The ability to generate quantum entangled photon pairs through spontaneous parametric down conversion (SPDC) is playing a pivotal role in many applications in quantum technologies, including quantum communications, quantum computation, and quantum imaging. Metasurfaces, two-dimensional arrays of nanostructures with subwavelength thickness, have recently shown extraordinary capability in manipulating classical and quantum states of light and realizing miniaturized SPDC sources. Whereas previous studies have primarily focused on periodic metasurfaces, we uncover the potential of spatially modulated nanopatterns for further SPDC enhancement and state engineering. Specifically, we propose a nonlinear metasurface featuring a lateral guided-mode cavity formed by two distributed Bragg reflectors. With the quasi-normal mode theory, we predict an SPDC rate up to 157 Hz/mW, which represents about 30 times rate enhancement when compared to the metasurface without a cavity. Furthermore, the cavity supports a continuous set of high-Q resonances across a broad bandwidth, allowing the all-optical tuning of the photon-pair emission directions transverse to the cavity by controlling the pump wavelength, which can find applications in quantum imaging.
The ability to generate quantum entangled photon pairs through spontaneous parametric down conversion (SPDC) is playing a pivotal role in many applications in quantum technologies, including quantum communications, quantum computation, and quantum imaging. Metasurfaces, two-dimensional arrays of nanostructures with subwavelength thickness, have recently shown extraordinary capability in manipulating classical and quantum states of light and realizing miniaturized SPDC sources. Whereas previous studies have primarily focused on periodic metasurfaces, we uncover the potential of spatially modulated nanopatterns for further SPDC enhancement and state engineering. Specifically, we propose a nonlinear metasurface featuring a lateral guided-mode cavity formed by two distributed Bragg reflectors. With the quasi-normal mode theory, we predict an SPDC rate up to 157 Hz/mW, which represents about 30 times rate enhancement when compared to the metasurface without a cavity. Furthermore, the cavity supports a continuous set of high-Q resonances across a broad bandwidth, allowing the all-optical tuning of the photon-pair emission directions transverse to the cavity by controlling the pump wavelength, which can find applications in quantum imaging.
Entangled photon-pairs generated through spontaneous parametric down-conversion (SPDC) have evolved into an indispensable resource for various quantum communication, imaging, sensing and computing schemes. While photon-pair sources are so far mostly based on bulk-optical nonlinear components, resonant nonlinear metasurfaces with sub-wavelength thickness have recently been demonstrated as a new platform for photon-pair generation [1]–[3]. Contrary to bulk and integrated nonlinear structures where phase matching steers the pair generation efficiency in different directions and wavelengths, nanostructures can control these properties via their optical resonances. However, the tunability of spatial photon emission in multiple directions remained an open challenge.
Entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) are the cornerstone of many quantum optical applications. Nonlinear metasurfaces have recently been introduced as a new class of compact SPDC sources [1], and the next step is to enable active tunability of their resulting quantum state. Here, we demonstrate the generation of narrow-band yet frequency tunable, spectrally entangled photon pairs from a nonlinear metasurface. For this, we use guided-mode resonances (GMR) with high angular dispersion. In previous works that use Mie- or BIC-type resonances, for a fixed pump wavelength, the demonstrated SPDC spectrum remained fixed by the respective resonance wavelength [1], [2]. Here, we theoretically and experimentally show that in a metasurface supporting GMRs, the non-degeneracy of the generated photon pairs can be tuned by tilting the metasurface while the pump wavelength can remain constant, see the conceptual rendering in Fig. 1(a).
Tunable photonic quantum entanglement promises to advance information processing beyond classical approaches, facilitating applications in quantum communications, imaging and computing. A practical approach to preparing quantum entanglement at room temperature is to engineer photon-pair generation through spontaneous parametric down-conversion (SPDC). Nonlinear flat-optics devices, incorporating subwavelength-thick nonlinear materials, are emerging as new platforms for generating photon pairs at the nanoscale. In particular, nanostructured metasurfaces supporting optical resonances can enhance and tailor SPDC processes, allowing for the generation of spectral, polarization, and spatial entanglement [1]. However, the tunability of polarization entanglement is subject to the symmetry of nonlinear tensors of materials, where the pump photon polarization serves as the only tuning parameter [2].
