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Optical and Quantum Sensing and Precision Metrology II

Characterization of second-harmonic generation in silver nanoparticles for spontaneous parametric down-conversion

Ariel Ashkenazy, Eliahu Cohen, Dror Fixler

Published Mar 3, 2022• 1 citation
Publisher

Abstract

Energy-time entangled photon pairs (EPPs), which are at the heart of numerous quantum light applications, are commonly generated in nonlinear crystals. Some highly sensitive quantum applications require the use of ultra-broadband entangled photons that cannot be generated in nonlinear crystals due to phase-matching requirements. Here, we investigate the possibility of using metallic nanoparticles (MNPs) as a means for generating entangled photons through spontaneous parametric down-conversion (SPDC). MNPs are known for their strong light-matter coupling at their localized surface plasmon resonance, and since the propagation length through them is negligible relative to optical wavelengths, we consider them as excellent candidates to serve as non-phase matched sources of ultra-broadband entangled photons. To that end, we report experimental results of classical-light second-harmonic generation in silver nanotriangles and nanocubes embedded in polyvinyl alcohol. Based on the results of our experiments, performed using the reference-free hyper-Rayleigh scattering method [A. Ashkenazy et al., J. Phys. B 145401, 2019], we present an estimation of the characteristics of SPDC in MNPs. We show that, despite of the metals' centrosymmetric structure, MNPs exhibit second order nonlinearity that is mainly of an electric dipole nature, and so they are suitable for SPDC. Moreover, we show that localized surface-plasmon resonance can play a significant role in enhancing the generated EPPs flux. Finally, we compare the SPDC capabilities of MNPs to that of commonly used nonlinear crystals and show that the expected EPPs flux from MNPs is weaker but the EPPs have very large bandwidth, which could be helpful for advanced quantum sensing, spectroscopy and communication applications.

