Laser-atom Interaction Research Articles

Measurement of photoionization yield in low-density barium photoplasma study

A finite-sized barium (Ba) photoplasma is generated through a two-step resonant photoionization method by shining laser pulses onto an atomic beam of Ba. The photoionization yield is estimated from the ratio of photoion density to neutral atom density measured in the laser–atom interaction region. The neutral atom density in the atomic beam is measured by an optical absorption technique where a Ba hollow cathode lamp (Ba-HCL) is used as an emission source. Since the photoplasma has a finite volume and it is pulsed in nature, the photoion density in the photoplasma is estimated through the collection of charge from the photoplasma. The measured ionization yield is compared with the calculated value that is obtained from an in-house developed code. It is observed that the values are in good agreement.

Synthetic Spin-Orbit Coupling in Two-Level Cold Atoms

We theoretically and computationally show the simplest realization of SOC using two-level cold atoms interacting with only one laser beam. The underlying mechanism is based on the non-adiabatic nature of laser-atom interaction, with the Rabi frequency being not much larger than the kinetic energy of the atom. We use Zitterbewegung oscillation to further illustrate the effects of the synthesized SOC on the quantum dynamics of the two-level cold atoms. We expect our proposal to be of experimental interest in the quantum simulation of SOC-related physics.

Ramsey effect of coherent population trapping in anti-relaxation-coated Rb vapor cells

We examine experimentally and theoretically the Ramsey effects on the coherent population trapping (CPT) resonance in a paraffin-coated Rb vapor cell. The observed CPT spectrum in the Hanle configuration of the 5S1/2(F = 2)−5P1/2(F′ = 1) transition of 87Rb atoms consists of a narrow structure with a spectral width of 0.10 mG due to the wall-induced Ramsey effect and a broad structure with a spectral width of 23 mG due to the transit effect. Considering the velocity distribution of atoms and the number of wall-atom collisions, we numerically calculated the transmittance of the laser beam as a function of the longitudinal magnetic field. The experimental results of the CPT Ramsey interference were in good agreement with the theoretical results, and the dependences of the CPT Ramsey effects on the atom-laser interaction time were investigated in detail.

Half-cycle-like field-enhanced harmonic radiation by femtosecond laser-atom interaction

A manipulatable half-cycle-like field (HCLF), homochromy with the driving femtosecond laser, is applied to enhance the harmonic emission by optimizing their relative delay time. We find that, by numerical computation, the HCLF can deliver a substantial momentum to the accelerated electrons as they return to their parent ions by a suitable phase delay, and also significantly increase the atomic ionization rate supplementarily. The results show that the harmonic order of the maximum or cut-off photon energy emitted in the presence of a half-cycle manipulating light pulse is risen to the 177th order, which is a significant increase compared with the 53rd order harmonics in the case of a single driving light pulse. To understand the essence from the wave-packet kinetics of the return electrons and their relation to the harmonic emission, we utilize the phase-space analysis of the coordinate momentum of an electronic wave packet by using a Gabor transformation, by which the presented numerical conclusions are further confirmed.

Generating Spin Squeezed State of Atom-Photon with Stimulated Raman Transition

We consider how to generate spin squeezed state of atom-photon with stimulated Raman transition. We first derive the effective Hamiltonian and then obtain the analytical solution for state vector, the optimally squeezed angle and then the squeezing parameter. It is shown that the spin squeezed state can be generated by appropriately adjusting the atom-laser dipole interaction and two-photon detuning.

Solving for the time-dependent multi-configuration hartree fock equations based on sine-discrete variable representation

A parallel quantum electrons wave packet computer code has been developed to study laser-atom interaction in the nonperturbative regime with attosecond resolution. The motion equations of the multi-configuration time-dependent hartree fock (MCTDHF) based on a sine discrete variable representation were solved by using an adaptive stepsize Runge-Kutta integrator of eight orders. Some efficient algorithms and strategies to accelerate the calculation velocity are introduced and discussed in details. Some illustrated imaginary time propagation and real time propagation have been respectively done in the paper. Single ionization probabilities calculated by using this one dimension MCTDHF model underestimate the accurate results calculated by solving time-dependent Schrodinger equation directly.

