[Paper Review] High-quality lithium niobate photonic crystal nanocavities

Hanxiao Liang, Rui Luo+3 more|arXiv (Cornell University)|Jun 27, 2017

Photonic and Optical Devices44 references3 citations

TL;DR

This paper demonstrates lithium niobate photonic crystal nanobeam resonators with optical quality factors (Q) up to 1.09×10⁵—over 100× higher than previous LN nanocavities—achieving extreme light confinement and enabling unprecedented nonlinear and optomechanical effects. The high Q and small mode volume lead to a photorefractive tuning rate of ~0.64 GHz/aJ and reveal novel quenching of photorefraction, while strong optomechanical coupling enables detection of gigahertz nanomechanical modes with a f·Q product of 1.47×10¹² Hz.

ABSTRACT

Lithium niobate (LN) exhibits unique material characteristics that have found many important applications. Scaling LN devices down to a nanoscopic scale can dramatically enhance light-matter interaction that would enable nonlinear and quantum photonic functionalities beyond the reach of conventional means. However, developing LN-based nanophotonic devices turns out to be nontrivial. Although significant efforts have been devoted in recent years, LN photonic crystal structures developed to date exhibit fairly low quality. Here we demonstrate LN photonic crystal nanobeam resonators with optical Q as high as 10^5, more than two orders of magnitude higher than other LN nanocavities reported to date. The high optical quality together with tight mode confinement leads to extremely strong nonlinear photorefractive effect, with a resonance tuning rate of 0.64 GHz/aJ, or equivalently 84 MHz/photon, three orders of magnitude greater than other LN resonators. In particular, we observed intriguing quenching of photorefraction that has never been reported before. The devices also exhibit strong optomechanical coupling with gigahertz nanomechanical mode with a significant f*Q product of 1.47*10^12 Hz. The demonstration of high-Q LN photonic crystal nanoresonators paves a crucial step towards LN nanophotonics that could integrate the outstanding material properties with versatile nanoscale device engineering for diverse intriguing functionalities.

Motivation & Objective

• To overcome the long-standing challenge of low optical quality factors (Q) in lithium niobate (LN) photonic crystal nanocavities, which limits their use in nonlinear and quantum photonics.
• To develop a nanophotonic platform based on LN that leverages its unique electro-optic, nonlinear, and piezoelectric properties at the nanoscale.
• To achieve high Q and strong light-matter confinement in LN nanobeam resonators through optimized design and fabrication.
• To explore and characterize nonlinear photorefractive effects and optomechanical coupling in LN nanocavities for potential applications in quantum optics and sensing.

Proposed method

• Design of a photonic crystal nanobeam structure with a defect cavity using finite element method simulations to optimize bandgap and mode confinement.
• Fabrication of LN nanobeam resonators via focused ion beam milling on bulk lithium niobate substrates, with precise control of sidewall angles and lattice periodicity.
• Use of a lattice constant gradient along the beam to minimize radiation losses and maximize the radiation-limited Q factor.
• Experimental characterization of optical Q using laser spectroscopy, measuring resonance shifts under optical illumination to probe photorefractive effects.
• Measurement of optomechanical coupling via sideband spectroscopy to extract mechanical mode frequencies, Q factors, and single-photon coupling rates.
• Numerical simulation of mechanical modes to correlate observed resonances with flexural vibrational patterns and effective masses.

Experimental results

Research questions

• RQ1 Can high-quality-factor (Q > 10⁵) photonic crystal nanocavities be achieved in bulk lithium niobate through nanofabrication and design optimization?
• RQ2 What is the extent of photorefractive nonlinearity in LN nanocavities, and can it be observed to saturate or quench under strong optical excitation?
• RQ3 To what extent does the optical mode confinement in LN nanocavities enhance optomechanical coupling, and can gigahertz mechanical modes be resolved?
• RQ4 How does air damping affect low-frequency mechanical modes in suspended LN nanobeams, and can high-Q mechanical modes be identified?
• RQ5 Can the strong coupling between optical, mechanical, and electrical degrees of freedom in LN be harnessed for multifunctional nanophotonic devices?

Key findings

• The fabricated lithium niobate photonic crystal nanobeam resonators achieved an optical Q factor as high as 1.09×10⁵, representing more than a 100-fold improvement over previously reported LN nanocavities.
• The effective mode volume was measured at ~1.03(λ/n)³, indicating strong spatial confinement of the optical mode.
• A photorefractive-induced resonance tuning rate of ~0.64 GHz/aJ was observed, corresponding to ~84 MHz/photon, three orders of magnitude higher than in other LN resonators.
• The devices exhibited novel quenching of the photorefractive effect under high optical power, a phenomenon not previously reported in LN nanophotonic systems.
• Optomechanical coupling was observed with a mechanical mode at 11.18 MHz showing a high mechanical Q of 6142 and a f·Q product of 1.47×10¹² Hz, comparable to state-of-the-art LN micromechanical devices.
• The single-photon optomechanical coupling rate |g₀|/(2π) reached 71 kHz, indicating strong interaction between the optical cavity and mechanical motion, despite no specific design for optomechanical applications.