Aluminum Nitride Microresonators Research Articles

Dual-mode microresonators as straightforward access to octave-spanning dissipative Kerr solitons

The Kerr soliton frequency comb is a revolutionary compact ruler of coherent light that allows applications from precision metrology to quantum information technology. The universal, reliable, and low-cost soliton microcomb source is key to these applications. As a development and extension of the direct creation of a soliton microcomb with the dual-mode scheme in an aluminum nitride microresonator, this paper thoroughly presents the design strategy to reliably attain such dual-modes in the silicon nitride (Si3N4) platform, separated by ∼10 GHz, which stabilizes soliton formation without using additional auxiliary laser or RF components. We demonstrate the deterministic generation of the refined single-solitons that span 1.5-octaves, i.e., near 200 THz, via adiabatic pump wavelength tuning. The ultra-wide soliton existence range up to 17 GHz not only suggests the robustness of the system but will also extend the applications of soliton combs. Moreover, the proposed scheme is found to easily give rise to multi-solitons as well as the soliton crystals featuring enhanced repetition rate (2 and 3 THz) and conversion efficiency greater than 10%. We also show the effective thermal tuning of mode separation to increase the possibility to access the single-soliton state. Our results are crucial for the chip-scale self-referenced frequency combs with a simplified configuration.

Octave-spanning Kerr frequency comb generation with stimulated Raman scattering in an AlN microresonator.

Octave-spanning optical frequency combs (OFCs) are essential for various applications, such as precision metrology and astrophysical spectrometer calibration. In this Letter, we demonstrate, for the first time to our knowledge, the generation of octave-spanning Kerr frequency combs ranging from 1150 to 2400 nm in aluminum nitride (AlN) microring resonators, by pumping the TM00 modes at 250 mW on-chip power. By simply adjusting the pump detuning, we observe the transition and coexistence of Kerr OFC and stimulated Raman scattering. For the TE00 mode in the same device, a broadband Raman-assisted frequency comb is demonstrated by adjusting the pump power and tuning. These results indicate a crucial development for the fundamentals of nonlinear dynamics and comb applications in AlN.

Photolithography allows high-Q AlN microresonators for near octave-spanning frequency comb and harmonic generation.

Single-crystal aluminum nitride (AlN) possessing both strong Pockels and Kerr nonlinear optical effects as well as a very large band gap is a fascinating optical platform for integrated nonlinear optics. In this work, fully etched AlN-on-sapphire microresonators with a high-Q of 2.1 × 106 for the TE00 mode are firstly demonstrated with the standard photolithography technique. A near octave-spanning Kerr frequency comb ranging from 1100 to 2150 nm is generated at an on-chip power of 406 mW for the TM00 mode. Due to the high confinement, the TE10 mode also excites a Kerr comb from 1270 to 1850nm at 316 mW. In addition, frequency conversion to visible light is observed during the frequency comb generation. Our work will lead to a large-scale, low-cost, integrated nonlinear platform based on AlN.

Generation of stable and breathing flat-top solitons via Raman assisted four wave mixing in microresonators

Flat-top-soliton (or platicon) dynamics in coherently pumped normal dispersion microresonators is important for both fundamental nonlinear physics and microcomb generation in the visible band. Here we numerically investigate the platicon generation that is initiated via Raman assisted four wave mixing instead of mode interaction. To show the possibility of generating coherent combs in the visible band, we design an aluminum nitride (AlN) microresonator with normal dispersion and investigate the comb generation dynamics in simulations. Stable platicon Kerr combs can be generated in this AlN microresonator using a 780-nm pump. Moreover, we also observe a breather platicon dynamics induced by the narrow Raman gain spectrum of crystalline AlN, which shows distinct dynamics from the dark soliton breathers reported in previous work that are dominated by Kerr effect. A phase diagram bearing the influence of the pump detuning and pump power on the breathing dynamics of the breather platicon is also computed. Furthermore, a transition to chaotic breathing is numerically observed.

