Double optical gating of high-order harmonic generation with carrier-envelope phase stabilized lasers

Characterizing inner-shell with spectral phase interferometry for direct electric-field reconstruction

In many atomic, molecular and solid systems, Lorentzian and Fano profiles are commonly observed in a broad research fields throughout a variety of spectroscopies. As the profile structure is related to the phase of the time-dependent dipole moment, it plays an important role in the study of quantum properties. Here we determine the dipole phase in the inner-shell transition using spectral phase interferometry for direct electric-field reconstruction (SPIDER) with isolated attosecond pulses (IAPs). In addition, we propose a scheme for pulse generation and compression by manipulating the inner-shell transition. The electromagnetic radiation generated by the transition is temporally compressed to a few femtoseconds in the extreme ultraviolet (XUV) region. The proposed pulse-compression scheme may provide an alternative route to producing attosecond pulses of light.

Characterizing and understanding the profile of atomic spectral lines tells us a great deal about an atom’s properties. Here, the authors combine attosecond transient spectroscopy and the SPIDER technique to reconstruct the dipole oscillation phase of the auto-ionisation inner-shell transition in neon.

Attosecond nonlinear optics using gigawatt-scale isolated attosecond pulses

High-energy isolated attosecond pulses required for the most intriguing nonlinear attosecond experiments as well as for attosecond-pump/attosecond-probe spectroscopy are still lacking at present. Here we propose and demonstrate a robust generation method of intense isolated attosecond pulses, which enable us to perform a nonlinear attosecond optics experiment. By combining a two-colour field synthesis and an energy-scaling method of high-order harmonic generation, the maximum pulse energy of the isolated attosecond pulse reaches as high as 1.3 μJ. The generated pulse with a duration of 500 as, as characterized by a nonlinear autocorrelation measurement, is the shortest and highest-energy pulse ever with the ability to induce nonlinear phenomena. The peak power of our tabletop light source reaches 2.6 GW, which even surpasses that of an extreme-ultraviolet free-electron laser.

The short duration of attosecond pulses makes them interesting for ultrafast experiments, although it has so far been difficult to generate isolated attosecond pulses with sufficiently high power. Here the authors achieve high-intensity isolated attosecond pulses with a tabletop setup, based on a scaled-up high-order harmonic generation process.

Generation of bright isolated attosecond soft X-ray pulses driven by multicycle midinfrared lasers

High harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, to date the shortest subfemtosecond (attosecond, 10(-18) s) pulses have been produced only in the extreme UV region of the spectrum below 100 eV, which limits the range of materials and molecular systems that can be explored. Here we experimentally demonstrate a remarkable convergence of physics: when midinfrared lasers are used to drive high harmonic generation, the conditions for optimal bright, soft X-ray generation naturally coincide with the generation of isolated attosecond pulses. The temporal window over which phase matching occurs shrinks rapidly with increasing driving laser wavelength, to the extent that bright isolated attosecond pulses are the norm for 2-µm driving lasers. Harnessing this realization, we experimentally demonstrate the generation of isolated soft X-ray attosecond pulses at photon energies up to 180 eV for the first time, to our knowledge, with a transform limit of 35 attoseconds (as), and a predicted linear chirp of 300 as. Most surprisingly, advanced theory shows that in contrast with as pulse generation in the extreme UV, long-duration, 10-cycle, driving laser pulses are required to generate isolated soft X-ray bursts efficiently, to mitigate group velocity walk-off between the laser and the X-ray fields that otherwise limit the conversion efficiency. Our work demonstrates a clear and straightforward approach for robustly generating bright isolated attosecond pulses of electromagnetic radiation throughout the soft X-ray region of the spectrum.

Quantum path selection in high-order harmonic generation from aligned molecules

We theoretically investigate high-order harmonic generation (HHG) from aligned N(2) molecules with a driving field composed of two-color circularly polarized laser pulses. It is shown that the combination of N(2) molecules and the waveform-controlled laser field allows us to select either long or short quantum path, depending on molecular alignment angles, while in atom Ar, two paths show comparable contribution to HHG. The selection of single quantum path in aligned N(2) molecules leads to an ultrabroad and smooth XUV supercontinuum, giving rise to isolated attosecond pulses generation. Moreover, we can control the intensity ratio of two attosecond pulses by adjusting the molecular alignment angles, providing an opportunity for attosecond pump-probe technique.

