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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Spontaneous Parametric Down-Conversion in Transition Metal Dichalcogenides

Maximilian A. Weissflog, Anna N. Fedotova, Yilin Tang, Benjamin Laudert, Fatemeh Alsadat Abtahi, Sai Shradha, Saniya Shinde, Sina Saravi, Isabelle Staude, Thomas Pertsch, Frank Setzpfandt, Yuerui Lu, Falk X. Eilenberger

Published Jun 26, 2023• 0 citations
Publisher

Abstract

Transition metal dichalcogenides (TMDCs) thinned-down to the monolayer level allow the realisation of very broad-band, phase-matching free nonlinear devices. A particularly interesting application would be the generation of entangled photon-pairs, which form the backbone of many recent implementations in quantum optics. However, the limited absolute efficiency of monolayers has so far only allowed seeded parametric amplification [1], experimental evidence for the generation of entangled photon-pairs through spontaneous parametric down-conversion (SPDC) remained elusive up to now [2]. A promising route for the realization of broad-band photon-pair sources in van der Waals materials are moderately thicker, yet still phase-matching free layer stacks with preserved noncentrosymmetry [3], [4]. Here, we close the gap of a missing entangled photon-pair source in TMDCs and demonstrate, for the first time to our knowledge, pair generation via SPDC in 3R-MoS2.

DOI
10.1109/cleo/europe-eqec57999.2023.10232137

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 26, 2023•0 citations
Transition metal dichalcogenides (TMDCs) thinned-down to the monolayer level allow the realisation of very broad-band, phase-matching free nonlinear devices. A particularly interesting application would be the generation of entangled photon-pairs, which form the backbone of many recent implementations in quantum optics. However, the limited absolute efficiency of monolayers has so far only allowed seeded parametric amplification [1], experimental evidence for the generation of entangled photon-pairs through spontaneous parametric down-conversion (SPDC) remained elusive up to now [2]. A promising route for the realization of broad-band photon-pair sources in van der Waals materials are moderately thicker, yet still phase-matching free layer stacks with preserved noncentrosymmetry [3], [4]. Here, we close the gap of a missing entangled photon-pair source in TMDCs and demonstrate, for the first time to our knowledge, pair generation via SPDC in 3R-MoS2.
We have theoretically investigated photon-pair generation from transition-metal dichalcogenide (TMDC) monolayers embedded in photonic crystals (PCs) by a rigorous quantum model of spontaneous parametric downconversion. The mean number of output photon pairs and the signal-field energy spectrum have been elaborated. Two different structures are studied: one is a monolayer WS2 embedded in the defective PC, and the other is a stack of WS2 layers in the dielectric multilayer structure. It is clearly shown that an obvious improvement of the generation rate of the correlated photon pairs from the monolayer WS2 can be realized in these two structures, and the enhancement can reach as much as 14 orders of magnitude. Such an effective improvement utilizes the improved field localization at the defect state or the band-edge state, the propagation of photons in the PCs, and coherent superposition; similar phenomena can occur for other TMDC monolayers. These phenomena are very beneficial for the design of optical devices.
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.
Efficient generation of entangled photons typically relies on spontaneous parametric down-conversion (SPDC) in phase-matched macroscopic nonlinear media. However, generating entanglement under phase-matching constraints requires additional bulk optics or interferometers. In contrast, ultrathin van der Waals semiconductors - such as transition metal dichalcogenides (TMDs) - exhibit strong enough optical nonlinearities for SPDC to be observed from subwavelength-thick media, thereby bypassing conventional phase-matching constraints. In this microscopic domain, the intrinsic crystal symmetry governs the nonlinear optical response, enabling the native generation of polarization-entangled photon pairs. However, generating these states efficiently has been fundamentally restricted by the material's coherence length (LcL_cLc​), which limits the attainable conversion efficiency. Here, we investigate periodically-poled TMDs (PPTMDs) designed to scale up this interaction via quasi-phase matching. We demonstrate that mechanically flipping the sign of the nonlinearity at precise intervals of LcL_cLc​ introduces quasi-phase matching, that scales the pair-production rate while preserving the pristine, symmetry-generated polarization entanglement, with fidelities exceeding 99%. Backed by a rigorous theoretical model, our work clarifies the interplay between crystal symmetry and propagation effects in thin nonlinear media, providing a new avenue for engineering quantum light in nanophotonic systems.
Abstract We present a detailed study of the second order nonlinearity of 2D (mono-atomic layer) dichalcogenide MoS2, both in the visible and in the IR regime, and test its potential for spontaneous parametric down-conversion (SPDC), the amplification of vacuum fluctuations mediated by optical nonlinearity. We develop a model of SPDC from a deeply subwavelength nonlinear medium, where phase matching conditions are completely relaxed, and make predictions about the rate of emitted photons, their momentum, polarisation and spectrum. We show that detection in the visible spectral region is hindered by the strong photoluminescence background. Moving to the IR regime we observe indications of SPDC by performing polarization, power dependence and lifetime measurements around 1560 nm. We show that the signal from a single monolayer is qualitatively different from that generated by multi-layer MoS2. Finally, we characterize the latter as a new kind of photo-luminescence emission which is enhanced at the edges of multi-layer MoS2.
The generation of entangled photons by spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM) attracted enormous interest in the field of quantum optics. Depending on applications, entangled photon sources prioritize either bandwidth (e.g. for quantum imaging) or brightness (e.g. for quantum key distribution). Layered materials offer unique advantages for both. They have already been used to realize thinnest SPDC sources [1], which, like other layered materials such as transition-metal dichalcogenides (TMDs) [2], offer nearly unlimited bandwidth thanks to relaxed phase-matching constraints. Further, their easy integration on photonic platforms is promising for bright on-chip entangled photon sources. In this context, SPDC-based solutions are limited by phase-matching, whereas SFWM would be an almost phase-matching-free process, as pump, idler, and signal photons can be generated at similar wavelengths, thus propagating at the same group velocity in integrated devices. Moreover, exploiting excitonic resonances could enhance FWM even more. However, to date, experiments with resonant FWM in TMDs have been limited to signals in the visible [3], which is unsuitable for integrated photonics and telecom systems due to reabsorption during propagation in the photonic device.