We report the size effects on the metal-insulator phase transition of vanadium dioxide (VO2) nanowires prepared by chemical vapor deposition. The phase transition temperature can be tuned from 67 °C in the bulk VO2 to as low as 29°C by reducing the diameter of VO2 nanowires to nanoscale. Temperature-dependent Raman spectra display a clear dynamic picture on the metal-insulator phase transition process of the VO2 nanowires. Whilst, Raman study shows no remarkable strain effect on the phase transition behaviors of our samples. The increasing surface defect density with reducing nanowire size facilitates the decreasing phase transition temperature. In addition, the polarized-photocurrent effect was observed, resulting from the anisotropy of the photoresponse and also caused by the reduced dimensionality.Our results indicate that size of VO2 nanostructures can dominate their thermoelectric and photoelectrical properties.

In this work, we report a novel and feasible strategy for the practical applications of one-dimensional ultrasensitive phototransistors made of tungsten-doped VO2 single nanowires. The photoconductive response of the single nanowire device was investigated under different visible light excitations (405 nm, 532 nm, and 660 nm). The phototransistor device exhibited ultrafast photoresponse, high responsivity, broad multispectral response, and rapid saturation characteristic curves. These promising results help to promote the applications of this material in nano-scale optoelectronic devices such as efficient multispectral phototransistors and optical switches.

We present a study on the composition-dependent electron transport in ternary CdS(x)Se(1-x) nanobelts at equilibrium and nonequilibrium conditions via THz spectroscopy. The measured spectra are analyzed using a Drude-Smith model combined with a harmonic oscillator. The physical origin of parameters in the Drude-Smith model is studied in detail. Under equilibrium conditions, the surface depletion region is the dominant factor to free-carrier backscattering. However, under nonequilibrium conditions, the influence of the surface depletion region is masked by the high bulk concentration and the free carriers are mainly localized by composition disorder. The contributions from different mechanisms to the carrier mobility are also explored. In equilibrium, alloy scattering is the most vital scattering mechanism for nanobelts with x=0.25→0.9 since composition disorder is significant in this range. On the other hand, the effect of electron-phonon interaction increases under photoexcitation.

We employ optical pump-terahertz probe spectroscopy to investigate the composition-dependent photoconductivity in ternary CdS x Se 1− x nanobelts. The observed carrier dynamics of CdS nanobelts display much shorter lifetime than those of ternary CdS x Se 1− x nanobelts. This indicates the implementation of CdS nanobelts as ultrafast switching devices with a switching speed potentially up to 46.7 GHz. Surprisingly, ternary CdS x Se 1− x nanobelts are found to exhibit much higher photoconductivity than binary CdS and CdSe. This is attributed to the higher photocarrier densities in ternary compounds. In addition, the presence of Se in samples resulted in prominent CdSe-like transverse optical (TO) phonon modes due to electron-phonon interactions. The strength of this mode shows a large drop upon photoexcitation but recovers gradually with time. These results demonstrated that growth of ternary nanostructures can be utilized to alleviate the high surface defect density in nanostructures and improve their photoconductivity.

The frequency- and fluence-dependent transient photoconductivity in ternary CdSSe nanobelts is investigated using time-resolved terahertz spectroscopy. The carrier density and mobility are extracted by modeling the measured complex photoconductivity using the Drude–Smith model. Within the first few picoseconds of excitation, both the carrier density and mobility reach their maximum values and then decay gradually over tens to hundreds of picoseconds. The decay of free carriers is mainly attributed to fast surface trapping and structural-defect-mediated recombination. The surface trapping saturates rapidly with increasing excitation fluence attributable to the low trapping density on the nanobelt surface caused by self-passivation of surface defects during the growth process.

Defects in nanomaterials often induce dramatic changes in the photoelectrical properties of semiconducting II-VI compound nanomaterials. The relationship between defects and carrier dynamics is pivotal in material engineering for potential applications. A thorough understanding of the dynamics of defect-related free carrier depletion is particularly important for the fabrication and optimization of nano-optoelectronic devices. In this work, optical pump-terahertz probe spectroscopy was employed to investigate the carrier dynamics in CdS and Se-alloyed CdS nanobelts. The dynamics are dominated by the surface defect trapping in the case of CdS and structural-defect-related recombination for the Se-alloyed CdS. The conclusion is also supported by temperature-dependent photoluminescence spectroscopic studies. Our results indicate that congeneric element replacement is an effective approach for defect-distribution restructuring, which modifies the physical properties of nanomaterials through defect engineering.

