Physics Faculty Research and Publications
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Item Open Access Anomalous ellipticity dependence of the generation of near-threshold harmonics in noble gases(2021-05-24) Wang, Bincheng; Zhang, Yinfu; Lan, Pengfei; Zhai, Chunyang; Li, Min; Zhu, Xiaosong; Chen, Jing; Lu, Peixiang; Lin, C. D.We measured the ellipticity dependence of the harmonic yield of He, Ne, Ar, Kr, and Xe. It is found that the harmonic yield in general decreases monotonically with increasing laser ellipticity but anomalous dependence does occur for each atom for some harmonic orders. We found that the degree of anomaly is weakest in helium but is stronger for the heavier atoms. The anomaly also depends on laser intensity. To explain these features, we developed a quantum trajectory Monte Carlo (QTMC) model based on the quantum path integral theory to study high-order harmonic generation (HHG). The model includes the effect of Coulomb potential from the target ion, thus it is capable of calculating near-threshold harmonics quantitatively. This model reveals that the presence of Coulomb potential would generate orbiting trajectories which are responsible for the anomalous ellipticity dependence near the threshold.Item Open Access The COLTRIMS Reaction Microscope—The Spyhole into the Ultrafast Entangled Dynamics of Atomic and Molecular Systems(2021-07-28) Schmidt-Böcking, Horst; Ullrich, Joachim; Dörner, Reinhard; Cocke, Charles LewisThe COLTRIMS Reaction Microscope C-REMI can image the momentum vectors of all emitted charged fragments in an atomic or molecular reactions similar to the bubble chamber in high energy particle physics. C-REMI can detect fragments with “zero” kinetic energy in an ultrahigh vacuum environment by projecting them with weak electromagnetic fields onto position-sensitive detectors. Geometrically a nearly 4π collection solid angle and a nearly 50% efficiency for a fivefold multi-coincidence can be achieved. Measuring time-of-flight and detector position the momenta of the fragments can be measured with excellent resolution (<0.01 a.u.; see A1 in the Appendix). Thus, multivector correlations in momentum space are measured, which provide insight into the entangled dynamics of atomic and molecular quantum systems. From these vector-correlations phases and energies can be deduced which allow for relative time measurements even in the zeptosecond range. C-REMI provides a “spyhole” into the secrets of ultrafast dynamics of atomic and molecular processes. It is applied today around the globe in numerous research projects in physics and chemistry. The purpose for writing this article is to demonstrate the universal application possibilities of C-REMI, and its high multi-coincidence efficiency and high momentum resolution. This paper will not give a review on all milestone experiments performed with C-REMI.Item Open Access Enhanced extreme ultraviolet high-harmonic generation from chromium-doped magnesium oxide(2021-05-17) Nefedova, V. E.; Fröhlich, S.; Navarrete, F.; Tancogne-Dejean, N.; Franz, D.; Hamdou, A.; Kaassamani, S.; Gauthier, D.; Nicolas, R.; Jargot, G.; Hanna, M.; Georges, P.; Ciappina, M. F.; Thumm, U.; Boutu, W.; Merdji, H.High-order harmonic generation (HHG) from crystals is emerging as a new ultrashort source of coherent extreme ultraviolet (XUV) light. Doping the crystal structure can offer a new way to control the source properties. Here, we present a study of HHG enhancement in the XUV spectral region from an ionic crystal, using dopant-induced vacancy defects, driven by a laser centered at a wavelength of 1.55 μm. Our numerical simulations based on solutions of the semiconductor Bloch equations and density-functional theory are supported by our experimental observations and demonstrate an increase in the XUV high harmonic yield from doped bulk magnesium oxide (MgO) compared to undoped MgO, even at a low defect concentration. The anisotropy of the harmonic emission as a function of the laser polarization shows that the pristine crystal's symmetry is preserved. Our study paves the way toward the control of HHG in solids with complex defects caused by transition-metal doping.Item Open Access Real-time observation of ultrafast molecular rotation in weakly bound dimers(2021-04-16) Zhou, Jiaqi; He, Chaoxiong; Liu, Ming-Ming; Wang, Enliang; Jia, Shaokui; Dorn, Alexander; Ren, Xueguang; Liu, YunquanThe fragmentation dynamics of dicationic dimers of acetylene molecules initiated upon strong-field laser ionization is studied. Time-resolved pump-probe