DOI
10.1117/12.2618308

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Energy-time entangled photon pairs (EPPs), which are at the heart of numerous quantum light applications, are commonly generated in nonlinear crystals. Some highly sensitive quantum applications require the use of ultra-broadband entangled photons that cannot be generated in nonlinear crystals due to phase-matching requirements. Here, we investigate the possibility of using metallic nanoparticles (MNPs) as a means for generating entangled photons through spontaneous parametric down-conversion (SPDC). MNPs are known for their strong light-matter coupling at their localized surface plasmon resonance, and since the propagation length through them is negligible relative to optical wavelengths, we consider them as excellent candidates to serve as non-phase matched sources of ultra-broadband entangled photons. To that end, we report experimental results of classical-light second-harmonic generation in silver nanotriangles and nanocubes embedded in polyvinyl alcohol. Based on the results of our experiments, performed using the reference-free hyper-Rayleigh scattering method [A. Ashkenazy et al., J. Phys. B 145401, 2019], we present an estimation of the characteristics of SPDC in MNPs. We show that, despite of the metals' centrosymmetric structure, MNPs exhibit second order nonlinearity that is mainly of an electric dipole nature, and so they are suitable for SPDC. Moreover, we show that localized surface-plasmon resonance can play a significant role in enhancing the generated EPPs flux. Finally, we compare the SPDC capabilities of MNPs to that of commonly used nonlinear crystals and show that the expected EPPs flux from MNPs is weaker but the EPPs have very large bandwidth, which could be helpful for advanced quantum sensing, spectroscopy and communication applications.
Spontaneous Parametric Down-Conversion (SPDC) is a quantum optical process where a photon spontaneously splits into a pair of lower energy entangled single-photons. Here, we demonstrate that the quantum effect of SPDC can be substantially boosted at room temperature by utilizing nonlinear plasmonic metasurfaces. The presented plasmonic metasurface design is composed of silver nanostripes grown on a bulk lithium niobate (LiNbO3) crystal terminated by a silver substrate. The quantum-classical correspondence principle is used to compute the SPDC entangled photon pair generation rate by the classical sum frequency generation (SFG) nonlinear optical process. The generated signal and idler frequencies are matched to the metasurface fundamental and higher order resonances leading to substantially boosted SFG efficiency and, consequently, significantly enhanced SPDC generation rate. The quantum SPDC process is at the heart of many quantum optical emerging apparatuses, since efficient single-photon generation at room temperature is required in a plethora of applications in quantum communications, cryptography, and sensing.
In the present paper, we theoretically consider the generation of entangled photon pairs in the process of spontaneous parametric downconversion of light by a nonlinear dielectric nanoparticle. This nanoparticle is the only source of nonlinearity in the considered model. We assume that the particle is small compared to the wavelength of light so that the process is non-resonant. An intuitive theoretical description is developed for the case when the nonlinear susceptibility tensor of the material of a nanoparticle has a form of the Levi-Civita symbol. In this case, a two-photon amplitude which characterizes a spatial distribution of entangled photon pairs can be interpreted as a result of interference between two induced dipoles, both being orthogonal to the direction of polarization of the pump photon. A generation of entangled surface plasmon-polaritons by a dielectric nanoparticle located near the metal-dielectric interface is also considered.
In the last few years, materials with strong second-order optical nonlinearity such as gallium arsenide, barium titanate and transition metal dichalcogenides have attracted significant attention, because they for the first time allowed efficient nonlinear optical interactions on the sub-micron scales. One of such nonlinear optical interactions - spontaneous parametric down-conversion (SPDC) - allows the generation of pairs of correlated photons and can enable photon entanglement. This is the foundation of many quantum optical applications ranging from secure communication to ultrafast quantum computing. The key challenges in this field are efficiency and the generation of on-demand quantum states.
The promises of quantum computing is to enable faster and more powerful computation than would ever be possible with a classical processor based on binary logic. Quantum linear optics is one of many promising platforms to build a universal quantum computers by leveraging the interaction between indistinguishable photons. A common technique to prepare such states is photon heralding, where many identical photons can be generated and used to encode and process information. Photon pairs are typically generated from a second-order nonlinear interaction known as spontaneous parametric downconversion (SPDC). In all existing platforms, materials are used as both a source of optical nonlinearity, and a phase-matching medium, often resulting in narrow-band and non-reconfigurable operation. 2D materials promise to change this, thanks to their high nonlinearity, inherent broadband phase-matching, and highly configurable electro-optical properties. To date SPDC has only been reported in structures with many millions of atoms, stimulating experimental efforts to validate its scaling laws in structures only a few atoms thick. In this thesis we investigate SPDC in group IV transition-metal dichalcogenides (TMDCs), and describe efforts towards the experimental observation of non-resonant SPDC from a diffraction-limited area. Because of the intimate connection between the classical second-harmonic generation (SHG) and the quantum SPDC, the efficiency of one process provides insights on the other. This guides the design of single-photon coincidence measurements required to demonstrate the strong temporal correlations of these entangled states. Measurements are hindered by the presence of a broadband, temporally uncorrelated background, attributed to photoluminescence. This work improves the understanding of the nonlinear quantum optical potential of these crystals, and provides a performance benchmark for these ultra-thin materials.
The Spontaneous Parametric Down-Conversion (SPDC) process is a fundamental method for the generation of entangled and correlated single-photon pairs. This research investigates the use of a plasmonic metasurface consisting of metallic nanostripes deposited on a bulk lithium niobate (LiNbO3) crystal to significantly increase efficiency of photon pair generation. The geometric parameters of the metasurface features can be set to create resonances with the signal and idler frequencies, boosting the electric field in the LiNbO3crystal and enhancing the SPDC process. The photon pair generation rate is found by invoking the quantum-classical correspondence principle and computing the rate for the classical sum frequency generation (SFG) process. Efficient room temperature SPDC single-photon nanophotonic sources show promise for developing free space quantum optical communications and have multiple applications across the field of quantum optics.
) crystal to realize a new scalable, ultrathin, and efficient SPDC source. By coinciding fundamental and higher order resonances of the metasurface with the generated signal and idler frequencies, respectively, the electric field in the nonlinear media is significantly boosted. This leads to a substantial enhancement in the SPDC process which, subsequently, by using the quantum-classical correspondence principle, translates to very high photon-pair generation rates. The emitted radiation is highly directional and perpendicular to the metasurface in contrast to relevant dielectric structures. The incorporation of circular polarized excitation further increases the photon-pair generation efficiency. The presented work will lead to the design of new efficient ultrathin SPDC single-photon nanophotonic sources working at room temperature that are expected to be critical components in free-space quantum optical communications. In a more general context, our findings can have various applications in the emerging field of quantum plasmonics.