arXiv subjects

Hui Zhao

Publications and source records attributed to Hui Zhao.

At least 127 records · Page 7Linked to original sources

arXiv2014

Tightly bound excitons in monolayer WSe2

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe2) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one- and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model. This result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors. The observed large exciton binding energy will also have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals.

cond-mat.mtrl-sci↗

arXiv2013

Exciton-exciton annihilation in MoSe2 monolayers

We investigate the excitonic dynamics in MoSe2 monolayer and bulk samples by femtosecond transient absorption microscopy. Excitons are resonantly injected by a 750-nm and 100-fs laser pulse, and are detected by a probe pulse tuned in the range of 790 - 820 nm. We observe a strong density-dependent initial decay of the exciton population in monolayers, which can be well described by the exciton-exciton annihilation. Such a feature is not observed in the bulk under comparable conditions. We also observe the saturated absorption induced by exciton phase-space filling in both monolayers and the bulk, which indicates their potential applications as saturable absorbers.

cond-mat.mes-hall↗

arXiv2013

Variational Bayes inference and Dirichlet process priors

This paper shows how the variational Bayes method provides a computational efficient technique in the context of hierarchical modelling using Dirichlet process priors, in particular without requiring conjugate prior assumption. It shows, using the so called parameter separation parameterization, a simple criterion under which the variational method works well. Based on this framework, its provides a full variational solution for the Dirichlet process. The numerical results show that the method is very computationally efficient when compared to MCMC. Finally, we propose an empirical method to estimate the truncation level for the truncated Dirichlet process.

stat.CO↗

arXiv2013

Diagnostics for Variational Bayes approximations

Variational Bayes (VB) has shown itself to be a powerful approximation method in many application areas. This paper describes some diagnostics methods which can assess how well the VB approximates the true posterior, particularly with regards to its covariance structure. The methods proposed also allow us to generate simple corrections when the approximation error is large. It looks at joint, marginal and conditional aspects of the approximate posterior and shows how to apply these techniques in both simulated and real data examples.

stat.CO↗

arXiv2013

Characterization of four-qubit states via Bell inequalities

A set of Bell inequalities classifying the quantum entanglement of four-qubit states is presented. These inequalities involve only two measurement settings per observer and can characterize fully separable, bi-separable and tri-separable quantum states. In addition, a quadratic inequality of the Bell operators for four-qubit systems is derived.

quant-ph↗

arXiv2013

Second harmonic microscopy of monolayer MoS2

We show that the lack of inversion symmetry in monolayer MoS2 allows strong optical second harmonic generation. Second harmonic of an 810-nm pulse is generated in a mechanically exfoliated monolayer, with a nonlinear susceptibility on the order of 1E-7 m/V. The susceptibility reduces by a factor of seven in trilayers, and by about two orders of magnitude in even layers. A proof-of-principle second harmonic microscopy measurement is performed on samples grown by chemical vapor deposition, which illustrates potential applications of this effect in fast and non-invasive detection of crystalline orientation, thickness uniformity, layer stacking, and single-crystal domain size of atomically thin films of MoS2 and similar materials.

cond-mat.mtrl-sci↗

arXiv2013

Charge carrier dynamics in bulk MoS2 crystal studied by transient absorption microscopy

We report a transient absorption microscopy study of charge carrier dynamics in bulk MoS2 crystals at room temperature. Charge carriers are injected by interband absorption of a 555-nm pulse, and probed by measuring differential reflection of a time-delayed and spatially scanned 660-nm pulse. We find an intervalley transfer time of about 0.35 ps, an energy relaxation time of hot carriers on the order of 50 ps, and a carrier lifetime of 180 ps. By monitoring the spatiotemporal dynamics of carriers, we obtained a diffusion coefficient of thermalized electrons of 4.2 cm2/s, corresponding to a mobility of 170 cm2/Vs. We also observed a time-varying diffusion coefficient of hot carriers.

cond-mat.mtrl-sci↗

arXiv2013

Third harmonic generation in graphene and few-layer graphite films

We observe optical third harmonic generation from graphene and few-layer graphite flakes produced by exfoliation. The emission scales with the cube of the intensity of the incident near-infrared femtosecond pulses and has a wavelength that is one-third of the incident wavelength, both consistent with third harmonic generation. We extract an effective third-order susceptibility for graphene that is on the order of 1e-16 square meter per square volt, which is comparable to that for materials that are resonantly excited, but larger than for materials that are transparent at the fundamental and third harmonic wavelengths. By measuring a set of flakes with different numbers of atomic layers, we find that the emission scales with the square of the number of atomic layers, which suggests that the susceptibility of graphene is independent of layer number, at least for a few layers.

