Faraday Geometry Research Articles

Far-infrared electrodynamics of thin superconducting NbN film in magnetic fields

We studied a thin superconducting NbN film in magnetic fields up to 8 T above the zero-temperature limit by means of time-domain terahertz and scanning tunneling spectroscopies in order to understand the vortex response. Scanning tunneling spectroscopy was used to determine the optical gap and the upper critical field of the sample. The values obtained were subsequently used to fit the terahertz complex conductivity spectra in the magnetic field in the Faraday geometry above the zero-temperature limit. These spectra are best described in terms of the Coffey–Clem self-consistent solution of a modified London equation in the flux creep regime.

Asymmetric bistable reflection and polarization switching in a magnetic nonlinear multilayer structure

Optical properties of one-dimensional photonic structures consisting of Kerr-type nonlinear and magnetic layers under the action of an external static magnetic field in the Faraday geometry are investigated. The structure is a periodic arrangement of alternating nonlinear and magnetic layers (a one-dimensional photonic crystal) with one of the layers doubled to create a defect where periodicity is violated. Strong enhancement of nonreciprocity is observed at the frequencies of the defect modes, where linearly polarized light incident from one side of the structure undergoes polarization rotation upon reflection, while light reflected from the other side has its polarization unchanged. Using the nonlinear transfer matrix calculations in the frequency domain, it is demonstrated that defect resonances in the nonlinear reflection spectra undergo bending, resulting in polarization bistability of reflected light. This bistability is shown to result in abrupt switching between linear polarization of the output reflected light when the input intensity is varied. This switching is confirmed in finite-difference time-domain simulations, and its hysteresis character is established.

Vector acoustic solitons from the coupling of long and short waves in a paramagnetic crystal

We investigate the propagation of a longitudinal-transverse elastic pulse in a statically deformed crystal containing paramagnetic impurities and placed in an external magnetic field. We derive a system of three nonlinear wave equations describing the interaction of the pulse with the paramagnetic impurities in the quasiresonance approximation in the Faraday geometry. We assume that the transverse components of the pulse, which cause quantum transitions, have carrier frequencies and are short-wave (acoustic), while the longitudinal component has no carrier frequency and is long-wave. We show that in the case of an equilibrium initial distribution of populations of quantum levels of paramagnetic impurities, the coupling between the longitudinal and transverse components is weak, the pulse is therefore strictly transverse, and its dynamics are described by the Manakov system. With a nonequilibrium initial distribution of populations, conditions of effective interaction between all components of the elastic pulse can be reached, and their nonlinear dynamics are described by a vector generalization of the Zakharov equations. In the case of a unidirectional propagation of the pulse, these equations reduce to the Yajima-Oikawa vector system. We show that the obtained system of equations and its version with an arbitrary number of short-wave components can be integrated using the inverse scattering transform. We construct infinite hierarchies of solutions of the Yajima-Oikawa vector system (including a solution on a nontrivial background). We consider stationary (complex-valued Garnier system) and self-similar reductions of that system, also admitting a representation in the form of compatibility conditions.

A table-top, repetitive pulsed magnet for nonlinear and ultrafast spectroscopy in high magnetic fields up to 30 T

We have developed a mini-coil pulsed magnet system with direct optical access, ideally suited for nonlinear and ultrafast spectroscopy studies of materials in high magnetic fields up to 30 T. The apparatus consists of a small coil in a liquid nitrogen cryostat coupled with a helium flow cryostat to provide sample temperatures down to below 10 K. Direct optical access to the sample is achieved with the use of easily interchangeable windows separated by a short distance of ~135 mm on either side of the coupled cryostats with numerical apertures of 0.20 and 0.03 for measurements employing the Faraday geometry. As a demonstration, we performed time-resolved and time-integrated photoluminescence measurements as well as transmission measurements on InGaAs quantum wells.

Three-Dimensional Magneto-Photoluminescence as a Probe of the Electronic Properties of Crystal-Phase Quantum Disks in GaAs Nanowires

Crystal-phase engineering has emerged as a novel method of bandgap engineering, made feasible by the high surface-to-volume ratio of nanowires. There remains intense debate about the exact characteristics of the band structure of the novel crystal phases, such as wurtzite GaAs, obtained by this approach. We attack this problem via a low-temperature angle-dependent magneto-photoluminescence study of wurtzite/zinc-blende quantum disks in single GaAs nanowires. The exciton diamagnetic coefficient is proportional to the electron-hole correlation length, enabling a determination of the spatial extent of the exciton wave function in the plane and along the confinement axis of the crystal-phase quantum disks. Depending on the disk nature, the diamagnetic coefficient measured in Faraday geometry ranges between 25 and 75 μeV/T(2). For a given disk, the diamagnetic coefficient remains constant upon rotation of the magnetic field. Along with our envelope function calculation accounting for excitonic effects, we demonstrate that the electron effective mass in wurtzite GaAs quantum disks is heavy, mostly isotropic and results from mixing of the two lower-energy conduction bands with Γ7 and Γ8 symmetries. Finally, we discuss the implications of the results of the angle dependent magneto-luminescence for the likely symmetry of the exciton states. This work provides important insight in the band structure of wurtzite GaAs for future nanowire-based polytypic bandgap engineering.

