THz Laser Pulses Research Articles

Quantum theory of the spin dynamics excited by ultrashort THz laser pulses in rare earth antiferromagnets. DyFeO3

A quantum theory of spin dynamics in the rare-earth orthoferrites excited by terahertz laser pulses is developed. The study demonstrates that dynamic magnetic configurations, triggered by a light pulse, exhibit stability even after the excitation source is ceased. The magnitude of post-excitation oscillations is linked to the ratio between the frequency of rare-earth ion excitations and the frequency of the external source. According to the analysis presented, dynamic response is significantly amplified when the system is exposed to ultrashort terahertz pulses. The physical characteristics of the oscillations emerging after the pulse are determined, and the factors governing their amplitude and phase are identified. The response signal is found to be dependent on the initial part of the pulse, specifically the half-period of the ultrashort light wave, while the subsequent part of the pulse contributes minimally to post-pulse magnetization dynamics. The findings highlight that in DyFeO3, terahertz dynamics primarily result from the influence of the magnetic field of the light, leading to excitations of electrons from the ground state to low-lying electronic levels of Dy3+ions. Additionally, the dynamic magnetoelectric effect excited by the electric field of the pulse is explored, revealing the emergence of odd magnetic modes.

Proton tunneling in the dissociation of H2+ and its asymmetric isotopologues driven by circularly polarized THz laser pulses.

Light-induced deprotonation of molecules is an important process in photochemical reactions. Here, we theoretically investigate the tunneling deprotonation of H2+ and its asymmetric isotopologues driven by circularly polarized THz laser pulses. The quasi-static picture shows that the field-dressed potential barrier is significantly lowered for the deprotonation channel when the mass asymmetry of the diatomic molecule increases. Our numerical simulations demonstrate that when the mass symmetry breaks, the tunneling deprotonation is significantly enhanced and the proton tunneling becomes the dominant dissociation channel in the THz driving fields. In addition, the simulated nuclear momentum distributions show that the emission of the proton is directed by the effective vector potential for the deprotonation channel and, meanwhile, the angular distribution of the emitting proton is affected by the alignment and rotation of the molecule induced by the rotating field.

Dynamics of Correlated Double-Ionization of Two-Electron Quantum Dots in Laser Fields.

Using an ab initio, time-dependent calculational method, we study the non-linear dynamics of a two-electron quantum dot in the presence of ultrashort Thz laser pulses. The analysis of the contribution of the various partial waves to two-electron joint radial and energy distribution patterns revealed strongly correlated electron ejection channels. In the double-ionization process, regardless of the photon energy, the two-electron wave packets are born and remain concentrated until the pulse's peak; at later times, and depending on the photon energy of the field, distinctly different patterns emerge. Our calculations also showed the gradual transition of the radial and energy patterns from a single-peak to a doubly peaked structure, associated with the direct and the sequential double-ionization mechanisms, respectively.

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.

Achieving high molecular alignment and orientation for CH_3F through manipulation of rotational states with varying optical and THz laser pulse parameters

Increasing interest in the fields of high-harmonics generation, laser-induced chemical reactions, and molecular imaging of gaseous targets demands high molecular “alignment” and “orientation” (A&O). In this work, we examine the critical role of different pulse parameters on the field-free A&O dynamics of the CH_3F molecule, and identify experimentally feasible optical and THz range laser parameters that ensure maximal A&O for such molecules. Herein, apart from rotational temperature, we investigate effects of varying pulse parameters such as, pulse duration, intensity, frequency, and carrier envelop phase (CEP). By analyzing the interplay between laser pulse parameters and the resulting rotational population distribution, the origin of specific A&O dynamics was addressed. We could identify two qualitatively different A&O behaviors and revealed their connection with the pulse parameters and the population of excited rotational states. We report here the highest alignment of langle {mathrm{cos}^2theta }rangle = 0.843 and orientation of langle {mathrm{cos}(theta )}rangle = 0.886 for CH_3F molecule at 2 K using a single pulse. Our study should be useful to understand different aspects of laser-induced unidirectional rotation in heteronuclear molecules, and in understanding routes to tune/enhance A&O in laboratory conditions for advanced applications.