experiments with femtosecond laser pulses, accompanied by ab initio dynamical calculations, allow us to evaluate the detailed behavior of molecular ions during the dissociation process. The dynamical properties of the intermediate C2H+2⋯C2H+2 state created by the pump pulse are probed by a second pulse which causes further ionization. The time-dependent yield of a coincident C2H+2 + C2H22+ ion pair exhibits an oscillation feature with a periodicity of 240 ± 30 fs. Our studies demonstrate that this is caused by an ultrafast rotation of C2H+2 cations driven by intermolecular Coulomb force together with the orientation-dependent ionization rate. We suggest that the present observation of ultrafast molecular rotation can be a general phenomenon occurring in a wide variety of systems.Item Open Access Molecular structure retrieval directly from laboratory-frame photoelectron spectra in laser-induced electron diffraction(2021-03-09) Sanchez, A.; Amini, K.; Wang, S.-J.; Steinle, T.; Belsa, B.; Danek, J.; Le, A. T.; Liu, X.; Moshammer, R.; Pfeifer, T.; Richter, M.; Ullrich, J.; Gräfe, S.; Lin, C. D.; Biegert, J.Ubiquitous to most molecular scattering methods is the challenge to retrieve bond distance and angle from the scattering signals since this requires convergence of pattern matching algorithms or fitting methods. This problem is typically exacerbated when imaging larger molecules or for dynamic systems with little a priori knowledge. Here, we employ laser-induced electron diffraction (LIED) which is a powerful means to determine the precise atomic configuration of an isolated gas-phase molecule with picometre spatial and attosecond temporal precision. We introduce a simple molecular retrieval method, which is based only on the identification of critical points in the oscillating molecular interference scattering signal that is extracted directly from the laboratory-frame photoelectron spectrum. The method is compared with a Fourier-based retrieval method, and we show that both methods correctly retrieve the asymmetrically stretched and bent field-dressed configuration of the asymmetric top molecule carbonyl sulfide (OCS), which is confirmed by our quantum-classical calculations.Item Open Access Formation of covalently bound C4H4+ upon electron-impact ionization of acetylene dimer(2021-04-08) Wang, Yingying; Wang, Enliang; Zhou, Jiaqi; Dorn, Alexander; Ren, XueguangWe investigate the formation mechanisms of covalently bound C4H4+ cations from direct ionization of hydrogen bonded dimers of acetylene molecules through fragment ion and electron coincident momentum spectroscopy and quantum chemistry calculations. The measurements of momenta and energies of two outgoing electrons and one ion in triple-coincidence allow us to assign the ionization channels associated with different ionic fragments. The measured binding energy spectra show that the formation of C4H4+ can be attributed to the ionization of the outermost 1πu orbital of acetylene. The kinetic energy distributions of the ionic fragments indicate that the C4H4+ ions originate from direct ionization of acetylene dimers while ions resulting from the fragmentation of larger clusters would obtain significantly larger momenta. The formation of C4H4+ through the evaporation mechanism in larger clusters is not identified in the present experiments. The calculated potential energy curves show a potential well for the electronic ground state of (C2H2)2+, supporting that the ionization of (C2H2)2 dimers can form stable C2H2⋅C2H2+(1πu−1) cations. Further transition state analysis and ab initio molecular dynamics simulations reveal a detailed picture of the formation dynamics. After ionization of (C2H2)2, the system undergoes a significant rearrangement of the structure involving, in particular, C–C bond formation and hydrogen migrations, leading to different C44+ isomers.Item Open Access Precise multispecies agricultural gas flux determined using broadband open-path dual-comb spectroscopy(2021-03-31) Herman, Daniel I.; Weerasekara, Chinthaka; Hutcherson, Lindsay C.; Giorgetta, Fabrizio R.; Cossel, Kevin C.; Waxman, Eleanor M.; Colacion, Gabriel M.; Newbury, Nathan R.; Welch, Stephen M.; DePaola, Brett D.; Coddington, Ian; Santos, Eduardo A.; Washburn, Brian R.Advances in spectroscopy have the potential to improve our understanding of agricultural processes and associated trace gas emissions. We implement field-deployed, open-path dual-comb spectroscopy (DCS) for precise multispecies emissions estimation from livestock. With broad atmospheric dual-comb