Sub-doppler Cooling Achieved with Programmable 780-nm Laser and PZT-on-SiN Resonator

Programmability and precise control of laser frequency are crucial for advancements in atomic clocks, quantum computing, and cold-atom research! Andrei Isichenko, Steven Carpenter, and Nick Montifiore, alongside colleagues from the University of California Santa Barbara and the University of Wisconsin-Madison, have developed an electrically controllable optical frequency source that addresses limitations in current bulky systems! Their research details a semiconductor laser stabilised to a photonic-integrated PZT-on-SiN resonator, enabling agile and programmable frequency tuning, demonstrated here with 780-nm laser control for rubidium spectroscopy! Significantly, the team achieved sub-Doppler cooling of rubidium-87 atoms to temperatures as low as 16 K, all without external modulators, paving the way for compact, low-power, and scalable atomic sensing technologies.

The team demonstrated a system where a semiconductor laser is stabilized to a photonic-integrated resonator cavity actuated by lead zirconate titanate (PZT), enabling precision programmable frequency control at 780nm. This innovative approach allows for periodic referencing to rubidium spectroscopy, followed by fast, programmable, arbitrary frequency tuning sequences crucial for quantum control applications. Experiments show the device achieves a tuning strength of up to 1GHz/V with an 11MHz modulation bandwidth, all while consuming a mere 10 nanowatts of electrical power.

This research establishes a novel method for achieving both long-term stability and short-term agility in laser frequency control, overcoming limitations of current bulky and power-hungry systems. The team successfully implemented sub-Doppler cooling of rubidium-87 atoms without relying on any external modulators, reaching atom-cloud temperatures as low as 16 μK. This was accomplished by utilizing the PZT-actuated resonator to dynamically control the laser frequency, enabling precise detuning sequences for polarization-gradient cooling within a 3D magneto-optical trap. The device’s ability to rapidly ramp frequencies, tens of megahertz in milliseconds and gigahertz-scale jumps, is fundamental to achieving ultra-low temperatures and coherence necessary for high-precision sensing and quantum-state readout.

The core innovation lies in integrating PZT on a silicon nitride (SiN) platform, leveraging its wide transparency window, ultra-low optical losses, and compatibility with heterogeneous integration. PZT integration introduces negligible optical loss and operates at nanowatt-level power consumption, providing significantly stronger electromechanical coupling for larger frequency shifts at lower voltages. This work extends stress-optic modulation into the visible spectrum, demonstrating agile and arbitrary frequency control in the 780nm range, a critical wavelength for rubidium atomic physics, and applying it successfully in an atomic experiment for the first time. Furthermore, the demonstrated system offers a pathway toward compact, low-power, and chip-scale laser systems, addressing a critical need for next-generation quantum and atomic sensing technologies. The PZT cavity’s ability to hold frequency stability between periodic re-referencing to rubidium spectroscopy highlights its potential for fully integrated, frequency-programmable laser systems, paving the way for field-deployable quantum devices and advanced atomic sensors with unprecedented performance and portability. This achievement represents a significant step towards miniaturizing and scaling quantum technologies for broader applications.

PZT Resonator Enables Compact Laser Frequency Control

Scientists engineered an electrically controllable optical frequency source utilising a semiconductor laser stabilised to a photonic-integrated lead zirconate titanate (PZT)-actuated resonator cavity! This innovative device circumvents the limitations of bulky, power-hungry modulators commonly used in applications like atomic clocks and computers, paving the way for portable and scalable systems. The research team demonstrated precision programmable frequency control of a 780-nm laser, periodically referencing it to rubidium spectroscopy before implementing fast, arbitrary frequency tuning sequences for control purposes. Fabrication of the device involved depositing a 1-μm thick layer of planar PZT on top of a Si3N4 waveguide, crucially without requiring an undercut, a significant improvement over previous methodologies.

The layer stack comprised a 15-μm SiO2 lower cladding, a 120-nm thick and 900-nm wide Si3N4 core, and a 4-μm SiO2 upper cladding, with a pair of platinum electrodes integrated for thermal tuning of the resonance. Characterisation revealed a high quality factor (Qi = 2.8 M), a loaded QL of 2.3 M, and a propagation loss of 20 dB/m, demonstrating exceptional performance. Electrical-to-optical modulation response measurements, conducted using a network analyser, showed a 6-dB cutoff frequency of 11MHz and a 180° phase-lag point of 5.8MHz. The PZT actuator enabled low-power frequency tuning with a linear tuning strength of 1GHz/V, while consuming less than 10 nW of electrical power.

