Showing papers on "High harmonic generation published in 2012"
TL;DR: In this paper, the FERMI free-electron laser operating in the high-gain harmonic generation regime was demonstrated, allowing high stability, transverse and longitudinal coherence and polarization control.
Abstract: Researchers demonstrate the FERMI free-electron laser operating in the high-gain harmonic generation regime, allowing high stability, transverse and longitudinal coherence and polarization control.
831 citations
TL;DR: The generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40 nanometres, is reported by coupling a high-power near-infrared frequency comb to a robust femtosecond enhancement cavity, and the absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy.
Abstract: By coupling a high-power, high-repetition-rate near-infrared frequency comb to a femtosecond optical cavity, a frequency comb operating in the extreme-ultraviolet spectral range has been produced, by high harmonic generation, and provides high-resolution spectroscopy in this spectral region. Laser-based optical frequency combs, so called because they emit evenly spaced spectral lines, are used in precision spectroscopy and other applications requiring accurate measurements, such as atomic clocks. Efforts to extend this capability to shorter wavelengths in the extreme ultraviolet — which would open up exciting new applications, including searches for variation in fundamental constants — have lacked sufficient power for the purpose until now. Jun Ye and co-workers demonstrate a new approach, using a high-power, high-repetition pulsed infrared laser coupled into an optical cavity, to produce an improved extreme UV comb. In a first precision spectroscopy demonstration, they use direct frequency comb spectroscopy to determine argon and neon atomic transitions with ultra-high precision. The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability to provide a precise and direct link between microwave and optical frequencies1,2. A further advance in frequency comb technology is the generation of frequency combs in the extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity3,4. Until now, combs produced by this method have lacked sufficient power for applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40 nanometres, by coupling a high-power near-infrared frequency comb5 to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82 nanometres and a neon transition at 63 nanometres, thus confirming the combs’ coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending ultrahigh-precision spectroscopy to the spectral region below 100 nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions in molecules6, experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium7,8,9, the development of next-generation ‘nuclear’ clocks10,11,12 and searches for variation of fundamental constants13 using the enhanced sensitivity of highly charged ions14.
417 citations
TL;DR: A review of femtosecond filamentation can be found in this paper with emphasis on the collective work of filamentation nonlinear optics, which is a new hot subject of current debate.
Abstract: This is a review of some recent development in femtosecond filamentation science with emphasis on our collective work. Previously reviewed work in the field will not be discussed. We thus start with a very brief description of the fundamental physics of single filamentation of powerful femtosecond laser pulses in air. Intensity clamping is emphasized. One consequence is that the peak intensity inside one or more filaments would not increase significantly even if one focuses the pulse at very high peak power even up to the peta-watt level. Another is that the clamped intensity is independent of pressure. One interesting outcome of the high intensity inside a filament is filament fusion which comes from the nonlinear change of index of refraction inside the filament leading to cross beam focusing. Because of the high intensity inside the filament, one can envisage nonlinear phenomena taking place inside a filament such as a new type of Raman red shift and the generation of very broad band supercontinuum into the infrared through four-wave-mixing. This is what we call by filamentation nonlinear optics. It includes also terahertz generation from inside the filament. The latter is discussed separately because of its special importance to those working in the field of safety and security in recent years. When the filamenting pulse is linearly polarized, the isotropic nature of air becomes birefringent both electronically (instantaneous) and through molecular wave packet rotation and revival (delayed). Such birefringence is discussed in detailed. Because, in principle, a filament can be projected to a long distance in air, applications to pollution measurement as well as other atmospheric science could be earned out. We call this filamentation atmospheric science. Thus, the following subjects are discussed briefly, namely, lightning control, rain making, remote measurement of electric field, microwave guidance and remote sensing of pollutants. A discussion on the higher order Kerr effect on the physics of filamentation is also given. This is a new hot subject of current debate. This review ends on giving our view of the prospect of progress of this field of filamentation in the future. We believe it hinges upon the development of the laser technology based upon the physical understanding of filamentation and on the reduction in price of the laser system.
