Ultra-low loss piezo-optomechanical low-confinement silicon nitride platform for visible wavelength quantum photonic circuits

Abstract

The stringent demands of photonic quantum computing protocols motivate photonic integrated circuit (PIC) platforms with passive optical properties such as extremely low losses and correspondingly large circuit depths, as well as active optical properties such as high reconfiguration rates, low power dissipation, and minimal crosstalk. At the same time, many quantum photonic resource state generators, such as single-photon sources and quantum memories, require operation in the visible wavelength range. These requirements make the passive optical properties of CMOS-fabricated, ultralow-loss, low-confinement silicon nitride waveguides especially attractive. However, the conventional active properties of these systems based on thermo-optic modulation are plagued by high levels of crosstalk, slow modulation rates, and high power dissipation. Although there have been recent demonstrations of CMOS-fabricated, visible wavelength, piezo-optomechanical PICs that solve the above challenges associated with implementing active functionality, these have made use of high-confinement waveguides with currently demonstrated losses of order $0.3$-$1~\mathrm{dB/cm}$, precluding circuit depths required for scalable quantum algorithms. Here, we demonstrate that combining piezo-optomechanical actuation with a low-confinement, ultra-low loss silicon nitride platform addresses the scalability challenge while enabling high-performance active functionality at visible wavelengths. This platform achieves a propagation loss $0.026~\mathrm{dB/cm}$ at $780~\mathrm{nm}$, modulation bandwidths in the MHz range, and a phase shifter voltage-length product ($V_πL$) of approximately $2.8~\mathrm{\mathrm{V}\cdot\mathrm{m}}$ and negligible hysteresis. We further demonstrate reconfigurable Mach-Zehnder interferometers based on spiral phase shifters with 0.63 dB loss per phase shifter.

Figures (13)

• Figure 1: Low-confinement silicon nitride piezo-optomechanical platform. (a) Schematic representation of the eight-mode linear programmable nanophotonic processor (LPNP) circuit, based on the Clements et al. Clements16 architecture, featuring a network of Mach-Zehnder interferometers (MZIs) and phase shifters. (b) Schematic of an MZI incorporating two Archimedean spiral phase shifters (SPSs) and two tunable directional couplers (TDCs). (c) Comparison of the voltage-loss product ($\mathrm{VLP}$) and propagation loss across existing visible wavelength aluminum nitride-based piezo-optomechanical modulator platforms Dong22_01Dong22_05. (d) Comparison of modulation speed and power consumption for visible wavelength silicon nitride ($\mathrm{Si}_3\mathrm{N}_4$) based modulator platforms Dong22_01Dong22_05Hosseini:15Yong:22nick26_Blumenthal. The $\mathrm{Si}_3\mathrm{N}_4$ platforms include devices fabricated via low-pressure chemical vapor deposition (LPCVD) and anneal-free plasma-enhanced chemical vapor deposition (PECVD). Details on the loss and $\mathrm{VLP}$ values for existing visible wavelength $\mathrm{Si}_3\mathrm{N}_4$ platforms are provided in Supplementary Table 1. (e) Scanning electron microscope (SEM) image showing the cross-section of a wafer diced through the center of an on-chip SPS, with a minimum bending radius of $150~\mu\mathrm{m}$ at the center. (f) Cross-sectional schematic of a low-confinement waveguide-based piezo-optomechanical phase modulator, highlighting the different material layers. Inset: x-component of the electric field of the fundamental optical mode in a low-confinement waveguide, simulated using the finite element method.
• Figure 1: Summary of propagation loss values from Table \ref{['table1']} for silicon nitride waveguide photonic devices, covering wavelengths from $400~\mathrm{nm}$–$1000~\mathrm{nm}$.
• Figure 2: Piezo-optomechanical phase shifter characterization. (a, b) Cross-sectional SEM images of the fabricated phase modulators. In (b), blue and orange false colors highlight the electrode and the AlN actuator layer, respectively, which is suspended by an undercut. (c) Schematic of the imbalanced MZI used for characterization, incorporating an on-chip spiral phase shifter in one arm, two off-chip 50:50 beam splitters, a 780 nm laser, and a photodetector. A ramp voltage (red) is applied to the piezo-optomechanical actuator, modulating the phase and producing a sinusoidal optical output. (d) Schematic cross-section of the phase shifter with FEM-simulated $s_{11}$ strain distribution and the corresponding (exaggerated) waveguide deformation for clarity. (e) Measured phase response of the MZI for increasing and decreasing voltage sweeps from -10 V to 120 V and back, showing negligible hysteresis. (f) Measured optical response of the MZI under a $200~\mathrm{Hz}$ ramp voltage applied to the piezo-optomechanical SPS. An $8.7~\mathrm{cm}$ SPS achieves a $\pi$ phase shift at $32.5~\mathrm{V}$. (g) Measured S-parameter spectra ($\mathrm{S}{11}$, $\mathrm{S}{21}$) of the SPS, showing strong $\mathrm{S}{21}$ resonances at 3.7, 5.7, 6.6, and 8.3 MHz and negligible $\mathrm{S}{11}$ response over 0–20 MHz. Inset: simulated mechanical eigenmode at 3.7 MHz.
• Figure 2: Strain-induced deformation and photoelastic refractive-index modulation in SPS waveguides: schematics, simulations, and wavelength shift experimental result. a Top- and side-view schematics illustrate the (exaggerated) strain-induced deformation of the released SPS section under an applied voltage. The accompanying plot presents the simulated displacement of the cladding edge along the spiral length. b A top-view schematic illustrates the exaggerated strain-induced deformation of the 4- and 8-released SPS sections under an applied voltage. The $\mathrm{V}_\pi$L reaches values as low as 0.17 V$\cdot$m. c FEM simulation of the strain-induced change in effective refractive index, showing that the photoelastic (PE) contribution ($\sim10^{-7}/\mathrm{V}$) exceeds the moving boundary (MB) contribution ($\sim10^{-9}/\mathrm{V}$) for different cladding widths. d Measured fringe shift when 40 V is applied to the SPS in MZI while scanning the laser wavelength at 780 nm, indicating a positive effective refractive index change under negative strain. This is possible only for the case when the dominant photoelastic contribution in these low-confinement waveguides comes from the silicon oxide cladding.
• Figure 3: Waveguide-loss measurement using top-view imaging. (a) Schematic of the experimental setup for measuring waveguide loss using a top‑down micrograph technique. The setup includes a $780~\mathrm{nm}$ laser, polarization controller (PC), and a camera equipped with a $12\times$ zoom lens and a $5\times$ microscope objective (MO). (b) Propagation loss characterization of an unetched $1.8~\mathrm{m}$ silicon nitride Archimedean spiral waveguide (no actuator). Image analysis of a spiral section shows the average pixel intensity versus propagation distance; linear fitting yields a loss of $2.57~\mathrm{dB/m}$. (c) Propagation loss analysis of an etched $8.2~\mathrm{cm}$‑long Archimedean spiral waveguide with an integrated bottom actuator. Image processing of the complete spiral structure and pixel‑intensity averaging along its length yield a propagation loss of $7.2~\mathrm{dB/m}$.