Lithium Niobate Resonators Research Articles

Ultra-flat electro-optic frequency comb based on a chirp-modulated lithium niobate resonator.

Chirp modulation can generate a relatively flat electro-optic frequency comb (EO comb) and offers the advantage of frequency reconfigurability, demonstrating significant potential in high-precision sensing and absorption spectroscopy measurements. However, nonresonant devices such as waveguides are susceptible to limitations in modulation efficiency and bandwidth during electro-optic modulation. In this paper, by utilizing chirp modulation resonance mode, we have realized an EO comb based on a lithium niobate resonator with small tooth spacing and high flatness. Theoretically, the chirp modulation of phase is achieved by modulating the dispersion coupling term in the resonant mode transmission equation. Compared with conventional waveguide-based EO combs, the resonant mode chirp modulation is capable of generating a multistage flat comb, and thus the bandwidth of the comb is significantly expanded. In the experiment, with a repetition rate as low as 20 kHz and a bias voltage of 1 V, the comb bandwidth extended to over 150 MHz, where the number of 3 dB flat comb teeth for a single stage exceeds 2,000. Finally, we evaluated the measurement capability of the frequency comb at different temperatures by utilizing the transmission spectrum of the germanium-doped silica waveguide cavity as the absorption spectrum, measuring a temperature sensitivity of 1505.00 MHz/K and a temperature instability of 1.13 mK/Hz1/2.

Precise wavelength alignment of second-harmonic generation in thin-film lithium niobate resonators.

Second-harmonic generation (SHG) plays a significant role in modern photonic technology. Integrated photonic resonators fabricated with thin-film lithium niobate can achieve ultrahigh efficiencies by combining small mode volumes with high material nonlinearity. Cavity-enhanced SHG requires accurate phase and frequency matching conditions, where fundamental and second-harmonic wavelengths are both on resonance. However, this double-resonance condition can typically be realized only at a fixed random wavelength due to the high sensitivity of photonic resonances to the device geometry and fabrication variations. Here, we propose a novel method that can achieve the double-resonance condition over a large wavelength range. We combine thermal-optic and electro-optic (EO) effects to realize the separate tuning of fundamental and second-harmonic resonances. We demonstrated that the optimum SHG efficiency can be maintained over a wavelength range that exceeds the limit achievable with only thermal tuning. With this flexible tuning capability, we further show the precise alignment of SHG wavelengths of two separate thin-film lithium niobate resonators without sacrificing efficiencies.

High-efficiency second-harmonic generation in reverse-polarization dual-layer lithium niobate waveguides.

Second-harmonic (SH) generation is an important way to generate short-wavelength light sources. Thin-film lithium niobate (TFLN) resonators and waveguides featuring tight light confinement are flexible to realize mode- or quasi-phase-matching (MPM or QPM) and thus efficient SH generation. Here, we report an efficient SH with an absolute conversion efficiency of 45% in a reverse-polarized double-layer TFLN waveguide. Such a high conversion efficiency without periodic poling was realized with the pump of 67 mW in the 1550 nm band, benefiting from the employment of the largest nonlinear coefficient d33 and the significant nonlinear interaction of the fundamental transverse electric (TE) mode and the first-order TE mode in the vertical direction. This work paves the way toward on-chip integrated nonlinear optics in terms of harmonic generation and quantum light sources across the telecom and visible bands.

Edge treatment for spurious mode suppression in thin-film lithium niobate resonators

Thin-film lithium niobate is an attractive material for RF acoustic devices because of its high electromechanical coupling. However, due to the large coupling and the high anisotropy, thin-film lithium niobate resonators are prone to accidental resonances called spurious modes. These modes compromise the frequency response of the resonators, limiting their use in filter and oscillator applications. In this work, we present a novel method of spurious mode suppression through a special edge treatment etch process. Two thin-film lithium niobate resonators were fabricated, one with smooth sidewalls and one with the edge treatment. It was found that the edge-treated resonators show a weaker spurious mode response. This is potentially a new way to mitigate spurious resonances, a major issue in lithium niobate Lamb wave devices.

