Fast response of photorefraction in lithium niobate microresonators

Enhanced Frequency Conversion in Parity-Time Symmetry Line

Non-Hermitian degeneracies reveal intriguing and nontrivial behaviors in open physical systems. Examples like parity-time (PT) symmetry breaking, topological encircling chirality, and enhanced sensing near an exceptional point (EP) are often associated with the abrupt nature of the phase transition around these degeneracies. Here we experimentally observe a cavity-enhanced second-harmonic frequency (SHG) conversion on a PT symmetry line, i.e., a set consisting of open-ended isofrequency or isoloss lines, both terminated at EPs on the Riemann surface in parameter space. The enhancement factor can reach as high as 300, depending on the crossing point whether in the symmetry or the broken phase of the PT line. Moreover, such enhancement of SHG enables sensitive distance sensing with a nanometer resolution. Our works may pave the way for practical applications in sensing, frequency conversion, and coherent wave control.

UV-enhanced photorefractive response rate in a thin-film lithium niobate microdisk

The photorefractive (PR) effect plays a critical role in emerging photonic technologies, including dynamic volume holography and on-chip all-optical functionalities. Nevertheless, its slow response rate has posed a significant obstacle to its practical application. Here, we experimentally demonstrate the enhancement of the PR response rate in a high-Q thin-film lithium niobate (TFLN) microdisk under UV light irradiation. At an irradiation intensity of 30 mW/cm2, the PR effect achieves a high response bandwidth of approximately 256 kHz. By employing this UV-assisted PR effect, we have achieved rapid laser-cavity locking and self-stabilization, where perturbations are automatically compensated. This technique paves the way toward real-time dynamic holography, editable photonic devices on a lithium niobate platform, and high-speed all-optical information processing.

Octave soliton microcombs in lithium niobate microresonators

Soliton microcombs are regarded as an ideal platform for applications such as optical communications, optical sensing, low-noise microwave sources, optical atomic clocks, and frequency synthesizers. Many of these applications require a broad comb spectrum that covers an octave, essential for implementing the f - 2f self-referencing techniques. In this work, we have successfully generated an octave-spanning soliton microcomb based on a z-cut thin-film lithium niobate (TFLN) microresonator. This achievement is realized under on-chip optical pumping at 340 mW and through extensive research into the broadening of dual dispersive waves (DWs). Furthermore, the repetition rate of the octave soliton microcomb is accurately measured using an electro-optic comb generated by an x-cut TFLN racetrack microresonator. Our results represent a crucial step toward the realization of practical, integrated, and fully stabilized soliton microcomb systems based on TFLN.

Efficient second harmonic generation in a high-Q Fabry-Perot microresonator on x-cut thin film lithium niobate

Microresonators facilitate enhanced light-matter interactions within a limited space, showing great promise for nonlinear optics. Here, we demonstrate a high-quality (Q) factor Fabry-Perot microresonator (FPR) for second harmonic generation (SHG) on an x-cut thin film lithium niobate (TFLN) platform. The FPR exhibits Q factors of Qpump = 1.09 × 105 and QSH = 1.15 × 104 at the 1560 nm pump wavelength and 780 nm second harmonic wavelength, respectively. Under low pump power, a normalized SHG efficiency of 158.5 ± 18.5%/W is attained. We experimentally verify that increased temperatures mitigate photorefractive effects that degrade SHG performance. This work highlights the immense capabilities of one-dimensional planar optical waveguide resonators for efficient on-chip nonlinear wavelength conversion.

Relaxation of the electro-optic response in thin-film lithium niobate modulators

Thin-film lithium niobate (TFLN) is a promising electro-optic (EO) photonics platform with high modulation bandwidth, low drive voltage, and low optical loss. However, EO modulation in TFLN is known to relax on long timescales. Instead, thermo-optic heaters are often used for stable biasing, but heaters incur challenges with cross-talk, high power, and low bandwidth. Here, we characterize the low-frequency (1 mHz to 1 MHz) EO response of TFLN modulators, investigate the root cause of EO relaxation and demonstrate methods to improve bias stability. We show that relaxation-related effects can enhance EO modulation across a frequency band spanning 1kHz to 20kHz in our devices - a counter-intuitive result that can confound measurement of half-wave voltage (V π) in TFLN modulators. We also show that EO relaxation can be slowed by more than 104-fold through control of the LN-metal interface and annealing, offering progress toward lifetime-stable EO biasing. Such robust EO biasing would enable applications for TFLN devices where cross-talk, power, and bias bandwidth are critical, such as quantum devices, high-density integrated photonics, and communications.

