systems have recently emerged as a promising platform for enhancing infrared detection sensitivity, owing to their ability to up-convert low-frequency infrared (IR) photons to visible frequency range. Generally, under red-detuned pumping in such systems, the ideal conversion efficiency of the IR signal approaches 1. To overcome this efficiency constraint, we propose a scheme that incorporates gain into the infrared cavity of a molecular cavity optomechanical system comprising two cavities and an ensemble of N molecules. The upconversion process, which relies on IR absorption and Raman scattering associated with specific vibrational modes, is significantly amplified by the incorporation of gain under the red-detuned conditions. Moreover, our analysis demonstrates that the added noise is maintained near 0.5.
ABSTRACT The mid‐infrared (MIR) spectral region, spanning from 2‐ to 25‐, provides access to rich chemical information through the vibrational modes of functional groups, lipids, and other complex molecules. However, conventional imaging in this range remains limited by challenges in generating MIR light and by the lack of mature detection technologies. Quantum imaging with undetected light (QIUL) offers an alternative approach by using photon‐pair sources in which the sample is illuminated with MIR light while detection and image reconstruction are carried out at visible wavelengths. We report the first QIUL technique that achieves diffraction‐limited resolution without relying on spatial correlations of the photon‐pair, thereby overcoming a fundamental limitation of all previous implementations. This work establishes induced coherence without induced emission as the essential physical resource underlying QIUL and redefines its operational boundary, extending its applicability to regimes where spatial correlations are weak or absent. Our results represent a conceptual advance in quantum imaging and open the way toward practical and cost‐effective MIR microscopy based on visible detection.
The development of accurate tools to characterize Intramolecular Vibrational Redistribution (IVR) is of major interest in chemistry. In this context, surface-enhanced vibrational spectroscopies stand up as well-established techniques to study molecular vibrational lines and populations with a sensitivity that can reach the single-molecule level. However, to date, this possibility has not been fully developed to address IVR. Here, we establish a quantum mechanical framework based on molecular optomechanics that accounts for IVR, and we adopt it to analyze strategies to optimize IVR characterization by surface-enhanced vibrational spectroscopy. In particular, we model two different pump-and-probe configurations where the vibrational pumping is provided either by infrared laser illumination or by Stokes scattering processes in surface-enhanced Raman spectroscopy (SERS). We show for the two pumping configurations the existence of clear signatures on the anti-Stokes SERS spectra of population transfer between coupled vibrational modes in a molecule. Our calculations adopt realistic molecular and SERS parameters, suggesting that these signatures of IVR could be accessible at the single-molecule level within realistic experimental platforms.