Attosecond pulse trains generated using two color laser fields

Highly versatile, two-color setup for high-order harmonic generation using spatial light modulators

We present a novel, interferometric, two-color, high-order harmonic generation setup based on a turn-key Ytterbium-doped femtosecond laser source and its second harmonic. Each interferometer arm contains a spatial light modulator with individual capabilities to manipulate the spatial beam profiles and to stabilize the relative delay between the fundamental and the second harmonic. In addition, separate control of the relative power and focusing geometries of the two color beams is implemented to conveniently perform automated scans of multiple parameters. A live diagnostics system gives continuous information during ongoing measurements.

Ultrafast control of the LnF+/LnO+ ratio from Ln(hfac)3

The photo-induced dissociative ionization of lanthanide complexes Ln(hfac)3 (Ln = Pr, Er, Yb) is studied using ultrafast shaped laser pulses in a time-of-flight (TOF) mass spectrometry setup. Various fluorine and Ln-containing mass fragments were observed, which can be interpreted by the photo-fragmentation mechanistic pathway involving C-C bond rotation processes proposed previously. A set of experiments used pulse shaping guided by closed-loop feedback control to identify pulses that optimize the ratio of LnF+/LnO+. In agreement with previous studies in which very little LnO+ was observed, broad pulses were found to maximize the LnF+/LnO+ ratio, which involves metal-ligand bond-breaking followed by bond rotation and bond rearrangement. In contrast, a transform limited (TL) pulse favored the formation of LnO+. Finally, the recently developed experimental control pulse slicing (CPS) technique was applied to elucidate the dynamics induced by fields that either maximize or minimize the LnF+/LnO+ ratio, which also indicates that longer laser pulses facilitate LnF+ formation during the C-C bond rotation dissociative-ionization process.

Vortex harmonic generation in indium tin oxide thin film irradiated by a two-color field

When a two-color Laguerre-Gaussian laser beam propagates through an indium tin oxide (ITO) material, the spatial distributions of odd- and even-order vortex harmonics carrying orbital angular momentum (OAM) are studied. The origin of vortex harmonics can be directly clarified by investigating their dependence on the incident laser field amplitude and frequency. In addition, it is shown that the spectral intensities of vortex harmonics are sensitive to the epsilon-near-zero nonlinear enhancing effects and the thickness of ITO materials. Thus the vortex harmonics can be conveniently tunable, which provides a wider potential application in optical communications based on high-order OAM coherent vortex beams.

Control of the electron dynamics in solid-state high harmonic generation on ultrafast time scales by a polarization-skewed laser pulse

Since high-order harmonic generation (HHG) from atoms depends sensitively on the polarization of the driving laser field, the polarization gating (PG) technique was developed and applied successfully to generate isolated attosecond pulses from atomic gases. The situation is, however, different in solid-state systems as it has been demonstrated that due to collisions with neighboring atomic cores of the crystal lattice strong HHG can be generated even by elliptically- and circularly-polarized laser fields. Here we apply PG to solid-state systems and find that the conventional PG technique is inefficient for the generation of isolated ultrashort harmonic pulse bursts. In contrast, we demonstrate that a polarization-skewed laser pulse is able to confine the harmonic emission to a time window of less than one-tenth of the laser cycle. This method provides a novel way to control HHG and to generate isolated attosecond pulses in solids.

High Photon Number Entangled States and Coherent State Superposition from the Extreme Ultraviolet to the Far Infrared

We present a theoretical demonstration on the generation of entangled coherent states and of coherent state superpositions, with photon numbers and frequencies orders of magnitude higher than those provided by the current technology. This is achieved by utilizing a quantum mechanical multimode description of the single- and two-color intense laser field driven process of high harmonic generation in atoms. It is found that all field modes involved in the high harmonic generation process are entangled, and upon performing a quantum operation, lead to the generation of high photon number optical cat states spanning from the far infrared to the extreme ultraviolet spectral region. This provides direct insights into the quantum mechanical properties of the optical field in the intense laser matter interaction. Finally, these states can be considered as a new resource for fundamental tests of quantum theory, quantum information processing, or sensing with nonclassical states of light.

