Intense Few-cycle Pulses Research Articles

Effective soliton order approach for scaling of pulse self-compression in hollow-core fibers

Optical solitons can undergo extreme temporal and spectral reshaping at high pulse intensities. These dynamics can be utilized to extract few-cycle pulses in the negative dispersion region in the case of higher-order spatial modes in hollow fibers. One can achieve self-compression up to few-cycle duration in hollow capillary fibers without any external optical post-compression schemes using this technique. These intense few-cycle pulses are an essential tool for revealing the ultrafast dynamics in the femtosecond to attosecond timescale, where the tunability and scalability of few-cycle pulses are a necessity. Utilizing the soliton dynamics, we can scale the pulses up to multi-millijoule energies as well as terawatts of peak power. Scaling relations are relations that can be used to produce propagation dynamics that are invariant and essentially identical for multiple sets of input conditions. But, for the same input soliton order, the scaling relations derived under different dispersion conditions, such as different gas pressure, result in somewhat different scaling laws. This leads to an ambiguity in the compression factor and compression length for any particular soliton order N. It is thus necessary to find an accurate soliton order which can scale the self-compression dynamics over different dispersion conditions. We numerically simulate soliton self-compression in an argon gas-filled HCF across a wide range of dispersion conditions and present a set of efficient scaling laws for soliton dynamics. We introduce an effective soliton order Neff for explaining the behavior of the dynamics in systems with high third-order dispersion (TOD) such as the HCF. The proficiency of the new scaling laws over the earlier scaling laws in literature is elucidated in this work. Thus, an inclusive set of scaling laws for generating high-energy few-cycle pulses are defined which are critical for generating single and trains of attosecond pulses, as well as electron and ion acceleration strategies in intense laser pulses.

Charge Migration in HCCI Cations Probed by Strong Field Ionization: Time-Dependent Configuration Interaction and Vibrational Wavepacket Simulations.

Strong field ionization of neutral iodoacetylene (HCCI) can produce a coherent superposition of the X and A cations and results in charge migration between the CC π orbital and the iodine π-type lone pair. This charge migration causes oscillations in the rate of strong field ionization of the cation to the dication that can be monitored using intense few-cycle probe pulses. The dynamics and strong field ionization of the coherent superposition the X and A states of HCCI+ have been modeled by time-dependent configuration interaction (TDCI) simulations. When the nuclei are allowed to move, the electronic wavefunctions need to be multiplied by vibrational wavefunctions. Nuclear motion has been modeled by vibrational packets moving on quadratic approximations to the potential energy surfaces for the X and A states of the cation. The overlap of the vibrational wavepackets decays in about 10-15 fs. Consequently, the oscillations in the strong field ionization decay on the same time scale. A revival of the vibrational overlap and in the oscillations of the strong field ionization is seen at 60-110 fs. TDCI simulations show that the decay and revival of the charge migration can be monitored by strong field ionization with intense 2- and 4-cycle linearly polarized 800 nm pulses. The revival is also seen with 7-cycle pulses.

Efficient generation of mid-infrared few-cycle pulses by the intrapulse difference-frequency generation in YCOB.

Yttrium calcium oxyborate (YCOB) crystals have been widely applied for generating intense near-infrared laser pulses by optical parametric amplification. Here, we show that the YCOB crystals oriented in both the XZ and XY principal planes possess broadband phase-matching property of intrapulse difference-frequency generation in the mid-infrared region. Few-cycle pulses tunable from 2 to 4 µm are experimentally produced by using a 7.5-fs pump laser at 800 nm, in which the conversion efficiency can be as high as 2.5%. With a large-size crystal and high-power pump laser, intrapulse difference-frequency generation based on YCOB may provide a new route for directly producing intense few-cycle mid-infrared pulses.

Self-Stopping of Light.

Here, we show that light can bring itself to a complete standstill (self-stop) via self-interaction mediated by the resonant nonlinearity in a fully homogeneous medium. An intense few-cycle pulse, entering the medium, may reshape to form a strongly coupled light-matter bundle, in which the energy is transferred from light to the medium and back periodically on the single-cycle scale. Such oscillating structure can decelerate, alter its propagation direction, and even completely stop, depending on the state of its internal degrees of freedom. This phenomenon is expected to occur in the few-cycle strong-field regime when the Rabi oscillation frequency becomes comparable with the frequency of the incoming light.