Single-mode squeezed-light generation and tomography with an integrated optical parametric oscillator.

Quantum optical technologies promise advances in sensing, computing, and communication. A key resource is squeezed light, where quantum noise is redistributed between optical quadratures. We introduce a monolithic, chip-scale platform that exploits the χ(2) nonlinearity of a thin-film lithium niobate (TFLN) resonator device to efficiently generate squeezed states of light. Our system integrates all essential components-except for the laser and two detectors-on a single chip with an area of one square centimeter, reducing the size, operational complexity, and power consumption associated with conventional setups. Using the balanced homodyne measurement subsystem that we implemented on the same chip, we measure a squeezing of 0.55 decibels and an anti-squeezing of 1.55 decibels. We use 20 milliwatts of input power to generate the parametric oscillator pump field by using second harmonic generation on the same chip. Our work represents a step toward compact and efficient quantum optical systems posed to leverage the rapid advances in integrated nonlinear and quantum photonics.

Electromagnetically induced transparency-like effect in a lithium niobate resonator via electronic control

In this study, we theoretically proposed a method to achieve an electromagnetically induced transparency (EIT)-like effect in a whispering gallery mode resonator (WGMR) and experimentally validated the method in a lithium niobate (LN) device. Benefitting from the electro-optic and inverse piezoelectric effects of the LN material, two modes of the LN WGMR that are close in frequency can be tuned at different tuning rates, resulting in EIT-like resonance lineshapes. By varying the electric field applied to the LN WGMR, the full dynamic of the EIT-like phenomenon can be precisely controlled. The experimental results agreed well with the calculations based on the coupled mode theory. Moreover, we observed a hysteresis resulting from the photorefractive effect of LN. We believe our proposed method and demonstrated devices offer a way to control an EIT-like effect, which could have potential applications in light storage, quantum information processing, and enhanced sensing techniques.

1Pa4-3 Direct observation of aggregation reaction of α- synuclein under shear stress using total internal reflection fluorescence microscopy with lithium-niobate resonator

1Pa4-3 Direct observation of aggregation reaction of α- synuclein under shear stress using total internal reflection fluorescence microscopy with lithium-niobate resonator

Soliton based χ(2) combs in high-Q optical microresonators.

Investigations of the frequency combs in χ(3) microresonators have passed a critical point when the soliton based regimes are well established and realized on different platforms. For χ(2) microresonators, where the first harmonic (FH) and second harmonic (SH) envelopes are coupled via the SH generation and optical parametric oscillation, the comb-soliton studies are just starting. Here we report on a vast accessible dual χ(2) soliton-comb family in high-Q microresonators with the SH and FH combs centered at the pump frequency ωp and its half ωp/2. Vicinity of the point of equal FH and SH group velocities λc, available via proper radial poling, is found to be the most advantageous for the generation of spectrally broad dual FH-SH combs. Our predictions as applied to lithium niobate resonators include the dependence of comb and dissipative soliton parameters on the pump power, the deviation λp - λc, the modal quality factors and frequency detunings, and the necessary parameters of radial poling of the resonator. These predictions form a solid basis for the realization of χ(2) frequency combs.

