F. Q. Liu

Strong photoluminescence of nanostructured crystalline tungsten oxide thin films

Strong photoluminescence (PL) is observed in nanostructured crystalline tungsten oxide thin films that are prepared by thermal evaporation. Two kinds of films are investigated—one made of nanoparticles and another of nanowires. At room temperature, strong PL emissions at ultraviolet-visible and blue regions are found in both of the films. Compared with the complete absence of emission of bulk phase tungsten oxide powder under the same excitation conditions, our results clearly demonstrate the quantum-confinement-effect-induced photoluminescence in nanostructured tungsten oxides.Strong photoluminescence (PL) is observed in nanostructured crystalline tungsten oxide thin films that are prepared by thermal evaporation. Two kinds of films are investigated—one made of nanoparticles and another of nanowires. At room temperature, strong PL emissions at ultraviolet-visible and blue regions are found in both of the films. Compared with the complete absence of emission of bulk phase tungsten oxide powder under the same excitation conditions, our results clearly demonstrate the quantum-confinement-effect-induced photoluminescence in nanostructured tungsten oxides.

Controlled self-assembled nanoaeroplanes, nanocombs, and tetrapod-like networks of zinc oxide

We report on the controlled growth of novel self-assembled nanostructures (nanoaeroplanes, nanocombs, and tetrapod-like networks of zinc oxide) by a simple thermal evaporation technique in a single-stage furnace. Scanning electron microscopy and transmission electron microscopy are used to investigate their morphologies and structures. Shapes are regular and unit sizes vary from 80 nm to 5 µm. The relationship between the preparation conditions and the nanomorphologies in the experiments is helpful for controlling the physical properties of the nanosystems and, hence, is described in some detail. A possible growth mechanism is discussed.

Thermal redistribution of photocarriers between bimodal quantum dots

We study the photoluminescence (PL) properties of InAs/GaAs self-assembled quantum dots (QDs) by varying excitation power and temperature. Excitation power-dependent PL shows that there exists bimodal size distribution in the QD ensemble. Thermal carrier redistribution between the two branches of dots is observed and investigated in terms of the temperature dependence of their relative PL intensity. Based on a model in which carrier transfer between dots is facilitated by the wetting layer, the experimental results are well explained

Strong terahertz radiation from relativistic laser interaction with solid density plasmas

We report a plasma-based strong THz source generated in intense laser-solid interactions at relativistic intensities > 10(18) W/cm(2). Energies up to 50 mu J/sr per THz pulse is observed when the laser pulses are incident onto a copper foil at 67.5 degrees. The temporal properties of the THz radiation are measured by a single shot, electro-optic sampling method with a chirped laser pulse. The THz radiation is attributed to the self-organized transient fast electron currents formed along the target surface. Such a source allows potential applications in THz nonlinear physics and provides a diagnostic of transient currents generated in intense laser-solid interactions

Sequential coupling transport for the dark current of quantum dots-in-well infrared photodetectors

The dark current characteristics and temperature dependence for quantum dot infrared photodetectors have been investigated by comparing the dark current activation energies between two samples with identical structure of the dots-in-well in nanoscale but different microscale n-i-n environments. A sequential coupling transport mechanism for the dark current between the nanoscale and the microscale processes is proposed. The dark current is determined by the additive mode of two activation energies: Ea,micro from the built-in potential in the microscale and Ea,nano related to the thermally assisted tunneling in nanoscale. The activation energies Ea,micro and Ea,nano decrease exponentially and linearly with increasing applied electric field, respectively.

Quantum-confined Stark effect and built-in dipole moment in self-assembled InAs/GaAs quantum dots

Quantum-confined Stark effect and built-in dipole moment in self-assembled InAs/GaAs quantum dots (QDs), which are grown at relative low temperature (460degreesC) and embedded in GaAs p-i-n structure, have been studied by dc-biased electroreflectance. Franz-Keldysh oscillations from the undoped GaAs layer are used to determine the electric field under various bias voltages. Stark shift of -34 meV for the ground-state interband transition of the QDs is observed when the electric field increases from 105 to 308 kV/cm. The separation of the electron and hole states in the growth direction of 0.4 nm, corresponding to the built-in dipole moment of 6.4x10(-29) C m, is determined. It is found that the electron state lies above that of the hole, which is the same as that predicted by theoretical calculations for ideal pyramidal InAs QDs

High-performance uncooled distributed-feedback quantum cascade laser without lateral regrowth

We demonstrate, uncooled, room-temperature continuous-wave (cw) operation of single-mode distributed-feedback (DFB) quantum cascade lasers (QCLs) emitting around 4.6 μm without lateral regrowth. The effects of cavity length on device performance are studied. A record low threshold electrical power consumption of 2.3 W for the entire laser system with a 1.5-mm-long cavity is realized. For the 2-mm-long laser, high cw output power of 125 mW and very low threshold current density of 0.86 kA/cm2 are obtained. Our devices represent an important step towards using uncooled DFB QCLs in mid-infrared spectral range for practical applications.

Demonstration of self-truncated ionization injection for GeV electron beams.

Ionization-induced injection mechanism was introduced in 2010 to reduce the laser intensity threshold for controllable electron trapping in laser wakefield accelerators (LWFA). However, usually it generates electron beams with continuous energy spectra. Subsequently, a dual-stage target separating the injection and acceleration processes was regarded as essential to achieve narrow energy-spread electron beams by ionization injection. Recently, we numerically proposed a self-truncation scenario of the ionization injection process based upon overshooting of the laser-focusing in plasma which can reduce the electron injection length down to a few hundred micrometers, leading to accelerated beams with extremely low energy-spread in a single-stage. Here, using 100 TW-class laser pulses we report experimental observations of this injection scenario in centimeter-long plasma leading to the generation of narrow energy-spread GeV electron beams, demonstrating its robustness and scalability. Compared with the self-injection and dual-stage schemes, the self-truncated ionization injection generates higher-quality electron beams at lower intensities and densities, and is therefore promising for practical applications.

Temperature dependence of electron redistribution in modulation-doped InAs/GaAs quantum dots

In this work we report the photoluminescence (PL) and interband absorption study of Si-modulation-doped multilayer InAs/GaAs quantum dots grown by molecular beam epitaxy (MBE) on (100) oriented GaAs substrates. Low-temperature PL shows a distinctive double-peak feature. Power-dependent PL and transmission electron microscopy (TEM) confirm that they stem from the ground states emission of islands of bimodal size distribution. Temperature-dependent PL study indicates that the family of small dots is ensemble effect dominated while the family of large dots is likely to be dominated by the intrinsic property of single quantum dots (QDs). The temperature-dependent PL and interband absorption measurements are discussed in terms of thermalized redistribution of the carriers among groups of QDs of different sizes in the ensemble

Influence of strain on annealing effects of In(Ga)As quantum dots

Post-growth rapid thermal annealing has been performed with In(Ga)As quantum dots (QDs) at different strain statuses. It is confirmed that the strain-enhanced interdiffusion decreases the inhomogeneous size distribution. The preferential lateral interdiffusion of QDs during annealing was observed. we attribute it to the naturally anisotropic strain distribution in/around the dots and the saturation of strain difference between the base boundary and the top of the dots. There exist strain-enhanced mechanism and vacancy diffusion enhanced mechanism during the annealing. As to which one dominates the QD interdiffusion depends on the thickness of capping layer and the annealing temperature