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• Photonics Research
• Vol. 10, Issue 2, 02000401 (2022)
Ang Gao1、2、†, Chen Yang1、†, Likun Chen2、†, Ru Zhang1, Qiang Luo1, Wei Wang2, Qitao Cao2, Zhenzhong Hao1, Fang Bo1、3、*, Guoquan Zhang1、4、*, and Jingjun Xu1、5、*
Author Affiliations
• 1MOE Key Laboratory of Weak-Light Nonlinear Photonics, TEDA Institute of Applied Physics and School of Physics, Nankai University, Tianjin 300457, China
• 2State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China
• 3e-mail: bofang@nankai.edu.cn
• 4e-mail: zhanggq@nankai.edu.cn
• 5e-mail: jjxu@nankai.edu.cn
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Abstract

We systematically investigate the field distribution of the transverse electric modes in X-cut lithium niobate disks as an example of circular microcavities with anisotropic refractive index. A conserved quantity is discovered, which indicates the absence of chaos that generally exists in deformed microcavities and leads to a nontrivial directional emission. The emission directionality was theoretically investigated and experimentally verified by exciting high-order modes of an X-cut lithium niobate microresonator assisted with second harmonics. The field distribution analysis can enrich the knowledge in designing photonic devices that need precise control of field distribution, such as phase matching in nonlinear processes. Furthermore, the discovered emission phenomenon is momentous in enhancing and controlling communications between on-chip photonic devices.

1. INTRODUCTION

Whispering-gallery-mode (WGM) microcavities are well known for enhancing light intensity in a tremendously small spatial region [1,2], showing excellent performance in photonics applications such as lasing [3], sensing [4], and frequency combs [5,6]. Meanwhile, from the perspective of optical materials, thin-film crystals represented by lithium niobate on insulator (LNOI) which features superior $χ(2)$ nonlinearity in optics [7,8] and nice accessibility in integration, dominate the construction of chip-scale photonic devices such as optical parametric oscillators [912] and electro-optic modulators [1316]. To fully use the $χ(2)$ property, LNOI with crystal axis that lies in the plane of a thin LN layer, also known as X-cut LNOI, is introduced to construct photonic components, which paves the way for a better combination of LN and WGM microcavities, leading to the advent of a series of novel devices. Specifically, X-cut LN circular microcavities have been intensively studied for their effectiveness in elevating the efficiency of electro-optic modulation as well as broadening the bandwidth of the phase-matching window in the parametric process [1719].