Isolated attosecond pulse generation with the chirped two-color laser field

We propose a scheme to generate isolated attosecond pulse using a linearly chirped two-color laser field, which includes a fundamental laser field and a weak infrared control laser field in the multicycle regime. The fundamental laser field consists of one linearly up-chirped and one linearly down-chirped pulses. The control pulse is chirped free. We compare the attosecond pulse generated in the chirped two-color field and the chirp-free field. It is found that an IAP can be generated even without carrier envelop phase stabilization in the chirped two-color laser field with a duration of 40 fs. We also discuss the influence of the relative intensity, relative phase, time delay, and chirping parameters on the generation of IAPs.     

Generation of carrier-envelope phase-controlled isolated attosecond pulses using chirped femtosecond excitation lasers

ABSTRACT

We present an efficient approach for producing a carrier-envelope phase controlled isolated attosecond pulse by an optimized intense driving laser pulse. High-order harmonics are produced by numerically solving the time-dependent Schrödinger equation for the one-dimensional hydrogen atom in an ultrashort laser pulse. We define an efficient cost function to optimize the laser pulse by a genetic algorithm scheme. Our approach produces single attosecond pulses with desired properties, including the carrier-envelope phase, central frequency, and duration. Also, we analyze the time–frequency profiles of the attosecond emissions to gain a deeper insight into the underlying physical mechanism.     

Optimal control of high harmonics for the generation of double attosecond pulses by using a chirped laser pulse

An efficient scheme for the optimization of ultrashort femtosecond pulse shapes interacting with an atom to control high harmonics spectrum and double attosecond pulse generation is presented. The time-dependent Schrödinger equation of one-dimensional hydrogen atom is solved numerically to obtain electric field emission. The genetic algorithm optimization method is used to control the phase and amplitude of ultrashort excitation laser pulses to generate the desired attosecond-shaped pulses. An appropriate cost function is introduced for genetic algorithm optimization of double attosecond pulse generation. It is shown that the relative intensity of two generated pulses, their delay time and duration can be controlled in this approach. Finally, the parameters of the optimized emitted attosecond pulse are compared with those of desired pulses, and the underlying physical mechanisms are discussed in detail.     

Quantum wavepacket exploration of isolated high-order harmonic attosecond pulse generation: from two-color scheme to three-color scheme

Isolated attosecond generation under the two-color scheme and three-color scheme are investigated theoretically by use of wave-packet dynamics calculation. Optimized relative phase under the two-color field is obtained and comparison is taken between the second-harmonic control and half-harmonic control. A further exploration of the three-color scheme by adding a weak second-harmonic pulse and a half-harmonic pulse onto 8×10¹⁴?W?cm^?2 6?fs/800?nm pulse shows the potential for generating the 337?eV continuum supporting creation of an isolated 11 as pulse.     

Isolated sub-50 attosecond pulse generation driven by tricolor multi-cycle pulses

We theoretically investigate the high-order harmonic generation (HHG) driven by laser pulses with tri-color carrier wave in the multi-cycle regime. Through the modulation of the carrier wave, the peak of the return kinetic energy of the electron near the pulse center extends dramatically and the other peaks are suppressed by the envelope. Thus, a very broad continuum spectrum appears in the HHG. Moreover, due to the propagation effect, the long path of the electron for the continuum spectrum is eliminated effectively. Hence, the continuum spectrum is well-phase locked, from which an isolated sub-50 attosecond pulse could be obtained even for the driver pulse with duration of 30 fs.     

Single attosecond light pulses from multi-cycle laser sources

High harmonic generation provides a means of producing attosecond pulses of light which are the shortest, controllable probes available to science for time-resolving ultrafast dynamics. We review techniques based on high harmonic generation for generating single attosecond pulses using high-power, multi-cycle laser sources, including optical-, polarisation-, and ionisation-gating schemes as well as techniques based on field synthesis. By significantly reducing the technical demands placed on the driving laser, these techniques have the potential to greatly broaden the application base for attosecond pulses.     

An intuitive model for on-axis pulse evolution of ultrashort pulsed Gaussian beams diffracted from a circular aperture

The diffraction of ultrashort pulsed Gaussian beams from a circular aperture is studied by means of Fresnel diffraction integral and Fourier transform method. A uniform analytical expression is derived for temporal pulse form of ultrashort pulsed Gaussian beams in two cases, i.e. with constant beam waist and with constant diffraction length. It is shown that the on-axis pulse can be formulated as a superposition of an unapertured pulse and an aperture-induced pulse. The superposition of these two pulses leads to an enhanced pulse intensity for small truncation parameters at certain distances in the near field. Our results may find applications in high-intensity laser waveform control.     