Integrated lithium niobate optical phased array for two-dimensional beam steering

Optical phased arrays (OPAs) with high speed, low power consumption, and low insertion loss are appealing for various applications, including light detection and ranging, free-space communication, image projection, and imaging. These OPAs can be achieved by fully harnessing the advantages of integrated lithium niobate (LN) photonics, which include high electro-optical modulation speed, low driving voltage, and low optical loss. Here we present an integrated LN OPA that operates in the near-infrared regime. Our experimental results demonstrate 24 × 8° two-dimensional beam steering, a far-field beam spot with a full width at half maximum of 2 × 0.6°, and a sidelobe suppression level of 10 dB. Furthermore, the phase modulator of our OPA exhibits a half-wave voltage of 6 V. The low power consumption exhibited by our OPA makes it highly attractive for a wide range of applications. Beyond conventional applications, our OPA's high speed opens up the possibility of novel applications such as high-density point cloud generation and tomographic holography.

110 GHz, 110 mW hybrid silicon-lithium niobate Mach-Zehnder modulator

High bandwidth, low voltage electro-optic modulators with high optical power handling capability are important for improving the performance of analog optical communications and RF photonic links. Here we designed and fabricated a thin-film lithium niobate (LN) Mach-Zehnder modulator (MZM) which can handle high optical power of 110 mW, while having 3-dB bandwidth greater than 110 GHz at 1550 nm. The design does not require etching of thin-film LN, and uses hybrid optical modes formed by bonding LN to planarized silicon photonic waveguide circuits. A high optical power handling capability in the MZM was achieved by carefully tapering the underlying Si waveguide to reduce the impact of optically-generated carriers, while retaining a high modulation efficiency. The MZM has a [Formula: see text] product of 3.1 V.cm and an on-chip optical insertion loss of 1.8 dB.

Integrated silicon carbide electro-optic modulator

Owing to its attractive optical and electronic properties, silicon carbide is an emerging platform for integrated photonics. However an integral component of the platform is missing—an electro-optic modulator, a device which encodes electrical signals onto light. As a non-centrosymmetric crystal, silicon carbide exhibits the Pockels effect, yet a modulator has not been realized since the discovery of this effect more than three decades ago. Here we design, fabricate, and demonstrate a Pockels modulator in silicon carbide. Specifically, we realize a waveguide-integrated, small form-factor, gigahertz-bandwidth modulator that operates using complementary metal-oxide-semiconductor (CMOS)-level voltages on a thin film of silicon carbide on insulator. Our device is fabricated using a CMOS foundry compatible fabrication process and features no signal degradation, no presence of photorefractive effects, and stable operation at high optical intensities (913 kW/mm²), allowing for high optical signal-to-noise ratios for modern communications. Our work unites Pockels electro-optics with a CMOS foundry compatible platform in silicon carbide.

Electro-optic modulator is used to encode electrical signals onto light. Here the authors demonstrate an electro-optic modulator, based on Silicon Carbide, which can be useful for quantum and optical communications.

Quantum-enabled operation of a microwave-optical interface

Solid-state microwave systems offer strong interactions for fast quantum logic and sensing but photons at telecom wavelength are the ideal choice for high-density low-loss quantum interconnects. A general-purpose interface that can make use of single photon effects requires < 1 input noise quanta, which has remained elusive due to either low efficiency or pump induced heating. Here we demonstrate coherent electro-optic modulation on nanosecond-timescales with only [Formula: see text] microwave input noise photons with a total bidirectional transduction efficiency of 8.7% (or up to 15% with [Formula: see text]), as required for near-term heralded quantum network protocols. The use of short and high-power optical pump pulses also enables near-unity cooperativity of the electro-optic interaction leading to an internal pure conversion efficiency of up to 99.5%. Together with the low mode occupancy this provides evidence for electro-optic laser cooling and vacuum amplification as predicted a decade ago.

