Half-cycle Pulses Research Articles

Dephasingless two-color terahertz generation

A laser pulse composed of a fundamental and an appropriately phased second harmonic can drive a time-dependent current of photoionized electrons that generates broadband THz radiation. Over the propagation distances relevant to many experiments, dispersion causes the relative phase between the harmonics to evolve. This “dephasing” slows the accumulation of THz energy and results in a multi-cycle THz pulse with significant angular dispersion. Here, we introduce a novel optical configuration that compensates the relative phase evolution, allowing for the formation of a half-cycle THz pulse with almost no angular dispersion. The configuration uses the spherical aberration of an axilens to map a prescribed radial phase variation in the near field to a desired longitudinal phase variation in the far field. Simulations that combine this configuration with an ultrashort flying focus demonstrate the formation of a half-cycle THz pulse with a controlled emission angle and 1/4 the angular divergence of the multi-cycle pulse created by a conventional optical configuration.

Analytical and numerical study of light-induced optical microcavity generation by half-cycle light pulses in the resonant medium

An optical Bragg-like microcavity can be formed when extremely short light pulses interacting coherently with the medium collide in the resonant medium. Here we present an analytical approach, based on the approximate solution of the time-dependent Schrödinger equation, that reveals both the cavity formation and its control by the half-cycle pulses colliding in the medium. This approach allows the calculation of cavity properties in the weak-field regime without the need for complex numerical simulations. Beyond the approximate analytical solutions, our analytical results are confirmed by numerical simulations of the Maxwell-Bloch equations. Grating formation is also studied in the strong-field regime, where collisions of self-induced transparency half-cycle pulses occur. The results presented here reveal, to our knowledge, a new type of spatio-temporal photonic crystal in the form of micro-cavities in a simple atomic medium. The results obtained demonstrate the feasibility of attosecond optical switching in a simple atomic medium with half-cycle pulses.

Generation and control of population difference gratings in a three-level hydrogen atomic medium using half-cycle attosecond pulses nonoverlapping in the medium

Generation and control of population difference gratings in a three-level hydrogen atomic medium using half-cycle attosecond pulses nonoverlapping in the medium

Optical microcavity formation and ultrafast control using half-cycle attosecond pulses in two- and three-level media

Light induced population difference gratings have numerous applications in modern optics. The conventional approach of creating them by using monochromatic laser radiation makes it virtually impossible to rapidly control their properties. In recent years, there has been active discussion about using ultra-short light pulses to control such gratings. We show the possibility of ultrafast control of population difference structures, such as dynamic microresonators and Bragg-like grating formations, by the collision of extremely short light pulses in a resonant medium. This work focuses on changes in microcavity properties based on the different parameters, such as the atomic density of the medium, the form of the pulses, and whether the medium is two- or three-level.

Electromagnetically induced gratings created by extremely short non-overlapping pulses of light in a three-level resonant medium

In a fixed spectral range, single- and half-cycle electromagnetic pulses have the shortest duration. Half-cycle pulses are promising tools for ultrafast control of quantum systems. Previously, the possibility of using a sequence of single- and half-cycle attosecond pulses to generate and ultrafast control light-induced population difference gratings has been demonstrated. However, such studies have been carried out using different approximations, such as the sudden perturbation theory and the two-level model for the resonant medium. In this paper, based on the numerical solution of constitutive equations for elements of the density matrix and wave equation it is shown that it is possible to generate and control population gratings in a three-level medium without using the approximation of sudden perturbations used in previous studies. It is shown that taking into account the additional level of the medium does not lead to a violation of the effect of generating such gratings. This extends the applicability of previous results.

Excitation and control of level populations in rectangular quantum wells by unipolar half-cycle attosecond pulses

Unipolar and quasi-unipolar half-cycle pulses represent the limit of the pulse shortening in a given spectral range. So far such pulses have become a subject of active research due to their potential in the ultrafast optics and study of new regimes of light–matter interactions with subcycle resolution. Here, we show the possibility of the effective ultrafast control of the level populations in 1D rectangular quantum wells (such a model is used for modeling semiconductor nanostructures and nanoparticles) by the half-cycle unipolar attosecond light pulses in comparison to the single-cycle ones. It is shown that the population dynamics is determined by the electric pulse area divided into its characteristic “scale,” which is given by the quantum well width when pulse duration is smaller than the orbital period of electrons in the ground state. Both the selective excitation of quantum states and the feasibility of the population inversion by subcycle unipolar pulses are demonstrated.

