Long lifetime plasma channel in air generated by multiple femtosecond laser pulses and an external electrical field

The lifetime of a plasma channel produced by self-guiding intense femtosecond laser pulses in air is largely prolonged by adding a high voltage electrical field in the plasma and by introducing a series of femtosecond laser pulses. An optimal lifetime value is realized through adjusting the delay among these laser pulses. The lifetime of a plasma channel is greatly enhanced to 350 ns by using four sequential intense 100fs( FWHM) laser pulses with an external electrical field of about 350kV/m, which proves the feasibility of prolonging the lifetime of plasma by adding an external electrical field and employing multiple laser pulses. (c) 2006 Optical Society of America.

Formation of X-waves at fundamental and harmonics by infrared femtosecond pulse filamentation in air

We experimentally observe the formation of X-waves at fundamental, third harmonic, and fifth harmonic wavelengths by infrared (central wavelength at similar to 1500 nm) femtosecond laser pulse filamentation in air. By fitting the angularly resolved spectra of the fundamental and harmonic waves using X-wave relations, we confirm that all the X-waves have nearly the same group velocity, indicating that they are locked in space and time during their propagation in filament.

Spectral evolution of angularly resolved third-order harmonic generation by infrared femtosecond laser-pulse filamentation in air

We experimentally investigate the evolution of an angularly resolved spectrum of third harmonic generated by infrared femtosecond laser pulse filamentation in air. We show that at low pump intensity, phase matching between the fundamental and third-harmonic waves dominates the nonlinear optical effect and induces a ring structure of the third-harmonic beam, whereas at high pump intensity, the dispersion properties of air begin to affect the angular spectrum, leading to the formation of a nonlinear X wave at third harmonic.

X-shaped third harmonic generated by ultrashort infrared pulse filamentation in air

The authors report the measurement of the angularly resolved spectrum of the third harmonic generated in a femtosecond filament in air and its evolution with increasing pump power. Pumped by a focused infrared ultrashort pulse with a carrier wavelength of 1270 nm, a pulse duration of similar to 20 fs, and pulse energy up to 487 mu J, the generated third harmonic is composed of an on-axis emission and a conical ring emission. When the pump power is sufficiently high, angularly resolved spectra with significant X-like feature could be observed, indicating the formation of nonlinear X wave at third harmonic. (c) 2008 American Institute of Physics.

Transverse evolution of a plasma channel in air induced by a femtosecond laser

We investigate the evolution of filamentation in air by using a longitudinal diffraction method and a plasma fluorescence imaging technique. The diameter of a single filament in which the intensity is clamped increases as the energy of the pump light pulse increases, until multiple filaments appear. (c) 2006 Optical Society of America.

ZnSe nanowires grown on the crystal surface by femtosecond laser ablation in air

Uniform ZnSe nanowires are observed on the ablation crater on ZnSe crystal surface irradiated by femtosecond lasers in air, while other parts of the sample surface are not polluted. The nanowire growth rate is about 5 mu m/s, it is higher than that fabricated by chemical vapor deposition method by a factor of 10(4). The nanowire length and diameter can be controlled by varying laser pulse energy and pulse number. The formation mechanism is studied and found to be self-catalyzed vapor-liquid-solid process. (c) 2006 American Institute of Physics.

Control of filamentation induced by femtosecond laser pulses propagating in air

Filamentation formed by self-focusing of intense laser pulses propagating in air is investigated. It is found that the position of filamentation can be controlled continuously by changing the laser power and divergence angle of the laser beam. An analytical model for the process is given.

Polarization-dependent supercontinuum generation from light filaments in air

We investigate polarization-dependent properties of the supercontinuum emission generated from filaments produced by intense femtosecond laser pulses propagating through air over a long distance. The conversion efficiency from the 800-nm fundamental to white light is observed to be higher for circular polarization than for linear polarization when the laser intensity exceeds the threshold of the breakdown of air. (C) 2005 Optical Society of America.

Quantum interference effects of two coherent population trapping states on the atomic spectral lines of a F-e=0 -> F-g=1 transition

We investigate the fluorescence spectrum in a nearly degenerate atomic system of a F-e = 0 -> F-g = 1 transition by analytically solving Schrodinger equations. An ultranarrow fluorescence spectral line in between the two coherent population trapping windows has been found. Our analytic solutions clearly show the origin of the ultranarrow spectral line. Due to quantum interference effects between two coherent population trapping states, the width and intensity of the central spectral line can be controlled by an external magnetic field. Such an effect may be used to detect a magnetic field.

Temperature dependence of dark lines in coherent population trapping

We measure the signal amplitude and linewidth of a dark line in coherent population trapping in the Rb vapour cell filled with mixed buffer gas N-2 and Ar as a function of cell temperature. We find that the dark line signal amplitude increases with temperature up to a maximum at 49 degrees C and then drops at higher temperatures due to quenching effects of N-2. The linewidth of the dark line remains basically constant, at 1080 Hz. We also measure the linewidth of the dark line as a function of laser intensity. The linewidth increases linearly with laser intensity. An intrinsic linewidth (FWHM=896 Hz at 3.4 GHz) of the Rb cell is obtained.

Determination and analysis of the optical constants of thin films of nickel(II) and copper(II) hydrazone complexes by spectroscopic ellipsometry

Thin films of four nickel(II) and copper(II) hydrazone complexes, which will hopefully be used as recording layers for the next-generation of high-density recordable disks, were prepared by using the spin-coating method. Absorption spectra of the thin films on K9 optical glass substrates in the 300-700 nm wavelength region were measured. Optical constants (complex refractive indices N) and thickness d of the thin films prepared on single-crystal silicon substrates in the 275-675 nm wavelength region were investigated on a rotating analyzer-polarizer scanning ellipsometer by fitting the measured ellipsometric angles (Psi(lambda) and Delta(lambda)) with a 3-layer model (Si/dye film/air). The dielectric functions epsilon and absorption coefficients alpha as a function of the wavelength were then calculated. Additionally, a design to achieve high reflectivity and optimum dye film thickness with an appropriate reflective layer was performed with the Film Wizard software using a multilayered model (PC substrate/reflective layer/dye film/air) at 405 nm wavelength.

Tuning the Eu luminescence in glass materials synthesized in air by adjusting glass compositions

The doped Eu3+ ions can be partly reduced to Eu2+ in a series of MO-B2O3: Eu (M=Ba, Sr, Ca) glasses synthesized in air atmosphere, but not in the 12CaO-7Al(2)O(3): Eu glass. The different redox-behavior of Eu ions in these two glass systems is attributed to the different host optical basicity. It is found that a lower valence state of Eu2+ is more favorable in acidic glasses, which have lower optical basicities. A notion of the critical value of optical basicity is introduced empirically, which can be used as a guide for the selection of glass composition suitable to incorporate Eu2+ for luminescence. (C) 2006 Elsevier B.V. All rights reserved.

A new method of modeling cladding band structure of air-guiding photonic crystal fibers

Cladding band structure of air-guiding photonic crystal fibers with high air-filling fraction is calculated in terms of fiber shape variation. The fundamental photonic band gap dependence on structure parameters, air-filling fraction and spacing, is also investigated. The numerical results show that the band gap edges shift toward longer wavelength as the air-filling fraction is increased, whereas the relative band gap width increases linearly. For a fixed air-filling fraction, the band gap edges with respect to spacing keep constant. With this method, the simulation results agree well with the reported data. © 2007 Elsevier B.V. All rights reserved.