Combination Of Laser Pulses Research Articles

Laser pulse design using optimal control theory-based adaptive simulated annealing technique: vibrational transitions and photo-dissociation

We have designed and optimised a combined laser pulse using optimal control theory-based adaptive simulated annealing technique for selective vibrational excitations and photo-dissociation. Since proper choice of pulses for specific excitation and dissociation phenomena is very difficult, we have designed a linearly combined pulse for such processes and optimised the different parameters involved in those pulses so that we can get an efficient combined pulse. The technique makes us free from choosing any arbitrary type of pulses and makes a ground to check their suitability. We have also emphasised on how we can improve the performance of simulated annealing technique by introducing an adaptive step length of the different variables during the optimisation processes. We have also pointed out on how we can choose the initial temperature for the optimisation process by introducing heating/cooling step to reduce the annealing steps so that the method becomes cost effective.

Coherent polarization beam combination of four mode-locked fiber MOPAs in picosecond regime

In this manuscript, we report on coherent polarization beam combination (CPBC) of a four-channel pulsed amplifier array in the picosecond regime by using single frequency dithering technique. By employing a photo-detector with low-pass bandwidth (8.5 MHz at 10 dB gain) to filter the intensity fluctuation and obtain phase errors for feedback, a combined laser pulse with~480 ps pulse width at~60 MHz repetition rate is obtained with an average power of 88 W. By adjusting the optical path differences (OPDs) and controlling the pump power to ensure the synchronizations and alleviate the influence of nonlinear phases among each channel, more than 90% combining efficiency is achieved with excellent beam quality (M(2)~1.1). Finally, the efficiency loss of the system along with the power scaling process is discussed.

Vibronic and Vibrational Coherences in Two-Dimensional Electronic Spectra of Supramolecular J-Aggregates

In J-aggregates of cyanine dyes, closely packed molecules form mesoscopic tubes with nanometer-diameter and micrometer-length. Their efficient energy transfer pathways make them suitable candidates for artificial light harvesting systems. This great potential calls for an in-depth spectroscopic analysis of the underlying energy deactivation network and coherence dynamics. We use two-dimensional electronic spectroscopy with sub-10 fs laser pulses in combination with two-dimensional decay-associated spectra analysis to describe the population flow within the aggregate. Based on the analysis of Fourier-transform amplitude maps, we distinguish between vibrational or vibronic coherence dynamics as the origin of pronounced oscillations in our two-dimensional electronic spectra.

Evolution of dopant-induced helium nanoplasmas

Two-component nanoplasmas generated by strong-field ionization of doped helium nanodroplets are studied in a pump–probe experiment using few-cycle laser pulses in combination with molecular dynamics simulations. High yields of helium ions and a pronounced resonance structure in the pump–probe transients which is droplet size dependent reveal the evolution of the dopant-induced helium nanoplasma with an active role for He shells in the ensuing dynamics. The pump–probe dynamics is interpreted in terms of strong inner ionization by the pump pulse and resonant heating by the probe pulse which controls the final charge states detected via the frustration of electron–ion recombination.

Momentum spectra for dynamically assisted Schwinger pair production

Recently the dynamically assisted Schwinger mechanism, i.e., electron–positron pair production from vacuum by a combination of laser pulses with different time scales has been proposed. The corresponding results, which suggest that the rate of produced pairs is significantly enhanced by dynamical effects, are verified. Employing the framework of quantum kinetic theory intrinsically enables us to additionally provide momentum space information on the generated positron spectrum.

Ionization of one- and three-dimensionally-oriented asymmetric-top molecules by intense circularly polarized femtosecond laser pulses

We present a combined experimental and theoretical study on strong-field ionization of a three-dimensionally oriented asymmetric top molecule, benzonitrile (C$_7$H$_5$N), by circularly polarized, nonresonant femtosecond laser pulses. Prior to the interaction with the strong field, the molecules are quantum-state selected using a deflector, and 3-dimensionally (3D) aligned and oriented adiabatically using an elliptically polarized laser pulse in combination with a static electric field. A characteristic splitting in the molecular frame photoelectron momentum distribution reveals the position of the nodal planes of the molecular orbitals from which ionization occurs. The experimental results are supported by a theoretical tunneling model that includes and quantifies the splitting in the momentum distribution. The focus of the present article is to understand strong-field ionization from 3D-oriented asymmetric top molecules, in particular the suppression of electron emission in nodal planes of molecular orbitals. In the preceding article [Dimitrovski et al., Phys. Rev. A 83, 023405 (2011)] the focus is to understand the strong-field ionization of one-dimensionally-oriented polar molecules, in particular asymmetries in the emission direction of the photoelectrons.

