Nonresonant Laser Pulses Research Articles

The management of response time Stark echo and its intensity in the presence of non-resonant laser pulses with spatial inhomogeneity

The possibility of management of the response time Stark echo and the influence of a transverse irreversible relaxation on its intensity by change of the mutual orientation of external spatial non-uniform non-resonant laser pulses are analyzed.

Theory of strong-field injection and control of photocurrent in dielectrics and wide band gap semiconductors

We propose a theory of optically induced currents in dielectrics and wide gap semiconductors exposed to a nonresonant ultrashort laser pulse with a stabilized carrier-envelope phase. To describe strong-field electron dynamics, equations for the density matrix have been solved self-consistently with equations for the macroscopic electric field inside the medium, which we model by a one-dimensional potential. We provide a detailed analysis of physically important quantities (band populations, macroscopic polarization, and transferred charge), which reveals that carrier-envelope phase control of the electric current can be interpreted as a result of quantum-mechanical interference of multiphoton excitation channels. Our numerical results are in good agreement with experimental data.

Formation of stimulated photon echo by nonresonant laser pulses

pulses. We show that a slight difference between the wavelengths of the nonresonant laser pulses leads to the effect of locking the stimulated photon echo responses. Introduction. In designing optical memory devices, echo holographic processing of information is of interest, which presumes an efficient mechanism for erasing and associative sampling of information. In (1, 2) it is shown that for these purposes, the most promising is the effect of "locking" recorded echo holographic information i.e., creating conditions such that it cannot develop as a response of the resonant medium, which can be accomplished by disrupt- ing the frequency-time correlation of the inhomogeneous broadening of the resonance line in different time intervals. The frequency-time correlation of the inhomogeneously broadened line for the resonant transition is associated with strict correspondence of individual isochromats of the line in different time intervals. Each isochromat of the inhomo- geneously broadened line is formed by an ensemble of optical centers found under identical conditions but distributed randomly within the volume of the sample. The process of formation of photon echo responses consists of two main stages: dephasing of the oscillating dipole moments of the optical centers, followed by their rephasing, leading to the appearance of macroscopic polarization in the medium, observed as a coherent response. A slight disruption of the strict frequency-time correlation for the inhomogeneous broadening in different time intervals should lead to significant weakening of the intensity of the response. In other words, we are dealing with reversible disruption of the phase memory of the resonant medium, with the possibility of recovering it. Such an effect can be achieved by application of different spatially inhomogeneous external perturbations to the resonant medium (in different time intervals), leading to random shifts or splitting of the original isochromats of the inhomogeneously broadened line. We note that in (1), we theoretically predicted and experimentally confirmed the effect of locking a long-lived photon echo in a LaF 3 :Pr 3+ crystal (the transition 3 H4(0)- 3 P0, λ = 477.7 nm) when an inhomogeneous electric field is applied in the time interval between the first and second laser pulses. In (2), we studied the effectiveness of suppressing the stimulated photon echo (SPE) response for different schemes for application of spatially inhomogeneous electric fields to a resonant medium. In this work, we study the effect of locking the stimulated photon echo in the case when nonresonant laser radiation (standing wave) acts as the inhomogeneous external perturbation leading to random shifts or splittings of the original isochromats for an inhomogeneously broadened line. In this case, the spatial inhomogeneity is connected with the variation in the electric field strength for the laser radiation within one wavelength (alternation of antinodes and nodes in the standing wave). Reversible Disruption of the Phase Memory of the Resonant Medium in the Presence of Nonresonant

The Long-Lived Photon Echo Response Locking Effect in the Presence of External Non-Resonant Laser Pulses with a Different Spatial Orientation

The correlation of the inhomogeneous broadening of the resonance transition at different time intervals and the efficiency of long-lived photon echo response locking by the action of standing waves of non-resonant laser pulses are considered. It is shown that the long-lived photon echo response locking effect may be observed even at the angles of the relative orientation of the non-resonant standing wave laser pulses of less than one degree, due to the change in the correlation coefficient of inhomogeneous broadening at time intervals between the first and the second and after the third resonant laser pulses.

