Integrated photonics plays a central role in modern science and technology, enabling experiments from nonlinear science to quantum information, ultraprecise measurements and sensing, and advanced applications like data communication and signal processing. Optical materials with favorable properties are essential for nanofabrication of integrated-photonics devices. Here we describe a material for integrated nonlinear photonics, tantalum pentoxide (Ta$_2$O$_5$, hereafter tantala), which offers low intrinsic material stress, low optical loss, and efficient access to Kerr-nonlinear processes. We utilize >800-nm thick tantala films deposited via ion-beam sputtering on oxidized silicon wafers. The tantala films contain a low residual tensile stress of 38 MPa, and they offer a Kerr index $n_2$=6.2(23)$\times10^{-19}$ m$^2$/W, which is approximately a factor of three higher than silicon nitride. We fabricate integrated nonlinear resonators and waveguides without the cracking challenges that are prevalent in stoichiometric silicon nitride. The tantala resonators feature an optical quality factor up to $3.8\times10^6$, which enables us to generate ultrabroad-bandwidth Kerr-soliton frequency combs with low threshold power. Moreover, tantala waveguides enable supercontinuum generation across the near-infrared from low-energy, ultrafast seed pulses. Our work introduces a versatile material platform for integrated, low-loss nanophotonics that can be broadly applied and enable heterogeneous integration.
Designing integrated photonics, especially to leverage Kerr-nonlinear optics, requires accurate and precise knowledge of refractive index across the visible to infrared spectral ranges. Tantala (Ta_2O_5) is an emerging material platform for integrated photonics and nanophotonics that offers broadband ultralow loss, moderately high nonlinearity, and advantages for scalable and heterogeneous integration. We present refractive-index measurements on a thin film of tantala, and we explore the efficacy of this data for group-velocity dispersion (GVD) engineering with waveguide and ring-resonator dev…
Non-volatile integrated photonics on lithium tantalate-on-insulator
Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology（武汉光电国家实验室与华中科技大学光学与电子信息学院）；State Key Laboratory of Photonics and Communications, Department of Electronic Engineering, Shanghai Jiao Tong University（上海交通大学电子工程系光子与通信国家重点实验室）；School of Physics, Harbin Institute of Technology（哈尔滨工业大学物理学院）
Hybrid Integration of Quantum Dot Single Photon Sources with Lithium Tantalate Photonics for On Chip Routing
National University of Defense Technology（国防科技大学）；Zhejiang University（浙江大学）；Harbin Institute of Technology (Shenzhen)（哈尔滨工业大学（深圳））；Sun Yat-sen University（中山大学）
Robust and Active Visible-Light Integrated Photonics on Thin-Film Lithium Tantalate for Underwater Optical Wireless Communications
National Center for International Research on Green Optoelectronics, South China Normal University（华南师范大学绿色光电子国际联合研究中心）；Jiaxing Research Institute, Zhejiang University（浙江大学嘉兴研究院）
Low voltage and high-bandwidth thin-film lithium tantalate modulator on a silicon dioxide substrate
Institute of Physics, Swiss Federal Institute of Technology, Lausanne (EPFL)（洛桑联邦理工学院物理研究所）；Institute of Electrical and Micro Engineering (IEM), EPFL（洛桑联邦理工学院电气与微工程研究所）；Institute of Photonics and Quantum Electronics (IPQ), Karlsruhe Institute of Technology (KIT)（卡尔斯鲁厄理工学院光子与量子电子学研究所）
On-chip efficient nonlinear functions are instrumental in escalating the utilities and performance of photonic integrated circuits (PICs), especially for a wide range of classical and quantum applications, such as tunable coherent radiation, optical frequency conversion, spectroscopy, quantum science, etc. Lithium tantalate (LT) has been widely used in nonlinear wavelength converters, surface acoustic wave resonators, and electro-optic, acoustic-optic devices owing to its excellent optical properties. Here, we fabricated a Z-cut lithium tantalate on insulator (LTOI) microdisk with high quality…
Toward scalable and bias-stable optical phased arrays on lithium tantalate
State Key Laboratory of Optoelectronic Materials and Technologies, School of Electronics and Information Technology, Sun Yat-sen University（中山大学电子与信息学院光电材料与技术国家重点实验室）；State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University（清华大学精密仪器系精密测量技术与仪器国家重点实验室）；State Key Laboratory of Materials for Integrated Circuits, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences（中国科学院上海微系统与信息技术研究所集成材料电路国家重点实验室）；Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences（中国科学院大学材料科学与工程中心）；University of Science and Technology of China（中国科学技术大学）
Integrated Kerr micro-combs, a powerful source of many wavelengths for photonic RF and microwave signal processing, are particularly useful for transversal filter systems. They have many advantages including a compact footprint, high versatility, large numbers of wavelengths, and wide bandwidths. We review recent progress on photonic RF and microwave high bandwidth temporal signal processing based on Kerr micro-combs with spacings from 49-200GHz. We cover integral and fractional Hilbert transforms, differentiators as well as integrators. The potential of optical micro-combs for RF photonic app…
Reliable operation of photonic integrated circuits at cryogenic temperatures would enable new capabilities for emerging computing platforms, such as quantum technologies and low-power cryogenic computing. The silicon-on-insulator platform is a highly promising approach to developing large-scale photonic integrated circuits due to its exceptional manufacturability, CMOS compatibility and high component density. Fast, efficient and low-loss modulation at cryogenic temperatures in silicon, however, remains an outstanding challenge, particularly without the addition of exotic nonlinear optical mat…
We review recent work on broadband RF channelizers based on integrated optical frequency Kerr micro-combs combined with passive micro-ring resonator filters, with microcombs having channel spacings of 200GHz and 49GHz. This approach to realizing RF channelizers offers reduced complexity, size, and potential cost for a wide range of applications to microwave signal detection.