Ionization Process of Atoms by Intense Femtosecond Chirped Laser Pulses

We numerically investigate the ionization mechanism in a real hydrogen atom under intense fem to second chirped laser pulses. The central carrier frequency of the pulses is chosen to be 6.2 eV (λ = 200 nm), which corresponds to the fourth-harmonic of the Ti:Sapphire laser. Our simulation of the laser-atom interaction consists on numerically solving the three-dimensional time-dependent Schrodinger equation with a spectral method. The unperturbed wave functions and electronic energies of the atomic system were found by using an L2 discretization technique based on the expansion of the wave functions on B-spline functions. The presented results of kinetic energy spectra of the emitted electrons show the sensitivity of the ionization process to the chirp parameter. Particular attention is paid to the important role of the excited bound states involved in the ionization paths.

Control of the Atomic Ionization with Short and Intense Chirped Laser Pulses

We investigate a two-photon ionization process in a real hydrogen atom by short and intense chirped laser pulses. Our simulation of the laser-atom interaction consists on numerically solving the three-dimensional time-dependent Schrodinger equation with a spectral method. The unperturbed wave functions and electronic energies of the atomic system were found by using an accurate L2 discretisation technique based on the expansion of the wave functions on B-spline functions. We show the efficiency of chirped laser pulses to control the ionization yield and the transfer of the population to the 2p bound state involved in the ionization path.

Particle-number fractionalization of a one-dimensional atomic Fermi gas with synthetic spin-orbit coupling

We propose an experimental scheme to simulate the fractionalization of particle number by using a one-dimensional spin-orbit coupled ultracold fermionic gas. The wanted spin-orbit coupling, a kink-like potential, and a conjugation-symmetry-breaking mass term are properly constructed by laser-atom interactions, leading to an effective low-energy relativistic Dirac Hamiltonian with a topologically nontrivial background field. The designed system supports a localized soliton excitation with a fractional particle number that is generally irrational and experimentally tunable, providing a direct realization of the celebrated generalized-Su-Schrieffer-Heeger model. In addition, we elaborate on how to detect the induced soliton mode with the FPN in the system.

Electron rescattering at metal nanotips induced by ultrashort laser pulses

We investigate plateau and cutoff structures in photoelectron spectra from nanoscale metal tips interacting with few-cycle near-infrared laser pulses. These hallmarks of electron rescattering, well-known from atom-laser interaction in the strong-field regime, appear at remarkably low laser intensities with nominal Keldysh parameters of the order of $\ensuremath{\gtrsim}\phantom{\rule{-0.16em}{0ex}}\phantom{\rule{-0.16em}{0ex}}10$. Quantum and quasiclassical simulations reveal that a large field enhancement near the tip and the increased backscattering probability at a solid-state target play a key role. Plateau electrons are by an order of magnitude more abundant than in comparable atomic spectra, reflecting the high density of target atoms at the surface. The position of the cutoff serves as an in situ probe for the locally enhanced electric field at the tip apex.

Laser-assisted nuclear photoeffect

Proton emission from nuclei via the nuclear photoeffect in the combined electromagnetic fields of a $\ensuremath{\gamma}$-ray photon and an intense laser wave is studied. An $S$-matrix approach to the process is developed by utilizing methods known from the theory of nonperturbative laser-atom interactions. As a specific example, photo-proton ejection from halo nuclei is considered. We show that, due to the presence of the laser field, rich sideband structures arise in the photo-proton energy spectra. Their dependence on the parameters and relative orientation of the photon fields is discussed.

Fano interference in classical oscillators

We seek to illustrate Fano interference in a classical coupled oscillator by using classical analogues of the atom–laser interaction. We present an analogy between the dressed state picture of coherent atom–laser interaction and a classical coupled oscillator. The Autler–Townes splitting due to the atom–laser interaction is analogous to the splitting of normal-mode frequencies of a coupled oscillator. Using this analogy, we simulate and experimentally demonstrate Fano interference and the associated phenomena in three-level atoms in a coupled electrical resonator circuit. This work aims to highlight analogies between classical and quantum systems for students at the postgraduate and graduate levels. Also, the reported technique can be easily realized in undergraduate laboratories.

