We develop a fully vectorial and non-paraxial formalism to describe spontaneous parametric down-conversion in nonlinear thin films. The formalism is capable of treating slabs with a sub-wavelength thickness, describe the associated Fabry-Pérot effects, and even treat absorptive nonlinear materials. With this formalism, we perform an in-depth study of the dynamics of entangled photon-pair generation in nonlinear thin films, to provide a needed theoretical understanding for such systems that have recently attracted much experimental attention as sources of photon pairs. As an important example, we study the far-field radiation properties of photon pairs generated from a high-refractive-index nonlinear thin-film with zinc-blende structure that is deposited on a linear low-refractive-index substrate. In particular, we study the thickness-dependent effect of Fabry-Pérot interferences on the far-field radiation pattern of the photon pairs. We also pay special attention to study of entanglement generation, and find the conditions under which maximally polarization-entangled photon pairs can be generated and detected in such nonlinear thin-films.
We develop a fully vectorial and non-paraxial formalism to describe spontaneous parametric down-conversion in nonlinear thin films. The formalism is capable of treating slabs with a sub-wavelength thickness, describe the associated Fabry-Pérot effects, and even treat absorptive nonlinear materials. With this formalism, we perform an in-depth study of the dynamics of entangled photon-pair generation in nonlinear thin films, to provide a needed theoretical understanding for such systems that have recently attracted much experimental attention as sources of photon pairs. As an important example, we study the far-field radiation properties of photon pairs generated from a high-refractive-index nonlinear thin-film with zinc-blende structure that is deposited on a linear low-refractive-index substrate. In particular, we study the thickness-dependent effect of Fabry-Pérot interferences on the far-field radiation pattern of the photon pairs. We also pay special attention to study of entanglement generation, and find the conditions under which maximally polarization-entangled photon pairs can be generated and detected in such nonlinear thin-films.
The process of spontaneous parametric down-conversion (SPDC) inX(2)-materials, in which a pump (P) photon of wavelength λpcan split into a pair of entangled signal (S) and idler (I) photons of lower energy, is an instrumental tool in optical quantum technologies. Recently, there has been a growing interest in pair generation in sub-wavelength-thin nonlinear crystals [1], [2], especially because they are not restricted by the phase-matching condition and exhibit SPDC in a very wide spectral and angular range. This also motivates their use in quantum imaging applications, for pushing the resolution of such schemes to the limit [3], [4]. Despite its seemingly simple appearance, there has not yet been a comprehensive theoretical analysis of photon-pair generation in a thin unstructured nonlinear slab. Specifically, a proper theory should embed the Fabry-Pérot type of interferences that the signal and idler photons experience in such a nonlinear etalon during SPDC. In our work, we develop the theory for description of SPDC in thin nonlinear etalons and calculate far-field properties of the generated pairs. We use the Green's function (GF) quantization approach for description of pairgeneration [5], combined with analytical expressions for the GF of multilayer systems [6], to develop our formalism. Because of the generality of the GF method, this formalism can describe any thickness of the slab, ultra-thin or thick, and far-field radiation in the non-paraxial regime. Fig. 1 (a) schematic of spdc in a nonlinear etalon.ϵ1,ϵ2andϵ3are linear permittivities of the mediums. Medium 2 with thicknesshhas a second-orderχ(2)nonlinearity. (b-e) Joint detection probabilities of different detection configurations and etalon thicknesses.
Miniaturised entangled photon sources are highly demanded for the development of integrated quantum photonics. Since the invention of subwavelength optical metasurfaces and their successes at replacing bulky optical components, the possibility of implementing entangled photon sources on such devices is actively investigated. Here, as a first step towards the development of quantum optical metasurfaces (QOM), we demonstrate photon pair generation via spontaneous parametric down-conversion (SPDC) from subwavelength films. We achieved photon pair generation with a high coincidence-to-accidental ratio in lithium niobate and gallium phosphide nanofilms. In addition, we have measured the SPDC frequency spectrum via fibre spectroscopy, obtaining photon pairs with a spectral bandwidth of 500\;nm, limited only by the overall detection efficiency. Moreover, we have observed the vacuum field enhancement due to a Fabry-Perot resonance inside the nonlinear films. Our experiments lay the groundwork for the future development of flat SPDC sources, including QOM.
