Sub-cycle Pulses Research Articles

Selective ultrafast control of multi-level quantum systems by subcycle and unipolar pulses.

The most typical way to optically control population of atomic and molecular systems is to illuminate them with radiation, resonant to the relevant transitions. Here we consider a possibility to control populations with the subcycle and even unipolar pulses, containing less than one oscillation of electric field. Despite the spectrum of such pulses covers several levels at once, we show that it is possible to selectively excite the levels of our choice by varying the driving pulse shape, duration or time delay between consecutive pulses. The pulses which are not unipolar, but have a peak of electric field of one polarity much higher (and shorter) than of the opposite one, are also capable for such control.

On the Possibility of Holographic Recording in the Absence of Coherence between a Reference Beam and a Beam Scattered by an Object

For the holographic recording of the wave field of an object, a pattern created by the interference of the reference wave and the wave scattered by the object is used. Traditional holography requires mutual coherence of the reference and object-related beams. However, it has been shown in this work that the holographic recording of information concerning the object under study is possible without coherence between them, even by using radiation from different sources. Such recording can be carried out by employing subcycle or unipolar radiation pulses, when a resonant medium with a large phase memory time T2 is used as the recording medium. In this case, the medium records the interference pattern formed by the subcycle or unipolar pulse reflected from the object and the polarization wave created by the same short pulse. Coherence is ensured by a polarization wave, which interacting with radiation reflected from the object induces a population grating in such a medium. This grating mimics the interference pattern in a similar holographic process with a monochromatic source having the wavelength equal to that corresponding to the resonant transition in the medium.

Simulations on the single attosecond pulse generation by optimized multicolor laser field

The attosecond (as) pulses are of great significance to observe and control the ultrafast electron dynamics in atoms, molecules and condensed matter. By utilizing some sub-cycle gating methods, i.e., amplitude gating, polarization gating, and double optical gating, isolated attosecond pulses (IAP) have been generated experimentally from high-order harmonics generation (HHG) by the interaction of intense femtosecond laser fields and gas targets. At present, about 50-as IAPs are now available only in few laboratories in the world. Thus, searching for the ways to generate more short IAPs is still desirable and important for the strong field physics and attosecend science. The color gating is another potential technique generating ultrashort IAP, which could generate continuum high-order harmonics without using sub-cycle laser pulses. On the theoretical side, many schemes with two- and three-color combination field have been proposed to generate short IAP, and some works propose that in this way the IAP with width less than 10 as may be generated. However, these investigations are performed only on the single-atom level, and the real experimental conditions and strength of IAPs are not considered in their simulations. In this work, we perform a numerical experiment on the generation of strong IAP from HHG driven by multicolor driving fields. Unlike the previous studies, the macroscopic effects are included in the present simulations. Considering the detection range of spectrograph, the centering energy of IAP is confined as ~200 eV. To obtain single attosecond pulse with short width, a genetic algorithm is used to optimize waveforms of two- and three-color excitation laser pulses, which is determined by intensity, wavelength and carrier envelope phase for each component. We choose the fundamental pulse as Ti:sapphire laser field with the wavelength of 800 nm to ensure the intensity of the generated attosecond pulse, and the second or third component is not confined as the commensurate harmonics of fundamental pulse. In order to possess an acceptable computation time, the model of strong field approximation is adopted in the simulations of HHG. Our simulations show that by coherent synthesizing two or three multi-cycle pulses (the full width at half maximum of each laser field is 6 fs) we obtain sub-optical-cycle laser pulse, which can easily generate supercontinuum high-order harmonic spectra with wide bandwidth. Different from the single-color driving field, which produces the attosecond pulse train, the single attosecond pulses are generated from optimized laser fields. The width of IAP decreases with the increase of intensity of driving laser field. On the single atom level, our simulations show that about 50 as and 40 as IAP can be obtained by superimposing appropriate harmonics generated by optimized two- and three-color laser pulses, respectively. More importantly, after considering the propagation effects, about 50 as IAPs are obtained with spherical spatial filtering of harmonics in the far field, under reasonable conditions, i.e., the length and position of gas jet, gas pressure. With great advent of the ultrafast lasers technology, such as coherent waveform synthesizing, optical parametric amplification and optical parametric chirped-pulse amplification, this work gives helpful guidance for intense short IAP generation in the experiment.

Multi-octave, CEP-stable source for high-energy field synthesis.

The development of high-energy, high-power, multi-octave light transients is currently the subject of intense research driven by emerging applications in attosecond spectroscopy and coherent control. We report on a phase-stable, multi-octave source based on a Yb:YAG amplifier for light transient generation. We demonstrate the amplification of a two-octave spectrum to 25 μJ of energy in two broadband amplification channels and their temporal compression to 6 and 18 fs at 1 and 2 μm, respectively. In this scheme, due to the intrinsic temporal synchronization between the pump and seed pulses, the temporal jitter is restricted to long-term drift. We show that the intrinsic stability of the synthesizer allows subcycle detection of an electric field at 0.15 PHz. The complex electric field of the 0.15-PHz pulses and their free induction decay after interaction with water molecules are resolved by electro-optic sampling over 2 ps. The scheme is scalable in peak and average power.