Magnetic field induced nutation of exciton-polariton polarization in (Cd,Zn)Te crystals

We study the polarization dynamics of exciton-polaritons propagating in sub-mm thick (Cd,Zn)Te bulk crystals using polarimetric time-of-flight techniques. The application of a magnetic field in Faraday geometry leads to synchronous temporal oscillations of all Stokes parameters of an initially linearly or circularly polarized, spectrally broad optical pulse of 150 fs duration propagating through the crystal. Strong dispersion for photon energies close to the exciton resonance leads to stretching of the optical pulse to a duration of 200$-$300 ps and enhancement of magneto-optical effects such as the Faraday rotation and the non-reciprocal birefringence. The oscillation frequency of the exciton-polariton polarization increases with magnetic field $B$, reaching 10 GHz at $B\sim 5$T. Surprisingly, the relative contributions of Faraday rotation and non-reciprocal birefringence undergo strong changes with photon energy, which is attributed to a non-trivial spectral dependence of Faraday rotation in the vicinity of the exciton resonance. This leads to polarization nutation of the transmitted optical pulse in the time domain. The results are well explained by a model that accounts for Faraday rotation and magneto-spatial dispersion in zinc-blende crystals. We evaluate the exciton $g$-factor $|g_{\rm exc}|=0.2$ and the magneto-spatial constant $V= 5 \times 10^{-12}$ eVcm$\textup{T}^{-1}$.

Microscopic origin of magnetic linear dichroism in the antiferromagnetic insulator MnF2

Magnetically ordered crystals often show unique magneto-optical phenomena that are not found without magnetic order. For example, MnF${}_{2}$, a two-sublattice antiferromagnet with different sublattice symmetries shows magnetic linear dichroism (MLD), which is odd with respect to the external magnetic field in the Faraday geometry. In particular, the magnon sideband of an exciton with a particular orbital function exhibits MLD, and the commonly found magnetic circular dichroism (MCD) is almost absent. In this study, we clarify the microscopic origin of this characteristic feature. We found that the symmetry of the exciton orbital determines the polarization eigenstate of the light to create the exciton magnon pair. These eigenstates can even be two cross-linear polarizations, depending in which sublattice the exciton is created. In this case, only MLD is allowed and MCD is forbidden.

The set of photoelectromagnetic methods for determination of recombination and diffusion parameters of p-MCT thin films

In this paper the set of photoelectromagnetic methods for determination of recombination and diffusion parameters of charge carriers in p -type mercury cadmium telluride epitaxial thin films at temperature range 77–125 K is offered. The set of methods includes the photoconductivity in magnetic field for Faraday and Voigt geometries, the photoelectromagnetic effect, the Hall effect and the measurements of magnetoconductivity. Such films parameters as concentrations and mobilities of heavy and light holes, mobility of minor electrons, electrons lifetime and ratio between holes and electrons lifetimes, surface recombination velocities can be determined with help of offered set.

Solid state electrically injected exciton-polariton laser.

Inversionless ultralow threshold coherent emission, or polariton lasing, can be obtained by spontaneous radiative recombination from a degenerate polariton condensate with nonresonant excitation. Such excitation has, hitherto, been provided by an optical source. Coherent emission from a GaAs-based quantum well microcavity diode with electrical injection is observed here. This is achieved by a combination of modulation doping of the wells, to invoke polariton-electron scattering, and an applied magnetic field in the Faraday geometry to enhance the exciton-polariton saturation density. These measures help to overcome the relaxation bottleneck and to form a macroscopic and degenerate condensate as evidenced by angle-resolved luminescence, light-current characteristics, spatial coherence, and output polarization. The experiments were performed at 30 K with an applied field of 7 T.

