Showing papers on "High harmonic generation published in 2003"

Colloquium: Aligning molecules with strong laser pulses

Abstract: We review the theoretical and experimental status of intense laser alignment---a field at the interface between intense laser physics and chemical dynamics with potential applications ranging from high harmonic generation and nanoscale processing to stereodynamics and control of chemical reactions After placing the intense laser approach in context with other alignment techniques, we proceed with a discussion of the physics underlying this technique and a description of methods of observing it in the laboratory The roles played by the laser frequency, the pulse duration, and the system temperature are illustrated numerically and experimentally Alignment is extended to three-dimensional orientational control, a method of hindering the rotation about all three axes of polyatomic molecules We conclude with a discussion of potential applications of intense laser alignment

Multiphoton photonics: quasi-phase-matched generation of coherent extreme-ultraviolet light

Abstract: We demonstrate quasi-phase-matched frequency conversion of laser light into the extreme-ultraviolet via high harmonic generation in a microstructured hollow waveguide. This engineered waveguide extends the cutoff phase-matched energy by 100 eV over a straight waveguide.

Analysis of second-harmonic generation of Lamb modes using a modal analysis approach

Abstract: An effective technique for analyzing the generation of second harmonics of Lamb modes in elastic plates is presented. The nonlinearity of the wave equation governing the wave propagation ensures that there is second-harmonic generation accompanying primary Lamb mode propagation. This nonlinearity may be treated as a second-order perturbation of the linear elastic response. Using a second-order perturbation approximation and a modal analysis approach, the complicated problems of the generation of second harmonics of Lamb modes have been investigated. The fields of the second harmonics of Lamb modes in elastic plates are considered as superpositions of the fields of a series of double-frequency Lamb modes. The solutions provide physical insight into the generation of second harmonics with a cumulative growth effect, and the corresponding second-harmonic solutions. Although Lamb modes are dispersive, the cumulative growth effect of the second harmonics does exist under some conditions. The influence of Lamb mode dispersion on second-harmonic generation is considered.

Phase-controlled amplification of few-cycle laser pulses

Abstract: Intense ultrashort waveforms of light that can be produced with an exactly predetermined electromagnetic field are essential in a number of applications of extreme nonlinear optics, most prominently in laser-driven sources of high-energy attosecond radiation. Field reproducibility in each laser shot requires stabilization of the carrier-envelope phase. The authors analyze different schemes of phase-stable pulse amplification and identify constraints limiting the precision with which the phase can be maintained. Next, they describe a phase-stabilized laser system based on a 20-fs multipass Ti:sapphire amplifier supplemented with a fiber compression stage for producing pulses in the few-cycle regime. It is shown that the amplifier introduces only a slow millihertz phase drift and, therefore, can be seeded by a standard phase-stabilized oscillator. This residual phase drift is assigned primarily to the beam pointing instability and can be precompensated in the phase-control loop of the seed oscillator using a feedback signal from a phase detector placed in the amplifier output. The phase stability of the resultant 5-fs 400-/spl mu/J pulses at a 1-kHz repetition rate is subsequently independently verified by higher order harmonic generation, in which different carrier-envelope phase settings are shown, both theoretically and experimentally, to produce distinctly different spectral shapes of the XUV radiation. From a series of such spectral patterns, the authors succeed in calibrating the value of the carrier envelope phase (with a /spl plusmn//spl pi/ ambiguity), which in turn allows them to fully characterize the temporal structure of the electric field of the laser pulses. The estimated precision of the phase control on the XUV target is better than /spl pi//5, which reduces the timing jitter between the driving laser pulse and the XUV bursts to /spl sim/ 250 as and opens the way to generate stable isolated attosecond pulses.

First ultraviolet high-gain harmonic-generation free-electron laser.

Abstract: We report the first experimental results on a high-gain harmonic-generation (HGHG) free-electron laser (FEL) operating in the ultraviolet. An 800 nm seed from a Ti:sapphire laser has been used to produce saturated amplified radiation at the 266 nm third harmonic. The results confirm the predictions for HGHG FEL operation: stable central wavelength, narrow bandwidth, and small pulse-energy fluctuation.

