Showing papers on "High harmonic generation published in 2009"
TL;DR: In this article, the history of the discovery of the nonlinear optical crystal KBe2BO3F2 (KBBF) is described, with a full description of its growth, space structure, optical properties, and capability to generate deep-ultraviolet (UV) harmonic generation.
Abstract: This review introduces in detail the history of the discovery of the nonlinear optical crystal KBe2BO3F2 (KBBF), with a full description of its growth, space structure, optical properties, and capability to generate deep-ultraviolet (UV) harmonic generation. Several important applications developed recently using all-solid-state deep-UV light sources, such as super-high-resolution laser photoemission spectrograph and 193 nm photolithography, will be reviewed.
542 citations
TL;DR: In this paper, the authors survey the development of high-order harmonic generation of femtosecond laser pulses by means of laser-produced plasmas and discuss the prospects for applying HHG as a short-wavelength coherent optical tool.
Abstract: The investigation of high-order harmonic generation (HHG) of femtosecond laser pulses by means of laser-produced plasmas is surveyed. This kind of harmonic generation is an alternative to the HHG in gases and shows significantly higher conversion efficiency. Furthermore, with plasma targets there is no limitation on applicable laser intensity and thus the generated harmonics can be much more intense. In principle, harmonic light may also be generated at relativistic laser intensity, in which case their harmonic intensities may even exceed that of the focused laser pulse by many orders of magnitude. This phenomenon presents new opportunities for applications such as nonlinear optics in the extreme ultraviolet region, photoelectron spectroscopy, and opacity measurements of high-density matter with high temporal and spatial resolution. On the other hand, HHG is strongly influenced by the laser-plasma interaction itself. In particular, recent results show a strong correlation with high-energy electrons generated during the interaction process. The harmonics are a promising tool for obtaining information not only on plasma parameters such as the local electron density, but also on the presence of large electric and magnetic fields, plasma waves, and the (electron) transport inside the target. This paper reviews the theoretical and experimental progress on HHGmore » via laser-plasma interactions and discusses the prospects for applying HHG as a short-wavelength, coherent optical tool.« less
363 citations
TL;DR: The rapidly decreasing microscopic single-atom yield, predicted for harmonics driven by longer-wavelength lasers, is compensated macroscopically by an increased optimal pressure for phase matching and a rapidly decreasing reabsorption of the generated X-rays, making tabletop, fully coherent, multi-keV X-ray sources feasible.
Abstract: We show how bright, tabletop, fully coherent hard X-ray beams can be generated through nonlinear upconversion of femtosecond laser light. By driving the high-order harmonic generation process using longer-wavelength midinfrared light, we show that, in theory, fully phase-matched frequency upconversion can extend into the hard X-ray region of the spectrum. We verify our scaling predictions experimentally by demonstrating phase matching in the soft X-ray region of the spectrum around 330 eV, using ultrafast driving laser pulses at 1.3-μm wavelength, in an extended, high-pressure, weakly ionized gas medium. We also show through calculations that scaling of the overall conversion efficiency is surprisingly favorable as the wavelength of the driving laser is increased, making tabletop, fully coherent, multi-keV X-ray sources feasible. The rapidly decreasing microscopic single-atom yield, predicted for harmonics driven by longer-wavelength lasers, is compensated macroscopically by an increased optimal pressure for phase matching and a rapidly decreasing reabsorption of the generated X-rays.
347 citations
TL;DR: An experimental study of the high harmonic yield over a wavelength range of 800-1850 nm found that the scaling at constant laser intensity is lambda(-6.3+/-1.1) in Xe and lambda(-(5-6)) in Kr over the wavelength range, somewhat worse than the theoretical predictions.
Abstract: Using longer wavelength laser drivers for high harmonic generation is desirable because the highest extreme ultraviolet frequency scales as the square of the wavelength. Recent numerical studies predict that high harmonic efficiency falls dramatically with increasing wavelength, with a very unfavorable lambda(-(5-6)) scaling. We performed an experimental study of the high harmonic yield over a wavelength range of 800-1850 nm. A thin gas jet was employed to minimize phase matching effects, and the laser intensity and focal spot size were kept constant as the wavelength was changed. Ion yield was simultaneously measured so that the total number of emitting atoms was known. We found that the scaling at constant laser intensity is lambda(-6.3+/-1.1) in Xe and lambda(-6.5+/-1.1) in Kr over the wavelength range of 800-1850 nm, somewhat worse than the theoretical predictions.