X-Band Miniature Filters Using Lithium Niobate Acoustic Resonators and Bandwidth Widening Technique

This work presents a class of micro-electromechanical system (MEMS)-driven radio frequency filters in the X-band. The X-band center frequencies are achieved by resorting to the third-order antisymmetric Lamb wave mode (A3) in a 650-nm-thick Z-cut lithium niobate thin film. A novel bandwidth (BW) widening technique based on using the self-inductance of the top interdigital transducers and bus lines is proposed to overcome the limitations set by the electromechanical coupling (k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> ) and satisfy the demands in miniaturization and wide BW. Four different designs of the filters are designed and fabricated to show the trade-off among BW, insertions loss (IL), out-of-band rejections, and footprint. Due to the spurious-free and highQ performance of the A3 lithium niobate resonators, the fabricated A3 lithium niobate filters have demonstrated small in-band ripples and sharp roll-offs. One of these fabricated has demonstrated a 3-dB BW of 190 MHz, an IL of 1.5 dB, and a compact footprint of 0.56 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Another design is fabricated to demonstrate a 3-dB BW of 170 MHz, an IL of 2.5 dB, an outof-band rejection of 28 dB, and a compact footprint of 1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Fixed-Frequency Control of Piezoelectric Resonator DC-DC Converters for Spurious Mode Avoidance

Piezoelectric resonators have shown promise as efficient, power-dense energy storage element alternatives to continue the miniaturization of DC-DC converters. However, practical piezoelectric resonators diverge from their ideal behavior due to spurious modes that cause high loss regions throughout their operating frequency range. Typically, control of piezoelectric resonator DC-DC converters is constrained to unidirectional power flow each resonant cycle for maximum efficiency operation. However, output power depends on the frequency with such control, which means the converter cannot operate at loads corresponding to spurious mode frequencies. This paper presents a fixed-frequency control mode utilizing the high-quality factor of piezoelectric resonators to avoid spurious modes. The fixed-frequency control enables efficient operation spanning the converters full load range, demonstrated through a prototype DC-DC converter with a custom fabricated lithium niobate resonator. At a conversion ratio of 60 V to 30 V, spurious modes limit the converters operating range from 33 W to 51 W using unidirectional power-constrained control, yet the fixed-frequency control extends operation from 33 W to 2 W.

Athermal lithium niobate microresonator

Lithium niobate (LN), possessing wide transparent window, strong electro-optic effect, and large optical nonlinearity, is an ideal material platform for integrated photonics application. Microring resonators are particularly suitable as integrated photonic components, given their flexibility of device engineering and their potential for large-scale integration. However, the susceptibility to temperature fluctuation has become a major challenge for their implementation in a practical environment. Here, we demonstrate an athermal LN microring resonator. By cladding an x-cut LN microring resonator with a thin layer of titanium oxide, we are able to completely eliminate the first-order thermo-optic coefficient (TOC) of cavity resonance right at room temperature (20°C), leaving only a small residual quadratic temperature dependence with a second-order TOC of only 0.37 pm/K2. It corresponds to a temperature-induced resonance wavelength shift within 0.33 nm over a large operating temperature range of (-10 - 50)°C that is one order of magnitude smaller than a bare LN microring resonator. Moreover, the TiO2-cladded LN microring resonator is able to preserve high optical quality, with an intrinsic optical Q of 5.8 × 105 that is only about 11% smaller than that of a bare LN resonator. The flexibility of thermo-optic engineering, high optical quality, and device fabrication compatibility show great promise of athermal LN/TiO2 hybrid devices for practical applications, elevating the potential importance of LN photonic integrated circuits for future communication, sensing, nonlinear and quantum photonics.

A Ku-Band Oscillator Utilizing Overtone Lithium Niobate RF-MEMS Resonator for 5G

This letter presents a 12.9-GHz silicon germanium (SiGe) Pierce oscillator using a third antisymmetric overtone in a lithium niobate (LiNbO3) resonator for 5G communications. Quarter-wave resonators are used to satisfy the Barkhausen oscillation conditions for the third overtone while suppressing the fundamental and higher order resonances. The oscillator achieves a measured phase noise of -70 and -111 dBc/Hz at 1 and 100-kHz offsets from a 12.9-GHz carrier while consuming 20 mW of dc power. The oscillator achieves a figure-of-merit of 200 dBc/Hz at 100-kHz offset, surpassing the state-of-the-art electromagnetic (EM) and overtone MEMS oscillators. The achieved oscillation frequency is the highest reported to date for an MEMS oscillator. The demonstrated performance shows the strong potential of microwave acoustic oscillators for 5G frequency synthesis. This work will enable low-power 5G transceivers featuring high speed, high sensitivity, and high selectivity in small form factors.