240 citations
TL;DR: In this article, the authors show how using spatiotemporally coupled light fields in high harmonic generation experiments enables the production of attosecond lighthouses, i.e., sources emitting a collection of angularly well-separated light beams, each consisting of an isolated femtosecond pulse.
Abstract: Under the effect of even simple optical components, the spatial properties of femtosecond laser beams can vary over the duration of the light pulse. We show how using such spatiotemporally coupled light fields in high harmonic generation experiments (e.g., in gases or dense plasmas) enables the production of attosecond lighthouses, i.e., sources emitting a collection of angularly well-separated light beams, each consisting of an isolated attosecond pulse. This general effect opens the way to a new generation of light sources, particularly suitable for attosecond pump-probe experiments, and provides a new tool for ultrafast metrology, for instance, giving direct access to fluctuations of the carrier-envelope relative phase of even the most intense ultrashort lasers.
184 citations
TL;DR: In this article, the echo-enabled harmonic generation scheme was used for the first time to produce coherent lasing in X-ray free-electron laser at short wavelengths, and the experiment was conducted at a test facility that combines a 135.4 MeV electron accelerator with an amplifier consisting of a series of undulator magnets.
Abstract: Free-electron lasers have been successfully operated with ultrahigh brightness and excellent transverse coherence at X-ray wavelengths1,2,3,4. One of the next goals for further improvements is full coherence5,6. An obvious approach is to seed the free-electron laser interaction using a conventional source that has good temporal coherence7,8,9,10,11,12. Here, we show the first lasing of a free-electron laser with an echo-enabled harmonic generation scheme11, which shows great promise for producing coherent lasing at short wavelengths, even in the X-ray regime. The experiment was conducted at a test facility13,14 that combines a 135.4 MeV electron accelerator with an amplifier consisting of a series of undulator magnets. Lasing was achieved at the third harmonic of the seed with a gain of ∼100,000 over spontaneous radiation. The measurements show typical exponential growth and excellent spectral characteristics, as well as good intensity stability. Lasing in X-ray free-electron lasers is typically achieved by self-amplification of spontaneous emission, which is known to have non-ideal temporal coherence and suffer from beam fluctuations. Here researchers report lasing based on echo-enabled harmonic generation at the Shanghai Deep Ultraviolet free-electron laser facility.
145 citations
TL;DR: Stable broad mid-IR frequency combs can be derived from commercially available near-IRfrequency combs without an extra stabilization mechanism with mid-infrared supercontinuum generation.
Abstract: We demonstrate mid-infrared (mid-IR) supercontinuum generation (SCG) with instantaneous bandwidth from 2.2 to 5 μm at 40 dB below the peak, covering the wavelength range desirable for molecular spectroscopy and numerous other applications. The SCG occurs in a tapered As2S3 fiber prepared by in-situ tapering and is pumped by femtosecond pulses from the subharmonic of a mode-locked Er-doped fiber laser. Interference with a narrow linewidth c.w. laser verifies that the coherence properties of the near-IR frequency comb have been preserved through these cascaded nonlinear processes. With this approach stable broad mid-IR frequency combs can be derived from commercially available near-IR frequency combs without an extra stabilization mechanism.
133 citations
TL;DR: It is demonstrated that the broad-band field enhancement leads to efficient third harmonic generation in a simplified wire trapezoidal geometry when a Kerr medium is introduced, due to the lightning rod effect at the fundamental and the Purcell effect atThe induced third harmonic.
