[Reseach on THz Time Domain Spectrum of Photo-Induced Insulator-Metal Phase Transition of VO2 Films].

Vanadium dioxide (VO2) film will be phase-transitioned from insulator into metal, accompanied with dramatic change on conductivity, which is named as photo-induced insulator-metal phase transition. Such phase transition of VO2 film has important application potentials in modulators or other functional devices for terahertz waves. In this paper, the transmission spectrum variations before and after the photo-induced insulator-metal phase transition of vanadium dioxide film are investigated, and the phase transition properties in terahertz(THz) region are analyzed. In the experiment, the phase transition of the VO2 film was induced by a continuous wave (CW) laser source and a femtosecond (fs) laser source, respectively. Obvious changes on the THz waveforms were observed for the both mentioned means of excitation, and the amplitude attenuation, as well as the signal distortion, was intensified with the increase of the impinging optical power. The fast Fourier transform (FFT) spectra of the transmitted THz time-domain signals were analyzed and it was found that the amplitude of the transmitted spectrum decreased synchronously with the increase of the optical power, accompanied with deformation of the spectrum line shape at the same time. The reason was that the macroscopic dielectric properties of the VO2 film approached gradually to that of a metal as laser power was increased. A parameter, transmission modulation function, was defined in the paper as the amplitude difference between the transmission spectra of the VO2 film before and after the laser excitation, to describe the dispersivity of the photo-induced phase transition more clearly. From the curve of the transmission modulation function, strong frequency-dependent properties at THz frequencies were found to vary regularly with the incident light power. After furthermore comparison, it was found that, though the insulator-metal phase transition could be trigged by both CW laser source and fs laser source, the corresponding impinging optical power values were obviously alternative for the equivalent transmission modulation function. At the end of the paper, the difference of the phase transition efficiency between the two excitation methods was analyzed and discussed.

[Study on Spectral Characteristics of Two Kinds of Home-Made Novel Yb-Doped Fluoride Laser Crystals].

Yb-doped fluoride crystals are of important another Yb-doped laser materials besides Yb-doped oxide, which are becoming one of interests for developing tunable lasers and ultrafast lasers. In this paper, the systematic and contrastive experiments of the optical spectral characteristics are presented for two types of home-made novel Yb-doped fluoride laser crystals, namely, Yb-doped CaF2-SrF2 mixed crystal and co-doped Yb, Y:CaF2 single crystal. The fluorescent features of Yb-doped CaF2-SrF2 mixed crystal and co-doped Yb, Y:CaF2 single crystal are apparently different by the fluorescence experiment. The physical mechanism of these fluorescence spectra were analyzed and proposed. The influence of doping concentrations of active Yb(3+) ions or co-doping Y ions on the absorption of Yb-doped CaF2-SrF2 mixed crystal and co-doped Yb, Y:CaF2 single crystal was experimentally investigated, and the optimal values of doping concentrations of active Yb(3+) ions or co-doping Y ions in the two types of fluoride laser crystals were obtained. Continuous-wave laser operation for the two novel fluoride laser crystals has been achieved in three-mirror-folded resonator using a laser diode as the pump source. Therein, the laser operation for the co-doped Yb, Y:CaF2 crystal is demonstrated for the first time. For the two types of fluoride laser crystals (four samples), the input-output power relational curves, the optical slope efficiencies and the laser spectra were demonstrated by the laser experiments. By comparisons between the two types of fluoride laser crystals in the absorbability, fluorescence and laser spectra, laser threshold and slope efficiency of the continuous-wave laser operation, the results show that the best one of the four samples in spectral and laser characteristics is co-doped 3at%Yb, 6at% Y:CaF2 single crystal, which has an expected potential in the application. The research results provide available references for improving further laser performance of Yb-doped fluoride crystals.

Raman detection of cell proliferation probes with antiresonance-guiding hollow fibers.

Antiresonance-guiding hollow-core fibers are shown to enable highly sensitive detection of cell proliferation probes using Raman scattering within the region where the cellular Raman activity is minimal. We demonstrate that such fibers can substantially reduce the level of the background compared to standard index-guiding optical fibers, thus radically improving the sensitivity of Raman detection of DNA synthesis in cells and offering a powerful tool for fiber-based live-cell imaging.

Multiwatt octave-spanning supercontinuum generation in multicore photonic-crystal fiber.