309 citations
09 Dec 2009
TL;DR: In this article, the effect of coherent and incoherent synchrotron radiation in the dispersion sections is addressed, and an echo-enabled harmonic generation (EEHG) scheme is proposed.
Abstract: The Echo-Enabled Harmonic Generation (EEHG) FEL uses two modulators in combination with two dispersion sections to generate a high-harmonic density modulation starting with a relatively small initial energy modulation of the beam. After presenting the concept of the EEHG, we address several practically important issues, such as the effect of coherent and incoherent synchrotron radiation in the dispersion sections. Using a representative realistic set of beam parameters, we show how the EEHG scheme enhances the FEL performance and allows one to generate a fully (both longitudinally and transversely) coherent radiation. We then discuss application of the echo modulation for generation of attosecond pulses of radiation, and also using echo for generation of terahertz radiation. We present main parameters of a proof-of-principle experiment currently being planned at SLAC for demonstration of the echo modulation mechanism.
285 citations
TL;DR: A one-dimensional model of the echo effect is developed which allows us to optimize the amplitude of the modulation for a given harmonic number.
Abstract: We propose to use the mechanism of an echo effect previously observed in hadron accelerators for up-frequency conversion of density modulation in an electron beam. We show that, for generation of high harmonics, this method is much more efficient in comparison with the currently used approach. A one-dimensional model of the effect is developed which allows us to optimize the amplitude of the modulation for a given harmonic number.
281 citations
TL;DR: In this paper, a quantitative rescattering theory for high-order harmonic generation (HHG) by intense laser pulses is presented, which can be expressed as a product of a returning electron wave packet and the photorecombination differential cross section of the laser free continuum electron back to the initial bound state.
Abstract: The quantitative rescattering theory (QRS) for high-order harmonic generation (HHG) by intense laser pulses is presented. According to the QRS, HHG spectra can be expressed as a product of a returning electron wave packet and the photorecombination differential cross section of the laser-free continuum electron back to the initial bound state. We show that the shape of the returning electron wave packet is determined mostly by the laser. The returning electron wave packets can be obtained from the strong-field approximation or from the solution of the time-dependent Schr\"odinger equation (TDSE) for a reference atom. The validity of the QRS is carefully examined by checking against accurate results for both harmonic magnitude and phase from the solution of the TDSE for atomic targets within the single active electron approximation. Combining with accurate transition dipoles obtained from state-of-the-art molecular photoionization calculations, we further show that available experimental measurements for HHG from partially aligned molecules can be explained by the QRS. Our results show that quantitative description of the HHG from aligned molecules has become possible. Since infrared lasers of pulse durations of a few femtoseconds are easily available in the laboratory, they may be used for dynamic imaging of a transient molecule with femtosecond temporal resolutions.
274 citations
TL;DR: The perfect waveform is presented which, during a strong field interaction, generates the maximum possible electron recollision energy for any given oscillation period, over 3 times as high as that for a pure sinusoidal wave.
Abstract: We present the perfect waveform which, during a strong field interaction, generates the maximum possible electron recollision energy for any given oscillation period, over 3 times as high as that for a pure sinusoidal wave. This ideal waveform has the form of a linear ramp with a dc offset. A genetic algorithm was employed to find an optimized practically achievable waveform composed of a longer wavelength field, to provide the offset, in addition to higher frequency components. This second waveform is found to be capable of generating electron recollision energies as high as those for the perfect waveform while retaining the high recollision amplitudes of a pure sinusoidal wave. Calculations of high harmonic generation demonstrate this enhancement, by increasing the cutoff energy by a factor of 2.5 while maintaining the harmonic yield, providing an enhanced tool for attosecond science.
