Highly Unidirectional Emission and Ultralow‐Threshold Lasing from On‐Chip Ultrahigh‐Q Microcavities

Abstract

Prominent examples are whispering gallery mode (WGM) microcavities, [ 2 , 3 ] which confi ne photons by means of continuous total internal refl ection along a curved and smooth surface. The long photon lifetime (described by high Q factors), strong fi eld confi nement, and in-plane emission characteristics make them promising candidates for novel light sources [ 4–9 ] and biochemical sensors with the ability of detecting few or even single nanoparticles. [ 10 , 11 ] The principal disadvantage of circular WGM microcavities is their intrinsic isotropy of emission due to their rotational symmetry. In addition to the photonic structures consisting of two or more perfectly spherical microcavities, [ 12 ] one of vital solutions is to use deformed microcavities by breaking the rotational symmetry, [ 13–16 ] which can provide not only the directional emission but also the effi cient and robust excitation of WGMs by a free-space optical beam. [ 17–20 ] Deformed microcavities fabricated on a chip are particularly desired for high-density optoelectronic integration, but they suffer from low Q factors in experiments. The Q factors are typically around or even smaller than ten thousand [ 21–27 ] limited by the large scattering losses from the involuntary surface roughness. The high Q factor is of great importance in fundamental studies and on-chip photonic applications. Here, with a pattern transfer technique and a refl ow process ensuring a nearly atomic-scale microcavity surface, we demonstrate experimentally on-chip undoped silica deformed microcavities which support both nearly unidirectional emission and ultrahigh Q factors exceeding 100 million. Consequently, low-threshold, unidirectional microlasing in such a microcavity with Q factor about 3 million is realized by erbium doping and a convenient free-space excitation.