arXiv subjects

Min Xiao

Publications and source records attributed to Min Xiao.

At least 109 records · Page 6Linked to original sources

arXiv2014

Superlinear density dependence of singlet fission rate in tetracene films

We experimentally show that the rate of singlet fission in tetracene films has a superlinear dependence on the density of photo-excited singlet excitons with ultrafast transient absorption spectroscopy. The spectrotemporal features of singlet and triplet dynamics can be disentangled from experimental data with the algorithm of singular value decomposition. The correlation between their temporal dynamics indicates a nonlinear density dependence of fission rate, which leads to a conjecture of coherent singlet fission process arising from superradiant excitons in crystalline tetracene. This hypothesis might be able to resolve some long-standing controversies.

cond-mat.mtrl-sci↗

arXiv2014

Two-photon dynamics in coherent Rydberg atomic ensemble

We study the interaction of two photons in a Rydberg atomic ensemble under the condition of electromagnetically induced transparency, combining a semi-classical approach for pulse propagation and a complete quantum treatment for quantum state evolution. We find that the blockade regime is not suitable for implementing photon-photon cross-phase modulation due to pulse absorption and dispersion. However, approximately ideal cross-phase modulation can be realized based on relatively weak interactions, with counter-propagating and transversely separated pulses.

quant-ph↗

arXiv2013

PT-symmetric optical potentials in a coherent atomic medium

We demonstrate that a coherently-prepared four-level atomic medium can provide a versatile platform for realizing parity-time (PT) symmetric optical potentials. Different types of PT-symmetric potentials are proposed by appropriately tuning the exciting optical fields and the pertinent atomic parameters. Such reconfigurable and controllable systems can open up new avenues in observing PT-related phenomena with appreciable gain/loss contrast in coherent atomic media.

physics.optics↗

arXiv2013

Synchronous control of dual-channel all-optical multi-state switching

We have experimentally observed optical multistabilities (OMs) simultaneously on both the signal and generated Stokes fields in an optical ring cavity with a coherently-prepared multilevel atomic medium. The two observed OMs, which are governed by different physical processes, are coupled via the multilevel atomic medium and exhibit similar threshold behaviors. By modulating the cavity input (signal) field with positive or negative pulses, dual-channel all-optical multi-state switching has been realized and synchronously controlled, which can be useful for increasing communication and computation capacities.

physics.atom-ph↗

arXiv2013

Reducing the efficiency droop by lateral carrier confinement in InGaN/GaN quantum-well nanorods

Efficiency droop is a major obstacle facing high-power application of InGaN/GaN quantum-well (QW) light-emitting diodes. In this letter, we report the suppression of efficiency droop induced by density-activated defect recombination in nanorod structure of a-plane InGaN/GaN QWs. In the high carrier density regime, the retained emission efficiency in a dry-etched nanorod sample is observed to be over two times higher than that in its parent QW sample. We further argue that the improvement is a combined effect of the amendment contributed by lateral carrier confinement and the deterioration made by surface trapping.

cond-mat.mes-hall↗

arXiv2013

Enhanced Hot-Carrier Luminescence in Multilayer Reduced Graphene Oxide Nanospheres

We report a method to promote photoluminescence emission in graphene materials by enhancing carrier scattering instead of directly modifying band structure in multilayer reduced graphene oxide (rGO) nanospheres. We intentionally curl graphene layers to form nanospheres by reducing graphene oxide with spherical polymer templates to manipulate the carrier scattering. These nanospheres produce hot-carrier luminescence with more than ten-fold improvement of emission efficiency as compared to planar nanosheets. With increasing excitation power, hot-carrier luminescence from nanospheres exhibits abnormal spectral redshift with dynamic feature associated to the strengthened electron-phonon coupling. These experimental results can be well understood by considering the screened Coulomb interactions. With increasing carrier density, the reduced screening effect promotes carrier scattering which enhances hot-carrier emission from such multilayer rGO nanospheres. This carrier-scattering scenario is further confirmed by pump-probe measurements.