196 citations
TL;DR: In this paper, the authors developed an experimental procedure for characterizing fatigue damage in metallic plates using nonlinear guided waves, and the normalized acoustic nonlinearity of low cycle fatigue damaged aluminum specimens was measured with Lamb waves.
Abstract: This research develops an experimental procedure for characterizing fatigue damage in metallic plates using nonlinear guided waves. The work first considers the propagation of nonlinear waves in a dispersive medium and determines the theoretical and practical considerations for the generation of higher order harmonics in guided waves. By using results from the nonlinear optics literature, it is possible to demonstrate that both phase and group velocity matching are essential for the practical generation of nonlinear guided elastic waves. Next, the normalized acoustic nonlinearity of low cycle fatigue damaged aluminum specimens is measured with Lamb waves. A pair of wedge transducers is used to generate and detect the fundamental and second harmonic Lamb waves. The results show that the normalized acoustic nonlinearity measured with Lamb waves is directly related to fatigue damage in a fashion that is similar to the behavior of longitudinal and Rayleigh waves. This normalized acoustic nonlinearity is then compared with the measured cumulative plastic strain to confirm that these two parameters are related, and to reinforce the notion that Lamb waves can be used to quantitatively assess plasticity driven fatigue damage using established higher harmonic generation techniques.
189 citations
TL;DR: Experiments on the propagation of intense, femtosecond, self-bending Airy laser beams in water reveal the changing character of the laser-pulse evolution on propagation.
Abstract: We report experiments on the propagation of intense, femtosecond, self-bending Airy laser beams in water. The supercontinuum radiation generated along the curved beam path is angularly resolved in the far field. Spectral maps of this radiation reveal the changing character of the laser-pulse evolution on propagation.
184 citations
TL;DR: A laser frequency comb allows the conversion of the very rapid oscillations of visible light of some 100's of THz down to frequencies that can be handled with conventional electronics as mentioned in this paper.
Abstract: A laser frequency comb allows the conversion of the very rapid oscillations of visible light of some 100’s of THz down to frequencies that can be handled with conventional electronics. This capability has enabled the most precise laser spectroscopy experiments yet that allowed to test quantum electrodynamics, to determine fundamental constants and to search for possible slow changes of these constants. Using an optical frequency reference in combination with a laser frequency comb has made it possible to construct all optical atomic clocks, that are now outperforming even the best cesium atomic clocks. In future direct frequency comb spectroscopy might enable high resolution laser spectroscopy in the extreme ultraviolet for the first time. Frequency combs are also used to calibrate astronomical spectrographs and might reach an accuracy that is sufficient to observe the expansion of the universe in real time.
TL;DR: In this paper, the echo-enabled harmonic generation (EEHG) free electron laser (FEL) was studied and the effect of incoherent synchrotron radiation in the dispersion sections, and the beam transverse size effect in the modulator.
Abstract: In this paper, we systematically study the echo-enabled harmonic generation (EEHG) free electron laser (FEL). The EEHG FEL uses two modulators in combination with two dispersion sections that allow to generate in the beam a high harmonic density modulation starting with a relatively small initial energy modulation of the beam. After presenting analytical theory of the phenomenon, we address several practically important issues, such as the effect of incoherent synchrotron radiation in the dispersion sections, and the beam transverse size effect in the modulator. Using a representative realistic set of beam parameters, we show how the EEHG scheme enhances the FEL performance and allows to generate a fully (both longitudinally and transversely) coherent radiation. As an example, we demonstrate that 5 nm coherent soft x-rays with GW peak power can be generated directly from the 240 nm seeding laser using the proposed EEHG scheme.
TL;DR: In this article, the transfer of frequency combs to the vacuum-ultraviolet with potential applications in spectroscopy has been discussed, where high-harmonic generation in atoms has led to fast sources of short-wavelength photons.
Abstract: Frequency combs have revolutionized frequency metrology. High-harmonic generation in atoms has led to fast sources of short-wavelength photons. Combining these two technologies enables the transfer of frequency combs to the vacuum-ultraviolet with potential applications in spectroscopy.
TL;DR: The spectra exhibit a deep minimum that is shown to be independent of the laser intensity, and is thus a clear measure of the electronic structure of the atom, suggesting that electronic structure can be accurately determined in high-harmonic experiments despite the presence of the strong laser field.