Orientation and Alignment dynamics of polar molecule driven by shaped laser pulses

We review the theoretical status of intense laser induced orientation and alignment-a field of study which lies at the interface of intense laser physics and chemical dynamics and having potential applications such as high harmonic generation, nano-scale processing and control of chemical reactions. The evolution of the rotational wave packet and its dynamics leading to orientation and alignment is the topic of the present discussion. The major part of this article primarily presents an overview of recent theoretical progress in controlling the orientation and alignment dynamics of a molecule by means of shaped laser pulses. The various theoretical approaches that lead to orientation and alignment such as static electrostatic field in combination with laser field(s), combination of orienting and aligning field, combination of aligning fields, combination of orienting fields, application of train of pulses etc. are discussed. It is observed that the train of pulses is quite an efficient tool for increasing the orientation or alignment of a molecule without causing the molecule to ionize. The orientation and alignment both can occur in adiabatic and non-adiabatic conditions with the rotational period of the molecule taken under consideration. The discussion is mostly limited to non-adiabatic rotational excitation (NAREX) i.e. cases in which the pulse duration is shorter than the rotational period of the molecule. We have emphasised on the so called half-cycle pulse (HCP) and square pulse (SQP). The effect of ramped pulses and of collision on the various laser parameters is also studied. We summarize the current discussion by presenting a consistent theoretical approach for describing the action of such pulses on movement of molecules. The impact of a particular pulse shape on the post-pulse dynamics is also calculated and analysed. In addition to this, the roles played by various laser parameters including the laser frequency, the pulse duration and the system temperature etc. are illustrated and discussed. The concept of alignment is extended from one-dimensional alignment to three-dimensional alignment with the proper choice of molecule and the polarised light. We conclude the article by discussing the potential applications of intense laser orientation and alignment.

Quantum Interference Visibility Spectroscopy in Two-Color Photoemission from Tungsten Needle Tips

When two-color femtosecond laser pulses interact with matter, electrons can be emitted through various multiphoton excitation pathways. Quantum interference between these pathways gives rise to a strong oscillation of the photoemitted electron current, experimentally characterized by its visibility. In this Letter, we demonstrate the two-color visibility spectroscopy of multiphoton photoemissions from a solid-state nanoemitter. We investigate the quantum pathway interference visibility over an almost octave-spanning wavelength range of the fundamental (ω) femtosecond laser pulses and their second harmonic (2ω). The photoemissions show a high visibility of 90% ±  5%, with a remarkably constant distribution. Furthermore, by varying the relative intensity ratio of the two colors, we find that we can vary the visibility between 0% and close to 100%. A simple but highly insightful theoretical model allows us to explain all observations, with excellent quantitative agreements. We expect this work to be universal to all kinds of photo-driven quantum interference, including quantum control in physics, chemistry, and quantum engineering.

A multi-terawatt two-color beam for high-power field-controlled nonlinear optics

Two-color laser beams are instrumental in light-field control and enhancement of high-order harmonic, spectral supercontinuum, and terahertz radiation generated in gases, plasmas, and solids. We demonstrate a multi-terawatt two-color beam produced using a relativistic plasma mirror, with 110 mJ at 800 nm and 30 mJ at 400 nm. Both color components have high spatial quality and can be simultaneously focused, provided that the plasma mirror lies within a Rayleigh range of the driving fundamental beam. Favorable scaling of second-harmonic generation by plasma mirrors at relativistic intensities suggests them as an excellent tool for multi-color waveform synthesis beyond the petawatt level.

Generation and measurement of isolated attosecond pulses with enhanced flux using a two colour synthesized laser field

We have generated isolated attosecond pulses and performed attosecond streaking measurements using a two-colour synthesized laser field consisting of a strong near-infrared few-cycle pulse and a weaker multi-cycle pulse centred at 400 nm. An actively stabilized interferometer was used to coherently combine the two pulses. Using attosecond streaking we characterised the electric fields of the two pulses and accurately retrieved the spectrum of the multi-cycle pulse. We demonstrated a two-fold increase in the flux of isolated attosecond pulses produced and show that their duration was minimally affected by the presence of the weaker field due to spectral filtering by a multilayer mirror.

