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arXiv:2401.07185 · Optics Letters

Non-resonant Bragg scattering four-wave mixing at near-visible wavelengths in low-confinement silicon nitride waveguides

Nicholas Jaber, Scott E. Madaras, Andrew Starbuck, Andrew T. S. Pomerene, Christina M. Dallo, Douglas C. Trotter, Michael R. Gehl, Nils T. Otterstrom

Published Apr 11, 2024• 4 citations
arXivHTMLPDF

Abstract

Quantum state coherent frequency conversion processes—such as Bragg-scattering four-wave mixing (BSFWM)—hold promise as a flexible technique for networking heterogeneous and distant quantum systems. In this Letter, we demonstrate BSFWM within an extended (1.2-m) low-confinement silicon nitride waveguide and show that this system has the potential for near-unity frequency conversion in visible and near-visible wavelength ranges. Using sensitive classical heterodyne laser spectroscopy at low optical powers, we characterize the Kerr coefficient (∼1.55 W−1m−1) and linear propagation loss (∼0.0175 dB/cm) of this non-resonant waveguide system, revealing a record-high nonlinear figure of merit (NFM = γ/α ≈ 3.85 W−1) for BSFWM of near-visible light in non-resonant silicon nitride waveguides. We predict how, at high yet achievable on-chip optical powers, this NFM would yield a comparatively large frequency conversion efficiency, opening the door to near-unity flexible frequency conversion without cavity enhancement and resulting bandwidth constraints.

