Ultrashort Pulse Irradiation Research Articles

Optimization of the flare angle of a TEM horn for efficient radiation of ultrashort pulses

The finite-difference time-domain method is used to develop the complete electrodynamic model of a TEM horn with exponential longitudinal distribution of the wave impedance. This model is used for optimization of the antenna flare angle, which is aimed at the improvement of the radiation characteristics of an ultrashort pulse with a given shape.

Reproducibility of Observables and Coherent Control in Molecular Photoionization: From Continuous Wave to Ultrashort Pulsed Radiation

One-photon single ionization of molecules has been at the focus of several discussions concerning the reconstruction of observables obtained with ultrashort pulses from those obtained from continuous wave radiation (and vice versa). A related controversy on the conditions and observables that allow for coherent control in one-photon processes has been recently revisited (Science 2006, 313, 1257; J. Chem. Phys. 2010, 133, 151101). Our benchmark to investigate these issues is photoionization of the hydrogen molecule, where the autoionization events are the time-dependent processes in field-free evolution that could serve as a target for coherent control. We show that the variation of one-photon ionization probabilities with pulse duration are solely due to spectral effects and thus cannot be coherently controlled. We then discuss for which observables and under which conditions phase control of autoionization dynamics is possible.

Scattering of an ultrashort electromagnetic radiation pulse by an atom in a broad spectral range

The scattering of an ultrashort electromagnetic pulse by atomic particles is described using a consistent quantum-mechanical approach taking into account excitation of a target and nondipole electromagnetic interaction, which is valid in a broad spectral range. This approach is applied to the scattering of single- and few-cycle pulses by a multielectron atom and a hydrogen atom. Scattering spectra are obtained for ultrashort pulses of different durations. The relative contribution of “elastic” scattering of a single-cycle pulse by a hydrogen atom is studied in the high-frequency limit as a function of the carrier frequency and scattering angle.

Electrophysical phenomena accompanying femtosecond impacts of laser radiation on semiconductors

The space–time concentration distribution of nonequilibrium charge carriers is numerically modelled under the action of ultrashort laser radiation pulses on semiconductors, taking into account the external emission of electrons. The results are compared with the experimental conditions for the excitation and propagation of waveguide modes in silicon under the action of femtosecond pulses with a quantum energy of about 0.98 eV.

All-photonic-crystal-fiber coherent black-light source

Photonic-crystal fibers (PCFs) are widely recognized as efficient sources of white-light supercontinuum. Here, we show that a combination of an ultrashort-pulse ytterbium PCF laser with a highly nonlinear PCF enables all-fiber generation of ultrashort pulses of coherent UV radiation in the black-light spectral region with all the key processes, such as lasing, amplification, and conversion of the amplified PCF-laser output to the UV range, taking place in specifically designed fiber components.

Investigation of THz Emission by p-GaAsSb

With a suitable electric field imposed and under illumination by ultrashort pulses of near-infrared radiation, GaAsSb emits THz-frequency electromagnetic radiation. We conclude that high-temperature-grown GaAsSb is a new class of THz source.

Semiclassical description of high harmonic generation during the above-threshold ionization of atoms

A simple analytical formula for evaluating the intensity of high-frequency emission from an atom is obtained proceeding from the classical notions about the motion of an electron in the field of the atomic core and an external laser radiation field. Calculations using this formula are performed for a hydrogen atom that occurs initially in the ground state and is exposed to the action of ultrashort radiation pulses involving various numbers of field oscillations.

New modes of THz generation by low-temperature-grown GaAsSb

The low-temperature growth of GaAs 1− y Sb y with y = 0.4 and 0.85 has been reported recently along with characterization by X-ray diffraction, Hall, and current–voltage measurements. Here we extend the characterization by employing reflectance spectroscopy in the range 5–18 THz to confirm the compositions of the grown layers. In the course of this work we established for the first time that GaAs 1− y Sb y may serve as an emitter of THz radiation under optical excitation by ultrashort pulses of near-infrared radiation in two distinct experimental arrangements: THz is generated when an electrical bias is applied through a simple electrode structure, attributed to a photoconductive effect; and THz is generated by the pristine layers themselves, attributed to a surface-field effect. In each case the THz emission is compared directly with that from low-temperature-grown GaAs. The results presented here are for as-grown material. Suitable annealing may improve the THz emission even further.

