arXiv subjects

Kaikai Liu

Publications and source records attributed to Kaikai Liu.

28 records · Page 2Linked to original sources

arXiv2022

Silicon nitride stress-optic microresonator modulator for optical control applications

The silicon nitride integration platform has been successful at realizing extremely low waveguide losses across the visible to infrared and components including high performance lasers, filters, resonators, stabilization cavities, and optical frequency combs. Yet, progress towards implementing low loss, low power modulators in the silicon nitride platform, while maintaining the planar, wafer-scale process compatibility has been limited. Here we report a significant advance in integration of a piezo-electric (PZT) actuated micro-ring modulation in a fully-planar, wafer-scale silicon nitride platform, that maintains low optical loss (0.03 dB/cm in a 625 um resonator) at 1550 nm, with an order of magnitude increase in bandwidth (DC to 20 MHz 3-dB) and order of magnitude lower power consumption of 20 nW improvement over prior PZT modulators. The modulator provides a >14 dB ER and 7.1 million Q over the entire 4 GHz tuning range, a tuning efficiency of 200 MHz/V, and delivers the linearity required for control applications with 65.1 dBHz2/3 and 73.8 dBHz2/3 IMD3 SFDR at 1 MHz and 10 MHz respectively. We demonstrate two control applications, laser stabilization in a PDH lock loop, reducing laser frequency noise by 40 dB, and as a laser carrier tracking filter. This PZT modulator design can be extended to the visible in the ultra-low loss silicon nitride platform with minor waveguide design changes. This integration of PZT modulation in the ultra-low loss silicon nitride waveguide platform enables modulator control functions in a wide range of visible to IR applications such as atomic and molecular transition locking for cooling, controllable optical frequency combs, low-power external cavity tunable lasers, atomic clocks, and tunable ultra-low linewidth lasers and ultra-low phase noise microwave synthesizers.

physics.optics↗

arXiv2021

36 Hz integral linewidth laser based on a photonic integrated 4.0-meter coil resonator

Laser stabilization sits at the heart of many precision scientific experiments and applications, including quantum information science, metrology and atomic timekeeping. These systems narrow the laser linewidth and stabilize the carrier by use of Pound-Drever-Hall (PDH) locking to a table-scale, ultra-high quality factor (Q), vacuum spaced Fabry-Perot reference cavity. Integrating these cavities, to bring characteristics of PDH stabilization to the chip-scale, is critical to reduce their size, cost, and weight, and enable a wide range of portable and system-on-chip applications. We report a significant advance in integrated laser linewidth narrowing, stabilization and noise reduction, by use of a photonic integrated 4.0-meter-long coil resonator to stabilize a semiconductor laser. We achieve a 36 Hz 1/π-integral linewidth, an Allan deviation (ADEV) of 1.8x10^{-13} at 10 ms measurement time, and a 2.3 kHz/sec drift, to the best of our knowledge the lowest integral linewidth and highest stability demonstrated for an integrated reference cavity. Two coil designs, stabilizing lasers operating at 1550 nm and 1319 nm are demonstrated. The resonator is bus coupled to a 4.0-meter-long coil, with a 49 MHz free spectral range (FSR), a mode volume of 1.0x10^{10} μm^3 and a 142 million intrinsic Q, fabricated in a CMOS compatible, ultra-low loss silicon nitride waveguide platform. Our measurements and simulations show that the thermorefractive noise floor for this particular cavity is reached for frequencies down to 20 Hz in an ambient environment with simple passive vibration isolation and without vacuum or thermal isolation. The TRN limited performance is estimated to be an 8 Hz 1/π integral linewidth and ADEV of 5x10^{-14} at 10 ms, opening a stability regime that heretofore has only been available in fundamentally un-integrated systems.

physics.optics↗

arXiv2021

Thermal and driven noise in Brillouin Lasers

Owing to their highly coherent emission and compact form factor, Brillouin lasers have been identified as a valuable asset for applications including portable atomic clocks, precision sensors, coherent microwave synthesis and energy-efficient approaches to coherent communications. While the fundamental emission linewidth of these lasers can be very narrow, noise within dielectric materials leads to drift in the carrier frequency, posing vexing challenges for applications requiring ultra-stable emission. A unified understanding of Brillouin laser performance may provide critical insights to reach new levels of frequency stability, however existing noise models focus on only one or a few key noise sources, and do not capture the thermo-optic drift in the laser frequency produced by thermal fluctuations or absorbed power. Here, we develop a coupled mode theory of Brillouin laser dynamics that accounts for dominant forms of noise in non-crystalline systems, capturing the salient features of the frequency and intensity noise for a variety of systems. As a result, theory and experiment can be directly compared to identify key sources of noise and the frequency bands they impact, revealing strategies to improve the performance of Brillouin lasers and pave the way for highly-coherent sources of light on a chip.