Abstract: We propose a broad-band near-infrared trapezoidal plasmonic nanoantenna, analyze it numerically using finite integration and difference time domain techniques, and explain qualitatively its performance via a multidipolar scenario as well as a conformal transformation. The plasmonic nanoantenna reported here intercepts the incoming light as if it were of cross-sectional area larger than double its actual physical size for a 1500 nm bandwidth expanding from the near-infrared to the visible spectrum. Within this bandwidth, it also confines the incoming light to its center with more than 1 order of magnitude field enhancement. This wide-band operation is achieved due to the overlapping of the different dipole resonances excited across the nanoantenna. We further demonstrate that the broad-band field enhancement leads to efficient third harmonic generation in a simplified wire trapezoidal geometry when a Kerr medium is introduced, due to the lightning rod effect at the fundamental and the Purcell effect at the...
124 citations
TL;DR: In this article, a generalized phase-matching model is proposed for the generation of high-order harmonics using laser pulse energies in the few-μJ range at high repetition rates.
Abstract: We investigate the generation of high-order harmonics using laser pulse energies in the few-μJ range at high repetition rates. We analyse how the conversion efficiency is influenced by the tight focusing geometry required for the generation of high-order harmonics under these conditions. A generalized phase-matching model allows us to discuss macroscopic phase effects independent of focal length. We present experimental results using the example of a 100 kHz laser system to generate harmonics up to the 27th order in Ar with a photon flux up to 3 × 109 photons s−1 into one harmonic order. High-repetition-rate femtosecond or even attosecond light sources open new possibilities for a broad range of applications such as time-resolved photoelectron spectroscopy and microscopy in the extreme ultraviolet regime.
120 citations
TL;DR: In this paper, the authors present theoretical studies of high-order harmonic generation (HHG) produced by nonhomogeneous fields resulting from the illumination of plasmonic nanostructures with a short laser pulse.
Abstract: We present theoretical studies of high-order-harmonic generation (HHG) produced by nonhomogeneous fields resulting from the illumination of plasmonic nanostructures with a short laser pulse. We show that both the inhomogeneity of the local fields and the confinement of the electron movement play an important role in the HHG process and lead to the generation of even harmonics and a significantly increased cutoff, more pronounced for the longer-wavelength cases studied. In order to understand and characterize the new HHG features, we employ two different approaches: the numerical solution of the time-dependent Schr\"odinger equation and the semiclassical approach known as the strong-field approximation (SFA). Both approaches predict comparable results and show the new features, but by using the semiclassical arguments behind the SFA and time-frequency analysis tools, we are able to fully understand the reasons for the cutoff extension.
114 citations
TL;DR: The numerical solution of the time-dependent Schrödinger equation (TDSE) with the electric fields obtained from 3D finite element simulations and time-frequency analysis are employed to extract more detailed information from the TDSE results and classical tools to explain the extended harmonic spectra.
Abstract: We study high-order harmonic generation (HHG) resulting from the illumination of plasmonic nanostructures with a short laser pulse of long wavelength. We demonstrate that both the confinement of the electron motion and the inhomogeneous character of the laser electric field play an important role in the HHG process and lead to a significant increase of the harmonic cutoff. In particular, in bow-tie nanostructures with small gaps, electron trajectories with large excursion amplitudes experience significant confinement and their contribution is essentially suppressed. In order to understand and characterize this feature, we combine the numerical solution of the time-dependent Schrodinger equation (TDSE) with the electric fields obtained from 3D finite element simulations. We employ time-frequency analysis to extract more detailed information from the TDSE results and classical tools to explain the extended harmonic spectra. The spatial inhomogeneity of the laser electric field modifies substantially the electron trajectories and contributes also to cutoff increase.
112 citations
TL;DR: Kim et al. as discussed by the authors proposed an efficient scheme to generate a broadband extreme-ultraviolet (xuv) continuum from high-order harmonic generation emerging from the concept of plasmonic field enhancement in the vicinity of metallic nanostructures.
