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Physical Review A

Enhancement of the photon pair generation rate by spontaneous four-wave mixing in bent waveguides

Maria Paszkiewicz, Carsten Rockstuhl

Published Jan 7, 2025• 3 citations
Publisher

Abstract

Enhancing nonlinear optical effects is critical to improving the performance of many functional devices for nonlinear and quantum optical applications. Here, we study the possibility of bending a waveguide to enhance the photon pair generation rate in a spontaneous four-wave mixing process. Whereas intuition might suggest that bending makes the process less efficient because the waveguides are more lossy when bent, we show that the increase in the effective nonlinearity outperforms the loss disadvantage for moderate bending radii. That can be explained by a better localization of the guided eigenmodes, leading to a reduced mode area. By studying selected waveguide devices with a fixed length, we demonstrate an improvement in the photon pair generation rate between 5% and 210% for an optimal bending radius. These findings have implications for the future design of integrated photonic devices for quantum optical applications, especially in cases where the chip estate tends to be a limiting factor.

DOI
10.1103/physreva.111.013507

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arXiv:2208.13955•Aug 30, 2022•0 citations
Enhancing nonlinear optical effects is critical to improving the performance of many functional devices for nonlinear and quantum optical applications. Here, we study the possibility of bending a waveguide to enhance the photon pair generation rate in a spontaneous four-wave mixing process. Whereas intuition might suggest that bending makes the process less efficient because the waveguides are more lossy when bent, we show that the increase in the effective nonlinearity outperforms the loss disadvantage for moderate bending radii. That can be explained by a better localization of the guided eigenmodes, leading to a reduced mode area. By studying selected waveguide devices with a fixed length, we demonstrate an improvement in the photon pair generation rate between 5% and 210% for an optimal bending radius. These findings have implications for the future design of integrated photonic devices for quantum optical applications, especially in cases where the chip estate tends to be a limiting factor.
Enhancing nonlinear optical effects is critical to improving the performance of many functional devices for nonlinear and quantum optical applications. Here we study the possibility of bending a waveguide to enhance the photon pair generation rate in a spontaneous four-wave mixing process. Whereas intuition might suggest that bending makes the process less efficient, because the waveguides are more lossy when bent, we show that the increase in the effective nonlinearity outperforms the disadvantage for moderate bending radii. That can be explained by a better localization of the guided eigenmodes, leading to a reduced mode area. By studying selected waveguide devices with a fixed length, we demonstrate an optimal improvement between 5 % and 270 % in the photon pair generation rate for an optimal bending radius. These findings have implications for the future design of integrated photonic devices for quantum optical applications, especially in cases where the chip estate tends to be a limiting factor.
We theoretically model pair generation and evolution via the nonlinear process of spontaneous four-wave mixing in a coupled-resonator optical waveguide in a photonic crystal slab. Using the adjoint master equation for a system of lossy coupled cavities, we calculate a symmetrized second-order coherence function to determine pair detection probability. We find that the scattering loss can have as large an effect on pair generation as waveguide dispersion. In particular, the wave-vector dependence of the loss can shift the frequency of the maximum detection probability and therefore cannot, in general, be ignored or treated via a simple overall loss factor.
Third-order nonlinear processes require phase matching between the interacting fields to achieve high efficiencies. Typically in guided-wave χ(3)χ^{(3)}χ(3) platforms this is achieved by engineering the dispersion of the modes through the transverse profile of the device. However, this limits the flexibility of the phase matching that can be achieved. Instead, we analyze four-wave mixing in a pair of asymmetric waveguides and show that phasematching may be achieved in any χ(3)χ^{(3)}χ(3) waveguide by coupling of a nondegenerate pump from an adjacent waveguide. We demonstrate the additional flexibility that this approach yields in the case of photon-pair generation by spontaneous FWM, where the supermode dispersion may be modified to produce pure heralded single photons -- a critical capability required for example by silicon platforms for chip-scale quantum photonics.
Abstract An on‐chip quantum light source based on spontaneous four‐wave mixing is an essential element for developing quantum photonic integrated circuit technology, which has the advantage of no connection loss owing to the integration of the source into photonic circuits. The waveguide‐based quantum light source inevitably causes propagation loss owing to imperfections in the fabrication process, but the propagation loss effects on photon‐pair generation have not been extensively studied. In this study, propagation loss effects were examined using theoretical and experimental methods. In theory, the performance of quantum light sources, such as brightness, heralding efficiency, and coincidence‐to‐accidental ratio, strongly depend on propagation loss. We fabricate several waveguides with a moderate propagation loss of 2.2 dB/cm to investigate the loss dependence and ascertain that the brightness, heralding efficiency, and coincidence‐to‐accident ratio strongly correlate with the length of the optical waveguide. The maximum coincidence‐count brightness occurred at an optimization length of 1/ α , where α is the absorption coefficient. In contrast, the single‐count brightness shows slightly different waveguide length dependence owing to loss‐induced one‐photon states. We expect that the results obtained in this study will greatly assist in determining the proper waveguide length for photon‐pair generation according to the source’s application fields. The results will be helpful in the development of a quantum light source suitable for practical and quantum optical integrated circuits and will lead to the development of high‐fidelity quantum technologies.
On-chip quantum light sources are a fundamental component of integrated photonic quantum information systems. However, their performance is fundamentally limited by weak nonlinear interactions and high propagation losses, which hinder efficient generation. Here, we propose a fishbone-modulated grating structure on a silicon-nanowire waveguide to significantly enhance spontaneous four-wave mixing. The proposed structure achieves a quantum-light generation efficiency two orders of magnitude higher than conventional silicon-nanowire waveguides. Specifically, we exploit a Su-Schrieffer-Heeger-like topological interface to slow light in the waveguide, thereby enhancing the nonlinear coefficient. Moreover, we leverage the principles of bound states in the continuum to optimize the modulated gratings on the waveguide, achieving ultra-low waveguide propagation loss. The simultaneous realization of the slow-wave effect and ultralow loss effectively enhances the spontaneous four-wave mixing process in silicon. Our approach opens new possibilities for designing enhanced nonlinear nanophotonic devices using various nonlinear optical materials.
Integrated Silicon photonics has played an important role in advancing the applications of quantum information and quantum science. However, due to different material properties, it is challenging to integrate all components with excellent performance based on homogeneous material. Here, by combining high nonlinearity and low losses in a heterogeneous silicon platform, we efficiently generate high-quality photon pairs through spontaneous four-wave mixing in hydrogenated amorphous silicon waveguide and route them off-chip through low loss silicon nitride waveguide. A record high coincidence- to- accidental rate value of 1632.6 (±\pm± 260.4) is achieved in this heterogeneous design with a photon pair generation rate of 1.94 MHz. We also showcase a wide range of multi-channel photon sources with coincidence- to- accidental rate consistently at 200. Lastly, we measure heralded single-photons with a lowest gH(2)(0)g^{(2)}_H(0)gH(2)​(0) of 0.1085 ±\pm± 0.0014. Our results demonstrate the heterogeneous silicon platform as an ideal platform for efficient generation of photon pairs and routing them off-chip with low losses. It also paves a way for the future hybrid photonic integrated circuit by collecting distinct features from different materials.