Global optimization of high harmonic generation.

Abstract: We investigate the relevance of the absorption limit concept in the optimization of high harmonic generation. Thanks to the first direct observation of the coherence length of the process from high-contrast Maker fringes, we unravel experimental conditions for which the harmonic dipole response is enhanced when phase matching is realized within the absorption limit, leading to record conversion efficiencies in argon. Moreover, we show that harmonic generation in guided or freely propagating geometries are equivalent in the loose focusing regime. This analysis is generalized to other advanced phase-matching schemes, thereby predicting the possibility to boost the conversion efficiencies using light noble gases.

Photoemission and ionization of He+ under simultaneous irradiation of fundamental laser and high-order harmonic pulses.

Abstract: We theoretically study the response of He+ exposed simultaneously to an intense Ti:sapphire laser and its 27th or 13th harmonic pulses. High-order harmonic emission from He+ is enhanced by many orders of magnitude compared with the case of the fundamental pulse alone. Moreover, while an individual 10 fs laser (wavelength lambda(F)=800 nm) or its 27th harmonic pulse with a peak intensity of 3 x 10(14) and 10(13) W/cm(2), respectively, ionizes no more than 5 x 10(-6) of He+, their combined pulse leads to a surprisingly high He2+ yield of 17%.

X-ray generation in an ion channel

Abstract: X-ray generation by relativistic electrons in an ion channel is studied. The emission process is analyzed in the regime of high harmonic generation when the plasma wiggler strength is large. Like for the conventional free electron laser, the synchrotron-like broadband spectrum is generated in this regime. An asymptotic expression for the radiation spectrum of the spontaneous emission is derived. The radiation spectrum emitted from an axisymmetric monoenergetic electron beam is analyzed. The stimulated emission in the ion channel is studied and the gain of the ion-channel synchrotron-radiation laser is calculated. It is shown that the use of laser-produced ion channels leads to a much higher power of x-ray radiation than the one in a self-generated channel. In addition, the mean photon energy, the number of emitted photons and the brilliance of the photon beam increase dramatically. Three-dimensional particle-in-cell simulations of a 25-GeV electron bunch propagating in a laser-produced ion channel are mad...

High-order above-threshold ionization with few-cycle pulse: a meter of the absolute phase

Abstract: Angle-resolved energy spectra of high-order above-threshold ionization are calculated in the direction of the laser polarization for a linearly polarized four-cycle laser pulse (two cycles FWHM) as a function of the carrier-envelope relative phase (absolute phase). The spectra exhibit a characteristic left-right (backward-forward) asymmetry, which should allow one to determine the value of the absolute phase in a given experiment by comparison with the theoretical spectra. A classical analysis of the spectra calculated is presented. High-energy electron emission is found to occur in one or two ultrashort (< inverted exclamation mark<< 0.7 fs) bursts. In the latter case, the spectra display a peak structure whose analysis reveals a time-domain image of electron emission.

Phase-matched third harmonic generation in microstructured fibers

Abstract: Strong guiding provided by the high-delta microstructured fibers allows for efficient intermodally phase-matched harmonic generation with femtosecond pumping at telecom wavelengths. Visible harmonics are generated in a number of distinct transverse modes of the structure. We present a detailed experimental and theoretical study of the third harmonic generation in such fibers including phase-matching wavelengths, far-field intensity distributions and polarization dependence. Good agreement between the theory and experiment is achieved.

Time-gated high-order harmonic generation

Abstract: We generate high-order harmonics by using an 800-nm fundamental pulse whose polarization evolves with time. Controlling the ellipticity modulation of the fundamental field allows us to continuously confine the harmonic emission from an estimated minimum value of 7 fs $(1\mathrm{fs}{=10}^{\ensuremath{-}15}\mathrm{s})$ up to more than the 35 fs input pulse duration. Depending on the observed harmonic, the harmonic spectrum can show either a narrowing or a broadening when the ellipticity is changed in good agreement with an effective confinement of the high-order harmonic generation.