Temperature Stability Analysis of Thin-Film Lithium Niobate SH0 Plate Wave Resonators

This work presents an extensive study on the temperature coefficient of frequency (TCF) for thin-film lithium niobate (LiNbO3) resonators. To capture the temperature-frequency behavior for each vibration mode, a one-dimensional (1D) multi-section TCF model is proposed and verified by both the finite-element method (FEM) and experiments. By partitioning the metallized and non-metallized sections of the interdigitated transducers (IDT) as distinct TCF blocks and properly considering the spatial energy distribution, the TCF of each mode can be accurately predicted. Two higher lateral-order resonators based on the fundamental shear-horizontal mode (SH0) are designed and fabricated on a LiNbO3-on-SiO2 wafer to provide odd- and even-order harmonics. TCFs of the first 6 overtones (from 84 to 515 MHz) are calculated using the material properties given in the literature, exhibiting a maximum TCF difference of 3 ppm/K between the theoretical prediction and measurement results. Although only SH0 resonators are selected for experimental verification, the proposed multi-section TCF model can be applied to other plate wave MEMS resonators. [2019-0112]

Self-starting bi-chromatic LiNbO3 soliton microcomb

For its many useful properties, including second and third-order optical nonlinearity as well as electro-optic control, lithium niobate is considered an important potential microcomb material. Here, a soliton microcomb is demonstrated in a monolithic high-Q lithium niobate resonator. Besides the demonstration of soliton mode locking, the photorefractive effect enables mode locking to self-start and soliton switching to occur bi-directionally. Second-harmonic generation of the soliton spectrum is also observed, an essential step for comb self-referencing. The Raman shock time constant of lithium niobate is also determined by measurement of soliton self-frequency-shift. Besides the considerable technical simplification provided by a self-starting soliton system, these demonstrations, together with the electro-optic and piezoelectric properties of lithium niobate, open the door to a multi-functional microcomb providing f-2f generation and fast electrical control of optical frequency and repetition rate, all of which are critical in applications including time keeping, frequency synthesis/division, spectroscopy and signal generation.

Nanowatt-Level Wakeup Receiver Front Ends Using MEMS Resonators for Impedance Transformation

This paper presents the first demonstration of nanowatt-level CMOS wakeup receiver (WuRx) front ends (FEs) that utilize microelectromechanical system (MEMS)-based matching networks (MNs). The first FE uses a fully MEMS-based MN (MMN) that operates at 88.8 MHz. It consists of a large array of lithium niobate resonators that are fabricated on the same die. Measurements of the integrated WuRx FE with the MMN indicate that its loaded voltage gain achieves a bandwidth of 0.78 MHz with a quality factor of 114. The second FE uses a hybrid MN (HMN) that operates at 457 MHz. It consists of an aluminum nitride laterally vibrating resonator as well as eight discrete inductors and capacitors surrounding the MEMS device. Measurements of the integrated WuRx FE with the HMN indicate that it achieves a sensitivity of -54 dBm with 7 nW of average dc power consumption without a digital correlator.

High-Q chaotic lithium niobate microdisk cavity.

Lithium niobate (LN) is the workhorse for modern optoelectronics industry and nonlinear optics. High quality (Q) factor LN microresonators are promising candidates for applications in optical communications, quantum photonics, and sensing. However, the phase-matching requirement of traditional evanescent coupling methods poses significant challenges to achieve high coupling efficiencies of the pump and signal light simultaneously, ultimately limiting the practical usefulness of these high Q factor LN resonators. Here, for the first time, to the best of our knowledge, we demonstrate deformed chaotic LN microcavities that feature directional emission patterns and high Q factors simultaneously. The chaotic LN microdisks are created using conventional semiconductor fabrication processes, with measured Q factors exceeding 106 in the telecommunication band. We show that our devices can be free-space-coupled with high efficiency by leveraging directional emission from the asymmetric cavity. Using this broadband approach, we demonstrate a 58-fold enhancement of free-space collection efficiency of a second harmonic generation signal, compared with a circular microdisk.

