Low-loss waveguides on Y-cut thin film lithium niobate: towards acousto-optic applications

Abstract

We investigate the dependence of photonic waveguide propagation loss on the thickness of the buried oxide layer in Y-cut lithium niobate on insulator substrate to identify trade-offs between optical losses and electromechanical coupling of surface acoustic wave (SAW) devices for acousto-optic applications. Simulations show that a thicker oxide layer reduces the waveguide loss but lowers the electromechanical coupling coefficient of the SAW device. Optical racetrack resonators with different lengths were fabricated by argon plasma etching to experimentally extract waveguide losses. By increasing the thickness of the oxide layer from 1 µm to 2 µm, propagation loss of 2 µm (1 µm) wide waveguide was reduced from 1.85 dB/cm (3 dB/cm) to as low as 0.37 dB/cm (0.77 dB/cm), and, resonators with quality factor greater than 1 million were demonstrated. An oxide thickness of approximately 1.5 µm is sufficient to significantly reduce propagation loss due to leakage into the substrate and simultaneously attain good electromechanical coupling in acoustic devices. This work not only provides insights on the design and realization of low-loss photonic waveguides in lithium niobate, but most importantly offers experimental evidence on how the oxide thickness directly impacts losses and guides its selection for the synthesis of highperformance acousto-optic devices in Y-cut lithium niobate on insulator.

High performance fully etched isotropic microring resonators in thin-film lithium niobate on insulator platform

“…By carefully designing the optical mode and the coupling between microrings and straight waveguides we demonstrate very high resonance extinction (~ 25 dB) for a very small radius (~ 30 µm) and high Q-factor (~ 31000). The extinction for our thin-film LN microring resonators is higher than previous results in the literature, e.g., ~ 58 dB was reported in [36], ~ 12 dB was reported in [42], ~ 4 dB was reported in [40], and ~ 6.5 dB was reported in [37]. Our work will pave the way for realizing ultra-compact and isotropic microring modulators with speeds of ~ 10 Gb/s in thinfilm LNOI.…”

“…To overcome this and induce more optical confinement in LN, partially etched LN has been recently proposed to create optical waveguides as a path to realizing high-performance electro-optic traveling wave structures [35] and resonant devices [36][37][38]. Due to the optical anisotropy of LN [39], the performance of the demonstrated structures based on X-cut or Ycut crystals (in-plane extraordinary axis) [40] and TE optical mode (in-plane polarization) has an inherent dependence on their orientation. This is especially pronounced for microringbased structures [37], where their optical or electro-optic response can significantly degrade if the structures have some in-plane rotation or misplacement.…”

Rejuvenating a Versatile Photonic Material: Thin‐Film Lithium Niobate

“…More recently, promising works [45][46][47][48][49] have been reported on low-loss dry-etching of LN with reported loss values as low as 0.027 dB cm −1 [46] for X-cut, and <2-nm sidewall roughness [47] for Z-cut TFLN. Following the dry-etching of LN, the waveguides have to undergo a thorough RCA cleaning step in order to remove the organic residue and chemical byproducts of the etching process.…”

“…Following the dry-etching of LN, the waveguides have to undergo a thorough RCA cleaning step in order to remove the organic residue and chemical byproducts of the etching process. [45,49] In a recent work, [48] a combination of PE and dry etching is presented for efficient direct-etching of TFLN, that is, faster etch rates while avoiding re-deposition of byproducts. By using this method, low-loss channel waveguides with large etch depths (∼900 nm) and improved verticality of sidewalls are reported in X-cut TFLN.…”

“…In order to achieve low-loss submicron waveguides, several methods have been demonstrated on the TFLN technology. They include rib-loading with a refractive-index-matched material, [29,[40][41][42][43][44] dry etching, [45][46][47][48][49][50] ion-beam-enhanced etching, [51] PE, [52,53] Ti indiffusion, [54] direct-or BCB-bonding on silicon-on-insulator (SOI), [38,55,56] plasma-enhanced chemical deposition (PECVD) of other materials, such as amorphous Si, [57] optical-grade dicing, [58] and mechanical thinning. [39] The optical mode size in these waveguides is typically reduced by about two orders of magnitude compared to their bulk counterparts (see Figure 1c).…”