Real-time second-harmonic-generation microscopy based on a 2-GHz repetition rate Ti:sapphire laser

Abstract: The problem of weak harmonic generation signal intensity limited by photodamage probability in optical microscopy and spectroscopy could be resolved by increasing the repetition rate of the excitation light source. Here we demonstrate the first photomultiplier-based real-time second-harmonic-generation microscopy taking advantage of the strongly enhanced nonlinear signal from a high-repetition-rate Ti:sapphire laser. We also demonstrate that the photodamage possibility in common biological tissues can be efficiently reduced with this high repetition rate laser at a much higher average power level compared to the commonly used ~80- MHz repetition rate lasers.

High-order harmonic generation at a repetition rate of 100 kHz

Abstract: We report high-order harmonic generation (HHG) in rare gases using a femtosecond laser system with a very high repetition rate (100 kHz) and low pulse energy $(7\ensuremath{\mu}\mathrm{J}).$ To our knowledge, this is the highest repetition rate reported to date for HHG. The tight focusing geometry required to reach sufficiently high intensities implies low efficiency of the process. Harmonics up to the 45th order are nevertheless generated and detected. We show evidence of clear separation and selection of quantum trajectories by moving the gas jet with respect to the focus, in agreement with the theoretical predictions of the semiclassical model of HHG.

Generation of vacuum-ultraviolet light by an optically contacted, prism-coupled KBe2BO3F2 crystal.

Abstract: We have demonstrated the second harmonic of a frequency-tripled Nd:YVO4 laser with 25-mW quasi-cw output by using an optically contacted, prism-coupled KBe2BO3F2 crystal We also achieved the second harmonic with a frequency-doubled single-mode Ti:sapphire laser at 1725 nm and sum-frequency mixing with a dual-wavelength Ti:sapphire laser at 1633 nm These wavelengths are to our knowledge the shortest obtained by use of nonlinear crystals for second-harmonic generation and sum-frequency mixing, respectively

Native tissue imaging at superharmonic frequencies

Abstract: The second harmonic imaging mode has been adapted to image tissue and shown considerable improvements in image quality in several applications compared to the fundamental mode. The improvements were attributed to the effects of wave distortion due to nonlinear propagation in tissue. However, imaging tissue at the second harmonic frequency only has various drawbacks. Because the energy in the second harmonic frequency band is much lower than that in the fundamental frequency band, there must be excellent sensitivity and dynamic range in the receiving system to achieve an acceptable amount of signal-to-noise ratio. To increase the sensitivity, the spectral overlap between the fundamental and the second harmonic has to be diminished, which in return deteriorates the imaging resolution. Consequently, a trade-off is mandatory between resolution and sensitivity. Using simulations and measurements, we show that, at appropriate scanning acoustic settings, higher harmonics are generated in tissue. The higher harmonics represent additional, relevant information for tissue imaging and characterization. An elegant way to take advantage of the higher harmonics and to bring all the information together is to combine and incorporate all the multiple higher harmonics into a single component that we call the superharmonic component. Using a newly developed array transducer having a wide frequency band, B-mode images of a phantom were made in the superharmonic mode transmitting at 1.2 MHz. These images have exceptionally improved clarity and yield a dramatically cleaner and sharper contrast between the different structures being imaged. In addition to increased signal-to-noise ratio, superharmonic imaging shows better contrast and axial resolution as well as acceptable penetration depth.