Attosecond science based on high harmonic generation from gases and solids

Recent progress in high power ultrafast short-wave and mid-wave infrared lasers has enabled gas-phase high harmonic generation (HHG) in the water window and beyond, as well as the demonstration of HHG in condensed matter. In this Perspective, we discuss the recent advancements and future trends in generating and characterizing soft X-ray pulses from gas-phase HHG and extreme ultraviolet (XUV) pulses from solid-state HHG. Then, we discuss their current and potential usage in time-resolved study of electron and nuclear dynamics in atomic, molecular and condensed matters.

Different methods are demonstrated in recent years to produce attosecond pulses. Here, the authors discuss recent development and future prospects of the generation of such pulses from gases and solids and their potential applications in spectroscopy and ultrafast dynamics in atoms, molecules and other complex systems.

Ultrafast Photofragmentation of Ln(hfac)3 with a Proposed Mechanism for forming High Mass Fluorinated Products

The photo-induced dissociative-ionization of lanthanide complexes Ln(hfac)₃ (Ln = Pr, Er, Yb) is studied using intense ultrafast transform limited (TL) and linearly chirped laser pulses in a time-of-flight (TOF) mass spectrometry setup. Various fluorine and Ln-containing high-mass fragments were observed in this experiment, including the molecular parent ion, which have not been seen with previous studies relying on relatively long-duration laser pulses (i.e., ns or longer). These new high-mass observations provide important formerly missing information for deducing a set of photo-fragmentation mechanistic pathways for Ln(hfac)₃. An overall ultrafast control mechanism is proposed by combining insights from earlier studies and the fragments observed in this research to result in three main distinct photo-fragmentation processes: (a) ligand-metal charge transfer, (b) CF₃ elimination, and (c) C-C bond rotation processes. We conclude that ultrafast dissociative-ionization could be a promising technique for generating high-mass fragments for potential use in material science applications.

Giant Isolated Attosecond Pulses from Two-Color Laser-Plasma Interactions

A new regime in the interaction of a two-color (ω,2ω) laser with a nanometer-scale foil is identified, resulting in the emission of extremely intense, isolated attosecond pulses-even in the case of multicycle lasers. For foils irradiated by lasers exceeding the blow-out field strength (i.e., capable of fully separating electrons from the ion background), the addition of a second harmonic field results in the stabilization of the foil up to the blow-out intensity. This is then followed by a sharp transition to transparency that essentially occurs in a single optical cycle. During the transition cycle, a dense, nanometer-scale electron bunch is accelerated to relativistic velocities and emits a single, strong attosecond pulse with a peak intensity approaching that of the laser field.

Double optical gating for generating high flux isolated attosecond pulses in the soft X-ray regime

Double optical gating (DOG) technique was implemented with a two-cycle, 1.7 µm driving field to generate isolated attosecond pulses in the 100-250 eV spectrum range. The strong ellipticity dependency of the high harmonics from the 1.7 µm driving field makes polarization based gating method very efficient. When a second harmonic (SH) field is introduced, complete gating can be achieved with less ionization from the leading edge of the driving field, which yields supercontinua with a pulse energy of 0.3 nJ. We perform an attosecond streaking measurement to confirm the generation of isolated attosecond pulses.

Application of Quasi-Phase Matching Concept for Enhancement of High-Order Harmonics of Ultrashort Laser Pulses in Plasmas

Novel methods of coherent short-wavelength sources generation require thorough analysis for their further amendments and practical implementations. In this work, we report on the quasi-phase matching (QPM) of high-order harmonics generation during the propagation of single- and two-color femtosecond pulses through multi-jet plasmas, which allows the enhancement of groups of harmonics in different ranges of extreme ultraviolet. The role of the number of coherent zones; sizes of plasma jets and the distance between them; plasma formation conditions, and the characteristics of the fundamental radiation on the harmonic efficiency at quasi-phase matching (QPM) conditions are analyzed. We demonstrate the ~40× enhancement factor of the maximally-enhanced harmonic with respect to the one generated at ordinary conditions in the imperforated plasma.