Excitation with effective subcycle laser pulses

We have used laser pulses with a temporally shaped polarization to demonstrate the multiphoton excitation of the xenon 5g state within a subcycle of a laser pulse. Our polarization gated laser pulses are composed of circularly polarized sections at the leading and trailing edges of the pulse and of an experimentally defined linearly polarized central part. Only the linear part (the gate) of the pulse can excite neutral xenon in the 5g state. The transition cannot be driven with circularly polarized light because the number of photons needed would cause a violation of selection rules for the change of the magnetic quantum number. We show that the linearly polarized central part can be reduced to a subcycle pulse. This allows us to study excitation with an effective pulse as short as 2.3 fs at 800 nm. Electron imaging spectroscopy has been used to visualize the presence of excited states as a function of the pulse duration of the gate.

Intense supercontinuum generation exceeding 300eV using a two-color field in combination with a 400-nm few-cycle control pulse

We propose a method to control the harmonic process by using a two-color field in combination with a 400-nm few-cycle control pulse for the generation of an ultra-broadband supercontinuum with high efficiency. The ionization and acceleration steps in the harmonic process can be simultaneously controlled by using a three-color field synthesized by a 2000-nm driving pulse and two weak 800-nm and 400-nm control pulses. Then an intense supercontinuum covered by the spectral range from 140 eV to 445 eV is produced. The 3D macroscopic propagation is also employed to select the short quantum path of the supercontinuum, then intense isolated sub-100-as pulses with tunable central wavelengths are directly obtained within water window region. In addition, the generation of isolated attosecond pulses in the far field is also investigated. An isolated 52-as pulse can be generated by using a filter centered on axis to select the harmonics in the far field.

Quantum-orbit analysis for yield and ellipticity of high order harmonic generation with elliptically polarized laser field

We perform a quantum-orbit analysis for the dependence of high-order-harmonic yield on the driving field ellipticity and the polarization properties of the generated high harmonics. The electron trajectories responsible for the emission of particular harmonics are identified. It is found that, in elliptically polarized driving field, the electrons have ellipticity-dependent initial velocities, which lead to the decrease of the ionization rate. Thus the harmonic yield steeply decreases with laser ellipticity. Besides, we show that the polarization properties of the harmonics are related to the complex momenta of the electron. The physical origin of the harmonic ellipticity is interpreted as the consequence of quantum-mechanical uncertainty of the electron momentum. Our results are verified with the experimental results as well as the numerical solutions of the time dependent Schrödinger equation from the literature.

In situ calibration of an extreme ultraviolet spectrometer for attosecond transient absorption experiments

We report a method for calibrating an extreme ultraviolet spectrometer based on a flat-field grazing incidence spherical grating in the energy range of 20-30 eV. By measuring absorption lines corresponding to singly excited states in helium atoms and autoionizing states in argon atoms, the photon energy of the detected light was determined. The spectral resolution of the spectrometer, 60 meV, was obtained by deconvolving the Fano resonance profile of argon autoionizing states from the measured absorption line profiles.

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.

Broadband isolated attosecond pulse with high spatiotemporal quality in pre-excited medium by multi-cycle two-color fields

We propose a scheme that combines multi-cycle two-color pump pulses and pre-excited medium to simultaneously control the ionization and acceleration steps of electron wave packet. Intense two-plateau supercontinua are observed at microscopic level, which support isolated attosecond pulse with tunable central wavelength. We further investigate the propagation effects and find that the macroscopic response is mainly associated with the initial population of the excited state. Large initial population can distort the pump field and weaken the phase-matching of the supercontinua due to the high density of free electrons. On the contrary, small initial population of 3% can generate well phase-matched supercontinua, and an efficient isolated attosecond pulse with gaussian-like spatial distribution is directly achieved. The divergence angle of the generated pulse is less than 0.2 mrad, which indicates its beam quality is good. This pulse benefits for some potential applications of ultrafast measurements with high spatiotemporal resolutions.