This work reports the characterization of antimony doping effects on the electron transportation in SnO2 nanonets via a contact (field-effect transistor) and a non-contact (terahertz time-domain spectroscopy) approach. The doping influence is well demonstrated by the contact method through exploring the output characteristics of the devices. In addition, through the analysis of the terahertz time-domain spectra using Drude-Smith model, the non-contact method provides more precise characterization ascribed to the absence of extra effects such as contact resistance and nanowire-nanowire junction barriers.

By employing femtosecond pump-probe configuration, we successfully realized narrowband terahertz wave generation and detection in both photorefractive periodically poled lithium niobate (PPLN) and periodically poled Mg:LiNb(3) (PP-Mg:LN) crystal. Using an applied magnetic field, we achieved modulation of the terahertz wave in a photorefractive PPLN crystal. The terahertz wave depends strongly on the magnitude of the applied magnetic field in the photorefractive PPLN crystal. Terahertz wave independence of the magnetic field in PP-Mg:LN crystal was also identified. The interaction of the magnetic field and photorefractive PPLN crystal is believed to occur due to the Lorentz force, which results in the buildup of a space-charge field in a photorefractive PPLN crystal.

Magnetic field induced phase shift of terahertz wave in PPLN crystal was investigated with femtosecond pump-probe method. A pi phase shift occurs in an undoped PPLN with applied magnetic field reaches ~0.45 T, but in the case of MgO-doped PPLN crystal, THz phase shows magnetic field independence. The magnetic field induced pi-phase shift comes from the photorefractive effect in PPLN crystal.

Excitation photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal was studied by time resolved pump-probe reflectivity technique at room temperature. The results show that spin relaxation time decreases monotonously. While with increasing excitation carrier density, the time constants increases initially then decreases after reaching a maximum value. Our experimental results reveal that both D’yakonov–Perel’ [M. I. D’yakonov and V. I. Perel’, Sov. Phys. JETP 38, 177 (1974)] and Elliot–Yafet [R. J. Elliott, Phys. Rev. 96, 266 (1954); Y. Yafet, Solid State Phys. 14, 1 (1963)] mechanisms dominate the spin relaxation process in CdTe crystal.

The interactions between electromagnetic wave and photonic quasicrystals are investigated. A terahertz (THz) source with multi-frequency modes in an optical LiTaO(3) superlattice produced by quasiperiodic (Fibonacci) domain-inverted ferroelectric material is demonstrated experimentally. Using the canonical pump-probe experimental technique, THz radiations in both forward and backward propagations are in-situ detected simultaneously. Four pronounced THz frequencies at 1.18, 0.78, 0.59 and 0.37 THz in Fourier transform spectrum are observed. The physical properties of THz waves inside quasiperiodic superlattice are discussed.

Gold nanoparticles were conjugated with transferrin molecules for targeting, imaging and therapy of breast cancer cells (Hs578T, ATCC). Results show that, the transferrin-transferrin receptor-mediated cellular uptake of gold nanoparticles is six times of that in the absence of this interaction. As a consequence, the laser power effective for photothermal therapy of the cancer cells was reduced to values of two orders of magnitude lower. To demonstrate the efficiency of the conjugated gold nanoparticles in selectively targeting cancer cells, the cellular uptake of the gold nanoparticles by noncancerous cells (3T3, ATCC) was also investigated. The cellular uptake by the normal cells is only one fourth of that by the cancerous cells indicating that the transferrin-transferrin receptor interaction plays an important role in controlling the cellular uptake of the gold nanoparticles.

Considering two beams propagate in semiconductor crystal, this paper discusses the polarization dependence of pump beam-induced intensity attenuation of probe beam due to two-photon absorption (TPA). Numerical calculation and experimental measurement demonstrate that TPA coefficient is polarization dependent. For homogeneous materials, probe beam attenuation arises from the imaginary part of diagonal and off-diagonal components of third-order nonlinear susceptibilities.