Abstract: We address an efficient scheme to generate a broadband extreme-ultraviolet (xuv) continuum from high-order harmonic generation emerging from the concept of plasmonic field enhancement in the vicinity of metallic nanostructures [Kim et al., Nature (London) 453, 757 (2008)]. Based on the numerical solution of a timedependent Schrödinger equation, for moderate field intensities and depending on the inhomogeneity of the field, we are able to increase the plateau region roughly by a factor of two and generate a broadband xuv continuum. The underlying physics of the plasmon enhancement in harmonic generation is investigated in terms of the semiclassical trajectories of strong field-electron dynamics, and perfect consistency is found between quantum mechanical simulations. It is found that the field inhomogeneity plays a critical role in quantum path selection. After a critical value, we observe a systematic suppression in the long trajectories, suggesting the generation of a single isolated attosecond pulse. Finally, we investigate the dependence of cutoff position on the order of field inhomogeneity and find a β2.3∓0.2 scaling.
TL;DR: Observations reveal that the absolute efficiency of the harmonics declines for the steepest plasma density scale length L(p)→0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses.
Abstract: Harmonic generation in the limit of ultrasteep density gradients is studied experimentally. Observations reveal that, while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale lengths (${L}_{p}/\ensuremath{\lambda}l1$), the absolute efficiency of the harmonics declines for the steepest plasma density scale length ${L}_{p}\ensuremath{\rightarrow}0$, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the relativistic oscillating mirror was estimated to be in the range of ${10}^{\ensuremath{-}4}--{10}^{\ensuremath{-}6}$ of the laser pulse energy for photon energies ranging from 20--40 eV, with the best results being obtained for an intermediate density scale length.
TL;DR: Measurements reveal that, for the conditions optimum for harmonic generation, the orientation is produced by preferential ionization which depletes the sample of molecules of one orientation.
Abstract: We produce oriented rotational wave packets in CO and measure their characteristics via high harmonic generation. The wave packet is created using an intense, femtosecond laser pulse and its second harmonic. A delayed 800 nm pulse probes the wave packet, generating even-order high harmonics that arise from the broken symmetry induced by the orientation dynamics. The even-order harmonic radiation that we measure appears on a zero background, enabling us to accurately follow the temporal evolution of the wave packet. Our measurements reveal that, for the conditions optimum for harmonic generation, the orientation is produced by preferential ionization which depletes the sample of molecules of one orientation.
TL;DR: In this article, the effect of a non-resonant intense laser field on the optical rectification and second and third harmonic generation in a Poschl-Teller quantum well is theoretically investigated.
TL;DR: It is demonstrated that the two OPOs show stable spatial and temporal interference and are mutually locked in frequency and in phase and deterministically choose between the two frequency states corresponding to the two sets of modes shifted with respect to each other by half of the laser pulse repetition rate.
Abstract: We study coherence properties of a χ(2) optical parametric oscillator (OPO), which produces 2/3-octave-wide spectrum centered at the subharmonic (3120 nm) of the femtosecond pump laser. Our method consists of interfering the outputs of two identical, but independent OPOs pumped by the same laser. We demonstrate that the two OPOs show stable spatial and temporal interference and are mutually locked in frequency and in phase. By observing a collective heterodyne beat signal between the two OPOs we show that one can deterministically choose, by cavity length adjustment, between the two frequency states corresponding to the two sets of modes shifted with respect to each other by half of the laser pulse repetition rate. Moreover, we observe that the existence of two opposite phase states, a known common feature of a parametrically driven n = 2 subharmonic oscillator, reveals itself in our experiment as a common phase, 0 or π, being established through the whole set of some 300 thousand longitudinal modes.
TL;DR: The amplification of nearly octave-spanning ultrabroad pulses without spectral broadening results in good stability in output energy and CEP and high harmonics in the water window from a neon cell that corresponds to a laser intensity of 4.1×10(14) W/cm(2).