Invited review article: technology for attosecond science

We describe a complete technological system at Imperial College London for Attosecond Science studies. The system comprises a few-cycle, carrier envelope phase stabilized laser source which delivers sub 4 fs pulses to a vibration-isolated attosecond vacuum beamline. The beamline is used for the generation of isolated attosecond pulses in the extreme ultraviolet (XUV) at kilohertz repetition rates through laser-driven high harmonic generation in gas targets. The beamline incorporates: interferometers for producing pulse sequences for pump-probe studies; the facility to spectrally and spatially filter the harmonic radiation; an in-line spatially resolving XUV spectrometer; and a photoelectron spectroscopy chamber in which attosecond streaking is used to characterize the attosecond pulses. We discuss the technology and techniques behind the development of our complete system and summarize its performance. This versatile apparatus has enabled a number of new experimental investigations which we briefly describe.

A compact collinear polarization gating scheme for many cycle laser pulses

We demonstrate the generation of a broadband coherent continuum extreme-ultraviolet (XUV) radiation produced by the interaction of gases with a many-cycle infrared (IR) laser field, utilizing a compact collinear many cycle-polarization gating (CMC-PG) device. The spectral width of the XUV radiation can support isolated pulses of 200 asec duration. The CMC-PG device forms a high energy content ultra-short temporal gate in a many-cycle laser pulse, within which the XUV emission is taking place. The gate width has been measured and is in agreement with the theoretical calculations. The simplicity, the compactness, the long term stability, and the high IR energy output within the gate, make the CMC-PG device an ideal tool for generating energetic isolated attosecond pulses and measure the carrier-envelope phase of a high-power many-cycle laser field.

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.

Carrier-envelope phase stabilization of a terawatt level chirped pulse amplifier for generation of intense isolated attosecond pulses

We demonstrate the first carrier-envelope phase (CEP)-stabilized chirped pulse amplification system with pulse peak-powers in the terawatt regime. The system, which eventually is intended to be used in the generation of isolated attosecond pulses, consists of two consecutive multipass amplification stages. The first amplification stage is a commercial CEP-stable kHz system including a single 13-pass amplifier reaching a pulse energy of 2.3 mJ. Pulses are picked after the first stage at a repetition rate of 50 Hz and are further amplified in a 5-pass power-amplifier to pulse energies that reach up to 80 mJ before compression. After compression the pulse energy is 35mJ at a pulse duration of 32 fs, signifying a peak power of 1.1 terawatt. Peak-powers exceeding 1.5 TW should easily be achievable by improving the efficiency of the grating compressor. The CEP-stability of the terawatt system is demonstrated by single shot measurements of the residual CEP jitter at the full repetition rate and show an excellent root-mean-square value of 315 mrad.

Mid-infrared laser-driven broadband water-window supercontinuum generation from pre-excited medium

We theoretically investigate the broadband water-window supercontinuum generation from pre-excited medium with a mid-infrared pulse. We find that the wavelength scaling of the harmonic yield from near-visible (0.8 μm) to mid-infrared (1.8 μm) in single-atom level is λ(-2.7). Using an intense phase-stabilized few-cycle 1.6 μm laser pulse, a broadband water window supercontinuum with bandwidth of approximately 140 eV is obtained. We also investigate the macroscopic effects and find that large initial population of the excited state leads to the high-density of free electrons, which shift the carrier-envelop phase of the driving pulse and further diminish the water-window supercontinuum generation. The highly-ionized medium also results in poor temporal and spatial properties of the attosecond pulse. Instead, small initial population of the excited state can produce well phase-matched xuv supercontinuum in water-window region and an 100-as pulse with central wavelength of 2.8 nm and pulse energy of 0.15 nJ can be filtered out.