Ultrafast Control of Magnetic Anisotropy by Resonant Excitation of 4f Electrons and Phonons in Sm0.7Er0.3FeO3

We compare the ultrafast dynamics of the spin reorientation transition in the orthoferrite ${\mathrm{Sm}}_{0.7}{\mathrm{Er}}_{0.3}{\mathrm{FeO}}_{3}$ following two different pumping mechanisms. Intense few-cycle pulses in the midinfrared selectively excite either the $f\text{\ensuremath{-}}f$ electronic transition of ${\mathrm{Sm}}^{3+}$ or optical phonons. With phonon pumping, a finite time delay exists for the spin reorientation, reflecting the energy transfer between the lattice and $4f$ system. In contrast, an instantaneous response is found for resonant $f\text{\ensuremath{-}}f$ excitation. This suggests that $4f$ electronic pumping can directly alter the magnetic anisotropy due to the modification of $4f\text{\ensuremath{-}}3d$ exchange at femtosecond timescales, without involving lattice thermalization.

Efficiency enhancement of ion acceleration from thin target irradiated by multi-PW few-cycle laser pulses

We investigate the energy conversion efficiency from the laser to the accelerated ion beams in the interaction of different duration laser pulses with a simple plane foil, while the laser energy is kept constant. By employing multi-dimensional particle-in-cell simulations, we demonstrate that an intense few-cycle pulse is more beneficial for the energy transferring from the laser to the generated ions. Owing to the short acceleration time window of few-cycle pulses, the overwhelming majority of laser energy could be transferred to ions through a high-efficiency radiation pressure acceleration mechanism before the rupture of the target caused by transverse instabilities and finite spot effects. More importantly, it is found that a laser pulse with a too short duration also fails to reach the highest energy conversion efficiency for a given laser energy. The optimal pulse duration is related to laser energy and ion species. Our three-dimensional particle-in-cell simulations show that the highest energy conversion efficiency is obtained with a laser pulse with a duration of two cycles, which, unlike a single-cycle pulse, is the optimal duration for the given laser energy (49 J). Furthermore, when the laser energy is reduced to 12 J, four-cycle becomes the optimal duration.

Iteration of semiconductor Bloch equations for ultrashort laser pulse propagation

The numerical propagation of intense laser pulses through bulk material requires the recurrent calculation of the nonlinear material response. To describe the optical Kerr effect and the current in the conduction band for macroscopic propagation distances, very simplified models are typically used. Recent studies of the response of dielectrics to intense few-cycle pulses have revealed that ionization does not accumulate monotonically, but conduction bands are populated both irreversibly and reversibly during a laser cycle. The reversible (or transient or virtual) population of the conduction bands is not captured by simple response models. Here, an efficient iteration based on the semiconductor Bloch equations for three bands is developed, which consistently captures the laser cycle resolved interband polarization and intraband current. The full calculation of the nonlinear material response at each propagation step is avoided, instead only the incremental modification of the previous propagation step is calculated. The iteration is particularly well-suited for very short pulses and can be applied for intensities above the critical value at which perturbation theory does not converge. Furthermore, it is shown that virtual currents and dynamic Bloch oscillations are mechanisms which are missing in the Drude model, but these two mechanisms prevail for short intense pulses. Therefore, a generalized Drude model is derived from the SBEs, which is capable to account for arbitrary band shapes and both real and virtual ionization.

Single-shot intense few-cycle pulse interaction with polycrystalline ZnSe.

The interaction of high-intensity few-cycle laser pulses with solids opens a new area of fundamental light-material interaction research. The applied research extends from extreme nonlinearity in solids to the next-generation high laser light damage resistance optical design. In this Letter, 11 fs infrared, carrier-envelope-phase (CEP) stable, two-cycle laser pulses were applied to investigate the process of laser-material interaction on the ZnSe surface. A systematic study of a few-cycle pulse laser-induced damage threshold on ZnSe was performed for a single-pulse regime (1-on-1). Laser damage morphologies were carefully characterized. Our experiment demonstrated the very beginning of laser-induced structures on the ZnSe surface by using the shortest infrared few-cycle laser pulse currently available with a stable CEP.

Photoelectron Circular Dichroism at the Few-cycle Limit

Photoelectron circular dichroism (PECD) was investigated for methyloxirane with intense few-cycle pulses. The observed PECD shows a dependence on electron energy. This may signify an influence of the chiral potential on the outgoing electron’s trajectory.

Above-threshold ionization by few-cycle Bessel pulses carrying orbital angular momentum

We investigate theoretically the above-threshold ionization (ATI) of localized atomic targets by intense few-cycle Bessel pulses that carry orbital angular momentum (OAM), known also as twisted light. More specifically, we use the strong-field approximation (SFA) to compute the photoelectron energy spectra. While for plane-wave laser pulses the outgoing photoelectron is typically described by Volkov states within the SFA, no equivalent is known for an electron in a twisted laser field. Here, we therefore introduce a local dipole approximation for the (continuum) state of the photoelectron that is justified for few-cycle pulses. Based on this approximation, we demonstrate that the photoelectrons can also be emitted into the propagation direction of the pulse. When measured in propagation direction, moreover, we show that the magnitude of the ATI peaks depend on the opening angle and the (projection of) total angular momentum of the Bessel pulse.