Laser-parameter effects on the generation of ultrabroad harmonic and ultrashort attosecond pulse in a long-plus-short scheme

By numerically solving the time-dependent Schrödinger equation for helium gas in a special two-color laser field, which is synthesized by a long (9?fs) driving pulse and a short (6?fs) controlling pulse, we discuss the influence of the carrier-envelope phase, frequency, and the intensity of the controlling pulse on the generation of harmonic spectra and isolated attosecond pluses. In the cutoff region, two or three plateaus can be controlled by optimizing these laser parameters, and an ultrabroad supercontinuum harmonic spectrum with a bandwidth of 800?eV can be produced, which can support an ultrashort isolated 4.5 as pulse generation by Fourier transformation. Furthermore, using classical ionizing and returning energy maps, time–frequency analyses are presented to explain the underlying physical mechanisms.     

Caustics of a Light Beam Passing Through a Time-varying Dielectric Wedge

Abstract

A caustic of cusp type in space-time is found to arise in a laser beam deflected by a wedge with time-varying refractive index. Asymptotic formulae describing the wave field under the assumption that the refractive index varies slowly compared with the oscillations of the light wave are given. The intensity of the wave field at the cusp point can be, say, 10³ times greater than the initial one. The temporal variation of the field at the cusp gives rise to an ultrashort pulse. The lengths of these pulses can be 10⁵ times less than the characteristic time of modulation of the refractive index, i.e., of subpicosecond range.     

Quantitative evaluation of B1 insensitivity in nonadiabatic frequency‐selective fat‐suppression RF pulse techniques

Nonadiabatic frequency‐selective fat‐suppression radiofrequency (RF) pulses are simpler than adiabatic RF pulses because nonadiabatic RF pulses are only amplitude modulated. The specific absorption rate (SAR) is lower. However, nonadiabatic RF pulses tend to be sensitive to B₁ inhomogeneity. The purpose of this research was to evaluate whether conventional adiabatic RF pulses can replace nonadiabatic RF pulse techniques. The B₁ insensitivities of nonadiabatic frequency‐selective fat‐suppression RF pulse techniques were calculated by using the Bloch equation, and their effectiveness was evaluated by simulation using the measured B₁ field map. The B₁ insensitivities were compared quantitatively. The B₁ insensitivities of the nonadiabatic RF pulse techniques were ±5% (CHESS), a maximum of ±20% (inversion recovery RF pulse with the best inversion time), ±25% (RF pulse train with two subpulses), and ±44% (RF pulse train with three subpulses). The RF pulse train was the most effective. The B₁ insensitivity of different nonadiabatic RF pulse techniques was specified quantitatively. These results can be used to judge which nonadiabatic RF pulses can replace adiabatic RF pulses. Nonadiabatic RF pulses can reduce the SAR without compromising the image quality and would be useful in higher field‐strength MRI. © 2015 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 25, 86–91, 2015     

A multifunctional, reconfigurable pulse generator for high-frequency ultrasound imaging

High-frequency (>20 MHz) ultrasound (HFUS) imaging systems have made it possible to image small structures with fine spatial resolution. They find a variety of biomedical applications in dermatology, ophthalmology, intravascular imaging, and small-animal imaging. One critical technical challenge of HFUS is to generate high-voltage, high-frequency pulsed signals to effectively excite the transducer for a high SNR. This paper presents the development of a multifunctional, reconfigurable pulse generator for HFUS imaging. The pulse generator can produce a high-voltage unipolar pulse, a bipolar pulse, or arbitrary pulses for B-mode imaging, Doppler measurement, and modulated excitation imaging. The characteristics of the pulses, such as timing, waveform, and frequency are reconfigurable by a high-speed field-programmable gate array (FPGA). Customized software was developed to interface with the FPGA through a USB connector for pulse selection, and easy, flexible, real-time pulse management. The hardware was implemented in a compact, printed circuit board (PCB)-based scheme using state-of-the-art electronics for costeffectiveness and fully digital control. Testing results show that the unipolar pulse can reach over 165 Vpp with a 6-dB bandwidth of 70 MHz, and the bipolar pulse and arbitrary pulses can reach 150 and 60 Vpp with central frequencies of 60 and 120 MHz, respectively.     

On possibility of high-power terahertz emission from target under the action of powerful laser pulses

The possibility of terahertz (THz) emission from a target irradiated by short (∼0.1 ns) high-intensity (I ∼ 10¹⁸–10¹⁹ W/cm²) laser pulses has been studied by numerical simulations using a relativistic electromagnetic PIC code. The laser pulse action on the target generates plasma and the runaway electrons form a virtual cathode, which oscillates in the intrinsic field of electrons and the field of plasma ions. These oscillations account for the emission of radiation in a THz range. The generation efficiency is about three times as high as that in the absence of ions (according to the conventional reditron mechanism). Explanation of the observed phenomena is proposed.     