Anderson pseudospin and Superradiant Superconductivity revisited

Using Anderson pseudospins (Anderson, 1958; Bardeen et al., 1957), I suggested in 2012 (Baskaran, 2012) superradiant superconductivity (SRSC) and related Dicke phenomena in bulk superconductors. It was a response to a report from Cavalleri’s lab (Kaiser et al., 2012; Fausti et al., 2011), of transient room temperature superconductivity in YBCO, when pumped by 20 THz laser pulses. In my minimal model, Anderson pseudospins in k-space in a bulk superconductor were coupled to single polariton mode. The model was adapted to the pseudogap normal state of YBCO to explain observed transient superconductivity. We also pointed out, possibility of other Dicke phenomena, (i) spontaneous superradiance above a critical polariton matter coupling, with no pumping, (ii) enhanced quantum entanglement near the critical coupling and (iii) superfluoresence. Here we revisit our early work. Recent discovery by Ebbesens group (Thomas et al., 2019) of large increase of superconducting Tc from 30 K to 45 K in K3C60 via specially designed plasmon–phonon-polariton mode, but with no external pumping seems to be in line with our early prediction. Relevance of SRSC to developments, including other superradiant matters and Higgs Spectroscopy is pointed out.

Shaping of the time evolution of field-free molecular orientation by THz laser pulses

We present a theoretical study of the shaping of the time-evolution of field-free orientation of linear molecules. We show the extend to which the degree of orientation can be steered along a desired periodic time-dependent signal. The objective of this study is not to optimize molecular orientation but to propose a general procedure to precisely control rotational dynamics through the first moment of the molecular axis distribution. Rectangular and triangular signals are taken as illustrative examples. At zero temperature, we compute the quantum states leading to such field-free dynamics. A TeraHertz laser pulse is designed to reach these states by using optimal control techniques. The investigation is extended to the case of non-zero temperature. Due to the complexity of the dynamics, the control protocol is derived with a Monte-Carlo simulated annealing algorithm. A figure of merit based on the Fourier coefficients of the degree of orientation is used. We study the robustness of the control process against temperature effects and amplitude variations of the electric field.

The photoionization dynamics based on molecular pre-orientation.

We study theoretically the photoionization dynamics of pre-oriented NaK molecule. Firstly, a THz laser pulse is utilized to orient the ground state molecule. And then the pump and probe laser pulses are used to excite and ionize the molecule, respectively. We study the influence of molecular orientation duration and degree on the ionization probability, angle-resolved photoelectron spectrum and photoelectron angular distribution (PAD). It is shown that we could obtain more stable ionization signal and PAD when the molecules are ionized in molecular orientation duration. We could increase the ionization probability and obtain more concentrated ionization signal and photoelectron distribution by increasing the orientation degree in the ground state. Moreover, we discuss the splitting pattern in the angle-resolved photoelectron spectrum.

Basic mechanisms in the laser control of non-Markovian dynamics

Referring to a Fano-type model qualitative analogy we develop a comprehensive basic mechanism for the laser control of the non-Markovian bath response in strongly coupled Open Quantum Systems (OQS). A converged Hierarchy Equations Of Motion (HEOM) is worked out to numerically solve the master equation of a spin-boson Hamiltonian to reach the reduced electronic density matrix of a heterojunction in the presence of strong THz laser pulses. Robust and efficient control is achieved increasing by a factor ?2 non-Markovianity measured by the time evolution of the volume of accessible states. The consequences of such fields on the central system populations and coherence are examined, putting the emphasis on the relation between the increase of non- Markovianity and the slowing down of decoherence processes.

Influence factor analysis of field-free molecular orientation**Project supported by the National Natural Science Foundation of China (Grant No. 11674198), the Taishan Scholar Project of Shandong Province, China, and the Natural Science Foundation of Shandong Province, China (Grant No. ZR2014AM002).

The effects of the characteristics of molecules and external fields on field-free molecular orientation are investigated through the comparison of HBr with LiH driven by the combination of a two-color laser pulse and a time-delayed THz laser pulse. It is shown that the dipole interaction has greater influence on field-free orientation than the hyperpolarizability interaction. In addition to the temperature dependence of orientation degree, the effects of the amplitudes of the two-color laser pulse and THz laser pulse, rising time, and THz laser frequency on molecular orientation are also discussed.