Advances in integrated ultra-wideband electro-optic modulators [Invited]

Increasing data traffic and bandwidth-hungry applications require electro-optic modulators with ultra-wide modulation bandwidth for cost-efficient optical networks. Thus far, integrated solutions have emerged to provide high bandwidth and low energy consumption in compact sizes. Here, we review the design guidelines and delicate structures for higher bandwidth, applying them to lumped-element and traveling-wave electrodes. Additionally, we focus on candidate material platforms with the potential for ultra-wideband optical systems. By comparing the superiority and mechanism limitations of different integrated modulators, we design a future roadmap based on the recent advances.

Theory of χ(2)-microresonator-based frequency conversion

Microresonator-based platforms with ${\chi ^{(2)}}$ nonlinearities have the potential to perform frequency conversion at high efficiencies and ultralow powers with small footprints. The standard doctrine for achieving high conversion efficiency in cavity-based devices requires "perfect matching," that is, zero phase mismatch while all relevant frequencies are precisely at a cavity resonance, which is difficult to achieve in integrated platforms due to fabrication errors and limited tunabilities. In this Letter, we show that the violation of perfect matching does not necessitate a reduction in conversion efficiency. On the contrary, in many cases, mismatches should be intentionally introduced to improve the efficiency or tunability of conversion. We identify the universal conditions for maximizing the efficiency of cavity-based frequency conversion and show a straightforward approach to fully compensate for parasitic processes such as thermorefractive and photorefractive effects that, typically, can limit the conversion efficiency. We also show the design criteria that make these high-efficiency states stable against nonlinearity-induced instabilities.

Resonant Stimulated Photorefractive Scattering

We present the first observations, and a complete theoretical explanation, of stimulated photorefractive scattering in a high- Q crystalline cavity. The standing-wave light field in the cavity induces an ultranarrow and long-lived Bragg grating through the photorefractive effect. The spatial phase of the grating is automatically matched to that of the standing wave. The scattering from the grating strengthens the standing wave, which then further reinforces the grating itself. Eventually, the mode is seen to split into a doublet, thereby disrupting the usual strict periodicity of the mode spectrum.

High-Q microresonators on 4H-silicon-carbide-on-insulator platform for nonlinear photonics

The realization of high-quality (Q) resonators regardless of the underpinning material platforms has been a ceaseless pursuit, because the high-Q resonators provide an extreme environment for confining light to enable observations of many nonlinear optical phenomenon with high efficiencies. Here, photonic microresonators with a mean Q factor of 6.75 × 10⁶ were demonstrated on a 4H-silicon-carbide-on-insulator (4H-SiCOI) platform, as determined by a statistical analysis of tens of resonances. Using these devices, broadband frequency conversions, including second-, third-, and fourth-harmonic generations have been observed. Cascaded Raman lasing has also been demonstrated in our SiC microresonator for the first time, to the best of our knowledge. Meanwhile, by engineering the dispersion properties of the SiC microresonator, we have achieved broadband Kerr frequency combs covering from 1300 to 1700 nm. Our demonstration represents a significant milestone in the development of SiC photonic integrated devices.

On-chip erbium-doped lithium niobate microring lasers

Lithium niobate on insulator (LNOI), regarded as an important candidate platform for optical integration due to its excellent nonlinear, electro-optic, and other physical properties, has become a research hotspot. A light source, as an essential component for an integrated optical system, is urgently needed. In this Letter, we reported the realization of 1550 nm band on-chip LNOI microlasers based on erbium-doped LNOI ring cavities with loaded quality factors higher than 1 million at ∼970nm, which were fabricated by using electron beam lithography and inductively coupled plasma reactive ion etching processes. These microlasers demonstrated a low pump threshold of ∼20µW and stable performance under the pump of a 980 nm band continuous laser. Comb-like laser spectra spanning from 1510 to 1580 nm were observed in a high pump power regime, which lays the foundation of the realization of pulsed laser and frequency combs on a rare-earth ion-doped LNOI platform. This Letter effectively promotes the development of on-chip integrated active LNOI devices.

Mitigating photorefractive effect in thin-film lithium niobate microring resonators

Thin-film lithium niobate is an attractive integrated photonics platform due to its low optical loss and favorable optical nonlinear and electro-optic properties. However, in applications such as second harmonic generation, frequency comb generation, and microwave-to-optics conversion, the device performance is strongly impeded by the photorefractive effect inherent in thin-film lithium niobate. In this paper, we show that the dielectric cladding on a lithium niobate microring resonator has a significant influence on the photorefractive effect. By removing the dielectric cladding layer, the photorefractive effect in lithium niobate ring resonators can be effectively mitigated. Our work presents a reliable approach to control the photorefractive effect on thin-film lithium niobate and will further advance the performance of integrated classical and quantum photonic devices based on thin-film lithium niobate.

