Blue to Near-IR Integrated PZT Silicon Nitride Modulators for Quantum and Atomic Applications

Technical Summary: Blue to Near-IR Integrated PZT Silicon Nitride Modulators for Quantum and Atomic Applications

Problem Statement
The realization of portable, robust, and compact quantum technologies—specifically trapped-ion and neutral atom quantum computers, atomic clocks, and sensors—requires high-efficiency integrated optical modulators. Current challenges stem from the diversity of atomic species and experimental architectures, which necessitate modulation capabilities across a wide spectrum from the visible (blue) to the near-infrared (NIR). Existing solutions often lack the combination of wavelength independence, CMOS compatibility, low waveguide loss, high resonator quality factor ($Q$), DC-coupled broadband frequency response, and low power consumption required for scalable photonic integration. While stress-optic modulation has been demonstrated at communication wavelengths, progress in achieving these specific characteristics across the visible-to-NIR range within a CMOS-compatible platform has been limited.

Methodology
The authors utilize a CMOS-compatible silicon nitride ($Si_3N_4$) integration platform known for ultra-low optical loss across the blue to NIR spectrum. The core methodology involves the monolithic integration of lead zirconate titanate (PZT) actuators with $Si_3N_4$ waveguides.

• Fabrication: The process employs ultra-low loss $Si_3N_4$ waveguides (20 nm thick for 493/532 nm devices; 120 nm thick for 780 nm devices). PZT actuators and platinum electrodes are deposited as a back-end process after the upper cladding oxide deposition, avoiding the undercut structures required in aluminum nitride (AlN) stress-optic modulators.
• Actuation Mechanism: The PZT actuators are offset from the waveguide (0–5 µm) to induce lateral strain across the nitride core via the stress-optic effect, changing the refractive index without significantly overlapping the optical mode or increasing waveguide loss.
• Device Designs: Four distinct modulator architectures were fabricated and tested:
  1. Coil Mach-Zehnder Modulator (MZM): Features a 5 cm long PZT actuator on one arm.
  2. Coil Pure Phase Modulator: A straight waveguide with a 5 cm PZT actuator (derived from the MZM design).
  3. Bus-Coupled Ring Resonator: A critically coupled ring with a 750 µm radius.
  4. Add-Drop Ring Resonator: An add-drop configuration operating at 780 nm.

Key Contributions and Results
The paper demonstrates four types of integrated stress-optic modulators operating at 493 nm, 532 nm, and 780 nm, achieving the following performance metrics:

• Coil MZM (532 nm):

  • Operates with a half-wave voltage ($V_\pi$) of 2.8 V.
  • Achieves an extinction ratio (ER) of 21.5 dB.
  • Exhibits a DC-coupled 3-dB bandwidth of 0.4 MHz and a 6-dB bandwidth of 0.6 MHz.
  • Shows a 90/10 rise time of 1.7 µs.
  • Maintains low optical waveguide loss ($\alpha = 0.24$ dB/cm) and consumes only ~5 nW of power.
  • The tuning coefficient is 716 MHz/V ($0.68$ pm/V), with a $V_\pi L$ of 14.0 V·cm.

• Coil Phase Modulator (493 nm):

  • Operates with a $V_\pi$ of 2.8 V.
  • Demonstrates a 180° phase lag bandwidth of 1 MHz.
  • Achieves a low residual amplitude modulation (RAM) of -34 dB at a 1 kHz offset, a critical metric for applications like PDH locking.
  • Power consumption is negligible (~20 nW total for the device).

• Bus-Coupled Ring Resonator (493 nm):

  • Intrinsic quality factor ($Q_i$) of 3.4 million; loaded $Q$ ($Q_L$) of 1.9 million.
  • Linear tuning strength of 0.92 GHz/V (outside the hysteresis regime).
  • DC-coupled 3-dB bandwidth of 2.6 MHz.
  • Extinction ratio of 18.7 dB.

• Add-Drop Ring Resonator (780 nm):

  • $Q_i$ of 1.9 million; $Q_L$ of 0.7 million.
  • Linear tuning strength of 1.01 GHz/V.
  • DC-coupled 3-dB bandwidth of 10 MHz.
  • Extinction ratio of 12.1 dB.

All devices operate with DC coupling and broadband frequency response. The PZT actuators are driven with voltages up to 20 V, with leakage currents below 1 nA, resulting in power consumption in the tens of nanowatts per actuator.

Significance and Claims
The authors claim that this work establishes a clear path toward fully integrated quantum technologies on the silicon nitride platform. By demonstrating versatile modulators that operate from 493 nm to 780 nm with high efficiency, low power consumption, and compatibility with CMOS foundry processes, the platform addresses the specific needs of diverse atomic systems (e.g., Barium ions at 493/532 nm and Rubidium at 780 nm).

The significance lies in the ability to maintain the native low optical loss and high $Q$ of $Si_3N_4$ while adding active modulation capabilities. The strong DC stress-optic coefficient of PZT provides quiescent set points and tuning strength orders of magnitude higher than other stress-optic actuators. The authors posit that these solutions enable the integration of modulators with other precision silicon nitride components, such as lasers, to realize chip-scale atomic and quantum systems. The demonstrated bandwidths (up to 10 MHz) and low RAM are specifically noted as suitable for atomic quantum control functions, including polarization gradient cooling, Zeeman qubits, and PDH locking of integrated lasers to references.