High-Harmonic Spectroscopy of Oriented OCS Molecules: Emission of Even and Odd Harmonics

We study the emission of even and odd high-harmonic orders from oriented OCS molecules. We use an intense, nonresonant femtosecond laser pulse superimposed with its phase-controlled second harmonic field to impulsively align and orient a dense sample of molecules from which we subsequently generate high-order harmonics. The even harmonics appear around the full revivals of the rotational dynamics. We demonstrate perfect coherent control over their intensity through the subcycle delay of the two-color fields. The odd harmonics are insensitive to the degree of orientation, but modulate with the degree of axis alignment, in agreement with calculated photorecombination dipole moments. We further compare the shape of the even and odd harmonic spectra with our calculations and determine the degree of orientation.

Effect of inhomogeneous electric field intensities on stark echo formation

We show that it is possible to control the appearance time of the Stark echo response by varying the intensity of inhomogeneous electromagnetic fields. We found that when the system is exposed to nonresonant laser pulses with spatial inhomogeneity, we observe a shift of the Stark echo appearance time in the nanosecond range.

Enhancing strong-field-induced molecular vibration with femtosecond pulse shaping

This work investigates the utility of femtosecond pulse shaping in increasing the efficiency of Raman excitation of molecules in the strong-field interaction regime. We study experimentally and theoretically the effect of pulse shaping on the strength of non-resonant coherent anti-Stokes Raman scattering in iodine vapor at laser intensities exceeding $10^{13}$ W/cm$^2$. We show that unlike the perturbative case, shaping strong non-resonant laser pulses can increase the signal strength beyond that observed with the transform-limited excitation. Both adiabatic and non-adiabatic schemes of excitation are explored, and the differences of their potential in increasing the excitation efficiency are discussed.

Nuclear spin selective laser control of rotational and torsional dynamics

We explore the possibility of controlling rotational-torsional dynamics of non-rigid molecules with strong, non-resonant laser pulses and demonstrate that transient, laser-induced torsional alignment depends on the nuclear spin of the molecule. Consequently, nuclear spin isomers can be manipulated selectively by a sequence of time-delayed laser pulses. We show that two pulses with different polarization directions can induce either overall rotation or internal torsion, depending on the nuclear spin. Nuclear spin selective control of the angular momentum distribution may open new ways to separate and explore nuclear spin isomers of polyatomic molecules.

Modifying molecule-surface scattering by ultrashort laser pulses

In recent years it became possible to align molecules in free space using ultrashort laser pulses. Here we explore two schemes for controlling molecule-surface scattering process, which are based on the laser-induced molecular alignment. In the first scheme, a single ultrashort non-resonant laser pulse is applied to a molecular beam hitting the surface. This pulse modifies the angular distribution of the incident molecules, and causes the scattered molecules to rotate with a preferred sense of rotation (clockwise or counter-clockwise). In the second scheme, two properly delayed laser pulses are applied to a molecular beam composed of two chemically close molecular species (isotopes, or nuclear spin isomers). As the result of the double pulse excitation, these species are selectively scattered to different angles after the collision with the surface. These effects may provide new means for the analysis and separation of molecular mixtures.

Selective control of vibrational modes with sequential femtosecond-scale laser pulses

For nonresonant femtosecond-scale laser pulses, we showed earlier that the optimum FWHM pulse duration is 0.42 T if one wishes to achieve maximum relative excitation of a specific vibrational mode with period T. Here we show that much greater enhancement of a specific mode can be achieved with a sequence of laser pulses which have both this optimized duration and optimized delays between pulses. One can thus enhance a selected set of modes that are particularly useful in identifying or characterizing a chemical system, to lift them out of the background of less informative modes.