Two-color field for the generation of an isolated attosecond pulse in water-window region

For the investigation of various ultrafast electron dynamics, an isolated attosecond pulse in a broad spectral range is necessary. The generation of isolated attosecond pulses demands the manipulation of the electric field of a laser. We propose a two-color field scheme for generating an isolated attosecond pulse in the water-window region. Two-color fields are generated by mixing two equally-strong pulsed color fields. The investigation shows that an isolated attosecond pulse with a photon energy of near 500 eV and a pulse duration of 125 - 188 attoseconds can be generated using 10 - 15 fs FWHM laser fields.

Observation of non-odd order harmonics by sub-2-cycle laser pulses

High order harmonics generation from argon gas was comprehensively investigated by using multi-cycle and few-cycle laser pulses. Non-odd order harmonics were observed for sub-5-fs pulses, compare to the normal odd-order harmonics in the multi-cycle case. Theoretic analysis shows that the new spectral structure origins from the asymmetry of laser field in few-cycle pulses. This asymmetry induced both amplitude and phase difference between attosecond pulses from consecutive half-cycle of the laser field, which change the interference property of attosecond pulses and result in complex spectrum.

Time-resolved EUV photoelectron spectroscopy of dissociating I2 by laser harmonics at 80 nm

Generation of single-order laser harmonics in extreme ultraviolet (EUV) and the application to the time-resolved photoelectron spectroscopy of I(2) are demonstrated. The EUV pulses at 80 nm were generated from Kr as the 5th order harmonics of intense 400 nm laser pulses and then separated from other harmonic orders by a thin indium foil. The pump-probe photoelectron spectroscopy of I(2) in the B (3)Π(0(u)(+)) and B" (1)Π(1(u)) states excited by visible laser pulses at 490 nm showed a rapid increase in the yield of atomic iodine (~400 fs), reflecting the dissociation dynamics evolving simultaneously in the two excited states.

Generation of isolated single attosecond hard X-ray pulse in enhanced self-amplified spontaneous emission scheme

The generation of isolated attosecond hard x-ray pulse has been studied under the enhanced self-amplified spontaneous emission (ESASE) scheme with the density and energy modulation of an electron bunch. It is demonstrated in simulation that an isolated attosecond hard x-ray pulse of a high contrast ratio can be produced by adjusting a driver laser wavelength and the energy distribution of an electron bunch. An isolated attosecond pulse of ~146 attosecond full-width half-maximum (FWHM) at 0.1 nm wavelength is obtained with a saturation length of 34 meter for the electron beam parameters of Korean X-ray Free Electron laser.

Continuously adjustable gate width setup for attosecond polarization gating: theory and experiment

We demonstrate an alternative approach for attosecond polarization gating. A setup composed of four quartz wedges and a quarter-wave plate allows an easy adjustment of the temporal gate-width and of the total dispersion. A numerical simulation of the pulse propagation beyond the carrier-envelope approximation enables a calibration of the setup and provides a flexible choice of the desired temporal polarization. An electron imaging spectrometer is used to measure the electron momentum distribution resulting from the ionization of xenon with our optical gated laser pulses. This allows us to measure the orientation of the polarization plane in the most intense temporal slice of the laser pulse. We compare the experimental results to theory and we numerically show the robustness of the method against non-ideal laser parameters.

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.

Monitoring and controlling the electron dynamics in helium with isolated attosecond pulses

Helium atoms in the presence of extreme ultraviolet light pulses can lose electrons through direct photoionization or through two-electron excitation followed by autoionization. Here we demonstrate that, by combining attosecond extreme ultraviolet pulses with near infrared femtosecond lasers, electron dynamics in helium autoionization can be not only monitored but also controlled. Furthermore, the interference between the two ionization channels was modified by the intense near infrared laser pulse. To the best of our knowledge, this is the first time that double excitation and autoionization were studied experimentally by using isolated attosecond pulses.

Route to attosecond nonlinear spectroscopy

We demonstrate generation of coherent microjoule-scale, low-order harmonic supercontinua in the deep and vacuum ultraviolet (4-9 eV), resulting from the nonlinear transformations of near-single-cycle laser pulses in a gas cell. We show theoretically that their formation is connected to a novel nonlinear regime, holding promise for the generation of powerful deep-UV and vacuum ultraviolet subfemtosecond pulses. Our work opens the route to pump-probe spectroscopy of subfemtosecond-scale valence-shell phenomena in atoms, molecules, and condensed matter.