Stable tuning of photorefractive microcavities using an auxiliary laser

Cavity nonlinear optics enables intriguing physical phenomena to occur at micro- or nano-scales with modest input powers. While this enhances capabilities in applications such as comb generation, frequency conversion, and quantum optics, undesired nonlinear effects including photorefraction and thermal bistability are exacerbated. In this Letter, we propose and demonstrate a highly effective method of achieving cavity stabilization using an auxiliary laser for controlling photorefraction in a z-cut periodically poled lithium niobate (LN) microcavity system. Our numerical study accurately models the photorefractive effect under high input powers, guiding future analyses and development of LN microcavity systems.

Photorefraction-induced Bragg scattering in cryogenic lithium niobate ring resonators

We report intracavity Bragg scattering induced by the photorefractive (PR) effect in high-Q lithium niobate ring resonators at cryogenic temperatures. We show that when a cavity mode is strongly excited, the PR effect imprints a long-lived periodic space-charge field. This residual field in turn creates a refractive index modulation pattern that dramatically enhances the back scattering of an incoming probe light, and results in selective and reconfigurable mode splittings. This PR-induced Bragg scattering effect, despite being undesired for many applications, could be utilized to enable optically programmable photonic components.

On-chip tunable microdisk laser fabricated on Er3+-doped lithium niobate on insulator

We demonstrate a C-band wavelength-tunable microlaser with an Er³⁺-doped high quality (∼1.8×10⁶) lithium niobate microdisk resonator. With a 976 nm continuous-wave pump laser, lasing action can be observed at a pump power threshold lower than 400 µW at room temperature. Furthermore, the microdisk laser wavelength can be tuned by varying the pump laser power, showing a tuning efficiency of ∼-17.03pm/mW at low pump power below 13 mW, and 10.58 pm/mW at high pump power above 13 mW.

Wafer-scale low-loss lithium niobate photonic integrated circuits

Thin-film lithium niobate (LN) photonic integrated circuits (PICs) could enable ultrahigh performance in electro-optic and nonlinear optical devices. To date, realizations have been limited to chip-scale proof-of-concepts. Here we demonstrate monolithic LN PICs fabricated on 4- and 6-inch wafers with deep ultraviolet lithography and show smooth and uniform etching, achieving 0.27 dB/cm optical propagation loss on wafer-scale. Our results show that LN PICs are fundamentally scalable and can be highly cost-effective.

High optical damage threshold on-chip lithium tantalate microdisk resonator

Lithium tantalate (LT) is one of the most attractive optical nonlinear materials, as it possesses a high optical damage threshold and great UV transparency (0.28-5.5 µm). Recently, optical grade LT nanoscale film was developed. Here a high-quality-factor (∼10⁵) LT microdisk resonator based on LT-on-insulator (LTOI) film is fabricated by utilizing focused ion beam (FIB) milling. 2 µW output second-harmonic waves are achieved in the LTOI microdisk at about 500 mW input power. Cascaded third-harmonic generation is also observed in the fabricated device. This work may pave the way for LTOI in integrated photonic chips.

Polarization-independent one-dimensional grating coupler design on hybrid silicon/LNOI platform

We report here on one-dimensional (1D) grating couplers based on hybrid silicon/LNOI platform for polarization-independent and high-efficient single-polarization coupling efficiencies. A low index oxide buffer layer was introduced in between the top silicon high index grating coupler and bottom LNOI waveguide. With optimal design of the buffer layer thicknesses, modal and index matches can be tuned for either single polarization or both TE/TM polarization coupling applications. Over 70% coupling efficiency can be achieved for single polarization based on the basic uniform 1D grating coupler design without any bottom reflectors incorporated. Polarization independent coupling efficiency of 51% was also achieved. The spectral bandwidth is over 50 nm with polarization dependent loss of 0.1 dB. The proposed structure is simple to fabricate. Detailed modal and loss analysis suggest different dominant loss mechanisms in the proposed hybrid structure, where the introduction of the bottom mirror may not result in significant improvement in coupling efficiency, as the dominant loss mechanism arises from the top reflection loss.