Two-photon double ionization of He(1s2) and He(1s2s1S) by xuv short pulses

This paper addresses the problem of two-photon double ionization (TPDI) of He($1{s}^{2}$) and He($1s2{s}^{1}\phantom{\rule{-0.16em}{0ex}}S$). First, we reconsider TPDI of He($1{s}^{2}$) with a photon energy of 2.1 a.u.; it is well known that TPDI is mainly a sequential process, resulting in an electron spectrum dominated by two peaks. In the past, we have noticed that the peaks are shifted as the pulse duration shortens; they move toward each other. The first objective of the present work is to clarify the origin of the shift and to evaluate it quantitatively. In parallel with the resolution of the time-dependent Schr\odinger equation (TDSE), we have developed a model calculation based on lowest-order perturbation theory for the laser-atom interaction and for the atomic structure representation. The model agrees with TDSE calculations, and it also explains the physical origin of the shifts. Furthermore, it provides a quantitative evaluation of these shifts. The second objective of the present work is to extend our study to TPDI of He($1s2{s}^{1}\phantom{\rule{-0.16em}{0ex}}S$), which has received much less attention until now. We have investigated the cases of photon energies of 1.8 and 2.5 a.u. We show that TPDI is dominated by a sequential process where the first ionization leaves a remaining He${}^{+}(2s)$ ion, which is ionized by a second photon. As in the case of He($1{s}^{2}$), the model agrees with TDSE calculations, and it leads to better insights into the physics underlying the double-ionization process.

Generalized space-translated Dirac and Pauli equations for superintense laser-atom interactions

We obtain a generalization of the nonrelativistic space-translation transformation to the Dirac equation in the case of a unidirectional laser pulse. This is achieved in a quantum-mechanical representation connected to the standard Dirac representation by a unitary operator $T$ transforming the Foldy-Wouthuysen free-particle basis into the Volkov spinor basis. We show that a solution of the transformed Dirac equation containing initially low momenta $p$ ($p/mc\ensuremath{\ll}1)$ will maintain this property at all times, no matter how intense the field or how rapidly it varies (within present experimental capabilities). As a consequence, the transformed four-component equation propagates independently electron and positron wave packets, and in fact the latter are propagated via two two-component Pauli equations, one for the electron, the other for the positron. These we shall denote as the Pauli low-momentum regime (LMR) equations, equivalent to the Dirac equation for the laser field. Successive levels of dynamical accuracy appear depending on how accurately the operator $T$ is approximated. At the level of accuracy considered in this paper, the Pauli LMR equations contain no spin matrices and are in fact two-component Schr\"odinger equations containing generalized time-dependent potentials. The effects of spin are nevertheless included in the theory because, in the calculation of observables which are formulated in the laboratory frame, use is made of the spin-dependent transformation operator $T$. In addition, the nonrelativistic limit of our results reproduces known results for the laboratory frame with spin included. We show that in intense laser pulses the generalized potentials can undergo extreme distortion from their unperturbed form. The Pauli LMR equation for the electron is applicable to one-electron atoms of small nuclear charge$\phantom{\rule{0.28em}{0ex}}(\ensuremath{\alpha}Z\ensuremath{\ll}1)$ interacting with lasers of all intensities and frequencies $\ensuremath{\omega}\ensuremath{\ll}m{c}^{2}.$

Floquet analysis of real-time wave functions without solving the Floquet equation

We propose a method to obtain Floquet states---also known as light-induced states---and their quasi-energies from real-time wavefunctions without solving the Floquet equation. This is useful for the analysis of various phenomena in time-dependent quantum dynamics if the Hamiltonian is not strictly periodic, as in short laser pulses, for instance. There, the population of the Floquet states depends on the pulse form and is automatically contained in the real-time wavefunction but not in the standard Floquet approach. Several examples in the area of intense laser-atom interaction are exemplarily discussed: (i) the observation of even harmonics for an inversion-symmetric potential with a single bound state; (ii) the dependence of the population of Floquet states on (gauge) transformations and the emergence of an invariant, observable photoelectron spectrum; (iii) the driving of resonant transitions between dressed states, i.e., the dressing of dressed states, and (iv) spectral enhancements at channel closings due to the ponderomotive shift of above-threshold ionization peaks.