Orientation dependence of high-order harmonic generation in molecules

Abstract: We present two- and three-dimensional model calculations of high-order harmonic generation in ${\mathrm{H}}_{2}^{+}.$ The harmonic spectra exhibit clear signatures of intramolecular interference. An interference minimum appears at a harmonic order that depends on the molecular orientation. Harmonic generation in three-center molecules is studied on the basis of two-dimensional calculations for a ${\mathrm{H}}_{3}^{2+}$ model system. From analytical considerations, the orientation dependence of the harmonic intensities in three-center molecules exhibits a double minimum due to intramolecular interference. In the numerical results, the double minimum is broadened into a single wide minimum. The effect of nonzero laser ellipticity on harmonic generation is investigated by means of two-dimensional simulations for ${\mathrm{H}}_{2}^{+}.$ We find that harmonic generation with elliptical polarization is governed by interference effects similar to linear polarization.

Attosecond angle-resolved photoelectron spectroscopy.

Abstract: We report experiments on the characterization of a train of attosecond pulses obtained by high- harmonic generation, using mixed-color (XUVIR) atomic two-photon ionization and electron detection on a velocity map imaging detector. We demonstrate that the relative phase of the harmonics is encoded both in the photoelectron yield and the angular distribution as a function of XUV-IR time delay, thus making the technique suitable for the detection of single attosecond pulses. The timing of the attosecond pulse with respect to the field oscillation of the driving laser critically depends on the target gas used to generate the harmonics.

Roles of resonances and recollisions in strong-field atomic phenomena. II. High-order harmonic generation

Abstract: The theoretical developments presented in a preceding companion paper by Wassaf et al. [Phys. Rev. A $67,$ 053405 (2003)], for simulating photoelectron spectra, are used to address several issues regarding the harmonic generation process. Both above-threshold Ionization (ATI) and high-order harmonic generation are observed when atoms are submitted to a laser field with intensity around ${I=10}^{14}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2}.$ Here, we demonstrate that the resonances, together with multiple recollisions processes, which have been shown to be at the origin of enhancements of the magnitudes of ATI peaks in the high-energy range, can also play a determining role on the magnitudes of harmonic lines within the plateau. These findings have been obtained via a set of quantum and classical simulations for two classes of one-dimensional model potentials, i.e., either long range (Coulomb-like) or short range (with an exponentially decreasing tail). They are confirmed by following the time evolution of the emission rate of selected harmonics with the help of a (waveletlike) Gabor time-frequency analysis.

Anomalies in high-order harmonic generation at relativistic intensities

Abstract: High-order harmonic generation from a solid target surface has been investigated using femtosecond laser pulses focused to intensities greater than ${10}^{18}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ The experiments show that the harmonics are very intense, with a conversion efficiency that is one or two orders of magnitude larger than that of harmonics generated in gases. Beside the observation of presently the shortest wavelength harmonics from femtosecond-laser solid target interaction, i.e., down to 22 nm, an anomaly has been observed in the harmonic spectrum. In contrast to the expected well-known continuous ``roll off'' of the high-harmonic orders, the harmonic intensity decreases with the increase of harmonic order, but in between shows minima which are significantly less intense than the neighboring harmonics. Furthermore, the order of the harmonic minima depend on target material. Additional calculations using numerical kinetic particle simulations and a simpler oscillating mirror model show that the physical origin of these modulations is an intricate interplay of resonance absorption and ponderomotive force which leads to a complex electron density profile evolution. Furthermore, this is emphasized by a spectral line analysis of the harmonics. In agreement with the theory, broad lines have been observed and, in particular for the harmonics in the minima, a complex interference structure is present.