Abstract: We report on the generation of 9.0 fs, 550 μJ, carrier-envelope phase (CEP)-stabilized optical pulses around 1.6 μm at 1 kHz. Few-cycle IR pulses are obtained from a BiB3O6 optical parametric chirped-pulse amplifier. The amplification of nearly octave-spanning ultrabroad pulses without spectral broadening results in good stability in output energy (0.85% rms) and CEP (160 mrad rms). We observed high harmonics in the water window from a neon cell that corresponds to a laser intensity of 4.1×1014 W/cm2.
TL;DR: These findings demonstrate that laser excitation drives the valence band structure out of magnetic equilibrium.
Abstract: The temporal evolution of the exchange-split Δ(2)-like Σ valence bands of the 4f-ferromagnet gadolinium after femtosecond laser excitation has been studied using angle-resolved photoelectron spectroscopy based on high-order harmonic generation. The ultrafast drop of the exchange splitting reflects the magnetic response seen in femtosecond magnetic dichroism experiments. However, while the minority valence band reacts immediately, the response of the majority counterpart is delayed by 1 picosecond and is only half as fast. These findings demonstrate that laser excitation drives the valence band structure out of magnetic equilibrium.
TL;DR: In this article, a generalized approach is presented to analyze the nature of guided wave mode interactions in isotropic weakly nonlinear elastic plates, which facilitates systematic analysis of mode interaction in general and that of higher harmonic generation in particular.
Abstract: A generalized approach is presented to analyze the nature of guided wave mode interactions in isotropic weakly nonlinear elastic plates. The problem formulation is carried out in terms of the displacement gradient, which facilitates systematic analysis of mode interactions in general and that of higher harmonic generation in particular. Only cumulative harmonics are analyzed; these (1) have nonzero power flow and (2) are phase matched. Results indicate that the interaction of Rayleigh-Lamb modes of the same nature (symmetric or antisymmetric) can generate only cumulative harmonics that are symmetric modes, while the interaction between modes of opposite nature can generate only cumulative harmonics that are antisymmetric modes. A methodology for assessing cumulative higher harmonic generation (e.g., the third harmonic) is also proposed.
TL;DR: In this article, the authors describe the generation of high-order elliptically and circularly polarized harmonic spectra in an aligned H+2 molecule ion by a combination of two-colour ultrashort intense laser fields from numerical solutions of the corresponding time-dependent Schrodinger equation (TDSE).
Abstract: We describe the generation of high-order elliptically and circularly polarized harmonic spectra in an aligned H+2 molecule ion by a combination of two-colour ultrashort intense laser fields from numerical solutions of the corresponding time-dependent Schrodinger equation (TDSE). In intense bichromatic circularly and linearly or circularly polarized laser pulses with intensity I0 and angular frequencies ω0 and 2ω0, it is found that maximum molecular high-order harmonic generation (MHOHG) energies are functions of the molecular internuclear distance. Based on a classical model of laser-induced electron collisions with neighbouring ions, the optimal values of the pulse relative carrier envelope phase , the molecular internuclear distance R and the angle of molecular alignment to the laser polarization axis are obtained for efficiently producing MHOHG spectra with the maximum harmonic energy Ip + 13.5Up, where Ip is the ionization potential of the molecule and Up = I0/4meω20 is the ponderomotive energy of the continuum electron at intensity I0 and frequency ω0 of the laser pulse. The results have been confirmed from corresponding TDSE nonperturbative numerical simulations. The polarization property of the generated harmonics is also presented. The mechanism of MHOHG is further characterized with a Gabor time frequency analysis. It is confirmed that a single collision trajectory of the continuum electron with neighbouring ions dominates in the MHOHG processes. The high efficiency of the proposed MHOHG scheme provides a possible source for production of elliptically and/or circularly polarized attosecond extreme ultraviolet pulses. Circularly polarized attosecond pulses can also be generated by using intense ultrashort circularly polarized laser pulses in combination with static electric fields of comparable intensity for H+2 at equilibrium. A time frequency analysis also confirms the role of single recollisions as the dominant mechanism of the generation of circularly polarized harmonics.
