Optical Absorption Spectra and Intraband Dynamics in Terahertz-Driven Semiconductor Superlattice

We have theoretically investigated the optical absorption spectrum and intraband dynamics by subjecting a superlattice to both a terahertz (THz)-frequency driving field and an optical pulse by using an excitonic basis.In the presence of a THz dc field, the satellite structures in the absorption spectra are presented. The satellite structure is a result from the THz nonlinear dynamics of Wannier-Stark ladder excitons. On the other hand, the coherent intraband polarization is investigated. We find that the excitonic Bloch oscillation is driven by the THz field and yields an intraband polarization that continues to oscillate at times much longer than the intraband dephasing time. The temporal evolution of the slowly varying components of the intraband polarization is dependent on the THz frequency.     

Intraband Dynamics and Terahertz Emission in Biased GaAs/In0.53Ga0.47As Semiconductor Superlattices

Using an excitonic basis, we investigate the intraband polarization, opticalabsorption spectra, and terahertz emission of semiconductor superlattice withthe density matrix theory. The excitonic Bloch oscillation is driven by the dcand ac electric fields. The slow variation in the intraband polarizationdepends on the ac electric field frequency. The intraband polarizationincreases when the ac electric field frequency is below the Bloch frequency.When the ac electric field frequency is above the Bloch frequency, theintraband polarization downwards and its intensity decreases. The satellitestructures in the optical absorption spectra are presented. Due to excitonicdynamic localization, the emission lines of terahertz shift in different acelectric field and dc electric field.     

Coherent delocalization of atomic wave packets in driven lattice potentials

Atomic wave packets loaded into a phase-modulated vertical optical-lattice potential exhibit a coherent delocalization dynamics arising from intraband transitions among Wannier-Stark levels. Wannier-Stark intraband transitions are here observed by monitoring the in situ wave-packet extent. By varying the modulation frequency, we find resonances at integer multiples of the Bloch frequency. The resonances show a Fourier-limited width for interrogation times up to 2 s. This can also be used to determine the gravity acceleration with ppm resolution.     

The density matrix picture of laser coherent control current

The photocurrent has been widely investigated for a long time. Conventionally the current results from the directional motion of the photo-electrons accelerated by dc bias. In recent years, the development of the optical information science calls for the Tbit per second transport speed, so the related devices must work on a time scale of picosecond even femtosecond. But this requirement is limited by the lifetime of the conventional photo-electrons. With the development of ultrafast laser pul…     

Poles of structural factor and secondary emission from a semi-infinite crystal

Secondary emission from a semi-infinite crystal is considered. The contribution to the emission amplitude originating from structural factor singularities is analyzed. Poles of the structural factor determine the main contribution to the emission amplitude. The energy and angular distributions of the emission depend on the crystallographic orientation of the crystal surface and are characterized by a set of allowed vectors of the reciprocal lattice. It is shown that emission anisotropy related to the crystallographic directions is due to the coherent character of the photoabsorption process, and diffraction scattering leads only to relative change in the emission intensity in allowed low-index directions. The concrete calculations are performed for (100) and (111) surfaces of a diamond-like lattice.     

Self-consistent simulations of quantum cascade laser structures for frequency comb generation

Maxwell–Bloch equations are widely used to model the dynamics due to coherent light-matter interaction in quantum cascade laser (QCL) structures, which plays an essential role especially for the generation of frequency combs and mode-locked pulses. While the modest numerical complexity of the Maxwell–Bloch system allows for a full spatiotemporal treatment, its main disadvantage is the inclusion of dissipation by empirical dephasing rates and electron lifetimes. We present a self-consistent multi-domain approach which couples the Maxwell–Bloch equations to advanced carrier transport simulations based on a density matrix Monte Carlo technique, yielding the scattering and dephasing rates. In this way, the compact spatiotemporal modeling of the carrier-light dynamics by the Maxwell–Bloch system can be combined with the versatility and reliability of self-consistent carrier transport approaches. Simulation results are shown for a QCL-based terahertz frequency comb source, and good agreement with experiment is obtained.     

Influence of intraband motion on the interband excitation and high harmonic generation

Tunnelling, acceleration, and collision of electrons are the basic events in the process of high harmonic generation(HHG) in strong-field interaction with atoms.However, the periodic array of atoms in semiconductor structure makes three steps become interatomic coherent process which leads to complicated carrier dynamics and two sources of high harmonic emission: interband polarization and intraband current.The difference of features of high harmonic generation between semiconductors and atoms is strongly linked to the unique presence of intraband motion which manifests itself a nontrivial role in intertwined two dynamics.Here, we review recent experimental and theoretical advances of understanding coupled interband and intraband mechanisms of HHG in semiconductors.Particularly we focus on the influence of intraband motion on the interband excitation, and on the subsequent HHG emission and attosecond pulse generation.     