Nonadiabatic alignment of van der Waals--force-bound argon dimers by femtosecond laser pulses

We demonstrated that the weak van der Waals–force-bound argon dimer can be nonadiabatically aligned by nonresonant femtosecond laser pulses, showing periodic alignment and anti-alignment revivals after the extinction of the laser pulse. Based on the measured nonadiabatic alignment trace, the rotational constant of the argon dimer ground state is determined to be B0 = 0.05756 ± 0.00004 cm −1 . Noticeable alignment dependence of frustrated tunneling ionization and bond-softening induced dissociation of the argon dimer are observed.

Nuclear spin selective alignment of ethylene and analogues

We investigate the alignment of ethylene and of some of its analogues via short, non-resonant laser pulses and show that it depends crucially on the nuclear spin of the molecules. We calculate the time-dependent alignment factors of the four nuclear spin isomers of ethylene and analyze them by comparison with the symmetric top molecule allene. Moreover, we explore how the nuclear spin selective alignment depends on the asymmetry of the molecules and on the intensity of the laser pulse. As an application, we discuss how nuclear spin selective alignment could be applied in order to separate different isotopomers of ethylene.

Field-Free Molecular Orientation Induced by Nonresonant Square Laser Pulses

We propose a scheme to achieve field-free molecular orientation by a nonresonant square laser pulse. Using CO molecules as an example, we show that, differing from the conventional Gaussian pulse, field-free molecular orientation excited by square pulses can be realized in both nonadiabatic and adiabatic cases. We also show that the maximum degree of the molecular orientation can be further enhanced by separating one square pulse into two time delayed subpulses and by applying the second subpulse at the beginning of the rotational wave packet rephrasing created by the first subpulse and the optimal intensity ratio between the two subpulses is about I1/I2 = 1 : 1.5.

Redistribution of vibrational population in a molecular ion with nonresonant strong-field laser pulses

We present an experimental demonstration of nonresonant manipulation of vibrational states in a molecule by an intense ultrashort laser pulse. A vibrational wave packet is generated in ${\mathrm{D}}_{2}{}^{+}$ through tunnel ionization of ${\mathrm{D}}_{2}$ by a few-cycle pump pulse. A similar control pulse is applied as the wave packet begins to dephase so that the dynamic Stark effect distorts the electronic environment of the nuclei, transferring vibrational population. The time evolution of the modified wave packet is probed via the ${\mathrm{D}}_{2}{}^{+}$ photodissociation yield that results from the application of an intense probe pulse. Comparing the measured yield with a quasiclassical trajectory model allows us to determine the redistribution of vibrational population caused by the control pulse.

Nonresonant Femtosecond Laser Vaporization with Electrospray Postionization for ex vivo Plant Tissue Typing Using Compressive Linear Classification

Laser electrospray mass spectrometry (LEMS) with offline classification is used to discriminate plant tissues at atmospheric pressure using an intense (10(13) W cm(-2)), nonresonant (800 nm) femtosecond laser pulse to vaporize cellular content for subsequent mass analysis. The tissue content of the plant within the 0.05 mm(2) laser interaction region is vaporized into the electrospray plume where the molecules are ionized prior to transfer into the mass spectrometer. The measurements for a flower petal, leaf, and stem of an impatiens plant reveal mass spectral signatures that enable discrimination as performed using a compressive linear classifier. The statistical analysis of the plant tissue samples reveals reproducibility of the data for replicate tissue samples and within a single tissue sample. A similar degree of discrimination was achieved for the green and white regions of aphelandra squarrosa (zebra plant) leaves.

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.

Mass Analysis of Biological Macromolecules at Atmospheric Pressure Using Nonresonant Femtosecond Laser Vaporization and Electrospray Ionization

A nonresonant femtosecond laser pulse, with an intensity of 10(13) Wcm(-2), vaporizes proteins and biomolecules intact, regardless of molecular structure, size or electronic structure for subsequent electrospray ionization and transfer into a mass spectrometer. Rapid, direct analysis from dried sample, aqueous solution and cellular material is demonstrated at atmospheric pressure using laser electrospray mass spectrometry (LEMS). Measurements are presented for lysozyme (14.3 kDa), hemoglobin from human blood, ovalbumin (45 kDa) from hen egg white and phospholipids from hen egg yolk. Mass analysis of biological material is performed without dilution, extraction or sample preparation, other than placing the biological material onto the sample plate.