A simplified calculation of power-broadened linewidths, with application to resonance ionization mass spectrometry

I present a simple derivation of the power-broadened linewidth of an optical transition in a two-level atom. The novelty of the approach lies in its flexibility, as the approach described here can be applied to any spectral lineshape, corresponding to either homogeneous or inhomogeneous broadening mechanisms. For a Lorentzian profile, I recover the same dependence of linewidth on laser power that one calculates from the optical Bloch equations. The present treatment makes explicit that power broadening arises because of the saturability of the atomic response to intense lasers, rather than to explicitly quantum mechanical aspects of the laser–atom interaction, all of which are ignored. I discuss the utility of power broadening in resonance ionization mass spectrometry, where it is an important means of mitigating unwanted instrumental isotope fractionation.

Gauge independent theory applied to a model of atomic ionization by an intense laser pulse

An explicitly gauge invariant strong field theory is introduced and tested using a model laser-atom interaction. The theory relies on a power series in the target potential. Transitions amplitudes are obtained by using a corresponding series in the momentum space wave function. We demonstrate that this approach is explicitly gauge invariant to all orders. A well know 1D delta function potential model is used to test the convergence of the series in the evaluation of total ionization probabilities and ionization spectra. Actually, the convergence is verified when both, the perturbation as well as the order of the expansion, are increased.

Unconventional Bose—Einstein Condensations from Spin-Orbit Coupling

According to the “no-node" theorem, the many-body ground state wavefunctions of conventional Bose—Einstein condensations (BEC) are positive-definite, thus time-reversal symmetry cannot be spontaneously broken. We find that multi-component bosons with spin-orbit coupling provide an unconventional type of BECs beyond this paradigm. We focus on a subtle case of isotropic Rashba spin-orbit coupling and the spin-independent interaction. In the limit of the weak confining potential, the condensate wavefunctions are frustrated at the Hartree—Fock level due to the degeneracy of the Rashba ring. Quantum zero-point energy selects the spin-spiral type condensate through the “order-from-disorder" mechanism. In a strong harmonic confining trap, the condensate spontaneously generates a half-quantum vortex combined with the skyrmion type of spin texture. In both cases, time-reversal symmetry is spontaneously broken. These phenomena can be realized in both cold atom systems with artificial spin-orbit couplings generated from atom-laser interactions and exciton condensates in semi-conductor systems.

Time-dependent method in the laser–atom interactions

We introduce a recent developed time-dependent method used in the study of laser–atom interactions. The key ingredients of the method are that (1) we propagate the wave function in real space in a finite region. The region is split into two parts, an inner-region and an outer-region. Once the electron moved into the outer-region, the wave function is projected into momentum space and propagated analytically to avoid the reflection from the boundary. (2) To increase the numerical accuracy and make the physical processes more transparent, we solve the time-integral Schrödinger equation, instead of time-differential equation. Then we apply the method to study the infrared laser assisted photoionization of helium by a coherent extreme-ultraviolet light. The ionization process can be controlled by tuning the time delay between the two pulses.

Analytic solutions of the susceptibility for Doppler-broadened two-level atoms

We derive the rigorous analytic solutions for the susceptibility of Doppler-broadened two-level atoms, valid for arbitrary laser intensities. As is usually the case with real atoms, we observe that, when the homogeneous linewidth is much narrower than the Doppler width, the real part of the susceptibility is given by the product of an imaginary error function and a Gaussian, whereas the imaginary part consists simply of a Gaussian function. By employing the analytic results, in particular, we study the effects of optical pumping and saturation on the atomic susceptibility. We also find that the real (imaginary) part of the susceptibility is weakly (strongly) dependent on such parameters as the laser intensity or the optical pumping rate. The availability of analytic solutions for the simple laser-atom interaction may provide convenient tools for its understanding as well as application to more general and realistic situations.