Intraband Polarization Research Articles

Polarization-dependent intervalence band absorption in quantum dots

In this work, we compute the electronic band structure of ZnCdSe/ZnS semiconductor quantum dots (QDs) using k. p method, and studied the dependence of QDs parameters on the in-plane polarization anisotropy and degree of polarization (DOP) of intersubband absorption spectra of QDs. Higher energy levels in the valence band show nonlinear dependence on the parabolic confinement strength and exhibit strong anti-crossing. We find that the optical transitions for specific polarization directions are defined by the size, strain, and shape anisotropy of the QDs. We observe that increasing the shape anisotropy, strain, and QDs radius enhanced the in-plane polarization anisotropy, whereas increasing QDs height reduces the in-plane polarization anisotropy. The DOP is found to increase with increasing QDs radius and strain and reducing the shape anisotropy and QDs height. Furthermore, the QDs parameters play a key role in deciding the absorption peak of the specific polarization. The present study suggests that the combination of shape, size, and strain could provide an optimal approach to tune the intraband polarization response from QDs.

Tunable intraband optical conductivity and polarization-dependent epsilon-near-zero behavior in black phosphorus.

Black phosphorus (BP) offers considerable promise for infrared and visible photonics. Efficient tuning of the bandgap and higher subbands in BP by modulation of the Fermi level or application of vertical electric fields has been previously demonstrated, allowing electrical control of its above-bandgap optical properties. Here, we report modulation of the optical conductivity below the bandgap (5 to 15 μm) by tuning the charge density in a two-dimensional electron gas induced in BP, thereby modifying its free carrier-dominated intraband response. With a moderate doping density of 7 × 1012 cm-2, we were able to observe a polarization-dependent epsilon-near-zero behavior in the dielectric permittivity of BP. The intraband polarization sensitivity is intimately linked to the difference in effective fermionic masses along the two crystallographic directions, as confirmed by our measurements. Our results suggest the potential of multilayer BP to allow new optical functions for emerging photonics applications.

Interband and intraband transition, dynamical polarization and screening of the monolayer and bilayer silicene in low-energy tight-binding model

We investigate the interband and intraband transition of the monolayer and AB-stacked bilayer silicene in low-energy tight-binding model under the electric field, where we focus on the dynamical polarization function, screening due to the charged impurity, and the plasmon dispersion. We obtain the logarithmically divergen polarization function within the random-phase-approximation (RPA) whose logarithmic singularities corresponds to the discontinuities of the first derivative which is at the momentum ${\bf q}=2{\bf k}_{F}$ in static case and indicate the topological phase transition point from the gapless semimetal to the gapped band insulator. We also obtain the power-law-dependent Friedel oscillation which can be enhanced by increasing the Rashba-coupling, that can contribute to the screened potential of the charged impurity which scale as $\sim r^{-1/2}$ in the short distance from the impurity and scale as $\sim r^{-1/3}$ in the long distance from the impurity. In the single-particle excitation regime with the electron-hole continuum, the interband and intraband transition happen, and the plasmon dispersion, which we mainly focus on the optical plasmon (which $\sim\sqrt{{\bf q}}$ in long-wavelength limit) in this paper, start to damped into the electron-hole pairs due to the nonzero imaginary part of the polarization function.

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

Using an excitonic basis, we investigate the intraband polarization, optical absorption spectra, and terahertz emission of semiconductor superlattice with the density matrix theory. The excitonic Bloch oscillation is driven by the dc and ac electric fields. The slow variation in the intraband polarization depends on the ac electric field frequency. The intraband polarization increases when the ac electric field frequency is below the Bloch frequency. When the ac electric field frequency is above the Bloch frequency, the intraband polarization downwards and its intensity decreases. The satellite structures in the optical absorption spectra are presented. Due to excitonic dynamic localization, the emission lines of terahertz shift in different ac electric field and dc electric field.