Isolated intense 38 attosecond pulse generation from the combined laser pulse irradiating the coherent superposition state

A scheme to efficiently generate an intense short isolated attosecond pulse is presented by the combined laser pulse irradiating the coherent superposition state. The combined pulse is composed of a multi-cycle 800 nm laser and a 1600 nm one. Compared with the case of the fundamental laser irradiating the ground state, the cutoff position is significantly extended from 70th to 280th harmonic, a broadband supercontinuum spectrum with 108 eV spectral width can be produced which directly generates an isolated 38 attosecond pulse, the intensity of which is increased eleven orders of magnitude. Our simulation also shows that the value of the time delay has a large range to keep the isolated sub-50-as pulse generation. Moreover, the value of the time delay in the range between ±0.05π is optinal. This result shows that the scheme appears feasible for an experimental demonstration.

An integrated approach to ultraintense laser sciences: The PLASMON-X project

In this paper we discuss recent results in the design configuration and modelling of the PLASMON-X project aimed at the development of an innovative, high-gradient acceleration with super-intense and ultra-short laser pulses, and a tuneable, hard X/γ-ray source, based upon Thomson scattering of optical photons by energetic electrons. Both experiments require very high power, ultra-short laser pulses in combination with very bright and short electron bunches generated either by conventional acceleration (LINAC) or by laser-driven, self injection acceleration in plasmas. The main issues concerning the integrated use of unique laser and linear accelerator installations, and the complementary use of all-optical configurations will be briefly examined.

Controlling the rotation of asymmetric top molecules by the combination of a long and a short laser pulse

We describe theoretically and experimentally a laser-based method to control the rotations of asymmetric top molecules in three-dimensional space. Our method relies on keeping one axis of a molecule essentially fixed in space along the polarization vector of a nanosecond laser pulse (termed the long pulse) and forcing the molecule to rotate about the aligned axis by an orthogonally polarized, femtosecond laser pulse (termed the short pulse). Experimentally, we use femtosecond timed Coulomb explosion to image the three-dimensional (3D) alignment of the 3,5-difluoroiodobenzene molecule as a function of time after the short pulse. Strong 3D alignment is observed a few picoseconds after the short pulse and is repeated periodically, reflecting directly the revolution of the molecular plane about the aligned axis. Our numerical results, based on nonperturbative solution of the time-dependent Schr\odinger equation, are in good agreement with the experimental findings and serve to unravel the underlying physical mechanism of the observations. The experiments and theory explore the influence of the laser parameters on the rotational control, in particular the role played by the intensity of the long and the short laser pulses. To illustrate the generality of our method, we illustrate its applicability to a molecule (3,4-dibromothiophene), with significantly different inertia and polarizability tensors. Finally, our theory shows that the strong 3D alignment obtained by the combined laser pulse method can be converted in to field-free alignment by rapid truncation of the long laser pulse.

Laser mode effects on the ion acceleration during circularly polarized laser pulse interaction with foil targets

Ion acceleration by circularly polarized laser pulses interacting with foil targets is studied using two dimensional particle-in-cell simulations. It is shown that laser pulses with transverse super-Gaussian profile help in avoiding target deformation as compared with the usual Gaussian pulse. This improves monochromaticity of the accelerated ion spectrum. Two kinds of surface instabilities have been found during the interaction. These instabilities can potentially break the target and destroy the quasimonoenergetic character of the final ion spectrum. Combined laser pulses with super-Hermite and Gaussian modes are used to improve the ion acceleration and transverse collimation.

Development of an OPO system at 1.57 μm for integrated path DIAL measurement of atmospheric carbon dioxide

Active remote sensing is a promising technique to close the gaps that exist in global measurement of atmospheric carbon dioxide sources, sinks and fluxes. Several approaches are currently under development. Here, an experimental setup of an integrated path differential absorption lidar (IPDA) is presented, operating at 1.57 μm using direct detection. An injection seeded KTP-OPO system pumped by a Nd:YAG laser serves as the transmitter. The seed laser is actively stabilized by means of a CO2 reference cell. The line-narrowed OPO radiation yields a high spectral purity, which is measured by means of a long path absorption cell. First measurements of diurnal variations of the atmospheric CO2 mixing ratio using a topographic target were performed and show good agreement compared to simultaneously taken measurements of an in situ device. A further result is that the required power reference measurement of each laser pulse in combination with the spatial beam quality is a critical point of this method. The system described can serve as a testbed for further investigations of special features of the IPDA technique.

Quantum Control for Broadband Attosecond Pulse Generation

In our work, we propose a scheme for coherent control the HHG with an ultrashort laser pulse in combination with a controlling pulse. It is shown that this method can significantly modulate the electron dynamics and a broadband supercontinuous high harmonics can be generated. Then an isolated sub-100 attosecond pulse can be obtained, which allows one to measure a wide range of ultrafast dynamics not normally accessible before.

Efficient extension and enhancement of high-order harmonics of He+ by combined laser pulses

We theoretically study the interaction of a He+ ion with fundamental laser pulses and different combined laser pulses. For low laser intensity the cut-off energy is extended; however, for higher laser intensity the cut-off of the enhanced harmonics is extended under the combination of two fundamental laser pulses whose wavelengths are 800 nm and 394 nm, respectively. We investigate the yield of ionization as a function of peak laser intensity and find that the yield of ionization is different for low and higher laser intensity regimes. The efficient extension of cut-off energy of enhanced harmonics is demonstrated by a classical analysis.