TL;DR: In this article, a detailed analysis of high-order harmonic generation in atoms within the strong field approximation (SFA) by considering spatially inhomogeneous monochromatic laser fields was performed.
Abstract: We perform a detailed analysis of high-order harmonic generation (HHG) in atoms within the strong field approximation (SFA) by considering spatially inhomogeneous monochromatic laser fields. We investigate how the individual pairs of quantum orbits contribute to the harmonic spectra. We show that in the case of inhomogeneous fields the electron tunnels with two different canonical momenta. One of these momenta leads to a higher cutoff and the other one develops a lower cutoff. Furthermore, we demonstrate that the quantum orbits have a very different behavior in comparison to the homogeneous field. We also conclude that in the case of the inhomogeneous fields both odd and even harmonics are present in the HHG spectra. Within our model, we show that the HHG cutoff extends far beyond the standard semiclassical cutoff in spatially homogeneous fields. Our findings are in good agreement both with quantum-mechanical and classical models.
TL;DR: In this article, a new high-order harmonic generation mechanism reaching the "water window" spectral region was demonstrated with multiterawatt femtosecond lasers irradiating gas jets.
Abstract: We demonstrate a new high-order harmonic generation mechanism reaching the "water window" spectral region in experiments with multiterawatt femtosecond lasers irradiating gas jets. A few hundred harmonic orders are resolved, giving μJ/sr pulses. Harmonics are collectively emitted by an oscillating electron spike formed at the joint of the boundaries of a cavity and bow wave created by a relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by particle-in-cell simulations.
TL;DR: In this paper, a parametrization of the solution of the eigenvalue equation for lasing at odd harmonics is presented, and an explicit expression for FEL gain length, taking into account all essential effects.
Abstract: Harmonic lasing in a free electron laser with a planar undulator (under the condition that the fundamental frequency is suppressed) might be a cheap and efficient way of extension of wavelength ranges of existing and planned x-ray free electron laser (FEL) facilities. Contrary to nonlinear harmonic generation, harmonic lasing can provide much more intense, stable, and narrow-band FEL beam which is easier to handle due to the suppressed fundamental frequency. In this paper we perform a parametrization of the solution of the eigenvalue equation for lasing at odd harmonics, and present an explicit expression for FEL gain length, taking into account all essential effects. We propose and discuss methods for suppression of the fundamental harmonic. We also suggest a combined use of harmonic lasing and lasing at the retuned fundamental wavelength in order to reduce bandwidth and to increase brilliance of x-ray beam at saturation. Considering 3rd harmonic lasing as a practical example, we come to the conclusion that it is much more robust than usually thought, and can be widely used in the existing or planned x-ray FEL (XFEL) facilities. In particular, Linac Coherent Light Source (LCLS) after a minor modification can lase to saturation at the 3rd harmonic up to the photon energy of 25--30 keV providing multigigawatt power level and narrow bandwidth. As for the European XFEL, harmonic lasing would allow one to extend operating range (ultimately up to 100 keV), to reduce FEL bandwidth and to increase brilliance, to enable two-color operation for pump-probe experiments, and to provide more flexible operation at different electron energies. Similar improvements can be realized in other x-ray FEL facilities with gap-tunable undulators like FLASH II, SACLA, LCLS II, etc. Harmonic lasing can be an attractive option for compact x-ray FELs (driven by electron beams with a relatively low energy), allowing the use of the standard undulator technology instead of small-gap in-vacuum devices. Finally, in this paper we discover that in a part of the parameter space, corresponding to the operating range of soft x-ray beam lines of x-ray FEL facilities (like SASE3 beam line of the European XFEL), harmonics can grow faster than the fundamental wavelength. This feature can be used in some experiments, but might also be an unwanted phenomenon, and we discuss possible measures to diminish it.
TL;DR: In this article, the authors review the current state of tabletop extreme ultraviolet (XUV) sources based on high harmonic generation (HHG) in femtosecond enhancement cavities (fsEC), including current technical challenges to increasing the photon flux and maximum photon energy produced by this type of system.
