Sub-cycle Pulses Research Articles

Superradiance of an Extended Resonant Medium Excited by Half-Cycle Attosecond Pulses

Here we demonstrate an unusual type of superradiance in an extended layer of a two-level medium excited by a pair of unipolar pulses. It is shown that, due to the interference of secondary waves emitted by the resonant medium, the response of such a layer in reflection represents a pair of half-cycle unipolar pulses of different polarity. They follow with a time delay proportional to the layer thickness. The source of such superradiance is a monopolar half-cycle pulse of the stopped polarization induced by the first excitation pulse and switched off by the second excitation pulse. The predicted effect can be used to control the shape of few- and subcycle pulses in resonant media.

Carrier-envelope-phase measurement of sub-cycle UV pulses using angular photofragment distributions

Carrier-envelope-phase (CEP) of sub-cycle ultraviolet (UV) pulse strongly influences the dynamics of quantum systems, but its characterization is not accessible experimentally. Here we investigate photodissociation of a diatomic molecule from its ground-rovibrational state in a linearly polarized weak sub-cycle UV pulse with a controlled CEP. The angular distribution of photofragments shows an asymmetric profile deviating from the well-known {cos }^{2}- or {sin }^{2}-like ones, which can be identified as a way to imprint CEP. We unveil that such an effect stems from the temporal neighboring rotational excitation by molecular permanent dipole interaction through the joint contributions between counter-rotating and rotating terms. This in turn, opens different pathways in photodissociation dynamics. Given that the temporal excitation between various states with close energies can be manipulated by CEP of sub-cycle UV pulses, our results pave ways for understanding and manipulating electron, nuclear and their joint dynamics with variation of CEP of attosecond pulses.

Interference of areas of subcycle light pulses

Progress in reducing the duration of light pulses (down to one cycle of field oscillations or less), achieved in the last decades, has made it possible to experimentally observe and study a number of atomic-scale phenomena that occur at ultrafast times. Their study and observation are unavailable with conventional sources of monochromatic laser radiation. When using few- or subcycle pulses, a number of well-known phenomena in optics, which are well studied when using long monochromatic radiation sources, either lose their meaning, or their underlying physical mechanisms require revision. For example, the direct interference of subcycle pulses with their direct overlap is not possible due to their short duration. This note addresses the interference phenomenon, when a pair of subcycle pulses act on a medium, while do not simultaneously overlap inside it. In this case, it is not the pulses themselves that interfere in the medium, but the probability amplitudes of the bound states of the medium. Therefore the result of their impact on the medium can be interpreted as the interference of the areas of pulses—electric pulse areas (integral of the electric field strength over time) and envelope areas (integral of the slowly varying envelope of the electric field strength over time).

Dispersive propagation of trans-ionospheric pulse pairs in ionosphere

As the most powerful natural radio waves on earth, trans-ionospheric pulse pairs (TIPPs) are emitted by lightning and have been recorded by several satellites. TIPPs commonly consist of two radio pulses with a clear ionospheric dispersion. Here, we calculate the propagation of TIPPs across the ionosphere by use of a method accurate for sub-cycle radio pulses. One can directly obtain dispersed waveforms in the time domain and then reproduce satellite-recorded frequency spectra. Our work affords a simple and robust tool to directly compare with observation data, which should be helpful to deduce original undispersed waveforms of TIPPs and characteristics of high-energy electrons produced by lightning.

Carrier-envelope phase control of synthesized waveforms with two acousto-optic programmable dispersive filters.

We demonstrate the scanning and control of the carrier-envelope phases (CEPs) of two adjacent spectral components totally spanning more than one-octave in the short-wave infrared (SWIR) wavelength region by operating two individual acousto-optic programmable dispersive filters (AOPDFs) applied to each of the two spectral components. The total CEP shift of the synthesized sub-cycle pulse composed of the two spectral components is controlled with simultaneous scans of the two CEPs. The resultant error of the controlled CEP was 642 mrad, so that this technique is useful for searching zero CEP of the synthesized pulse with the maximum field amplitude. In addition, we conduct a closed feedback loop to compensate for the CEP fluctuation by using the two AOPDFs together. As a result, we succeed to reduce the rms error of the CEP from 399 mrad to 237 mrad.