Optical study of electron-electron exchange interaction in CdTe/ZnTe quantum dots

We present an experimental study of electron-electron exchange interaction in self-assembled CdTe/ZnTe quantum dots based on the photoluminescence measurements. The character and strength of this interaction are obtained by simultaneous observation of various recombination channels of a doubly negatively charged exciton X${}^{2\ensuremath{-}}$, including previously unrecognized emission lines related to the electron-singlet configuration in the final state. A typical value of the electron singlet-triplet splitting, which corresponds to the exchange integral of electron-electron interaction, has been determined as 20.4 meV with a spread of 1.4 meV across the wide population of quantum dots. We also evidence an unexpected decrease of energy difference between the singlet and triplet states under a magnetic field in Faraday geometry.

Spin‐polarized exciton transfer in Zn‐Cd‐Mn‐Se multi quantum wells

AbstractFour‐quantum‐well structure consisting of a ZnCdMnSe diluted magnetic semiconductor (DMS) well and three ZnCdSe non‐magnetic semiconductor (NMS) wells separated by 15 nm‐thick ZnSe barrier layers were prepared by using ALE and MBE technique. The sample was excited by CW and pulse lasers with photon energy larger than the barrier layer band gap, and circularly polarized PL spectra due to recombination of excitons localized in each well were measured as a function of external magnetic field in the Faraday geometry up to 8 T at 4 K. The PL from the NMS well next to the DMS well and from the NMS well next‐but‐one to the DMS well showed positive circular polarization due to LO phonon assisted transfer of positively spin polarized excitons from the DMS well to the NMS wells. The two NMS wells showed different magnetic field dependence of the degree of circular polarization of PL, which suggested that the exciton transfer from the DMS well to the NMS well next‐but‐one was governed by direct interaction between the two wells, not by cascade transfer process. The degree of circular polarization of PL from the NMS wells increased from zero to 20∼30% after pulse excitation. Raise time of the polarization degree was discussed on the basis of the difference of distance between the NMS well and the DMS well. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence studies of Zeeman effect in type-II InSb/InAs nanostructures

Electron spin polarization up to 100$%$ has been observed in type-II narrow-gap heterostructures with ultrathin InSb insertions in an InAs matrix via investigation of circularly polarized photoluminescence in an external magnetic field applied in Faraday geometry. The polarization degree decreases drastically, changes its sign, and saturates finally at the value of 10$%$ in the limit of either high temperature or strong excitation. The observed effect is explained in terms of strong Zeeman splitting of the electron conduction band in the InAs matrix and a heavy-hole state confined in the InSb insertion, due to a large intrinsic $g$ factor of both types of carriers. The hole ground state in a monolayer-scale InSb/InAs quantum well, calculated using a tight-binding approach, fits well the observed emission wavelength. Temperature dependence of the emission polarization degree is in good agreement with its theoretical estimation performed in the framework of a proposed phenomenological model.

Compensation of dipolar-exciton spin splitting in magnetic field

Magnetoluminescence of spatially indirect dipolar excitons in 25nm GaAs/AlGaAs single quantum well collected within a lateral potential trap has been studied in Faraday geometry. The paramagnetic spin splitting of the luminescence line of the heavy-hole excitons in the trap centre is completely compensated at magnetic field below critical value ≈2Т. The effect of spin splitting compensation is caused by the exchange interaction in dense exciton Bose gas which is in qualitative agreement with the existing theoretical concepts.

Resonant spin amplification of resident electrons in CdTe/(Cd,Mg)Te quantum wells subject to tilted magnetic fields

Electron spin coherence in CdTe/(Cd,Mg)Te quantum wells is studied experimentally and theoretically in tilted external magnetic fields generated by a superconducting vector magnet. The long-lived spin coherence is measured by pump-probe Kerr rotation in the resonant spin amplification (RSA) regime. The shape of RSA signals is very sensitive to weak magnetic field components deviating from the Voigt or Faraday geometries.

Magneto-optical studies of ensembles of semimagnetic self-organized Cd(Mn)Se/Zn(Mn)Se quantum dots

Ensembles of Cd(Mn)Se/Zn(Mn)Se semimagnetic self-organized quantum dots with different Mn content have been studied by photoluminescence and Raman scattering under strong magnetic fields in Faraday and Voigt geometries and with selective excitation. Electron spin-flip transitions have been observed in Voigt geometry in the structures with large Mn content, characterized by a large magnetic shift of photoluminescence bands. Narrow exciton peaks completely ${\ensuremath{\sigma}}^{\ensuremath{-}}{\ensuremath{\sigma}}^{+}$ polarized have been observed under selective excitation in the Faraday geometry in the structures with medium and small Mn content. A number of effects due to s,p-d exchange interaction and exciton magnetic polaron formation manifested themselves in the structures with the smallest Mn content, where no Zeeman shift of the photoluminescence bands was observed.