Significantly enhanced conversion efficiency of high-order harmonic generation by introducing chirped laser pulses into scheme of spatially inhomogeneous field

An effective scheme to enhance the yield of high-order harmonic generation originated from spatially inhomogeneous field through the interaction between few-cycle chirped laser pulses and a nano-tip structure is demonstrated. The conversion efficiency of harmonics from chirped laser pulses was significantly improved by nearly three more orders of magnitude than that of chirp-free pulses, and the cutoff energy of the corresponding harmonics was dramatically enhanced. By superimposing a series of properly selected orders of harmonics, isolated attosecond pulses of high intensity can be obtained. Furthermore, we compared the effects of different types of chirps on harmonics.

Refined Ptychographic Reconstruction of Attosecond Pulses

Advanced applications of attosecond pulses require the implementation of experimental techniques for a complete and accurate characterization of the pulse temporal characteristics. The method of choice is the frequency resolved optical gating for the complete reconstruction of attosecond bursts (FROG-CRAB), which requires the development of suitable reconstruction algorithms. In the last few years, various numerical techniques have been proposed and implemented, characterized by different levels of accuracy, robustness, and computational load. Many of them are based on the central momentum approximation (CMA), which may pose severe limits in the reconstruction accuracy. Alternative techniques have been successfully developed, based on the implementation of reconstruction algorithms which do not rely on this approximation, such as the Volkov-transform generalized projection algorithm (VTGPA). The main drawback is a notable increase of the computational load. We propose a new method, called refined iterative ptychographic engine (rePIE), which combines the advantages of a robust algorithm based on CMA, characterized by a fast convergence, with the accuracy of advanced algorithms not based on such approximation. The main idea is to perform a first fast iterative ptychographic engine (ePIE) reconstruction and then refine the result with just a few iterations of the VTGPA in order to correct for the error introduced by the CMA. We analyse the accuracy of the novel reconstruction method by comparing the residual error (i.e., the difference between the reconstructed and the simulated original spectrograms) when VTGPA, ePIE, and rePIE reconstructions are employed. We show that the rePIE approach is particularly useful in the case of short attosecond pulses characterized by a broad spectrum in the vacuum-ultraviolet (VUV)–extreme-ultraviolet (XUV) region.

Complete reconstruction of ultra-broadband isolated attosecond pulses including partial averaging over the angular distribution

Attosecond streaking is a powerful tool to investigate ultrafast electron dynamics on the attosecond time scale. To obtain the highest temporal resolution in a pump-probe experiment, soft-X-ray (SXR) and infrared (IR) pulses have to be carefully characterized. Here, we present a detailed description of our recent generalization of the Volkov-transform generalized projection algorithm (VTGPA) and its application to multiple overlapping photoelectron bands. This method allows for the complete temporal reconstruction of both IR and SXR pulses under the inclusion of accurate complex photoionization matrix elements (PMEs). In this article, we compare the performance of our new method with traditional algorithms. We particularly focus on the important role played by the photoelectron angular distribution (PAD) which needs to be taken into account for the highest fidelity of attosecond pulse reconstruction. For this purpose, we investigate numerically the influence of the finite collection angle of the electron spectrometer on the retrieval and the obtained pulse parameters. We further theoretically demonstrate the reliability of the reconstruction for pulse durations even shorter than the atomic unit of time.

Frequency gating to isolate single attosecond pulses with overdense plasmas using particle-in-cell simulations

We present the isolation of single attosecond pulses for multi-cycle and few-cycle laser pulses from high harmonic generation in overdense plasmas, calculated with particle-in-cell simulations. By the combination of two laser pulses of equal amplitude and a small frequency shift between them, we demonstrate that it is possible to shorten the region in which the laser pulse is most intense, therefore restricting the generation of high harmonic orders in the form of attosecond pulses to a narrower time window. The creation of this window is achieved due to the combination of the laser pulse envelope and the slow oscillating wave obtained from the coherent sum of the two pulses. A parametric scan, performed with particle-in-cell simulations, reveals how the pulse isolation behaves for different input laser pulse lengths and which are the optimal frequency shifts between the two laser pulses in each case, giving the conditions for having a good isolation of an attosecond pulse when working with laser-plasma interaction in overdense targets.