We demonstrate Kerr-frequency-comb generation with nanofabricated Fabry-Perot resonators with photonic-crystal-reflector (PCR) end mirrors. The PCR group-velocity-dispersion (GVD) is engineered to counteract the strong normal GVD of a rectangular waveguide fabricated on a thin, 450 nm silicon nitride device layer. The reflectors provide the resonators with both the high optical quality factor and anomalous GVD required for Kerr-comb generation. We report design, fabrication, and characterization of devices in the 1550 nm wavelengths bands, including the GVD spectrum and quality factor. Kerr-co…
The exponential growth of computing needs for artificial intelligence and machine learning has had a dramatic impact on data centre energy consumption, which has risen to environmentally significant levels. Using light to send information between compute nodes can dramatically decrease this energy consumption while simultaneously increasing bandwidth. Through wavelength-division multiplexing with chip-based microresonator Kerr frequency combs, independent information channels can be encoded onto many distinct colours of light in the same optical fibre for massively parallel data transmission w…
Sub-kHz linewidth integrated extended-DBR Pockels lasers using lithium tantalate
Institute of Physics, Swiss Federal Institute of Technology Lausanne (EPFL)（洛桑联邦理工学院物理研究所）；Deeplight SA（Deeplight公司）；State Key Laboratory of Materials for Integrated Circuits, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences（中国科学院上海微系统与信息技术研究所集成电路材料重点实验室）；Institute of Electrical and Micro engineering, Swiss Federal Institute of Technology, Lausanne (EPFL)（洛桑联邦理工学院电气与微电子工程研究所）
We report a 92 channel RF channelizer based on a 48.9 GHz integrated micro-comb that operates via soliton crystals, together with a passive high-Q ring resonator that acts as a periodic filter with an optical 3dB bandwidth of 121.4 MHz. We obtain an instant RF bandwidth of 8.08 GHz and 17.55 GHz achieved through temperature tuning. These results represent a major advance to achieving fully integrated photonic RF spectrum channelizers with reduced low complexity, size, and high performance for digital-compatible signal detection and broadband analog signal processing.
High-Efficiency Acousto-Optic Modulation on Non-Suspended Thin-Film Lithium Tantalate
University of Science and Technology of China（中国科学技术大学）；Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences（中国科学院上海微系统信息技术研究所）；University of Chinese Academy of Sciences（中国科学院大学）
We demonstrate advanced transversal radio frequency (RF) and microwave functions based on a Kerr optical comb source generated by an integrated micro-ring resonator. We achieve extremely high performance for an optical true time delay aimed at tunable phased array antenna applications, as well as reconfigurable microwave photonic filters. Our results agree well with theory. We show that our true time delay would yield a phased array antenna with features that include high angular resolution and a wide range of beam steering angles, while the microwave photonic filters feature high Q factors, w…
Microresonator Kerr frequency combs, which rely on third-order nonlinearity ($χ^{(3)}$), are of great interest for a wide range of applications including optical clocks, pulse shaping, spectroscopy, telecommunications, light detection and ranging (LiDAR) and quantum information processing. Many of these applications require further spectral and temporal control of the generated frequency comb signal, which is typically accomplished using additional photonic elements with strong second-order nonlinearity ($χ^{(2)}$). To date these functionalities have largely been implemented as discrete off-ch…
Soliton crystal micro-combs are powerful tools as sources of multiple wavelength channels for radio frequency (RF) signal processing. They offer a compact device footprint, large numbers of wavelengths, very high versatility, and wide Nyquist bandwidths. Here, we demonstrate integral order RF signal processing functions based on a soliton crystal micro-comb, including a Hilbert transformer and first- to third-order differentiators. We compare and contrast results achieved and the tradeoffs involved with varying comb spacing, tap design methods, as well as shaping methods.
