Toward whispering-gallery-mode microdroplet sensing: Surface charge accumulation, microdroplet interface evolution and forward-photovoltaic-jetting on C-cut LiNbO3:Fe crystals

Abstract

LiNbO3-based photovoltaic jetting can generate dielectric microdroplets with volume down to fL, showing great potential in producing whispering-gallery-mode (WGM) microresonators. However, so far no successful work on the photovoltaic generation of microdroplet resonators has been reported due to the lack of the knowledge about the association of the jetting behaviors with the accumulation and confinement of photovoltaic charges. Here we study the microdroplet interface evolution caused by the central and boundary illumination of a focused laser beam, report that the photovoltaic charges may spread along LiNbO3:Fe surface, and reinforce it through the dependences of photovoltaic current curve on probing position and illumination intensity. By properly configuring LiNbO3:Fe crystals we confine the photovoltaic charges strictly resulting in strong electrodynamic flow to trigger the directional jetting of the dielectric liquid from LiNbO3:Fe surface toward target substrates. Through this forward-photovoltaic-jetting we realize an all-optical, in-situ printing of WGM microdroplets containing fluorescent quantum dots onto substrates without thickness limitation. The WGM modes in the microdroplets are found highly dependent on the microdroplet size, shape, and especially, the contact angles of the microdroplets on modified substrates. Applications of printed WGM microdroplets as anti-counterfeiting spectral barcode and temperature/gas sensors are demonstrated by detecting the change of resonant modes.