Effect of aligning pulse train on the orientation and alignment of a molecule in presence of orienting pulse

Field-free molecular alignment is studied theoretically in presence of orienting laser pulse and a delayed Infrared laser (IRL) pulse train. The pulse shapes taken are sine square (sin²) and square. The degree of alignment can be significantly enhanced by the combination of orienting pulse and IRL pulse train compared with only IRL pulse train. Special emphasis is laid on time delay between orienting and aligning pulse, the width and shape of the pulse train. By adjusting the time delay, width and intensity of coupling laser one can suppress a population of particular state while simultaneously enhancing the population of desired states.

Ultrashort Pulses for Far-Field Nanoscopy

We show that ultrashort pulses can be focused, in a particular instant, to a spot size given by the wavelength associated with its spectral width. For attosecond pulses this spot size is within the nanometer scale. Then we show that a two-level system can be left excited after interacting with an ultrashort pulse whose spectral width is larger than the transition frequency, and that the excitation probability depends not on the field amplitude but on the field intensity. The latter makes the excitation profile have the same spot size as the ultrashort pulse. This unusual phenomenon is caused by quantum electrodynamics in the ultrafast light-matter interaction regime since the usually neglected counterrotating terms describing the interaction with the free electromagnetic modes are crucial for making the excitation probability nonzero and depend on the field intensity. These results suggest that a train of coherent attosecond pulses could be used to excite fluorescent markers with nanoscale resolution. The detection of the light emitted after fluorescence-or any other method used to detect the excitation-could then lead to a new scheme for far-field light nanoscopy.

Two-color high-harmonic generation in plasmas: efficiency dependence on the generating particle properties

The high-order harmonic generation (HHG) in silver, gold, and zinc plasma plumes irradiated by orthogonally polarized two-color field is studied theoretically and experimentally. We find an increase of the HHG efficiency in comparison with the single-color case, which essentially depends on the plasma species and harmonic order. An increase of more than an order of magnitude is observed for silver plasma, whereas for gold and zinc it is lower; these results are reproduced in our calculations that include both propagation and microscopic response studies. We show that the widely used theoretical approach assuming the 1s ground state of the generating particle fails to reproduce the experimental results; the agreement is achieved in our theory using the actual quantum numbers of the outer electron of the generating particles. Moreover, our theoretical studies highlight the redistribution of the electronic density in the continuum wave packet as an important aspect of the HHG enhancement in the two-color orthogonally polarized fields with comparable intensities: in the single-color field the electronic trajectories with almost zero return energy are the most populated ones; in the two-color case the total field maximum can be shifted in time so that the trajectories with high return energies (in particular, the cut-off trajectory) become the most populated ones.

Controlling attosecond angular streaking with second harmonic radiation

High harmonic generation, which produces a coherent burst of radiation every half cycle of the driving field, has been combined with ultrafast wavefront rotation to create a series of spatially separated attosecond pulses, called the attosecond lighthouse. By adding a coherent second harmonic beam with polarization parallel to the fundamental, we decrease the generating frequency from twice per optical cycle to once. The increased temporal separation increases the pulse contrast. By scanning the carrier envelope phase, we see that the signal is 2π periodic.

Enhanced attosecond bursts of relativistic high-order harmonics driven by two-color fields

We study the generation of attosecond x-ray and ultraviolet pulses from relativistically driven overdense plasma targets with two-color incident light. Particle-in-cell simulations show that significant improvement in pulse intensity and isolation is achievable with appropriate laser and plasma parameters. Conversion of 5% of incident laser energy to its second harmonic can enhance the intensity of generated attosecond pulses by an order of magnitude. This approach allows the generation of higher attosecond pulse intensities with existing experimental laser technology and offers a powerful tool for the analysis of the dynamics of relativistic laser-plasma interaction.

Route to optimal generation of soft X-ray high harmonics with synthesized two-color laser pulses

High harmonics extending to X-rays have been generated from gases by intense lasers. To establish these coherent broadband radiations as an all-purpose tabletop light source for general applications in science and technology, new methods are needed to overcome the present low conversion efficiencies. Here we show that the conversion efficiency may be drastically increased with an optimized two-color pulse. By employing an optimally synthesized 2-µm mid-infrared laser and a small amount of its third harmonic, we show that harmonic yields from sub- to few-keV energy can be increased typically by ten-fold over the optimized single-color one. By combining with favorable phase-matching and together with the emerging high-repetition MHz mid-infrared lasers, we anticipate efficiency of harmonic yields can be increased by four to five orders in the near future, thus paving the way for employing high harmonics as useful broadband tabletop light sources from the extreme ultraviolet to the X-rays, as well as providing new tools for interrogating ultrafast dynamics of matter at attosecond timescales.

