On-chip thermo-optic tuning of suspended microresonators

Magnetic tuning with minimal thermal drift in high-Q microspheres coated with magnetorheological polydimethylsiloxane

Integration of whispering-gallery-mode (WGM) resonators with high-quality factors (Q) into advanced timing, oscillator, and sensing systems demands a platform that enables precise resonance frequency modulation. This study investigates the tuning characteristics of magnetorheological polydimethylsiloxane (MR-PDMS) coated microspheres (µ-spheres) employed as magnetic microresonators, achieving a Q value of 107 at the 1550 nm wavelength. Magnetic WGM resonators not only endow the device with magnetic adjustability but also markedly improve thermal resistance. Experimental findings reveal that the magnetic µ-sphere demonstrates a sensitivity of -32.53 MHz/mT, outperforming conventional magnetic WGM resonators. Furthermore, analysis of the temperature dependence shows a reduction in fluctuation to -2.85 MHz/K, thereby greatly enhancing the sensor's practical detection limit.

Enhanced sensing of optomechanically induced nonlinearity by linewidth suppression and optical bistability in cavity-waveguide systems

We study the enhanced sensing of optomechanically induced nonlinearity (OMIN) in a cavity-waveguide coupled system. The Hamiltonian of the system is anti-PT symmetric, with the two involved cavities being dissipatively coupled via the waveguide. The anti-PT symmetry may break down when a weak waveguide-mediated coherent coupling is introduced. However, we find a strong bistable response of the cavity intensity to the OMIN near the cavity resonance, benefiting from linewidth suppression caused by the vacuum induced coherence. The joint effect of optical bistability and the linewidth suppression is inaccessible by the anti-PT symmetric system involving only dissipative coupling. Due to that, the sensitivity measured by an enhancement factor is greatly enhanced by two orders of magnitude compared to that for the anti-PT symmetric model. Moreover, the enhancement factor shows resistance to a reasonably large cavity decay and robustness to fluctuations in the cavity-waveguide detuning. Based on the integrated optomechanical cavity-waveguide systems, the scheme can be used for sensing different physical quantities related to the single-photon coupling strength and has potential applications in high-precision measurements with systems involving Kerr-type nonlinearity.

Tuning of silicon nitride micro-cavities by controlled nanolayer deposition

Integration of single-photon emitters (SPEs) with resonant photonic structures is a promising approach for realizing compact and efficient single-photon sources for quantum communications, computing, and sensing. Efficient interaction between the SPE and the photonic cavity requires that the cavity's resonance matches the SPE's emission line. Here we demonstrate a new method for tuning silicon nitride (Si₃N₄) microring cavities via controlled deposition of the cladding layers. Guided by numerical simulations, we deposit silicon dioxide (SiO₂) nanolayers onto Si₃N₄ ridge structures in steps of 50 nm. We show tuning of the cavity resonance exceeding a free spectral range (FSR) of 3.5 nm without degradation of the quality-factor (Q-factor) of the cavity. We then complement this method with localized laser heating for fine-tuning of the cavity. Finally, we verify that the cladding deposition does not alter the position and spectral properties of nanoparticles placed on the cavity, which suggests that our method can be useful for integrating SPEs with photonic structures.

Compact thermo-optic modulator based on a titanium dioxide micro-ring resonator

Thermo-optic (TO) modulators with the ability of working from the visible to the infrared spectrum are promising for many emerging applications. However, current technologies suffer from either a limited operating spectrum range or weak TO effect. In this work, we present an effective TO modulator based on a titanium dioxide TiO₂ micro-ring resonator with solgel SiO₂ as the cladding. Taking advantage of the large negative TO coefficients of TiO₂ and solgel SiO₂, the fabricated device demonstrates a temperature-dependent wavelength shift of 58.3 pm/°C and a π-shift power consumption of 7.8 mW. Since both TiO₂ and solgel SiO₂ have a broad transmission window, the demonstrated device will have wide applications in integrated optics from the visible to the infrared wavelength range.

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.

Performance of integrated optical switches based on 2D materials and beyond

Applications of optical switches, such as signal routing and data-intensive computing, are critical in optical interconnects and optical computing. Integrated optical switches enabled by two-dimensional (2D) materials and beyond, such as graphene and black phosphorus, have demonstrated many advantages in terms of speed and energy consumption compared to their conventional silicon-based counterparts. Here we review the state-of-the-art of optical switches enabled by 2D materials and beyond and organize them into several tables. The performance tables and future projections show the frontiers of optical switches fabricated from 2D materials and beyond, providing researchers with an overview of this field and enabling them to identify existing challenges and predict promising research directions.

