We demonstrate single-mode microdisk lasers in the telecom band with ultralow thresholds on erbium-ytterbium co-doped thin-film lithium niobate (TFLN). The active microdisk was fabricated with high-Q factors by photolithography-assisted chemomechanical etching. Thanks to the erbium-ytterbium co-doping providing high optical gain, the ultralow loss nanostructuring, and the excitation of high-Q coherent polygon modes, which suppresses multimode lasing and allows high spatial mode overlap between pump and lasing modes, single-mode laser emission operating at 1530 nm wavelength was observed with an ultralow threshold, under a 980-nm-band optical pump. The threshold was measured as low as 1 µW, which is one order of magnitude smaller than the best results previously reported in single-mode active TFLN microlasers. The conversion efficiency reaches 4.06 × 10^-3, which is also the highest value reported in single-mode active TFLN microlasers.

Single-frequency ultranarrow linewidth on-chip microlasers with a fast wavelength tunability play a game-changing role in a broad spectrum of applications ranging from coherent communication, light detection and ranging, to metrology and sensing. Design and fabrication of such light sources remain a challenge due to the difficulties in making a laser cavity that has an ultrahigh optical quality (Q) factor and supports only a single lasing frequency simultaneously. Here, we demonstrate a unique single-frequency ultranarrow linewidth lasing mechanism on an erbium ion-doped lithium niobate (LN) microdisk through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths. As the polygon modes are sparse within the optical gain bandwidth compared with the whispering gallery mode counterpart, while their Q factors (above 10 million) are even higher due to the significantly reduced scattering on their propagation paths, single-frequency lasing with a linewidth as narrow as 322 Hz is observed. The measured linewidth is three orders of magnitude narrower than the previous record in on-chip LN microlasers. Finally, enabled by the strong linear electro-optic effect of LN, real-time electro-optical tuning of the microlaser with a high tuning efficiency of ∼50 pm / 100 V is demonstrated.

We demonstrate integrated lithium niobate (LN) microring resonators with Q factors close to the intrinsic material absorption limit of LN. The microrings are fabricated on pristine LN thin-film wafers thinned from LN bulk via chemo-mechanical etching without ion slicing and ion etching. A record-high Q factor up to 108 at the wavelength of 1550 nm is achieved because of the ultra-smooth interface of the microrings and the absence of ion-induced lattice damage, indicating an ultra-low waveguide propagation loss of ∼0.0034dB/cm. The ultra-high Q microrings will pave the way for integrated quantum light source, frequency comb generation, and nonlinear optical processes.

We demonstrate high-quality (intrinsic Q factor ∼2.8 × 10⁶) racetrack microresonators fabricated on lithium niobate thin film with a free spectral range (FSR) of ∼86 pm. By integrating microelectrodes alongside the two straight arms of the racetrack resonator, the resonance wavelength around 1550 nm can be red shifted by 92 pm when the electric voltage is raised from -100 V to 100 V. The microresonators with the tuning range spanning over a full FSR are fabricated using photolithography assisted chemo-mechanical etching.

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.