Optical whispering gallery mode (WGM) microcavities are promising candidates for basic research and optoelectronic applications. Due to the isotropic emission property resulting from the rotational symmetry, traditional WGM microcavities have to rely on external couplers to excite the modes and collect their emission. One of the most possible solutions is to deform microcavities from rotational symmetry, which could provide directional emission instead of isotropic characteristic. Here we report the first experimental realization of on-chip microcavities which support both highly unidirectional emission and ultra-high Q factors. The demonstrated Q factor exceeds 100 million in near infrared. By doping erbium into the deformed microcavity, lasing action in 1550 nm band was observed under convenient freespace optical pumping, with the threshold as low as 2 μW. Remarkably, the lasing emission is along a single direction with a narrow divergence angle about 10 degrees.
Fano resonances, sometimes behaving like electromagnetic induced transparency (EIT)-like resonances in whisperinggallery-
mode (WGM) microcavities are attracting much attention due to the important applications in high-sensitivity
biosensing, optical switch, and slow light. In this paper, Fano resonances in several WGM systems are reported,
including in a single WGM photonic microcavity and in coupled WGM microcavities. Specifically, in a single WGM
microcavity system, Fano or EIT-like resonances in both polydimethylsiloxane (PDMS) coated silica microtoroid and
bare silica microtoroid are experimentally investigated. In coupled microcavity system, we report a theoretical study of
EIT-like resonance in an array of microcavities indirectly coupled through a parallel waveguides system. Finally, we
experimentally investigate the Fano resonance in two microcavities indirectly coupled via a single fiber taper.
A high-sensitivity temperature sensor is demonstrated by coating a layer of polydimethylsiloxane (PDMS) on the
surface of a silica toroidal microresonator on a silicon chip. Combining both the advantages of the WGM microcavity
(with ultrahigh Q factor) and PDMS (with large thermal effect), the PDMS-coated microresonator is highly sensitive to
the temperature change of the surroundings. We find that, when the PDMS layer becomes thicker, the fundamental
WGM experiences a transition from red- to blue-shift with temperature increasing due to the negative thermal-optic
coefficient of PDMS. The measured sensitivity (0.151 nm/K) is one order of magnitude higher than pure silica
microcavity sensors. The ultra-high resolution of the temperature sensor is also analyzed to reach 10-4 K. With both high
sensitivity and resolution, the thermal sensor can be employed to monitor a slight temperature variation which cannot be
realized with conventional temperature sensor. Its on-chip feature can also fulfill the demand for integration and
miniaturization in optics.
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