Giant self-induced transparency of intense few-cycle terahertz pulses in n-doped silicon.

The results of high-field terahertz transmission experiments on n-doped silicon (carrier concentration of 8.7×1016 cm-3) are presented. We use terahertz pulses with electric field strengths up to 3.1 MV cm-1 and a pulse duration of 700fs. A huge transmittance enhancement of ∼90 times is observed with increasing of the terahertz electric field strengths within the range of 1.5-3.1 MV cm-1.

Nonlinear pulse compression to 43 W GW-class few-cycle pulses at 2 μm wavelength.

High-average power laser sources delivering intense few-cycle pulses in wavelength regions beyond the near infrared are promising tools for driving the next generation of high-flux strong-field experiments. In this work, we report on nonlinear pulse compression to 34.4μJ-, 2.1-cycle pulses with 1.4 GW peak power at a central wavelength of 1.82μm and an average power of 43W. This performance level was enabled by the combination of a high-repetition-rate ultrafast thulium-doped fiber laser system and a gas-filled antiresonant hollow-core fiber.

High harmonic generation experiments based on solid-state supercontinuum

Intense few-cycle pulses are widely used in transient light synthesis,high harmonic generation (HHG) and especially in isolated attosecond pulse generation.To obtain intense few-cycle pulses,the intense supercontinuum is needed.The traditional way to generate intense supercontinuum is using rare gas filled hollow-core fibers.Since the input energy of hollow-core fiber system is limited to a level of tens of mJ,it is necessary to find new ways to achieve energy scaling.In this paper we demonstrate the efficient generation of supercontinuum by solid thin plates,compression and its application in HHG. The Ti:sapphire laser used in the present experiment emits 0.8 mJ in energy with a duration of 30 fs at 1 kHz.After passing through a 3:1 telescope,the beam has a diameter changed from 12 mm to 4 mm.Then the laser is focused by an f=2000 mm lens into a 600 m-diameter spot.After propagating through 7 fused silica plates placed at Brewster's angle (55.5) with a thickness of 0.1 mm,the 0.7 mJ octave spanning supercontinuum is achieved,corresponding to an efficiency of 87.5%.The first three plates are placed at 31,11,2.5 mm in front of the beam waist,and the last four plates are placed at 2,7,12,17 mm behind the beam waist respectively.With a pair of wedges and 4 pairs of chirped mirrors,the 0.68 mJ supercontinuum is compressed to a duration of 6.3 fs,which is measured by TG-FROG. The 0.5 mJ,6.3 fs pulse is used to perform high-harmonic generation experiment.The beam diameter is 150 m when focused by an f=400 mm lens,with a laser intensity of 8.11014 W/cm2.The 1 mm Ne gas jet is used to perform HHG experiment with a back pressure of 300 mbar.To block the near-infrared light,a 150 m Zirconium foil is placed behind the gas jet.Then the XUV spectrum is detected by a spectrometer,which consists of a flat field grating and a CCD camera.For driving pulses of few-cycle regime without dispersion,the cutoff spectrum of HHG is continuous.But when the pulse is stretched by positive or negative dispersion,the cutoff spectrum turns discrete.The HHG result is that the cutoff region is continuous when the wedge is in a certain place.Then by increasing or reducing the insertion of the wedge,the cutoff spectrum becomes discrete.Our result is consistent with HHG generated by few-cycle pulses. In conclusion,we demonstrate high-harmonic generation based on supercontinuum generated by solid thin plates. The 0.7 mJ supercontinuum is achieved when 0.8 mJ pulses are injected to 7 thin fused silica plates.The supercontinuum is compressed to 0.68 mJ,6.3 fs.The 0.5 mJ,6.3 fs pulse is used to perform HHG experiments.The HHG result was consistent with few-cycle driving pulses.Our research indicates that solid state supercontinuum has great potential applications in HHG and isolated attosecond pulse generation.