Electron dynamics controlled via self-interaction

The dynamics of an electron in a strong laser field can be significantly altered by radiation reaction. This usually results in a strongly damped motion, with the electron losing a large fraction of its initial energy. Here we show that the electron dynamics in a bichromatic laser pulse can be indirectly controlled by a comparatively small radiation reaction force through its interplay with the Lorentz force. By changing the relative phase between the two frequency components of the bichromatic laser field, an ultrarelativistic electron bunch colliding head-on with the laser pulse can be deflected in a controlled way, with the deflection angle being independent of the initial electron energy. The effect is predicted to be observable with laser powers and intensities close to those of current state-of-the-art petawatt laser systems.

Pulse train induced rotational excitation and orientation of a polar molecule

We investigate theoretically the rotational excitation and field free molecular orientation of polar HBr molecule, interacting with train of ultrashort laser pulses. By adjusting the number of pulses, pulse period and the intensity of the pulse, one can suppress a population while simultaneously enhancing the desired population in particular rotational state. We have used train of laser pulses of different shaped pulse envelopes. The dynamics and orientation of molecules in the presence of pulse train of different shapes is studied and explained.

Macroscopic effects in noncollinear high-order harmonic generation

We study two-color high-order harmonic generation using an intense driving field and its weak second harmonic, crossed under a small angle in the focus. Employing sum- and difference-frequency generation processes, such a noncollinear scheme can be used to measure and control macroscopic phase matching effects by utilizing a geometrical phase mismatch component, which depends on the noncollinear angle. We further show how spatial phase effects in the generation volume are mapped out into the far field allowing a direct analogy with temporal carrier envelope effects in attosecond pulse generation.

Quasi-phase-matching of only even-order high harmonics

High harmonic spectrum of a quasi-monochromatic pump that interacts with isotropic media consists of only odd-order harmonics. Addition of a secondary pump, e.g. a static field or the second harmonic of the primary pump, can results with generation of both odd and even harmonics of the primary pump. We propose a method for quasi-phase matching of only the even-order harmonics of the primary pump. We formulate a theory for this process and demonstrate it numerically. We also show that it leads to attosecond pulse trains with constant carrier envelop phase and high repetition rate.

Efficient high-order harmonic generation boosted by below-threshold harmonics

High-order harmonic generation (HHG) in gases has been established as an important technique for the generation of coherent extreme ultraviolet (XUV) pulses at ultrashort time scales. Its main drawback, however, is the low conversion efficiency, setting limits for many applications, such as ultrafast coherent imaging, nonlinear processes in the XUV range, or seeded free electron lasers. Here we introduce a novel scheme based on using below-threshold harmonics, generated in a “seeding cell”, to boost the HHG process in a “generation cell”, placed further downstream in the focused laser beam. By modifying the fundamental driving field, these low-order harmonics alter the ionization step of the nonlinear HHG process. Our dual-cell scheme enhances the conversion efficiency of HHG, opening the path for the realization of robust intense attosecond XUV sources.

Attosecond pulse characterization

In this work we propose a novel procedure for the characterization of attosecond pulses. The method relies on the conversion of the attosecond pulse into electron wave-packets through photoionization of atoms in the presence of a weak IR field. It allows for the unique determination of the spectral phase making up the pulses by accurately taking into account the atomic physics of the photoionization process. The phases are evaluated by optimizing the fit of a perturbation theory calculation to the experimental result. The method has been called iPROOF (improved Phase Retrieval by Omega Oscillation Filtering) as it bears a similarity to the PROOF technique [Chini et al. Opt. Express 18, 13006 (2010)]. The procedure has been demonstrated for the characterization of an attosecond pulse train composed of odd and even harmonics. We observe a large phase shift between consecutive odd and even harmonics. The resulting attosecond pulse train has a complex structure not resembling a single attosecond pulse once per IR period, which is the case for zero phase. Finally, the retrieval procedure can be applied to the characterization of single attosecond pulses as well.