spectra, we interrogate upwind and downwind paths from pens containing approximately 300 head of cattle, providing time-resolved concentration enhancements and fluxes of CH4, NH3, CO2, and H2O. The methane fluxes determined from DCS data and fluxes obtained with a colocated closed-path cavity ring-down spectroscopy gas analyzer agree to within 6%. The NH3 concentration retrievals have sensitivity of 10 parts per billion and yield corresponding NH3 fluxes with a statistical precision of 8% and low systematic uncertainty. Open-path DCS offers accurate multispecies agricultural gas flux quantification without external calibration and is easily extended to larger agricultural systems where point-sampling-based approaches are insufficient, presenting opportunities for field-scale biogeochemical studies and ecological monitoring.Item Open Access Inner-Shell-Ionization-Induced Femtosecond Structural Dynamics of Water Molecules Imaged at an X-Ray Free-Electron Laser(2021-12-03) Jahnke, T.; Guillemin, R.; Inhester, L.; Son, S.-K.; Kastirke, G.; Ilchen, M.; Rist, J.; Trabert, D.; Melzer, N.; Anders, N.; Mazza, T.; Boll, R.; De Fanis, A.; Music, V.; Weber, Th.; Weller, M.; Eckart, S.; Fehre, K.; Grundmann, S.; Hartung, A.; Hofmann, M.; Janke, C.; Kircher, M.; Nalin, G.; Pier, A.; Siebert, J.; Strenger, N.; Vela-Perez, I.; Baumann, T. M.; Grychtol, P.; Montano, J.; Ovcharenko, Y.; Rennhack, N.; Rivas, D. E.; Wagner, R.; Ziolkowski, P.; Schmidt, P.; Marchenko, T.; Travnikova, O.; Journel, L.; Ismail, I.; Kukk, E.; Niskanen, J.; Trinter, F.; Vozzi, C.; Devetta, M.; Stagira, S.; Gisselbrecht, M.; Jäger, A. L.; Li, X.; Malakar, Y.; Martins, M.; Feifel, R.; Schmidt, L. Ph. H.; Czasch, A.; Sansone, G.; Rolles, D.; Rudenko, A.; Moshammer, R.; Dörner, R.; Meyer, M.; Pfeifer, T.; Schöffler, M. S.; Santra, R.; Simon, M.; Piancastelli, M. N.The ultrafast structural dynamics of water following inner-shell ionization is a crucial issue in high-energy radiation chemistry. We have exposed isolated water molecules to a short x-ray pulse from a free-electron laser and detected momenta of all produced ions in coincidence. By combining experimental results and theoretical modeling, we can image dissociation dynamics of individual molecules in unprecedented detail. We reveal significant molecular structural dynamics in H2O2+, such as asymmetric deformation and bond-angle opening, leading to two-body or three-body fragmentation on a timescale of a few femtoseconds. We thus reconstruct several snapshots of structural dynamics at different time intervals, which highlight dynamical patterns that are relevant as initiating steps of subsequent radiation-damage processes.Item Open Access Importance of one- and two-photon transitions in the strong-field dissociation of NO2+(2021-11-18) Jochim, Bethany; Zohrabi, M.; Gaire, B.; Uhlíková, Tereza; Carnes, K. D.; Wells, E.; Esry, B. D.; Ben-Itzhak, I.Employing a coincidence three-dimensional momentum imaging technique, we investigate the ultrafast, intense laser-induced dissociation of a metastable NO2+ ion beam into N++O+. Based on the kinetic energy release and angular distributions, measured using both 774-nm and second-order-harmonic 387-nm pulses, we show that the main processes driving dissociation in pulses of about 1014W/cm2 peak intensity are one- and two-photon transitions from the X2Σ+ ground state to the A2Π first-excited state. First-order perturbation theory calculations also corroborate these findings.Item Open Access Laser-induced electron diffraction of the ultrafast umbrella motion in ammonia(2021-01-13) Belsa, B.; Amini, K.; Liu, X.; Sanchez, A.; Steinle, T.; Steinmetzer, J.; Le, A. T.; Moshammer, R.; Pfeifer, T.; Ullrich, J.; Moszynski, R.; Lin, C. D.; Gräfe, S.; Biegert, J.Visualizing molecular transformations in real-time requires a structural retrieval method with Ångström spatial and femtosecond temporal atomic resolution. Imaging of hydrogen-containing molecules additionally requires an imaging method sensitive to the atomic positions of hydrogen nuclei, with most methods possessing relatively low sensitivity to hydrogen scattering. Laser-induced electron diffraction (LIED) is a table-top technique that can image ultrafast structural changes of gas-phase polyatomic molecules with sub-Ångström and femtosecond spatiotemporal resolution together with relatively high sensitivity to hydrogen scattering. Here, we image the umbrella motion of an isolated ammonia molecule (NH3) following its strong-field ionization. Upon ionization of a neutral ammonia molecule, the