Wafer-scale uniformity was assessed across a 4″ wafer, with all seven measured devices exhibiting tuning strengths exceeding 100MHz/V, and the tuning polarity dictated by the PZT actuator’s lateral offset. This approach enabled the team to pioneer sub-Doppler cooling of rubidium-87 without any external modulators, achieving atom-cloud temperatures as low as 16 K. To demonstrate this capability, the researchers replaced conventional acousto-optic modulators (AOMs) with the PZT-on-SiN resonator for both magneto-optical trap (MOT) formation and sub-Doppler cooling. A Vescent D2-125-PL servo controller facilitated rapid switching between spectroscopy-referenced operation and free-running, resonator-stabilized operation, allowing arbitrary voltage waveforms to be applied to the PZT for precise frequency control. The 780-nm cooling laser was PHD-stabilised to the ring resonator, with the PZT voltage alternating between rubidium disciplining and agile frequency control, while a fiber-coupled semiconductor optical amplifier (SOA) provided programmable power ramping and shuttering.

Integrated PZT Resonator Enables Agile Laser Control

Scientists have achieved electrically controllable, agile optical frequency control using a semiconductor laser stabilized to a photonic-integrated lead zirconate titanate (PZT)-actuated resonator cavity! This breakthrough delivers a compact and low-power laser system, paving the way for next-generation atomic sensing technologies. The team measured sub-Doppler cooling of rubidium-87 atoms to temperatures as low as 16 K, demonstrating the system’s capability without any external modulators. This remarkable feat was accomplished by precisely controlling the laser frequency using the integrated PZT resonator, effectively replacing traditional bulky acousto-optic modulators.

Experiments revealed a tuning strength of up to 1GHz/V with an 11MHz modulation bandwidth, while the device consumed only 10 nW of electrical power! The PZT-on-SiN resonator, with a radius of 750μm and a free-spectral range of 38GHz, provides an integrated platform for locked laser frequency control, combining short-term stability with fast, programmable tuning. Researchers characterized the resonator, measuring an intrinsic quality factor (Qi) of 2.8 M and a loaded quality factor (QL) of 2.3 M, alongside a propagation loss of 20 dB/m. These measurements confirm the high performance and efficiency of the integrated photonic circuit.

Data shows the small-signal electrical-to-optical modulation response (S21) exhibited a 6-dB cutoff frequency of 11MHz and a 180° phase-lag point of 5.8MHz. The optical transmission spectrum demonstrated resonance tuning as a function of DC control voltage, achieving a linear tuning strength of 1GHz/V. Wafer-scale uniformity was evaluated across a 4″ wafer, with all seven measured devices exhibiting tuning strengths exceeding 100MHz/V. The device with the highest QL and tuning strength was then packaged and successfully used in the atom cooling demonstration. The team implemented a dual-stage lock to a rubidium saturation absorption spectroscopy (SAS) module, anchoring the laser to an atomic transition and minimizing long-term frequency drift.

By periodically disabling the Rb spectroscopy lock and driving the PZT with an arbitrary waveform generator, they generated laser frequency ramps and jumps essential for magneto-optical trap (MOT) formation and polarization gradient cooling (PGC). This approach allows for rapid red-detuning of the cooling light, achieving sub-Doppler temperatures below the 146 μK Doppler limit. Results demonstrate the potential for fully integrated, frequency-programmable laser systems for quantum sensing and atom interferometry.

Compact Laser Cools Rubidium to 16K, a new

Scientists have developed an electrically controllable optical frequency source utilising a semiconductor laser stabilised to a photonic-integrated lead zirconate titanate (PZT)-actuated resonator cavity! This innovative device allows for precise and programmable control of laser frequency, overcoming limitations associated with bulky and power-intensive conventional systems. Researchers demonstrated sub-Doppler cooling of rubidium-87 atoms without external modulators, achieving temperatures as low as 16 K, showcasing the device’s capabilities in atom manipulation! The achievement centres on a compact, low-power laser system capable of tuning up to 1GHz/V with an 11MHz modulation bandwidth, consuming only 10 nW of electrical power, a significant advancement for portable and scalable atomic sensing technologies.

The authors acknowledge a current limitation in data availability, stating that the underlying data is not publicly accessible but can be requested from them! Future work could focus on broader device characterisation and exploring applications in more complex quantum systems, as suggested by the demonstrated cooling capabilities. This research establishes a promising pathway towards chip-scale laser systems with implications for atomic clocks, quantum computing, and cold-atom experiments, all while drastically reducing power consumption and system size.