AN ULTRA WIDEBAND IMPULSE OPTOELECTRONIC RADAR: RUGBI

An ultra wideband radar system based on a coherent emission of an ultra-wideband antenna array using photoconductive switching devices is proposed. The triggering process is obtained by the excitation of semiconductor samples in linear mode using a picosecond laser source. The emitting antenna system and the receiving antenna developed by the Research Institute XLIM present some specific qualities suitable for radiation and measurement of ultrashort pulses. The optical control of the sources allows to sum the

Phase Fresnel lens recorded in photo-thermo-refractive glass by selective exposure to infrared ultrashort laser pulses

A new two-step approach for fabricating phase optical elements in photo-thermo-refractive glass by exposure to IR ultrashort laser pulses followed by thermal development is shown. A binary phase Fresnel lens was designed to focus light at 632.8 nm to a focal length of 400 cm. Conditions of ultrashort pulse irradiation and thermal development were chosen to achieve pi phase shift between zone boundaries. The focusing efficiency of the element was measured to be close to 50%.

Changes in the human nuclear chromatin induced by ultra wideband pulse irradiation

AbstractThe effects of ultra wideband pulse radiation on human cells were investigated. The density of the flow of energy on the surface of irradiated object varied from 10−6 to 10−2 W/cm2 with exposure of 10 s. It was shown that heterochromatin granule quantity in cell nuclei increased under the influence of radiation from 10−4 to 10−2 W/cm2. In some intervals the effect increased with irradiation dose. At irradiation intensity 10−3 W/cm2 the process of heterochromatin granule formation was fully reversible after 2 h of recovery; at intensity 10−2 W/cm2 the reversion of irradiation effects was not full. The data obtained indicated the strong biological activity of ultra wideband ultra short pulse radiation.

Gamma ray sources using imperfect relativistic mirrors

The collective backscattering of intense laser radiation by energetic electron beams is considered. Exact solutions for the radiation field are obtained, for arbitrary electron pulse shapes and laser intensities. The electron beams act as imperfect nonlinear mirrors on the incident laser radiation. This collective backscattering process can lead to the development of new sources of ultrashort pulse radiation in the gamma-ray domain. Numerical examples show that, for plausible experimental conditions, intense pulses of gamma rays, due to the double Doppler shift of the harmonics of the incident laser radiation, can be produced using the available technology, with durations less than one attosecond.

Investigation of p-GaAsSb as a THz Emitter

Our purpose was to determine if GaAsSb might be made to emit THz-frequency electromagnetic radiation. Our result is that, with a suitable electric field imposed and under illumination by ultrashort pulses of near-infrared radiation, GaAsSb indeed emits THz radiation. To the best of our knowledge this is the first report of high-temperature-grown GaAsSb acting as a THz source. THz emission has been observed both by incoherent (pneumatic Golay cell) and by coherent (time-domain spectroscopy using electro-optic detection) experimental configurations. Both simple Ag paint and photolithographically formed Au on Ti antenna structures have been found to produce THz radiation. We compare our results with those from the better-known THz emitter, GaAs. In the case of GaAs, THz radiation is emitted even in the absence of applied bias. This is not the case for GaAsSb. We conclude that the photoconductive mechanism dominates in the emission of THz radiation from epitaxial single-crystal GaAsSb. A further proposal is made that such materials can be used with nanostructured surfaces for special THz-emission characteristics.

UWB corner‐slot antenna

AbstractA new bended slot antenna for ultra‐wideband applications is proposed in this article. A prototype has been fabricated and results show that the proposed design exhibits very low voltage standing wave ratio (<1.5) and broadside radiation pattern from 3 to 9 GHz. By processing frequency domain measurements with an IFFT algorithm, the time domain performance of the antenna has been evaluated. The fidelity is better than 0.98 for boresight direction and greater than 0.82 for the rest. All this results reveal that the antenna as an excellent candidate for ultra short microwave pulse radiation. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 663–665, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23146

Multi-time-scale heat transfer modeling of turbid tissues exposed to short-pulsed irradiations

A combined hyperbolic radiation and conduction heat transfer model is developed to simulate multi-time-scale heat transfer in turbid tissues exposed to short-pulsed irradiations. An initial temperature response of a tissue to an ultrashort pulse irradiation is analyzed by the volume-average method in combination with the transient discrete ordinates method for modeling the ultrafast radiation heat transfer. This response is found to reach pseudo steady state within 1 ns for the considered tissues. The single pulse result is then utilized to obtain the temperature response to pulse train irradiation at the microsecond/millisecond time scales. After that, the temperature field is predicted by the hyperbolic heat conduction model which is solved by the MacCormack's scheme with error terms correction. Finally, the hyperbolic conduction is compared with the traditional parabolic heat diffusion model. It is found that the maximum local temperatures are larger in the hyperbolic prediction than the parabolic prediction. In the modeled dermis tissue, a 7% non-dimensional temperature increase is found. After about 10 thermal relaxation times, thermal waves fade away and the predictions between the hyperbolic and parabolic models are consistent.

