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Optical frequency combs consist of equally spaced discrete optical frequency components and are essential tools for optical communication, precision metrology, timing and spectroscopy 1 – 9 . At present, combs with wide spectra are usually generated by mode-locked lasers 10 or dispersion-engineered resonators with third-order Kerr nonlinearity 11 . An alternative method of comb production uses electro-optic (EO) phase modulation in a resonator with strong second-order nonlinearity, resulting in combs with excellent stability and controllability 12 – 14 . Previous EO combs, however, have been limited to narrow widths by a weak EO interaction strength and a lack of dispersion engineering in free-space systems. Here we overcome these limitations by realizing an integrated EO comb generator in a thin-film lithium niobate photonic platform that features a large EO response, ultralow optical loss and highly co-localized microwave and optical fields 15 , while enabling dispersion engineering. Our measured EO comb spans more frequencies than the entire telecommunications L-band (over 900 comb lines spaced about 10 gigahertz apart), and we show that future dispersion engineering can enable octave-spanning combs. Furthermore, we demonstrate the high tolerance of our comb generator to modulation frequency detuning, with frequency spacing finely controllable over seven orders of magnitude (10 hertz to 100 megahertz), and we use this feature to generate dual-frequency combs in a single resonator. Our results show that integrated EO comb generators are capable of generating wide and stable comb spectra. Their excellent reconfigurability is a powerful complement to integrated Kerr combs, enabling applications ranging from spectroscopy 16 to optical communications 8 . Using a thin-film lithium niobate photonic platform, an electro-optic frequency comb generator is realized that is capable of producing wide and stable spectra, spanning more frequencies than the entire telecommunications L-band.

The commercialization of lithium niobate on insulator (LNOI) wafer has resulted in significant on-chip photonic integration application owing to its remarkable photonic, acousto-optic, electro-optic, and piezoelectric nature. In recent years, a variety of high-performance on-chip LNOI-based photonic devices have been realized. In this study, we developed a 1-mol% erbium-doped lithium niobate crystal and its LNOI on a silicon substrate and fabricated an erbium-doped LNOI microdisk with high quality factor (∼ 1.05×10 5 ). C-band laser emission at ∼1530 and ∼1560 nm (linewidth 0.12 nm) from the high- Q erbium-doped LNOI microdisk was demonstrated with 974- and 1460-nm pumping, with the latter having better thermal stability. This microlaser would play an important role in the photonic integrated circuits of the lithium niobate platform.

Electro-optic modulators translate high-speed electronic signals into the optical domain and are critical components in modern telecommunication networks 1 , 2 and microwave-photonic systems 3 , 4 . They are also expected to be building blocks for emerging applications such as quantum photonics 5 , 6 and non-reciprocal optics 7 , 8 . All of these applications require chip-scale electro-optic modulators that operate at voltages compatible with complementary metal–oxide–semiconductor (CMOS) technology, have ultra-high electro-optic bandwidths and feature very low optical losses. Integrated modulator platforms based on materials such as silicon, indium phosphide or polymers have not yet been able to meet these requirements simultaneously because of the intrinsic limitations of the materials used. On the other hand, lithium niobate electro-optic modulators, the workhorse of the optoelectronic industry for decades 9 , have been challenging to integrate on-chip because of difficulties in microstructuring lithium niobate. The current generation of lithium niobate modulators are bulky, expensive, limited in bandwidth and require high drive voltages, and thus are unable to reach the full potential of the material. Here we overcome these limitations and demonstrate monolithically integrated lithium niobate electro-optic modulators that feature a CMOS-compatible drive voltage, support data rates up to 210 gigabits per second and show an on-chip optical loss of less than 0.5 decibels. We achieve this by engineering the microwave and photonic circuits to achieve high electro-optical efficiencies, ultra-low optical losses and group-velocity matching simultaneously. Our scalable modulator devices could provide cost-effective, low-power and ultra-high-speed solutions for next-generation optical communication networks and microwave photonic systems. Furthermore, our approach could lead to large-scale ultra-low-loss photonic circuits that are reconfigurable on a picosecond timescale, enabling a wide range of quantum and classical applications 5 , 10 , 11 including feed-forward photonic quantum computation. Chip-scale lithium niobate electro-optic modulators that rapidly convert electrical to optical signals and use CMOS-compatible voltages could prove useful in optical communication networks, microwave photonic systems and photonic computation.