Transient gain in photonic crystals

Using the optical response formula for photonic crystals and the Bloch equations, we analyze coherent emission in arbitrary two-dimensional photonic crystals. The transient emission depends not only on the photon density of the states, but also on the intensity and duration of the pump laser pulses. A transient gain exists if the pump pulse area is a certain value and the transition frequency is tuned to the band edge. The sharp structure of the density of states near the band edge leads to a dramatic enhancement of the transient emission intensity and an abrupt change in the oscillation of the emission field. PACS 42.70.Qs; 32.80.-t; 42.25.Bs     

Anomalous electronic correlations in the ground state momentum density of Al(97)Li(3)

We report high resolution Compton scattering measurements on an Al(97)Li(3) disordered alloy single crystal for momentum transfer along the [100], [110], and [111] symmetry directions. The results are interpreted via corresponding Korringa-Kohn-Rostoker coherent potential approximation first-principles computations. By comparing spectra for Al(97)Li(3) and Al, we show that the momentum density in the alloy differs significantly from the predictions of the conventional Fermi-liquid picture and that the ground state of Al is modified anomalously by the addition of Li.     

An alternative method for the exact calculation of Wannier–Stark localization in superlattices

We present an alternative method for the exact calculations of the Wannier–Stark (WS) localization in a long periodic potential corresponding to a (50 Å/30 Å) GaAs/Ga_0.7Al_0.3As superlattice. We show that the electric field dependence of the electron wavefunction has unique localization dynamics. One interesting prediction is a small effect involving the change of the dipole field with increasing WS field. It is argued that this may give rise to parasitic effects in Bloch oscillations and, therefore, to noise in coherent terahertz emission.     

Magnetic inverted photonic crystals: A polarized neutron scattering study

This work is devoted to a small-angle polarized neutron scattering study of the structure and magnetic properties of nickel inverted photonic crystals. Depending on the intensity of the small-angle scattering, diffraction maximums up to fourth-order reflections, which correspond to scattering from the highly ordered structures of the test samples, are observed. Several contributions to the scattering are analyzed: a nuclear contribution; a magnetic contribution; a contribution depending on an external magnetic field; and a nuclear magnetic interference, which shows a correlation between magnetic and nuclear structures. It is found that a magnetization reversal process, which was represented by a standard hysteresis curve, for weak fields was accompanied by both domain formation and coherent magnetization rotation from the field direction to directions caused by geometric structure peculiarities.     

Selection rules for Bloch wave scattering for HREM imaging of imperfect crystals along symmetry axes

A Bloch wave analysis is used to investigate high-resolution electron microscope (HREM) imaging of crystals containing atomic displacements due to strain. In the absence of interband scattering, the shifts of peaks and troughs in the image will correspond to the displacements of the atoms in the exit surface. Interband scattering will shift the image peaks away from the actual atom positions and modify the apparent magnitude of the displacement identified by the observed image peak positions. By considering the case of seven-beam imaging of a cubic crystal aligned along a ?111? axis, it is shown that the symmetry of the Bloch waves leads to selection rules for the interband scattering, similar to those seen for dipole electron excitations in atoms. It is also shown that, to first order, no intraband scattering can occur.     

Classical and quantum mechanical resonance fluorescence

We study resonance fluorescence from a two-level atom illuminated by coherent and incoherent light. Especially, we treat the case of an intense incoherent component which is broad band and chaotic in character.New insights into the phenomenon of resonance fluorescence are obtained by constructing certain analogies with the precession of a classical (Bloch) vector around a classical stochastic field. The analogies are based on a representation of the density operator of the two-level atoms as a diagonal mixture of directed angular momentum states.As long as the whole light field is an imposed one the weight function of the mixture mentioned above describes a random sequence of rotations of the Bloch vector and obeys a simple Fokker Planck equation. If, however, the incoherent component of the light field acts as a zero- or finite temperature heat bath, the equation of motion for the weight function is no longer a Fokker Planck equation. Nontheless, we find the exact solution and calculate the correlation functions relevant to a discussion of the spectrum and of antibunching effects.     

Super Bloch oscillations in the Peyrard-Bishop-Holstein model

Recently, polarons in the Peyrard-Bishop-Holstein model under DC electric fields were established to perform Bloch oscillations, provided the charge-lattice coupling is not large. In this work, we study this model when the charge is subjected to an applied field with both DC and AC components. Similarly to what happens in the rigid lattice, we find that the carrier undergoes a directed motion or coherent oscillations when the AC field is resonant or detuned with respect to the Bloch frequency, respectively. The electric density current and its Fourier spectrum are also studied to reveal the frequencies involved in the polaron dynamics.     