ammonia cation (NH3+) undergoes an ultrafast geometrical transformation from a pyramidal (ΦHNH=107°) to planar (ΦHNH=120°) structure in approximately 8 femtoseconds. Using LIED, we retrieve a near-planar (ΦHNH=117 ± 5°) field-dressed NH3+ molecular structure 7.8−9.8 femtoseconds after ionization. Our measured field-dressed NH3+ structure is in excellent agreement with our calculated equilibrium field-dressed structure using quantum chemical ab initio calculations.Item Open Access XUV-Initiated Dissociation Dynamics of Molecular Oxygen (O2)(2021-12-02) Rebholz, Marc; Ding, Thomas; Aufleger, Lennart; Hartmann, Maximilian; Meyer, Kristina; Stooß, Veit; Magunia, Alexander; Wachs, David; Birk, Paul; Mi, Yonghao; Borisova, Gergana Dimitrova; da Costa Castanheira, Carina; Rupprecht, Patrick; Magrakvelidze, Maia; Thumm, Uwe; Roling, Sebastian; Butz, Marco; Zacharias, Helmut; Düsterer, Stefan; Treusch, Rolf; Brenner, Günter; Ott, Christian; Pfeifer, ThomasWe performed a time-resolved spectroscopy experiment on the dissociation of oxygen molecules after the interaction with intense extreme-ultraviolet (XUV) light from the free-electron laser in Hamburg at Deutsches Elektronen-Synchrotron. Using an XUV-pump/XUV-probe transient-absorption geometry with a split-and-delay unit, we observe the onset of electronic transitions in the O2+ cation near 50 eV photon energy, marking the end of the progression from a molecule to two isolated atoms. We observe two different time scales of 290 ± 53 and 180 ± 76 fs for the emergence of different ionic transitions, indicating different dissociation pathways taken by the departing oxygen atoms. With regard to the emerging opportunities of tuning the central frequencies of pump and probe pulses and of increasing the probe–pulse bandwidth, future pump–probe transient-absorption experiments are expected to provide a detailed view of the coupled nuclear and electronic dynamics during molecular dissociation.Item Open Access Extended quantitative rescattering model for simulating high-order harmonic streaking spectra by synchronization of an intense IR laser and a time-delayed attosecond XUV pulse(2021-09-01) Wang, Kan; Li, Baochang; Tang, Xiangyu; Xu, Chenhui; Lin, C. D.; Jin, ChengWe theoretically investigate the modulated high-harmonic generation (HHG) driven by an intense few-cycle infrared (IR) laser field and a weak extreme-ultraviolet (XUV) pulse at a delayed time. We establish an extended quantitative rescattering (EQRS) model to simulate the HHG streaking spectra, with the ideas of correcting the IR ionization and the transition from the ground to continuum states in the strong-field approximation. The EQRS model has an accuracy comparable to that from “exactly” solving the time-dependent Schrödinger equation (TDSE). We reveal that the fringes in the streaking spectra are caused by the interference between the attosecond XUV pulse and harmonics resulting from different recombination pathways under the intense IR laser. We then demonstrate that the XUV pulse can be accurately retrieved by treating the single-atom TDSE results or macroscopic propagation results as the “input” data. This work provides with a tool for efficiently simulating and thoroughly analyzing the XUV-assisted HHG, which could also enhance its capability for tracing the electron dynamics involved in the strong-field phenomena.Item Open Access Strong-field ionization of the triplet ground state of O2(2021-10-25) Wangjam, Tomthin Nganba; Lam, Huynh Van Sa; Kumarappan, VinodUsing strong-field ionization as a probe, we observe highly nonperiodic evolution of the spin-rotation wave packet launched by a nonionizing femtosecond pulse in oxygen. The nonperiodicity is readily apparent only in rotationally cold molecules that are pumped with a weak alignment pulse. We show that this behavior is a consequence of the spin-rotation and the spin-spin couplings in the triplet ground state of the neutral molecule. A model that includes these couplings in the field-free Hamiltonian but in neither the alignment nor the ionization step explains most of the observed dynamics, suggesting that neither process depends explicitly on the electronic spin. We also show that the angle dependence of strong-field ionization can be retrieved from the delay-dependent signal even when coupling to spin complicates the rotational dynamics.Item Open Access Simple model for sequential multiphoton ionization by ultraintense x rays(2021-09-22) Li, Xiang; Boll, Rebecca; Rolles, Daniel; Rudenko, ArtemA simple model for sequential multiphoton