We present theoretical aspects of spin polarized two dimensional electron gas (SP2DEG) which can be achieved in doped semimagnetic quantum wells. This original model system has been recently studied by magneto Raman scattering experiments has given a new access to spin resolved excitations and spectrum of the SP2DEG. Starting from the Diluted Magnetic Semiconductor (DMS) Hamiltonian in presence of the Coulomb interaction between conduction electrons, we define the conditions to reach such a SP2DEG. The equilibrium state is studied at low temperature; in particular a theory for the degree of spin polarization is derived. Dynamical spin susceptibilities are further calculated in the framework of a spin density functional formalism already developed in the past. We then derive spin conserving and spin flip excitations dispersions using a recent determination of the SP2DEG correlation energy corrected from the thickness of the well. The SP2DEG presents two key features: the spin flip wave, which existence is a direct consequence of the Coulomb interaction between the spin polarized electrons, with a dispersion and energy range typical to the SP2DEG obtained in DMS, the spin density fluctuations exhibiting a specific collective behaviour when the spin polarization is increased. The dissipation spectrum through these excitations is studied in detail. Particular attention is given to the spectrum determined by resonant Raman scattering. We show, indeed, that the latter gives unique access to the spin-fluctuations spectrum of the SP2DEG.
Owing to their integer spin, exciton-polaritons in microcavities can be used for observation of non-equilibrium Bose-Einstein condensation in solid state. However, spin-related phenomena of such condensates are difficult to explore due to the relatively small Zeeman effect of standard semiconductor microcavity systems and the strong tendency to sustain an equal population of two spin components, which precludes the observation of condensates with a well defined spin projection along the axis of the system. The enhancement of the Zeeman splitting can be achieved by introducing magnetic ions to …
Orthorhombic tungsten ditelluride (WTe2), with a distorted 1T structure, exhibits a large magnetoresistance that depends on the orientation, and its electrical characteristics changes rom semimetallic to insulating as the thickness decreases. Through polarized Raman spectroscopy in combination with transmission electron diffraction, we establish a reliable method to determine the thickness and crystallographic orientation of few-layer WTe2. The Raman spectrum shows a pronounced dependence on the polarization of the excitation laser. We found that the separation between two Raman peaks at ~90 c…
Polarized Raman spectroscopy on few-layer ReS2 and ReSe2 was carried out to determine the crystallographic orientations. Since monolayer ReX2 (X=S or Se) has a distorted trigonal structure with only an inversion center, there is in-plane anisotropy and the two faces of a monolayer crystal are not equivalent. Since many physical properties vary sensitively depending on the crystallographic orientation, it is important to develop a reliable method to determine the crystal axes of ReX2. By comparing the relative polarization dependences of some representative Raman modes measured with three diffe…
Few-layer tungsten diselenide (WSe2) is investigated using circularly polarized Raman spectroscopy with up to eight excitation energies. The main E2g1 and A1g modes near 250 cm-1 appear as a single peak in the Raman spectrum taken without consideration of polarization but are resolved by using circularly polarized Raman scattering. The resonance behaviors of the E2g1 and A1g modes are examined. Firstly, both the E2g1 and A1g modes are enhanced near resonances with the exciton states. The A1g mode exhibits Davydov splitting for trilayers or thicker near some of the exciton resonances. The low-f…
We are proposing a novel method of laser heating control only through change in polarization of the incident light, keeping its power density constant. The idea combines antenna effect found in isolated multi-walled carbon nanotubes and the possibility of their heating by light illumination. To observe this we used Raman spectroscopy technique, where the heating manifests itself in a pronounced downshift of the Raman G and 2D lines as a function of the polarization angle. Our method can be useful in field electron emission devices or in selective nanotubes heating and destruction. It can also …
From our theoretical studies of resonant Raman transitions in two-electron quantum dots (artificial helium atoms) we show that in this system, the singlet-triplet Raman transitions are allowed (in polarized configuration) only in the presence of spin-orbit interactions. With an increase of the applied magnetic field this transition dominates over the singlet-singlet and triplet-triplet transitions. This intriguing effect can therefore be utilized to tune Raman transitions as well as the spin-orbit coupling in few-electron quantum dots.
