The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Lithium niobate (LN), an outstanding and versatile material, has influenced our daily life for decades—from enabling high-speed optical communications that form the backbone of the Internet to realizing radio-frequency filtering used in our cell phones. This half-century-old material is currently embracing a revolution in thin-film LN integrated photonics. The successes of manufacturing wafer-scale, high-quality thin films of LN-on-insulator (LNOI) and breakthroughs in nanofabrication techniques have made high-performance integrated nanophotonic components possible. With rapid development in the past few years, some of these thin-film LN devices, such as optical modulators and nonlinear wavelength converters, have already outperformed their legacy counterparts realized in bulk LN crystals. Furthermore, the nanophotonic integration has enabled ultra-low-loss resonators in LN, which has unlocked many novel applications such as optical frequency combs and quantum transducers. In this review, we cover—from basic principles to the state of the art—the diverse aspects of integrated thin-film LN photonics, including the materials, basic passive components, and various active devices based on electro-optics, all-optical nonlinearities, and acousto-optics. We also identify challenges that this platform is currently facing and point out future opportunities. The field of integrated LNOI photonics is advancing rapidly and poised to make critical impacts on a broad range of applications in communication, signal processing, and quantum information.

/pdf/integrated-photonics-on-thin-film-lithium-niobate-3tfumu49q8.pdf

Integrated photonics on thin-film lithium niobate

Lithium niobate (LN), an outstanding and versatile material, has influenced our daily life for decades: from enabling high-speed optical communications that form the backbone of the Internet to realizing radio-frequency filtering used in our cell phones. This half-century-old material is currently embracing a revolution in thin-film LN integrated photonics. The success of manufacturing wafer-scale, high-quality, thin films of LN on insulator (LNOI), accompanied with breakthroughs in nanofabrication techniques, have made high-performance integrated nanophotonic components possible. With rapid development in the past few years, some of these thin-film LN devices, such as optical modulators and nonlinear wavelength converters, have already outperformed their legacy counterparts realized in bulk LN crystals. Furthermore, the nanophotonic integration enabled ultra-low-loss resonators in LN, which unlocked many novel applications such as optical frequency combs and quantum transducers. In this Review, we cover -- from basic principles to the state of the art -- the diverse aspects of integrated thin-film LN photonics, including the materials, basic passive components, and various active devices based on electro-optics, all-optical nonlinearities, and acousto-optics. We also identify challenges that this platform is currently facing and point out future opportunities. The field of integrated LNOI photonics is advancing rapidly and poised to make critical impacts on a broad range of applications in communication, signal processing, and quantum information.

/pdf/integrated-photonics-on-thin-film-lithium-niobate-55z73oui6u.pdf

Modern advanced photonic integrated circuits require dense integration of high-speed electro-optic functional elements on a compact chip that consumes only moderate power. Energy efficiency, operation speed, and device dimension are thus crucial metrics underlying almost all current developments of photonic signal processing units. Recently, thin-film lithium niobate (LN) emerges as a promising platform for photonic integrated circuits. Here, we make an important step towards miniaturizing functional components on this platform, reporting high-speed LN electro-optic modulators, based upon photonic crystal nanobeam resonators. The devices exhibit a significant tuning efficiency up to 1.98 GHz V-1, a broad modulation bandwidth of 17.5 GHz, while with a tiny electro-optic modal volume of only 0.58 μm3. The modulators enable efficient electro-optic driving of high-Q photonic cavity modes in both adiabatic and non-adiabatic regimes, and allow us to achieve electro-optic switching at 11 Gb s-1 with a bit-switching energy as low as 22 fJ. The demonstration of energy efficient and high-speed electro-optic modulation at the wavelength scale paves a crucial foundation for realizing large-scale LN photonic integrated circuits that are of immense importance for broad applications in data communication, microwave photonics, and quantum photonics.

