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arXiv:2403.08588 · Physical Review A

Quantum plasmonics model of refractive index sensing using photon correlations

Luke C. Ugwuoke, Tjaart P. J. Krüger, Mark Tame

Published Oct 10, 2024• 6 citations
arXivHTMLPDF

Abstract

The interaction between the electric dipole moments of a quantum emitter and a metal nanoparticle gives rise to unique optical properties, such as interference-induced photon correlations, that could be useful for enhanced intensity-based sensing. Using the quantum theory of photodetection, we propose a nanosensor system comprising a quantum emitter and a metal nanoparticle that explores the possibility of utilizing higher-order photon correlations for refractive index sensing. Both the refractive index sensitivity and resolution of the nanosensor, whose scattering spectrum lies within the visible region, are predicted. The sensor is supported by a substrate and driven weakly by a coherent field. By calculating the mean photocount and its second factorial moment resulting from the scattered field of the system, the sensing performance of the intensity and intensity-intensity correlation ${g}^{(2)}(0)$ are compared at optimal driving wavelengths. The mean photocount was found to be inherently low, inhibiting the role of interference-induced photon antibunching in minimizing the sensor's intensity shot noise. However, a regime in which the noise could be reduced below the shot noise limit is identified, leading to a quantum enhancement in the sensing performance.

DOI
10.1103/physreva.110.043506

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The interaction between the electric dipole moments of a quantum emitter and a metal nanoparticle gives rise to unique optical properties, such as interference-induced photon correlations, that could be useful for enhanced intensity-based sensing. Using the quantum theory of photodetection, we propose a nanosensor system comprising a quantum emitter and a metal nanoparticle that explores the possibility of utilizing higher-order photon correlations for refractive index sensing. Both the refractive index sensitivity and resolution of the nanosensor, whose scattering spectrum lies within the visible region, are predicted. The sensor is supported by a substrate and driven weakly by a coherent field. By calculating the mean photocount and its second factorial moment resulting from the scattered field of the system, the sensing performance of the intensity and intensity-intensity correlation g(2)(0){g}^{(2)}(0)g(2)(0) are compared at optimal driving wavelengths. The mean photocount was found to be inherently low, inhibiting the role of interference-induced photon antibunching in minimizing the sensor's intensity shot noise. However, a regime in which the noise could be reduced below the shot noise limit is identified, leading to a quantum enhancement in the sensing performance.
In the field of optics and photonics, plasmonic sensors have garnered considerable interest due to their outstanding sensitivity and impressive capacity to control light at the nanoscale. These sensors enable the integration of biochemical sensing platforms by exploiting the combination of enhanced and localized electromagnetic fields at the metal–dielectric interface, and they operate effectively below the diffraction limit. The performance of conventional plasmonic sensors, despite their practical development and commercial success, remains inherently constrained by classical noise constraints, owing to the quantum fluctuations of light, known as the shot-noise limit. Recent advances have introduced quantum optical resources such as entangled and squeezed light states to overcome inherent classical restrictions, thus giving rise to the quantum plasmonic sensing field. This interdisciplinary field combines the ability to enhance electric field capabilities with the precision of quantum metrology, enabling ultrasensitive chemical and biological detection that surpasses classical precision thresholds. In the last decades, theoretical models and experimental demonstrations have greatly advanced the development of quantum plasmonic sensors, improving our comprehension of quantum light–matter interactions and facilitating novel nanoscale sensing designs. This chapter provides a comprehensive overview of the fundamental principles, latest developments, and major concepts that have driven progress in quantum plasmonic sensing and shows that quantum components are being integrated into plasmonic systems, thereby transforming optical sensing to the quantum level.
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Using a scattered field interference mechanism, we theoretically demonstrate the quantum statistics control with a hybrid system comprised of a quantum emitter and a plasmonic multimode nanocavity. Enhanced through multimode interactions, destructive interference between scattered fields from the emitter and nanocavity is able to change the photon statistics from bunching to antibunching. This transition cannot be explained by treating the plasmonic nanocavity in the dipole approximation. In some specific regions, an effective single-mode model, which is equivalent to the multimode model, is derived by simply shifting the transition frequency and modifying decay rates of the nearby quantum emitter. Superior to the closed optical microcavity, this hybrid system can be used to control the photon statistics without the need for strong coupling, and may find applications in nanoscale refractive index sensing.
Quantum sensing is an emerging field of quantum optics that seeks to take advantage of quantum correlations available in quantum states of light to enable sensitivities beyond the fundamental classical limits. \nThe sensitivity of measurements and sensing apparatus when using classical states of light is limited to the shot-noise limit (SNL), which is achieved with coherent states of light. \n \nTwo-mode squeezed states of light (twin beams) have quantum correlations both in time and space, leading to temporal and spatial squeezing properties. \nSeveral applications can benefit from such noise reduction to enable new approaches, such as quantum-enhanced interferometry, quantum imaging, and quantum sensing. \nThe emergence of quantum technologies has been referred to as the second quantum revolution. \nFor metrology and sensing applications, in particular, it has led to new state-of-the-art sensitivity limits. \n \nIn this thesis, we discuss the implementation of quantum sensing based on squeezed states of light and plasmonic sensors as a platform for the demonstration of real-life quantum sensing. \nWe present a quantum-enhanced plasmonic sensing setup that can detect changes in the refractive index of air beyond the SNL. \nFurthermore, we generalize such experimental apparatus to probe an array of sensors using the quantum correlations present in different spatial locations to demonstrate a parallel quantum-enhanced plasmonic sensing scheme that can simultaneously detect changes in the refractive index of air in multiple locations with a single probing beam. \nThese results prove the applicability of twin beams for real-life applications based on plasmonic sensors. \nThe spatially resolved sensing scheme can be extended to pixel-size sensing of multiple sensors for multi-parameter estimation and detection applications to reach more complex sensing architectures.
Photonic sensors have many applications in a range of physical settings, from measuring mechanical pressure in manufacturing to detecting protein concentration in biomedical samples. A variety of sensing approaches exist, and plasmonic systems in particular have received much attention due to their ability to confine light below the diffraction limit, greatly enhancing sensitivity. Recently, quantum techniques have been identified that can outperform classical sensing methods and achieve sensitivity below the so-called shot-noise limit. Despite this significant potential, the use of definite photon number states in lossy plasmonic systems for further improving sensing capabilities is not well studied. Here, we investigate the sensing performance of a plasmonic interferometer that simultaneously exploits the quantum nature of light and its electromagnetic field confinement. We show that, despite the presence of loss, specialized quantum resources can provide improved sensitivity and resolution beyond the shot-noise limit within a compact plasmonic device operating below the diffraction limit.
Abstract We propose a quantum plasmonic sensor using Hong–Ou–Mandel (HOM) interferometry that measures the refractive index of an analyte, embedded in a plasmonic beam splitter composed of a dual-Kretschmann configuration, which serves as a frustrated total internal reflection beamsplitter (BS). The sensing performance of the HOM interferometry, combined with single-photon detectors, is evaluated through Fisher information for estimation of the refractive index of the analyte. This is subsequently compared with the classical benchmark that considers the injection of a coherent state of light into the plasmonic BS. By varying the wavelength of the single photons and the refractive index of the analyte, we identify a wide range where a 50% quantum enhancement is achieved and discuss the observed behaviors in comparison with the classical benchmark. We expect this study to provide a useful insight into the advancement of quantum-enhanced sensing technologies, with direct implications for a wide range of nanophotonic BS structures.