High-performance, adiabatically nanotapered fiber-chip couplers in silicon at 2 microns wavelength.

Fiber optic technology connects the world through the Internet, enables remote sensing, and connects disparate functional optical devices. Highly confined silicon photonics promises extreme scale and functional integration. However, the optical modes of silicon nanowire waveguides and optical fibers are very different, making efficient fiber-chip coupling a challenge. Vertical grating couplers, the dominant coupling method today, have limited optical bandwidth and are naturally out-of-plane. Here we demonstrate a new method that is low-loss, broadband, manufacturable, and naturally planar. We adiabatically couple a tapering silicon nanowire waveguide to a conic nanotapered optical fiber, measuring transmission between 2.0 µm and 2.2 µm wavelength. The silicon chip is fabricated at a commercial foundry and then post-processed to release the tapering nanowires. We estimate an optimal per-coupler transmission of -0.48 dB (maximum; 95% confidence interval [+0.46, -1.68] dB) and a 1-dB bandwidth of at least 295 nm. With automated measurements, we quantify the device tolerance to lateral misalignment, measuring a flat response within ±0.968 µm. This new design can enable low-loss modular systems of integrated photonics irrespective of material and waveband.

10-GHz-clock time-multiplexed non-degenerate optical parametric oscillator network with a variable planar lightwave circuit interferometer.

A coherent XY machine (CXYM) is a physical spin simulator that can simulate the XY model by mapping XY spins onto the continuous phases of non-degenerate optical parametric oscillators (NOPOs). Here, we demonstrated a large-scale CXYM with >47,000 spins by generating 10-GHz-clock time-multiplexed NOPO pulses via four-wave mixing in a highly nonlinear fiber inside a fiber ring cavity. By implementing a unidirectional coupling from the ith pulse to the (i + 1)th pulse with a variable 1-pulse delay planar lightwave circuit interferometer, we successfully controlled the effective temperature of a one-dimensional XY spin network within two orders of magnitude.

Evaluation of graphene optical nonlinearity with photon-pair generation in graphene-on-silicon waveguides.

We evaluate the nonlinear coefficient of graphene-on-silicon waveguides through the coincidence measurement of photon-pairs generated via spontaneous four-wave mixing. We observed the temporal correlation of the photon-pairs from the waveguides over various transfer layouts of graphene sheets. A simple analysis of the experimental results using coupled-wave equations revealed that the atomically-thin graphene sheets enhanced the nonlinearity of silicon waveguides up to ten-fold. The results indicate that the purely χ (3)-based effective nonlinear refractive index of graphene is on the order of 10-13 m 2/W, and provide important insights for applications of graphene-based nonlinear optics in on-chip nanophotonics.

Efficient Single-Photon Coupling from a Nitrogen-Vacancy Center Embedded in a Diamond Nanowire Utilizing an Optical Nanofiber.

Nitrogen-Vacancy (NV) centers in diamond are promising solid-state quantum emitters that can be utilized for photonic quantum applications. Various diamond nanophotonic devices have been fabricated for efficient extraction of single photons emitted from NV centers to a single guided mode. However, for constructing scalable quantum networks, further efficient coupling of single photons to a guided mode of a single-mode fiber (SMF) is indispensable and a difficult challenge. Here, we propose a novel efficient hybrid system between an optical nanofiber and a cylindrical-structured diamond nanowire. The maximum coupling efficiency as high as 75% for the sum of both fiber ends is obtained by numerical simulations. The proposed hybrid system will provide a simple and efficient interface between solid-state quantum emitters and a SMF suitable for constructing scalable quantum networks.