Zhanghai Chen

Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS2 and MoS2 Monolayer and Few-Layer Films

The optical nonlinearity of WS2 and MoS2 monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near-infrared range. The nonlinear absorption of few- and multilayer WS2 and MoS2 films and their dependences on excitation wavelength were studied. WS2 films with 1-3 layers exhibited a giant two-photon absorption (TPA) coefficient as high as (1.0 ± 0.8) × 10(4) cm/GW. TPA saturation was observed for the WS2 film with 1-3 layers and for the MoS2 film with 25-27 layers. The giant nonlinearity of WS2 and MoS2 films is attributed to a two-dimensional confinement, a giant exciton effect, and the band edge resonance of TPA.

Direct Observation of Whispering Gallery Mode Polaritons and their Dispersion in a ZnO Tapered Microcavity

We report direct observation of the strong exciton-photon coupling in a ZnO tapered whispering gallery (WG) microcavity at room temperature. By scanning excitations along the tapered arm of the ZnO tetrapod using a micro-photoluminescence spectrometer with different polarizations, we observed a transition from the pure WG optical modes in the weak interaction regime to the excitonic polariton in the strong coupling regime. The experimental observations are well described by using the plane wave model including the excitonic polariton dispersion relation. This provides a direct mapping of the polariton dispersion, and thus a comprehensive picture for coupling of different excitons with differently polarized WG modes.

Enhancing Perovskite Solar Cell Performance by Interface Engineering Using CH3NH3PbBr0.9I2.1 Quantum Dots

To improve the interfacial charge transfer that is crucial to the performance of perovskite solar cells, the interface engineering in a device should be rationally designed. Here we have developed an interface engineering method to tune the photovoltaic performance of planar-heterojunction perovskite solar cells by incorporating MAPbBr3-xIx (MA = CH3NH3) quantum dots (QDs) between the MAPbI3 perovskite film and the hole-transporting material (HTM) layer. By adjustment of the Br:I ratio, the as-synthesized MAPbBr3-xIx QDs show tunable fluorescence and band edge positions. When the valence band (VB) edge of MAPbBr3-xIx QDs is located below that of the MAPbI3 perovskite, the hole transfer from the MAPbI3 perovskite film to the HTM layer is hindered, and hence, the power conversion efficiency decreases. In contrast, when the VB edge of MAPbBr3-xIx QDs is located between the VB edge of the MAPbI3 perovskite film and the highest occupied molecular orbital of the HTM layer, the hole transfer from the MAPbI3 perovskite film to the HTM layer is well-facilitated, resulting in significant improvements in the fill factor, short-circuit photocurrent, and power conversion efficiency.

Single-crystalline tower-like ZnO microrod UV lasers

High quality tower-like ZnO microrods with hexagonal cross-sections were fabricated by a simple reduction–oxidation process. By using the spatially resolved spectroscopic technique, the stable UV lasing was directly observed and further investigated systematically. Our results demonstrated that such tower-like microrods can function as novel cavity based microlasers, which could have potential applications in optoelectronics.

The rf magnetic-field vector detector based on the spin rectification effect

Ferromagnetic resonances on three Permalloy strips under an in-plane external magnetic field are detected electrically. By measuring and analyzing the angular dependence of the photovoltage induced by the spin rectification effect, an approach is demonstrated for making microwave detectors capable of detecting the rf magnetic field vector at subwavelength scale.

Whispering gallery modes in indium oxide hexagonal microcavities

The work is funded by the NSFC 973 projects and STCSM of China Grant Nos. 2004CB619004 and 2006CB921506. The authors thank the Australian Government Department of Innovation, Industry, Science and Research for funding this collaborative research under the International Science Linkages China Program.

Quasi-whispering gallery modes of exciton-polaritons in a ZnO microrod

We report the photoluminescence (PL) investigations of quasi-whispering gallery mode (quasi-WGM) polaritons in a ZnO microrod at room temperature. By using the confocal micro-PL spectroscopic technique, we observe the clear optical quasi-WGMs. These quasi-WGMs appeared in the ultraviolet (UV) emission region where the cavity modes strongly couple with excitons and form polaritons. The quasi-WGMs polaritons can be well described by the plane wave interference and the coupling oscillator model.

Raman scattering in InAs nanowires synthesized by a solvothermal route

Micro-Raman scattering experiment has been performed to study solvothermally synthesized InAs nanowires with different diameters. Strong low frequency branch of the LO-phonon-plasmon mode (L−) was observed. The longitudinal optic (LO) mode was found to be dependent on diameter of the nanowires. The wave vector uncertainty and the carrier concentration related Fermi-Thomas screening effect were proposed to play important roles for this phenomenon. Laser heating, phonon confinement, surface defects, together with the diameter distribution effects were also discussed.

Spin-Resolved Purcell Effect in a Quantum Dot Microcavity System

We demonstrate the spin selective coupling of the exciton state with cavity mode in a single quantum dot (QD)-micropillar cavity system. By tuning an external magnetic field, each spin polarized exciton state can be selectively coupled with the cavity mode due to the Zeeman effect. A significant enhancement of spontaneous emission rate of each spin state is achieved, giving rise to a tunable circular polarization degree from -90% to 93%. A four-level rate equation model is developed, and it agrees well with our experimental data. In addition, the coupling between photon mode and each exciton spin state is also achieved by varying temperature, demonstrating the full manipulation over the spin states in the QD-cavity system. Our results pave the way for the realization of future quantum light sources and the quantum information processing applications.

Electric detection of ferromagnetic resonance in single crystal iron film

We report electric detection of ferromagnetic resonance (FMR) in epitaxially grown single crystal iron film through microwave photovoltage generation technique. The experimental results agree well with the established theory about FMR in iron films, showing excellent extendability of such a technique onto different ferromagnets as an effective way to study magnetocrystalline anisotropy and spin excitations. Furthermore, the information about the phase of magnetization precession is implicated in the lineshape of photovoltage, which makes it possible to probe in details into magnetic phase dynamics that is of significance for devising spintronic devices.