Optothermal dynamics in whispering-gallery microresonators

Optical whispering-gallery-mode microresonators with ultrahigh quality factors and small mode volumes have played an important role in modern physics. They have been demonstrated as a diverse platform for a wide range of applications in photonics, such as nonlinear optics, optomechanics, quantum optics, and information processing. Thermal behaviors induced by power build-up in the resonators or environmental perturbations are ubiquitous in high-quality-factor whispering-gallery-mode resonators and have played an important role in their operation for various applications. In this review, we discuss the mechanisms of laser-field-induced thermal nonlinear effects, including thermal bistability and thermal oscillation. With the help of the thermal bistability effect, optothermal spectroscopy and optical nonreciprocity have been demonstrated. By tuning the temperature of the environment, the resonant mode frequency will shift, which can also be used for thermal sensing/tuning applications. The thermal locking technique and thermal imaging mechanisms are discussed briefly. Finally, we review some techniques employed to achieve thermal stability in a high-quality-factor resonator system.

High-Q microresonators integrated with microheaters on a 3C-SiC-on-insulator platform

We demonstrate, to the best of our knowledge, the first thermally reconfigurable high-Q silicon carbide (SiC) microring resonators with integrated microheaters on a 3C-SiC-on-insulator platform. We extract a thermo-optic coefficient of around 2.67×10^-5/K for 3C-SiC from wavelength shift of a resonator heated by a hot plate. Finally, we fabricate a 40-μm-radius microring resonator with intrinsic Q of 139,000 at infrared wavelengths (∼1550  nm) after integration with a NiCr microheater. By applying current through the microheater, a resonance shift of 30 pm/mW is achieved in the microring, corresponding to ∼50  mW per π phase shift. This platform offers an easy and reliable way for integration with electronic devices as well as great potential for diverse integrated optics applications.

Field-programmable silicon temporal cloak

Temporal cloaks have aroused tremendous research interest in both optical physics and optical communications, unfolding a distinct approach to conceal temporal events from an interrogating optical field. The state-of-the-art temporal cloaks exhibit picosecond-scale and static cloaking window, owing to significantly limited periodicity and aperture of time lens. Here we demonstrate a field-programmable silicon temporal cloak for hiding nanosecond-level events, enabled by an integrated silicon microring and a broadband optical frequency comb. With dynamic control of the driving electrical signals on the microring, our cloaking windows could be stretched and switched in real time from 0.449 ns to 3.365 ns. Such a field-programmable temporal cloak may exhibit practically meaningful potentials in secure communication, data compression, and information protection in dynamically varying events.

Free spectral range electrical tuning of a high quality on-chip microcavity

Reconfigurable photonic circuits have applications ranging from next-generation computer architectures to quantum networks, coherent radar and optical metamaterials. Here, we demonstrate an on-chip high quality microcavity with resonances that can be electrically tuned across a full free spectral range (FSR). FSR tuning allows resonance with any source or emitter, or between any number of networked microcavities. We achieve it by integrating nanoelectronic actuation with strong optomechanical interactions that create a highly geometry-dependent effective refractive index. This allows low voltages and sub-nanowatt power consumption. We demonstrate a basic reconfigurable photonic network, bringing the microcavity into resonance with an arbitrary mode of a microtoroidal optical cavity across a telecommunications fibre link. Our results have applications beyond photonic circuits, including widely tuneable integrated lasers, reconfigurable optical filters for telecommunications and astronomy, and on-chip sensor networks.

Digitally controlled multiplexed silicon photonics phase shifter using heaters with integrated diodes

We present a silicon side heater with integrated diode to provide multiplexed control of different elements in a photonic circuit based on the polarity of the driving signal. The diode introduces an asymmetric electrical response where the heater is only active under forward bias. This can be used to address multiple heaters through the same electrical electrical contacts. We demonstrate push-pull operation on a Mach-Zehnder interferometer with heaters in both arms, as well as time-multiplexed operation of multiple heaters by modulating the driving signal. We extend this work by demonstrating how pulse width modulation (PWM) and duobinary-PWM can be used to improve the linearity of the response of the phase shifters.