Abstract: We review the current state of tabletop extreme ultraviolet (XUV) sources based on high harmonic generation (HHG) in femtosecond enhancement cavities (fsEC). Recent developments have enabled generation of high photon flux (1014 photons s?1) in the XUV, at high repetition rates (>50?MHz) and spanning the spectral region from 40 to 120?nm. This level of performance has enabled precision spectroscopy with XUV frequency combs and promises further applications in XUV spectroscopic and photoemission studies. We discuss the theory of operation and experimental details of the fsEC and XUV generation based on HHG, including current technical challenges to increasing the photon flux and maximum photon energy produced by this type of system. Current and future applications for these sources are also discussed.
TL;DR: In this article, an intuitive semiclassical model that makes use of the recently measured initial transverse momentum of tunneling ionization was developed to predict the dependence of the high-orderharmonic yield on driving laser ellipticity.
Abstract: High-order-harmonic-generation yield is remarkably sensitive to driving laser ellipticity, which is interesting from a fundamental point of view as well as for applications. The most well-known example is the generation of isolated attosecond pulses via polarization gating. We develop an intuitive semiclassical model that makes use of the recently measured initial transverse momentum of tunneling ionization. The model is able to predict the dependence of the high-order-harmonic yield on driving laser ellipticity and is in good agreement with experimental results and predictions from a numerically solved time-dependent Schr\"odinger equation.
06 May 2012
TL;DR: Experiments on resonant second-harmonic generation from planar gold split-ring-resonator arrays under normal incidence of light as a function of the lattice constant show that optimum nonlinear conversion occurs at intermediate lattice constants.
Abstract: We perform experiments on resonant second-harmonic generation from planar gold split-ring-resonator arrays under normal incidence of light as a function of the lattice constant. Optimum nonlinear conversion occurs at intermediate lattice constants.
TL;DR: In this article, a nonlinear convolutional signal analysis for non-destructive testing of nonlinear systems using vibrations and elastic waves is proposed, which allows to extract the vibrational/acoustical responses of the system, at the excitation frequency and importantly, also at higher harmonics, with the help of a non linear convolution signal analysis.
Abstract: A method developed for the analysis of nonlinear systems is applied for the first time to non-destructive testing of diverse materials using vibrations and elastic waves. This method allows to extract the vibrational/acoustical responses of the system, at the excitation frequency and importantly, also at higher harmonics, with the help of a nonlinear convolution signal analysis. It is then possible to make use of the robust nonlinear resonance method together with the harmonic generation method in order to analyze the nonlinear elastic resonances of a sample at excitation frequency harmonics. Definitions of the nonlinear hysteretic parameters associated to higher harmonic resonances are provided. The bases of the signal analysis method are also described. A higher sensitivity to the presence of gradual damage compared to the classical nonlinear resonance method is demonstrated experimentally for diverse materials and configurations.
TL;DR: Ohyabu et al. as mentioned in this paper proposed a cavity mode to avoid mode competition with a neighboring fundamental harmonic mode, which achieved an oscillation power of 83 εkW at about 389 GHz.
Abstract: New power records of second harmonic gyrotron oscillation have been demonstrated in the sub-THz band. The first step gyrotron of demountable type had succeeded in oscillation with power more than 50 kW at 350 GHz and nearly 40 kW at 390 GHz [T. Notake et al., Phys. Rev. Lett. 103, 225002 (2009)]. Then, the second step gyrotron of sealed-off type was manufactured. A cavity mode was carefully selected to avoid mode competition with a neighboring fundamental harmonic mode. Matching of the selected mode with the electron gun was also circumspectly considered. The second step gyrotron has attained higher power radiation than the first gyrotron. The maximum single mode power was 62 kW at 388 GHz. Then, the electron gun was modified for use of a different cavity mode with a higher coupling coefficient than that for the 62 kW mode. The new mode proved single mode oscillation power of 83 kW at about 389 GHz. These results are new second-harmonic-oscillation power records for sub-THz gyrotrons. The present study constitutes foundations of development of high power second harmonic sub-THz gyrotron for application to collective Thomson scattering measurement on fusion plasmas, especially on high-density plasmas such as those produced in LHD [N. Ohyabu et al., Phys. Rev. Lett. 97, 055002 (2006)]. This paper reports the design consideration to realize high power single mode gyrotron oscillation at second harmonic and the examination of oscillation characteristics of the gyrotron.