We demonstrate an RF photonic fractional Hilbert transformer based on an integrated Kerr micro-comb source featuring a record low free spectral range of 48.9 GHz, yielding 75 microcomb lines across the C-band. By programming and shaping the comb lines according to calculated tap weights, we demonstrate that the Hilbert transformer can achieve tunable bandwidths ranging from 1.2 to 15.3 GHz, switchable centre frequencies from baseband to 9.5 GHz, and arbitrary fractional orders. We experimentally characterize the RF amplitude and phase response of the tunable bandpass and lowpass Hilbert transf…
We report a photonic radio frequency (RF) fractional differentiator based on an integrated Kerr micro-comb source. The micro-comb source has a free spectral range (FSR) of 49 GHz, generating a large number of comb lines that serve as a high-performance multi-wavelength source for the differentiator. By programming and shaping the comb lines according to calculated tap weights, arbitrary fractional orders ranging from 0.15 to 0.90 are achieved over a broad RF operation bandwidth of 15.49 GHz. We experimentally characterize the frequency-domain RF amplitude and phase responses as well as the tem…
Microcavity solitons enable miniaturized coherent frequency comb sources. However, the formation of microcavity solitons can be disrupted by stimulated Raman scattering (SRS), particularly in the emerging crystalline microcomb materials with high Raman gain. Here, we propose and implement dissipation control---tailoring the energy dissipation of selected cavity modes---to purposely raise/lower the threshold of Raman lasing in a strongly Raman-active lithium niobate microring resonator, and realize on-demand soliton mode-locking or Raman lasing. Numerical simulations are carried out to confirm …
Electro-optic silicon-based modulators contribute to ease the integration of high-speed and low-power consumption circuits for classical optical communications or quantum computers. However, the inversion symmetry in the silicon crystal structure inhibits the use of Pockels effect. An electric field-induced optical modulation equivalent to a Pockels effect can nevertheless be achieved in silicon by the use of DC Kerr effect. Although some theoretical and experimental studies have shown its existence in silicon, the DC Kerr effect in optical modulation have led to a negligible contribution so f…
Nonreciprocal devices such as isolators and circulators are key enabling technologies for communication systems, both at microwave and optical frequencies. While nonreciprocal devices based on magnetic effects are available for free-space and fibre-optic communication systems, their on-chip integration has been challenging, primarily due to the concomitant high insertion loss, weak magneto-optical effects, and material incompatibility. We show that Kerr nonlinear resonators can be used to achieve all-passive, low-loss, bias-free, broadband nonreciprocal transmission and routing for application…
We demonstrate two categories of photonic radio frequency (RF) filters based on integrated optical micro-combs. The first one is based on the transversal filtering structure and the second one is based on the channelization technique. The large number of wavelengths brought about by the microcomb results in a significantly increased RF spectral resolution and a large instantaneous bandwidth for the RF filters. For the RF transversal filter, we demonstrated Q factor enhancement, improved out-of-band rejection, tunable centre frequency, and reconfigurable filtering shapes. While a high resolutio…
We demonstrate a photonic RF integrator based on an integrated soliton crystal microcomb source. By multicasting and progressively delaying the input RF signal using a transversal structure, the input RF signal is integrated discretely. Up to 81 wavelengths are provided by the microcomb source, which enable a large integration time window of 6.8 ns, together with a time resolution as fast as 84 ps. We perform signal integration of a diverse range of input RF signals including Gaussian pulses with varying time widths, dual pulses with varying time intervals and a square waveform. The experiment…
We demonstrate the use of integrated micro-resonator based optical frequency comb sources as the basis for transversal filtering functions for microwave and radio frequency photonic filtering and advanced functions.
