Enhancement and broadening of extreme-ultraviolet supercontinuum in a relative phase controlled two-color laser field

We experimentally demonstrate the generation of an extreme-ultraviolet (XUV) supercontinuum in argon with a two-color laser field consisting of an intense 7 fs pulse at 800 nm and a relatively weak 37 fs pulse at 400 nm. By controlling the relative time delay between the two laser pulses, we observe enhanced high-order harmonic generation as well as spectral broadening of the supercontinuum. A method to produce isolated attosecond pulses with variable width and intensity is proposed. (C) 2008 Optical Society of America.

Generation of intense extreme supercontinuum radiation via resonant propagation effects

Confinement of electromagnetic energy into a single well-controlled oscillation of light is very important for generation of intense supercontinuum radiation. We find that the pulse breakup of few-cycle ultrashort laser pulses via resonant propagation effects can achieve this aim. By extracting such pulses and then focusing them to drive the He atoms, about 200 eV intense supercontinuum radiation can be generated, which is capable of supporting similar to 20 attosecond isolated pulse generation.

High-conversion-efficiency and broadband tunable femtosecond noncollinear optical parametric amplifier

We build a compact high-conversion-efficiency and broadband tunable noncollinear optical parametric amplifier (OPA) in the infra-red (IR) pumped by a femtosecond Ti:sapphire CPA laser. The OPA consists of an internal seed of white-light continuum generator (WLG) and two noncollinear optical parametric amplifiers. The tunable wavelength range is from 1.2 mu m to 2.4 mu m for both signal and idle pulses. The total OPA efficiency in the last OPA stage reaches about 40% in a wider tunable spectral range (from 1.3 mu m to 1.7 mu m for signal pulse, from 1.5 mu m to 2.0 mu m for idle pulse respectively).

Linear chirp control of infrared signal in a biased semiconductor thin film

Ultrashort light-matter interactions between a linear chirped pulse and a biased semiconductor thin film GaAs are investigated. Using different chirped pulses, the dependence of infrared spectra on chirp rate is demonstrated for a 5 fs pulse. It is found that the infrared spectra can be controlled by the linear chirp of the pulse. Furthermore, the infrared spectral intensity could be enhanced by two orders of magnitude via appropriately choosing values of the linear chirp rates. Our results suggest a possible scheme to control the infrared signal.

Active phase control and frequency chirp effects on supercontinuum generation in high birefringence photonic crystal fiber

An acoustic-optics programmable dispersive filter (AOPDF) was first employed to actively control the linearly polarized femtosecond pump pulse frequency chirp for supercontinuum (SC) generation in a high birefringence photonic crystal fiber (PCF). By accurately controlling the second order phase distortion and polarization direction of incident pulses, the output SC spectrum can be tuned to various spectral energy distributions and bandwidths. The pump pulse energy and bandwidth are preserved in our experiment. It is found that SC with broader bandwidth can be generated with positive chirped pump pulses except when the chirp value is larger than the optimal value, and the same optimal value exists for the pump pulses polarized along the two principal axes. With optimal positive chirp, more than 78% of the pump energy can be transferred to below 750 nm. Otherwise, negative chirp will weaken the blue-shift broadening and the SC bandwidth. (C) 2007 Elsevier B.V. All rights reserved.

Nonlinear laser focusing using a conical guide and generation of energetic ions

Using conventional methods, a laser pulse can be focused down to around 6-8 mu m, but further reduction of the spot size has proven to be difficult. Here it is shown by particle-in-cell simulation that with a hollow cone an intense laser pulse can be reduced to a tiny, highly localized, spot of around 1 mu m radius, accompanied by much enhanced light intensity. The pulse shaping and focusing effect is due to a nonlinear laser-plasma interaction on the inner surface of the cone. When a thin foil is attached to the tip of the cone, the cone-focused light pulse compresses and accelerates the ions in its path and can punch through the thin target, creating highly localized energetic ion bunches of high density.

Pulse temporal cleaner based on nonlinear ellipse rotation by using BK7 glass plate

We report a new pulse cleaning technique to enhance the contrast ratio of intense ultra-short laser pulses. A pulse temporal cleaner based on nonlinear ellipse rotation by using BK7 glass plate is developed, and a contrast ratio improvement of two orders of magnitude for the milli-joule level femtosecond input pulses is demonstrated, the total transmission efficiency of the pulse cleaner is 16.7%.

An ultra-broadband spatially dispersed regenerative amplifier free from spatial chirp

An ultra-broadband Ti:sapphire regenerative amplifier based on spatially dispersed amplification is demonstrated experimentally. Departing from previous reports, a new design of the cavity gets the amplified pulse free from spatial chirp. Utilizing this new regenerative amplifier, chirped pulses with bandwidth (FWHM) of about 80 nm are obtained, and the bandwidth is limited only by that of the incident seed pulses.