Abstract: We report detailed measurements of the high-harmonic spectra generated from argon atoms. The spectra exhibit a deep minimum that is shown to be independent of the laser intensity, and is thus a clear measure of the electronic structure of the atom. We show that exact field-free continuum wave functions reproduce the minimum, but plane wave and Coulomb wave functions do not. This remarkable observation suggests that electronic structure can be accurately determined in high-harmonic experiments despite the presence of the strong laser field. Our results clarify the relation between high-harmonic generation and photoelectron spectroscopy. The use of exact continuum functions also resolves the ambiguity associated with the choice of the dispersion relation.
TL;DR: In this article, the sizeable crystals of RbBe2(BO3)F2 (RBBF) were obtained by the flux method and the crystal structure was determined by x-ray data and the space group was proven to be R32.
Abstract: Sizeable crystals of RbBe2(BO3)F2 (RBBF) were obtained by the flux method. The crystal structure was determined by x-ray data and the space group was proven to be R32, belonging to the uniaxial class. The linear and nonlinear optical parameters, including the cutoff wavelength, refractive indices, phase-matching angles, and effective nonlinear optical coefficients were determined for the first time to our knowledge, and then the Sellmeier equations were also constructed. By using an RBBF prism coupling device (PCD), tunable fourth-harmonic output from a Ti:sapphire laser and the sixth harmonic of an Nd-based laser were also obtained with relatively high power.
TL;DR: In this paper, the generation of intense terahertz radiation in ultrafast laser-gas interactions is studied on a basis of transient electron current model, and the generation mechanism is examined with an analytic derivation and numerical simulations, in which tunneling ionization and subsequent electron motion in the combined laser field play a key role.
Abstract: The generation of intense terahertz radiation in ultrafast laser-gas interactions is studied on a basis of transient electron current model. When an ultrashort pulse laser’s fundamental and its second harmonic fields are mixed to ionize a gas, a nonvanishing, directional photoelectron current can be produced, which simultaneously emits terahertz radiation in the far field. Here, the generation mechanism is examined with an analytic derivation and numerical simulations, in which tunneling ionization and subsequent electron motion in the combined laser field play a key role. In the simulations, three types of laser-gas interactions are considered: (i) mixing the fundamental and its second harmonic fields, (ii) mixing nonharmonic, two-color fields, and (iii) focusing single-color, few-cycle pulses. In these interactions, terahertz generation and other nonlinear effects driven by the transient current are investigated. In particular, anticorrelation between terahertz and second (or third) harmonic generation ...
TL;DR: In this paper, a classical electrodynamic theory was developed to study the optical nonlinearities of metallic nanoparticles, where quasi free electrons inside the metal were approximated as a classical Coulomb-interacting electron gas, and their motion under the excitation of an external electromagnetic field was described by the plasma equations.
Abstract: In this paper, we develop a classical electrodynamic theory to study the optical nonlinearities of metallic nanoparticles. The quasi free electrons inside the metal are approximated as a classical Coulomb-interacting electron gas, and their motion under the excitation of an external electromagnetic field is described by the plasma equations. This theory is further tailored to study second-harmonic generation. Through detailed experiment-theory comparisons, we validate this classical theory as well as the associated numerical algorithm. It is demonstrated that our theory not only provides qualitative agreement with experiments but it also reproduces the overall strength of the experimentally observed second-harmonic signals.
TL;DR: In this article, the authors presented a method of producing single attosecond pulses by using a few-cycle (5 fs) driving pulse with two additional weak control pulses, based on the high-order harmonic spectrum, classical ionizing and returning energy maps, time-frequency maps, and time profiles of the pulses.
Abstract: We present a method of producing single attosecond pulses by using a few-cycle (5 fs) driving pulse with two additional weak control pulses. We discuss how single attosecond pulses produced from high-order harmonic generation processes in a synthesized three-colour laser field are similar to those processes in a much shorter single-colour laser field. Based on the high-order harmonic spectrum, classical ionizing and returning energy maps, time-frequency maps and time profiles of the attosecond pulses, the actions of the synthesized three-colour laser field are analogous to a 3 fs field although some differences still exist, and our method is proved to be a potential way to reduce the attosecond pulse duration from high-order harmonic generation with a currently available ultrafast laser source instead of a shorter pulse.
