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Applied Physics Letters

Nonlinear coupling of linearly uncoupled resonators through a Mach–Zehnder interferometer

Federico Andrea Sabattoli, Houssein El Dirani, Laurène Youssef, Linda Gianini, Luca Zatti, Francesco Garrisi, Davide Grassani, Camille Petit-Étienne, E. Pargon, John E. Sipe, Marco Liscidini, Corrado Sciancalepore, Daniele Bajoni, Mattéo Galli

Published Nov 14, 2022• 7 citations
Publisher

Abstract

Optical nonlinear processes in linearly uncoupled resonators are being actively studied as a convenient way to engineer and control the generation of non-classical light. In these structures, one can take advantage of the independent combs of resonances of two linearly uncoupled ring resonators for field enhancement, with the phase-matching condition being significantly relaxed compared to a single resonator. However, previous implementations of this approach have shown a limited operational bandwidth along with a significant reduction of the generation efficiency. Here, we experimentally demonstrate that a Mach–Zehnder interferometer can be used to effectively linearly uncouple two resonators and, at the same time, allows for their efficient nonlinear coupling. We demonstrate that this structure can lead to an unprecedented control over the rings' interaction and can operate over more than 160 nm, covering the S-, C-, and L-telecom bands. In addition, we show that the photon pair generation efficiency is increased by a factor of four with respect to previous implementations.

DOI
10.1063/5.0109392

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•Oct 5, 2017•1 citation
Optical nonlinear processes in linearly uncoupled resonators are being actively studied as a convenient way to engineer and control the generation of non-classical light. In these structures, one can take advantage of the independent combs of resonances of two linearly uncoupled ring resonators for field enhancement, with the phase-matching condition being significantly relaxed compared to a single resonator. However, previous implementations of this approach have shown a limited operational bandwidth along with a significant reduction of the generation efficiency. Here, we experimentally demonstrate that a Mach–Zehnder interferometer can be used to effectively linearly uncouple two resonators and, at the same time, allows for their efficient nonlinear coupling. We demonstrate that this structure can lead to an unprecedented control over the rings' interaction and can operate over more than 160 nm, covering the S-, C-, and L-telecom bands. In addition, we show that the photon pair generation efficiency is increased by a factor of four with respect to previous implementations.
We present a detailed study of the generation of photon pairs by spontaneous four-wave mixing in a structure composed of two linearly uncoupled resonators, where energy can be transferred from one resonator to another only through a nonlinear interaction. Specifically, we consider the case of two racetrack-shaped resonators connected by a coupler designed to guarantee that the resonance comb of each resonator can be tuned independently and to allow the nonlinear interaction between modes that belong to different combs. We show that such a coupler can be realized in at least two ways: A directional coupler or a Mach-Zehnder interferometer. For these two scenarios, we derive analytic expressions for the pair-generation rate via single-pump spontaneous four-wave mixing and compare these results with that achievable in a single-ring resonator.
In this Letter, we theoretically study spontaneous parametric downconversion (SPDC) in a periodically poled structure composed of two linearly uncoupled resonators that are nonlinearly coupled via a Mach-Zehnder interferometer. The device does not require dispersion engineering to achieve efficient doubly resonant SPDC, and, unlike the case of a single resonator, one can reconfigure the system to generate photon pairs over a bandwidth of hundreds of nm. We consider the case of SPDC pumped at 775 nm in a periodically poled lithium niobate (PPLN) device compatible with up-to-date technological platforms. We calculated pair generation rates of up to 250 MHz/mW pump power for a single resonance and integrated pair generation rates of up to 100 THz/mW pump power over 170 nm. When properly reconfigured, a single device can efficiently generate over a bandwidth of some 300 nm, covering the S, C, L, and U infrared bands.
Integrated micro- and nanostructures allow for the efficient generation of photon pairs via parametric fluorescence, thanks to the enhancement of the light-matter interaction associated with light confinement in small volumes. Yet the advantages of using integrated devices go well-beyond the improvement of generation efficiency, for micro-structures can be designed to enable a precise control over the properties of the generated non-classical light. Particularly interesting are structures composed of more than one micro-resonator. In such systems one can control the spectral properties of the generated light, and suppress unwanted phenomena, with a careful engineering of the linear and nonlinear coupling of the resonant elements. Here we show that we can exploit this structure to generate photon pairs that are highly energy entangled or completely uncorrelated.
We demonstrate a reconfigurable photon pair source based on spontaneous parametric downconversion in a periodically poled aluminum nitride doubly resonant system. The device features two linearly uncoupled microring resonators integrated in a Mach-Zehnder interferometer, enabling independent tuning of pump and signal/idler resonances without complex dispersion engineering. Using a silicon nitride strip-loaded waveguide on poled aluminum nitride, we achieve quasi-phase-matching and resonant enhancement over a broad spectral range. Simulations predict efficient generation across a bandwidth of over 200 nm, with rates exceeding 4 MHz/mW per resonance. This platform offers high performance, spectral tunability, and fabrication tolerance for integrated quantum photonic applications.
Ring resonators are used as photon pair sources by taking advantage of the materials second or third order non- linearities through the processes of spontaneous parametric downconversion and spontaneous four wave mixing respectively. Two materials of interest for these applications are silicon for the infrared and aluminum nitride for the ultraviolet through the infrared. When fabricated into ring type sources they are capable of producing pairs of indistinguishable photons but typically suffer from an effective 50% loss. By slightly decoupling the input waveguide from the ring, the drop port coincidence ratio can be significantly increased with the trade-off being that the pump is less efficiently coupled into the ring. Ring resonators with this design have been demonstrated having coincidence ratios of 96% but requiring a factor of ~10 increase in the pump power. Through the modification of the coupling design that relies on additional spectral dependence, it is possible to achieve similar coincidence ratios without the increased pumping requirement. This can be achieved by coupling the input waveguide to the ring multiple times, thus creating a Mach-Zehnder interferometer. This coupler design can be used on both sides of the ring resonator so that resonances supported by one of the couplers are suppressed by the other. This is the ideal configuration for a photon-pair source as it can only support the pump photons at the input side while only allowing the generated photons to leave through the output side. Recently, this device has been realized with preliminary results exhibiting the desired spectral dependence and with a coincidence ratio as high as ~ 97% while allowing the pump to be nearly critically coupled to the ring. The demonstrated near unity coincidence ratio infers a near maximal heralding efficiency from the fabricated device. This device has the potential to greatly improve the scalability and performance of quantum computing and communication systems.