The rapid development of 5G, cloud-based service, artificial intelligence, and the Internet of Things has led to explosive growth in data communication traffic, resulting in a dramatic increase in energy demand. To meet this ever-growing demand, energy-efficient optical transmission systems are required to support high-speed optical links. The key component of such systems, ranging from long-reach applications to short-reach interconnects in data centers and on-chip optical interconnects, is the electro-optical (EO) modulator. In this regard, EO modulators have continuously been the focus of research in this field since the emergence of optical communications. Given the challenges posed by data traffic and the energy crisis, it is imperative to develop high-performance EO modulators to support high-speed data transmission with low-power consumption, targeting several femtojoules per bit for next-generation transceivers. EO modulators can convert signals from the electrical domain to the optical domain at high speed. The amplitude, phase, frequency, and polarization of the optical carrier can be exploited to encode information. EO modulators are categorized into free-space and integrated types. In the former category, optical waves propagate freely, and free-space modulators can be based on multi-layers, metasurfaces, and diffraction grating structures. In contrast, integrated modulators in the latter category utilize guided modes within photonic integrated circuits. In this review, we exclusively focus on the integrated EO modulation. In recent decades, various EO modulators integrated on different photonic integrated circuits have been extensively studied, including silicon, indium phosphide (InP), and silicon-organic hybrid (SOH) structures. Pure silicon-based modulators operate through carrier injection or depletion within p-n junctions integrated into optical waveguides, which leads to an inherent trade-off between modulation efficiency and optical loss. InP-based modulators, while capable of achieving high data transmission rates, are constrained by intrinsic modulation nonlinearity, substantial optical loss, and high costs, thus limiting their widespread application. SOH modulators leverage the ultra-high EO coefficient of engineered polymers, yet they often suffer from considerable optical loss and susceptibility to temperature variations. Lithium niobate (LN) is a ferroelectric crystal prized for its linear Pockels effect, broad transparency across wavelengths, and stable physical and chemical properties. Over recent decades, LN has stood out as a highly promising material for photonic devices. Notably, its linear EO effect (r₃₃≈33 pm/V) has enabled the development and commercial availability of high-speed LN EO modulators, crucial for long-distance telecommunications systems. More recently, thin-film lithium niobate (TFLN) has emerged as a topic of extensive interest. Unlike conventional LN waveguides, TFLN waveguides exploit high refractive-index contrast to tightly confine optical and electric fields, thereby supporting compact footprints and optimizing EO modulation efficiency. In this review, we mainly concentrate on the TFLN-based EO modulators and their applications.

TFLN-based EO modulators, which offer advantages such as a small footprint, high bandwidth, and low power consumption, could outperform counterparts based on bulk LN crystal, making them highly competitive in optical communications. In the first section (Sec. 1) of this review, we briefly describe current bottlenecks in optical communication systems and introduce EO modulators across various integrated photonic platforms. To better understand LN crystal materials’ characteristics, we summarize their development history and manufacturing processes (Fig. 2) in Sec. 2, which covers both LN crystal and TFLN-based wafers. The subsequent focus is on recently demonstrated TFLN-based modulators with various structures, including non-resonator types (Fig. 5), resonator types (Fig. 9), and others (Fig. 12). Moreover, various heterogeneous integration technologies of TFLN with other material platforms are summarized in detail, such as die-to-wafer bonding (Fig. 13), rib-loaded waveguides, and micro-transfer-printing (Fig. 14). The end of Sec. 3 describes the TFLN-based EO modulators designed for multi-channel operation (Fig. 16) and diverse operating wavelengths (Fig. 15). Modulation performances of different EO modulator types are comprehensively compared and evaluated in Tables 1 to 4. Finally, Sec. 4 discusses applications of TFLN-based modulators, including EO comb generation, tunable and mode-locked lasers, EO isolators, microwave processing engines, and EO programmable optical switches.

TFLN has emerged as the leading EO integration platform in recent years. TFLN-based modulators boast ultrahigh speed and ultralow power consumption, poised to noticeably influence optical communications, microwave photonics, and quantum information applications. Beyond its linear EO effect, TFLN also exhibits acousto-optical, second-order nonlinear, piezoelectric, and pyroelectric properties. Recent advancements have showcased a range of high-performance devices such as periodically poled LN, acousto-optical modulators, and surface acoustic wave filters. Thus, TFLN is expected to drive rapid progress in optical communication, computing, sensing, and other photonic information processing fields.