Transparency and perfect absorption are two contradictory terms; a perfect absorber never permits waves to transmit through. However, this statement only remains true in the linear regime, where the nonlinearity has been omitted and the physical system like the perfect absorber is not affected by the incoming waves. Here we experimentally demonstrate an intriguing self-induced transparency effect in a perfectly absorbing optical microcavity, which perfectly absorbs any incoming waves at the low power level, but allows a portion of waves to be transmitted at the higher power due to the nonlinear coupling between the fundamental and its second harmonic modes. Moreover, the asymmetric scattering nature of the microcavity enables a chiral and unidirectional reflection in one of the input ports, this leads to asymmetric and chiral coherent control of the perfect absorption states through phase varying. More importantly, such chiral behaviors also empower the chiral emission of second-harmonic generation with a high distinct ratio in the transparency state. These results pave the way for controllable transparency in a wide range of fields in optics, microwaves, acoustics, mechanics, and matter waves.
Whispering-gallery-mode (WGM) microresonators can dramatically boost the interaction between light and matter, making them an ideal platform for studying nonlinear optics. Efficient wavelength conversion by nonlinear parametric processes is key to integrated optics in many applications, where WGM microresonators play an important role. Here, we demonstrate efficient second-harmonic generation (SHG) and sum-frequency generation (SFG) in an x-cut thin film lithium niobate microdisk with a high intrinsic quality factor over 106. The normalized conversion efficiency of cyclic quasi-phase-matched (CQPM) SHG and SFG is measured to be 1.53%/mW and 2.52×10−4/mW, respectively. Our work expands nonlinear optics applications of CQPM WGM microresonators as efficient wavelength convertors for light upconversion.
Two coupled resonance modes can lead to exotic transmission spectra due to internal interference processes. Examples include electromagnetically induced transparency (EIT) in atoms and mode splitting in optics. The ability to control individual modes plays a crucial role in controlling such transmission spectra for practical applications. Here we experimentally demonstrate a controllable EIT-like mode splitting in a single microcavity using a double-port excitation. The mode splitting caused by internal coupling between two counter-propagating resonances can be effectively controlled by varying the power of the two inputs, as well as their relative phase. Moreover, the presence of asymmetric scattering in the microcavity leads to chiral behaviors in the mode splitting in the two propagating directions, manifesting itself in terms of a Fano-like resonance mode. These results may offer a compact platform for a tunable device in all-optical information processing.
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