Coherent control of a multilevel resonant medium by subcycle pulses

We theoretically demonstrate the possibility of coherent control of the response of a multilevel resonant medium by means of half-cycle unipolar or quasi-unipolar attosecond pulses. We show that despite the ultrabroad spectrum of half-cycle pulses the induced medium polarization can be efficiently steered through the proper choice of the parameters for the excitation pulses. Besides, we found that the response of a multilevel medium for the amplitude of driving subcycle pulses below a certain threshold can be quantitatively well approximated by the two-level model.

Generation of an ultrahigh-repetition-rate optical half-cycle pulse train in the nested quantum wells.

We propose a simple quantum system, namely, a nested quantum-well structure, which is able to generate a train of half-cycle pulses of a few-femtosecond duration when driven by a static electric field. We theoretically investigate the emission of such a structure and its dependence on the parameters of the quantum wells. It is shown that the production of a regular output pulse train with tunable properties and the pulse repetition frequencies of tens of terahertz is possible in certain parameter ranges. We expect the suggested structure can be used as an ultra-compact source of subcycle pulses in the optical range.

Analytical solutions for the sliding displacement of model slopes simulating both the frictional and rotational effects under piecewise linear acceleration pulses

AbstractSimplified methods predicting earthquake‐induced displacement of slopes are based on the Newmark sliding‐block model. This model does not consider the decrease in inclination of the sliding mass as a result of its downward motion and this effect is usually modeled by improving the above model by assuming that the critical acceleration value for relative motion of the block increases linearly with the distance moved. This work, for the first time, derives analytical solutions predicting the sliding displacement under idealized acceleration pulses of the model described above simulating both the frictional and rotational effects. First, a general solution is derived in dimensionless form for the motion of the sliding mass under a piecewise linearly varying pulse for the cases of both static stability and instability. The solution is obtained in each interval by (a) solving for the displacement and velocity in terms of these quantities at the end of the previous interval, (b) estimating the time corresponding to a sliding velocity equal to zero, and (c) investigating if the estimated time solution is consistent with the time interval and thus if it can be considered as the global solution. Then, based on this solution and the conditions where downward and upward motion starts, a procedure predicting the final displacement of the sliding mass for a piecewise linearly varying pulse is proposed. Finally, analytical solutions are derived, given in graphical form, partly validated, and discussed for the particular cases of half and full cycle pulses of rectangular, triangular, and trapezoidal shapes.

On the peak acceleration of a beam in response to a shock and its validity for cylindrical pipes

This paper presents a closed-form solution for the peak acceleration in an infinite beam as a response to a half-cycle sine pulse excitation. The limits of its validity for a finite length beam are discussed. Numerical and experimental results are presented for a drilling collar where the excitation of shell modes may lead to underestimating the maximum acceleration if beam theory is used. A condition is suggested to decide if beam theory is appropriate for thick cylinders and the contribution from shell modes can be neglected.

Half-cycle electromagnetic pulses and pulse electric area

Recent progress in the generation of attosecond optical pulses raises the question of the limits of pulse duration reduction. Such pulses are needed to penetrate and control the world of intra-atomic and potentially intra-nuclear processes, radically increase the productivity of information transmission and processing and for other applications. The pulse duration is inversely proportional to the width of the pulse spectrum. When the possibilities for the width expansion are exhausted, the duration can be reduced by reducing the number of field cycles in the pulse. Extremely short pulses are half-cycle and unipolar. They have a more effective, unidirectional effect on electrical charges of the same sign. Experiments show the half-cycle pulses in the terahertz region of the spectrum; ways to obtain them in the optical region are discussed. The unusual properties of half-cycle pulses require the use of strict Maxwell equations.

Giant isolated half-cycle attosecond pulses generated in coherent bremsstrahlung emission regime.