DOI
10.1364/ol.519793

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Quantum state coherent frequency conversion processes—such as Bragg-scattering four-wave mixing (BSFWM)—hold promise as a flexible technique for networking heterogeneous and distant quantum systems. In this Letter, we demonstrate BSFWM within an extended (1.2-m) low-confinement silicon nitride waveguide and show that this system has the potential for near-unity frequency conversion in visible and near-visible wavelength ranges. Using sensitive classical heterodyne laser spectroscopy at low optical powers, we characterize the Kerr coefficient (∼1.55 W−1m−1) and linear propagation loss (∼0.0175 dB/cm) of this non-resonant waveguide system, revealing a record-high nonlinear figure of merit (NFM = γ/α ≈ 3.85 W−1) for BSFWM of near-visible light in non-resonant silicon nitride waveguides. We predict how, at high yet achievable on-chip optical powers, this NFM would yield a comparatively large frequency conversion efficiency, opening the door to near-unity flexible frequency conversion without cavity enhancement and resulting bandwidth constraints.
Low-noise, tunable wavelength-conversion through nondegenerate four-wave mixing Bragg scattering in SiN(x) waveguides is experimentally demonstrated. Finite element method simulations of waveguide dispersion are used with the split-step Fourier method to predict device performance. Two 1550 nm wavelength band pulsed pumps are used to achieve tunable conversion of a 980 nm signal over a range of 5 nm with a peak conversion efficiency of ≈5%. The demonstrated Bragg scattering process is suitable for frequency conversion of quantum states of light.
We describe a chip-scale, telecommunications-band frequency conversion interface designed for low-noise operation at wavelengths desirable for common single photon emitters. Four-wave-mixing Bragg scattering in silicon nitride waveguides is used to demonstrate frequency upconversion and downconversion between the 980 nm and 1550 nm wavelength regions, with signal-to-background levels > 10 and conversion efficiency of ≈ -60 dB at low continuous wave input pump powers (< 50 mW). Finite element simulations and the split-step Fourier method indicate that increased input powers of ≈ 10 W (produced by amplified nanosecond pulses, for example) will result in a conversion efficiency > 25 % in existing geometries. Finally, we present waveguide designs that can be used to connect shorter wavelength (637 nm to 852 nm) quantum emitters with 1550 nm.
Quantum frequency conversion serves a key role in the realization of hybrid quantum networks by interfacing between wavelength-incompatible platforms. Here we present what is believed to be the first quantum frequency converter connecting visible and telecom domains on a silicon nitride (SiN) chip, using Bragg-scattering four-wave mixing to upconvert heralded single photons from 1260 to 698 nm, which covers a 192 THz span. We examine the noise sources in SiN and devise approaches to suppress noise photons at the source and target frequencies to enable measurements at the single-photon level. We demonstrate an on-chip conversion efficiency of 5% in photon flux and describe design modifications that can be implemented to significantly improve it. Our results pave the way for the implementation of complementary metal-oxide-semiconductor (CMOS)-compatible devices in quantum networks.
Optical microcavity based four-wave mixing (FWM) and its various applications, such as entangled photon pair generation, have been intensively studied in the past few years. Compared to the microring cavities that most researchers have been focusing on, here, we report the generation of FWM photons with Fabry–Perot Bragg grating cavities (FPBG) using the Si 3 N 4 /SiO 2 platform. By leveraging the large, nonuniform and symmetrical dispersion and tunability of our grating cavity, a unique strategy is established to compensate the strong normal dispersion of the waveguide and phase matching condition is achieved on a thin Si 3 N 4 film, which is critical for efficient wavelength conversion. We develop an analytical model to optimize the nonlinear conversion efficiency of FWM, and we also introduce tapered grating in the FPBG to reduce the grating loss from 3.5 dB/cm to 0.36 dB/cm. With a 5.3 dBm pump power, the nonlinear conversion efficiency is characterized to be −37.7 dB. Our work demonstrates that FPBG is a promising on-chip platform for entangled photon pair generation and quantum communication applications.
Silicon photonics has drawn a lot of attention over the last decades, mainly in telecom-related application fields where the nonlinear optical properties of silicon are ignored or minimized. However, silicon’s high χ(3) Kerr optical nonlinearity in sub-micron-scale high-confinement waveguides can enable significant improvements in traditional nonlinear devices, such as for wavelength conversion, and also enable some device applications in quantum optics or for quantum key distribution. In order to establish the viability of silicon photonics in practical applications, some big challenges are to improve the optical performance (e.g., optimize nonlinearity or minimize loss) and integration of optics with microelectronics. In this context, we discuss how electronic PIN diodes improve the performance of wavelength conversion in a microring resonator based four-wave mixing device, which achieves a continuous-wave four-wave mixing conversion efficiency of −21.3 dB at 100 mW pump power, with enough bandwidth for the wavelength conversion of a 10 Gbps signal. In the regime of quantum optics, we describe a coupled microring device that can serve as a tunable source of entangled photon pairs at telecommunications wavelengths, operating at room temperature with a low pump power requirement. By controlling either the optical pump wavelength, or the chip temperature, we show that the output bi-photon spectrum can be varied, with implications on the degree of frequency correlation of the generated quantum state.
Four-wave mixing (FWM) in silicon photonics offers promising applications in optical signal processing, wavelength conversion, and quantum photonics, necessitating a comprehensive understanding of its underlying physics and limitations. Stimulated four-wave mixing in silicon photonic wire waveguides has been investigated theoretically by considering two-photon absorption, free carrier absorption, and free carrier dispersion, etc., operating near λ∼1550 nm. It has been shown that these effects severely degrade the performance when operating at higher pump power levels. We present exact numerical calculations of idler wavelength conversion efficiency and bandwidth taking into account the abovementioned nonlinearities, to show the effects of various parameters within the waveguide structure. We extended to investigate silicon photonic microring resonators in terms of the four-wave mixing performance of high-Q-value microring resonators, which are very promising as photon sources (spontaneous four-wave mixing) for large-scale quantum photonic circuits. Our analysis illustrates the limitations of analytical models in describing FWM in ring resonator behavior at a moderate input power level, necessitating a rigorous numerical approach. Experimentally, we demonstrate stimulated FWM (1525 nm≤λp≤1575 nm) in a silicon photonic wire with a cross-section of 500 nm×220 nm and with a small waveguide length of 2 mm. The idler-to-signal conversion efficiency of about −35 dB has been measured for an approximate launched pump power of 30 mW. The conversion efficiency is improved further in the microring resonator as predicted by the simulation results.