Energy density and spectrum of single-cycle and sub-cycle electromagnetic pulses

Based on the exact solution of the Maxwell equations in the form of a collapsing spherical vector wave specified by an arbitrary function of time, we have calculated the maximum energy density that can be achieved when focusing extremely short pulses of various shapes. It is shown that our earlier formula expressing the maximum energy density in terms of the spectrum parameters for Gaussian quasi-monochromatic pulses is approximately (with an accuracy of ±40 %) valid for the main types of extremely short pulses.

Observation of extremely efficient terahertz generation from mid-infrared two-color laser filaments

Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here, we experimentally demonstrate that two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm allows one to generate ultrashort sub-cycle THz pulses with sub-milijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm−1. Our numerical simulations predict that the observed THz yield can be significantly upscaled by further optimizing the experimental setup. Finally, in order to demonstrate the strength of our THz source, we show that the generated THz pulses are powerful enough to induce nonlinear cross-phase modulation in electro-optic crystals. Our work paves the way toward free space extreme nonlinear THz optics using affordable table-top laser systems.

Однонаправленные одноцикловые и субцикловые импульсы

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.

On Some New Possibilities for Controlling Quantum Systems Using Unipolar Extremely Short Pulses

Based on an approximate solution of the Schrodinger equation, the effect that a pair of extremely short small-amplitude subcycle pulses has on a quantum system is studied. It is shown that, if the pulses are shorter than the inverse frequencies of atomic transitions, the probability of transitions between levels is determined by the electric area of the pulses and the time delay between them. As a consequence, a unipolar pulse can more efficiently affect the system than a bipolar one, while a sequence of pulses realizes a selective action on quantum objects, despite a nonresonant character of the interaction.

Селективное возбуждение и создание инверсной населенности в квантовых системах с помощью униполярных аттосекундных и терагерцовых импульсов

The possibility of selective population of the energy levels of quantum systems was studied using a single unipolar subcycle pulse and a pair of pulses. Selective population of quantum levels is clearly illustrated based on the numerical solution of the system of equations for the density matrix of a three-level medium interacting with a pair of subcycle attosecond and terahertz pulses. The possibility of creating an population inversion in a three-level medium is shown using a pair of such pulses. The dynamics of population density gratings in a three-level medium is studied at the impact on the system of a pair of large-amplitude Gaussian pulses. If in a weak field the shape of the gratings is harmonic, according to analytical calculations performed according to perturbation theory, then in in the case of a strong field, the spatial profile of the gratings can differ from the sinusoidal one and has complex spike structure.

Transportation of Extremely Short Radiation Pulses in a Waveguide with a Nonsimply Connected Cross Section

Specific features of the transportation of extremely short radiation pulses in a metal waveguide are analyzed. It is inferred that the mode dispersion can be suppressed, thereby preserving the shape of pulses, including subcycle and unipolar pulses, at distances limited by the absorption in the metals.

Population gratings produced in a quantum system by a pair of sub-cycle pulses

Possible generation of light-induced harmonic population gratings in a quantum system by two half-cycle pulses is studied using the approximate solution of the Schrödinger equation. The case is considered where the duration of pulses is shorter than the resonance transition periods in the system and the pulses do not overlap in the medium. The grating modulation depth is determined by the pulse electric area.

Effects of orbital angular momentum on few-cycle and sub-cycle pulse shapes: coupling between the temporal and angular momentum degrees of freedom.

We describe the effects of the coupling between the orbital angular momentum (OAM) and the temporal degrees of freedom in pulsed Laguerre-Gauss beams as a significant increase of the duration of few-cycle pulses when the OAM carried by the pulse is large, and as an enhancement to other previously known spatiotemporal couplings as radial red- and blue-shift of the mean frequency. In contrast to other detrimental, but retrievable, effects in the generation and propagation of ultrashort vortices such as spatial, group velocity, and topological charge dispersions, the OAM-temporal coupling effect is unavoidable and, therefore, has an impact on applications such as OAM-based optical communications, or on the generation of vortex-carrying attosecond pulses.

Chirped dispersive pulse propagation of arbitrary initial width: a saddle point perspective

The propagated field dynamics in chirped Gaussian pulse propagation of arbitrary initial width in a linear, causally dispersive, Lorentz-type dielectric are derived, validated, and elaborated. The performed asymptotic analysis relies on a two-term series expansion around the saddle points of the unified phase. As I show, the dynamics of each relevant saddle point are mapped into the characteristics of a respective pulse component contributing to the total propagated field. The accuracy of the description is verified upon comparison with depicted numerical results. This asymptotic approach provides unique insight that is accurate and valid from the quasi-monochromatic to the sub-cycle pulse regimes.

