Ultrashort Pulse Irradiation Research Articles

Three-photon induced photobleaching in a three-dimensional memory material

The photobleaching properties of a 3D memory material based on pyrylium salts are studied. It is shown that under ultrashort pulsed irradiation photobleaching occurs through two diverse mechanisms. One is major and causes fast bleaching, whereas the other is minor and causes slow bleaching. Furthermore, it is proved that both mechanisms are three- or more-photon processes. This fact gives the capability of significantly increasing data storage density.

Ultrabroadband infrared solid-state lasers

Ultrabroadband infrared transition metal ion-doped solid-state lasers have come of age and are increasingly being used in trace gas monitoring, remote sensing, telecommunications, ophthalmology, and neurosurgery. Operating at room temperature, they are stable, versatile, and easy to handle successors to the color center lasers. They are becoming the critical components in optical frequency standards, space-based remote sensing systems, and may soon find application in femtochemistry and attosecond science. The article reviews the principles and basic physics of these types of lasers, which are distinguished by their ability to support the shortest pulses down to single optical cycle durations and the ultimately broad tuning ranges. The paper further reviews the state of the art in the existing diode-pumped sources of broadly tunable continuous wave, and ultrashort pulsed radiation in the infrared, and provides examples of their successful application to supercontinuum generation, trace gas measurements, and ultrasensitive intracavity spectroscopy. Developments in such lasers as Cr:YAG, Cr:ZnSe, Cr:ZnS, as well as the recently proposed mixed Cr:ZnS/sub x/Se/sub 1-x/ laser, are discussed in more detail. These lasers nearly continuously cover the infrared spectral region between 1.3 and 3.1 /spl mu/m. The gain spectra of these lasers perfectly match and extend toward the infrared spectra of such established ultrabroadband lasers, operating at shorter wavelengths between /spl sim/0.7-1.3 /spl mu/m, as Ti:sapphire, Cr:LiSAF/Cr:LiSGaF and Cr:forsterite. This opens up new opportunities for synthesis of single-cycle optical pulses and frequency combs in the infrared.

Resonant radiation and collapse of ultrashort pulses in planar waveguides

We study the spatiotemporal dynamics of ultrashort pulses close to a point of zero group-velocity dispersion in planar waveguides with focusing nonlinearities. We find that the process of pulse collapse enhances the emission of so-called resonant radiation, providing an efficient mechanism of energy transfer from solitonic to dispersive waves and leading to suppression of the collapse.

Reverse Monte Carlo Method for Transient Radiative Transfer in Participating Media

Abstract The Monte Carlo (MC) method has been widely used to solve radiative transfer problems due to its flexibility and simplicity in simulating the energy transport process in arbitrary geometries with complex boundary conditions. However, the major drawback of the conventional (or forward) Monte Carlo method is the long computational time for converged solution. Reverse or backward Monte Carlo (RMC) is considered as an alternative approach when solutions are only needed at certain locations and time. The reverse algorithm is similar to the conventional method, except that the energy bundle (photon ensemble) is tracked in a time-reversal manner. Its migration is recorded from the detector into the participating medium, rather than from the source to the detector as in the conventional MC. There is no need to keep track of the bundles that do not reach a particular detector. Thus, RMC method takes up much less computation time than the conventional MC method. On the other hand, RMC will generate less information about the transport process as only the information at the specified locations, e.g., detectors, is obtained. In the situation where detailed information of radiative transport across the media is needed the RMC may not be appropriate. RMC algorithm is most suitable for diagnostic applications where inverse analysis is required, e.g., optical imaging and remote sensing. In this study, the development of a reverse Monte Carlo method for transient radiative transfer is presented. The results of non-emitting, absorbing, and anisotropically scattering media subjected to an ultra short light pulse irradiation are compared with the forward Monte Carlo and discrete ordinates methods results.