Intraband dynamics and terahertz emission in biased semiconductor superlattices coupled to double far-infrared pulses

This paper studies both the intraband polarization and terahertz emission of a semiconductor superlattice in combined dc and ac electric fields by using the superposition of two identical time delayed and phase shifted optical pulses. By adjusting the delay between these two optical pulses, our results show that the intraband polarization is sensitive to the time delay. The peak values appear again for the terahertz emission intensity due to the superposition of two optical pulses. The emission lines of terahertz blueshift and redshift in different ac electric fields and dynamic localization appears. The emission lines of THz only appear to blueshift when the biased superlattice is driven by a single optical pulse. Due to excitonic dynamic localization, the terahertz emission intensity decays with time in different dc and ac electric fields. These are features of this superlattice which distinguish it from a superlattice generated by a single optical pulse to drive it.

Coherent control of Bloch oscillations by means of optical pulse shaping

We excite excitonic wave packets in biased semiconductor superlattices with spectrally shaped ultrashort optical pulses. We tailor the shape and phase of the pulse spectrum in order to control the coherent dynamics of the excitonic wave packets formed from a superposition of three excitonic states. Via careful shaping, we are able to excite either wave packets that exhibit standard Bloch oscillations (BO's) or breathing-mode (BM) motion. These two types of motion are characterized by the presence (BO) or absence (BM) of an internal intraband polarization caused by the electron-hole separation within the excitonic wave packet. The wave packet evolution is monitored using spectrally resolved four-wave mixing. This ability to control the BO's provides a way to control the emitted THz radiation.

The Interplay of Intraband and Interband Polarization in the Ultrafast Response of Biased Semiconductor Superlattices

The Interplay of Intraband and Interband Polarization in the Ultrafast Response of Biased Semiconductor Superlattices

Optical Absorption Spectra and Intraband Dynamics inTerahertz-Driven Semiconductor Superlattice

We have theoretically investigated the optical absorption spectrumand intraband dynamics by subjecting a superlattice to both aterahertz (THz)-frequency driving field and an optical pulse byusing an excitonic basis. In the presence of a THz dc field, thesatellite structures in the absorption spectra are presented. Thesatellite structure is a result from the THz nonlinear dynamics ofWannier–Stark ladder excitons. On the other hand, the coherentintraband polarization is investigated. We find that the excitonicBloch oscillation is driven by the THz field and yields an intrabandpolarization that continues to oscillate at times much longer thanthe intraband dephasing time. The temporal evolution of the slowlyvarying components of the intraband polarization is dependent onthe THz frequency.

Excitonic intraband relaxation and polarization anisotropies in PTCDA on femtosecond and picosecond timescales

We report on investigations of optical excitations in polycrystalline organic molecular crystals with quasi-1D-stacked crystal structure and negative exciton dispersion. As model system, we choose thin films of the perylene derivative 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). Using pump–probe spectroscopy, we show how the relaxation from the absorbing state towards the border of the Brillouin zone occurs on a 120 fs timescale. Time-resolved luminescence anisotropy gives evidence that as a result of the coherent coupling between adjacent stacks, populations of the Davydov-split states that are prepared during photo-excitation relax into the emitting states in less than 5 ps. The behavior of the luminescence anisotropy can be explained by the orientation of the two PTCDA molecules in the unit cell. However, a full understanding of the ultrafast pump–probe anisotropy requires novel explanations beyond current models.

Intraband polarization and terahertz emission in biased semiconductor superlattices with full excitonic basis

We report theoretical and experimental results for the intraband dynamics of biased semiconductor superlattices excited by ultrashort optical pulses. The theoretical model used employs an excitonic basis that includes $1s$ and all higher-energy in-plane excitonic states. These excitonic states are used to calculate the intraband polarization and terahertz emission of the superlattice system in response to excitation via an ultrashort optical pulse. Our results show that the higher in-plane excitonic states often modify considerably the terahertz emission relative to the results obtained using a $1s$ exciton basis, but that under some excitation conditions a $1s$ exciton basis gives accurate results. Good agreement between experimental and theoretical results is obtained.

