Monolithic ultra-high-Q lithium niobate microring resonator

Abstract

We demonstrate an ultralow loss monolithic integrated lithium niobate photonic platform consisting of dry-etched subwavelength waveguides. We show microring resonators with a quality factor of 10 7 and waveguides with propagation loss as low as 2.7 dB/m.Lithium niobate (LN) is a material with wide range of applications in optical and microwave technologies, owing to its unique properties that include large second order nonlinear susceptibility (χ (2) = 30 pm/V), large piezoelectric response (C 33 ∼ 250 C/m 2 ), wide optical transparency window (350 nm − 5 µm) and high refractive index (∼ 2.2) [1]. Conventional LN components, including fiber-optic modulators and periodically poled frequency converters have been the workhorse of the optoelectronic industry. The performances of these components have the potential to be dramatically improved as optical waveguides in bulk LN crystals are defined by ion-diffusion or proton-exchange methods which result in low index contrast and weak optical confinement. Integrated LN platform, featuring sub-wavelength scale light confinement and dense integration of optical and electrical components, has the potential to revolutionize optical communication and microwave photonics [1][2][3][4][5][6][7].The major road-block for practical applications of integrated LN photonics is the difficulty of fabricating devices that simultaneously achieve low optical propagation loss and high confinement. Recently developed thin-film LN-on-insulator technology makes this possible, and has resulted in the development of two complementary approaches to define nanoscale optical waveguides: hybrid and monolithic. The hybrid approach integrates an easyto-etch material (e.g. silicon or silicon nitride) with LN thin films to guide light [2-4] with a relatively low propagation loss (0.3 dB/cm) [4]. However, the resulting optical modes only partially reside in the active material region (i.e. LN), reducing the nonlinear interaction efficiency. The monolithic approach relies on direct etching of LN to achieve high optical confinement in the active region, but has suffered from a relatively high propagation loss. While freestanding LN microdisk resonators have achieved optical quality factors (Q) of ∼ 10 6 [5, 6], integrated microring resonators typically feature Q ∼ 10 5 with waveguide propagation loss on the order of 3 dB/cm [7]. Since LN is perceived as a difficult-to-etch material, it is commonly accepted that low-loss propagation in a monolithic waveguide is not possible, and that therefore it is not the most promising path forward.Here we challenge the status quo and show that subwavelength scale lithium niobate waveguides can be fabricated with a propagation loss as low as 2.7 ± 0.3 dB/m through an optimized etching process. We also demonstrate ultra-high Q factor optical cavities with intrinsic Q = 10 7 . We fabricate waveguide coupled microring and racetrack resonators with a bending radius r = 80 µm, and various straight arm lengths l and waveguide widths w (Fig. 1). The optical modes in these reso...

Status and Potential of Lithium Niobate on Insulator (LNOI) for Photonic Integrated Circuits

“…From this review, the potential benefits of LNOI are clear, however, it is worth considering whether LNOI will be widely adopted as a fundamental platform for photonic integration, or whether sufficient benefits could be achieved by hybrid integrating local islands of LiNbO 3 onto a more mature wafer platform such as SiN and SOI. The extremely low loss (2.7 dB m −1 ) and the tight waveguide bending radii available in the LNOI platform are comparable to well established SiN platforms, so there is no compromise in terms of insertion loss and device compactness to take advantage of the strong electro‐optic and nonlinear optical properties of LN. This virtuous combination of properties has maintained the industrial relevance of diffused lithium niobate waveguides for decades, strongly suggesting that LNOI will become an important and potentially widely used PIC platform in the near future.…”

“…This process is very promising as it was successfully used to achieve wire waveguides with losses as low as 3 dB/cm and bending radii <20 μm for telecommunication C‐band wavelengths. The optical losses of the waveguide were further improved by optimizing the waveguide fabrication process, which resulted in an outstanding result with waveguide losses as low as 2.7 dB/m . This indicates that this fabrication technique can be used for integrating many optical components on a single PIC .…”

“…was not to achieve very high Q factors, as they used the ring resonators to modulate light . The same group demonstrated shortly afterwards exceptional Q factors up to 10 7 in LNOI waveguides using an optimized design and waveguide fabrication technique …”

Lithium Niobate Metasurfaces

“…Mechanical sculpture is also difficult given the hardness of the material. Fortunately, in recent years, there has been significant progress in high quality LN micro‐ or nanostructure fabrication, for example, thermal diffusion or proton exchange, laser writing, focused ion milling, plasma dry etching, and so on. Until now, the waveguides, photonic crystals, micro‐cavities, optical frequency converters, superbroad‐bandwidth modulators with the feature sizes of several to hundreds of micrometers have been fabricated.…”

A Theoretical Study on Rib‐Type Photonic Wires Based on LiNbO3 Thin Film on Insulator

“…The LNOI structure has been pioneered by the groups of Osgood [4] and Günter [5] on smaller substrates. To date, many LNOI PW devices have been demonstrated, such as (CMOS-compatible) electro-optic modulator, [10][11][12][13][14][15][16][17][18][19] (electrooptically tunable) micro-ring or micro-disk resonator, [7,[19][20][21][22][23][24][25][26][27][28][29] LNOI heterogeneous photonic device, [19] grating coupler, [30,31] photonic crystal, [7,[32][33][34] transverse-electric/magnetic (TE/TM)pass polarizer, [35] quantum optics device, [36] as well as some nonlinear devices based on periodically poled LNOI (PPLNOI) PWs. in diameter) LNOI thin-film is commercially available.…”