Weihang Zhou

Electrically pumped waveguide lasing from ZnO nanowires

Ultraviolet semiconductor lasers are widely used for applications in photonics, information storage, biology and medical therapeutics. Although the performance of gallium nitride ultraviolet lasers has improved significantly over the past decade, demand for lower costs, higher powers and shorter wavelengths has motivated interest in zinc oxide (ZnO), which has a wide direct bandgap and a large exciton binding energy. ZnO-based random lasing has been demonstrated with both optical and electrical pumping, but random lasers suffer from reduced output powers, unstable emission spectra and beam divergence. Here, we demonstrate electrically pumped Fabry-Perot type waveguide lasing from laser diodes that consist of Sb-doped p-type ZnO nanowires and n-type ZnO thin films. The diodes exhibit highly stable lasing at room temperature, and can be modelled with finite-difference time-domain methods.

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.

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.

Thermodynamic-effect-induced growth, optical modulation and UV lasing of hierarchical ZnO Fabry–Pérot resonators

High quality ZnO multilayer hexagonal microplates and microtubes were fabricated via a simple carbothermal method without any catalysts, carrier gases, or low pressure. The formation of the ZnO microstructures with different morphologies was discussed in detail, and a possible thermodynamic viewpoint was proposed. Fabry–Perot (FP) modes and UV lasing were directly observed using a spatially resolved spectroscopic technique. All the resonant modes observed experimentally were identified using finite-difference-time-domain simulations. Excitation power and temperature dependence of lasing properties of the ZnO microplate were further studied. Compared with the conventional one-dimensional nanowire/nanobelt FP cavities, such ZnO multilayer vertical-FP resonators have much less optical loss and excellent optical responses and may find potential applications in UV microlasers.

Relative Ordering between Bright and Dark Excitons in Single-walled Carbon Nanotubes

The ordering and relative energy splitting between bright and dark excitons are critical to the optical properties of single-walled carbon nanotubes (SWNTs), as they eventually determine the radiative and non-radiative recombination processes of generated carriers. In this work, we report systematic high-field magneto-optical study on the relative ordering between bright and dark excitons in SWNTs. We identified the relative energy position of the dark exciton unambiguously by brightening it in ultra-high magnetic field. The bright-dark excitonic ordering was found to depend not only on the tube structure, but also on the type of transitions. For the 1st sub-band transition, the bright exciton appears to be higher in energy than its dark counterpart for any chiral species and is robust against environmental effect. While for the 2nd sub-band, their relative ordering was found to be chirality-sensitive: the bright exciton can be either higher or lower than the dark one, depending on the specific nanotube structures. These findings provide new clues for engineering the optical and electronic properties of SWNTs.

Survey of exciton-phonon sidebands by magneto-optical spectroscopy using highly specified (6,5) single-walled carbon nanotubes

We report high-field magneto-optical study on the exciton-phonon sideband of single-walled carbon nanotubes consisting only of (6,5) species. Both energy and intensity of the observed phonon sideband were found to be independent of the external magnetic field. Comparing with theoretical calculations, we confirmed that these sidebands originate from the optically forbidden K-momentum singlet excitons. Energy of these K-momentum dark excitons was estimated to be ∼21.5 meV above the bright Γ-momentum singlet excitons, in close agreement with recent theoretical predictions and experimentally determined values.

Band-edge exciton states in a single-walled carbon nanotube revealed by magneto-optical spectroscopy in ultrahigh magnetic fields

We report high field magneto-optical study on the first and second sub-band transitions of single-chirality single-walled carbon nanotubes. The ordering and relative energy splitting between bright and dark excitonic states were found to be inverse between the first and second subbands. We verified that the zero-momentum dark singlet exciton lies below the bright exciton for the first subband transitions, while for the second sub-band transitions, it was found to have higher energy than the bright excitonic state. Effect of this peculiar excitonic structure was found to manifest itself in distinctive Aharonov-Bohm splitting in ultra-high magnetic fields up to 190 T.

Magnetic field control of the quantum chaotic dynamics of hydrogen analogs in an anisotropic crystal field.

We report magnetic field control of the quantum chaotic dynamics of hydrogen analogues in an anisotropic solid state environment. The chaoticity of the system dynamics was quantified by means of energy level statistics. We analyzed the magnetic field dependence of the statistical distribution of the impurity energy levels and found a smooth transition between the Poisson limit and the Wigner limit, i.e., transition between regular Poisson and fully chaotic Wigner dynamics. The effect of the crystal field anisotropy on the quantum chaotic dynamics, which manifests itself in characteristic transitions between regularity and chaos for different field orientations, was demonstrated.

Exciton-phonon bound complex in single-walled carbon nanotubes revealed by high-field magneto-optical spectroscopy

High-field magneto-optical spectroscopy was performed on highly enriched (6,5) single-walled carbon nanotubes. Spectra of phonon sidebands in both 1st and 2nd sub-bands were unchanged by an external magnetic field up to 52 T. The dark K-momentum singlet (D-K-S) exciton, which plays an important role for the external quantum efficiency of the system for both sub-bands in the near-infrared and the visible light region, respectively, was clarified to be the origin of the phonon sidebands.

Magneto-optical study of Dirac fermion in quartz CVD-grown graphene above 100 T

The cyclotron resonance of CVD-grown graphene on a quartz substrate was investigated under the ultra-high magnetic fields generated by the single-turn coil method. We successfully observed the resonance peak due to n = 0 → 1 intra-LL transition around 110 T, which is the first observation of the physical property of graphene over 100 T.