Laser-induced nonlinear excitation of collective electron motion in a cluster

Abstract: We consider the nonlinear collective electron dynamics inside a large cluster irradiated by a strong linearly polarized short (0.1 ?ps) laser pulse. The equation of the centre-of-mass motion of the electron cloud driven by the strong laser field is derived using the approximation of an incompressible medium. The analysis of this equation demonstrates the presence of odd harmonics of the fundamental frequency in the cluster dipole moment, and in both the internal and the scattered electric field. Both neutral and ionized clusters are considered. For clusters with radii R 100?? irradiated by a femtosecond titanium?sapphire laser with a peak intensity of I 1016?W?cm?2, the internal electric field strength near the tripled fundamental frequency is shown to be of the same order as the field of the fundamental. The reason is that both for metallic and for laser-ionized van der Waals clusters the Mie surface-plasmon energy ?M is around 5 eV, which is close to three times the energy of a titanium?sapphire laser-field quantum. On the other hand, the condition for first-order resonance with the Mie frequency is not met during the presence of the main laser pulse, but only temporarily, either at the first onset of inner ionization on the leading edge of the pulse (for van der Waals clusters) or during the subsequent Coulomb explosion. In both cases, the ion density is reduced. The presence of a strong third harmonic leads, in particular, to the enhanced production of multiply charged ions in clusters irradiated by a strong laser field, as compared with isolated atoms. This point is discussed in the light of recent experimental results on the production of multiply charged ions in laser?cluster experiments. Third-harmonic generation by a cluster in a strong laser field, as a function of both the cluster and the laser-field parameters, is also considered.

Generation of high-order harmonics in a self-guided beam

Abstract: Generation of high-order harmonics in Xe was studied in a static cell at high pump energies, 5 m focal length, and up to 14 cm interaction lengths. Self-guided propagation of the pulse was observed experimentally and confirmed by a three-dimensional model. Phase-matched generation was demonstrated in the self-guided beam, and the high energy and low divergence of the harmonic radiation were explained. Harmonic field calculations, in good agreement with experimental results, allow for the explanation of the higher-order harmonic generation dynamics in the self-guided region.

Direct measurement of the voltage sensitivity of second-harmonic generation from a membrane dye in patch-clamped cells.

Abstract: We report what is to our knowledge the first optical imaging of voltage-clamped cells by second-harmonic generation. For the membrane-staining styryl dye di-4-ANEPPS, we determined the sensitivity of second-harmonic generation to be 18%/100 mV at an excitation wavelength of 850 nm. This sensitivity is significantly better than the optimal 10%/100 mV under fluorescence and further establishes the importance of second-harmonic generation for the functional imaging of membrane potential in living cells.

Ionization and high-order harmonic generation in aligned benzene by a short intense circularly polarized laser pulse

Abstract: We present a first-principles study of ionization and high-order harmonic generation by benzene aligned in the polarization plane of a short circularly polarized laser pulse. Time-dependent density-functional theory within the adiabatic local-density approximation is employed to describe the 30 valence-electron dynamics in three dimensions. The multielectron approach enables us to study the effect of very strong laser fields, 10 14 ‐1 0 15 Wc m 22 , where multiple ionization and high-order harmonic generation interplay. Large ionization currents are formed, causing ionization of 1‐ 4 electron charges, while strong high-order harmonic generation is observed. The well-known recollision mechanism of high-order harmonic generation plays a part for moderate laser intensities but is fully suppressed for strong laser fields. The harmonic generation spectra are characterized by two distinguishable plateaus, where the structure of the first plateau is dominated by the 6k 6 1( k50,1,...) selection rule. The number of harmonics in the second plateau is insensitive to the duration of the pulse. The peaks appear in pairs or in threesomes, depending on the pulse duration.

Controlling harmonic generation in molecules with intense laser and static magnetic fields: Orientation effects

Abstract: We solve exactly the three-dimensional time-dependent Schr\"odinger equation for the ${\mathrm{H}}_{2}^{+}$ molecular ion in a combination of intense short laser pulses and static magnetic fields using a newly developed adaptive grid method. This allows for a study of the control of high-order harmonic generation in molecules by various orientations of the magnetic and laser fields in order to control electron recollision with nuclei. In particular, we find regimes of pure even harmonic or even and odd harmonic generation for particular field and frequency configurations. Furthermore, magnetic fields are found to extend harmonic generation plateaus to higher order as a result of further nonlinear coupling between charges and fields.

Continuously tunable high-order harmonics from atoms in an intense femtosecond laser field

Abstract: Through the control of laser energy and chirp, continuous wavelength tuning of high-order harmonics was achieved without sacrificing spectral sharpness. This process was implemented after the amplification stage in a chirped-pulse amplification Ti:sapphire laser, without changing laser spectrum. It opens a new pathway for the realization of a continuously tunable, coherent, femtosecond x-ray source, i.e., a tabletop synchrotron.

