Showing papers on "High harmonic generation published in 2002"
TL;DR: By calculating the spatial dependence of the harmonic amplitudes and phases, this work is able to explain these effects in terms of interfering contributions from various regions within the molecule.
Abstract: We study numerically the generation of high-order harmonics by two-center molecules for arbitrary angles between the molecular axis and the laser polarization axis. For fixed angle, the harmonic spectrum exhibits a minimum at a frequency which is independent of the laser parameters. The amplitude of each harmonic is strongly angle dependent, and a pronounced minimum is found at the same angle where a sudden jump in the harmonic phase occurs. By calculating the spatial dependence of the harmonic amplitudes and phases, we are able to explain these effects in terms of interfering contributions from various regions within the molecule.
388 citations
TL;DR: An analytical model for the second harmonic generation (SHG) in the rat-tail tendon depends strongly on the polarization of the input laser beam and is applied in determining the orientation of collagen fibrils in the fascicle and the ratio gamma between the two independent elements of the second-order nonlinear susceptibility tensor.
Abstract: Using scanning confocal microscopy, we measure the backscattered second harmonic signal generated by a 100 fs laser in rat-tail tendon collagen. Damage to the sample is avoided by using a continuous scanning technique, rather than measuring the signal at discrete points. The second harmonic signal varies by about a factor of 2 across a single cross section of the rat-tail tendon fascicle. The signal intensity depends both on the collagen organization and the backscattering efficiency. This implies that we cannot use intensity measurements alone to characterize collagen structure. However, we can infer structural information from the polarization dependence of the second harmonic signal. Axial and transverse scans for different linear polarization angles of the input beam show that second harmonic generation (SHG) in the rat-tail tendon depends strongly on the polarization of the input laser beam. We develop an analytical model for the SHG as a function of the polarization angle in the rat-tail tendon. We apply this model in determining the orientation of collagen fibrils in the fascicle and the ratio gamma between the two independent elements of the second-order nonlinear susceptibility tensor. There is a good fit between our model and the measured data.
313 citations
TL;DR: In this article, the second and third-harmonic generation (SHG, THG) were imaged on single zinc oxide (ZnO) nanowires using near-field scanning optical microscopy (NSOM).
Abstract: The nonlinear optical response of semiconductor nanowires has potential application for frequency conversion in nanoscale optical circuitry. Here, second- and third-harmonic generation (SHG, THG) are imaged on single zinc oxide (ZnO) nanowires using near-field scanning optical microscopy (NSOM). The absolute magnitudes of the two independent (2) elements of a single wire are determined, and the nanowire SHG and THG emission patterns as a function of incident polarization are attributed to the hexagonal nanowire geometry and (2) tensor symmetry. Semiconductor nanowires are of current interest because of their unique electrical and optical properties. 1-3 In particular, their nonlinear optical properties suggest potential applications as frequency converters or logic/routing elements in nanoscale optoelectronic circuitry. A linear optical property of nanowires, photoluminescence (PL) polarization, has recently been studied in single indium phosphide nanowires. 2 In that case, the PL polarization is based upon the classical electromagnetic properties of a dielectric cylinder and averages ca. 91%. In contrast, coherent nonlinear optical phenomena, such as second- and third-harmonic generation (SHG and THG, respectively), depend explicitly on the crystal lattice structure of the medium, which could yield a very high (nearly 100%) polarization selectivity. In addition, the temporal response of the nonresonant harmonic generation is similar to the pulse width of the pump laser, in some cases 20 fs, 4 while incoherent processes are at least 2-4 orders of magnitude slower. Moreover, nonresonant SHG is essentially independent of wavelength below the energy band gap of semiconductor materials, most often including the 1.3-1.5 Im wavelength region typically used in optical fiber
310 citations
TL;DR: In this article, high-order harmonic generation for molecules in linearly polarized laser pulses by numerical solution of the Schr\"odinger equation was studied and maxima and minima due to intramolecular interference were found in the dependence of the harmonic intensities on the internuclear distance and on the orientation of the molecules.
Abstract: We study high-order harmonic generation for ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{H}}_{2}$ model molecules in linearly polarized laser pulses by numerical solution of the Schr\"odinger equation. Maxima and minima due to intramolecular interference are found in the dependence of the harmonic intensities on the internuclear distance and on the orientation of the molecules. These extrema can be approximately predicted by regarding them as the result of interference between two radiating point sources located at the positions of the nuclei.
