High-order harmonic generation from plasma mirrors

We discuss the two mechanisms involved in high-order harmonic generation from plasma mirrors, and show that they can be clearly identified experimentally. The very different phase properties of the corresponding harmonics lead to light beams with different divergences. This can be exploited to select a particular type of harmonic by spatial filtering in the far-field.     

High-order optical harmonic generation from solid surfaces

During the interaction of an intense ultrashort laser pulse with solid targets, a thin layer of surface plasma is generated in which the density drops to the vacuum level in a distance much shorter than the wavelength. This sharp plasma-vacuum boundary performs an oscillatory motion in response to the electromagnetic forces of the intense laser light. It is shown that the generation of reflected harmonics can be interpreted as a phase modulation experienced by the light upon reflection from the oscillating boundary. The modulation side-bands of the reflected frequency spectrum correspond to odd and even harmonics of the laser frequency. Retardation effects lead to a strong anharmonicity for high velocities of the plasma-vacuum boundary. As a result, harmonic generation is strongly enhanced in the relativistic regime of laser intensities.Prof. F. P. Schäfer on the occasion of his 65th birthday.     

High-order harmonic generation from organic molecules in ultra-short pulses

We have studied high-order harmonic generation (HHG) from organic molecules irradiated with near-infrared high intensity laser pulses of 70 fs and 240 fs duration. The molecular systems studied were the aromatics benzene and naphthalene and the alkanes cyclopropane and cyclohexane (cyclic) and n-hexane (linear). Harmonic intensities were measured both as a function of laser intensity (in the range 5×10¹³-5×10¹⁵ W cm^-2) and as a function of ellipticity of the laser field polarisation. The results were compared with those from the xenon atom. For 70 fs pulses, harmonic generation from the organic systems was similar to that of xenon, revealing an atom-like behaviour for molecules when the laser pulse duration is shorter than the fragmentation timescale of the molecule. We note significant differences between molecules with respect to HHG efficiencies and the suppression of HHG in larger species. We discuss these differences in the context of the molecular properties, electronic structure and behaviour of ionisation and fragmentation that result in enhancement of field ionisation in larger systems. Study of the polarisation ellipticity dependence of HHG shows that the harmonic yield in molecules is less sensitive to the polarisation than for atoms (xenon). This is consistent with the expected behaviour given the larger recollision cross-section presented by the core in the molecular system compared to the atom. Our results suggest that study of HHG from molecules exposed to ultra-short pulses is potentially a powerful tool for understanding the electron dynamics of molecules exposed to an intense field. Received 14 September 2000 and Received in final form 6 December 2000     

High-order harmonic generation in sub-one-cycle regime

We theoretically investigate the high-order harmonic generation from the hydrogen atom driven by the laser pulses with the durations less than the optical cycle.It is found that the switching term of the laser field may have an obvious influence on the cutoff,intensity or plateau structure of the high-order harmonic spectrum.Generally speaking,the switching term can shorten the cutoff of the high-order harmonic spectrum for a relatively longer pulse and extend the cutoff for a relatively shorter pulse.     

High-order harmonic generation from ions in a capillary discharge

We demonstrate a significant extension of the high-order harmonic cutoff by using a fully-ionized capillary discharge plasma as the generation medium. The preionized plasma dramatically reduces ionization-induced defocusing and energy loss of the driving laser due to ionization. This allows for significantly higher photon energies, up to 150 eV, to be generated from xenon ions, compared with the 70 eV observed previously. We also demonstrate enhancement of the harmonic flux of nearly 2 orders of magnitude at photon energies around 90 eV when the capillary discharge is used to ionize xenon, compared with harmonic generation in a hollow waveguide. The use of a plasma as a medium for high-order harmonic generation shows great promise for extending efficient harmonic generation to much shorter wavelengths using ions.     

High-order harmonic generation from boron plasma in the extreme-ultraviolet range

We demonstrate the generation of up to the 63rd harmonic (lambda = 12.6 nm) of a Ti:sapphire laser pulse (150 fs, 10 mJ), using a prepulse- (210-ps,24-mJ) produced boron plasma as the nonlinear medium. The influence of various parameters on the harmonic conversion efficiency was analyzed. The steep decrease in intensity for low-order harmonics (up to 19th order) was followed by a plateau. Typical conversion efficiencies were evaluated to be 10(-4) (for a 3rd harmonic) to 10(-7) (within the plateau region). Harmonic generation appeared to be efficient for the plasma that comprised neutral atoms and singly ionized boron.     

High-order harmonic generation from solid targets with 2 mJ pulses

Harmonics up to the 18th order are generated from solid targets by focusing 2 mJ, 50 fs pulses at 800 nm to a spot size of 1.7 μm (FWHM). To our knowledge, this is the first demonstration of high-harmonic generation with a very short focal length paraboloid (f/1.4) and kilohertz laser system. The harmonics have a low divergence (<4°) compared to the driving beam and conversion efficiencies (>10(-7) per harmonic) comparable to gas harmonics. No contrast enhancement techniques are employed, and the system is capable of operating at 500 Hz.     

High-order harmonic generation from solid surfaces in intense laser fields

We give a Floquet analysis of high-order harmonic generation from a solid surface interacting with intense laser fields. A general theoretical algorithm, within a three-dimensional quasi-free electron model, is presented. It takes account of the effective masses of the electron both within and outside the solid and treats both reflected and refracted laser fields. The high-order harmonic spectrum is obtained from the off-diagonal photon components of the expectation value of the time-dependent current operator. We present the formation of a plateau, well known in the atomic case, in the spectrum of the high-order harmonic radiation, which arises from the evanescent part of the electron wave function at the surface.     