High Homogeneity of Magnesium Doped LiNbO3 Crystals Grown by Bridgman Method

A series of LiNbO3 crystals doped with various MgO concentrations (0, 3%, and 5 mol%) was simultaneously grown in one furnace by the modified vertical Bridgman method. The wet chemistry method was used to prepare the polycrystalline powders, and the growth conditions were optimized. The full width at half maximum of high-resolution X-ray rocking curves for (001) reflection of 5 mol% Mg doped lithium niobate (LN) crystal was about 8″, which meant it possessed high crystalline quality. The OH− absorption spectra shifted to 3534.7 cm−1, and the UV absorption edge violet shift indicated that 5 mol% MgO successfully doped in LN and exceeded the threshold. The extraordinary refractive index gradient of 5 mol% Mg doped LN crystal was as small as 2.5 × 10−5/cm, which exhibited high optical homogeneity.

Recent Progress in Lithium Niobate: Optical Damage, Defect Simulation, and On-Chip Devices

Lithium niobate (LN) is one of the most important synthetic crystals. In the past two decades, many breakthroughs have been made in material technology, theoretical understanding, and application of LN crystals. Recent progress in optical damage, defect simulation, and on-chip devices of LN are explored. Optical damage is one of the main obstacles for the practical usage of LN crystals. Recent results reveal that doping with ZrO₂ not only leads to better optical damage resistance in the visible but also improves resistance in the ultraviolet region. It is still awkward to extract defect characteristics and their relationship with the physical properties of LN crystals directly from experimental investigations. Recent simulations provide detailed descriptions of intrinsic defect models, the site occupation of dopants and the variation of energy levels due to extrinsic defects. LN is considered to be one of the most promising platforms for integrated photonics. Benefiting from advances in smart-cut, direct wafer bonding and layer transfer techniques, great progress has been made in the past decade for LNs on insulators. Recent progress on on-chip LN micro-photonic devices and nonlinear optical effects, in particular photorefractive effects, are briefly reviewed.

Microdisk resonators with lithium-niobate film on silicon substrate

In this paper, we report the fabrication of lithium niobate (LN) microdisk resonators on a pulsed-laser deposited polycrystalline LN film on a silicon substrate rather than commercially provide LN film on insulator. The quality factor of these polycrystalline LN microdisks were measured above 3.4×10⁴ in the 1550-nm band. Second harmonic generation was demonstrated in the fabricated microresonators. Because the properties of homemade LN film can be easily tuned by doping various ions, LN devices on homemade LN film may have more flexible functions and broad applications.

Lithium niobate micro-disk resonators of quality factors above 107

We report on the fabrication of crystalline lithium niobate microresonators with quality factors above 10⁷, as measured around 770 nm wavelength. Our technique relies on femtosecond laser micromachining for patterning a mask coated on the lithium niobate on insulate (LNOI) into a microdisk, followed by a chemo-mechanical polishing process for transferring the disk-shaped pattern to the LNOI. Nonlinear processes including second-harmonic generation and Raman scattering have been demonstrated in the fabricated microdisk.

Photorefractive Properties of Molybdenum and Hafnium Co-Doped LiNbO3 Crystals

A series of LiNbO3: Mo, Hf crystals with 0.5 mol % fixed MoO3 and various HfO2 concentrations (0.0, 2.0, and 3.5 mol %) were grown by the Czochralski technique. The photorefractive properties of the LiNbO3: Mo, Hf crystals were investigated by two-wave coupling measurements and the beam distortion method was employed to obtain the optical damage resistance ability. The UV-visible and OH− absorption spectra were also studied. The experimental results imply that the photorefractive properties of LiNbO3: Mo crystals at laser wavelengths of 532, 488, and 442 nm can be greatly enhanced by doping HfO2 over the threshold concentration. At 442 nm especially, the response time of LN: Mo, Hf3.5 can be shortened to 0.9 s with a diffraction efficiency of 46.07% and a photorefractive sensitivity reaching 6.28 cm/J. Besides this, the optical damage resistance at 532 nm is 3 orders of magnitude higher than that of the mono-doped LiNbO3: Mo crystal, which is beneficial for applying it in the field of high-intensity lasers.