Yisheng Gao

All-Dielectric Full-Color Printing with TiO2 Metasurfaces

Recently, color generation in resonant nanostructures have been intensively studied. Despite of their exciting progresses, the structural colors are usually generated by the plasmonic resonances of metallic nanoparticles. Due to the inherent plasmon damping, such plasmonic nanostructures are usually hard to create very distinct color impressions. Here we utilize the concept of metasurfaces to produce all-dielectric, low-loss, and high-resolution structural colors. We have fabricated TiO2 metasurfaces with electron-beam lithography and a very simple lift-off process. The optical characterizations showed that the TiO2 metasurfaces with different unit sizes could generate high reflection peaks at designed wavelengths. The maximal reflectance was as high as 64% with full width at half-maximum (fwhm) around 30 nm. Consequently, distinct colors have been observed in bright field and the generated colors covered the entire visible spectral range. The detailed numerical analysis shows that the distinct colors were generated by the electric resonance and magnetic resonances in TiO2 metasurfaces. Based on the unique properties of magnetic resonances, distinct colors have been observed in bright field when the metasurfaces were reduced to a 4 × 4 array, giving a spatial resolution around 16000 dpi. Considering the cost, stability, and CMOS-compatibility, this research will be important for the structural colors to reach real-world industrial applications.

Room temperature three-photon pumped CH3NH3PbBr3 perovskite microlasers

Hybrid lead halide perovskites have made great strides in next-generation light-harvesting and light emitting devices. Recently, they have also shown great potentials in nonlinear optical materials. Two-photon absorption and two-photon light emission have been thoroughly studied in past two years. However, the three-photon processes are rarely explored, especially for the laser emissions. Here we synthesized high quality CH3NH3PbBr3 perovskite microstructures with solution processed precipitation method and studied their optical properties. When the microstructures are pumped with intense 1240 nm lasers, we have observed clear optical limit effect and the band-to-band photoluminescence at 540 nm. By increasing the pumping density, whispering-gallery-mode based microlasers have been achieved from CH3NH3PbBr3 perovskite microplate and microrod for the first time. This work demonstrates the potentials of hybrid lead halide perovskites in nonlinear photonic devices.

Miscellaneous Lasing Actions in Organo-Lead Halide Perovskite Films

Lasing actions in organo-lead halide perovskite films have been heavily studied in the past few years. However, due to the disordered nature of synthesized perovskite films, the lasing actions are usually understood as random lasers that are formed by multiple scattering. Herein, we demonstrate the miscellaneous lasing actions in organo-lead halide perovskite films. In addition to the random lasers, we show that a single or a few perovskite microparticles can generate laser emissions with their internal resonances instead of multiple scattering among them. We experimentally observed and numerically confirmed whispering gallery (WG)-like microlasers in polygon shaped and other deformed microparticles. Meanwhile, owing to the nature of total internal reflection and the novel shape of the nanoparticle, the size of the perovskite WG laser can be significantly decreased to a few hundred nanometers. Thus, wavelength-scale lead halide perovskite lasers were realized for the first time. All of these laser behaviors are complementary to typical random lasers in perovskite film and will help the understanding of lasing actions in complex lead halide perovskite systems.

Formation of Lead Halide Perovskite Based Plasmonic Nanolasers and Nanolaser Arrays by Tailoring the Substrate

Hybrid plasmonic nanolasers are intensively studied due to their nanoscale mode confinement and potentials in highly integrated photonic and quantum devices. Until now, the characteristics of plasmonic nanolasers are mostly determined by the crystal facets of top semiconductors, such as ZnO nanowires or nanoplates. As a result, the spasers are isolated, and their lasing wavelengths are random and difficult to tune. Herein, we experimentally demonstrate the formation of lead halide perovskite (MAPbX3) based hybrid plasmonic nanolasers and nanolaser arrays with arbitrary cavity shapes and controllable lasing wavelengths. These spasers are composed of MAPbX3 perovskite nanosheets, which are separated from Au patterns with a 10 nm SiO2 spacer. In contrast to previous reports, here, the spasers are determined by the boundary of Au patterns instead of the crystal facets of MAPbX3 nanosheets. As a result, whispering gallery mode based circular spasers and spaser arrays were successfully realized by patterning the Au substrate into circles and gratings, respectively. The standard wavelength deviation of spaser arrays is as small as 0.3 nm. Meanwhile, owing to the anion-exchangeable property of MAPbX3 perovskite, the emission wavelengths of spasers were tuned more than 100 nm back and forth by changing the stoichiometry of perovskite postsynthetically.