Tunable frequency-controlled isolated attosecond pulses characterized by either 750 nm or 400 nm wavelength streak fields

A compact and robust Mach-Zehnder type interferometer coupled with the double optical gating technique provides tunable isolated attosecond pulses and streak field detection with fields centered at either 750 nm or 400 nm. Isolated attosecond pulses produced at 45 eV (with measured pulse duration of 114 ± 4 as) and 20 eV (with measured pulse duration of 395 ± 6 as) are temporally characterized with a 750 nm wavelength streak field. In addition, an isolated 118 ± 10 as pulse at 45 eV is measured with a 400 nm wavelength streak field. The interferometer design used herein provides broad flexibility for attosecond streak experiments, allowing pump and probe pulses to be specified independently. This capability is important for studying selected electronic transitions in atoms and molecules.

Broadband supercontinuum generation method combining mid-infrared chirped-pulse modulation and generalized polarization gating

We present a method to control the harmonic process by a mid-infrared modulated generalized polarization gating for the generation of the broadband supercontinuum. Using a mid-IR generalized polarization gating modulated by a weaker mid-IR linearly polarized chirped field, the ionization, acceleration and recombination steps in the HHG process are simultaneously controlled, leading to the efficient generation of an ultra-broadband supercontinuum covered by the spectral range from ultraviolet to water window x-ray. Using this method we expect that isolated sub-100 attosecond pulses with tunable wavelength could be obtained straightforwardly.

Isolated attosecond pulse generation without the need to stabilize the carrier-envelope phase of driving lasers

Single isolated attosecond pulses can be extracted from a pulse train with an ultrafast gate in the generation target. By setting the gate width sufficiently narrow with the generalized double optical gating, we demonstrate that single isolated attosecond pulses can be generated with any arbitrary carrier-envelope phase value of the driving laser. The carrier-envelope phase only affects the photon flux, not the pulse duration or contrast. Our results show that isolated attosecond pulses can be generated using carrier-envelope phase unstabilized 23 fs pulses directly from chirped pulse amplifiers.

Infrared two-color multicycle laser field synthesis for generating an intense attosecond pulse

We propose and demonstrate the generation of a continuum high-order harmonic spectrum by mixing multicycle two-color (TC) laser fields with the aim of obtaining an intense isolated attosecond pulse. By optimizing the wavelength of a supplementary infrared pulse in a TC field, a continuum harmonic spectrum was created around the cutoff region without carrier-envelope phase stabilization. The obtained harmonic spectra clearly show the possibility of generating isolated attosecond pulses from a multicycle TC laser field, which is generated by an 800 nm, 30 fs pulse mixed with a 1300 nm, 40 fs pulse. Our proposed method enables us not only to relax the requirements for the pump pulse duration but also to reduce ionization of the harmonic medium. This concept opens the door to create an intense isolated attosecond pulse using a conventional femtosecond laser system.

Mid-infrared modulated polarization gating for ultra-broadband supercontinuum generation

We propose a method to control the harmonic process by a mid-IR modulated polarization gating for the effective generation of an ultra-broadband supercontinuum in the neutral rare-gas media. Using a mid-IR polarization gating pulse modulated by a weaker 800-nm linearly polarized pulse, the ionization, acceleration and recombination steps in the harmonic process are simultaneously controlled, leading to the efficient generation of an ultra-broadband supercontinuum covered by the spectral range from ultraviolet to water window x ray under the low ionization rate. The right phase-matching technique is employed to macroscopically select the short quantum path of the supercontinuum, then isolated sub-100-as pulses with tunable wavelengths are directly obtained. This supercontinuum also supports the pulse duration far below one atomic unit of time.