273 citations
TL;DR: Using specific characteristics of the contrast bubbles will open new perspectives in imaging and analysis for medical diagnosis, including emitted frequency, bubble size, and nonlinear propagation effects have significant influence on the harmonic generation.
Abstract: Small (encapsulated) gas bubbles in a contrast medium react to an external oscillating pressure field with volume pulsations. Depending on the magnitude of the ultrasound wave, the vibrations will be related either linearly or nonlinearly to the applied acoustic pressure. For low acoustic pressures, the instantaneous radius oscillates linearly in relation to the amplitude of the applied external pressure field. The oscillation of the bubble is governed by parameters such as resonance frequency, damping coefficients, and shell properties. For higher amplitudes of the external field, the pulsation of the bubbles becomes nonlinear. The spectrum of the scattered ultrasound wave also contains higher harmonics of the emitted frequency in addition to the fundamental frequency. The emitted frequency, bubble size, and nonlinear propagation effects have significant influence on the harmonic generation. For encapsulated bubbles exposed to even higher acoustic amplitudes, their scattering effectiveness increases dramatically and becomes transient. The scattered frequency spectrum broadens, containing higher harmonics. This consequence is due to rupture, disappearance, change of gas content, etc. Using these specific characteristics of the contrast bubbles will open new perspectives in imaging and analysis for medical diagnosis.
252 citations
01 Nov 2002
TL;DR: In this article, the authors present a general description of wave propagation in nonlinear media, including high-resolution nonlinear optical spectroscopy, and four-wave mixing and mixing.
Abstract: Introduction. Nonlinear Optical Susceptibilities. General Description of Wave Propagation in Nonlinear Media. Electrooptical and Magnetooptical Effects. Optical Rectification and Optical Field-Induced Magnetization. Sum-Frequency Generation. Harmonic Generation. Difference Frequency Generation. Parametric Amplification and Oscillation. Stimulated Raman Scattering. Stimulated Light Scattering. Two-Photon Absorption. High-Resolution Nonlinear Optical Spectroscopy. Four-Wave Mixing. Four-Wave Mixing Spectroscopy. Optical-Field-Induced Birefringence. Self-Focusing. Multiphoton Spectroscopy. Detection of Rare Atoms and Molecules. Laser Manipulation of Particles. Transient Coherent Optical Effects. Strong Interaction of Light with Atoms. Infrared Multiphoton Excitation and Dissociation of Molecules. Laser Isotope Separation. Surface Nonlinear Optics. Nonlinear Optics in Optical Waveguides. Optical Breakdown. Nonlinear Optical Effects in Plasmas. Index.
229 citations
TL;DR: It is demonstrated, in this study, that the CTR increases as a function of the order of the harmonic frequency, in agreement with the calculations using existing models for the response of encapsulated bubbles and known theory of nonlinear propagation.
Abstract: For ultrasound contrast agents (UCA), nonlinear imaging now has become fundamental. All of the current contrast-imaging methods are dominantly based on the nonlinear response of UCA bubbles. The discrimination between the perfused tissue and the UCA is the challenge in the field of UCA-imaging. This differentiation is usually associated or expressed by the ratio of the scattered power from the contrast agent to the scattered power from the tissue and is termed "contrast-to-tissue ratio" (CTR). Second harmonic imaging showed a better discrimination between tissue and UCA than fundamental imaging because of a higher CTR. We demonstrate, in this study, that the CTR increases as a function of the order of the harmonic frequency. Currently, due to the limited bandwidth of the transducers, only the second harmonic is selectively imaged, resulting in images with a superior quality to fundamental images, but still degraded and not optimal because of the harmonic generation in the underlying tissue (due to nonlinear propagation) and hence giving a limited CTR. To increase the CTR and to take advantage of the higher harmonics (third, fourth, fifth and the ultraharmonics and termed here super harmonics), we have developed a new phased array transducer. The array transducer contains two different types of elements arranged in an interleaved pattern (odd and even elements). The total number of elements is 96. The elements can operate separately and at a distinct frequency, enabling separate transmission and reception modes. The odd elements (48) operate at typically 2.8 MHz center frequency and 80% bandwidth. The even elements (48) have a center frequency of 900 kHz with a bandwidth of 50%. In vitro measurements using the dual frequency probe show an increase of 40 dB in the CTR for super harmonic components over the conventional second harmonic system. The increase in CTR is in agreement with the calculations using existing models for the response of encapsulated bubbles and known theory of nonlinear propagation. Animal experiments have demonstrated the feasibility of this approach using commercially avail- able UCA and showed a similar increase of the CTR (E-mail: bouakaz@tch.fgg.eur.nl). © 2002 World Federation for Ultrasound in Medicine & Biology.