ionization by ultraintense x rays is presented. The derived scaling of the ion yield with pulse energy quantitatively reproduces the experimental data, which shows that the ion yield increases according to the “power-law” behavior typical of multiphoton ionization, followed by saturation at high pulse energies. The calculated average time interval between ionizations for producing ions at a certain charge state is found to be proportional to the pulse duration and independent of all other x-ray pulse parameters. This agrees with previous studies where the kinetic energy of fragment ions with a given charge state produced by the intense x-ray ionization of molecules was found to be independent of the pulse energy, but to increase with a smaller pulse duration due to the smaller time interval between ionizations.Item Open Access Time-resolved relaxation and fragmentation of polycyclic aromatic hydrocarbons investigated in the ultrafast XUV-IR regime(2021-10-20) Lee, J. W. L.; Tikhonov, D. S.; Chopra, P.; Maclot, S.; Steber, A. L.; Gruet, S.; Allum, F.; Boll, R.; Cheng, X.; Düsterer, S.; Erk, B.; Garg, D.; He, L.; Heathcote, D.; Johny, M.; Kazemi, M. M.; Köckert, H.; Lahl, J.; Lemmens, A. K.; Loru, D.; Mason, R.; Müller, E.; Mullins, T.; Olshin, P.; Passow, C.; Peschel, J.; Ramm, D.; Rompotis, D.; Schirmel, N.; Trippel, S.; Wiese, J.; Ziaee, F.; Bari, S.; Burt, M.; Küpper, J.; Rijs, A. M.; Rolles, D.; Techert, S.; Eng-Johnsson, P.; Brouard, M.; Vallance, C.; Manschwetus, B.; Schnell, M.Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10–100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH*, PAH+* and PAH2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH2+ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.Item Open Access Fragmentation of CF4q+ (q=2,3) induced by 1-keV electron collisions(2021-09-10) Chen, Lei; Wang, Enliang; Shan, Xu; Shen, Zhenjie; Zhao, Xi; Chen, XiangjunWe report on an investigation on the fragmentation dynamics of CF4q+ (q=2,3) induced by 1-keV electron collisions utilizing an ion momentum imaging spectrometer. From the time-of-flight correlation maps five dominating dissociation channels of CF42+ as well as one three-body fragmentation channel of CF43+ are identified. The kinetic energy release (KER) distributions for these channels are obtained and compared with the data available in the literature. The Dalitz-like momentum diagram and the Newton diagram are employed to analyze the breakup mechanism in the three-body fragmentation channel. We found that, for CF42+ dissociation into F++CF2++F, F++CF++2F, and F++F++CF2, the concerted breakup is the dominating process. Channel C++F++3F is dominated by the initial charge separation, i.e., CF42+→F++CF++2F→C++F++3F. With the help of the native frame method, we assigned one sequential pathway and two concerted pathways for channel CF43+→F++F++CF2+. The branching ratios of these pathways are determined. The momentum correlation of the fragments and the deduced KER distribution indicate that different excited states of CF43+ with different geometries are responsible for these three pathways. The Coulomb explosion model simulation shows that most of the events in this channel are produced by CF43+ ions that have deformed geometries from the neutral CF4 molecule.Item Open Access Pulse Energy and Pulse Duration Effects in the Ionization and Fragmentation of Iodomethane by Ultraintense Hard X Rays(2021-08-27) Li, X.; Inhester, L.; Robatjazi, S. J.; Erk, B.; Boll, R.; Hanasaki, K.; Toyota, K.; Hao, Y.; Bomme, C.; Rudek, B.; Foucar, L.; Southworth, S. H.; Lehmann, C. S.; Kraessig, B.; Marchenko, T.; Simon, M.; Ueda, K.; Ferguson, K. R.; Bucher, M.; Gorkhover, T.; Carron, S.; Alonso-Mori, R.; Koglin, J. E.; Correa, J.; Williams, G. J.; Boutet, S.; Young, L.; Bostedt, C.; Son, S.-K.; Santra, R.; Rolles, D.; Rudenko, A.The interaction of intense femtosecond x-ray pulses with molecules sensitively depends on the interplay between multiple photoabsorptions, Auger decay, charge rearrangement, and nuclear motion. Here, we report on a combined experimental and theoretical study of the ionization and fragmentation of iodomethane (CH3I) by ultraintense (∼1019 W/cm2) x-ray pulses at 8.3 keV, demonstrating how these dynamics depend on the x-ray pulse energy and duration. We show that the timing of multiple ionization steps leading to a particular reaction product and, thus, the product’s final kinetic energy, is determined by the pulse duration rather than the pulse energy or intensity. While the overall degree of ionization