Resonant raman scattering at the E1 energy gap of semiconductor crystals

We present a discussion of resonant Raman scattering by optical phonons at the E₁ energy gap of group IV and groups III–V compound semiconductor crystals (e.g., Ge and InSb). For allowed scattering by TO and LO phonons, the q-dependent “double resonant” two-band calculation of the Raman tensor may display destructive interference effects when the intermediate electron-hole pairs are uncorrelated. We also discuss the Franz-Keldysh mechanism of resonant electric field induced Raman scattering by LO phonons. The double resonance terms due to this mechanism will, for large electric fields, broaden and have its largest resonance enhancement at the energy gap.     

Wavelength and concentration dependence of Raman scattering from CdS1−xSex

We present measurement of the resonant Raman effect in the alloy system CdS_1?xSe_x for laser photon energies both above and below the concentration-dependent energy gap. Our data show that the Raman intensities are not simply functions of the difference between the photon energy and the gap energy. These results may be partially understood in terms of a simple theoretical model which elucidates the role of local electric fields in determining Raman intensities in alloys.     

Heavy and light hole subbands exchange induced mixing in CdTe-(Cd,Mn)Te magnetic superlattices

We present the first calculation of the electronic structure of hole subbands in a semimagnetic superlattice in the presence of an applied magnetic field, taking into account the existence of a preferential axis of quantization for the hole spin and the fourfold degeneracy of the top of the valence band. We show that, for fields applied parallel to the layers, there is a strong exchange induced mixing of heavy and light holes subbands for any value of k, the Bloch wave vector parallel to the layers. This mixing dominates over the usual k·p induced mixing and should be observable in resonant tunneling and magnetotransport measurements on p-type samples.     

Resonant Inelastic X-ray Scattering: From band mapping to inter-orbital excitations

Resonant inelastic X-ray scattering (also known as resonant X-ray Raman spectroscopy when only valence and conduction states are involved in the final state excitation) has developed into a major tool for understanding the electronic properties of complex materials. Presently it provides access to electron excitations in the few hundred meV range with element and bulk selectivity. Recent progress in X-ray optics and synchrotron radiation engineering have opened up new perspectives for this powerful technique to improve resolving power and efficiency. We briefly present the basics of the method and illustrate its potential with examples chosen from the literature. To cite this article: J. Lüning, C.F. Hague, C. R. Physique 9 (2008).     

Magnetotunneling spectroscopy of ring-shaped (InGa)As quantum dots: Evidence of excited states with 2pz character

We study the effect of a high magnetic field (B) on the current–voltage characteristics, I(V), of a GaAs/(AlGa)As resonant tunneling diode incorporating a layer of ring-shaped quantum dots (QDs) in the quantum well (QW). The dots give rise to a series of four unusual resonances in I(V) which show a high degree of reproducibility across the epitaxial wafer. By combining data for B parallel and perpendicular to the growth axis z, we identify that the unusual resonances arise from resonant tunneling into QD excited states with 2p_z-like symmetry. The two series of magneto-oscillations in I for Bz allow us to determine the resonant charging and discharging of the QW with varying bias.     

Multiphonon resonant Raman scattering in N‐doped ZnO

Multiphonon resonant Raman scattering in N‐doped ZnO films was studied, and an enhancement of the resonant Raman scattering process as well as longitudinal optical (LO) phonon overtones up to the sixth order were observed at room temperature. The resonant Raman scattering intensity of the 1LO phonon in N‐doped ZnO appears three times as strong as that of undoped ZnO, which mainly arises from the defect‐induced Raman scattering caused by N‐doping. The nature of the 1LO phonon at 578 cm⁻¹ is interpreted as a quasimode with mixed A₁ and E₁ symmetry because of the defects formed in the ZnO lattice. In addition, the previously neglected impurity‐induced two‐LO‐phonon scattering process was clearly observed in N‐doped ZnO. Copyright © 2009 John Wiley & Sons, Ltd.     

Dynamics of F2 centers in LiF: One phonon sideband and Raman scattering

The vibrational structure of the F₂⁺ emission in LiF was investigated, together with the resonant Raman scattering from the first excited electronic state. The one phonon sideband of the emission and the resonant Raman spectrum were found to be very similar, as expected for transitions involving non degenerate electronic states.     