226 citations
DSM1
TL;DR: In this article, high-order harmonic generation at a high pumping energy using a long focal length lens was studied, revealing the interplay between phase matching, absorption, and laser defocusing.
Abstract: We study high-order harmonic generation at a high pumping energy using a long focal length lens. We identify different saturation regimes of the harmonic emission, revealing the interplay between phase matching, absorption, and laser defocusing. In the optimal conditions, high conversion efficiencies are obtained, resulting in an increase of at least one order of magnitude of the harmonic energies compared to previously reported values. In xenon, microjoule energies are reached, opening new perspectives for the applications of this ultrashort coherent radiation. The generation of the high-order harmonics of intense laser pulses in gases @1# has recently opened new perspectives for probing matter in the extreme-ultraviolet ~XUV! pulses on an unprecedented time scale. The ultrashort harmonic pulse duration is used in pump-probe experiments in atomic @2,3# and molecular @ 4‐6 # spectroscopy, as well as in solidstate physics @7,8#. Combined to the high intrinsic beam coherence, it has allowed ultrafast diagnosis of laser-produced plasmas through XUV interferometry @9,10#. However, the harmonic beam energy is still relatively low and many applications would become possible if the number of generated photons were increased: ultrafast XUV holography, diagnosis of dense bright plasmas, or even study of nonlinear processes in the XUV, limited so far to low harmonic orders @11#. Recently a number of studies have demonstrated high conversion efficiencies, using ultrashort laser pulses focused in hollow core fibers @12‐14# or cells @15,16#. However, these efficiencies were obtained at a very low laser energy ~less than 1 mJ in most cases! that imposed a relatively tight focusing geometry in order to reach saturation intensities of the generating rare gases. This resulted in a low harmonic energy, in the nanojoule range. The fact that much larger energies are now available on ultrashort laser systems raises a number of questions: using higher laser energies and looser focusing, is it possible to achieve similar efficiencies, and thus to generate microjoule harmonic pulses? In particular, how will phase matching be affected by these unusual generating conditions? In this Rapid Communication, we report a thorough study of harmonic generation at a high pumping energy using a long focal length lens. Using a pumping energy of 27 mJ and a f 52 m lens, we study the influence of the beam aperture, the medium length, and the atomic density on the harmonic yield produced in a pulsed gas jet. We identify different saturation regimes, thanks to the excellent quantitative agreement obtained with detailed three-dimensional ~3D! simulations. In particular, we clearly observe the interplay between phase matching, absorption, and defocusing, the main limiting factors of the macroscopic emission. Absolute photon number measurements in the optimal conditions give conversion efficiencies as high as those reported using ultrashort (,20 fs) laser pulses, but now with ten times more energy. Using a f 55 m lens, we show that even higher conversion efficiencies can be obtained, resulting in harmonic energies in excess of 1 mJ. The experiments were performed on the LUCA laser facility with an amplified Ti:sapphire system delivering 60 fs pulses at 800 nm, with an energy of up to 100 mJ at 20 Hz. In our experiment, a pumping energy of 27 mJ was focused with either a f 52 m or 5 m lens in a pulsed gas jet. The nozzle was formed by a slit of dimensions 300 mm3 3m m producing a jet at pressure 10‐100 Torr characterized by Mach-Zehnder interferometry. By rotating the jet relative to the laser axis, we can change the length of the generating medium, while keeping the same peak density. Harmonics produced in the jet are analyzed by an XUV spectrometer without entrance slit @17#, and detected with a calibrated XUV photodiode blinded for diffused IR light with two 100 nm Al filters. The absolute spectrometer response~as well as the filter transmission! was measured using the harmonic radiation as a source further monochromatized with another spectrometer, like a synchrotron beam line. Since the total aperture of the laser beam was 40 mm, the
214 citations
TL;DR: It is argued that the third-harmonic pulse maintains both its peak intensity and energy over distances much longer than the characteristic coherence length due to a nonlinear phase-locking mechanism between the two pulses in the filament and is independent of the initial material wave-vector mismatch.