BiFeO$_3$ thin films can be epitaxially stabilized in a nearly-tetragonal phase under a high biaxial compressive strain. Here we investigate the polarized Raman spectra of constrained BiFeO$_3$ films with tetragonal-like (BFO-T), rhombohedral-like (BFO-R) and multiphase (BFO-T+R) structure. Based on analysis of the number and symmetry of the Raman lines, we provide strong experimental evidence that the nearly-tetragonal films are monoclinic ($Cc$ symmetry) and not tetragonal $(P4mm)$. Through the Raman mapping technique we show localized coexistence of BFO-T and BFO-R phases with the relative …
We investigated polarization dependence of the Raman modes in black phosphorus (BP) using five different excitation wavelengths. The crystallographic orientation was determined by comparing polarized optical microscopy with high-resolution transmission electron microscope analysis. In polarized Raman spectroscopy, the B2g mode shows the same polarization dependence regardless of the excitation wavelength or the sample thickness. On the other hand, the Ag1 and Ag2 modes show a peculiar polarization behavior that depends on the excitation wavelength and the sample thickness. The thickness depend…
The resonant Raman scattering of a quantum wire in a strong magnetic field is studied, focused on the effect of long range Coulomb interaction and the spin-charge separation. The energy-momentum dispersions of charge and spin excitation obtained from Raman cross-section show the characteristc cross-over behaviour induced by inter-edge Coulomb interaction. The "SPE" peak near resonance in polarized spectra becomes broad due to the momentum dependence of charge velocity. The broad peak in the depolarized spectra is shown to originate from the disparity between charge and spin excitation velocity…
Under homogeneous uniaxial strains, the Raman 2D band of graphene involving two-phonon double-resonance scattering processes splits into two peaks and they altogether redshift strongly depending on the direction and magnitude of the strain. Through polarized micro- Raman measurements and first-principles calculations, the effects are shown to originate from significant changes in resonant conditions owing to both the distorted Dirac cones and anisotropic modifications of phonon dispersion under uniaxial strains. Quantitative agreements between the calculation and experiment enable us to determ…
Group III nitride based two-dimensional multi-quantum well (MQW) nanostructures find remarkable applications in the visible to ultraviolet light sources. The interface optical (IFO) phonon modes in a c-axis oriented superlattice of [Al0.35Ga0.65N (~1.75 nm)/Al0.55Ga0.45N (~2nm)]20 MQWs are observed using tip-enhanced Raman spectroscopic (TERS) studies. The near-field studies using TERS probe with an Au spherical nanoparticle of ~ 200 nm diameter were carried out at ambient conditions showing approximately two to three orders of enhancement in the Raman intensities. The interface phonon mode be…
An optical characterization tool of Raman spectroscopy with extremely weak scattering cross section tool is not popular to analyze scattered signal from a single nanostructure in the sub-diffraction regime. In this regard, plasmonic assisted characterization tools are only relevant in spectroscopic studies of nanoscale object in the sub-diffraction limit. We have reported polarized resonance Raman spectroscopic (RRS) studies with strong electron-phonon coupling to understand the crystalline orientation of a single AlGaN nanowire of diameter about 100 nm. AlGaN nanowire is grown by chemical vap…
The group IV-VI compound SnSe, with an orthorhombic lattice structure, has recently attracted particular interest due to its unexpectedly low thermal conductivity and high power factor, showing great promise for thermoelectric applications. SnSe displays intriguing anisotropic properties due to the puckered low-symmetry in-plane lattice structure. Low-dimensional materials have potential advantages in improving the efficiency of thermoelectric conversion, due to the increased power factor and decreased thermal conductivity. A complete study of the optical and electrical anisotropies of SnSe na…
A theoretical description of electronic Raman scattering from GaAs/Al_{x}Ga_{1-x}As artificial atoms under the influence of an external magnetic field is presented. Raman spectra with laser excitation energy in the interval E_{gap}-30 meV to E_{gap} are computed in the polarized and depolarized geometry. The polarization ratios for the collective and single-particle excitations indicate a breakdown of the Raman polarization selection rules once the magnetic field is switched on. A Raman intensity jump rule at the band gap is predicted in our calculations. This rule can be a useful tool for ide…
The class of Ruddlesden-Popper type (PEA)$_2$PbI$_4$ perovskites comprises two-dimensional (2D) structures whose optical properties are determined by excitons with a large binding energy of about 260 meV. It complements the family of other 2D semiconductor materials by having the band structure typical for lead halide perovskites, that can be considered as inverted compared to conventional III-V and II-VI semiconductors. Accordingly, novel spin phenomena can be expected for them. Spin-flip Raman scattering is used here to measure the Zeeman splitting of electrons and holes in a magnetic field …