We present theoretical and experimental advances on photon-pair generation through spontaneous parametric down-conversion (SPDC) in nonlinear nanostructures and metasurfaces, underpinning quantum entanglement engineering at sub-wavelength scale for photon shaping with tailored polarization and spatial correlations. We derive explicit theoretical expressions for the measurable photon counts and quantum correlations, incorporating material dispersion and absorption characteristics [Phys. Rev. Lett. 117, 123901 (2016)], and demonstrate experimentally the generation of quantum heralded photon pairs from a single AlGaAs nanodisk [Optica 6, 1416 (2019)]. We further reveal the opportunities to enhance the generation rate through high-Q resonances including bound states in the continuum [doi:10.1117/12.2539888 (2019)]. We also outline the applications of metasurfaces for the manipulation and measurement of multi-photon quantum states for free-space quantum imaging and communications.
Photon pairs generated by spontaneous parametric down-conversion (SPDC) are playing a pivotal role in many quantum applications such as quantum computing, quantum imaging and quantum communication. Metasurfaces, which are two-dimensional arrays of nanostructures, have been studied intensively to enhance SPDC and provide additional functionalities over the bulky optical systems [1]. Van der Waals (vdWs) layered materials have emerged as promising platform for quantum light sources, owing to their unique properties [2], [3]. For example, vdWs materials offer high refractive index, a high nonlinear susceptibility, and compatibility with on-chip integration.
Entangled photons have acted as a workhorse in many quantum technologies, including cryptography, imaging, sensing, and computation. With the growing demands of quantum technology, the entangled photon pair sources are required to be compact and integrable. For the last few years, the field of quantum optics in the nanoscale has demonstrated photon pair generation in ’flat’ optics such as thin layers, Van der Waals crystals, micro-, nano-antennas, and metasurfaces. The word ’flat’ is used because such optical platforms have sub-wavelength thicknesses. Due to the short travel length inside the sources, they can possess multiple functions that their bulky counterparts cannot have. This doctoral thesis addresses photon pair generation in ’flat’ optics via second- and third-order nonlinear processes, which are spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM). The main aims of the thesis are to understand how the photon pair generation is realized via SPDC and SFWM on subwavelength-scale platforms, and to make a bridge between photon pair generation via two nonlinear processes under the presence of optical resonances. The thesis covers two experimental parts related to SPDC and SFWM, and one future expectation of the SFWM in metamaterials. The first part of the thesis addresses one of the multi-functionalities that has recently been reported: the emission direction control of the photon pairs. A 150 nm-thick gallium phosphide (GaP) metasurface is used as a source. The metasurface supports a quasi-bound state in the continuum (quasi-BIC) resonance, enhancing the electric field inside the material. I generate the entangled photons through the SPDC process. The generated photons are in two different wavelengths under the given pump wavelength: signal photon at the resonance, and idler photon out of the resonance. The photon pair generation rate is enhanced by 66 times compared to the unpatterned GaP slab since the vacuum field in the signal mode is enhanced by the resonance. The generated photons obey the transmission spectrum of the metasurface. As a result, the signal photons at resonance are mostly reflected, and the idler photons are transmitted after generation. This result is the first work of photon demultiplexing by a metasurface. The second part of the thesis covers the photon pair generation through SFWM in thin nonlinear films. Silicon-rich silicon nitride (SiN) layers are selected as the sources. The layers have an intriguing characteristic that the third-order nonlinear susceptibility (chi-3) differs with the nitrogen doping rate. I generate the photon pairs through chi-3 variant SiN layers. The thicknesses of the samples are all sub-wavelength level, having a few hundred nanometers. Time-energy correlation measurements are conducted for all samples, and the second-order correlation functions (g(2)(0)) are calculated. The maximum g(2)(0) of the samples exceeds 2. Interestingly, I observed two-photon interference effects between photon pairs generated in the layers and the fused-silica substrate, which allows me to estimate the chi-3 values of the samples by knowing the reference: chi-3 value of the substrate. The last part of the thesis introduces designs of metasurfaces and possible experimental schemes for photon pair generation through SFWM. Yet, due to its weak light-matter interaction in third-order nonlinear processes, the photon pair generation through SFWM has not been observed in metasurfaces. However, it is fairly possible to enhance the vacuum fields at the output photon modes and generate photon pairs because preliminary work in unpatterned slabs has already been observed in this thesis. Finally, I introduce a fabricated metasurface. It needs improvements, but it is quite promising for the ongoing project.