Giant half-cycle attosecond pulse generation in the coherent bremsstrahlung emission regime is proposed for laser pulses with normal incidence on a double-foil target, where the first foil is transparent and the second foil is opaque. The presence of the second opaque target contributes to the formation of a relativistic flying electron sheet (RFES) from the first foil target. After the RFES has passed through the second opaque target, it is decelerated sharply, and bremsstrahlung emission occurs, which results in the generation of an isolated half-cycle attosecond pulse having an intensity of ∼1.4×10^{22}W/cm^{2} and a duration of 3.6as. The generation mechanism does not require extra filters and may open a regime of nonlinear attosecond science.

Isolated attosecond pulse generation from the interaction of intense laser pulse with solid density plasma

This article gives an overview on recent progress in the generation of isolated attosecond pulse and isolated half-cycle attosecond pulse. As an isolated attosecond pulse is preferred in the pump-probe experiments for the dynamics of electrons in atom, molecule, or solid, we focus on the isolated attosecond pulses generation from the intense laser pulses interaction with solid density plasma, which have higher intensity and narrower pulse width than that generated in the interaction of laser pulse with gas target. We have firstly discussed the physical mechanism of isolated attosecond pulse generation, such as polarization gating, two-color laser pulses, attosecond light houses, and capacitor target mechanism. In the polarization gating mechanism, we have discussed the physical mechanism that the higher-order harmonic efficiency decreases with the increase of ellipticity. Both the coherent synchrotron radiation mechanism and the relativistic oscillation mechanism can control the intensity of high-order harmonic generation by controlling ellipticity of the incident laser pulse. We also discussed other mechanism to enhance the isolated attosecond pulse bursts in detail. Secondly, we focus on the isolated half-cycle attosecond pulses, which can also be generated from the intense laser pulses interaction with solid density plasma by double foil target mechanism, gas-foil target mechanism, cascaded generation mechanism, microstructured target mechanism, and three-color laser pulse mechanism. The half-cycle attosecond pulses can be useful for probing ultrafast electron dynamics in matter via asymmetric manipulation. Accordingly we discussed the physcial mechanism, experimental feasibility, calibration measurement, and application prospect of half-cycle attosecond pulse in this article. The above mechanism can directly generate ultra-intense isolated attosecond pulses in the transmission direction without requiring extra filters and gating techniques. The dense electron sheet is crucial for the generation of intense attosecond pulses in different mechanisms, such as coherent wake emission (CWE), relativistic oscillating mirror (ROM) and coherent synchrotron emission (CSE). In this article, all the mechanism for half-cycle attosecond pulses generation can ensure only one electron sheet contributing to the transmitted radiation. We discuss the theoretical model of nanobunching of the electron sheet, which shows that the relativistic oscillation is crucial for the formation of electron sheet.

Comparison of the Effects of Unipolar Half-Cycle and Resonant Multi-Cycle Electromagnetic Pulses on quantum systems

Comparison of the Effects of Unipolar Half-Cycle and Resonant Multi-Cycle Electromagnetic Pulses on quantum systems

A New Proxy for Near-Fault Acceleration Pulses and Implications on Inelastic Displacement Ratio

ABSTRACT This study develops new equations for predicting the constant-strength inelastic displacement ratio (C R ) by considering nonlinear single-degree-of-freedom systems subjected to near-fault ground motions. To this aim, an energy-based parameter, the MEA/MEV ratio, is firstly considered for characterizing the properties of pulse-like records. “MEA” and “MEV” represent the Maximum Energy of the half-cycle pulse in Acceleration and Velocity time histories, respectively. The effects of the MEA/MEV ratio on the C R spectra are investigated in terms of elastic-perfectly plastic hysteretic behavior. Finally, a simplified equation for estimating the C R spectra is developed, and its accuracy is evaluated by calculating the error measures.

Revival structure of rotational wave packets in the process of the field-free molecular orientation

We investigate theoretically the relationship between the revival structure of rotational wave packets and the orientation of the LiH molecule induced by the half-cycle THz laser pulse (HCP). The revival structure of oriented rotational wave packets can be further controlled through the choice of the delay time and the phase difference between two HCPs. We found the higher absolute values of the orientation corresponding to the higher asymmetries and more centralization of the time-dependent probability density ψθ,t2. We also found a similar population of the adjacent rotational states gives a higher ceiling for the absolute value of molecular orientation. Finally, the effect of the system temperature is discussed, the higher temperature makes a lower degree of orientation steered by two HCPs.