High-energy pulse self-compression and ultraviolet generation through soliton dynamics in hollow capillary fibres

Optical soliton dynamics can cause the extreme alteration of the temporal and spectral shape of a propagating light pulse. They occur at up to kilowatt peak powers in glass-core optical fibres and the gigawatt level in gas-filled microstructured hollow-core fibres. Here we demonstrate optical soliton dynamics in large-core hollow capillary fibres. This enables scaling of soliton effects by several orders of magnitude to the multi-mJ energy and terawatt peak power level. We experimentally demonstrate two key soliton effects. First, we observe self-compression to sub-cycle pulses and infer the creation of sub-femtosecond field waveforms - a route to high-power optical attosecond pulse generation. Second, we efficiently generate continuously tunable high-energy (1 to 16 $\mu$J) pulses in the vacuum and deep ultraviolet (110 nm to 400 nm) through resonant dispersive-wave emission.These results promise to be the foundation of a new generation of table-top light sources for ultrafast strong-field physics and advanced spectroscopy.

Electron Beam Driven Generation of Frequency-Tunable Isolated Relativistic Subcycle Pulses.

We propose a novel scheme for frequency-tunable subcycle electromagnetic pulse generation. To this end a pump electron beam is injected into an electromagnetic seed pulse as the latter is reflected by a mirror. The electron beam is shown to be able to amplify the field of the seed pulse while upshifting its central frequency and reducing its number of cycles. We demonstrate the amplification by means of 1D and 2D particle-in-cell simulations. In order to explain and optimize the process, a model based on fluid theory is proposed. We estimate that using currently available electron beams and terahertz pulse sources, our scheme is able to produce millijoule-strong midinfrared subcycle pulses.

Enhanced terahertz radiation by an intense femtosecond laser field assisted with sub-cycle pulses.

In this work, we theoretically investigate the enhanced Terahertz (THz) radiation by an intense laser pulse assisted with sub-cycle pulses (SCP) in the framework of quantum theory. By numerically solving the Schrödinger equation, the production and the dynamics of ionized electrons are analyzed. The simulations show that the SCP plays different roles for different time delays in the generation of THz radiation, such as increasing the production of the ionized electrons and manipulating their trajectories. The time-frequency analysis of the THz radiation is also carried out, which indicates that the THz radiation mainly occurs where the SCP is launched, and the THz radiation mainly comes from the formation of the asymmetric electric current. Finally, the scheme of dual sub-cycle pulses is studied, and we find that the THz radiations can constructively or destructively interfere, which leads to the formation of the streaky structures of radiation spectra.

Unipolar subcycle pulse-driven nonresonant excitation of quantum systems

The interaction of subcycle pulses with quantum systems is considered when the pulse duration becomes much smaller than the timescales of electron oscillations. We show analytically that the interaction process in this case is governed by the electric pulse area. The efficient nonresonant excitation of quantum systems by subcycle pulses with a high degree of unipolarity is demonstrated. The results are confirmed by direct numerical solution of multilevel Bloch equations.

Spectra of Ultrabroadband Squeezed Pulses and the Finite-Time Unruh-Davies Effect.

We study spectral properties of quantum radiation of ultimately short duration. In particular, we introduce a continuous multimode squeezing operator for the description of subcycle pulses of entangled photons generated by coherent-field driving in a thin nonlinear crystal with second-order susceptibility. We find the ultrabroadband spectra of the emitted quantum radiation perturbatively in the strength of the driving field. They can be related to the spectra expected in an Unruh-Davies experiment with a finite time of acceleration. In the time domain, we describe the corresponding behavior of the normally ordered electric field variance.

High harmonic generation in solids driven by sub-cycle mid-infrared pulses from laser filamentation

Carrier-envelope phase (CEP) controlled subcycle mid-infrared pulses from two-color laser filamentation have been applied for high harmonic (HH) generation in a crystalline silicon membrane. The HH spectrum reaches the ultraviolet region (<300 nm), beyond the direct band gap of the silicon. The shape of the HH spectrum shows the strong dependency on the CEP of the input pulse. The complete waveform characterization of the sub-cycle driver pulse with frequency-resolved optical gating capable of CEP determination is the effective method for the studies of the sub-cycle dynamics.

Транспортировка предельно коротких импульсов излучения в волноводах с неодносвязным поперечным сечением

The analysis of the features of transportation of few cycle radiation pulses in metal waveguides is carried out. It is concluded that mode dispersion can be suppressed and thereby the shape of pulses, including subcycle and unipolar pulses is preserved, at distances limited by absorption in metals.