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.
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.
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.
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.
Miniaturization of nonlinear optical elements is one of the major trends of modern photonics. In particular, in the ‘flatoptics’ geometry, the sample is ultrathin along the light propagation direction. For nonlinear optical effects in such samples, phase matching is satisfied automatically, which allows for the use of materials with giant nonlinear susceptibilities. The tiny thickness reduces the efficiency but it can be compensated for by using geometric and material resonances. Recently also spontaneous parametric down-conversion (SPDC), leading to the generation of entangled photons, has been implemented in ultrasmall sources: subwavelength layers, metasurfaces, even nanoantennas. The lifted constraint of phase matching gives to SPDC even more freedom than to classical frequency conversion effects. The reason is that SPDC, as a spontaneous effect, is stimulated by quantum vacuum fluctuations, which populate all modes uniformly. SPDC in ultrathin samples demonstrates very broad spectral and angular width, extremely high degrees of continuous-variable entanglement, tunable polarization entanglement, and orders of magnitude enhancement of photon pair production rate due to the geometric resonances of dielectric metasurfaces.
Non-phase-matched spontaneous parametric down-conversion (SPDC) in atomically thin materials provides new degrees of freedom and enhanced quantum information capacity compared to conventional phase-matched sources. These systems emerged as promising platforms for quantum computing, communication, and imaging, with the potential to support higher-order nonlinear processes. However, direct observation of photon-pair emission from a monolayer has remained experimentally challenging. In this work, we theoretically modeled SPDC emission across the full angular space from a monolayer GaSe film and experimentally validated the model through measurements of both co- and counter-propagating photon pairs. We demonstrated two-photon quantum correlations in the telecom C-band from the thinnest SPDC source reported to date. The spatially symmetric, broadband emission predicted by theory was confirmed experimentally. Furthermore, we observed high-fidelity Bell states in the counter-propagating configuration, marking the first realization of polarization-entangled photon pairs from a monolayer. Our results revealed the emission characteristics of SPDC in the deeply subwavelength, non-phase-matched regime, and introduced atomically thin, counterpropagating SPDC as a scalable and integrable platform for programmable quantum state generation, extendable via moiré superlattice engineering.
Non-phase-matched spontaneous parametric down-conversion (SPDC) in atomically thin materials provides new degrees of freedom and enhanced quantum information capacity compared to conventional phase-matched sources. These systems emerged as promising platforms for quantum computing, communication, and imaging, with the potential to support higher-order nonlinear processes. However, direct observation of photon-pair emission from a monolayer has remained experimentally challenging. In this work, we theoretically modeled SPDC emission across the full angular space from a monolayer GaSe film and experimentally validated the model through measurements of both co- and counter-propagating photon pairs. We demonstrated two-photon quantum correlations in the telecom C-band from a monolayer SPDC source. The spatially symmetric, broadband emission predicted by theory was confirmed experimentally. Furthermore, we observed high-fidelity Bell states in the counter-propagating configuration. Our results revealed the emission characteristics of SPDC in the deeply subwavelength, non-phase-matched regime, and introduced atomically thin, counter-propagating SPDC as a scalable and integrable platform for programmable quantum state generation, extendable via moiré superlattice engineering. The emission of entangled photon pairs from layered materials is attractive for quantum applications, but its observation in monolayers has remained elusive. Here, the authors report co- and counter-propagating photon pair emission in the telecom range from monolayer GaSe and the observation of high-fidelity Bell states in the counterpropagating configuration.
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 (Lc), 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 Lc 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.