A high-flux high-order harmonic source

We develop and implement an experimental strategy for the generation of high-energy high-order harmonics (HHG) in gases for studies of nonlinear processes in the soft x-ray region. We generate high-order harmonics by focusing a high energy Ti:Sapphire laser into a gas cell filled with argon or neon. The energy per pulse is optimized by an automated control of the multiple parameters that influence the generation process. This optimization procedure allows us to obtain energies per pulse and harmonic order as high as 200 nJ in argon and 20 nJ in neon, with good spatial properties, using a loose focusing geometry (f#≈400) and a 20 mm long medium. We also theoretically examine the macroscopic conditions for absorption-limited conversion efficiency and optimization of the HHG pulse energy for high-energy laser systems.

Synchronizing terahertz wave generation with attosecond bursts

We perform a joint measurement of terahertz waves and high-harmonics generated from argon atoms driven by a fundamental laser pulse and its second harmonic. By correlating their dependence on the phase delay between the two pulses, we determine the generation of THz waves in tens of attoseconds precision. Compared with simulations and models, we find that the laser-assisted soft collision of the electron wave packet with the atomic core plays a key role. It is demonstrated that the rescattering process, being indispensable in high-harmonic generation processes, dominates THz wave generation as well in a more elaborate way. The new finding might be helpful for the full characterization of the rescattering dynamics.

Attosecond control of orbital parity mix interferences and the relative phase of even and odd harmonics in an attosecond pulse train

We experimentally demonstrate that atomic orbital parity mix interferences can be temporally controlled on an attosecond time scale. Electron wave packets are formed by ionizing argon gas with a comb of odd and even high-order harmonics, in the presence of a weak infrared field. Consequently, a mix of energy-degenerate even and odd parity states is fed in the continuum by one- and two-photon transitions. These interfere, leading to an asymmetric electron emission along the polarization vector. The direction of the emission can be controlled by varying the time delay between the comb and infrared field pulses. We show that such asymmetric emission provides information on the relative phase of consecutive odd and even order harmonics in the attosecond pulse train.

Dramatic cutoff extension and broadband supercontinuum generation in multi-cycle two color pulses

We propose a method to markedly increase the electron-recollision kinetic energy, using the combination of a 0.8 μm/13 fs driving pulse with a much weaker multi-cycle mid-infrared pulse at 10.4 μm. The results show that the synthesized field effectively lengthens the accelerated distance of electron wave packet and the harmonic cutoff is significantly extended to IP+26UP, which is covered with the water-window spectral region. In addition, only one single quantum path contributes to harmonics, and those higher than IP+15UP become continuous. This supercontinuum can support the generation of an isolated sub-100 as pulse with tunable central wavelength and also the pulse with the duration below one atomic unit of time (24 as). Moreover, our scheme can further extend to more longer driving pulses, which provides a dramatic approach for cutoff extension and broadband supercontinuum generation with multi-cycle pump pulses.

Generation of two attosecond pulses with tunable delay using orthogonally-polarized chirped laser pulses

We investigate theoretically the high-order harmonic generation (HHG) by two orthogonally-polarized linearly chirped laser pulses. We show that such generating field has a specific temporal variation of the ellipticity which provides generation of two XUV attosecond pulses with tunable delay between them. This delay is controlled by the delay between the two generating pulses. Perspectives of application of this technique for the attosecond pump - attosecond probe experiments are discussed.

Efficient generation of isolated attosecond pulses with high beam quality by two-color Bessel-Gauss beams

The generation of isolated attosecond pulses with high efficiency and high beam quality is essential for attosecond spectroscopy. We numerically investigate the supercontinuum generation in a neutral rare-gas medium driven by a two-color Bessel-Gauss beam. The results show that an efficient smooth supercontinuum in the plateau is obtained after propagation and the spatial profile of the generated attosecond pulse is Gaussian-like with the divergence angle of 0.1° in the far-field. This bright source with high beam quality is beneficial for detecting and controlling the microscopic processes on attosecond time scale.