Self-induced transmission on intersubband resonance in multiple quantum wells

We investigate the nonlinear propagation of ultrashort pulses on resonant intersubband transitions in multiple semiconductor quantum wells. It is shown that the nonlinearity rooted from electron-electron interactions destroys the condition giving rise to self-induced transparency. However, by adjusting the area of input pulse, we find the signatures of self-induced transmission due to a full Rabi flopping of the electron density, and this phenomenon can be approximately interpreted by the traditional standard area theorem via defining the effective area of input pulse.

The effect of electric field maximum on the Rabi flopping and generated higher frequency spectra

We investigate the effect of the electric field maximum on the Rabi flopping and the generated higher frequency spectra properties by solving Maxwell-Bloch equations without invoking any standard approximations. It is found that the maximum of the electric field will lead to carrier-wave Rabi flopping (CWRF) through reversion dynamics which will be more evident when the applied field enters the sub-one-cycle regime. Therefore, under the interaction of sub-one-cycle pulses, the Rabi flopping follows the transient electric field tightly through the oscillation and reversion dynamics, which is in contrast to the conventional envelope Rabi flopping. Complete or incomplete population inversion can be realized through the control of the carrier-envelope phase (CEP). Furthermore, the generated higher frequency spectra will be changed from distinct to continuous or irregular with the variation of the CEP. Our results demonstrate that due to the evident maximum behavior of the electric field, pulses with different CEP give rise to different CWRFs, and then different degree of interferences lead to different higher frequency spectral features.

Theoretical investigation of single attosecond pulse generation in an orthogonally polarized two-color laser field

We theoretically demonstrate the generation of extreme ultraviolet supercontinua in an orthogonally polarized two-color few-cycle laser field. We show that the ionized electrons can be driven back to their parent ion after traveling along curved trajectories in a plane perpendicular to the beam propagation direction, giving rise to a train of attosecond pulses at different polarization angles. A single isolated attosecond pulse can be obtained by blocking the low-order high harmonics, which contribute to the formation of the satellite pulses. (C) 2008 Optical Society of America.

Tunable high-order harmonic generation and the role of the folded quantum path

We theoretically demonstrate the selective enhancement of high-order harmonic generation (HHG) in two-color laser fields consisting of a single-cycle fundamental wave (800 nm wavelength) and a multicycle subharmonic wave (2400 nm wavelength). By performing time-frequency analyses based on a single-active-electron model, we reveal that such an enhancement is a result of the modified electron trajectories in the two-color field. Furthermore, we show that selectively enhanced HHG gives rise to a bandwidth-controllable extreme ultraviolet supercontinuum in the plateau region, facilitating the generation of intense single isolated attosecond pulses.

Single sub-100 attosecond pulse generation in a two-colour time-gating laser field

We theoretically propose a method of generating a single sub-100 attosecond (as) pulse with a two-colour time-gating laser field. The field is synthesized by an 8 fs/800 nm (three optical cycles) pulse and a 24 fs/2400 nm (three optical cycles) pulse with an optimal time delay between them. In our simulation, we obtain a supercontinuum with an extremely broad spectrum of 150 eV and generate an isolated attosecond pulse with 96 as pulse duration without any dispersion compensation.

Carrier-envelope phase control of carrier-wave Rabi flopping in asymmetric semiparabolic quantum well

We investigate the carrier-wave Rabi flopping effects in an asymmetric semiparabolic semiconductor quantum well (QW) with few-cycle pulse. It is found that higher spectral components of few-cycle ultrashort pulses in the semiparabolic QW depend crucially on the carrier-envelope phase (CEP) of the few-cycle ultrashort pulses: continuum and distinct peaks can be achieved by controlling the CEP. Our results demonstrate that by adjusting the CEP of few-cycle ultrashort pulses, the intersubband dynamics in the asymmetric semiparabolic QW can be controlled in an ultrashort timescale with moderate laser intensity. (c) 2008 Optical Society of America.

Micromodification of element distribution in glass using femtosecond laser irradiation

We report micromodification of Eu element distribution in a silicate glass with femtosecond laser irradiation. Elemental analysis shows that the content of Eu decreased at the focal point and increased in a ring-shaped region around the focal point, which indicates migration of Eu ions has been induced by the femtosecond laser irradiation. Confocal fluorescence spectra demonstrate that the fluorescence intensity of Eu3+ ions increased by 20% in the laser-induced, Eu-enriched, ring-shaped region compared with that for nonirradiated glass. The mechanism for the laser induced change in fluorescence properties of Eu3+ has been investigated. (C) 2009 Optical Society of America