Electro-optic characterization of synthesized infrared-visible light fields

The measurement and control of light field oscillations enable the study of ultrafast phenomena on sub-cycle time scales. Electro-optic sampling (EOS) is a powerful field characterization approach, in terms of both sensitivity and dynamic range, but it has not reached beyond infrared frequencies. Here, we show the synthesis of a sub-cycle infrared-visible pulse and subsequent complete electric field characterization using EOS. The sampled bandwidth spans from 700 nm to 2700 nm (428 to 110 THz). Tailored electric-field waveforms are generated with a two-channel field synthesizer in the infrared-visible range, with a full-width at half-maximum duration as short as 3.8 fs at a central wavelength of 1.7 µm (176 THz). EOS detection of the complete bandwidth of these waveforms extends it into the visible spectral range. To demonstrate the power of our approach, we use the sub-cycle transients to inject carriers in a thin quartz sample for nonlinear photoconductive field sampling with sub-femtosecond resolution.

Optimization of pulse self-compression in hollow capillary fibers using decreasing pressure gradients.

The improvement of techniques for the generation of near-infrared (NIR) few-cycle pulses is paving the way for new scenarios in time-resolved spectroscopy and the generation of ultrashort extreme-ultraviolet pulses through high-harmonic generation. In this work, we numerically study how to optimize the self-compression of NIR pulses using decreasing pressure gradients in hollow capillary fibers (HCFs). We identify a moderate nonlinear regime in which sub-cycle pulses are obtained with very good temporal quality from an input 30 fs pulse centered at a 800 nm wavelength and coupled as the fundamental mode of an argon-filled HCF fully evacuated at the output end. Surprisingly, we observe that there is a relatively broad region of parameters for which the optimum self-compression takes place, defined by a simple relation between the input pulse energy and the initial gas pressure.

Parametric study of laser wakefield driven generation of intense sub-cycle pulses

Intense sub-cycle electromagnetic pulses allow one to drive nonlinear processes in matter with unprecedented levels of control. However, it remains challenging to scale such sources in the relativistic regime. Recently, a scheme that utilizes laser-driven wakes in plasmas to amplify and compress seed laser pulses to produce tunable, carrier-envelope-phase stable, relativistic sub-cycle pulses has been proposed. Here, we present parametric studies of this process using particle-in-cell simulations, showing its robustness over a wide range of experimentally accessible laser-plasma interaction parameters, spanning more than two orders of magnitude of background plasma density. The method is shown to work with different gas-jet profiles, including structured density profiles and is robust over a relatively wide range of driver laser intensities. Our study shows that sub-cycle pulses of up to 10mJ of energy can be produced.

Acceleration of electrons by tightly focused azimuthally polarized ultrashort pulses in a vacuum.

Using the complex sink-source model (CSSM) and the Hertz potential method (HPM), the electromagnetic field expressions of tightly focused ultrashort azimuthally polarized pulses can be obtained. By numerically solving the relativistic Newton-Lorentz equation, the acceleration and confinement of electrons by the sub-cycle and few-cycle azimuthally polarized ultrashort pulses in vacuum are studied. Considering the radiation reaction force, it is found that electrons with an initial kinetic energy of less than 1MeV can be accelerated to hundreds of MeV and can be confined in the range of less than 1 micron for hundreds of femtoseconds in the direction perpendicular to the pulse propagation (transverse direction) by the pulses. With the increase of the beam waist and the intensity of the pulse, the electrons can obtain the exit kinetic energy exceeding 1GeV. When electrons are accelerated by the few-cycle pulses, the confined time of the electrons in the transverse direction is three times longer than that of the sub-cycle pulse. When the initial velocity of the electron points to a point in front of the focus, the electron can obtain the maximum exit kinetic energy. The change of the angular frequency corresponding to the spectral peak of the electromagnetic radiation from the electron acceleration with the electric field amplitude parameter E0 of the pulse is studied. The phenomena of redshift and blueshift of the spectrum peak frequency of the electron radiation with the E0 are found. These studies provide the methods to confine the movement of electrons in certain directions and accelerate electrons in the same time.