Investigation of Electromechanical Coupling and Quality Factor of X-Cut Lithium Niobate Laterally Vibrating Resonators Operating Around 400 MHz

In this paper we present laterally vibrating resonators (LVRs) based on X-cut lithium niobate (LN) exhibiting a maximum quality factor $({Q})$ around 1200 and an electromechanical coupling ( $k_{t}^{2}$ ) of 30%. This paper shows how the highest $k_{t}^{2}$ ever attained in X-cut LN resonators can be combined with very high ${Q}$ to achieve a resonator figure of merit (FoM $= k_{t}^{2}\cdot {Q}$ ) of 360. In this paper, we validate the theoretical prediction of $k_{t}^{2}$ versus the resonator orientation in the YZ plane and investigate what geometrical parameters mostly impact the ${Q}$ of these devices. [2017–0181]

High-quality lithium niobate photonic crystal nanocavities

Lithium niobate (LN) exhibits unique material characteristics that have found many important applications. Scaling LN devices down to a nanoscopic scale can dramatically enhance light–matter interaction that would enable nonlinear and quantum photonic functionalities beyond the reach of conventional means. However, developing LN-based nanophotonic devices turns out to be nontrivial. Although significant efforts have been devoted to this in recent years, the LN photonic crystal structures developed to date exhibit fairly low quality (Q). Here we demonstrate LN photonic crystal nanobeam resonators with optical Q as high as 105, more than two orders of magnitude higher than other LN photonic crystal nanocavities reported to date. The high optical Q, together with tight mode confinement, leads to an extremely strong nonlinear photorefractive effect, with a resonance tuning rate of ∼0.64 GHz/aJ, or equivalently ∼84 MHz/photon, three orders of magnitude greater than other LN resonators. In particular, we observed an intriguing quenching of photorefraction that has never been reported before. The devices also exhibit strong optomechanical coupling with a gigahertz nanomechanical mode with a significant f·Q product of 1.47×1012 Hz. The demonstration of high-Q LN photonic crystal nanoresonators paves a crucial step toward LN nanophotonics that could integrate the outstanding material properties with versatile nanoscale device engineering for diverse and intriguing functionalities.

Fast response of photorefraction in lithium niobate microresonators.

We present a detailed characterization of photorefraction in on-chip high-Q lithium niobate (LN) microresonators. We show that the photorefractive effect in these devices exhibits very distinctive temporal relaxation dynamics compared with those in bulk crystals and in mm-sized LN resonators. The relaxation of photorefraction is dominated by a fast time response with a time constant as small as 20.85ms that is more than three-orders of magnitude faster than those observed in macroscopic devices. The observed fast response of photorefraction is of great potential as a convenient and energy-efficient approach for on-chip all-optical functionalities.

Second harmonic generation in a high-Q lithium niobate microresonator fabricated by femtosecond laser micromachining

We report on fabrication of high Q lithium niobate (LN) whispering-gallery-mode (WGM) microresonators suspended on silica pedestals by femtosecond laser microfabrication. The micrometer-scale (diameter ~82 {\mu}m) LN resonator possesses a Q factor of 2.5x10^5 around 1550 nm wavelength range. Moreover, second harmonic generation with a continuous-wave tunable single-longitudinal-mode pump laser in the on-chip LN microresonator is demonstrated in the on-chip LN microresonator. A fiber taper is employed to couple the pump laser into the microresonator, showing a normalized conversion efficiency of 1.35x10^-5/mW.