High-power supercontinuum spanning over more than an octave was generated using a high power femtosecond fiber laser amplifier and a multicore nonlinear photonic crystal fiber (PCF). Long multicore PCFs (as long as 20 m in our experiments) are shown to enable supercontinuum generation in an isolated fundamental supermode, with the manifold of other PCF modes suppressed due to the strong evanescent fields coupling between the cores, providing a robust 5.4 W coherent supercontinuum output with a high spatial and spectral quality within the range of wavelengths from 500 to 1700 nm.

Observation of multiphoton-induced fluorescence from graphene oxide nanoparticles and applications in in†vivo functional bioimaging.

Lightening organelles: A femtosecond laser can excite multiphoton-induced luminescence of graphene oxide nanoparticles. The flow, distributions, and clearance of intravenously injected GO-PEG nanoparticles in the blood vessel of mice could be observed clearly by two-photon imaging. The 3D distribution of microinjected GO-PEG nanoparticles in a mice brain could also be reconstructed with two-photon microscopy.

Plasmonic waveguiding in a hexagonally ordered metal wire array.

We propose the inclusion of a structured pattern of nanoscale metal wires in a silica fiber to form a symmetric plasmonic waveguide. The surface plasmon polariton modes within the waveguide are studied by varying the wire diameter and spacing. Simulation results show that hybridization of the single-wire mode and the gap plasmon mode can yield a hybrid mode with optimum propagation lengths comparable to those reported for other structures but with better light confinement. The fiber can be easily doped with a gain material to offset the loss so that the resultant waveguide will be useful for integration with electronic circuits at nanometer dimensions.

Hybrid multicore photonic-crystal fiber for in-phase supermode selection.

We examine a hybrid multicore photonic-crystal fiber, where the cores are separated by high-index solid rods and the microstructure cladding is built on a hexagonal lattice of air holes in silica. Antiresonant reflection from high-index solid rods is shown to assist the field confinement in the cores of such a fiber. When the cores are doped with a laser-active material, the maximum gain is achieved for the in-phase supermode, which translates into a high-quality Gaussian-like beam profile in the far field.

Generation of 150 MW, 110 fs pulses by phase-locked amplification in multicore photonic crystal fiber.

We use seven-core Yb-doped large-mode-area (LMA) photonic crystal fiber (PCF) to demonstrate phase-locked amplification of 0.7 W, 1 MHz, 1.9 ps laser pulses delivered by an LMA-PCF-laser-LMA-PCF-preamplifier system. Compression of the 24 W output of the multicore LMA-PCF amplifier with a grating compressor yields 110 fs pulses with a peak power up to 150 MW.

Two-dimensional coherent superposition of blue-shifted signals from an array of highly nonlinear waveguiding wires in a photonic-crystal fiber.

Frequency-shifted dispersive optical waves generated as a result of soliton dynamics of 30-fs Ti: sapphire-laser pulses in an array of waveguiding wires, implemented on a platform of a photonic-crystal fiber (PCF), are shown to produce regular stable interference patterns with high visibility, indicating a high coherence of frequency-shifted fields. For a hexagonal array of waveguides built into a silica PCF, the field intensity at the main peak of a six-beam interference pattern was found to be a factor of 22 higher than the intensity of a frequency-shifted signal from an individual waveguide in the array and 3.7 times higher than the field intensity attainable through an incoherent superposition of the same fields.

Stabilized soliton self-frequency shift and 0.1- PHz sideband generation in a photonic-crystal fiber with an air-hole-modified core.

Fiber dispersion and nonlinearity management strategy based on a modification of a photonic-crystal fiber (PCF) core with an air hole is shown to facilitate optimization of PCF components for a stable soliton frequency shift and subpetahertz sideband generation through four-wave mixing. Spectral recoil of an optical soliton by a red-shifted dispersive wave, generated through a soliton instability induced by high-order fiber dispersion, is shown to stabilize the soliton self-frequency shift in a highly nonlinear PCF with an air-hole-modified core relative to pump power variations. A fiber with a 2.3-microm-diameter core modified with a 0.9-microm-diameter air hole is used to demonstrate a robust soliton self-frequency shift of unamplified 50-fs Ti: sapphire laser pulses to a central wavelength of about 960 nm, which remains insensitive to variations in the pump pulse energy within the range from 60 to at least 100 pJ. In this regime of frequency shifting, intense high- and low-frequency branches of dispersive wave radiation are simultaneously observed in the spectrum of PCF output. An air-hole-modified-core PCF with appropriate dispersion and nonlinearity parameters is shown to provide efficient four-wave mixing, giving rise to Stokes and anti-Stokes sidebands whose frequency shift relative to the pump wavelength falls within the subpetahertz range, thus offering an attractive source for nonlinear Raman microspectroscopy.