Investigation of the spectral selectivity of volume holograms with femtosecond pulsed radiation

A method for studying the parameters of high-selectivity volume holograms and photonic crystal structures using ultrashort pulsed radiation in a wide spectral range is developed. The experimental system and measurement procedure are described. The results of determination of the diffraction efficiency and spectral selectivity of holographic gratings with spatial frequencies of 360 and 640 mm−1 are presented. The parameters of the holograms determined by the method proposed and by the conventional techniques based on the use of continuous laser radiation are found to coincide.

The area theorems for two-core nonlinear directional couplers

The area theorems, describing propagation of ultrashort pulses of resonant laser radiation in a two-core nonlinear directional coupler, are formulated for the first time. It is shown that such a system is characterized by a finite, bounded above set of areas of the stationary propagating pulses. We also predict quantization of the limit cycles and possible existence of the regime of periodic complete transfer of the radiation energy from one fiber to another.

On the diffraction of ultrashort pulses

The diffraction of ultrashort pulses of electromagnetic radiation is considered in terms of the model of an initial Gaussian time pulse and a Gaussian beam. A criterion for the short pulse duration and the average pulse wavelength are introduced, and the diffraction length and angular divergence of radiation are estimated. Along with the general expression for a field, simple expressions for the case of the far zone are derived. The results obtained are compared with the simplified delta pulse model.

Waveguide modes of electromagnetic radiation in hollow-core microstructure and photonic-crystal fibers

The properties of waveguide modes in hollow-core microstructure fibers with two-dimensionally periodic and aperiodic claddings are studied. Hollow fibers with a two-dimensionally periodic cladding support air-guided modes of electromagnetic radiation due to the high reflectivity of the cladding within photonic band gaps. Transmission spectra measured for such modes display isolated maxima, visualizing photonic band gaps of the cladding. The spectrum of modes guided by the fibers of this type can be tuned by changing cladding parameters. The possibility of designing hollow photonic-crystal fibers providing maximum transmission for radiation with a desirable wavelength is demonstrated. Fibers designed to transmit 532-, 633-, and 800-nm radiation have been fabricated and tested. The effect of cladding aperiodicity on the properties of modes guided in the hollow core of a microstructure fiber is examined. Hollow fibers with disordered photonic-crystal claddings are shown to guide localized modes of electromagnetic radiation. Hollow-core photonic-crystal fibers created and investigated in this paper offer new solutions for the transmission of ultrashort pulses of high-power laser radiation, improving the efficiency of nonlinear-optical processes, and fiber-optic delivery of high-fluence laser pulses in technological laser systems.

Reflection of ultrashort pulses from the boundary of a Drude-Lorentz medium

Reflection of ultrashort pulses of electromagnetic radiation from the boundary between a vacuum and a Drude-Lorentz medium (a set of classical linear oscillators with a low concentration) is considered. For the case of two incident pulses, the possibility of interference quenching of reflected radiation is revealed. The quenching occurs when the pulse amplitudes satisfy a certain relation and the pulses follow with an appropriate time delay.

Transient responses of SFG spectra of D 2O ice/CO/Pt(1 1 1) interface with irradiation of ultra-short NIR pump pulses

The behavior of D 2O ice on CO/Pt(1 1 1) and Pt(1 1 1) under the irradiation of near-IR pulses (NIR) was studied by sum-frequency generation (SFG) spectroscopy. The peaks assigned to the O–D stretching modes of ice were obtained for the first 30 molecular layers on Pt(1 1 1). When the D 2 O/ CO/ Pt(1 1 1) was irradiated, the signal of D 2O was weakened after 500 ps, but that of CO was weakened immediately after the pumping. A similar time response was observed for the D 2O peak in D 2 O/ Pt(1 1 1) . The weakening of SFG is attributed to the broadening of bands due to thermal excitation. This indicates that the energy of the pump pulse is deposited on the Pt(1 1 1) surface and diffused into the layers of D 2O ice in the 500 ps timescale.