We demonstrate a photonic radio frequency (RF) transversal filter based on an integrated optical micro-comb source featuring a record low free spectral range of 49 GHz yielding 80 micro-comb lines across the C-band. This record-high number of taps, or wavelengths for the transversal filter results in significantly increased performance including a QRF factor more than four times higher than previous results. Further, by employing both positive and negative taps, an improved out-of-band rejection of up to 48.9 dB is demonstrated using Gaussian apodization, together with a tunable centre frequen…
Optical frequency comb, with precisely controlled spectral lines spanning a broad range, has been the key enabling technology for many scientific breakthroughs. In addition to the traditional implementation based on modelocked lasers, photonic frequency microcombs based on dissipative Kerr and quadratic cavity solitons in high-Q microresonators have become invaluable in applications requiring compact footprint, low cost, good energy efficiency, large comb spacing, and access to nonconventional spectral regions. In this review, we comprehensively examine the recent progress of photonic frequenc…
Integrated Kerr microcombs are emerging as a powerful tool as sources of multiple wavelength channels for photonic RF and microwave signal processing mainly in the context of transversal filters. They offer a compact device footprint, very high versatility, large numbers of wavelengths, and wide Nyquist bands. Here, we review recent progress on Kerr microcomb-based photonic RF and microwave reconfigurable filters, based both on transversal filter methods and on RF to optical bandwidth scaling. We compare and contrast results achieved with wide comb spacing combs (200GHz) with more finely space…
THz oscillators generated via frequency-multiplication of microwaves are facing difficulty in achieving low phase noise. Photonics-based techniques, in which optical two tones are translated to a THz wave through opto-electronic conversion, are promising if the relative phase noise between the two tones is well suppressed. Here, a THz ($\approx$ 560 GHz) wave with a low phase noise is provided by a frequency-stabilized, dissipative Kerr microresonator soliton comb. The repetition frequency of the comb is stabilized to a long fiber in a two-wavelength delayed self-heterodyne interferometer, sig…
We stabilize a chip-scale Si3N4 phase-locked Kerr frequency comb via locking the pump laser to an independent stable high-Q reference microresonator and locking the comb spacing to an external microwave oscillator. In this comb, the pump laser shift induces negligible impact on the comb spacing change. This scheme is a step towards miniaturization of the stabilized Kerr comb system as the microresonator reference can potentially be integrated on-chip. Fractional instability of the optical harmonics of the stabilized comb is limited by the microwave oscillator used for comb spacing lock below 1…
We investigate numerically the effect of Kerr nonlinearity on the transmission spectrum of a one dimensional $δ$-function photonic crystal. It is found that the photonic band gap (PBG) width either increases or decreases depending on both sign and strength of Kerr nonlinearity. We found that any amount of self-focusing nonlinearity $(α>0)$ leads to an increase of the PBG width leading to light localization. However, for defocusing nonlinearity, we found a range of non-linearity strengths for which the photonic band gap width decreases when the nonlinearity strength increases and a critical non…
Nonlinear optical effects including stimulated Brillouin scattering (SBS) and four-wave mixing (FWM) play an important role in microwave photonics, optical frequency combs, and quantum photonics. Harnessing SBS and FWM in a low-loss and versatile integrated platform would open the path to building large-scale Brillouin/Kerr-based photonic integrated circuits. In this letter, we investigate the Brillouin and Kerr properties of a low-index (n=1.513 @ 1550 nm) silicon oxynitride (SiON) platform. We observed, for the first time, backward SBS in SiON waveguides with a Brillouin gain coefficient of …
Spontaneous symmetry breaking in nonlinear systems provides a unified method to understand vastly different phenomena, ranging from Higgs mechanism [1] and superconductivity [2] to ecological stability [3] and genome generation [4]. Spontaneous symmetry breaking is typically considered as the intrinsic property of nonlinear systems with fixed occurrence condition and property, as the form and magnitude of nonlinear interactions cannot be modified [5]. Here, we report the development of reconfigurable Kerr optical nonlinearity to control spontaneous symmetry breaking. This is achieved through t…
We report a broadband radio frequency (RF) channelizer with up to 92 channels using a coherent microcomb source. A soliton crystal microcomb, generated by a 49 GHz micro-ring resonator (MRR), is used as a multi-wavelength source. Due to its ultra-low comb spacing, up to 92 wavelengths are available in the C band, yielding a broad operation bandwidth. Another high-Q MRR is employed as a passive optical periodic filter to slice the RF spectrum with a high resolution of 121.4 MHz. We experimentally achieve an instantaneous RF operation bandwidth of 8.08 GHz and verify RF channelization up to 17.5…
We use our vector Maxwell's nonlinear eigenmode solver to study the stationary solutions in 2D cross-section waveguides with Kerr nonlinear cores. This solver is based on the fixed power algorithm within the finite element method. First, studying nonlinear plasmonic slot waveguides, we demonstrate that, even in the low power regime, 1D studies may not provide accurate and meaningfull results compared to 2D ones. Second, we study at high powers the link between the nonlinear parameter $γ_{nl}$ and the change of the nonlinear propagation constant $Δβ$. Third, for a specific type of photonic crys…