Coherent Electronic Wave Packet Motion inC60Controlled by the Waveform and Polarization of Few-Cycle Laser Fields

Strong laser fields can be used to trigger an ultrafast molecular response that involves electronic excitation and ionization dynamics. Here, we report on the experimental control of the spatial localization of the electronic excitation in the C_{60} fullerene exerted by an intense few-cycle (4fs) pulse at 720nm. The control is achieved by tailoring the carrier-envelope phase and the polarization of the laser pulse. We find that the maxima and minima of the photoemission-asymmetry parameter along the laser-polarization axis are synchronized with the localization of the coherent electronic wave packet at around the time of ionization.

Optimal control of high-harmonic generation by intense few-cycle pulses

At the core of attosecond science lies the ability to generate laser pulses of subfemtosecond duration. In tabletop devices the process relies on high-harmonic generation, where a major challenge is to obtain high yields and high cutoff energies required for the generation of attosecond pulses. We develop a computational method that can simultaneously resolve these issues by optimizing the driving pulses using quantum optimal control theory. Our target functional, an integral over the harmonic yield over a desired energy range, leads to a remarkable cutoff extension and yield enhancement for a one-dimensional model H atom. The physical enhancement process is shown to be twofold: the cutoff extension is of classical origin, whereas the yield enhancement arises from increased tunneling probability. The scheme is directly applicable to more realistic models and, within straightforward refinements, also to experimental verification.

Near infrared few-cycle pulses for high harmonic generation

We report on the development of intense tunable few-cycle pulses with central wavelengths ranging from to . These pulses were used as a proof of principle for high harmonic generation in atomic and molecular targets. In order to generate such pulses, we produced a filament in a four-bar krypton cell. Spectral broadening by a factor of two to three of a 40 fs near-infrared input pulse was achieved. The spectrally broadened output pulses were then compressed by fused silica plates down to the few-cycle regime close to the Fourier limit. The auto-correlation of these pulses revealed durations of ∼ three cycles for all investigated central wavelengths. Pulses with a central wavelength of and up to energy per pulse were employed to generate high-order harmonics in Xe, Ar, and N2. Moving to near-infrared few-cycle pulses opens the possibility of operating deeply in the non perturbative regime with a Keldysh parameter, . Hence, this source is suitable for the study of the non perturbative tunneling regime in most generating systems used for high-order harmonic generation and attoscience.

Intense Cr:forsterite-laser-based supercontinuum source

Supercontinuum pulses covering the range from 1100 to 1700 nm with energies >1.0 mJ and excellent beam quality are generated via nonlinear spectral broadening of Cr:forsterite (1240 nm, 110 fs) pulses in pressurized molecular nitrogen. Our spectra, which extend over more than half an octave, offer an attractive alternative to intense few-cycle pulse synthesis in the 1-2 μm range and lend themselves as an important add-on to Cr:forsterite laser technologies.

Intense isolated few-cycle attosecond XUV pulses from overdense plasmas driven by tailored laser pulses.

A new scheme to generate an intense isolated few-cycle attosecond XUV pulse is demonstrated using particle-in-cell simulations. By use of unipolarlike or subcycle laser pulses irradiating a thin foil target, a strong transverse net current can be excited, which emits a few-cycle XUV pulse from the target rear side. The isolated pulse is ultrashort in the time domain with duration of several hundred attoseconds. The pulse also has a narrow bandwidth in the spectral domain compared to other XUV sources of high-order harmonics. It has most energy confined around the plasma frequency and no low-harmonic orders below the plasma frequency. It is also shown that XUV pulse of peak field strength up to 8 × 10(12) Vm(-1) can be produced. Without the need for pulse selecting and spectral filtering, such an intense few-cycle XUV pulse is better suited to a number of applications.

Carrier-envelope-phase-induced asymmetries in double ionization of helium by an intense few-cycle XUV pulse

The carrier-envelope-phase (CEP) dependence of electron angular distributions in double ionization of He by an arbitrarily polarized, few-cycle, intense XUV pulse is formulated using perturbation theory (PT) in the pulse amplitude. Owing to the broad pulse bandwidth, interference of first- and second-order PT amplitudes produces asymmetric angular distributions sensitive to the CEP. The PT parametrization is shown to be valid by comparing with results of solutions of the full-dimensional, two-electron time-dependent Schr\odinger equation for the case of linear polarization.

Initial carrier-envelope phase of few-cycle pulses determined by terahertz emission from air plasma

The evolution of THz waveform generated in air plasma provides a sensitive probe to the variation of the carrier envelope phase (CEP) of propagating intense few-cycle pulses. Our experimental observation and calculation reveal that the number and positions of the inversion of THz waveform are dependent on the initial CEP, which is near 0.5{\pi} constantly under varied input pulse energies when two inversions of THz waveform in air plasma become one. This provides a method of measuring the initial CEP in an accuracy that is only limited by the stability of the driving few-cycle pulses.