Linear Response of Multiphoton Reaction: Three-Photon Cycloreversion of Anthracene Biplanemer in Solution by Intense Femtosecond Laser Pulses

The photocycloreversion of anthracene photodimers and biplanemer in solution was investigated by nonresonant intense femtosecond laser pulses. Cycloreversion of biplanemer showed a pseudolinear response to laser intensity whereas the formation of anthracene from photodimer was proportional to the cubic of laser intensity. The unusual intensity dependence of biplanemer was explained in terms of the sum of three-photon intramolecular cycloreversion and the recovery of reactant by a two-photon intramolecular cyclodimerization. The coexistence of high- and low-order multiphoton processes within the same laser pulse originated in the spatial distribution of the laser intensity. We observed white light emerging from the sample solution; however, the effect of solvated electrons was not observed in the present system. The saturation of both the photoreaction and white light due to a volume effect was observed at high intensity.

Coherent rotational excitation by intense nonresonant laser fields

The rotation of molecules in the gas phase can be coherently excited by irradiation with strong nonresonant short laser pulses, interacting with the molecular anisotropic polarisability. Such coherent rotational excitation has been attracting much attention because of the intriguing nature of the rotational wave packet thus created, and its wide applicability to dynamical studies and advanced optics. In this review article, we first survey various experimental schemes adopted so far for externally controlling molecular rotation, and then describe a new approach based on a quantum-state resolved spectroscopic probe for investigating coherent rotational excitation by intense nonresonant laser fields. Representative examples are given to show how the method provides detailed information on excitation pathways in wave-packet creation, and how it realises full quantum-state reconstruction of the rotational wave packet in a favourable case. We also describe an advanced wave-packet control, i.e. the creation and characterisation of a unidirectionally rotating wave packet, and discuss a further extension of this approach to explore coherent vibrational excitation.

Quasi-classical model of non-destructive wavepacket manipulation by intense ultrashort nonresonant laser pulses

A quasi-classical model (QCM) of nuclear wavepacket generation, modification and imaging by three intense ultrafast near-infrared laser pulses has been developed. Intensities in excess of 1013 W cm-2 are studied, the laser radiation is non-resonant and pulse durations are in the few-cycle regime, hence significantly removed from the conditions typical of coherent control and femtochemistry. The 1sσ ground state of the D2 precursor is projected onto the available electronic states in D2+ (1sσg ground and 2pσu dissociative) and D++D+ (Coulomb explosion) by tunnel ionization by an ultrashort ‘pump’ pulse, and relative populations are found numerically. A generalized non-adiabatic treatment allows the dependence of the initial vibrational population distribution on laser intensity to be calculated. The wavepacket is approximated as a classical ensemble of particles moving on the 1sσg potential energy surface (PES), and hence follow trajectories of different amplitudes and frequencies depending on the initial vibrational state. The ‘control’ pulse introduces a time-dependent polarization of the molecular orbital, causing the PES to be modified according to the dynamic Stark effect and the transition dipole. The trajectories adjust in amplitude, frequency and phase-offset as work is done on or by the resulting force; comparing the perturbed and unperturbed trajectories allows the final vibrational state populations and phases to be determined. The action of the ‘probe’ pulse is represented by a discrete internuclear boundary, such that elements of the ensemble at a larger internuclear separation are assumed to be photodissociated. The vibrational populations predicted by the QCM are compared to recent quantum simulations (Niederhausen and Thumm 2008 Phys. Rev. A 77 013404), and a remarkable agreement has been found. The applicability of this model to femtosecond and attosecond time-scale experiments is discussed and the relation to established femtochemistry and coherent control techniques are explored.