cond-mat.mes-hall↗

arXiv2012

Exciton diffusion in semiconducting single-wall carbon nanotubes studied by transient absorption microscopy

Spatiotemporal dynamics of excitons in isolated semiconducting single-walled carbon nanotubes are studied using transient absorption microscopy. Differential reflection and transmission of an 810-nm probe pulse after excitation by a 750-nm pump pulse are measured. We observe a bi-exponentially decaying signal with a fast time constant of 0.66 ps and a slower time constant of 2.8 ps. Both constants are independent of the pump fluence. By spatially and temporally resolving the differential reflection, we are able to observe a diffusion of excitons, and measure a diffusion coefficient of 200 cm2/s at room temperature and 300 cm2/s at lower temperatures of 10 K and 150 K.

cond-mat.mes-hall↗

arXiv2012

Ultrafast and spatially resolved studies of charge carriers in atomically-thin molybdenum disulfide

Atomically-thin molybdenum disulfide is emerging as a new nanomaterial with potential applications in the fields of electronic and photonics. Charge carrier dynamics plays an essential role in determining its electronic and optical properties. We report spatially and temporally resolved pump-probe studies of charge carriers in atomically-thin molybdenum disulfide samples fabricated by mechanical exfoliation. Carriers are injected by interband absorption of a 390-nm pump pulse and detected by measuring differential reflection of a time-delayed and spatially-scanned probe pulse that is tuned to an exciton transition. Several parameters on charge carrier dynamics are deduced, including carrier lifetime, diffusion coefficient, diffusion length, and mobility.

cond-mat.mes-hall↗

arXiv2012

Spatially resolved pump-probe study of single-layer graphene produced by chemical vapor deposition

Carrier dynamics in single-layer graphene grown by chemical vapor deposition (CVD) is studied using spatially and temporally resolved pump-probe spectroscopy by measuring both differential transmission and differential reflection. By studying the expansion of a Gaussian spatial profile of carriers excited by a 1500-nm pump pulse with a 1761-nm probe pulse, we observe a diffusion of hot carriers of 5500 square centimeter per second. We also observe that the expansion of the carrier density profile decreases to a slow rate within 1 ps, which is unexpected. Furthermore, by using an 810-nm probe pulse we observe that both the differential transmission and reflection change signs, but also that this sign change can be permanently removed by exposure of the graphene to femtosecond laser pulses of relatively high fluence. This indicates that the differential transmission and reflection at later times may not be directly caused by carriers, but may be from some residue material from the sample fabrication or transfer process.

cond-mat.mtrl-sci↗

arXiv2011

Second-harmonic generation induced by electric currents in GaAs

We demonstrate a new, nonlinear optical effect of electric currents. First, a steady current is generated by applying a voltage on a doped GaAs crystal. We demonstrate that this current induces second-harmonic generation of a probe laser pulse. Second, we optically inject a transient current in an undoped GaAs crystal by using a pair of ultrafast laser pulses, and demonstrate that it induces the same second-harmonic generation. In both cases, the induced second-order nonlinear susceptibility is proportional to the current density. This effect can be used for nondestructive, noninvasive, and ultrafast imaging of currents. These advantages are illustrated by the real-time observations of a coherent plasma oscillation and spatial resolution of current distribution in a device. This new effect also provides a mechanism for electrical control of the optical response of materials.

cond-mat.mtrl-sci↗

arXiv2011

Optical pump-probe studies of carrier dynamics in few-layer MoS2

Femtosecond optical pump-probe technique is used to study charge carrier dynamics in few-layer MoS2 samples fabricated by mechanical exfoliation. An ultrafast pump pulse excites carriers and differential reflection of a probe pulse tuned to an excitonic resonance is detected. We find that the spectrum of the differential reflection signal is similar to the derivative of the resonance. The decay time of the signal is in the range of 150 to 400 ps, increasing with the probe wavelength. With a fixed probe wavelength, the signal magnitude is proportional to the pump fluence, while the decay time is independent of it.