Generation of isolated attosecond pulses with large electric area in a dense resonant medium

Obtaining unipolar half-cycle optical pulses of femto- and attosecond duration with a large electrical area is an urgent but difficult task. The reason for the emerging difficulties lies in the existence of the rule of conservation of the electrical area of the pulse, which does not allow converting a bipolar pulse into a unipolar one. In this work, it is shown that in a resonant medium a few-cycle pulse can be converted into two unipolar pulses separated in time by a distance that is an order of magnitude or more longer than the duration of the initial pulse. This allows in a number of problems to consider such pulses separately as unipolar. The estimation of the electric area value relative to its "atomic scale" is carried out. Keywords: attosecond pulses, unipolar pulses, subcycle pulses, electrical area of pulses.

Pulse self-compression down to the sub-cycle regime in hollow capillary fibers with decreasing pressure gradients

We theoretically demonstrate an enhancement in the generation of clean, near-infrared sub-cycle laser pulses by soliton self-compression in gas-filled hollow capillary fibers using decreasing pressure gradients. Furthermore, we identify the optimal input parameters for high quality compression and the main advantages of this promising technique which paves the way towards ultrafast vacuum experiments.

Criterion for the yield of micro-object ionization driven by few- and subcycle radiation pulses with nonzero electric area

The photoionization yield is analyzed for three-dimensional quantum systems with finite number of discrete spectrum states driven by unipolar subcycle and few-cycle electromagnetic pulse with a duration much less than ``the Kepler period,'' electron oscillation period in the ground state. The yield for such objects---symmetric quantum dots and those described by the zero-radius potential---is compared with the yield for hydrogen atom. In all these cases, the standard Keldysh ionization theory is inapplicable. It is shown that ionization probability is determined by the ratio of the electric pulse area (integral of the electric field strength over time) and its characteristic value inversely proportional to the size of the electron localization, and not by the pulse energy or its maximum intensity.

Temporal characterization of two-octave infrared pulses by frequency resolved optical switching

We present the temporal characterization of infrared pulses with spectra extending from 0.55 to 2.5 μm by using the frequency resolved optical switching (FROSt) technique. The pulses are obtained by broadening femtosecond pulses at 1.75 μm central wavelength in a two-stage hollow core fiber setup. This work demonstrates the capability of the FROSt technique to temporally characterize pulses with ultra-broadband spectra. Being free of phase-matching constraints, it enables the characterization of pulses with very low energy at the limit of the detection threshold and with arbitrary long pulse duration. This strength of the FROSt technique is illustrated by the characterization of supercontinua pulses whose spectra span over two octaves and with only 150 nJ energy that is spread temporally over almost 40 ps. The FROSt capabilities provide a versatile tool for the characterization of sub-cycle pulses and to study nonlinear processes such as supercontinuum generation.

Population density gratings creation and control in resonant medium by half-cycle terahertz pulses

Electromagnetically induced gratings (EIG) are created by standing-wave laser field in resonant media. Such gratings can be also created by few-cycle electromagnetic pulses counter-propagating in the medium via coherent Rabi oscillations of atomic inversion. In this case, instantaneous cross-section of the pulses in the medium is not necessary for grating formation. In this paper, we revise our recent results in study of such grating formation and their control by few-cycle pulses coherently propagating in a resonant medium. We demonstrate the grating formation and their control in three-level medium excited by three subcycle THz pulses.

Laser Wakefield Driven Generation of Isolated Carrier-Envelope-Phase Tunable Intense Subcycle Pulses

Sources of intense, ultrashort electromagnetic pulses enable applications such as attosecond pulse generation, control of electron motion in solids, and the observation of reaction dynamics at the electronic level. For such applications, both high intensity and carrier-envelope-phase (CEP) tunability are beneficial, yet hard to obtain with current methods. In this Letter, we present a new scheme for generation of isolated CEP tunable intense subcycle pulses with central frequencies that range from the midinfrared to the ultraviolet. It utilizes an intense laser pulse that drives a wake in a plasma, copropagating with a long-wavelength seed pulse. The moving electron density spike of the wake amplifies the seed and forms a subcycle pulse. Controlling the CEP of the seed pulse or the delay between driver and seed leads to CEP tunability, while frequency tunability can be achieved by adjusting the laser and plasma parameters. Our 2D and 3D particle-in-cell simulations predict laser-to-subcycle-pulse conversion efficiencies up to 1%, resulting in relativistically intense subcycle pulses.