Effect of molecular orientation on predissociation of NaI molecules

Based on the time-depend quantum wave packet method, a scheme is proposed to investigate the effect of the molecular orientation on the predissociation of NaI molecules. The molecular orientation in the ground state is steered by THz half-cycle pulse. Then the oriented molecule is excited by femtosecond pump pulse. Molecular orientation has a significant effect on the angular distribution of wave packets. The excited wave packet is distributed in a more narrow region with the increase of molecular orientation degree. The angular distribution of dissociated products is essentially consistent with the angular distribution of excited wave packet. The dissociation probability is proportional to the excitation probability of pump pulse, which is related to the molecular orientations. Compared with the result of predissociation without molecular orientation, the maximal dissociation probability can increase by 6.5% in our work. The electric field intensity of the pump pulse also has a significant influence on dissociation probability.

Powerful laser-produced quasi-half-cycle THz pulses.

The Maxwell equations-based 3D-analytical solution for the terahertz (THz) half-cycle electromagnetic wave transition radiation pulse has been found. This solution describes generation and propagation of transition radiation into free space from laser-produced relativistic electron bunch which crosses a target-vacuum interface as a result of ultrashort laser pulse interaction with a thin high-conductivity target. The analytical solution found complements the theory of laser initiated transition radiation. It describes the THz wave half-cycle pulse at the arbitrary distance from a target surface including near-field zone rather than its standard far-field characterization. The analytical research has also been supplemented with the 3D simulations using the finite-difference time-domain method, which makes it possible for description of much wider spatial domain as compared to that from the particle-in-cell approach. The presented result sheds light fundamentally on the interference of the electron bunch field and the generated THz field of broadband transition radiation from laser-plasma interaction. The latter is studied for a long time in the experiments with solid density plasma and the theory developed may inspire to targeted measurements and investigations of unique super intense half-cycle THz radiation waves near the laser target.

Ultrahigh-Amplitude Isolated Attosecond Pulses Generated in the Transmission Regime from Ultrathin Foil

A coherent-synchrotron-emission regime of forward high-order harmonics generation (HHG) is proposed for the emission of an isolated unipolar half-cycle attosecond pulse by a three-color laser pulse impinging on ultrathin foil. A theoretical model is proposed for the electron nanobunching mechanism and the forward radiation, which is consistent with the numerical-simulation results. As the forward HHG does not need to penetrate from the front to the rear side of the target, the spectrum of the forward HHG has no low-frequency cutoff. The robustness of this regime is verified with different laser and foil plasma conditions as well as the two-dimensional effects. The robustness is also checked with similarity theory, which confirms that isolated attosecond pulses can be efficiently generated when the plasma density and the laser amplitude change simultaneously such that their ratio remains unchanged.

Glassy dynamics of the one-dimensional Mott insulator excited by a strong terahertz pulse

The elucidation of nonequilibrium states in strongly correlated systems holds the key to emergence of novel quantum phases. The nonequilibrium-induced insulator-to-metal transition is particularly interesting since it reflects the fundamental nature of competition between itinerancy and localization of the charge degrees of freedom. We investigate pulse-excited insulator-to-metal transition of the half-filled one-dimensional extended Hubbard model. Calculating the time-dependent optical conductivity with the time-dependent density-matrix renormalization group, we find that strong mono- and half-cycle pulses inducing quantum tunneling strongly suppress spectral weights contributing to the Drude weight σD, even if we introduce a large number of carriers Δnd. This is in contrast to a metallic behavior of σD∝Δnd induced by photon absorption and chemical doping. The strong suppression of σD in quantum tunneling is a result of the emergence of the Hilbert-space fragmentation, which makes pulse-excited states glassy.Received 9 February 2022Revised 15 May 2022Accepted 18 July 2022DOI:https://doi.org/10.1103/PhysRevResearch.4.L032019Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLocalizationNonequilibrium statistical mechanicsOptical conductivityPhotoinduced effectQuantum statistical mechanicsPhysical Systems1-dimensional systemsGlassy systemsMott insulatorsCondensed Matter, Materials & Applied PhysicsStatistical PhysicsQuantum Information