17 000%/W second-harmonic conversion efficiency in single-crystalline aluminum nitride microresonators

High quality factor optical microcavities have been employed in a variety of material systems to enhance nonlinear optical interactions. While single-crystalline aluminum nitride microresonators have recently emerged as a low loss platform for integrated nonlinear optics such as four wave mixing and Raman lasing, few studies have investigated this material for second-harmonic generation. In this letter, we demonstrate an optimized fabrication of dually resonant phase-matched ring resonators from epitaxial aluminum nitride thin films. An unprecendented second-harmonic generation efficiency of 17 000%/W is obtained in the low power regime, and pump depletion is observed at a relatively low input power of 3.5 mW. This poses epitaxial aluminum nitride as the highest efficiency second-harmonic generator among current integrated platforms.

High-fidelity cavity soliton generation in crystalline AlN micro-ring resonators.

Chip-scale mode-locked dissipative Kerr solitons have been realized on various materials platforms, making it possible to achieve a miniature, highly coherent frequency comb source with high repetition rates. Aluminum nitride (AlN), an appealing nonlinear optical material having both Kerr (χ3), and Pockels (χ2) effects, has immerse potential for comb self-referencing without the need for external harmonic generators. However, cavity soliton states have not yet been achieved in AlN microresonators. Here, we demonstrate mode-locked Kerr cavity soliton generation in a crystalline AlN microring resonator. By utilizing ultrafast tuning of the pump frequency through single-sideband modulation, in combination with an optimized wavelength scan and pump power-ramp patterns, we can deterministically elongate a ∼400 ns short-lived soliton to a time span as long as we wish to hold it.

Contour-Mode Ring-Shaped AlN Microresonator on Si and Feasibility of Its Application in Series-Resonant Converter

Microelectromechanical systems (MEMS) resonators on Si have the potential to replace the discrete passive components in a power converter. The main intention of this paper is to present a ring-shaped aluminum nitride (AlN) piezoelectric microresonator that can be used as an energy-transferring device to replace inductors/capacitors in low-power resonant converters for biomedical applications. Finite-element simulation results have been provided, showing the mode of vibration at resonant frequency. The zero-voltage switching (ZVS) condition for a series-resonant converter incorporating the proposed MEMS resonator has been presented analytically and verified through experiment. This ZVS condition can be found in terms of the equivalent circuit parameters of the resonator. To the knowledge of the authors, ZVS analysis based on the equivalent electrical circuit model of the thin piezoelectric film resonators has not yet been reported in the literature. A CMOS-compatible fabrication process has been proposed and implemented. In addition, the fabricated devices have been characterized and experimental results are included in this paper. The first contour-mode AlN MEMS resonator with moderately low resonant frequency and motional resistance is reported in this paper with measured resonant frequency and motional resistance of 87.28 MHz and 36.728 Ω, respectively.

Frequency Trimming of Aluminum Nitride Microresonators Using Rapid Thermal Annealing

To transition aluminum nitride (AlN) microresonator filters into a manufacturable technology, accurate control of the resonator frequency across a wafer is required. This paper describes a postfabrication rapid thermal anneal approach to trim resonator frequency over 27000 ppm with an accuracy of 500 ppm. Measurements made on 22.4 MHz resonators indicate that the effect of annealing on the resonators saturates in 5 min and upshift the resonator frequency super linearly with temperature. We replicate the frequency trimming effect on hermetically sealed wafer level packaged devices to reduce the across-wafer frequency distribution from 22000 to 4000 ppm. We confirm that this postannealing technique is permanent by temperature cycling the resonators from 50°C to 125°C. This technique provides a method to trim AlN microresonator frequency overcoming effects such as thin film variations, which are inherent to microsystems fabrication.

Elucidating the origin of spurious modes in aluminum nitride microresonators using a 2-D finite-element model.

