Linbo Shao

Detection of Single Nanoparticles and Lentiviruses Using Microcavity Resonance Broadening

A new label-free sensing mechanism is demonstrated experimentally by monitoring the whispering-gallery mode broadening in microcavities. It is immune to both noise from the probe laser and environmental disturbances, and is able to remove the strict requirement for ultra-high-Q mode cavities for sensitive nanoparticle detection. This ability to sense nanoscale objects and biological analytes is particularly crucial for wide applications.

Chaos-assisted broadband momentum transformation in optical microresonators

Harnessing chaos for enhanced coupling Functional optical devices typically require the coupling of light between different components. However, conservation of momentum usually limits the bandwidth of the coupling, often to a near-resonant effect. Jiang et al. show that slightly deformed microring resonators might be able to relax those restrictions. The chaotic scattering of the light within the deformed structure can transform optical modes of different angular momenta within a few picoseconds, providing a promising route to develop advanced nanophotonic circuits and devices. Science, this issue p. 344 Chaotic scattering of light in deformed microring resonators enhances the coupling of light into optical devices. The law of momentum conservation rules out many desired processes in optical microresonators. We report broadband momentum transformations of light in asymmetric whispering gallery microresonators. Assisted by chaotic motions, broadband light can travel between optical modes with different angular momenta within a few picoseconds. Efficient coupling from visible to near-infrared bands is demonstrated between a nanowaveguide and whispering gallery modes with quality factors exceeding 10 million. The broadband momentum transformation enhances the device conversion efficiency of the third-harmonic generation by greater than three orders of magnitude over the conventional evanescent-wave coupling. The observed broadband and fast momentum transformation could promote applications such as multicolor lasers, broadband memories, and multiwavelength optical networks.

Dynamical tunneling-assisted coupling of high-Q deformed microcavities using a free-space beam

We investigate the efficient free-space excitation of high-Q resonance modes in deformed microcavities via dynamical tunneling-assisted coupling. A quantum scattering theory is employed to study the free-space transmission properties, and it is found that the transmission includes the contribution from (1) the off-resonance background and (2) the on-resonance modulation, corresponding to the absence and presence of high-Q modes, respectively. The theory predicts asymmetric Fano-like resonances around high-Q modes in background transmission spectra, which are in good agreement with our recent experimental results. Dynamical tunneling across Kolmogorov-Arnold-Moser tori, which plays an essential role in the Fano-like resonance, is further studied. This efficient free-space coupling holds potential advantages to simplify experimental conditions and excite high-Q modes in higher-index-material microcavities.

Ultrahigh-Q, largely deformed microcavities coupled by a free-space laser beam

We experimentally report the fabrication of deformed silica microtoroids with only one-axial symmetry and very large deformation reaching 18%. The largely deformed microcavities are demonstrated to support ultrahigh-Q resonant modes exceeding 6 × 107, though much more chaos emerges compared with slightly deformed microcavities. The chaotic behavior of rays is beneficial to enhance dynamical tunneling to high-Q whispering-gallery modes, and therefore allows efficient coupling by a free-space laser beam. The largely deformed microcavities hold great potential in applied photonics and fundamental studies.

Ultrahigh-Q asymmetric microcavity photonics on a silicon chip

We experimentally realized on-chip deformed microcavities supporting both highly unidirectional emission and ultrahigh Q factors exceeding 108. This type of microcavity holds potential in ultralow-threshold laser and sensitive nanoparticle detection.

Laser spectroscopy of highly doped NV- centers in diamond

In this paper, prospects of using diamond with NV− centers as a gain medium have been studied. Spectroscopic characterization of NV− centers in diamond as well as absorption saturation and pump-probe experiments have been carried out. Absorption and emission cross-sections were estimated to be 2.8 × 10-17 cm2 and 4.3 × 10-17 cm2 at the maximum of absorption and emission bands, respectively. It was observed from emission spectra under pulse excitation that some NV− are photoionized to NV0 centers with ZPL at 575 nm. Room temperature luminescence lifetime of NV− centers was measured to be 12ns, which is close to the previously reported lifetime in bulk diamond (~13ns). Saturated transmission was only about 11% of calculated values even at energy fluence much higher than the saturation flux. Two excited state absorptions (ESAs) with different relaxation times (“fast-decay” and “slow-decay with relaxation times of ~500 ns and several tens of microseconds, respectively) were revealed in transmission decay kinetics at 632 nm. Kinetics of transmission at 670 nm was dominated by “slow-decay” ESA process. Kinetics of dk/k0 in shorter wavelength were strongly dominated by “fast-decay” ESA process. These results definitively indicate that stimulated emission of NV− centers is suppressed by photoionization and ESAs and the possibility of diamond lasers based on NV− centers is low.