Perturbative and phase-transition-type modification of mode field profiles and dispersion of photonic-crystal fibers by arrays of nanosize air-hole defects.

Based on the results of a fully vectorial finite-difference analysis, we identify three important regimes of field-profile and dispersion management of photonic-crystal fibers with a solid core modified by arrays of nanosize air-hole defects. In the first regime, very small air holes act as weak perturbations, slightly modifying the field profiles of fiber modes and red-shifting the wavelength of zero group-velocity dispersion (GVD). In the second regime, larger holes reduce the effective mode area, tightening the confinement of the light field in the fiber core and blue-shifting the zero- GVD wavelength. Finally, in the third regime, the nanosize air-hole defects with diameters above a critical value induce a phase-transition-type behavior of mode field profiles, dramatically reducing the localization of the field in the fiber core and increasing the radiation power in the fiber cladding. This phase transition in mode field profiles qualitatively modifies the wavelength dependence of the effective mode area and dispersion parameters of fiber modes, especially in the long-wavelength range, suggesting an attractive strategy for fiber dispersion and mode area engineering.

Mode-selective mapping and control of vectorial nonlinear-optical processes in multimode photonic-crystal fibers.

We demonstrate an experimental technique that allows a mapping of vectorial nonlinear-optical processes in multimode photonic-crystal fibers (PCFs). Spatial and polarization modes of PCFs are selectively excited in this technique by varying the tilt angle of the input beam and rotating the polarization of the input field. Intensity spectra of the PCF output plotted as a function of the input field power and polarization then yield mode-resolved maps of nonlinear-optical interactions in multimode PCFs, facilitating the analysis and control of nonlinear-optical transformations of ultrashort laser pulses in such fibers.

Tunable supercontinuum generation in a high-index-step photonic-crystal fiber with a comma-shaped core.

A fused silica high-index-step photonic-crystal fiber with a comma-shaped core is shown to upport two different types of guided modes with bell-shaped intensity profiles, efficiently transforming unamplified 30-fs Ti: sapphire laser pulses into supercontinuum emission through two different physical mechanisms. The modes of the first type provide broadly spanning supercontinuum emission with a smooth spectrum stretching from 450 to 1400 nm. The initial stage of supercontinuum generation in these modes involves four-wave mixing around the wavelength of zero group-velocity dispersion, leading to the depletion of the pump field. The modes of the second type generate supercontinuum with an enhanced short-wavelength wing, dominated by intense spectral lines centered at 400--450 nm. The two regimes of supercontinuum generation and the two types of output spectra are switched by displacing the input end of the fiber with respect to the laser beam in the transverse direction.

A hollow beam from a holey fiber.

A high-quality spectrally isolated hollow beam is produced through a nonlinear-optical transformation of Ti: sapphire laser pulses in a higher order mode of a photonic-crystal fiber (PCF). Instead of a doughnut shape, typical of hollow beams produced by other methods, the far-field image of the hollow-beam PCF output features perfect sixth-order rotation symmetry, dictated by the symmetry of the PCF structure. The frequency of the PCF-generated hollow beam can be tuned by varying the input beam parameters, making a few-mode PCF a convenient and flexible tool for the guiding and trapping of atoms and creation of all-fiber optical tweezers.

Polarization-demultiplexed two-color frequency conversion of femtosecond pulses in birefringent photonic-crystal fibers.

Birefringent photonic-crystal fibers provide efficient polarization-sensitive anti-Stokes frequency conversion of unamplified 35-fs Ti: sapphire laser pulses, giving rise to a doublet of intense blue-shifted emission spectral lines centered at 490 and 510 nm. We show that this anti-Stokes doublet can be wavelength-demultiplexed by a polarizationseparating prism. Generation of the 510-nm signal is decoupled from frequency conversion to 490 nm by accurately polarizing the pump field along one of the principal axes of the elliptically deformed fiber core.