Metal-dielectric multilayer films show high transmission at some specific wavelengths of light due to multiple Bragg reflections. By designing the multilayer structure, the high transmission position can be tuned to be on resonance with the laser wavelength at which the light can penetrate into the highly nonlinear metallic layers, leading to an enhanced nonlinear optical response. By employing a femtosecond optical Kerr technique, we experimentally investigated Ag-TiO2 multilayer stacks, and an enhanced nonlinear optical response was observed.

A widely tunable middle-infrared quasi-phase-matched optical parametric oscillation that uses a periodically poled magnesium-oxide-doped lithium niobate with a multigrating structure is reported. The optical parametric oscillation device is tuned by translating the crystal through the resonator and the pump beam, with no realignment needed. The results show that the idler range lies in 3.6–4.1μm in 0.1μm increment as the pump wavelength is fixed at 1.064μm. The grating periods on the crystal and the threshold pump energy in this experiment are verified, respectively.

Three novel Pb(II) coordination complexes, [Pb(dimb)(DMF)(NO3)(2)]n 1, [Pb(dimb)(SCN)(2)](n) 2 [dimb = 1,3-bis(imidazol-1-ylmethyl)-benzene], {[Pb(bimb)(1.5)(NO3)(2)](DMF)}(n) 3 [bimb = 4,4'-bis(imidazol-1-methyl)-biphenyl], were synthesized and characterized by X-ray crystallography. Complex 1 exhibits a one-dimensional (1D) zigzag chain structure, which has two crystallographically independent Pb(II) atom centers. Complex 2 possesses a two-dimensional (2D) corrugated network, which contains 24-membered M2L2 metallocyclic rings. Complex 3 has a 1D infinite non-interpenetrated molecular ladder, which has very large cavities with dimensions of 17.89 x 20.34 Angstrom, and DMF molecules fill the channel formed by adjacent two ladders. The third-order nonlinear optical (NLO) properties of the three complexes were measured by a Z-scan technique in DMF solution. All three complexes possess weak absorption and strong refraction. Their third-order NLO refractive coefficients, n(2), are -5.96 x 10(-19) m(2) W-1 for 1, -8.34 x 10(-19) m(2) W-1 for 2, and -7.15 x 10(-19) m(2) W-1 for 3. It is notable that two Pb( II) complexes 1 and 3 with 1D structure show strong self-defocusing behavior, which are different from the reported 1D coordination complexes containing d(10) metal ions; also, complex 2 is the first 2D coordination compound that possesses strong self-defocusing behavior. The chi((3)) values of complexes 1, 2 and 3 were calculated to be 4.46 x 10(-13), 6.25 x 10(-13) and 5.42 x 10(-13) esu, respectively.

In this paper, we present a study on quasi-phase matched (QPM) two-dimensional χ(2) lithium niobate (LN) nonlinear photonic crystal (NPC) for frequency doubling at λ = 1064nm. The NPCs were fabricated by electron beam lithography (EBL) through periodic polarization inversion of the ferroelectric domains and characterized with electrostatic force microscopy (EFM), atomic force microscopy and optical microscopy. Domain inversion occurred through the entire wafer thickness of 0.5mm as EFM images on the +c face of the z-cut wafer showed uniform domain structures throughout the corresponding electron beam irradiated regions of the -c face. In addition, the intended periodicity was observed. Moreover, domain inversion was also seen to have taken place in bulk from the optical images of the chemically etched samples. The EBL technique offers great flexibility in superlattice design and relative ease of fabrication as compared to the conventional poling techniques as pattern transfer is direct without the need for a mask and/or a coating of resist. Besides, micro- or sub-micro scale superlattices corresponding to wavelengths in the visible and into the ultraviolet are highly feasible, restricted only by the transparency of the crystals.

We examine the absorption of a weak probe beam by a laser driven V-type atom with a pair of closely lying excited levels, where both the driving and probe lasers interact simultaneously with the two transitions. The effects of quantum interference among decay channels on the absorption spectra are also investigated. We introduce dipole moments in the dressed-state representation and the Hamiltonian in terms of the dressed states describing the interaction between the probe and the atom. In the degenerate case, features similar to that of a driven two-level atomic system are found due to some dark transitions in the spontaneous emission and the fact that the probe beam only detects certain transitions. In the nondegenerate case, the absorption spectrum is strongly influenced by the degree of quantum interference, resulting in different line shapes for emission peaks, absorption peaks, and dispersionlike profiles. The effect of probe polarization on the absorption spectrum is also investigated.