Intraband versus interband decoherence times in biased semiconductor superlattices

We develop a theoretical method of calculating the dynamics of excitons in biased semiconductor superlattices, including exciton-LO-phonon interactions. We use this method to determine phonon-induced excitonic decoherence in the interband and intraband polarizations. We find that the intraband decoherence time is much longer than one would expect from a simple extrapolation from the interband decoherence time. The longer decoherence time is shown to be due to the persistence of intraband coherence after exciton-phonon scattering events. The results are found to be in qualitative agreement with recent experiments.

The density matrix picture of laser coherent control current

The physical substance of the coherent control current and the optical rectification have been analyzed based on density matrix perturbation theory. The analytical results demonstrate that they arise from the real and virtual manifestations of the same nonlinear process associated with diagonal and non-diagonal density matrix. And in terms of polarization, they respectively arise from the intraband and interband polarizations. Both the evolution of the coherent control current exited by ultrafast laser pulse and its dependence on frequency have been studied in time and frequency domains. In order to get an explicit knowledge of intraband polarization and the origination of the coherent control current, we have investigated the initial photo-carriers momentum distribution. The ultrafast decay of the polar momentum population in order of tens of femtosends is given to illustrate its instantaneous optical response.

Intraband polarization as the source of degenerate four-wave mixing signals in asymmetric semiconductor quantum well structures

We have developed a formalism for calculating the coherent response of asymmetric semiconductor quantum well structures to ultrashort optical pulses. We work in an excitonic basis and include exciton-exciton interactions via the long-wavelength portion of the excitonic intraband polarization. We apply this formalism to the calculation of degenerate four-wave mixing intensities of a biased semiconductor superlattice and find that many aspects of the four-wave mixing signals are most naturally interpreted as directly resulting from the scattering of excitons off of the intraband polarization grating. We furthermore develop an extremely accurate method of factoring the dynamical equations that does not suffer from the problem encountered by the semiconductor Bloch equations in the Hartree-Fock approximation.

Polarization revival of a Bloch-oscillating wave packet in conjunction with resonant Zener tunneling

We investigate the dynamics of a Bloch-oscillating wave packet in the presence of strong coupling to delocalized above barrier states (Zener tunneling), using time-resolved intraband polarization-sensitive measurements. At a threshold electric field, the resonance of localized and delocalized states causes a quantum beating which is observed as a revival in the intraband polarization. Our numerical simulation visualizes the spatial wave packet decomposition and reformation. The wave packet moves on a ps time scale over a distance of more than 100 nm and sequentially undergoes Bloch oscillations in the below- and above-barrier bands.

Synthesis and Magnetic Studies of Nanocrystalline Nickel Nitride Material

Single domain Ni 3 N nitride particles are synthesized by simultaneous decomposition and nitridation in ammonia atmosphere of [Ni(NH 3 ) 6 ](NO 3 ) 2 complex crystals at 650 K. The Ni 3 N phase crystallizes in the hexagonal structure with unit cell parameters a = 4.624(4) A and c = 4.316(4) A, and has a crystallite size of 16 nm. In TEM study, these nanosize particles show spherical shape, and form particle aggregates of 18 nm size. Ni 3 N particles exhibit a Curie temperature of T C = 634 K. In the field dependence magnetic measurements, the presence of hysteresis loop indicates ferromagnetic behavior with σ s = 1.678 emu/g, σ R = 0.50 emu/g and μ 0 H c = 0.022 T. In this phase, the density of states (DOS) of Ni is dominated by 3d states and is mixed with 2p DOS of N. The contribution of d-electrons in the intra-band polarization affects the magnetic moment of Ni and thus the magnetic moment of Ni 3 N.

Asymmetry in the excitonic Wannier-Stark ladder: A mechanism for the stimulated emission of terahertz radiation

Using an excitonic basis, we calculate the coherent intraband response of a photoexcited undoped semiconductor superlattice in combined along-axis static and terahertz electric fields. We find that the terahertz field drives the amplitude of the excitonic Bloch oscillations, yielding an intraband polarization that continues to oscillate at times much longer than the intraband dephasing time---an effect which is totally absent for noninteracting electron-hole pairs. Furthermore, these driven, Bloch-oscillating excitons are found to produce stimulated emission at terahertz frequencies as long as the Bloch oscillation amplitude is driven.