Population difference gratings created on vibrational transitions by nonoverlapping subcycle THz pulses

We study theoretically a possibility of creation and ultrafast control (erasing, spatial frequency multiplication) of population density gratings in a multi-level resonant medium having a resonance transition frequency in the THz range. These gratings are produced by subcycle THz pulses coherently interacting with a nonlinear medium, without any need for pulses to overlap, thereby utilizing an indirect pulse interaction via an induced coherent polarization grating. High values of dipole moments of the transitions in the THz range facilitate low field strength of the needed THz excitation. Our results clearly show this possibility in multi-level resonant media. Our theoretical approach is based on an approximate analytical solution of time-dependent Schrödinger equation (TDSE) using perturbation theory. Remarkably, as we show here, quasi-unipolar subcycle pulses allow more efficient excitation of higher quantum levels, leading to gratings with a stronger modulation depth. Numerical simulations, performed for THz resonances of the H_20 molecule using Bloch equations for density matrix elements, are in agreement with analytical results in the perturbative regime. In the strong-field non-perturbative regime, the spatial shape of the gratings becomes non-harmonic. A possibility of THz radiation control using such gratings is discussed. The predicted phenomena open novel avenues in THz spectroscopy of molecules with unipolar and quasi-unipolar THz light bursts and allow for better control of ultra-short THz pulses.

Решетки населенностей, создаваемые в газе атомов водорода с помощью ультрафиолетовых аттосекундных импульсов

The possibility of population density grating in a gas of hydrogen atoms using a pair ultraviolet (UV) attosecond pulses that do not overlap in the medium is studied. Wherein the central frequency of the pulses can both coincide with the frequency of the resonant transition 1−2 from the main state in the first excited state (the main line of the Lyman series), and be detuned from it. The results of numerical calculations are in agreement with the analytical values ​​obtained on the basis of approximate solution of Schrödinger equation using perturbation theory. It is shown that under resonant excitation the greatest efficiency of the grating is achieved with an increase in the pulse duration. When nonresonant excitation, on the contrary, the system is more efficiently excited by short quasi-unipolar subcycle pulses than bipolar multicycle pulses. The results obtained can be applicable to coherent excitation of a single atom (thin layer) using a pair of UV pulses. The possibility of controlling the modulation depth of the gratings by changing the carrier envelope phase (CEP) of attosecond pulses is shown.

Simple unidirectional finite-energy pulses

We obtain a family of simple closed-form exact solutions of the three-dimensional wave equation that are free of backward components. The solutions have no singularities and possess finite energy. They describe single-cycle and subcycle pulses. Depending on two parameters, the family yields pulses with different types of localization, in particular, focused pancakes, balls, and needles.

Unidirectional Single-Cycle and Sub-Cycle Pulses

We introduce a simple exact solution of the wave equation in free space. It describes unidirectional finite-energy single-cycle and sub-cycle pulses. Depending on the ratio of its free parameters, the solution can yield, in particular, pancake, ball, and needle pulses.

Excitation of molecular rotational levels by unipolar subcycle pulses

We study theoretically the action of subcycle unipolar pulses on the rotational dynamics of polar molecules. We show that efficient excitation of molecular rotational resonances by subcycle pump pulses with respect to bipolar single-cycle one is achieved, provided that duration of excitation pulses is much smaller than the rotational period of the molecule. The excitation probability for different rotational levels is analysed in dependence on the parameters of molecules and pump pulses. Remarkably, our analysis revealed the possibility of the almost single level excitation and the formation of the population inversion in the rotational levels excited by unipolar pulses, in spite of the nonresonant interaction and ultrabroad spectrum of subcycle pulses. This approach is entirely different from the traditional resonant excitation techniques with long quasi-monochromatic pulses tuned to the molecular transition. And from the other methods, when multi-color laser pulses or single-cycle THz pulses are used for control of the molecular properties. This opens new avenues in applications related to control of molecular systems and lasing creation, since it allows efficient ultrafast control over different rotational levels.