cond-mat.mes-hall↗

arXiv2013

Orbital-Selective Mottness in KxFe2-ySe2 Superconductors Revealed by Pump-Probe Spectroscopy

We report transient optical signatures of the orbital-selective Mottness in superconducting KxFe2-ySe2 crystals by using dual-color pump-probe spectroscopy. Besides multi-exponential decay recovery dynamics of photo-induced quasiparticles, a damped oscillatory component due to coherent acoustic phonons emerges when the superconducting phase is suppressed by increasing the temperature or excitation power. The oscillatory component diminishes with significant enhancement of a slow decay component upon raising temperature to 150-160 K. These results are in consistence with the picture of orbital-selective Mott phase transition, indicating a vital role played by electron correlation in the iron-based superconductors.

cond-mat.supr-con↗

arXiv2013

Defect recombination induced by density-activated carrier diffusion in nonpolar InGaN quantum wells

We report on the observation of carrier-diffusion-induced defect emission at high excitation density in a-plane InGaN single quantum wells. When increasing excitation density in a relatively high regime, we observed the emergence of defect-related emission together with a significant reduction in bandedge emission efficiency. The experimental results can be well explained with the density-activated carrier diffusion from localized states to defect states. Such a scenario of density-activated defect recombination, as confirmed by the dependences of photoluminescence on the excitation photon energy and temperature, is a plausible origin of efficiency droop in a-plane InGaN quantum-well light-emitting diodes.

cond-mat.mes-hall↗

arXiv2013

Entangler via Electromagnetically Induced Transparency with Atomic Ensembles

We present an efficient and convenient scheme to entangle multiple optical fields via electromagnetically induced transparency in an atomic ensemble. The atomic spin wave, produced through electromagnetically induced transparency in the Lambda-type configuration in an atomic ensemble, can be described by a Bose operator and acts as the entangler. By using the entangler, any desired number of nondegenerate narrow-band continuous-variable entangled fields, in principle, can be generated through stimulated Raman scattering processes, which holds great promise for applications in scalable quantum communication and quantum networks.

quant-ph↗

arXiv2013

Diffraction Interference Induced Superfocusing in Nonlinear Talbot Effect

We report a simple, novel sub-diffraction method, i.e. diffraction interference induced super-focusing in second-harmonic (SH) Talbot effect, to achieve focusing size of less than λ_pump/8 without involving evanescent waves or sub-wavelength apertures. By tailoring point spread functions with Fresnel diffraction interference, we observe periodic SH sub-diffracted spots over a hundred of micrometers away from the sample. Our demonstration is the first experimental realization of the proposal by Toraldo Di Francia pioneered 60 years ago for super-resolution imaging.

physics.optics↗

arXiv2013

Spatial-domain interactions between ultra-weak optical beams

We have observed the spatial interactions between two ultra-weak optical beams that are initially collinear and non-overlapping. The weak beams are steered towards each other by a spatially varying cross-Kerr refractive index waveguide written by a strong laser beam in a three-level atomic medium utilizing quantum coherence. After being brought together, the weak beams show controllable phase-dependent outcomes. This is the first observation of soliton-like interactions between weak beams and can be useful for all-optically tunable beam-combining, switching and gates for weak photonic signals.

physics.optics↗

arXiv2012

Ultrafast pump-probe spectroscopic signatures of superconducting and pseudogap phases in YBa2Cu3O7-δ films

Femtosecond pump-probe spectroscopy is applied to identify transient optical signatures of phase transitions in optimally doped YBa2Cu3O7-δ films. To elucidate the dynamics of superconducting and pseudogap phases, the slow thermal component is removed from the time-domain traces of photo-induced reflectivity in a high-flux regime with low frequency pulse rate. The rescaled data exhibit distinct signatures of the phase separation with abrupt changes at the onsets of TSC and TPG in excellent agreement with transport data. Compared to the superconducting phase, the response of the pseudogap phase is characterized by the strongly reduced reflectivity change accompanied by a faster recovery time.