Abstract: It is shown, both theoretically and experimentally, that during laser pulse filamentation in air an intense ultrashort third-harmonic pulse is generated forming a two-colored filament. The third-harmonic pulse maintains both its peak intensity and energy over distances much longer than the characteristic coherence length. We argue that this is due to a nonlinear phase-locking mechanism between the two pulses in the filament and is independent of the initial material wave-vector mismatch. A rich spatiotemporal propagation dynamics of the third-harmonic pulse is predicted. Potential applications of this phenomenon to other parametric processes are discussed.
207 citations
TL;DR: In this article, the authors considered single-atom high-order harmonic generation in the strong-field approximation, as formulated in the Lewenstein model, and analyzed in terms of quantum orbits.
Abstract: Single-atom high-order harmonic generation is considered in the strong-field approximation, as formulated in the Lewenstein model, and analyzed in terms of quantum orbits. Orbits are classified according to the solutions of the saddle-point equations. The results of a numerical integration are compared with the saddle-point approximation and the uniform approximation. Approximate analytical solutions for long orbits are presented. The formalism developed is used to analyze the enhancement of high-order harmonic generation near channel closings. The enhancements exactly at the channel closings are extremely narrow and built up by the constructive interference of a very large number of quantum orbits. Additional broader enhancements occur slightly below channel closings. They are generated by the interplay of a medium number of orbits.
142 citations
TL;DR: It is shown that the wave vector and frequency profiles of this x-ray analogue of coherent Raman spectroscopy provide an excellent real-space probe that carries most valuable structural and dynamical information, not available from spontaneous Raman techniques.
Abstract: Nonlinear x-ray four-wave mixing experiments are becoming feasible due to rapid advances in high harmonic generation and synchrotron radiation coherent x-ray sources. By tuning the difference of two x-ray frequencies across the valence excitations, it is possible to probe the entire manifold of molecular electronic excitations. We show that the wave vector and frequency profiles of this x-ray analogue of coherent Raman spectroscopy provide an excellent real-space probe that carries most valuable structural and dynamical information, not available from spontaneous Raman techniques.
TL;DR: In this article, the strong-field approximation, amended so as to allow for rescattering, is used to calculate high-order above-threshold ionization (ATI) spectra.
Abstract: The strong-field approximation, amended so as to allow for rescattering, is used to calculate high-order above-threshold ionization (ATI) spectra. The single-active-electron binding potential is modelled by a zero-range potential. The emphasis is on enhancements of groups of ATI peaks that occur for sharply defined laser intensities. The enhancements are traced to multiphoton resonance with the ponderomotively upshifted continuum threshold. Good agreement both with experimental data and with numerical simulations using the three-dimensional time-dependent Schrodinger equation for an optimized one-electron binding potential is observed. The physical reason for the close agreement of the results of the two, apparently so different, models is discussed. For quantitative agreement with the experimentally observed positions of the resonances, an `effective' continuum threshold has to be introduced. The effects of focal averaging are evaluated and discussed. Resonant enhancement of high-order harmonic generation is also considered.
Posted Content•
TL;DR: In this article, a theoretical description of femtosecond laser induced dynamics of the hydrogen molecule and of singly ionised sodium dimers, based on a real-space, real-time, implementation of time-dependent density functional theory (TDDFT), is presented.
Abstract: We present a theoretical description of femtosecond laser induced dynamics of the hydrogen molecule and of singly ionised sodium dimers, based on a real-space, real-time, implementation of time-dependent density functional theory (TDDFT). High harmonic generation, Coulomb explosion and laser induced photo-dissociation are observed. The scheme also describes non-adiabatic effects, such as the appearance of even harmonics for homopolar but isotopically asymmetric dimers, even if the ions were treated classically. This TDDFT-based method is reliable, scalable, and extensible to other phenomena such as photoisomerization, molecular transport and chemical reactivity.
TL;DR: In this paper, the second, third, fourth, and fifth harmonics of the output of a Yb-doped fiber amplifier seeded by a passively Q-switched Nd:YAG microchip laser were reported.
TL;DR: In this paper, the authors investigate resonance-enhanced high-order harmonic generation, with emphasis on the physical mechanism of the enhancement, and conclude that the channel closings typical of a short-range or zero-range potential are capable of generating essentially the same effects.