physics.optics↗

arXiv2021

Photonic circuits for laser stabilization with ultra-low-loss and nonlinear resonators

Laser-frequency stabilization with on-chip photonic integrated circuits will provide compact, low cost solutions to realize spectrally pure laser sources. Developing high-performance and scalable lasers is critical for applications including quantum photonics, precision navigation and timing, spectroscopy, and high-capacity fiber communications. We demonstrate a significant advance in compact, stabilized lasers to achieve a record low integral emission linewidth and precision carrier stabilization by combining integrated waveguide nonlinear Brillouin and ultra-low loss waveguide reference resonators. Using a pair of 56.4 Million quality factor (Q) Si$_3$N$_4$ waveguide ring-resonators, we reduce the free running Brillouin laser linewidth by over an order of magnitude to 330 Hz integral linewidth and stabilize the carrier to 6.5$\times$10$^{-13}$ fractional frequency at 8 ms, reaching the cavity-intrinsic thermorefractive noise limit for frequencies down to 80 Hz. This work demonstrates the lowest linewidth and highest carrier stability achieved to date using planar, CMOS compatible photonic integrated resonators, to the best of our knowledge. These results pave the way to transfer stabilized laser technology from the tabletop to the chip-scale. This advance makes possible scaling the number of stabilized lasers and complexity of atomic and molecular experiments as well as reduced sensitivity to environmental disturbances and portable precision atomic, molecular and optical (AMO) solutions.

physics.optics↗

arXiv2021

Fully symmetric controllable integrated three-resonator photonic molecule

Photonic molecules can be used to realize complex optical energy states and modes, analogous to those found in molecules, with properties useful for applications like spectral engineering and quantum optics. It is desirable to implement photonic molecules using high quality factor photonic integrated ring resonators due to their narrow atom-like spectral resonance, tunability, and the ability to scale the number of resonators on a photonic circuit. However, to take full advantage of molecule spectral complexity and tuning degree of freedom, resonator structures should have full symmetry in terms of inter-resonator coupling and resonator-waveguide coupling as well as independent resonance tuning, and low power dissipation operation, in a scalable integration platform. To date, photonic molecule symmetry has been limited to dual- and triple-cavity geometries coupled to single- or dual-busses, and resonance tuning limited to dual resonator molecules. In this paper, we demonstrate a three-resonator photonic molecule, consisting of symmetrically coupled 8.11 million intrinsic Q silicon nitride rings, where each ring is coupled to the other two rings. The resonance of each ring, and that of the collective molecule, is controlled using low power dissipation, monolithically integrated thin-film lead zirconate titanate (PZT) actuators that are integrated with the ultra-low loss silicon nitride resonators. This performance is achieved without undercut waveguides, yielding the highest Q to date for a PZT controlled resonator. This advance leads to full control of complex photonic molecule resonance spectra and splitting in a wafer-scale integration platform. The resulting six tunable supermodes can be fully controlled, including degeneracy, location and splitting as well as designed by a model that can accurately predict the energy modes and transmission spectrum and tunable resonance splitting.

physics.optics↗

arXiv2021

Visible light photonic integrated Brillouin laser

Narrow linewidth visible light lasers are critical for atomic, molecular and optical (AMO) applications including atomic clocks, quantum computing, atomic and molecular spectroscopy, and sensing. Historically, such lasers are implemented at the tabletop scale, using semiconductor lasers stabilized to large optical reference cavities. Photonic integration of high spectral-purity visible light sources will enable experiments to increase in complexity and scale. Stimulated Brillouin scattering (SBS) is a promising approach to realize highly coherent on-chip visible light laser emission. While progress has been made on integrated SBS lasers at telecommunications wavelengths, barriers have existed to translate this performance to the visible, namely the realization of Brillouin-active waveguides in ultra-low optical loss photonics. We have overcome this barrier, demonstrating the first visible light photonic integrated SBS laser, which operates at 674 nm to address the 88Sr+ optical clock transition. To guide the laser design, we use a combination of multi-physics simulation and Brillouin spectroscopy in a 2 meter spiral waveguide to identify the 25.110 GHz first order Stokes frequency shift and 290 MHz gain bandwidth. The laser is implemented in an 8.9 mm radius silicon nitride all-waveguide resonator with 1.09 dB per meter loss and Q of 55.4 Million. Lasing is demonstrated, with an on-chip 14.7 mW threshold, a 45% slope efficiency, and linewidth narrowing as the pump is increased from below threshold to 269 Hz. To illustrate the wavelength flexibility of this design, we also demonstrate lasing at 698 nm, the wavelength for the optical clock transition in neutral strontium. This demonstration of a waveguide-based, photonic integrated SBS laser that operates in the visible, and the reduced size and sensitivity to environmental disturbances, shows promise for diverse AMO applications.