Publisher's Note: Landau level spectroscopy of surface states in the topological insulator Bi0.91Sb0.09via magneto-optics [Phys. Rev. B85, 195440 (2012)

We have performed broad-band zero-field and magneto-infrared spectroscopy of the three dimensional topological insulator Bi$_{0.91}$Sb$_{0.09}$. The zero-field results allow us to measure the value of the direct band gap between the conducting $L_a$ and valence $L_s$ bands. Under applied field in the Faraday geometry (\emph{k} $||$ \emph{H} $||$ C1), we measured the presence of a multitude of Landau level (LL) transitions, all with frequency dependence $\omega \propto \sqrt{H}$. We discuss the ramification of this observation for the surface and bulk properties of topological insulators.

Landau level spectroscopy of surface states in the topological insulator Bi0.91Sb0.09via magneto-optics

We have performed broadband zero-field and magneto-infrared spectroscopy of the three dimensional topological insulator Bi${}_{0.91}$Sb${}_{0.09}$. The zero-field results allow us to measure the value of the direct band gap between the conducting ${L}_{a}$ and valence ${L}_{s}$ bands. Under applied field in the Faraday geometry ($k\ensuremath{\parallel}H\ensuremath{\parallel}\text{C1}$), we measured the presence of a multitude of Landau level (LL) transitions, all with frequency dependence $\ensuremath{\omega}\ensuremath{\propto}\sqrt{H}$. We discuss the ramification of this observation for the surface and bulk properties of topological insulators.

Hyperfine interaction mediated exciton spin relaxation in (In,Ga)As quantum dots

The population dynamics of dark and bright excitons in (In,Ga)As/GaAs quantum dots is studied by two-color pump-probe spectroscopy in an external magnetic field. With the field applied in Faraday geometry and at $T<20$ K, the dark excitons decay on a ten nanoseconds time scale unless the magnetic field induces a resonance with a bright exciton state. At these crossings their effective lifetime is drastically shortened due to spin flips of either electron or hole by which the dark excitons are converted into bright ones. Due to the quasielastic character we attribute the origin of these flips to the hyperfine interaction with the lattice nuclei. We compare the exciton spin relaxation times in the two resonances and find that the spin flip involving an electron is approximately 25 times faster than the one of the hole. A temperature increase leads to a considerable, nonmonotonic decrease of the dark exciton lifetime. Here phonon-mediated spin flips due to the spin-orbit interaction gradually become more important.

Quadrupolar spectra of nuclear spins in strained InxGa1−xAs quantum dots

Self-assembled quantum dots (QDs) are born out of lattice mismatched ingredients where strain plays an indispensable role. Through the electric quadrupolar coupling, strain affects the magnetic environment as seen by the nuclear spins. To guide prospective single-QD nuclear magnetic resonance (NMR) as well as dynamic nuclear spin polarization experiments, an atomistic insight to the strain and quadrupolar field distributions is presented. A number of implications of the structural and compositional profile of the QD have been identified. A high aspect ratio of the QD geometry enhances the quadrupolar interaction. The inclined interfaces introduce biaxiality and the tilting of the major quadrupolar principal axis away from the growth axis; the alloy mixing of gallium into the QD enhances both of these features while reducing the quadrupolar energy. Regarding the NMR spectra, both Faraday and Voigt geometries are investigated, unraveling in the first place the extend of inhomogeneous broadening and the appearance of the normally-forbidden transitions. Moreover, it is shown that from the main extend of the NMR spectra the alloy mole fraction of a single QD can be inferred. By means of the element-resolved NMR intensities it is found that In nuclei has a factor of five dominance over those of As. In the presence of an external magnetic field, the borderlines between the quadrupolar and Zeeman regimes are extracted as 1.5 T for In and 1.1 T for As nuclei. At these values the nuclear spin depolarization rates of the respective nuclei get maximized due to the noncollinear secular hyperfine interaction with a resident electron in the QD.

Nonreciprocal Anderson localization in magneto-optical random structures

We study both analytically and numerically disorder-induced localization of light in random-layered structures with magneto-optical materials. The Anderson localization in such structures demonstrates nonreciprocal features in the averaged localization length and individual transmission resonances. We employ the short-wavelength approximation where the localization effects are strong and consider both the Faraday and Voigt magneto-optical geometries. In the Faraday geometry, the transmission is strongly nonreciprocal for the circularly polarized waves, whereas in the Voigt geometry, the nonreciprocity is much weaker and it may appear only for the individual transmission resonances of the TM-polarized waves.