Ultrafast pulse propagation in direct gap semiconductor quantum well waveguide structures

Using the Luttinger formulations, we have analysed the propagation characteristics of an ultrashort (femtosecond) coherent radiation pulse in the transverse plane of a direct gap semiconductor quantum well waveguide structure (QWWS). The semiconductor QWWS is considered to possess degenerate valence subbands near the centre of the Brillouin zone. The photoinduced resonant transitions below the band edge to both 1s and 2s excitonic states from the light hole (lh) and heavy hole (hh) subbands have been considered. The cross-over between the lh and hh bands occurs in the case of the transverse plane where the light holes are found to play a more dominant role than the heavy holes. Efforts have been directed towards analysing the transient pulse propagation characteristics such as pulse break-up and optical nutation. We have also studied the transverse electric mode propagation in the QWWS assuming the same to behave as a three-layer asymmetric optical planar waveguide. Numerical analysis made for GaAs/AlGaAs QWWS illuminated by a 150 fs Ti : sapphire laser shows good qualitative agreement of the present results with those available in literature.

Generation of GW radiation pulses from a VUV free-electron laser operating in the femtosecond regime.

Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30-100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95-105 nm.

Optical continua generation in a coherently prepared Raman medium

It is shown that broadband optical continua may be generated as light propagates through a coherently prepared medium. Stimulated Raman scattering is a parametric process responsible for the spectral broadening. Even though the generation of spectral components is limited by the dispersion of the medium, numerical estimates for realistic parameters show feasibility of generating coherent optical continua with total bandwidth of several hundred terahertz without implementation of any dispersion compensating technique. Finally, a technique for producing broadband coherent optical spectra and, thus, ultrashort pulses of radiation is proposed.

Terahertz emission from GaAs and InAs in a magnetic field

We have studied terahertz (THz) emission from InAs and GaAs in a magnetic field, and find that the emitted radiation is produced by coupled cyclotron-plasma charge oscillations. Ultrashort pulses of THz radiation were produced at semiconductor surfaces by photoexcitation with a femtosecond Ti-sapphire laser. We recorded the integrated THz power and the THz emission spectrum as a function of magnetic field at fields up to 5.5 T, and as function of temperature for $T=10--280\mathrm{K}.$ The maximum observed THz power is $\ensuremath{\sim}1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\mathrm{J}/\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}$ (12 \ensuremath{\mu}W average power) from $n\ensuremath{-}\mathrm{InAs}(1.8\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})$ at $B=3.2\mathrm{T}.$ We compare our results to semiclassical models of magnetoplasma oscillations of bulk free carriers and damped motion of free carriers in a two-dimensional electron gas. The bulk model describes THz emission from $n\ensuremath{-}\mathrm{GaAs}$ at all magnetic fields, and InAs at $B=0.$ It fails to describe THz emission from InAs at nonzero magnetic fields. We show that a model including both bulk plasma oscillations and THz emission from a surface accumulation layer describes THz emission from InAs in a moderate magnetic field, but this model does not completely describe emission at fields $|B|g1.0\mathrm{T}.$

VUV and X-ray free-electron lasers

Recent advances in linear accelerators, new developments in laser driven low-emittance electron guns, and the feasibility of ultra-precise long undulators open up the exciting possibility of building single pass Free-Electron Lasers (FELs) based on self-amplified spontaneous emission (SASE). These FELs promise to provide extremely intense, polarised ultra-short pulse radiation for the Vacuum-Ultraviolet (VUV) and X-ray regimes. In addition their high peak and average brilliance, the tunability of the photon energy and the lateral coherence of the radiation will make the FELs to unique sources. World wide several projects for VUV and X-ray FELS have been initiated. At the Deutsches Elektronen Sychroton DESY (Hamburg) at the VUV–FEL (Phase I) for the first time laser emission was observed in the VUV regime. The principle of operation and the components of FELs based on SASE will be discussed in context with the VUV and X-FEL projects. Schemes to overcome the statistical fluctuations and to increase the longitudinal coherence will be presented and examples for basic and applied research with the FELs will be given.