Quantum path interferences of electron trajectories in two-center molecules

We report a new quantum path interference effect of electron trajectories in high-order harmonic generation (HHG) from two-center molecules, in which the interference minima are mainly located in the high-energy portion of HHG spectrum. The quantum calculations of the time-frequency analyses and the classical results of the electron trajectories demonstrate very good agreement and reveal that the positions of the interference minima are associated with the cutoff of various kinds of molecular electron trajectories. The interference fringes within a half optical cycle can be clearly seen in the time-frequency analysis spectrum. Moreover, the characteristics of both the HHG in frequency domain and the corresponding attosecond pulse generation in time domain permit tracing back the interference information of these electron trajectories. These interference phenomena offer new possibilities for getting insight into the attosecond electronic dynamics in molecules.

Calibration of electron spectrometer resolution in attosecond streak camera

We report a new method for determining the energy resolution of time-of-flight spectrometers for detecting photoelectrons produced with attosecond XUV pulses. By measuring the width of the 2s2p autoionization line of helium, we found the resolution of our spectrometer to be approximately 0.6 eV for electrons at 35.5 eV. Furthermore, the resolution in the 10 to 35 eV range was determined by applying a retarding potential at the entrance of the drift tube.

Delay control in attosecond pump-probe experiments

The time delay between the pump and probe pulses in attosecond time-resolved experiments, such as attosecond streaking, is commonly introduced by splitting and recombining the two pulses in an interferometer. This technique suffers from instability in the optical path lengths of the two arms due to mechanical vibration of the optical elements and fluctuating environmental conditions. We present a technique with which the instability of the unconventional interferometer is suppressed while at the same time the time delay is controlled to within 20 as RMS using a feedback loop. Using this scheme, the streaked spectrogram of an attosecond pulse was measured.

Extreme ultraviolet supercontinua supporting pulse durations of less than one atomic unit of time

Double optical gating of high-harmonic generation was used to obtain supercontinuous spectra in the extreme UV (XUV) region including the water window. The spectra supported a 16 as pulse duration that is below one atomic unit of time (24 as). The dependence of the gated spectra on the carrier-envelope phase of the laser provided evidence that isolated attosecond pulses were generated. In addition, to ensure the temporal coherence of the XUV light, the pulse shape and phase of isolated 107 as XUV pulses using a portion of the spectrum were characterized by attosecond streaking.

Generation of isolated attosecond pulses with 20 to 28 femtosecond lasers

Isolated attosecond pulses are powerful tools for exploring electron dynamics in matter. So far, such extreme ultraviolet pulses have only been generated using high power, few-cycle lasers, which are very difficult to construct and operate. We propose and demonstrate a technique called generalized double optical gating for generating isolated attosecond pulses with 20 fs lasers from a hollow-core fiber and 28 fs lasers directly from an amplifier. These pulses, generated from argon gas, are measured to be 260 and 148 as by reconstructing the streaked photoelectron spectrograms. This scheme, with a relaxed requirement on laser pulse duration, makes attophysics more accessible to many laboratories that are capable of producing such multicycle laser pulses.

Carrier-envelope phase stabilization and control of 1 kHz, 6 mJ, 30 fs laser pulses from a Ti:sapphire regenerative amplifier

Carrier-envelope (CE) phase stabilization of a two-stage chirped pulse amplifier laser system with regenerative amplification as the preamplifier is demonstrated. The CE phase stability of this laser system is found to have a 90 mrad rms error averaged over 50 laser shots for a locking period of 4.5 h. The CE phase locking was confirmed unambiguously by experimental observation of the 2pi periodicity of the high-order harmonic spectrum generated by double optical gating.

Broadband extreme ultraviolet multilayer mirror for supercontinuum light at a photon energy of 35-65 eV

We have developed a broadband multilayer mirror for photon energies between 35 and 65 eV. This extreme ultraviolet (EUV) mirror has an almost flat reflectivity profile at normal incidence, and the average reflectivity is 8%. The bandwidth of this mirror is 30 eV centered around 50 eV, 1.5 times broader than conventional multilayer mirrors in this energy region. This new mirror can be used in a variety of applications using supercontinuum EUV light sources including attosecond metrology.

The robustness of attosecond streaking measurements

We investigate attosecond streaking measurements, where a spectrogram is described by an ensemble of electron wave packets. Such a description may be required for processes more complex than direct photoemission from an isolated atom; an ensemble of wave packets may also be needed to describe the role of shot-to-shot fluctuations or a non-uniform spatio-temporal profile of attosecond light pulses. Under these conditions, we examine the performance of conventional (FROG) analysis of attosecond streaking measurements.