physics.optics↗

arXiv2020

422 Million Q Planar Integrated All-Waveguide Resonator with a 3.4 Billion Absorption Limited Q and Sub-MHz Linewidth

High Q optical resonators are a key component for ultra-narrow linewidth lasers, frequency stabilization, precision spectroscopy and quantum applications. Integration of these resonators in a photonic waveguide wafer-scale platform is key to reducing their cost, size and power as well as sensitivity to environmental disturbances. However, to date, the intrinsic Q of integrated all-waveguide resonators has been relegated to below 150 Million. Here, we report an all-waveguide Si3N4 resonator with an intrinsic Q of 422 Million and a 3.4 Billion absorption loss limited Q. The resonator has a 453 kHz intrinsic linewidth and 906 kHz loaded linewidth, with a finesse of 3005. The corresponding linear loss of 0.060 dB/m is the lowest reported to date for an all-waveguide design with deposited upper cladding oxide. These are the highest intrinsic and absorption loss limited Q factors and lowest linewidth reported to date for a photonic integrated all-waveguide resonator. This level of performance is achieved through a careful reduction of scattering and absorption loss components. We quantify, simulate and measure the various loss contributions including scattering and absorption including surface-state dangling bonds that we believe are responsible in part for absorption. In addition to the ultra-high Q and narrow linewidth, the resonator has a large optical mode area and volume, both critical for ultra-low laser linewidths and ultra-stable, ultra-low frequency noise reference cavities. These results demonstrate the performance of bulk optic and etched resonators can be realized in a photonic integrated solution, paving the way towards photonic integration compatible Billion Q cavities for precision scientific systems and applications such as nonlinear optics, atomic clocks, quantum photonics and high-capacity fiber communications systems on-chip.

physics.optics↗

arXiv2020

BAlN alloy for enhanced two-dimensional electron gas characteristics of GaN-based high electron mobility transistor

The emerging wide bandgap BAlN alloys have potentials for improved III-nitride power devices including high electron mobility transistor (HEMT). Yet few relevant studies have been carried. In this work, we have investigated the use of the B0.14Al0.86N alloy as part or entirety of the interlayer between the GaN buffer and the AlGaN barrier in the conventional GaN-based high electron mobility transistor (HEMT). The numerical results show considerable improvement of the two-dimensional electron gas (2DEG) concentration with small 2DEG leakage into the ternary layer by replacing the conventional AlN interlayer by either the B0.14Al0.86N interlayer or the B0.14Al0.86N/AlN hybrid interlayer. Consequently, the transfer characteristics can be improved. The saturation current can be enhanced as well. For instance, the saturation currents for HEMTs with the 0.5 nm B0.14Al0.86N/0.5 nm AlN hybrid interlayer and the 1 nm B0.14Al0.86N interlayer are 5.8% and 2.2% higher than that for the AlN interlayer when VGS-Vth= +3 V.

physics.app-ph↗

arXiv2020

Chance-Constrained and Yield-aware Optimization of Photonic ICs with Non-Gaussian Correlated Process Variations

Uncertainty quantification has become an efficient tool for uncertainty-aware prediction, but its power in yield-aware optimization has not been well explored from either theoretical or application perspectives. Yield optimization is a much more challenging task. On one side, optimizing the generally non-convex probability measure of performance metrics is difficult. On the other side, evaluating the probability measure in each optimization iteration requires massive simulation data, especially when the process variations are non-Gaussian correlated. This paper proposes a data-efficient framework for the yield-aware optimization of photonic ICs. This framework optimizes the design performance with a yield guarantee, and it consists of two modules: a modeling module that builds stochastic surrogate models for design objectives and chance constraints with a few simulation samples, and a novel yield optimization module that handles probabilistic objectives and chance constraints in an efficient deterministic way. This deterministic treatment avoids repeatedly evaluating probability measures at each iteration, thus it only requires a few simulations in the whole optimization flow. We validate the accuracy and efficiency of the whole framework by a synthetic example and two photonic ICs. Our optimization method can achieve more than $30\times$ reduction of simulation cost and better design performance on the test cases compared with a Bayesian yield optimization approach developed recently.

math.OC↗

arXiv2020

Polarization properties of wurtzite III nitride indicate the principle of polarization engineering

The spontaneous and piezoelectric polarizations of III-nitrides considerably affect the operation of various III-nitride-based devices. We report an ab initio study of the spontaneous polarization (SP) and piezoelectric (PZ) constants of the III-nitride binary and ternary alloys with the hexagonal reference structure. These calculated polarization properties offer us a profound principle for polarization engineering of nitride semiconductor devices, based on which we propose a few heterojunctions which have nearly-zero polarization effect at the junctions that can potentially enhance optical and power device performances. The polarization doping effect was investigated as well and by BAlN grading from AlN the polarization doping effect can be doubled.

physics.app-ph↗