In this work, an approach has been developed to predict the location of large spurious modes in the resonant response of aluminum nitride (AlN) microelectromechanical systems (MEMS) resonators over a wide range of desired operating frequencies. This addresses significant challenges in the design of more complex AlN devices, namely the prediction and elimination of spurious modes in the resonance response. Using the finite element method (FEM), the dispersion curves at wavelengths ranging from 8 to 20 μm were computed. It was determined that the velocities of symmetric Lamb (S0) and high-order antisymmetric (A) modes overlap at specific wavelengths. A 2-D FEM analysis showed that both the S0 and higher order A modes are mutually excited at a common operating wavelength. From this analysis, the coupling-of-modes (COM) parameters were extracted and used to compute the P-matrix and S-parameters using a 6-port transmission matrix. The P-matrix simulation was able to predict the electrical response of the S0 and nearby spurious modes. This work identified specific wavelength regions where COM has limited accuracy because of mode conversion. In these regions, the reflection (κ(p)) and transduction (ζ(p)) parameters change rapidly.

Elucidating the origin of spurious modes in aluminum nitride microresonators using a 2-D finite-element model

In this work, an approach has been developed to predict the location of large spurious modes in the resonant response of aluminum nitride (AlN) microelectromechanical systems (MEMS) resonators over a wide range of desired operating frequencies. This addresses significant challenges in the design of more complex AlN devices, namely the prediction and elimination of spurious modes in the resonance response. Using the finite element method (FEM), the dispersion curves at wavelengths ranging from 8 to 20 μm were computed. It was determined that the velocities of symmetric Lamb (S0) and high-order antisymmetric (A) modes overlap at specific wavelengths. A 2-D FEM analysis showed that both the S0 and higher order A modes are mutually excited at a common operating wavelength. From this analysis, the coupling-of-modes (COM) parameters were extracted and used to compute the P-matrix and S-parameters using a 6-port transmission matrix. The P-matrix simulation was able to predict the electrical response of the S0 and nearby spurious modes. This work identified specific wavelength regions where COM has limited accuracy because of mode conversion. In these regions, the reflection (κ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ) and transduction (ζ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ) parameters change rapidly.

AlN Microresonator-Based Filters With Multiple Bandwidths at Low Intermediate Frequencies

Filters with various bandwidths at low intermediate frequency (IF) have been demonstrated using aluminum nitride (AlN) microresonator technology. Specifically, at 13 MHz, 6 kHz, and 25 kHz bandwidth filters were implemented using a single resonator topology, and 250 kHz and 500 kHz bandwidth filters were constructed via the parallel lattice topology using four sub-resonators and L-matching networks. The bandwidths of these filters are from 0.046% to 3.8%, and particularly the 500 kHz bandwidth filter at 13 MHz is wider than that of the resonator kt <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> limit (40 kHz). The 100x variations in filter percent bandwidth were realized in a 1700 nm-thick aluminum nitride film on the same wafer through CMOS-compatible fabrication processes. Changes in the filter termination for proper filter matching were implemented in the Agilent Genesys RF and microwave design simulation software using actual measured filter responses with 50 Ω termination. The great flexibility in filter bandwidths and resonant frequencies, as well as other benefits such as size, manufacturing cost, isolation, and insertion loss provided by AlN microresonators will enable next generation multi-band, multi-waveform, and cognitive radios for defense and consumer wireless applications.

Oven-Based Thermally Tunable Aluminum Nitride Microresonators

Frequency tuning of aluminum nitride (AlN) microresonators has been demonstrated via localized heating (ovenization) of the resonator. Specifically, piezoelectrically driven ~ 100 MHz microresonators were heated by embedded joule heaters in vacuum. Three different designs with three different film stacks were tested, and among the tested devices, thermal resistances as large as 92 K/mW have been demonstrated, which corresponds to 1-mW power consumption to yield a temperature increase of 92°C. To minimize heat loss, the devices were suspended from the substrate by high thermal isolation beam-type supports. The beams exhibit very high thermal resistance not only due to their high length to cross-sectional area ratio but also because they are made of thin-film-deposited polycrystalline aluminum nitride. Film-deposited AlN has been shown to have thermal conductivity much lower than that measured in bulk materials. Thermal time constants for these devices were measured ranging from submilliseconds to 10 ms depending on the design and film stacks, and frequency tunability was measured as high as 2548 parts per million/mW. The availability of a power-efficient frequency tuning method, coupled with all other performance benefits, makes AlN microresonators a promising candidate for the next-generation timing devices and tunable filters for multiband communication systems.