10 Dec 2009
TL;DR: In this paper, it was shown that the ionization gating of high-harmonic emission on the leading edge of the driving pulse produces isolated attosecond pulses with a contrast ratio (the energy in the main pulse normalized to the energy in adjacent satellite pulses) c ¼ 3:3 � 0:2.
Abstract: Combining results from several techniques of attosecond spectroscopy, we show that ionization gating of high-harmonic emission on the leading edge of the driving pulse produces isolated attosecond pulses with a contrast ratio (the energy in the main pulse normalized to the energy in adjacent satellite pulses) c ¼ 3:3 � 0:2. Half-cycle cutoff analysis confirms that harmonic generation proceeds in the ionizationgated regime. The attosecond pulse contrast is measured using the technique of carrier–envelope phase (CEP)-scanning, recently developed by our group, in which photoelectrons generated from Ne atoms by the harmonic pulse are streaked as a function of CEP. Streaking of photoelectrons as a function of attosecond time delay also confirms the isolated nature of the harmonic pulse, which is measured to have a duration of 430 � 15 as, limited by the bandwidth of the reflective X-ray optics employed. The combined measurements imply that the experimental advantages of the ionization gating technique—tunable X-ray emission, relaxed sensitivity to the CEP and scalability to longer driver pulses—are also conferred on isolated attosecond pulse production. Published by Elsevier B.V.
TL;DR: Frolov et al. as discussed by the authors proposed a closed-form analytic formula for high-order harmonic generation (HHG) rates for atoms, which generalizes an HHG formula for negative ions.
Abstract: A closed-form analytic formula for high-order harmonic generation (HHG) rates for atoms (that generalizes an HHG formula for negative ions [M. V. Frolov, J. Phys. B 42, 035601 (2009)10.1088/0953-4075/42/3/035601]) is used to study laser wavelength scaling of the HHG yield for harmonic energies in the cutoff region of the HHG plateau. We predict increases of the harmonic power for HHG by Ar, Kr, and Xe with increasing wavelength lambda over atom-specific intervals of lambda in the infrared region, lambda approximately (0.8-2.0) microm.
TL;DR: In this paper, a crack detection technique based on nonlinear acoustics is investigated, where acoustic waves at a chosen frequency are generated using an actuating lead zirconate titanate (PZT) transducer, and they travel through the target structure before being received by sensing PZT wafer.
Abstract: A crack detection technique based on nonlinear acoustics is investigated in this study. Acoustic waves at a chosen frequency are generated using an actuating lead zirconate titanate (PZT) transducer, and they travel through the target structure before being received by a sensing PZT wafer. Unlike an undamaged medium, a cracked medium exhibits high acoustic nonlinearity which is manifested as harmonics in the power spectrum of the received signal. Experimental results also indicate that the harmonic components increase nonlinearly in magnitude with increasing amplitude of the input signal. The proposed technique identifies the presence of cracks by looking at the two aforementioned features: harmonics and their nonlinear relationship to the input amplitude. The effectiveness of the technique has been tested on aluminum and steel specimens. The behavior of these nonlinear features as crack propagates in the steel beam has also been studied.
TL;DR: The first experimental demonstration of high-order harmonic generation in rare gases driven by a state-of-the-art high-power Yb-doped-fiber chirped-pulse amplification system is reported, bringing ultrashort XUV coincidence experiments from synchrotron facilities to tabletop laboratories.
Abstract: We report the first experimental demonstration (to our knowledge) of high-order harmonic generation in rare gases driven by a state-of-the-art high-power Yb-doped-fiber chirped-pulse amplification system. The fiber laser delivers 270 fs pulses in the 30-100 μJ energy range at repetition rates varying from 100 kHz to 1 MHz. A proper focalization allows reaching several 1013 W/cm2 in a gas jet. We have been able to produce and detect harmonics up to order 31 (33.2 nm) in Ar at a 100 kHz repetition rate. High-order harmonic generation at 1 MHz is also demonstrated in Xe up to harmonic 15. The demonstrated extreme UV (XUV) source will bring ultrashort XUV coincidence experiments from synchrotron facilities to tabletop laboratories.