cond-mat.supr-con↗

arXiv2012

Cross Language Text Classification via Subspace Co-Regularized Multi-View Learning

In many multilingual text classification problems, the documents in different languages often share the same set of categories. To reduce the labeling cost of training a classification model for each individual language, it is important to transfer the label knowledge gained from one language to another language by conducting cross language classification. In this paper we develop a novel subspace co-regularized multi-view learning method for cross language text classification. This method is built on parallel corpora produced by machine translation. It jointly minimizes the training error of each classifier in each language while penalizing the distance between the subspace representations of parallel documents. Our empirical study on a large set of cross language text classification tasks shows the proposed method consistently outperforms a number of inductive methods, domain adaptation methods, and multi-view learning methods.

cs.CL↗

arXiv2012

Generation of Multipartite Continuous-variable Entanglements via Atomic Spin Wave

We present a proof-of-principle way to generate nondegenerate multipartite continuous-variable entanglements via atomic spin wave induced by the strong coupling and probe fields in the Lambda-type electromagnetically induced transparency configuration in an atomic ensemble. Quantum correlated/anti-correlated and entangled Stokes and anti-Stokes fields, simultaneously produced through scattering the applied laser fields off the atomic spin wave, can be achieved. This method can, in principle, be extended to flexibly and conveniently create multicolor multipartite entangled narrow-band fields to any desired order with long correlation time, which may find promising applications in quantum information processing and quantum networks.

quant-ph↗

arXiv2012

Temperature-dependent steady and transient emission properties of InGaN/GaN multiple quantum well nanorods

We observed different temperature-dependent behaviors of steady and transient emission properties in dry-etched InGaN/GaN multiple-quantum-well (MQW) nanorods and the parent MQWs. To clarify the impacts of nanofabrication on the emission properties, time-resolved photoluminescence spectra were recorded at various temperatures with carrier density in different regimes. The confinement of carrier transport was observed to play an important role to the emission properties in nanorods, inducing different temperature-dependent photoluminescence decay rates between the nanorods and MQWs. Moreover, together with other effects, such as surface damages and partial relaxation of the strain, the confinement effect causes faster recombination of carriers in nanorods.

cond-mat.mes-hall↗

arXiv2011

Measurement of two independent phase-shifts using coupled parametric amplifiers

In this article, we demonstrate a scheme capable of two-phase measurement, i.e. the simultaneous measurement of the two phase-shifts occurring in two independent Mach-Zehnder interferometers using one intensity detector. Our scheme utilizes dark-state-enhanced coupled parametric amplifiers in an atomic medium to mix the multiple fields probing the various arms of the interferometers in parallel. The two phase-differences are then encoded in separate continuous-variable parameters in the spectral waveform of the parametrically amplified atom-radiated signal field, which can be directly decoupled in a single intensity measurement. Besides resolving two phase differences in parallel, this method can also be used to increase the channel capacity in optical and quantum communication by the simultaneous use of phase-modulation and amplitude-modulation.

physics.optics↗

arXiv2011

Electromagnetically induced Talbot effect

By modulating transmission function of a weak probe field via a strong control standing wave, an electromagnetically induced grating can be created in the probe channel. Such a nonmaterial grating may lead to self-imaging of ultra-cold atoms or molecules in the Fresnel near-field regime. This work may offer a nondestructive and lensless way to image ultra-cold atoms or molecules.

physics.optics↗

arXiv2010

Engineering Biphoton Wave Packets with an Electromagnetically Induced Grating

We propose to shape biphoton wave packets with an electromagnetically induced grating in a four-level double-$Λ$ cold atomic system. We show that the induced hybrid grating plays an essential role in directing the new fields into different angular positions, especially to the zeroth-order diffraction. A number of interesting features appear in the shaped two-photon waveforms. For example, broadening or narrowing the spectrum would be possible in the proposed scheme even without the use of a cavity.

quant-ph↗