Elementary relaxation processes investigated by femtosecond photoelectron spectroscopy of two‐dimensional materials

Elementary scattering processes in solid matter occur on ultrafast timescales and photoelectron spectroscopy in the time domain represents an excellent tool for their analysis. Conventional photoemission accesses binding energies of electronic states and their momentum dispersion. The use of femtosecond laser pulses in pump‐probe experiments allows obtaining direct insights to the energy and momentum dependence of ultrafast dynamics. This article introduces the elementary interaction processes and emphasizes recent work performed in this rapidly developing field. Decay processes in the low excitation limit are addressed, where electrons decay according to their interaction with carriers in equilibrium. Here, hot electron relaxation in epitaxial metallic film is reviewed. In the limit of an intense optical excitation, scattering of the excited electrons among each other establishes a non‐equilibrium state. Results on charge‐density wave materials and the effect of coherent nuclear motion on the electronic structure, which can break low symmetry ground states, are discussed. Figure reprinted with permission from [71].     

Quasiclassical Describing of the Bloch Oscillation in a Superlattice by Fokker-Planck Equation

A theoretical study of Bloch electron transport in a superlattice miniband driven by an electric field parallel to the growth axis is carried out, by Fokker-Planck equation (FPE) in momentum space with the averaged momentum relaxation time (γ) approximation. Steadystate drift-velocity/field characteristics exhibit the expected maximum followed by negative differential conductivity (NDC), and then followed by drift-velocity oscillation when γ or electric field is large. The oscillation frequency is an increasing function of γ, and when γ → ∞, the limit of the oscillation frequency is the Bloch frequency as expected.     

Photoexcitation of graphene with twisted light

We study theoretically the interaction of twisted light with graphene. The light-matter interaction matrix elements between the tight-binding states of electrons in graphene are determined near the Dirac points. We examine the dynamics of the photoexcitation process by posing the equations of motion of the density matrix and working up to second order in the field. The time evolution of the angular momentum of the photoexcited electrons and their associated photocurrents are examined in order to elucidate the mechanisms of angular momentum transfer. We find that the transfer of spin and orbital angular momentum from light to the electrons is more akin here to the case of intraband than of interband transitions in semiconductors, due to the fact that the two relevant energy bands of graphene originate from the same atomic orbitals.     

Scattering of electrons on the Cu(001) surface by Co adatoms

Excited electrons at surfaces can be scattered by adsorbate atoms or defects, which changes the energy or momentum. Such scattering processes can be studied by energy, time and angle-resolved two-photon photoelectron spectroscopy. In this article the influence of statistically distributed Co adatoms on a Cu(001) surface on the dynamics of electrons in image-potential states is investigated. Different scattering mechanisms, such as interband, intraband, and bulk scattering are identified and analyzed quantitatively. Cobalt adatoms cause mainly quasielastic scattering of electrons in image-potential states. Inelastic processes are due to interactions with electrons in the substrate and are not significantly increased by Co adatoms. The results are compared to previous experimental and theoretical work on Cu adatoms. PACS 73.20.At; 68.49.Jk; 79.60.Ht     

Variance squeezing and information entropy squeezing via Bloch coherent states in two-level nonlinear spin models

The nonclassical squeezing effect emerging from a nonlinear coupling model (generalized Jaynes–Cummings model) of a two-level atom interacting resonantly with a bimodal cavity field via two-photon transitions is investigated in the rotating wave approximation. Various Bloch coherent initial states (rotated states) for the atomic system are assumed, i.e., (i) ground state, (ii) excited state, and (iii) linear superposition of both states. Initially, the atomic system and the field are in a disentangled state, where the field modes are in Glauber coherent states via Poisson distribution. The model is numerically tested against simulations of time evolution of the based Heisenberg uncertainty relation variance and Shannon information entropy squeezing factors. The quantum state purity is computed for the three possible initial states and used as a criterion to get information about the entanglement of the components of the system. Analytical expression of the total density operator matrix elements at t > 0 shows, in fact, the present nonlinear model to be strongly entangled, where each of the definite initial Bloch coherent states is reduced to statistical mixtures. Thus, the present model does not preserve the modulus of the Bloch vector.     

Dynamics of hydrogen bonds in water and consequences for the unusual behaviour of supercooled water

The dynamics of liquid water is evaluated by the coherent quasi-elastic scattering at two different momentum transfers, in order to discriminate hydrogen bond lifetime from molecular dynamics. The results indicate a possible issue for the puzzle of the behaviour of supercooled water.