/pdf/lithium-niobate-photonic-crystal-electro-optic-modulator-1z0pvp785q.pdf

Lithium niobate photonic-crystal electro-optic modulator

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

Pyroelectric materials enable the construction of high-performance yet low-cost and uncooled detectors throughout the infrared spectrum. These devices have been used as broadband sensors and, when combined with an interferometric element or filter, can provide spectral selectivity. Here we propose the concept of and demonstrate a new architecture that uses a multifunctional metamaterial absorber to directly absorb the incident longwave IR (8–12 μm) energy in a thin-film lithium niobate layer and also to function as the contacts for the two-terminal detector. Our device achieves a narrowband (560 nm FWHM at 10.73 μm), yet highly efficient (86%) absorption. The metamaterial creates high field concentration, reducing temperature fluctuation noise, and lowering device capacitance and loss tangent noise. The metamaterial design paradigm applied to detectors thus results in a very fast planar device with a thermal time constant of 28.9 ms with a room temperature detectivity, D*, of 107  cm W/Hz.

Multifunctional metamaterial pyroelectric infrared detectors

Pyroelectric thermal detectors are excellent candidates for detection of broadband radiation. Such detectors utilize
permanently poled ferroelectric single crystal lithium tantalate to generate a charge as the crystal heats up by absorbing
radiation. The charge, which results in a current output when connected to an external electrical circuit, is directly
proportional to the rate of change of temperature of the crystal. The fundamental approach toward enhancing pyroelectric
detector response is to form the pyroelectric material into a thin film. An elegant approach for producing bulk quality
thin films of pyroelectric materials is by crystal ion slicing. In this paper, we report on the formation of thin film lithium
tantalate (TFLT™) pyroelectric detector devices using the ion slicing process. The devices incorporate films less than 9
microns thin and feature apertures as large as 5 mm in diameter. To make functional detectors, ion sliced films were
transferred to ceramic carriers in TO-type can test packages. Test results have shown improvement in room temperature
detectivity about 20 times higher than the state-of-the-art lithium tantalate pyroelectric detectors.

Thin Film Lithium Tantalate (TFLT™) Pyroelectric Detectors

Most on-chip optical isolators utilize nonreciprocal magneto-optic (MO) garnet-materials that require an external bias magnetic field to operate. The magnetic field is applied through incorporation of either a permanent magnet or an active electromagnet. Permanent magnets add significantly to device bulk and electromagnets increase fabrication complexity and power consumption. Hence, it is highly desirable to reduce or eliminate the magnetizing component in these devices. Here, we experimentally demonstrate a first of its kind bias-magnet-free (BMF) on-chip waveguide optical isolator with integrated Polarcor UltraThin polarizers. The BMF isolator device obviates the need for magnetizing components while possessing superior performance, relative to other on-chip isolators, with ≥25 dB of isolation ratio (IR), and ≤3.5 dB of insertion loss (IL) across the entire infrared optical C-band.

/pdf/broadband-bias-magnet-free-on-chip-optical-isolators-with-zuo5v6cx3j.pdf

Broadband Bias-Magnet-Free On-Chip Optical Isolators With Integrated Thin Film Polarizers

Photonic methods for electric field sensing have been demonstrated across the electromagnetic spectrum from near-DC to millimeter waves, and at field strengths from microvolts-per-meter to megavolts-per-meter. The advantages of the photonic approach include a high degree of electrical isolation, wide bandwidth, minimum perturbation of the incident field, and the ability to operate in harsh environments. Aerospace applications of this technology span a wide range of frequencies and field strengths. They include, at the high-frequency/high-field end, measurement of high-power electromagnetic pulses, and at the low-frequency/low-field end, in-flight monitoring of electrophysiological signals. The demands of these applications continue to spur the development of novel materials and device structures to achieve increased sensitivity, wider bandwidth, and greater high-field measurement capability. This paper will discuss several new directions in photonic electric field sensing technology for defense applications. The first is the use of crystal ion slicing to prepare high-quality, single-crystal electro-optic thin films on low-dielectricconstant, RF-friendly substrates. The second is the use of two-dimensional photonic crystal structures to enhance the electro-optic response through slow-light propagation effects. The third is the use of ferroelectric relaxor materials with extremely high electro-optic coefficients.

Advanced Materials and Device Technology for Photonic Electric Field Sensors

This paper reports the first demonstration of a high-speed electro-optic modulator in crystal ion sliced thin film lithium niobate (TFLN™). The device exhibits a Vπ·L of 4.75 V·cm, 8dB RF loss at 65 GHz and 40 GHz electro-optic bandwidth.

Wide-band electro-optic modulator in thin-film lithium niobate on quartz substrate

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