TL;DR: In this paper, the authors report the extension of hollow-core fiber pulse compression to longer wavelengths and demonstrate the beneficial effect of few-cycle pulses which enable higher saturation intensities on target compared to multicycle pulses.
Abstract: We report the extension of hollow-core fibre pulse compression to longer wavelengths. High-energy multi-cycle infrared pulses are generated via optical parametric amplification and subsequently broadened in the fibre. 2.5-cycle pulses at the Signal wavelength (1.4 ?m) and 1.6-cycle pulses at the Idler wavelength (1.8 ?m) in the sub-millijoule regime have been generated. New compression schemes can be applied at 1.8 ?m and beyond. In this manner, 1.6-cycle carrier envelope phase stable pulses were generated by linear propagation in the anomalous dispersion regime of bulk glass which surprisingly enables compression below its third-order dispersion limit. Furthermore, a dispersion-free way of controlling the carrier envelope phase is demonstrated. Moreover, we experimentally confirm the increase in high-harmonic cut-off energy with driving laser wavelength and demonstrate the beneficial effect of few-cycle pulses which enable higher saturation intensities on target compared to multi-cycle pulses. It will be an ideal tool for future synthesis of isolated attosecond pulses in the sub-keV regime. With this laser source, we revealed for the first time multi-electron effects in high harmonic generation in xenon.
TL;DR: The methodology of orienting polar molecules is brought together with the phase sensitivity of high harmonic spectroscopy to experimentally compare the phase difference of attosecond bursts of radiation emitted upon electron recollision from different ends of a polar molecule.
Abstract: We bring the methodology of orienting polar molecules together with the phase sensitivity of high harmonic spectroscopy to experimentally compare the phase difference of attosecond bursts of radiation emitted upon electron recollision from different ends of a polar molecule. This phase difference has an impact on harmonics from aligned polar molecules, suppressing emission from the molecules parallel to the driving laser field while favoring the perpendicular ones. For oriented molecules, we measure the amplitude ratio of even to odd harmonics produced when intense light irradiates CO molecules and determine the degree of orientation and the phase difference of attosecond bursts using molecular frame ionization and recombination amplitudes. The sensitivity of the high harmonic spectrum to subtle phase differences in the emitted radiation makes it a detailed probe of polar molecules and will drive major advances in the theory of high harmonic generation.
TL;DR: In this paper, the effect of electron correlation and the character of the atomic orbital basis set on the electron dynamics associated with high-harmonic generation (HHG) spectra is analyzed.
Abstract: In this work, we present a study of H2 and N2 electron dynamics in intense laser fields with a specific focus on high-harmonic generation (HHG) spectroscopy. We performed this study with different theoretical methods: time-dependent configuration interaction singles (TD-CIS), perturbative doubles (TD-CIS(D)), time-dependent equation-of-motion coupled-cluster singles and doubles (TD-EOM-EE-CCSD) and time-dependent density-functional theory (TDDFT). All methods were implemented using a finite expansion in field-free eigenstates, and additionally direct propagation of the time-dependent density was employed for TDDFT. Within the sum over states approximation, the effect of electron correlation and the character of the atomic orbital basis set on the electron dynamics associated with HHG spectra is analysed. With respect to basis set, use of multiple sets of diffuse functions was found to be essential, while with respect to electron correlation, the treatment of double excitations in EOM-CCSD significantly af...