There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Abstract Lithium niobate (LiNbO3) on insulator (LNOI) is a promising material platform for integrated photonics due to single crystal LiNbO3 film’s wide transparent window, high refractive index, and high second-order nonlinearity. Based on LNOI, the fast-developing ridge-waveguide fabrication techniques enabled various structures, devices, systems, and applications. We review the basic structures including waveguides, cavities, periodically poled LiNbO3, and couplers, along with their fabrication methods and optical properties. Treating those basic structures as building blocks, we review several integrated devices including electro-optic modulators, nonlinear optical devices, and optical frequency combs with each device’s operating mechanism, design principle and methodology, and performance metrics. Starting from these integrated devices, we review how integrated LNOI devices boost the performance of LiNbO3’s traditional applications in optical communications and data center, integrated microwave photonics, and quantum optics. Beyond those traditional applications, we also review integrated LNOI devices’ novel applications in metrology including ranging system and frequency comb spectroscopy. Finally, we envision integrated LNOI photonics’ potential in revolutionizing nonlinear and quantum optics, optical computing and signal processing, and devices in ultraviolet, visible, and mid-infrared regimes. Beyond this outlook, we discuss the challenges in integrated LNOI photonics and the potential solutions.

/pdf/integrated-lithium-niobate-photonics-10qr21htqn.pdf

Integrated lithium niobate photonics

Electro-optic modulators (EOMs) convert signals from the electrical to the optical domain. They are at the heart of optical communication, microwave signal processing, sensing, and quantum technologies. Next-generation EOMs require high-density integration, low cost, and high performance simultaneously, which are difficult to achieve with established integrated photonics platforms. Thin-film lithium niobate (LN) has recently emerged as a strong contender owing to its high intrinsic electro-optic (EO) efficiency, industry-proven performance, robustness, and, importantly, the rapid development of scalable fabrication techniques. The thin-film LN platform inherits nearly all the material advantages from the legacy bulk LN devices and amplifies them with a smaller footprint, wider bandwidths, and lower power consumption. Since the first adoption of commercial thin-film LN wafers only a few years ago, the overall performance of thin-film LN modulators is already comparable with, if not exceeding, the performance of the best alternatives based on mature platforms such as silicon and indium phosphide, which have benefited from many decades of research and development. In this mini-review, we explain the principles and technical advances that have enabled state-of-the-art LN modulator demonstrations. We discuss several approaches, their advantages and challenges. We also outline the paths to follow if LN modulators are to improve further, and we provide a perspective on what we believe their performance could become in the future. Finally, as the integrated LN modulator is a key subcomponent of more complex photonic functionalities, we look forward to exciting opportunities for larger-scale LN EO circuits beyond single components.

Integrated lithium niobate electro-optic modulators: when performance meets scalability

Crystalline lithium niobate (LN) is an important optical material because of its broad transmission window that spans from ultraviolet to mid-infrared and its large nonlinear and electro-optic coefficients. Furthermore, the recent development and commercialization of LN-on-insulator (LNOI) technology has opened an avenue for the realization of integrated on-chip photonic devices with unprecedented performances in terms of propagation loss, optical nonlinearity, and electro-optic tunability. This review begins with a brief introduction of the history and current status of LNOI photonics. We then discuss the fabrication techniques of LNOI-based photonic structures and devices. The recent revolution in the LN photonic industry has been sparked and is still being powered by innovations of the nanofabrication technology of LNOI, which enables the production of building block structures, such as optical microresonators and waveguides of unprecedented optical qualities. The following sections present various on-chip LNOI devices categorized into nonlinear photonic and electro-optic tunable devices and photonic-integrated circuits. Some conclusions and future perspectives are provided.

/pdf/advances-in-on-chip-photonic-devices-based-on-lithium-461c9g3pmy.pdf

Advances in on-chip photonic devices based on lithium niobate on insulator

Controlling thermal transport has become relevant in recent years. Traditionally, this control has been achieved by tuning the scattering of phonons by including various types of scattering centres in the material (nanoparticles, impurities, etc). Here we take another approach and demonstrate that one can also use coherent band structure effects to control phonon thermal conductance, with the help of periodically nanostructured phononic crystals. We perform the experiments at low temperatures below 1 K, which not only leads to negligible bulk phonon scattering, but also increases the wavelength of the dominant thermal phonons by more than two orders of magnitude compared to room temperature. Thus, phononic crystals with lattice constants ≥1 μm are shown to strongly reduce the thermal conduction. The observed effect is in quantitative agreement with the theoretical calculation presented, which accurately determined the ballistic thermal conductance in a phononic crystal device.