TL;DR: A nonlinear contribution is unambiguously identified that originates specifically from the interplay of the local fields of the split-ring resonators and the bulk GaAs second-order nonlinear-susceptibility tensor.
Abstract: We study second-harmonic generation from gold split-ring resonators on a crystalline GaAs substrate. By systematically varying the relative orientation of the split-ring resonators with respect to the incident linear polarization of light and the GaAs crystallographic axes, we unambiguously identify a nonlinear contribution that originates specifically from the interplay of the local fields of the split-ring resonators and the bulk GaAs second-order nonlinear-susceptibility tensor. The experimental results are in good agreement with theoretical modeling.
TL;DR: High-order harmonic generation from C60 by an intense femtosecond Ti:sapphire laser is demonstrated for the first time, indicating fullerenes as the source of high-order harmonics.
Abstract: We demonstrate, for the first time, high-order harmonic generation from C60 by an intense femtosecond Ti:sapphire laser. Laser-produced plasmas from C60-rich epoxy and C60 films were used as the nonlinear media. Harmonics up to the 19th order were observed. The harmonic yield from fullerene-rich plasma is about 25 times larger compared with those produced from a bulk carbon target. Structural studies of plasma debris confirm the presence and integrity of fullerenes within the plasma plume, indicating fullerenes as the source of high-order harmonics.
TL;DR: In this paper, the authors proposed a new type of hollow core photonic crystal fiber (HC-PCF) for high harmonic generation (HHG) with high power diode-pumped multi-megahertz laser systems.
Abstract: High harmonic generation (HHG) of intense infrared laser radiation (Ferray et al., J. Phys. B: At. Mol. Opt. Phys. 21:L31, 1988; McPherson et al., J. Opt. Soc. Am. B 4:595, 1987) enables coherent vacuum-UV (VUV) to soft-X-ray sources. In the usual setup, energetic femtosecond laser pulses are strongly focused into a gas jet, restricting the interaction length to the Rayleigh range of the focus. The average photon flux is limited by the low conversion efficiency and the low average power of the complex laser amplifier systems (Keller, Nature 424:831, 2003; Sudmeyer et al., Nat. Photonics 2:599, 2008; Roser et al., Opt. Lett. 30:2754, 2005; Eidam et al., IEEE J. Sel. Top. Quantum Electron. 15:187, 2009) which typically operate at kilohertz repetition rates. This represents a severe limitation for many experiments using the harmonic radiation in fields such as metrology or high-resolution imaging. Driving HHG with novel high-power diode-pumped multi-megahertz laser systems has the potential to significantly increase the average photon flux. However, the higher average power comes at the expense of lower pulse energies because the repetition rate is increased by more than a thousand times, and efficient HHG is not possible in the usual geometry. So far, two promising techniques for HHG at lower pulse energies were developed: external build-up cavities (Gohle et al., Nature 436:234, 2005; Jones et al., Phys. Rev. Lett. 94:193, 2005) and resonant field enhancement in nanostructured targets (Kim et al., Nature 453:757, 2008). Here we present a third technique, which has advantages in terms of ease of HHG light extraction, transverse beam quality, and the possibility to substantially increase conversion efficiency by phase-matching (Paul et al., Nature 421:51, 2003; Ren et al., Opt. Express 16:17052, 2008; Serebryannikov et al., Phys. Rev. E (Stat. Nonlinear Soft Matter Phys.) 70:66611, 2004; Serebryannikov et al., Opt. Lett. 33:977, 2008; Zhang et al., Nat. Phys. 3:270, 2007). The interaction between the laser pulses and the gas occurs in a Kagome-type Hollow-Core Photonic Crystal Fiber (HC-PCF) (Benabid et al., Science 298:399, 2002), which reduces the detection threshold for HHG to only 200 nJ. This novel type of fiber guides nearly all of the light in the hollow core (Couny et al., Science 318:1118, 2007), preventing damage even at intensities required for HHG. Our fiber guided 30-fs pulses with a pulse energy of more than 10 μJ, which is more than five times higher than for any other photonic crystal fiber (Hensley et al., Conference on Lasers and Electro-Optics (CLEO), IEEE Press, New York, 2008).