Interaction of light with a semiconductor thin film, taking into account concentration gain of the dipole moment of the excitonic transition

The peculiarities of nonlinear transmission of ultrashort pulses of resonance laser radiation by a semiconductor thin film in the excitonic spectral region have been studied, taking into account the exciton-phonon interaction and concentration gain of the dipole moment of the excitonic transition. It has been shown that under exact resonance conditions the film transmits only the leading edge of the incident rectangular pulse and completely reflects the remaining part. For a nonzero off-resonance, a residual transmission occurs. A series of possibilities for transforming Gaussian pulses is predicted. The state equations for steady-state bistable transmission (reflection) have been derived, and the stability of the solutions has been studied. A theorem of areas, which predicts area restriction of the transmitted pulses, has been stated; namely, the transmitted pulse area cannot exceed π/2.

Quantum Control of Hole Transition Dynamics between Valence Bands

The problem of optimal quantum control of radiative transitions of a free charge carrier between energy bands in semiconductors is considered. Results on simulation of optimal intervalence transitions induced by an ultrashort radiation pulse are presented. Two methods of solving the problem are described. In the first method one assumes a fixed overall shape of an ultrashort optical electric field pulse characterized by a number of free parameters, and then finds an optimal set that maximizes the transition probability. In the second, more general method, the problem of finding an optimal radiation pulse is treated as a constrained minimization problem. A simple example of hole transitions between valence bands is used to illustrate both methods.

The Strathclyde terahertz to optical pulse source (TOPS)

We describe the newly created free-electron laser facility situated at the University of Strathclyde in Scotland, which will produce ultra-short pulses of high-power electromagnetic radiation in the terahertz frequency range. The FEL will be based on a 4 MeV photoinjector producing picosecond 1 nC electron pulses and driven by a frequency tripled Ti:sapphire laser thus ensuring synchronism with conventional laser based tuneable sources. A synchronised multi-terawatt Ti:sapphire laser amplifier will be used in the study of laser/plasma/electron beam interactions and as a plasma based X-ray source. A substantial user commitment has already been made in support of the programme.

Superradiance driven by coherent spontaneous emission in a Cherenkov free-electron maser amplifier

Superradiance (SR) initiated by coherent spontaneous emission (CSE) has been studied in a 35 GHz high gain free-electron Cherenkov maser. We present experimental results that show the development of ultra-short pulses of radiation in the non-linear superradiant regime which are characterised by a quadratic dependence of the intensity on the current. The self-similar pulses that develop have a duration that scales inversely with the fourth root of the intensity leading to three cycle long pulses at the highest intensity (few MW). The non-linear SR regime is preceded by a stage of linear exponential growth of the microwave pulses with a gain length of 1 cm. The superradiant pulse is shown to evolve from a CSE seed by extrapolating the growth curve. Further confirmation of CSE has been obtained by varying the current pulse shape. By varying the slope of the leading edge, and thus the Fourier components of the longitudinal spectral density, we are able to vary the strength of the CSE source. We compare the experimental results with a theoretical analysis.

Generation of ultrashort radiation pulses by injection locking a regenerative free-electron-laser amplifier

We demonstrate how a steady-state train of ultrashort radiation pulses can be produced utilizing a new free-electron laser (FEL) configuration, the injection-locked regenerative klystron amplifier (IRKA). This configuration consists of two elements: (1) a prebuncher, which microbunches the electron beams at the desired output wavelength, and (2) a multipass FEL operated at a very small cavity desynchronism and below the lasing threshold, in the regime of regenerative amplification. The regenerative amplifier is driven by the microbunched electron beam, so that the pulse-to-pulse stability is provided by the pre-buncher. The broad amplification bandwidth of this regenerative amplifier enables generation of ultrashort pulses, much shorter than a slippage length, with high efficiency. The IRKA configuration can produce such ultra-short radiation pulses while avoiding the chaotic dynamics that limits conventional FEL performance. {copyright} {ital 1997} {ital The American Physical Society}