Highly sensitive and multispectral responsive phototransistor using tungsten-doped VO2 nanowires.

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.

Composition-dependent electron transport in CdS(x)Se(1-x) nanobelts: a THz spectroscopy study.

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.

Influence of magnetic field on terahertz wave generation in photorefractive periodically poled lithium niobate crystal.

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.

In vitro cancer cell imaging and therapy using transferrin-conjugated gold nanoparticles.

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.

Ultrafast optical nonlinearity enhancement in metallodielectric multilayer stacks.

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.

Ultrafast all-optical switching in one-dimensional photonic crystal with two defects.

One-dimensional (1D) photonic crystals (PC) containing two-layer CdS defects are proposed and fabricated by using electron beam evaporation. Ultrafast nonlinear optical responses were characterized with the ultrafast pump-probe method in both time and spectral domains. Two-photon absorption coefficient enhancement and pump-beam-induced defect mode shift were reported. Both effects are attributed to the light localization in the defect layer of the multilayer structures. Our results demonstrated that defective photonic crystals are good candidates for fabrication of ultrafast all-optical switching devices.

Nonlinear frequency conversion with quasi-phase-mismatch effect.

In conventional frequency conversion based on chi(2)-induced nonlinear optical effects, a number of phase-matching techniques have been deployed to resolve the phase mismatch between the interacting waves. In this work, we report that in the quasi-phase-matching scheme, the accumulation of the phase mismatch can induce a quasi-phase-mismatch effect. And on the contrary, instead of eliminating the phase mismatch, modulating the quasi-phase-mismatch effect can lead to bandwidth enhancement and multiple-wavelength conversion. Both of these are essential factors for applications in nonlinear frequency conversion. As examples, designs with reset-periodic and cascaded-periodic optical superlattices based on such modulation are presented.

Defect-mode dependence of two-photon-absorption enhancement in a one-dimensional photonic bandgap structure.

A one-dimensional photonic crystal containing a single CdS defect layer of various thicknesses was fabricated. The dependence of the two-photon-absorption (TPA) coefficient on the defect mode was investigated by use of a femtosecond pump-probe method. Experimental results show that the TPA coefficient of the CdS defect layer depends strongly on the defect mode in the photonic bandgap. This is consistent with the predicted dependence of light intensity within the defect layer.

Size and dielectric dependence of the third-order nonlinear optical response of Au nanocrystals embedded in matrices.

The third-order nonlinear optical response of Au nanocrystals embedded in BaTiO(3) and ZrO(2) matrices was investigated through the off-resonance femtosecond optical Kerr effect. Compared with pristine films, the nonlinearity of composite films shows an enhancement that depends on the sizes of Au particles and the refractive index of the matrices. We performed a calculation based on Lorenz-Mie scattering theory to explain the experimental results. Additionally, the nonlinearity was optimized by determination of the values of the particle size and the refractive index of the matrix.