Enhancement of high harmonics from plasmas using two-color pump and chirp variation of 1 kHz Ti:sapphire laser pulses

We have investigated resonance effects in high-order harmonic generation (HHG) within laser-produced plasmas. We demonstrate a significantly improved harmonic yield by using two-color pump-induced enhancement and a 1 kHz pulse repetition rate. Together with an increased HHG output, the even harmonics in the cutoff region were enhanced with respect to odd harmonics. We report the observation of a resonance-induced growth in intensity of 20th harmonic in silver plasma (2×), 26th harmonic in vanadium plasma (4×), and 28th harmonic in chromium plasma (5×).

Nonresonant femtosecond laser vaporization of aqueous protein preserves folded structure

Femtosecond laser vaporization-based mass spectrometry can be used to measure protein conformation in vitro at atmospheric pressure. Cytochrome c and lysozyme are vaporized from the condensed phase into the gas phase intact when exposed to an intense (10(13) W/cm(2)), nonresonant (800 nm), ultrafast (75 fs) laser pulse. Electrospray postionization time-of-flight mass spectrometry reveals that the vaporized protein maintains the solution-phase conformation through measurement of the charge-state distribution and the collision-induced dissociation channels.

Attosecond twin-pulse control by generalized kinetic heterodyne mixing

Attosecond double-pulse (twin-pulse) production in high-order harmonic generation is manipulated by a combination of two-color and carrier-envelope phase-control methods. As we show in numerical simulations, both relative amplitude and phase of the double pulse can be independently set by making use of multidimensional parameter control. Two technical implementation routes are discussed: kinetic heterodyning using second-harmonic generation and split-spectrum phase-step control.

Ultrahigh-order wave mixing in noncollinear high harmonic generation

We show that noncollinear high harmonic generation (HHG) can be fully understood in terms of nonlinear optical wave mixing. We demonstrate this by superposing on the fundamental ω1 field its second harmonic ω2 of variable intensity in a noncollinear geometry. It allows us to identify, by momentum conservation, each field's contribution (n1,n2) to the extreme ultraviolet emission at frequency Ω = n1ω1 + n2ω2. We observe that the photon (Ω) yield follows an n2 power law on the ω2 intensity, before saturation. It demonstrates that, although HHG is a highly nonperturbative process, a perturbation theory can still be developed around it.

Frequency modulation of high-order harmonic fields with synthesis of two-color laser fields

We report periodical frequency modulation of high-order harmonic fields observed by changing the delay between the driving two-color laser fields consisting of the fundamental and its second harmonic (SH) field. The amplitude of modulation has been up to ∼0.4 eV, which is larger than the bandwidth of the fundamental field. Experimental results show that the intensity and chirp of the fundamental field can control this phenomenon. Numerical analysis by solving the time-dependent Schrödinger equation approves of these results and shows that anharmonic frequency components of the SH field have a crucial role in this phenomenon.

Control of electron localization in deuterium molecular ions using an attosecond pulse train and a many-cycle infrared pulse

We demonstrate an experimental control of electron localization in deuterium molecular ions created and dissociated by the combined action of an attosecond pulse train and a many-cycle infrared (IR) pulse. The attosecond pulse train is synthesized using both even and odd high order harmonics of the driving IR frequency so that it can strobe the IR field once per IR cycle. An asymmetric ejection of the deuterium ions oscillates with the full IR period when the APT-IR time-delay is scanned. The observed control is due to the creation of a coherent superposition of 1s sigma{g} and 2p sigma{u} states via interference between one-photon and two-photon dissociation channels.

Attosecond quantum stroboscope

Electron disco: A "quantum stroboscope" for capturing the electron motion on a subfemtosecond timescale for a particular class of problems is highlighted. The picture shows a diffraction pattern caused by wave packets obtained by synchronizing attosecond UV pulses to a near-IR field and ionizing rare-gas atoms.

Few-cycle attosecond pulses with stabilized-carrier-envelope phase in the presence of a strong terahertz field

High-order harmonic generation in the presence of a strong terahertz field is investigated, and phase stabilization of the few-cycle laser pulses is extended to the extremely ultraviolet region. It is found that the strong terahertz field significantly breaks the symmetry between the consecutive half-cycle and greatly extends the harmonic cutoff, producing both odd and even harmonics which are covered with an extremely broad bandwidth and well locked in phase. These results can support the generation of few-cycle attosecond pulse trains with stabilized carrier-envelope phase from pulse to pulse, and also enables the generation of phase-stabilized pulse train with tunable wavelength.