Review of Ultrafast Demagnetization After Femtosecond Laser Pulses: A Complex Interaction of Light with Quantum Matter

When a magnetic film is excited by a femtosecond laser pulse, either with THz or with optical frequencies, then there is at least a partial demagnetization within a few hundred femtoseconds, followed by a remagnetization to the original state on a bit longer time scale. This phenomenon is caused by a complex interaction of light with quantum matter. This paper gives a review of the present knowledge of the underlying physics. It discusses first the situation of a direct change of the magnetization by its interaction with the electromagnetic wave of the laser pulse, which appears during THz laser pulses with small field amplitudes. Then it considers the situation of an indirect change which appears after THz laser pulses with large field amplitudes and after optical laser pulses. In these cases the laser photons primarily excite electrons, with subsequent modifications of their spin-angular momenta by spin-flip scatterings of these electrons at quasiparticles, either at other electrons or at phonons or at magnons. The contributions of these various spin-flip scatterings to demagnetization are investigated. Then the transfer of angular momentum from the electronic spin system to the lattice during ultrafast demagnetization is discussed by describing the lattice vibrations in terms of magnetoelastic spin-phonon modes. Finally, the effect of electronic correlations in the sense of the density-matrix theory is investigated.

Coherent superpositions of three states for phosphorous donors in silicon prepared using THz radiation

Superposition of orbital eigenstates is crucial to quantum technology utilizing atoms, such as atomic clocks and quantum computers, and control over the interaction between atoms and their neighbours is an essential ingredient for both gating and readout. The simplest coherent wavefunction control uses a two-eigenstate admixture, but more control over the spatial distribution of the wavefunction can be obtained by increasing the number of states in the wavepacket. Here we demonstrate THz laser pulse control of Si:P orbitals using multiple orbital state admixtures, observing beat patterns produced by Zeeman splitting. The beats are an observable signature of the ability to control the path of the electron, which implies we can now control the strength and duration of the interaction of the atom with different neighbours. This could simplify surface code networks which require spatially controlled interaction between atoms, and we propose an architecture that might take advantage of this.

Field-free orientation dynamics of OCS molecule induced by utilizing two THz laser pulses

Field-free orientation of OCS molecule is studied theoretically by utilizing two THz laser pulses. The effects of the center frequency of THz laser pulses and the delay time between two THz laser pulses on molecular orientation are discussed. The molecular positive and negative orientation degrees can be changed to some extent by adjusting both the carrier envelope phases of two THz laser pulses. The maximal positive and negative orientation degrees obtained are and , respectively. Furthermore, the influence of the initial rotational temperature on molecular orientation is also investigated.

Extreme nonlinear terahertz electro-optics in diamond for ultrafast pulse switching

Polarization switching of picosecond laser pulses is a fundamental concept in signal processing [C. Chen and G. Liu, Annu. Rev. Mater. Sci. 16, 203 (1986); V. R. Almeida et al., Nature 431, 1081 (2004); and A. A. P. Pohl et al., Photonics Sens. 3, 1 (2013)]. Conventional switching devices rely on the electro-optical Pockels effect and work at radio frequencies. The ensuing gating time of several nanoseconds is a bottleneck for faster switches which is set by the performance of state-of-the-art high-voltage electronics. Here we show that by substituting the electric field of several kV/cm provided by modern electronics by the MV/cm field of a single-cycle THz laser pulse, the electro-optical gating process can be driven orders of magnitude faster, at THz frequencies. In this context, we introduce diamond as an exceptional electro-optical material and demonstrate a pulse gating time as fast as 100 fs using sub-cycle THz-induced Kerr nonlinearity. We show that THz-induced switching in the insulator diamond is fully governed by the THz pulse shape. The presented THz-based electro-optical approach overcomes the bandwidth and switching speed limits of conventional MHz/GHz electronics and establishes the ultrafast electro-optical gating technology for the first time in the THz frequency range. We finally show that the presented THz polarization gating technique is applicable for advanced beam diagnostics. As a first example, we demonstrate tomographic reconstruction of a THz pulse in three dimensions.

