Efficient and low noise single-photon-level frequency conversion interfaces using Si<sub>3</sub>N<sub>4</sub> microrings (2016)

Optical frequency conversion is an essential building block of nanophotonics, whose application ranges from tunable light sources for classical on-chip communications to miniaturized telecommunications-band interfaces for quantum information science. In the widely-used silicon photonics platform (including silicon nitride and silicon dioxide), a majority of reported frequency conversion experiments have used a degenerate four-wave mixing (FWM) process called parametric amplification (PA), as χ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> is absent due to the crystalline centrosymmetry and χ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(3)</sup> is the dominant nonlinearity. While PA-based frequency conversion works well for many classical applications, it offers a limited signal-to-noise ratio (SNR) when it comes down to the single-photon-level input light, largely because of the amplified vacuum fluctuation noise. Alternatively, there is another FWM process, termed four-wave-mixing Bragg scattering (FWMBS), that has been shown to be of low noise both theoretically and experimentally. The FWM-BS process is non-degenerate, and is typically operated in the normal dispersion region so it does not amplify vacuum fluctuations. Here we report our recent progress in developing an efficient and low-noise frequency conversion interface for single-photon-level inputs based on the FWM-BS process using silicon nanophotonics. For the demonstration purpose, the 1550nm band and 980nm band are chosen due to their relevance to low-loss transmission through optical fibers and quantum light generation by InAs/GaAs quantum dots, respectively. A compact silicon nitride microring resonator is employed (footprint <; 0.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), for which a careful design has been carried out to minimize the frequency mismatch of the underlying FWM-BS process and to optimize the coupling to the relevant resonant modes. Thanks to the resonance enhancement, the required pump power has been reduced significantly to less than 60mW continuous-wave pumps in all the following frequency conversion configurations using the same device. We first show the intraband conversion in the 980nm band with two strong pumps in the 1550nm band. The spectral translation is over a few nanometers and the conversion efficiency is > 25%. Next, we demonstrate wideband frequency conversion from the 1550nm band to the 980nm band (upconversion) and the 980nm band to the 1550nm band (downconversion), with one strong pump in each band. For both the upconversion and downconversion cases, the spectral translation is around 600 nanometer and the conversion efficiency is > 60%. To characterize the noise property of the FWM-BS process, we measure the background noise for both the 980nm intraband conversion and the wideband 980nm to 1550nm downconversion using single-photon-level inputs, which is found to be on the fW and pW level, respectively. In addition, we have adopted simulation techniques based on the Lugiato-Lefever equation, which has been typically used for microresonator frequency combs, for the FWM-BS process, and quantitative agreements with measurements have been obtained.