Ningbo Yi

Single Nanoparticle Detection Using Far-field Emission of Photonic Molecule around the Exceptional Point

Highly sensitive, label-free detection methods have important applications in fundamental research and healthcare diagnostics. To date, the detection of single nanoparticles has remained largely dependent on extremely precise spectral measurement, which relies on high-cost equipment. Here, we demonstrate a simple but very nontrivial mechanism for the label-free sizing of nanoparticles using the far-field emission of a photonic molecule (PM) around an exceptional point (EP). By attaching a nanoparticle to a PM around an EP, the main resonant behaviors are strongly disturbed. In addition to typical mode splitting, we find that the far-field pattern of the PM is significantly changed. Taking a heteronuclear diatomic PM as an example, we demonstrate that a single nanoparticle, whose radius is as small as 1 nm to 7 nm, can be simply monitored through the variation of the far-field pattern. Compared with conventional methods, our approach is much easier and does not rely on high-cost equipment. In addition, this research will illuminate new advances in single nanoparticle detection.

Enhanced second-harmonic generation from nonlinear optical metamagnetics

We present a numerical simulation of second-harmonic generation (SHG) from a nonlinear magnetic metamaterial. By inserting a second-order nonlinear material in the high local field area of magnetic metamaterial, which consists of periodic arrays of paired thin silver strips, the convertion efficiency of SHG has been significantly enhanced by almost four orders of magnitude. The corresponding field patterns and further studies on dependance between SHG and symmetry of nonlinear crystal show that the increase of the conversion efficiency is attributed to the local field enhancement caused by the magnetic resonnance of the structure. Our researches provide an additional way to further improve the optical nonlinearity in nanostructures.

Integrated photonic power divider with arbitrary power ratios.

Integrated optical power splitters are one of the fundamental building blocks in photonic integrated circuits. Conventional multimode interferometer-based power splitters are widely used as they have reasonable footprints and are easy to fabricate. However, it is challenging to realize arbitrary split ratios, especially for multi-outputs. In this Letter, an ultra-compact power splitter with a QR code-like nanostructure is designed by a nonlinear fast search method. The highly functional structure is composed of a number of freely designed square pixels with the size of 120×120  nm which could be either dielectric or air. The light waves are scattered by a number of etched squares with optimized locations, and the scattered waves superimpose at the outputs with the desired power ratio. We demonstrate 1×2 splitters with 1:1, 1:2, and 1:3 split ratios, and a 1×3 splitter with the ratio of 1:2:1. The footprint for all the devices is only 3.6×3.6  μm. Well-controlled split ratios are measured for all the cases. The measured transmission efficiencies of all the splitters are close to 80% over 30 nm wavelength range.

Tailoring the Performances of Lead Halide Perovskite Devices with Electron-Beam Irradiation

Lead halide perovskites are intensively studied in past few years due to their potential applications in optoelectronic devices such as solar cells, photodetectors, light-emitting diodes (LED), and lasers. In addition to the rapid developments in material synthesis and device fabrication, it is also very interesting to postsynthetically control the optical properties with external irradiations. Here, the influences of very low energy (10-20 keV) electron beam of standard electron beam lithography are experimentally explored on the properties of lead halide perovskites. It is confirmed that the radiolysis process also happens and it can selectively change the photoluminescence, enabling the direct formation of nanolaser array, microsized light emitter array, and micropictures with an electron beam writer. Interestingly, it is found that discontinuous metallic lead layers are formed on the top and bottom surfaces of perovskite microplate during the radiolysis process, which can act as carrier conducting layers and significantly increase the photocurrent of perovskite photodetector by a factor of 217%. By using the electron beam with low energy to modify the perovskite, this method promises to shape the emission patterns for micro-LED with well-preserved optical properties and improves the photocurrent of photodetector.

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.

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.

Absorption enhancement in thin-film organic solar cells through electric and magnetic resonances in optical metamaterial

Light absorption plays a key role in photovoltaic devices, especially in thin-film organic solar cells. Here we study the enhancement of optical absorption in a thin organic layer by embedding it into magnetic metamaterials. In a periodic metal-polymeric-metal sandwiched structure, the absorption of transverse magnetic polarized light has been significantly enhanced. The maximum enhancement is around 7 times, which is attributed to the strong local field enhancement in the organic thin film. Importantly, due to the coexistence of large electric resonance and magnetic resonance in magnetic metamaterials, the enhancement of light absorption has been obtained in a broadband from 350 nm to 750 nm, which is almost twice of that of the conventional plasmonic device and covers the strongest part of the solar spectrum. Moreover, such absorption enhancement is also valid for a wide range of incident angles. We believe that our finding can lead to a variety of important applications in solar cell technology.