/pdf/engineering-thermal-conductance-using-a-two-dimensional-a7jak7z5xw.pdf

Engineering thermal conductance using a two-dimensional phononic crystal

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]

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

Microwave photonics, a field that crosscuts microwave/millimeter-wave engineering with optoelectronics, has sparked great interest from research and commercial sectors. This multidisciplinary fusion can achieve ultrawide bandwidth and ultrafast speed that were considered impossible in conventional chip-scale microwave/millimeter-wave systems. Conventional microwave-to-photonic converters, based on resonant acousto-optic modulation, produce highly efficient modulation but sacrifice bandwidth and limit their applicability for most real-world microwave signal-processing applications. In this paper, we build highly efficient and wideband microwave-to-photonic modulators using the acousto-optic effect on suspended lithium niobate thin films. A wideband microwave signal is first piezoelectrically transduced using interdigitated electrodes into Lamb acoustic waves, which directly propagates across an optical waveguide and causes refractive index perturbation through the photoelastic effect. This approach is power-efficient, with phase shifts up to 0.0166  rad/√mW over a 45 μm modulation length and with a bandwidth up to 140 MHz at a center frequency of 1.9 GHz. Compared to the state-of-the-art, a 9× more efficient modulation has been achieved by optimizing the acoustic and optical modes and their interactions.

/pdf/efficient-and-wideband-acousto-optic-modulation-on-thin-film-3t7ycg384j.pdf

Efficient and wideband acousto-optic modulation on thin-film lithium niobate for microwave-to-photonic conversion

This paper reports a lithium niobate (LiNbO3) resonant transformer with an extracted passive open-circuit voltage gain of 38. The result has been attained thanks to a high figure of merit (FoM=k t 2 · Q) of 136 that results from a high electromechanical coupling (k t 2) of 28% and a high Q of 480. To our knowledge, this is the highest FoM that has been demonstrated for a two-port MEMS resonator. The fabricated transformer also features a factional bandwidth of 0.21%, thus providing a large operation temperature range (±18 K) over which a high gain can be maintained. This work has shown promising results for wake-up radio applications, where transformers can be exploited for detection of faint signals with very low or zero power consumption.

A high FoM lithium niobate resonant transformer for passive voltage amplification

This work presents a new class of microelectromechanical system (MEMS) resonators enabled by Y -cut lithium niobate (LiNb03) thin films. In operation, the devices are excited into the first order antisymmetric (A1) mode vibration. The fabricated MEMS resonator has demonstrated a large effective electromechanical coupling $(\pmb{k}_{\pmb{t}}^{2})$ of 6.3% and a high quality factor (Q) of 5341, hence resulting in an extremely high figure of merit $(\mathbf{FoM}= \pmb{k}_{\pmb{t}}^{2}\cdot \pmb{Q})$ of 336 at 1.7 GHz. Additionally, this work also marks the highest experimental/Q product of $\mathbf{9.15}\times \mathbf{10}^{\mathbf{12}}\mathbf{Hz}$ for state-of-the-art MEMS resonators based on transferred lithium niobate thin films.

1.7 GHz Y-Cut Lithium Niobate MEMS Resonators with FoM of336 andfQ of9.15×10 12

In this work, we present the low-loss acoustic delay lines (ADLs) at 5 GHz, using the first-order antisymmetric (A1) mode in 128° Y-cut lithium niobate thin films. The ADLs use a single-phase unidirectional transducer (SPUDT) design with a feature size of quarter acoustic wavelength. The design space is analytically explored and experimentally validated. The fabricated miniature A1 ADLs with a feature size of $0.45~\mu \text{m}$ show a high operating frequency at 5.4 GHz, a minimum insertion loss (IL) of 3 dB, a fractional bandwidth (FBW) of 1.6%, and a small footprint of 0.0074 mm2. The low IL and high operating frequency have significantly surpassed the state-of-the-art performance of ADLs. The propagation characteristics of A1 acoustic waves have also been extracted. The demonstrated designs can lead to low-loss and high-frequency transversal filters for future 5G applications in the sub-6-GHz bands.

/pdf/low-loss-5-ghz-first-order-antisymmetric-mode-acoustic-delay-yu8tpj1x4y.pdf

Low-Loss 5-GHz First-Order Antisymmetric Mode Acoustic Delay Lines in Thin-Film Lithium Niobate

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