High-order harmonic generation enhanced by XUV light

The combination of high-order harmonic generation (HHG) with resonant XUV excitation of a core electron into the transient valence vacancy that is created in the course of the HHG process is investigated theoretically. In this setup, the first electron performs a HHG three-step process, whereas the second electron Rabi flops between the core and the valence vacancy. The modified HHG spectrum due to recombination with the valence and the core is determined and analyzed for krypton on the 3d→4p resonance in the ion. We assume an 800?nm laser with an intensity of about 10(14)?W/cm2 and XUV radiation from the Free Electron Laser in Hamburg (FLASH) with an intensity in the range 10(13)-10(16)W cm2. Our prediction opens perspectives for nonlinear XUV physics, attosecond x rays, and HHG-based spectroscopy involving core orbitals.     

High-order harmonic generation in nanoparticle-containing laser-produced plasmas

Nanoparticle-containing media can be used for the efficient high-order harmonic generation (HHG) of laser radiation in the extreme ultraviolet range. We review the results of recent studies of the HHG in laser-produced plasmas containing Ag, Au, Pd, Pt, Ru, GaN, BaTiO₃, and SrTiO₃ nanoparticles. The harmonics of femtosecond radiation up to the 55th order were achieved using the nanoparticle-containing plumes, when the femtosecond radiation propagated through the preformed plasma. These results are compared with the high-order harmonics generated from the plasma produced on the surface of bulk targets at different delays between the subnanosecond heating prepulse and femtosecond pulse. We discuss a six-fold enhancement of the HHG yield, which was achieved in the case of nanoparticle-containing plumes with regard to the monoparticle-containing plasmas.     

High-order harmonic generation up to 250 eV from highly ionized argon

We demonstrate the generation of very high-order harmonics, up to 250 eV, using argon gas. This extends by 100 eV the highest harmonics previously observed using Ar and exceeds the energies observed using any other medium besides helium. This advance is made possible by using a waveguide geometry to limit plasma-induced laser beam defocusing, making it possible to generate high harmonics from Ar ions. This work shows that high harmonic emission from ions can extend laser-based coherent up-conversion into the soft x-ray region of the spectrum.     

High-order harmonic generation with a two-color laser pulse

We theoretically investigate the electron dynamics of the high-order harmonics generation process by combining a near-infrared 800 nm driving pulse with a mid-infrared 2000 nm control field.We also investigate the emission time of harmonics using time-frequency analysis to illustrate the physical mechanisms of high-order harmonic generation.We calculate the ionization rate using the Ammosov-Delone-Krainov model and interpret the variations in harmonic intensity for different control field strengths and delays.We find that the width of the harmonic plateau can be extended when the control electric field is added,and a supercontinuum from 198 to 435 eV is generated,from which an isolated 61-as pulse can be directly obtained.     

High-order harmonic generation by an ultrafast infrared pulse

Life science and biology have advanced with the progress of microscope technology. A strong desire of life scientists is to observe living cells with high spatial resolution. The development of soft x-ray microscopy using a special wavelength called the water window started in the 1970s in order to fulfill this desire. However, the photon fluxes realized so far are still insufficient for this application. Here, we review our work on a coherent water window x-ray generation based on high-order harmonics driven by an infrared pulse. Our proposed generation scheme, combining an infrared laser driver and a neutral gas medium, is efficient and scalable in output yields of the water window x-ray.     

High-order harmonic generation with a two-color laser pulse

We theoretically investigate the electron dynamics of the high-order harmonics generation process by combining a near-infrared 800 nm driving pulse with a mid-infrared 2000 nm control field. We also investigate the emission time of harmonics using time-frequency analysis to illustrate the physical mechanisms of high-order harmonic generation. We calculate the ionization rate using the Ammosov-Delone-Krainov model and interpret the variations in harmonic intensity for different control field strengths and delays. We find that the width of the harmonic plateau can be extended when the control electric field is added, and a supercontinuum from 198 to 435 eV is generated, from which an isolated 61-as pulse can be directly obtained.     

Second harmonic generation from closed packed hexagonal structure of nano-nickel arrays

A closed packed hexagonal structure of nickel nanoparticles was fabricated by deposition of thin nickel films on self-assembled polystyrene micron spheres, which were made of nanospherical lithography in conjunction with left-off the micro spheres. The optical second harmonic generation from these periodically arrayed nickel nanoparticles reveals intensity with a value much larger than that generated from a uniform nickel-film of the same thickness. Quasi phase matching contributed from Bragg wave vectors of the periodic arrayed nickel nanoparticles can satisfactorily express this enhancement.     

High-order harmonic generation in laser plasma: Recent achievements

Recent studies of high-order harmonic generation of laser radiation in laser-produced plasma show new attractive developments in this field. Those include generation of extended harmonics in plasma plumes, new approaches in application of two-color pump, generation of extremely broadened harmonics, further developments in harmonic generation in clusters (fullerenes, carbon nanotubes, in-situ produced nanoparticles), destructive interference of harmonics from different emitters, resonance-induced enhancement of harmonics, applications of high pulse repetition rate lasers for the enhancement of average power of generating harmonics, observation of quantum path signatures, etc. We review some of these recent developments.