Large-Scale and Defect-Free Silicon Metamaterials with Magnetic Response

All-dielectric metamaterials offer a potential low-loss alternative to plasmonic metamaterials at optical frequencies. Here, we experimentally demonstrate a silicon based large-scale magnetic metamaterial, which is fabricated with standard photolithography and conventional reactive ion etching process. The periodically arrayed silicon sub-wavelength structures possess electric and magnetic responses with low loss in mid-infrared wavelength range. We investigate the electric and magnetic resonances dependencies on the structural parameters and demonstrate the possibility of obtaining strong dielectric-based magnetic resonance through a broad band range. The optical responses are quite uniform over a large area about 2 × 2 cm2. The scalability of this design and compatibility fabrication method with highly developed semiconductor devices process could lead to new avenues of manipulating light for low-loss, large-area and real integrated photonic applications.

Polarization-independent metamaterial with broad ultrahigh refractive index in terahertz region

In this article, we report a broadband, isotropic three-dimensional metamaterial design with extremely high refractive index in the terahertz region. Two peaks of refractive index, 67.9 at 2.14 THz and 66.9 at 2.16 THz, are observed under TE and TM mode polarizations, respectively. The high refractive index metamaterial maintains low loss with figure of merit as high as 15 under both polarizations. Moreover, the refractive index does not decrease sharply at higher frequencies, and shows a very broadband behavior with a full-width at half-maximum (FWHM) of 2 THz.

Real-Time Tunable Colors from Microfluidic Reconfigurable All-Dielectric Metasurfaces

Structural colors arising from all-dielectric nanostructures are very promising for high-resolution color nanoprinting and high-density optical storage. However, once the all-dielectric nanostructures are fabricated, their optical performances are usually static or change slowly, significantly limiting the practical applications in advanced displays. Herein, we experimentally demonstrate the real-time tunable colors with microfluidic reconfigurable all-dielectric metasurfaces. The metasurface is composed of an array of TiO2 nanoblocks, which are embedded in a polymeric microfluidic channel. By injecting solutions with a different refractive index into the channel, the narrow band reflection peak and the corresponding distinct colors of a TiO2 metasurface can be precisely controlled. The transition time is as small as 16 ms, which is orders of magnitude faster than the current techniques. By varying the lattice size of TiO2 metasurfaces, the real-time tunable colors are able to span the entire visible spectrum. Meanwhile, the injection and ejection of solvent have also shown the capability of the erasion and the restoration of information encoded in TiO2 metasurfaces. The combination of all-dielectric nanostructures with microfluidic channels shall boost their applications in functional color display, banknote security, anticounterfeiting, and point-of-care devices.

Lead Halide Perovskite Nanostructures for Dynamic Color Display

Nanoprint-based color display using either extrinsic structural colors or intrinsic emission colors is a rapidly emerging research field for high-density information storage. Nevertheless, advanced applications, e. g., dynamic full-color display and secure information encryption, call for demanding requirements on in situ color change, nonvacuum operation, prompt response, and favorable reusability. By transplanting the concept of electrical/chemical doping in the semiconductor industry, we demonstrate an in situ reversible color nanoprinting paradigm via photon doping, triggered by the interplay of structural colors and photon emission of lead halide perovskite gratings. It solves the aforementioned challenges at one go. By controlling the pumping light, the synergy between interlaced mechanisms enables color tuning over a large range with a transition time on the nanosecond scale in a nonvacuum environment. Our design presents a promising realization of in situ dynamic color nanoprinting and will empower the advances in structural color and classified nanoprinting.

Inhibiting the Auger recombination of perovskite micro-rod laser through the monolayer graphene

Summary form only given. In past few years, while lead halide perovskite micro-lasers have been thoroughly studied, their performances at high excitation power have rarely explored. Here, we study the perovskite micro-lasers with high pumping density and explore the mechanism to improve their laser characteristics. By transferring a CH3NH3PbBr3 perovskite micro-rod onto a few-layered graphene slice, whispering gallery like lasing modes have been formed in the transverse plane of the microrod. Interestingly, the “gain saturation” of conventional CH3NH3PbBr3 perovskite micro-rod laser has been released. The maximal output intensity has been significantly improved more than 4 times and the threshold is reduced around 20%. The following experiments show that such improvements are caused by the electron acceptor property of graphene. By attracting electron with graphene, the electrons and holes are partially separated in the hybrid perovskite/graphene system and thus the Auger recombination at high pumping power has been inhibited. We believe that our finding will be important not only for the perovskite lasers but also for other semiconductor light emitting devices.

Observation of strong and weak couplings in a single hybrid plasmon-waveguide system

EIT-like and Fano-like resonances can be obtained in a single hybrid plasmonic system under the excitation by different polarization light. The LSPs and the Bragg resonance from the plasmonic grating under different polarizations interference couple with the waveguide mode and contribute to the strong and weak couplings, respectively.