Abstract: Solving the one-dimensional time-dependent Schr\"odinger equation for simple model potentials, we investigate resonance-enhanced high-order harmonic generation, with emphasis on the physical mechanism of the enhancement. By truncating a long-range potential, we investigate the significance of the long-range tail, the Rydberg series, and the existence of highly excited states for the enhancements in question. We conclude that the channel closings typical of a short-range or zero-range potential are capable of generating essentially the same effects.
TL;DR: These experimental results demonstrate for the first time the feasibility of using nonlinear harmonic SASE FEL radiation to produce coherent, femtosecond x rays.
Abstract: Nonlinear harmonic radiation was observed using the VISA self-amplified, spontaneous emission (SASE) free-electron laser (FEL) at saturation. The gain lengths, spectra, and energies of the three lowest SASE FEL modes were experimentally characterized. The measured nonlinear harmonic gain lengths and center spectral wavelengths decrease with harmonic number, n, which is consistent with nonlinear harmonic theory. Both the second and third nonlinear harmonics energies are about 1% of the fundamental energy. These experimental results demonstrate for the first time the feasibility of using nonlinear harmonic SASE FEL radiation to produce coherent, femtosecond x rays.
TL;DR: In this paper, the temporal profile and phase of the fifth harmonic of a Ti:sapphire laser were fully characterized by two-photon ionization frequency-resolved optical gating technique for the first time.
Abstract: The temporal profile and phase of the fifth harmonic of a Ti:sapphire laser were fully characterized by two-photon ionization frequency-resolved optical gating technique for the first time. The fifth harmonic was found to have negative chirp and the pulse compression was demonstrated. The negative chirp is well explained by using a zero-range potential model. This technique is scalable to extreme ultraviolet (XUV) and soft x-ray regions by using currently available light sources, making it possible to measure the pulse duration and phase of vacuum ultraviolet, XUV, and soft x-ray pulses.
TL;DR: Low-divergence, high-brightness harmonic emission has been generated by using a fundamental beam with a truncated Bessel intensity profile using the hollow-fiber compression technique, which allows one to optimize both temporal and spatial characteristics of the high-order harmonic generation process.
Abstract: Low-divergence, high-brightness harmonic emission has been generated by using a fundamental beam with a truncated Bessel intensity profile. Such a beam is directly obtained by using the hollow-fiber compression technique, which indeed allows one to optimize both temporal and spatial characteristics of the high-order harmonic generation process. This is particularly important for the applications of radiation, where extreme temporal resolution and high brightness are required.
TL;DR: In this paper, a laser-based instrument is described for the study of femtosecond dissociation dynamics of gas phase molecules via time-resolved vacuum ultraviolet and soft x-ray photoelectron spectroscopy.
Abstract: A laser-based instrument is described for the study of femtosecond dissociation dynamics of gas phase molecules via time-resolved vacuum ultraviolet and soft x-ray photoelectron spectroscopy. Visible or UV pump pulses are generated with nonlinear crystal techniques on a Ti:sapphire laser output, while soft x-ray probe pulses are created via high-order harmonic generation of the same laser in rare gases. Here we describe the optical layout of the pump-probe system, the means for separation of the high-order harmonics in the soft x-ray probe beam, including a description of the two grating setup used to compress the high-harmonic pulses and the magnetic bottle photoelectron spectrometer used for data collection. The feasibility of using the generated high-harmonic pulses for an array of gaseous phase photoelectron spectroscopy experiments is established. These include measurements of valence shell and core-level photoelectron transitions in atoms and molecules, the tunability of the soft x-ray harmonic thro...
TL;DR: In this article, the spin dynamics and its reaction on the particle motion were investigated for free and bound electrons in intense linearly polarized laser fields and the effect of spin-orbit coupling on the motion of the electron was analyzed.
Abstract: The spin dynamics and its reaction on the particle motion are investigated for free and bound electrons in intense linearly polarized laser fields. Employing both classical and quantum treatments we analytically evaluate the spin oscillation of free electrons in intense laser fields and indicate the effect of spin-orbit coupling on the motion of the electron, In Mott scattering an estimation for the spin oscillation is derived, In intense laser ion dynamics spin signatures are studied in detail with emphasis on high-order harmonic generation in the tunneling regime, First- and second-order calculations in the ratio of electron velocity and the speed of light show spin signatures in the radiation spectrum and spin-orbit effects in the electron polarization.
TL;DR: In this article, the second-harmonic conversion efficiency of two-dimensional hexagonally poled lithium niobate as a function of temperature and wavelength was measured using high-power nanosecond pulses.