Radiation safety aspects of the linac coherent light source project at [formula omitted

The linac coherent light source (LCLS) is a self-amplified spontaneous emission based free electron laser (FEL) that is being designed and built at the Stanford Linear Accelerator Center (SLAC) by a multi-laboratory collaboration. This facility will provide ultra-short pulses of coherent X-ray radiation with the fundamental harmonic energy tunable over the energy range of 0.82–8.2 keV. One-third of the existing SLAC linac will compress and accelerate the electron beam to energies ranging from 4.5 to 14.35 GeV. The beam will then be transported through a 130-m long undulator, emit FEL and spontaneous radiation. After passing through the undulator, the electron beam is bent to the main electron dump. The LCLS will have two experiment halls as well as X-ray optics and infrastructure necessary to make use of the FEL for research and development in a variety of scientific fields. The facility design will incorporate features that would make it possible to expand in future such that up to six independent undulators can be used. While some of the radiation protection issues for the LCLS are similar to those encountered at both high-energy electron linacs and synchrotron radiation facilities, LCLS poses new challenges as well. Some of these new issues include: the length of the facility and of the undulator, the experimental floor in line with the electron beam and the occupancy near 0 ∘ , and the very high instantaneous intensity of the FEL. The shielding design criteria, methodology, and results from Monte Carlo and analytical calculations are presented.

Ultrafast Fano Resonance between Optical Phonons and Electron-Hole Pairs at the Onset of Quasiparticle Generation in a Semiconductor

Based on the many-body time-dependent approach applied to the ultrafast time region, we investigate the dynamics of creation of an optical phonon incorporating with the electron-hole continuum in a semiconductor. In the transient Fano resonance, due to an interference between those sharp (optical phonon) and continuum (electron-hole pair) quasiparticles, we find the robust destructive interference at birth of them, i.e., tau approximately 0 if the created phonon is coherent under the irradiation of ultrashort optical pulses. The origin is found to be the potential scattering of the electron-hole pair by the q=0 coherent phonon. This finding agrees well with the recent experiment.

Holding molecular dications together in strong laser fields

Metastable channel of doubly ionized carbon monoxide, ${\mathrm{CO}}^{2+}$, was scantly seen in previous strong-field experiments at the visible wavelength region, but was commonly observed using single high-energy photon or electron excitation. For the first time with near-IR ultrashort-pulse radiation, we observe an abundance of ${\mathrm{CO}}^{2+}$. We show that ${\mathrm{CO}}^{2+}$ results from nonsequential double ionization, while its dissociation counterpart, ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$, results from sequential processes, and ${\mathrm{CO}}^{2+}$ can be obtained through either single high-energy photon or electron excitation or multiphoton ionization with ultrashort pulses before a critical internuclear distance is reached. Our study demonstrates the experimental conditions to converge the outcomes from two vastly different regimes, namely, multiphoton excitation and ionization in strong fields and single high-energy photon or electron excitation and ionization in weak fields.

Core holes, charge disorder, and transition from metallic to plasma properties in ultrashort pulse irradiation of metals

We study the details of a gradual change in electron properties from those of a nearly-free-electron (NFE) metal to those of a strongly-coupled plasma, in ultrashort pulse energy deposition in solid metal targets. Time scales shorter than those of a target surface layer expansion are considered. Both the case of an optical laser (visible or near infrared wavelengths range) and of a free electron laser (vacuum ultraviolet or X-ray) are treated. The mechanisms responsible for the change in electron behavior are isochoric melting, lattice charge disordering, and electron mean free path reduction. We find that the transition from metal to plasma usually occurs via an intermediate stage of a charge-disordered solid (solid plasma), in which ions are at their lattice sites but the ionization stages of individual ions differ due to ionization from localized bound states. Charge disordered state formation is very rapid (typically, few femtoseconds or few tens of femtoseconds). Pathway to charge-disordered state differs in simple metals and in noble metals. Probabilities are derived for electron impact ionization and 3-body recombination of a bound ionic state in solid-density medium, applicable both in metal and in plasma regime. An evolution of energy coupling between electron and ion subsystems, from metallic electron-phonon (e-ph) to plasma electron-ion (e-i) coupling, is considered. Substantial increase in coupling parameter is expected as a result of charge disorder.

Investigations of ultrafast phenomena in high-energy density physics using X-ray FEL radiation

The advent of highly intense and ultrashort pulses of short-wavelength radiation in the vacuum-ultraviolet to X-ray regime provides for the first time the possibility to study plasmas at the time scale of equilibration or even electron thermalization. The emerging radiation sources are free-electron lasers (FEL) based on high-energy electron accelerators. FELs provide a peak brilliance nine orders of magnitude higher than the best performing X-rays sources today. The FEL radiation parameters will enable the creation of high-energy density states of matter and the development of new diagnostics tools to investigate dense plasmas. As the first of the new sources the VUV-FEL at DESY, Hamburg becomes operational for high-energy density physics experiments during 2005.