cond-mat.mes-hall↗

arXiv2011

Spatially resolved femtosecond pump-probe study of topological insulator Bi2Se3

Carrier and phonon dynamics in Bi2Se3 crystals are studied by a spatially resolved ultrafast pump-probe technique. Pronounced oscillations in differential reflection are observed with two distinct frequencies, and are attributed to coherent optical and acoustic phonons, respectively. The rising time of the signal indicates that the thermalization and energy relaxation of hot carriers are both sub-ps in this material. We found that the thermalization and relaxation time decreases with the carrier density. The expansion of the differential reflection profile allows us to estimate an ambipolar carrier diffusion coefficient on the order of 500 square centimeters per second. A long-term slow expansion of the profile shows a thermal diffusion coefficient of 1.2 square centimeters per second.

cond-mat.mes-hall↗

arXiv2011

Observation of intrinsic inverse spin Hall effect

We report observation of intrinsic inverse spin Hall effect in un-doped GaAs multiple quantum wells with a sample temperature of 10 K. A transient ballistic pure spin current is injected by a pair of laser pulses through quantum interference. By time-resolving the dynamics of the pure spin current, the momentum relaxation time is deduced, which sets the lower limit of the scattering time between electrons and holes. The transverse charge current generated by the pure spin current via the inverse spin Hall effect is simultaneously resolved. We find that the charge current is generated well before the first electron-hole scattering event. Generation of the transverse current in the scattering-free ballistic transport regime provides unambiguous evidence for the intrinsic inverse spin Hall effect.

cond-mat.mes-hall↗

arXiv2011

Two-probe study of hot carriers in reduced graphene oxide

The energy relaxation of carriers in reduced graphene oxide thin films is studied using optical pump-probe spectroscopy with two probes of different colors. We measure the time difference between peaks of the carrier density at each probing energy by measuring a time-resolved differential transmission and find that the carrier density at the lower probing energy peaks later than that at the higher probing energy. Also, we find that the peak time for the lower probing energy shifts from about 92 to 37 fs after the higher probing energy peak as the carrier density is increased from 1.5E12 to 3E13 per square centimeter, while no noticeable shift is observed in that for the higher probing energy. Assuming the carriers rapidly thermalize after excitation, this indicates that the optical phonon emission time decreases from about 50 to about 20 fs and the energy relaxation rate increases from 4 to 10 meV/fs. The observed density dependence is inconsistent with the phonon bottleneck effect.

cond-mat.mes-hall↗

arXiv2011

Observation of second-harmonic generation induced by pure spin currents

Extensive efforts are currently being devoted to developing a new electronic technology, called spintronics, where the spin of electrons is explored to carry information. [1,2] Several techniques have been developed to generate pure spin currents in many materials and structures. [3-10] However, there is still no method available that can be used to directly detect pure spin currents, which carry no net charge current and no net magnetization. Currently, studies of pure spin currents rely on measuring the induced spin accumulation with optical techniques [5, 11-13] or spin-valve configurations. [14-17] However, the spin accumulation does not directly reflect the spatial distribution or temporal dynamics of the pure spin current, and therefore cannot monitor the pure spin current in a real-time and real-space fashion. This imposes severe constraints on research in this field. Here we demonstrate a second-order nonlinear optical effect of the pure spin current. We show that such a nonlinear optical effect, which has never been explored before, can be used for the non-invasive, non-destructive, and real-time imaging of pure spin currents. Since this detection scheme does not rely on optical resonances, it can be generally applied in a wide range of materials with different electronic bandstructures. Furthermore, the control of nonlinear optical properties of materials with pure spin currents may have potential applications in photonics integrated with spintronics.

cond-mat.mes-hall↗

arXiv2011

Ambipolar diffusion of photo-excited carriers in bulk GaAs

The ambipolar carrier diffusion in bulk GaAs is studied by using an ultrafast pump-probe technique with a high spatial resolution. Carriers with a point-like spatial profile are excited by a tightly focused pump laser pulse. The spatiotemporal dynamics of the carriers are monitored by a time-delayed and spatially scanned probe pulse. Ambipolar diffusion coefficients are deduced from linear fits to the expansion of the area of the profiles, and are found to decrease from about 170~$\mathrm{cm}^2 \mathrm{s}^{-1}$ at 10 K to about 20~$\mathrm{cm}^2 \mathrm{s}^{-1}$ at room temperature. Our results are consistent with those deduced from the previously measured mobilities.

cond-mat.mtrl-sci↗