Anneal-free ultra-low loss silicon nitride integrated photonics.

Heterogeneous and monolithic integration of the versatile low-loss silicon nitride platform with low-temperature materials such as silicon electronics and photonics, III-V compound semiconductors, lithium niobate, organics, and glasses has been inhibited by the need for high-temperature annealing as well as the need for different process flows for thin and thick waveguides. New techniques are needed to maintain the state-of-the-art losses, nonlinear properties, and CMOS-compatible processes while enabling this next generation of 3D silicon nitride integration. We report a significant advance in silicon nitride integrated photonics, demonstrating the lowest losses to date for an anneal-free process at a maximum temperature 250 °C, with the same deuterated silane based fabrication flow, for nitride and oxide, for an order of magnitude range in nitride thickness without requiring stress mitigation or polishing. We report record low anneal-free losses for both nitride core and oxide cladding, enabling 1.77 dB m-1 loss and 14.9 million Q for 80 nm nitride core waveguides, more than half an order magnitude lower loss than previously reported sub 300 °C process. For 800 nm-thick nitride, we achieve as good as 8.66 dB m-1 loss and 4.03 million Q, the highest reported Q for a low temperature processed resonator with equivalent device area, with a median of loss and Q of 13.9 dB m-1 and 2.59 million each respectively. We demonstrate laser stabilization with over 4 orders of magnitude frequency noise reduction using a thin nitride reference cavity, and using a thick nitride micro-resonator we demonstrate OPO, over two octave supercontinuum generation, and four-wave mixing and parametric gain with the lowest reported optical parametric oscillation threshold per unit resonator length. These results represent a significant step towards a uniform ultra-low loss silicon nitride homogeneous and heterogeneous platform for both thin and thick waveguides capable of linear and nonlinear photonic circuits and integration with low-temperature materials and processes.

Photonic integrated beam delivery for a rubidium 3D magneto-optical trap.

Cold atoms are important for precision atomic applications including timekeeping and sensing. The 3D magneto-optical trap (3D-MOT), used to produce cold atoms, will benefit from photonic integration to improve reliability and reduce size, weight, and cost. These traps require the delivery of multiple, large area, collimated laser beams to an atomic vacuum cell. Yet, to date, beam delivery using an integrated waveguide approach has remained elusive. Here we report the demonstration of a 87Rb 3D-MOT using a fiber-coupled photonic integrated circuit to deliver all beams to cool and trap > 1 ×106 atoms to near 200 µK. The silicon nitride photonic circuit transforms fiber-coupled 780 nm cooling and repump light via waveguides to three mm-width non-diverging free-space cooling and repump beams directly to the rubidium cell. This planar, CMOS foundry-compatible integrated beam delivery is compatible with other components, such as lasers and modulators, promising system-on-chip solutions for cold atom applications.

Integrated programmable strongly coupled three-ring resonator photonic molecule with ultralow-power piezoelectric control.

Photonic molecules can realize complex optical energy modes that simulate states of matter and have application to quantum, linear, and nonlinear optical systems. To achieve their full potential, it is critical to scale the photonic molecule energy state complexity and provide flexible, controllable, stable, high-resolution energy state engineering with low power tuning mechanisms. In this work, we demonstrate a controllable, silicon nitride integrated photonic molecule, with three high-quality factor ring resonators strongly coupled to each other and individually actuated using ultralow-power thin-film lead zirconate titanate (PZT) tuning. The resulting six tunable supermodes can be fully controlled, including their degeneracy, location, and degree of splitting, and the PZT actuator design yields narrow PM energy state linewidths below 58 MHz without degradation as the resonance shifts, with over an order of magnitude improvement in resonance splitting-to-width ratio of 58, and power consumption of 90 nW per actuator, with a 1-dB photonic molecule loss. The strongly coupled PZT-controlled resonator design provides a high-degree of resolution and controllability in accessing the supermodes. Given the low loss of the silicon nitride platform from the visible to infrared and the three individual bus, six-port design, these results open the door to novel device designs and a wide range of applications including tunable lasers, high-order suppression ultranarrow-linewidth lasers, dispersion engineering, optical parametric oscillators, physics simulations, and atomic and quantum photonics.

Visible light photonic integrated Brillouin laser.

Narrow linewidth visible light lasers are critical for atomic, molecular and optical (AMO) physics including atomic clocks, quantum computing, atomic and molecular spectroscopy, and sensing. Stimulated Brillouin scattering (SBS) is a promising approach to realize highly coherent on-chip visible light laser emission. Here we report demonstration of a visible light photonic integrated Brillouin laser, with emission at 674 nm, a 14.7 mW optical threshold, corresponding to a threshold density of 4.92 mW µm-2, and a 269 Hz linewidth. Significant advances in visible light silicon nitride/silica all-waveguide resonators are achieved to overcome barriers to SBS in the visible, including 1 dB/meter waveguide losses, 55.4 million quality factor (Q), and measurement of the 25.110 GHz Stokes frequency shift and 290 MHz gain bandwidth. This advancement in integrated ultra-narrow linewidth visible wavelength SBS lasers opens the door to compact quantum and atomic systems and implementation of increasingly complex AMO based physics and experiments.

Effect of dielectric claddings on the electro-optic behavior of silicon waveguides.

We fabricate silicon waveguides in silicon-on-insulator (SOI) wafers clad with either silicon dioxide, silicon nitride, or aluminum oxide and, by measuring their electro-optic behavior, we characterize the capacitively induced free-carrier effect. By comparing our results with simulations, we confirm that the observed voltage dependences of the transmission spectra are due to changes in the concentrations of holes and electrons within the semiconductor waveguides and show how strongly these effects depend on the cladding material that comes into contact with the waveguide. Waveguide loss is additionally found to have a high sensitivity to the applied voltage, suggesting that these effects may find use in applications that require low- or high-loss propagation. These phenomena, which are present in all semiconductor waveguides, may be incorporated into more complex waveguide designs in the future to create high-efficiency electro-optic modulators and wavemixers.