Field-Free Molecular Orientation Induced by a Single-Cycle THz Laser Pulse Train

AbstractWe investigate theoretically the field-free orientation of CO molecules induced by a single-cycle THz laser pulse train. It is shown that the molecular orientation can be obviously enhanced by applying the pulse train. The laser intensity and pulse number have some effects on the molecular orientation. The high degree of field-free molecular orientation |⟨cosθ⟩|max=0.9246 is obtained at temperature T=0 K. The variations of the maximum orientation degree with the experimentally available pulse number and peak intensity are given. Temperature T has a considerable influence on the field-free molecular orientation. The maximal field-free molecular orientation at T=0, 10, 20 and 30 K for N=14 and E0=1.8 MV/cm are |⟨cosθ⟩|max=0.9188, 0.7338, 0.6055 and 0.5154 in order, and the corresponding effective duration times of molecular orientation are Δt=0.759, 0.432, 0.297 and 0.117 ps.

Nonlinear response of graphene to a few-cycle terahertz laser pulse: Role of doping and disorder

The nonlinear response of graphene to a THz laser pulse is studied by solving the time-dependent Dirac equation and the time-dependent Schr\odinger equation within a tight-binding approximation applied to finite-sized structures. We compare predictions of these two approximations for the harmonic spectrum with the recent experiment by P. Bowlan et al. [Phys. Rev. B 89, 041408(R) (2014)]. We highlight the influence of short-range and long-range disorder which can be accounted for within the tight-binding description on a microscopic level. We find good agreement with the experiment. Most notably, the intensity of the second harmonic offers a quantitative indicator for the amount of short-range disorder.

Field-free orientation dynamics of CO molecule by combining two-color shaped laser pulse with THz laser pulse train

Field-free orientation of CO molecule is studied theoretically by combining a nonresonant two-color shaped laser pulse with a time-delayed THz laser pulse train. The molecular positive and negative orientation degrees can be significantly enhanced by the combination of two-color shaped laser pulse and THz laser pulse train compared with the single THz laser pulse train. The effects of the pulse number of THz laser pulse train and the delay time between two-color shaped laser pulse and THz laser pulse train on molecular orientation are discussed. Further, the influence of the initial rotational temperature on molecular orientation is investigated. The maximal positive and negative orientation degrees obtained are 〈cos θ〉pmax=0.91 and 〈cos θ〉nmax=0.89, respectively.

Field-free molecular orientation by two-color femtosecond laser pulse and time-delayed THz laser pulse

We propose a scheme for achieving molecular orientation steered by the combination of a nonresonant two-color femtosecond laser pulse and a time-delayed THz laser pulse. It is shown that the molecular negative and positive orientation can be significantly enhanced by applying this scheme compared to the single femtosecond or THz laser pulse. The effects of the intensity of the two-color femtosecond laser pulse and the delay time between the femtosecond laser pulse and the THz laser pulse on molecular orientation are discussed. The molecular orientation also depends on the carrier-envelope phase of the THz laser pulse and the initial rotational temperature of the molecule. The maximal negative and positive orientation degrees calculated are and , respectively.

Control of molecular orientation with combined near-single-cycle THz and optimally designed non-resonant laser pulses: Carrier-envelope phase effects

Molecular orientation with combinations of near-single-cycle THz and non-resonant laser pulses is studied by means of optimal control simulation with CO molecules at 5K. We optimize non-resonant laser pulses with optimally determined final times while assuming fixed THz pulses and focusing on the carrier-envelope phase (CEP) effects. Although almost the same high degrees of orientation are achieved by the combined control independent of CEPs, the structures of the optimal non-resonant laser pulses considerably depend on CEPs and the central frequencies of the THz pulses. From a practical viewpoint, simplified treatments are also discussed.

Tunneling dissociation ofH2+and its isotopes in THz laser pulses

Tunneling dissociation of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow><mml:mo>+</mml:mo></mml:msup></mml:math>and its isotopes in THz laser pulses