Attosecond ionization gating for isolated attosecond electron wave packet and broadband attosecond xuv pulses

An attosecond ionization gating is achieved using a few-cycle laser pulse in combination with its second harmonic. With this gating, the generation of the electron wave packet (EWP) is coherently controlled, and an isolated EWP of about $270\phantom{\rule{0.3em}{0ex}}\mathrm{as}$ is generated. An isolated broadband attosecond extreme ultraviolet pulse with a bandwidth of about $75\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ can also be generated using this gating, which can be used for EWP measurements as efficiently as a $50$-as pulse, allowing one to measure a wide range of ultrafast dynamics not normally accessible before.

Isolated sub-100‐aspulse generation via controlling electron dynamics

A new method to coherently control the electron dynamics is proposed using a few-cycle laser pulse in combination with a controlling field. It is shown that this method not only broadens the attosecond pulse bandwidth, but also reduces the chirp, then an isolated 80-as pulse is straightforwardly obtained and even shorter pulse is achievable by increasing the intensity of the controlling field. Such ultrashort pulses allow one to investigate ultrafast electronic processes which have never be achieved before. In addition, the few-cycle synthesized pulse is expected to manipulate a wide range of laser-atom interactions.

Triplet state and phenoxyl radical formation of 3, 4‐methylenedioxy phenol: a combined laser flash photolysis and pulse radiolysis study

AbstractFormations of triplet state, molecular cation radical, and phenoxyl radical of 3,4‐methylenedioxy phenol (sesamol, SOH) in organic solvents have been investigated by laser photolysis as well as pulse radiolysis techniques. Photolysis of SOH in cyclohexane has been found to produce both triplet state (λmax ∼ 480 nm) and phenoxyl radical (425–430 nm) of SOH by mono‐photonic processes. However, radical cation (λmax = 450 nm) and phenoxyl radical of SOH have been observed on radiolysis in cyclohexane. Further, radiolysis of SOH in benzene has been found to produce phenoxyl radical only. Mechanism of phenoxyl radical formation by photo‐excitation of SOH has been studied and triplet energy level of SOH is estimated to lie between 1.85 and 2.64 eV. Copyright © 2007 John Wiley & Sons, Ltd.

Efficient quantum logic with cold trapped ions for maximally entangled states

We propose the efficient combinations of laser pulses for constructing a multiqubit gate with cold trapped ions in order to implement maximally entangled states with quantum logic. Within the ion-trap quantum computation using the center of mass as the data bus, we can minimize the number of laser pulses as well as their operation time, which can improve the fidelity. We also present some numerical results of the operation time for different numbers of qubits and values of the Lamb-Dicke parameter.

Single X-Ray Attosecond Pulse Generation by Using CombinedPulses Irradiating on a United Two-Atom System

A scheme of a single x-ray attosecond pulse generation from a two-atomsystem exposed to the combined laser pulses is proposed. Our numericalresults show that a single x-ray attosecond pulse rather than a trainone can be produced by modulation of ionization. Furthermore,when we change the peak intensity Ih of the high-frequencypulse and keep the intensity of the low-frequency pulse constant, wecan find that a range of Ih where the intensity of theattosecond pulse is optimal is available, and a explanation by thestimulated property of the recombination is also presented.

Experimental study of a transient X-ray laser pumped by a picosecond pulsed laser

This paper presented the results of experimental studies on transient soft X-ray laser using a picosecond pulsed laser facility.A rather intensive Ni-like Ag X-ray laser at 13.9nm with output energy of about 5—10nJ was obtained by irradiating the solid flat targets with a several hundred picosecond long laser pulse in combination with an 1ps ultra-short laser pulse.

Laser safety aspects for refractive eye surgery with femtosecond laser pulses

Abstract We report on laser safety aspects for near infrared femtosecond laser refractive surgery. In particular, the transmittance of microjoule laser pulses at 1040 nm through the cornea during flap procedures based on femtosecond laser induced multiphoton ionization and photodisruption has been determined. When using focusing optics with a numerical aperture of 0.3, more than 20% of the incident NIR photons are propagating towards the retina. In addition, self-focusing, white light and second harmonic generation, and destructive photodisruptive side effects have to be considered when using such high energy laser pulses of amplified laser systems. Microjoule femtosecond laser pulses in combination with low NA objectives have the potential to induce destructive intraocular side effects. Further studies are required to evaluate the damage potential of the transmitted photons absorbed by the retinal pigment epithelium and other intraocular compartments. Because of the fact that flaps can be also generated with low nanojoule energy femtosecond laser pulses of non-amplified MHz lasers in combination with high NA objectives, a compromise between procedure time, pulse energy and numerical apertures has to be found for safe ocular femtosecond laser surgery.