31 May 2009
TL;DR: In this paper, the authors report visible (green) third-harmonic generation in silicon by launching near-infrared picosecond pulses into highly confined photonic crystal waveguides.
Abstract: We report visible (green) third-harmonic generation in silicon by launching near-infrared picosecond pulses into highly confined photonic crystal waveguides. We demonstrate slow light enhancement of this nonlinear process.
TL;DR: In this article, an experimental investigation of 18 cubic concrete specimens, caste with three different water-cement ratios, using non-linear ultrasonic technique is presented, where the specimens were ultrasonically evaluated both in damaged and undamaged conditions.
TL;DR: In this article, a time-dependent density-functional theory approach with proper long-range potential for an ab initio study of the effect of correlated multielectron responses on the multiphoton ionization (MPI) and high-order harmonic generation (HHG) of diatomic molecules was presented.
Abstract: We present a time-dependent density-functional theory approach with proper long-range potential for an ab initio study of the effect of correlated multielectron responses on the multiphoton ionization (MPI) and high-order harmonic generation (HHG) of diatomic molecules ${\text{N}}_{2}$ and ${\text{F}}_{2}$ in intense short laser pulse fields with arbitrary molecular orientation. We show that the contributions of inner molecular orbitals to the total MPI probability can be sufficiently large or even dominant over the highest-occupied molecular orbital, depending on detailed electronic structure and symmetry, laser field intensity, and orientation angle. The multielectron effects in HHG are also very important. They are responsible for enhanced HHG at some orientations of the molecular axis. Even strongly bound electrons may have a significant influence on the HHG process.
TL;DR: It is argued that the spatial resolution is limited by the lack of electron energy filtering in this particular demonstration experiment, and problems with extensive space charge effects are solved by reducing peak intensity while maintaining a sufficient average intensity to allow imaging.
Abstract: We report the first experiments carried out on a new imaging setup, which combines the high spatial resolution of a photoemission electron microscope (PEEM) with the temporal resolution of extreme ultraviolet (XUV) attosecond pulse trains. The very short pulses were provided by high-harmonic generation and used to illuminate lithographic structures and Au nanoparticles, which, in turn, were imaged with a PEEM resolving features below 300 nm. We argue that the spatial resolution is limited by the lack of electron energy filtering in this particular demonstration experiment. Problems with extensive space charge effects, which can occur due to the low probe pulse repetition rate and extremely short duration, are solved by reducing peak intensity while maintaining a sufficient average intensity to allow imaging. Finally, a powerful femtosecond infrared (IR) beam was combined with the XUV beam in a pump-probe setup where delays could be varied from subfemtoseconds to picoseconds. The IR pump beam could induce multiphoton electron emission in resonant features on the surface. The interaction between the electrons emitted by the pump and probe pulses could be observed.
TL;DR: In this article, a 8.2 W UV laser with the compact extra-cavity sum-frequency mixing was reported with a high power and high beam quality Q-switched Nd:YVO4 oscillator.
Abstract: A 8.2 W UV laser was reported with the compact extra-cavity sum-frequency mixing. The IR fundamental frequency source was a high power and high beam quality Q-switched Nd:YVO4 oscillator. 38 W fundamental frequency laser at 1064 nm was obtained at the pulse repetition rate of 450 kHz with the beam quality factors of M2x = 1.27, M2y = 1.21. The type I and type II phase-matched LBO crystals were used as the extra-cavity frequency doubling and mixing crystals respectively. At 38 kHz, 8.2 W UV laser at 355 nm was achieved with the pulse duration of 8 ns corresponding to the pulse peak power as high as 27 kW, and the optical-optical conversion efficiency from IR to UV was 25.6%. The output characteristics of the IR and the harmonic generations varying with the pulse repetition rate were also investigated detailedly.