Miniaturized entangled photon sources, in particular based on subwavelength metasurfaces, are highly demanded for the development of integrated quantum photonics. Here, as a first step towards the development of quantum optical metasurfaces (QOMs), we demonstrate generation of entangled photons via spontaneous parametric down-conversion (SPDC) from subwavelength films. We achieve photon pair generation with a high coincidence-to-accidental ratio in lithium niobate and gallium phosphide nanofilms. By implementing the fiber spectroscopy of SPDC in nanofilms, we measure a spectrum with a bandwidth of 500 nm, limited only by the overall detection efficiency. The spectrum reveals vacuum field enhancement due to a Fabry-Perot resonance inside the nonlinear films. It also suggests a strategy for observing SPDC from QOM. Our experiments lay the groundwork for future development of flat SPDC sources, including QOM.
There is a growing belief that we are currently in the midst of a Second Quantum Revolution. More and more quantum technologies, such as communication, computing, and imaging, are moving from academic exploration to practical industrial applications. These technologies often depend on entangled photons, and their increasing use requires the development of novel quantum light sources that are compact, versatile, and integrable into modern photonic systems. Additionally, the trend toward miniaturization and nano-optics is accelerating, driven by rapid advances in nanofabrication. Therefore, there is an urgent need to create new classes of miniaturized and tunable two-photon sources. This thesis is dedicated to the theoretical investigation and experimental demonstration of miniaturized sources of entangled photons based on spontaneous parametric down-conversion in thin nonlinear films. It focuses not only on traditional crystalline materials but also on the emerging platform of liquid crystals, marking the first-ever demonstration of photon-pair generation in a liquid crystal within this thesis. A significant achievement is the ability to relax phase-matching constraints in ultra-thin nonlinear layers, allowing the full utilization of the nonlinear tensor and enhancing the variety of possible entangled two-photon states. Several aspects of two-photon state engineering with relaxed phase-matching are discussed in the thesis. First, this relaxation facilitates the switching between different polarization two-photon states, with a varying degree of polarization entanglement and the ability to generate non-maximally entangled states. In terms of spectral properties, miniaturized two-photon sources generate exceptionally broad two-photon frequency spectra, leading to a high degree of frequency entanglement and an unprecedented number of frequency modes, which also leads to ultra-narrow time correlations. By considering the effects of Fabry-Perot resonances, it becomes possible to engineer the two-photon spectra and even achieve backward emission of photon pairs. Remarkably, the Fabry-Perot effect can be accurately described by a simple modification of the rigorous quantum theory, providing a powerful predictive tool for photon pair generation in advanced nanostructures, metasurfaces, and nonlinear nano-antennas when properly applied. Another central contribution of this work is the introduction of liquid crystals as tunable nonlinear media for photon pair generation. Controlling the molecular orientation through a static twist or applying an external electric field makes it possible to tune the generated two-photon states dynamically. A theoretical model of this process, verified through direct experimental measurements, predicts quasi-phase-matching-like behavior and enhanced conversion efficiency, which can be utilized to develop exceptionally tunable and integrable sources of photon pairs and squeezed light. Finally, a fundamental challenge addressed in this thesis is mitigating a parasitic photoluminescent background. This study demonstrates a two-order-of-magnitude reduction in photoluminescence-induced contamination of photon pairs generated from thin nonlinear films by switching from the commonly used continuous-wave regime to the pulsed regime and performing time-resolved distillation. It also shows a developed theory that relates measured experimental parameters to the intrinsic purity of the two-photon state. Overall, this thesis advances the quantum micro- and nano-photonics field by providing a deeper understanding and enhanced control over photon pair generation in thin nonlinear films. It bridges theoretical insights with experimental realizations and paves the way toward practical, tunable, and highly integrated quantum light sources.
Entangled photon pairs play a major role in various modern technologies such as quantum imaging, communication, and computing. Conventional photon-pair sources are often based on spontaneous parametric down-conversion in bulk nonlinear crystals. Recent advances have also shown entangled photon-pairs from transition metal dichalcogenide thin-films, however, these materials are not widely available and are not compatible with existing fabrication capabilities. We present a new thin-film lithium niobate source of polarization-entangled photon pairs at the telecom wavelength that requires no additional optical elements for entanglement generation and allows for easy application using the existing lithium niobate fabrication technologies. We demonstrate tunable entanglement generation using the three-fold rotational crystal symmetry of lithium niobate, allowing the generation of different maximally entangled Bell states or completely separable states depending on the polarization of the pump beam.