Laser Heating of Metals

Abstract

The Sprite KrF/Raman laser system has been developed, over the last 12 years, into one of the world's brightest laser sources. It is now a fully scheduled user facility delivering more than 1900 shots per year to a dedicated target chamber. A laser development programme is also supported, addressing the future requirements of the high-power laser community. Sprite has traditionally been operated as a KrF-pumped Raman laser, delivering 10 ps pulses of very high brightness (～ 10²⁰W cm^?2 sterad^?1) and exceptional prepulse contrast ratio (< 10¹⁰). Direct amplification of pulses as short as 3 ps is practical in the Sprite KrF chain, and a chirped pulse amplification scheme has now been implemented delivering 300 fs pulses to target with a power of 1 TW. The next major upgrade to the system will be the installation of a new 40 cm aperture amplifier, Titania, designed to deliver up to 400 J in four Raman pulses.     

Recent development and new ideas in the field of dispersive multilayer optics

A dispersive-mirror-based laser permits a dramatic simplification of high-power femtosecond and attosecond systems and affords promise for their further development toward shorter pulse durations, higher peak powers, and higher average powers with user-friendly systems. The result of the continuous development of dispersive mirrors permits pulse compression down to almost single cycle pulses of 3?fs duration. These design approaches together with the existing modern deposition technology pave the way for the manufacture of dielectric multilayer coatings able to compress pulses of tens of picoseconds duration down to a few femtoseconds.     

Enhancement of stability and peak power in a diode-pumped doubly QML YVO4/Nd:YVO4 laser with EO and Cr:YAG saturable absorber

A diode-pumped doubly Q-switched and mode-locked (QML) YVO₄/NdYVO₄ laser is realized with the electro-optic (EO) modulator and Cr⁴⁺:YAG saturable absorber, in which the repetition rate of the Q-switched envelope is controlled by the active EO modulation while the mode-locked pulses inside the Q-switched envelope depend on both the actively modulated loss and the passive saturable absorption. The experimental results show that the doubly QML laser can generate more stable and shorter pulses with higher peak power when compared with the singly passively QML laser with Cr⁴⁺:YAG. At the pump power of 20 W and the repetition rate 1 kHz, a 21 ns Q-switched pulse envelope with a average mode-locked peak power of 544 kW is obtained, which is the shortest Q-switched pulse envelope to my knowledge. In comparison to the singly passively QML laser with Cr⁴⁺:YAG, the doubly QML laser has compressed the Q-switched envelope pulse width 70% and improved the mode-locked pulsed peak power 27 times. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML laser is given and the numerical solutions of the equations are in good agreement with the experimental results.     

Experimental investigation of nonuniform heating and heat loss from a specimen for the measurement of thermal diffusivity by the laser pulse heating method

Nonuniform heating effect and heat loss effect from the specimen in the measurement of thermal diffusivity by the laser pulse heating method have been experimentally investigated using an axially symmetric Gaussian laser beam and a laser beam homogenized with an optical filter. The degree of error is theoretically estimated based on the solution of the two-dimensional heat conduction equation under the boundary condition of heat loss from the surface of the specimen in the axial direction and the initial conditions of axially symmetric nonuniform and uniform heating. A correction factor, which is determined by comparison of the entire experimental and the theoretical history curves, is introduced to correct the values obtained by the conventionalt _1,2 method. The applicability of this modified curve-fitting method has been experimentally tested using materials in the thermal diffusivity range 10⁻³ to 1 cm²·s⁻¹. The experimental error due to the nonuniform heating and heat loss was reduced to approximately 3%.     

The ablation threshold in amorphous diamondlike carbon films exposed to an ArF excimer laser radiation

An experimental study of the laser-induced ablation threshold in amorphous diamondlike carbon films is reported. The aim is to assess the possibility of using the material as a photoresist in vacuum-ultraviolet laser lithography. Grown on silicon substrates, 10-nm films were irradiated by 20-ns pulses of a 193-nm ArF excimer laser with variable pulse energy per unit area E _p. It is found that the etch rate is very low if E _p < 20 mJ/cm², whereas a single pulse suffices to remove the film completely if E _p=60 mJ/cm². This is attributed to an increase in the thermal ablation component.     

Pulse-duration dependent sequential double ionization by elliptically polarized laser pulses

Using a fully classical model, we have studied sequential double ionization of argon driven by elliptically polarized laser pulses at intensities well in the over-barrier ionization region. The results show that the joint electron momentum distributions in the minor elliptical direction depend strongly on the pulse duration. From pulse number N = 4 to 10, the clustering regions of the joint electron momentum increase with the pulse duration. For even larger pulse durations, the clustering region does not increase further but the population of the joint electron momentum in these regions changes with the pulse duration. Back analysis of double ionization trajectories shows the phenomenon of multiple ionization bursts and the pulse duration-dependent multiple ionization bursts of the second electron is responsible for the evolution of the joint electron momentum distribution with the pulse duration.