Abstract: Using high-power nanosecond pulses, we measured the second-harmonic conversion efficiency of two-dimensional hexagonally poled lithium niobate as a function of temperature and wavelength. These results were compared with theoretical estimates and with measurements in one-dimensional periodically poled lithium niobate. We found that for a substantial range of parameters a two-dimensional noncollinear interaction has a broader tuning response than a one-dimensional collinear interaction. We also observed and characterized third- and fourth-harmonic generation processes in the same crystal.
TL;DR: In this article, the authors report a systematic study of high order harmonic generation with an infrared laser apertured by an iris, as a function of the aperture size, and show that the aperture diameter which maximizes harmonic yield is the result of a compromise between considerations of focal geometry and ionization, which favour small apertures; and harmonic dipole amplitude and phase (which favour large aperture).
Abstract: We report a systematic study of high order harmonic generation with an infrared laser apertured by an iris, as a function of the aperture size. Measurements were made of harmonic generation efficiency for different gas species, laser energies and focal geometries. Harmonic efficiencies as a function of aperture show a characteristic peaked shape, which is independent of gas species and harmonic order. A one dimensional, time dependent simulation of harmonic generation in a gas cell, taking into account experimentally measured transverse coherence of the laser, closely reproduces the observed behaviours. We show that the aperture diameter which maximizes harmonic yield is the result of a compromise between considerations of focal geometry and ionization (which favour small apertures); and harmonic dipole amplitude and phase (which favour large apertures).
TL;DR: In this paper, the azimuthal dependence of third-order and cascaded second-order nonlinear coupling is used to measure the relative contributions of each to direct third-harmonic generation in β-barium borate.
Abstract: The azimuthal dependence of third-order and cascaded second-order nonlinear coupling are used to measure the relative contributions of each to direct third-harmonic generation in β-barium borate. This enabled the measurement of the values of χ10(3),χ11(3), and χ16(3) relative to the known χij(2). Finally, conversion efficiencies to 3ω of up to 6% from a single crystal were achieved with a femtosecond chirped-pulse-amplification laser with 200 GW/cm2 in collimated beams.
Book•
09 Sep 2002
TL;DR: In this article, the authors present a scheme for the determination of molecular properties and a scheme to determine invariants of molecules in the Euler Angle Matrix of tensors, which is the basis for our scheme.
Abstract: Preface. 1. Foundations of Molecular Harmonic Emission. 1.1 Classical Optics. 1.2 Quantum Electrodynamics. 1.3 Media Corrections. 2. Pertubation Theory. 2.1 Time-Dependent Pertubation Theory. 2.2 Time-Orderings and State Sequences. 3. Radiation Constructs. 3.1 Radiation Tensor Construction. 3.2 Quantum Optical Considerations. 3.3 Pump Photonics. 4. Molecular Properties. 4.1 Molecular Tensor Construction. 4.2 Symmetry. 4.3 Two-Level Systems. 5. Coherent and Incoherent Signals. 5.1 Regular Solids. 5.2 Gases, Liquids and Disordered Solids. 5.3 Macromolecules, Suspensions and Partially Ordered Solids. 5.4 Coherence and Wave-Vector Matching. 6. Coherent Harmonic Generation. 6.1 Harmonic Intensities. 6.2 Rotational Averaging and Symmetry Criteria. 6.3 Third Harmonic Generation. 7. Special Systems for Second Harmonic Generation. 7.1 Second Harmonic Generation at Surfaces and Interfaces. 7.2 Electric Field-Induced Second Harmonic Generation. 7.3 Optical Coherence in Dispersed Particles. 7.4 Six-Wave Second Harmonic Generation. 8. Incoherent Elastic Light Scattering. 8.1 General Principles. 8.2 Second Harmonic Scattering/Hyper-Rayleigh Scattering. 8.3 Third Harmonic Scattering. 9. Hyper-Raman Scattering. 9.1 Constructing the Signal. 9.2 Hyperpolarisability Theory. 9.3 Irreducible Tensors. 9.4 Symmetry Selection rules. 9.5 Scheme for the Determination of Molecular Invariants. 9.6 Reversal and Depolarisation Ratios. 9.7 Higher Multiple Effects. Appendix 1. Resonance Damping. Appendix 2. Rationale Averaging. Appendix 3. Isotropic Tensors and the Euler Angle Matrix. Appendix 4. Irreducible Cartesian Tensors. Appendix 5. Six-Wave Mixing and Secular Resonances. Appendix 6. Spectroscopic Selection Rules. Glossary of Symbols. References. Bibliography. Index.