is mainly defined by the pulse energy, our measurement reveals that the yield of the fragments with the highest charge states is enhanced for short pulse durations, in contrast to earlier observations for atoms and small molecules in the soft x-ray domain. We attribute this effect to a decreased charge transfer efficiency at larger internuclear separations, which are reached during longer pulses.Item Open Access Electron-ion coincidence measurements of molecular dynamics with intense X-ray pulses(2021-01-12) Li, Xiang; Inhester, Ludger; Osipov, Timur; Boll, Rebecca; Coffee, Ryan; Cryan, James; Gatton, Ave; Gorkhover, Tais; Hartman, Gregor; Ilchen, Markus; Knie, André; Lin, Ming-Fu; Minitti, Michael P.; Weninger, Clemens; Wolf, Thomas J. A.; Son, Sang-Kil; Santra, Robin; Rolles, Daniel; Rudenko, Artem; Walter, PeterMolecules can sequentially absorb multiple photons when irradiated by an intense X-ray pulse from a free-electron laser. If the time delay between two photoabsorption events can be determined, this enables pump-probe experiments with a single X-ray pulse, where the absorption of the first photon induces electronic and nuclear dynamics that are probed by the absorption of the second photon. Here we show a realization of such a single-pulse X-ray pump-probe scheme on N$$_2$$molecules, using the X-ray induced dissociation process as an internal clock that is read out via coincident detection of photoelectrons and fragment ions. By coincidence analysis of the kinetic energies of the ionic fragments and photoelectrons, the transition from a bound molecular dication to two isolated atomic ions is observed through the energy shift of the inner-shell electrons. Via ab-initio simulations, we are able to map characteristic features in the kinetic energy release and photoelectron spectrum to specific delay times between photoabsorptions. In contrast to previous studies where nuclear motions were typically revealed by measuring ion kinetics, our work shows that inner-shell photoelectron energies can also be sensitive probes of nuclear dynamics, which adds one more dimension to the study of light-matter interactions with X-ray pulses.Item Open Access Enhancing high-order harmonic generation by controlling the diffusion of the electron wave packet(2021-08-20) Severt, T.; Troß, J.; Kolliopoulos, G.; Ben-Itzhak, I.; Trallero-Herrero, C. A.We experimentally study the enhancement of high-order harmonic generation (HHG) driven by synthesized ω−3ω laser fields, where we control whether the ionization rate or the electron wave packet’s diffusion is the dominant enhancement mechanism. When minimizing the electron wave packet’s diffusion, the excursion times of the corresponding electron trajectories are reduced by a factor of 2 or more. This result is important for imaging techniques that use the returning electron wave packet to probe the remaining ion. Furthermore, we achieve a 10× to 3800× enhancement of the harmonic yield driven by the bichromatic fields relative to that of an optimized single-color field, showing that the bichromatic fields improve HHG’s capability as a light source. We also measure that the two-color field’s harmonics have half the divergence angle compared to their single-color counterpart, suggesting that the “short” electron trajectories play a more prominent role compared to their “long” trajectory counterparts, thus improving the wavefront of the emerging harmonic beam.Item Open Access Analysis of THz generation by multicolor laser pulses with various frequency ratios(2020-04-28) Zhou, Zhaoyan; lv, Zhihui; Zhang, Dongwen; Zhao, Zengxiu; Lin, C. D.Terahertz (THz) generation in a gas medium is simulated by quantum calculations with multicolor intense laser pulses of various frequency ratios. By correlating THz radiation with the above-threshold-ionization photoelectron spectrum, we confirm that our previously proposed free-free transition model for the generation of THz radiation is also applicable to multicolor pulses. THz photon emissions can proceed through transitions between continuous states with similar energies that have opposite parities. This mechanism predicts that THz waves can be efficiently generated at special two-color laser frequency ratios when the multiphoton mixing condition is satisfied. Applying this model to multicolor fields (sawtooth wave shape), we provide a quantum-mechanical interpretation for the reason for THz radiation enhancement. A scheme combining multiple lasers to raise THz radiation satisfying multiphoton mixing conditions is also proposed.