Photonics on thin-film lithium niobate (TFLN) has emerged as one of the most pursued disciplines within integrated optics. Ultracompact and low-loss optical waveguides and related devices on this modern material platform have rejuvenated the traditional and commercial applications of lithium niobate for optical modulators based on the electro-optic effect, as well as optical wavelength converters based on second-order nonlinear effects, e.g., second-harmonic, sum-, and difference-frequency generations. TFLN has also created vast opportunities for applications and integrated solutions for optical parametric amplification and oscillation, cascaded nonlinear effects, such as low-harmonic generation; third-order nonlinear effects, such as supercontinuum generation; optical frequency comb generation and stabilization; and nonclassical nonlinear effects, such as spontaneous parametric downconversion for quantum optics. Recent progress in nonlinear integrated photonics on TFLN for all these applications, their current trends, and future opportunities and challenges are reviewed.

Lithium niobate (LN) thin film has received much attention as an integrated photonic platform, due to its rich and great photoelectric characteristics, based on which various functional photonic devices, such as electro-optic modulators and nonlinear wavelength converters, have been demonstrated with impressive performance. As an important part of the integrated photonic system, the long-awaited laser and amplifier on the LN thin-film platform have made a series of breakthroughs and important progress recently. In this review paper, the research progress of lasers and amplifiers realized on lithium niobate thin film platforms is reviewed comprehensively. Specifically, the research progress on optically pumped lasers and amplifiers based on rare-earth ions doping of LN thin films is introduced. Some important parameters and existing limitations of the current development are discussed. In addition, the implementation scheme and research progress of electrically pumped lasers and amplifiers on LN thin-film platforms are summarized. The advantages and disadvantages of optically and electrically pumped LN thin film light sources are analyzed. Finally, the applications of LN thin film lasers and amplifiers and other on-chip functional devices are envisaged.

Femto-/nanosecond pulse-induced, red and near-infrared absorption is studied in LiNb 1 − x Ta x O 3 ( 0 ⩽ x ⩽ 1 , LNT) solid solutions with the aim of studying transient optical nonlinearities associated with the formation, transport and recombination of optically generated small bound electron polarons with strong coupling to the lattice. As a result, a pronounced, long-lived transient absorption is uncovered for LNT which exceeds lifetimes and starting amplitudes of LiNbO 3 (LN) and LiTaO 3 (LT) by a factor of up to 100 and 10, respectively. The transients reveal a stretched-exponential decay behavior and a thermally activated process which provide strong evidence for an underlying hopping transport mechanism of small bound polarons. All findings are discussed in comparison to the model systems LN and LT within the framework of appropriate band models and optical generation of polarons via two-photon excitation. To explain the significant differences, the simultaneous presence of Nb Li 5 + , Ta Li 5 + antisites, and Ta V 5 + interstitial defects, i.e. a mixture of the intrinsic defects widely established for LN and LT, is assumed for LNT.

Ultrashort mid-infrared laser pulses can drive atoms far from their equilibrium positions in LiNbO 3 , exciting high phonon harmonics and providing a way to map the interatomic potential. Solid display of phonon harmonics High-harmonic generation of electromagnetic radiation is a well-known example of a nonlinear process. It occurs when photons—usually from a strong laser pulse—interact with nonlinear systems, such as a gas, plasma or solid, in a way that generates new photons with energies that are multiples of the original. This idea can also be applied to phonons—the quasiparticles associated with lattice vibrations—but high-order phonon modes are much harder to generate. Andrea Cavalleri and colleagues now show that ultrashort mid-infrared laser pulses can induce field strengths in lithium niobate (LiNbO 3 ) that are large enough to drive atoms far away from their equilibrium positions. Such strong fields can excite up to five phonon harmonics and provide a way to map the interatomic potential, which can be used to benchmark ab initio calculations. Nonlinear optical techniques at visible frequencies have long been applied to condensed matter spectroscopy 1 . However, because many important excitations of solids are found at low energies, much can be gained from the extension of nonlinear optics to mid-infrared and terahertz frequencies 2 , 3 . For example, the nonlinear excitation of lattice vibrations has enabled the dynamic control of material functions 4 , 5 , 6 , 7 , 8 . So far it has only been possible to exploit second-order phonon nonlinearities 9 at terahertz field strengths near one million volts per centimetre. Here we achieve an order-of-magnitude increase in field strength and explore higher-order phonon nonlinearities. We excite up to five harmonics of the A 1 (transverse optical) phonon mode in the ferroelectric material lithium niobate. By using ultrashort mid-infrared laser pulses to drive the atoms far from their equilibrium positions, and measuring the large-amplitude atomic trajectories, we can sample the interatomic potential of lithium niobate, providing a benchmark for ab initio calculations for the material. Tomography of the energy surface by high-order nonlinear phononics could benefit many aspects of materials research, including the study of classical and quantum phase transitions.