TL;DR: In this article, high harmonics generated due to the scattering of relativistic electrons from high intensity laser light is studied in an Nd:glass laser system with a peak intensity of 2×1018 W cm−2 in underdense plasma.
Abstract: High harmonics generated due to the scattering of relativistic electrons from high intensity laser light is studied. The experiments are carried out with an Nd:Glass laser system with a peak intensity of 2×1018 W cm−2 in underdense plasma. It is shown that, at high intensities, when the normalized electric field approaches unity, in addition to the conventional atomic harmonics from bound electrons there is significant contribution to the harmonic spectrum from free electrons. The characteristic signatures of this are found to be the emission of even order harmonics, linear dependence on the electron density, significant amount of harmonics even with circular polarization and a much smaller spatial region over which these harmonics are produced as compared to the atomic case. Imaging of the harmonic beam shows that it is emitted in a narrow cone with a divergence of 2 to 3 degrees.
01 May 2002-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, an analytical estimate using the HGHG parameters in the DUVFEL project at BNL is presented, which shows that the effective energy spread in a chicane dispersion section is very small, and the effect of finite emittance can be neglected during the calculation of coherent harmonic generation.
Abstract: We discuss the theory of the High Gain Harmonic Generation (HGHG). First, we describe an analytical estimate using the HGHG parameters in the DUVFEL project at BNL as an example. We show that the effective energy spread in a chicane dispersion section is found to be very small, and the effect of finite emittance can be neglected during the calculation of coherent harmonic generation. Then we discuss some issues such as the intensity stability, and how to use HGHG to obtain information about local energy spread. We compare these issues with recent experimental results in the infrared. We discuss some of the key issues in the cascading HGHG scheme and its possible limitations.
TL;DR: In this article, the third-harmonic generation from solid samples of carbon nanotubes has been studied experimentally, using ultrashort pulses generated by a Cr:Forsterite laser, at a wavelength of 1250 nm.
Abstract: Third-harmonic generation from solid samples of carbon nanotubes has been studied experimentally, using ultrashort pulses generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The results show an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields, of ∼1010 W/cm2. This strong nonlinearity of the laser interaction with carbon nanoubes is also confirmed theoretically, in a full quantum-mechanical theory for harmonics generation from a single-walled carbon nanotube.
TL;DR: In this paper, the main physical processes behind the use of hollow fibers for efficient generation of unprecedentedly short light pulses, enhanced short-wavelength generation, and improvement of the sensitivity of nonlinear-optical gas-phase analysis are considered.
Abstract: Ultrafast-optics applications of nonlinear-optical processes in gas-filled hollow fibers are briefly reviewed. The main physical processes behind the use of hollow fibers for efficient generation of unprecedentedly short light pulses, enhanced short-wavelength generation, and improvement of the sensitivity of nonlinear-optical gas-phase analysis are considered. These processes include self- and cross-phase modulation, coherent four-wave mixing, high-order harmonic generation, and stimulated Raman scattering. The methods to generate extremely short pulses, including few-cycle field waveforms, and to control such pulses are discussed.
TL;DR: In this paper, the influence of the magnetic-field component of the incident pulse on the emission of photons by multiply charged ions interacting with intense, near-infrared laser pulses is investigated theoretically using a strong-field approximation that treats the coupling of the atom with the incident field beyond the dipole approximation.
Abstract: The influence of the magnetic-field component of the incident pulse on the emission of photons by multiply charged ions interacting with intense, near-infrared laser pulses is investigated theoretically using a strong-field approximation that treats the coupling of the atom with the incident field beyond the dipole approximation. For peak pulse intensities approaching ${10}^{17}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2},$ the electron drift in the laser propagation direction due to the magnetic-field component of the incident pulse strongly influences the photon emission spectra. In particular, emission is reduced and the plateau structure of the spectra modified, as compared to the predictions in the dipole approximation. Nondipole effects become more pronounced as the ionization potential of the ion increases. Photon emission spectra are interpreted by analysing classical electron trajectories within the semiclassical recollision model. It is shown that a second pulse can be used to compensate the magnetic-field induced drift for selected trajectories so that, in a well-defined spectral region, a single attosecond pulse is emitted by the ion.