The experimental Raman spectra of graphene exhibit a few intense two-phonon bands, which are enhanced through double-resonant scattering processes. Though there are many theoretical papers on this topic, none of them predicts the spectra within a single model. Here, we present results for the two-phonon Raman spectra of graphene calculated by means of the quantum perturbation theory. The electron and phonon dispersions, electronic lifetime, electron-photon and electron-phonon matrix elements, are all obtained within a density-functional-theory-based non-orthogonal tight-binding model. We study…
Starting from the Luttinger model for the band structure of GaAs, we derive an effective theory that describes the coupling of the fractional quantum Hall (FQH) system with photons in resonant Raman scattering experiments. Our theory is applicable in the regime when the energy of the photons $ω_0$ is close to the energy gap $E_G$, but $|ω_0-E_G|$ is much larger than the energy scales of the quantum Hall problem. In the literature, it is often assumed that Raman scattering measures the dynamic structure factor $S(ω,\mathbf{k})$ of the FQH. However, in this paper, we find that the light scatteri…
A consistent semiquantitative theoretical analysis of electronic Raman scattering from many-electron quantum dots under resonance excitation conditions has been performed. The theory is based on random-phase-approximation-like wave functions, with the Coulomb interactions treated exactly, and hole valence-band mixing accounted for within the Kohn-Luttinger Hamiltonian framework. The widths of intermediate and final states in the scattering process, although treated phenomenologically, play a significant role in the calculations, particularly for well above band gap excitation. The calculated p…
The selection rule for angle-resolved polarized Raman (ARPR) intensity of phonons from standard group-theoretical method in isotropic materials would break down in anisotropic layered materials (ALMs) due to birefringence and linear dichroism effects. The two effects result in depth-dependent polarization and intensity of incident laser and scattered signal inside ALMs and thus make a challenge to predict ARPR intensity at any laser incidence direction. Herein, taking in-plane anisotropic black phosphorus as a prototype, we developed a so-called birefringence-linear-dichroism (BLD) model to qu…
We present optical nanoantennas designed for applications that require processing temperatures larger than 800°C. The antennas consist of arrays of Re/Pt bilayer strips fabricated with a lift-off-free technique on top of etched trenches. Reflectance measurements show a clear plasmonic resonance at approximately 670 nm for light polarized orthogonal to the strip axis. The functionality of the antennas is demonstrated by growing single-walled carbon nanotubes (CNTs) on top of the antenna arrays and measuring the corresponding Raman signal enhancement of individual CNTs. The results of the measur…
We show that the low-energy peak in the polarized resonant Raman spectra of quantum wires, which is commonly associated with ``single particle excitations'', can be interpreted as signature of intra-band collective spin excitations. A broad maximum in the resonant depolarized spectra is predicted to exist above the frequency of the spin density excitation, due to simultaneous but independent propagation of spin- and charge-density modes.
Plasmon-enhanced Raman scattering (PERS) becomes nonlinear when phonon pumping and phonon-stimulated scattering come into play. It is fundamental to the understanding of PERS and its photobleaching behavior. By quantization of the molecular vibration coherent state into phonon number states, we theoretically predict a stepwise dependence of PERS intensity on laser power. Experimental evidence is presented by measuring a monolayer of malachite green isothiocyanate molecules sandwiched in individual gold nanosphere-plane antennas, under radially polarized laser excitation of sub-microWatt powers…
We use polarized magneto-Raman scattering to study purely electronic excitations and the electron-phonon coupling in bulk graphite. At a temperature of 4.2 K and in magnetic fields up to 28 T we observe $K$-point electronic excitations involving Landau bands with $Δ|n|=0$ and with $Δ|n|=\pm2$ that can be selected by controlling the angular momentum of the excitation laser and of the scattered light. The magneto-phonon effect involving the $E_{2g}$ optical phonon and $K$-point inter Landau bands electronic excitations with $Δ|n|=\pm1$ is revealed and analyzed within a model taking into account …
Excitation of four coherent phonon modes of different symmetries has been realized in copper metaborate CuB$_2$O$_4$ via impulsive stimulated Raman scattering (ISRS). Phonons were detected by monitoring changes in the linear optical birefringence using the balanced-detection (BD) technique. We compare the results of BD-ISRS experiment to the polarized spontaneous Raman scattering spectra. We show that the agreement between the two sets of data obtained by these allied techniques in a wide phonon frequencies range of 4-14 THz can be achieved by rigorously taking into account the symmetry of the…