Magnetic-field suppression of THz charge oscillations in a double quantum well

We have observed a suppression of the THz radiation emitted by charge oscillations photoexcited in the conduction band of ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ asymmetric double quantum wells in magnetic fields of order 1 T. Coherent charge oscillations are created by femtosecond excitation of a superposition of two states between which there is an electric dipole moment which oscillates in time as the wave packet evolves. The electric field radiated by this intraband polarization is coherently detected using an optically gated antenna. The nature of the suppression is qualitatively different for the two cases of magnetic field $B$ parallel and perpendicular to the plane of the quantum wells. For $B$ parallel the polarization dephasing rate increases with increasing field whilst the initial amplitude remains approximately constant. For $B$ perpendicular the dephasing rate does not change significantly with increasing field but there is a reduction in the initial amplitude. The behavior in parallel field can be understood in a semiclassical picture in which electrons are deflected as they tunnel, thus destroying the phase coherence. The observation of amplitude suppression in a moderate perpendicular field may be partly associated with magnetic confinement but the effect is much larger than expected. The effect of magnetic field on the charge oscillation frequency is also discussed.

Dielectric properties of multiband electron systems I. Tight-binding formulation

The screened electron-electron interaction in a multi-band electron system is calculated within the random phase approximation and in the tight-binding representation. The obtained dielectric matrix contains, beside the usual site-site correlations, also the site-bond and bondbond correlations, and thus includes all physically relevant polarization processes. The arguments are given that the bond contributions are negligible in the long wavelength limit. We analyse the system with two non-overlapping bands in this limit, and show that the corresponding dielectric matrix reduces to a 2 × 2 form. The intra-band and inter-band contributions are represented by diagonal matrix elements, while the off-diagonal elements contain the mixing between them. The latter is absent in insulators but may be finite in conductors. Performing the multipole expansion of the bare long-range interaction, we show that this mixing is directly related to the symmetry of the atomic orbitals participating in the tight-binding electronic states. In systems with forbidden atomic dipolar transitions, the intra-band and inter-band polarizations are separated. However, when the dipolar transitions are allowed, the off-diagonal elements of the dielectric matrix are of the same order as diagonal ones, due to a finite monopole-dipole interaction between the intra-band and inter-band charge fluctuations. We also calculate the macroscopic dielectric function and obtain an expression which interpolates between the well-known limits of oneband conductors and pure insulators. In particular, it is shown that the microscopic origin of the so-called selfpolarization corrections is the on-site interaction which exchanges two electrons at different orbitals, combined with a finite tunneling between neighboring sites.

Coherent electron–hole correlations in quantum dots

Using numerical time propagation of the electron–hole wave function, we demonstrate how various coherent correlation effects can be observed by laser excitation of a nanoscale semiconductor quantum dot. The lowest-lying states of an electron–hole pair, when appropriately excited by a laser pulse, give rise to charge oscillations that are manifested by beatings in the optical or intraband polarizations. A GaAs 5×25×25 nm3 dot in the effective-mass approximation, including the screened Coulomb interaction between the electron and a heavy or light hole, is simulated.

Magnetic properties of single domain ε-Fe3N synthesized by borohydride reduction route

Single domain ε-Fe3N nitride particles have been synthesized by chemical reduction followed by nitridation at 823 K. The ε-Fe3N phase crystallizes in hexagonal structure with unit cell parameters, a=2.70 Å and c=4.39 Å. There is a reduction of unpaired d electrons for intraband polarization as the nitrogen contributes to the density of states and thus results in lowering of magnetic moments. The reduction of intrinsic magnetizations and Curie temperatures with decreasing particle size is attributed to canted spin structure predominantly at the surface compared to the bulk of the particles. Chemisorption of oxygen results in the formation of oxynitride layer at the surface with a ferromagnetic coupling to the spins in the core of the particle. Mössbauer study of ε-Fe3N particles exhibits the coexistence of ferromagnetic and superparamagnetic particles and corroborates the observed magnetic properties.