Direct measurement of an electric field in femtosecond Bessel-Gaussian beams

We demonstrated the mapping of the spatial oscillation of electric fields in the transverse plane of a femtosecond Bessel-Gaussian laser beam from the first principle of classical electrodynamics. An attosecond burst of electrons for probing the electric force was placed in the Bessel beam through photoemission with single isolated 276 as extreme ultraviolet pulses. The direction reversal of the electric field in adjacent Bessel rings was directly confirmed by observing the momentum shift of the probe electrons.

Dynamic chirp control and pulse compression for attosecond high-order harmonic emission

We propose a scheme to compensate dynamically the intrinsic chirp of the attosecond harmonic pulses. By adding a weak second harmonic laser field to the driving laser field, the chirp compensation can be varied from the negative to the positive continuously by simply adjusting the relative time delay between the two-color pulses. Using this technique, the compensation of the negative chirp in harmonic emission is demonstrated experimentally for the first time and the nearly transform-limited attosecond pulse trains are obtained.

Measurement and optimization of isolated attosecond pulse contrast

An experimental method is presented to experimentally measure and control the carrier-envelope-phase (CEP)-dependent pulse-energy contrast of isolated attosecond pulses. By scanning the CEP and measuring the photoelectron spectrum produced by the combined action of the attosecond pulses and the high-harmonic driving laser pulses at zero relative time delay, one can extract the pulse-energy ratio between the main attosecond pulse and its neighboring satellite pulses arriving in preceding or subsequent half-cycles of the driver pulse. Moreover, this method allows fast and efficient in situ retrieval of the optimal CEP for high-contrast isolated attosecond pulse generation.

Spectral signature of short attosecond pulse trains

We report experimental measurements of high-order harmonic spectra generated in Ar using a carrier-envelope-offset (CEO) stabilized 12 fs, 800 nm laser field and a fraction (less than 10%) of its second harmonic. Additional spectral peaks are observed between the harmonic peaks, which are due to interferences between multiple pulses in the train. The position of these peaks varies with the CEO and their number is directly related to the number of pulses in the train. An analytical model, as well as numerical simulations, support our interpretation.

Quantum path selection in high-harmonic generation by a phase-locked two-color field

We report the results of our studies on the selection of the quantum path in high-harmonic generation (HHG) with a relative-phase-locked two-color laser field. It is shown that by tuning the relative phase between fundamental and second-harmonic fields, The timing of tunnel ionization and subsequent electron trajectories on the sub-cycle time scale can be controlled. We have clearly observed a phase-dependent two-step feature in the harmonic spectra that can be attributed to the selection of two major trajectories in the two-color field HHG.

Angularly-dispersed optical parametric amplification of optical pulses with one-octave bandwidth toward monocycle regime

We demonstrated experimentally the generation of 65-microJ, 5.8-fs optical pulses with an ultrabroad bandwidth (540-1000 nm) by the use of a double-pass angularly-dispersed non-collinear optical parametric amplifier. We also confirmed up to the 95-microJ output from the amplifier when seed pulses were not pre-compensated for. Furthermore, we confirmed that the broadband pump pulses brought in the broader gain bandwidth (from 520 to 1080 nm) than numerical estimation based on CW-pump approximation. To the best of our knowledge, this is the system with the broadest gain bandwidth.

Generation of 0.5 mJ, few-cycle laser pulses by an adaptive phase modulator

Previously, pulses shorter than 4 fs were generated by compressing white light from gas-filled hollow-core fibers with adaptive phase modulators; however, the energy of the few-cycle pulses was limited to 15 microJ. Here, we report the generation of 550 microJ, 5 fs pulses by using a liquid crystal spatial light modulator in a grating-based 4f system. The high pulse energy was obtained by improving the throughput of the phase modulator and by increasing the input laser energy. When the pulses were used in high harmonic generation, it was found that the harmonic spectra depend strongly on the high order spectral phases of the driving laser fields.