The influence of personal computer processing modes on line current harmonics

Abstract: The influence of the personal computer processing mode on the generation of line current harmonics has been investigated using a university library building containing over 370 PCs. The harmonics were monitored from the building switchboard using purpose-designed hardware. The PCs were set to execute four types of processing mode: idle, numerical processing, floppy, and hard disk accesses. The results recorded were indicative of harmonics caused by the PC box only, and secondary effects of harmonic production caused by distorted supply voltages were avoided. The results show that hard disk accesses cause the highest level of harmonic production affecting the 3rd and 5th components. Harmonic attenuation was very apparent when comparing the results between a single PC and 124 PCs connected to the same supply phase. However, attenuation and diversity effects due to changes in loading level on the PC power supply were not as predicted, showing an increase in harmonic generation with line current RMS level.

Measurement of the subcycle timing of attosecond XUV bursts in high-harmonic generation.

Abstract: The absolute timing of the high-harmonic attosecond pulse train with respect to the generating IR pump cycle has been measured for the first time. The attosecond pulses occur $190\ifmmode\pm\else\textpm\fi{}20\text{ }\mathrm{a}\mathrm{s}$ after each pump field maxima (twice per optical cycle), in agreement with the ``short'' quantum path of the quasiclassical model of harmonic generation.

Relativistic electron dynamics in intense crossed laser beams: acceleration and Compton harmonics.

Abstract: Electron motion and harmonic generation are investigated in the crossed-beam laser-accelerator scheme in a vacuum. Exact solutions of the equations of motion of the electron in plane-wave fields are given, subject to a restricted set of initial conditions. The trajectory solutions corresponding to axial injection are used to calculate precise emission spectra. Guided by hindsight from the analytic investigations, numerical calculations are then performed employing a Gaussian-beam representation of the fields in which terms of order epsilon(5), where epsilon is the diffraction angle, are retained. Present-day laser powers and initial conditions on the electron motion that simulate realistic laboratory conditions are used in the calculations. The analytic plane-wave work shows, and the numerical investigations confirm, that an optimal crossing angle exists, i.e., one that renders the electron energy gain a maximum for a particular set of parameters. Furthermore, the restriction to small crossing angles is not made anywhere. It is also shown that energy gains of a few GeV and energy gradients of several TeV/m may be obtained using petawatt power laser beams.

Pre-compensated excitation waveform to suppress second harmonic generation in MEMS electrostatic transducers

Abstract: Microelectromechanical systems (MEMS) electrostatic-based transducers inherently produce harmonics as the electrostatic force generated in the transmit mode is approximately proportional to the square of the applied voltage signal. This characteristic precludes them from being effectively used for harmonic imaging (either with or without the addition of microbubble-based contrast agents). The harmonic signal that is nonlinearly generated by tissue (or contrast agent) cannot be distinguished from the inherent transmitted harmonic signal. We investigated two precompensation methods to cancel this inherent harmonic generation in electrostatic transducers. A combination of finite element analysis (FEA) and experimental results are presented. The first approach relies on a calculation, or measurement, of the transducer's linear transfer function, which is valid for small signal levels. Using this transfer function and a measurement of the undesired harmonic signal, a predistorted transmit signal was calculated to cancel the harmonic inherently generated by the transducer. Due to the lack of perfect linearity, the approach does not work completely in a single iteration. However, with subsequent iterations, the problem becomes more linear and converges toward a very satisfactory result (a 18.6 dB harmonic reduction was achieved in FEA simulations and a 20.7 dB reduction was measured in a prototype experiment). The second approach tested involves defining a desired function [including a direct current (DC) offset], then taking the square root of this function to determine the shape of the required input function. A 5.5 dB reduction of transmitted harmonic was obtained in both FEA simulation and experimental prototypes test.