Lithium niobate (LN) has experienced significant developments during past decades due to its versatile properties, especially its large electro-optic (EO) coefficient. For example, bulk LN-based modulators with high speeds and a superior linearity are widely used in typical fiber-optic communication systems. However, with ever-increasing demands for signal transmission capacity, the high power and large size of bulk LN-based devices pose great challenges, especially when one of its counterparts, integrated silicon photonics, has experienced dramatic developments in recent decades. Not long ago, high-quality thin-film LN on insulator (LNOI) became commercially available, which has paved the way for integrated LN photonics and opened a hot research area of LN photonics devices. LNOI allows a large refractive index contrast, thus light can be confined within a more compact structure. Together with other properties of LN, such as nonlinear/acousto-optic/pyroelectric effects, various kinds of high-performance integrated LN devices can be demonstrated. A comprehensive summary of advances in LN photonics is provided. As LN photonics has experienced several decades of development, our review includes some of the typical bulk LN devices as well as recently developed thin film LN devices. In this way, readers may be inspired by a complete picture of the evolution of this technology. We first introduce the basic material properties of LN and several key processing technologies for fabricating photonics devices. After that, various kinds of functional devices based on different effects are summarized. Finally, we give a short summary and perspective of LN photonics. We hope this review can give readers more insight into recent advances in LN photonics and contribute to the further development of LN related research.

Many applications of metasurfaces require an ability to dynamically change their properties in the time domain. Electrical tuning techniques are of particular interest, since they pave a way to on-chip integration of metasurfaces with optoelectronic devices. In this work, we propose and experimentally demonstrate an electro-optic lithium niobate (EO-LN) metasurface that shows dynamic modulations to phase retardation of transmitted light. Quasi-bound states in the continuum (QBIC) are observed from this metasurface. By applying external electric voltages, the refractive index of lithium niobate (LN) is changed by Pockels EO nonlinearity, leading to efficient phase modulations to the transmitted light around the QBIC wavelength. The EO-LN metasurface developed in this study opens up new routes for potential applications in the field of displaying, pulse shaping, and spatial light modulating.

Lithium niobate has received interest in nonlinear frequency conversion due to its wide transparency window, from ultraviolet to mid-infrared spectral regions, and large second-order nonlinear susceptibility. However, its nanostructure is generally difficult to etch, resulting in low- Q resonance and lossy nanostructures for second harmonic generation. By applying the concept of bound states in the continuum, we performed theoretical and experimental investigations on high- Q resonant etchless thin-film lithium niobate with SiO 2 nanostructures on top for highly efficient second harmonic generation. In the fabricated nanostructured devices, a resonance with a Q factor of 980 leads to the strong enhancement of second harmonic generation by over 1500 times compared with that in unpatterned lithium niobate thin film. Although the pump slightly deviates from central resonance, an absolute conversion efficiency of 6.87×10 −7 can be achieved with the fundamental pump peak intensity of 44.65 MW/cm 2 , thus contributing to the normalized conversion efficiency of 1.54×10 −5 cm 2 /GW. Our work establishes an etchless lithium niobate device for various applications, such as integrated nonlinear nanophotonics, terahertz frequency generation, and quantum information processing.