Whispering gallery mode resonators covered by a ZnO nanolayer

Authors:

• Inga Brice,
• Roman Viter,
• Kristians Draguns,
• Karlis Grundsteins,
• Aigars Atvars,
• Janis Alnis,
• Luis Emerson Coy Romero,
• Igor Iatsunskyi

Abstract

The exceptional ability of whispering gallery mode resonators (WGMRs) to confine light within makes them interesting for sensing applications. The small size and high values of quality (Q) factors of the WGMR can be combined with a broad range of supporting optical elements. The surface of the resonator can be coated to enhance the desired attributes. In this paper, the impact of the ZnO layer thickness on the Q-factor of the WGMR has been studied. WGMRs were fabricated on a tip of a standard telecom fiber melted with an oxyhydrogen flame. The surface of the WGMR was coated with ZnO nanolayers of different thickness (5–100 nm) by atomic layer deposition (ALD). The Q-factor of as-prepared WGMR was in the range of 10⁷, whereas the Q-factor of ZnO-WGMR decreased 2–10 times. The optimal thickness of functional ZnO coating on WGMR was 10−60 nm. Mechanism of the ZnO layer thickness influence to Q-factor was based on two competing phenomena such as the change of refractive index and surface roughness due to ZnO growth. The effect of the ZnO as a protective coating against the Q-factor degradation was studied. The thicker ZnO layers prevented degradation of the Q-factor of the WGMR.