Magnetoabsorption in n-type InSb from 8 to 15 μm

Magnetoabsorption measurements in n-type InSb at T?30K have been made between ～ 8 and 15 μm using magnetic fields up to 150 kOe. The observed absorption peaks are identified as due to combined resonance, harmonics of cyclotron resonance and the corresponding LO phonon-assisted resonances. These resonant absorptions are considered to be important in the interpretation of the observed magnetic field behavior of the threshold and output power of the InSb spin-flip Raman laser pumped with 10.6 μm CO₂ laser.     

Triple‐resonant two‐phonon Raman scattering in graphene

The triple‐resonant (TR) second‐order Raman scattering mechanism in graphene is re‐examined. It is shown that the magnitude of the TR contribution to the photon‐G′ mode coupling function in graphene is one order of magnitude larger than the widely accepted two‐resonant coupling. Enhancement of the order of 100 in the Raman intensity, with respect to the usual double‐resonant model, is found for the G′ band in graphene, and is expected in the related sp²‐based carbon materials, as well. Copyright © 2011 John Wiley & Sons, Ltd.     

Polarization effects of picosecond CARS in liquids

Coherent Raman scattering of delayed probing pulses after ultrafast excitation is investigated under generalized polarization conditions. Three factors are shown to contribute to the scattering signal:

1. scattering off the isotropic part of the resonant material excitation via the isotropic component of the Raman polarizability
2. scattering via the anisotropic part of the Raman polarizability from a second, oriented component of the resonant material excitation
3. four-wave mixing via the non-resonant part χ_NR of the third-order susceptibility. We demonstrate theoretically and experimentally that different polarization conditions lead to drastic changes of the signal transients in liquids. For the ring breathing mode of C₆H₅Br the ratio of non-resonant to resonant contributions is measured to be χ_NR/χ_res=0.037±0.015.

     

Raman spectra of GaAs-AlxGa1-xAs superlattices

Systematic theoretical studies of Raman spectra of GaAs-Al_xGa_1-xAs superlattices are presented. The electronic states are described by an envelope-function method and the phonon modes are described in a microscopic rigid-ion model. Both resonant and nonresonant Raman scattering processes are considered. For resonant Raman scattering, the effects of discrete exciton states plus the continuum and the valence-band mixing are included via a k-space sampling method. Both the Fröhlich and deformation-potential mechanisms for electron-phonon coupling are considered. These two mechanisms are responsible for principal features in the z(x, x)z̄ and z(x, y)z̄ geometries, respectively. We find that the effects of exciton continuum states are quite important and the resonant Raman spectra so obtained are in much better agreement with experiment compared to those without including the exciton continuum states.     

Photoluminescence and multiphonon resonant Raman scattering in Ni-and Co-doped Zn1−x MnxTe crystals

Low-temperature photoluminescence, exciton reflection, and multiphonon resonant Raman scattering spectra of Ni-and Co-doped Zn_1−x Mn_xTe crystals were investigated. Intense emission occurs in a broad spectral region (1100–17 000 cm⁻¹) in the crystals containing Ni atoms. It is caused by intracenter transitions involving Mn²⁺ ions and transitions between the conduction band and a level of the doubly charged acceptor. The features of the exciton photoluminescence and multiphonon resonant Raman scattering involving longitudinal-optical (LO) phonons at various temperatures are investigated. The insignificant efficiency of the localization of excitons on potential fluctuations in the Zn_1−x Mn_xTe:Co crystals is established. A temperature-induced increase in the intensity of the 5LO multiphonon resonant Raman scattering line due to the approach of the conditions for resonance between this line and the ground exciton state is observed in these crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 616–621 (April 1998)     

Surface polariton dispersion measurement by Raman scattering excited in total reflection condition

We report the first measurement of phonon surface polariton (s.p.) dispersion by a non-forward Raman scattering geometry. The Raman scattering from surface modes was excited by the evanescent electromagnetic wave produced by total reflection at the interface between two transparent media. This Raman Scattering excited in condition of Total Reflection, RSTR, has been applied to the NaClO₃-sapphire interface for studying the surface polariton dispersion in the gap between the TO frequency at 966 cm^-1 and the LO frequency at 983 cm^-1 of sodium chlorate. Comparison is made between the experimental and theoretical dispersion curves.     

Cars probing of the Si Raman mode transformation during strong picosecond pulsed-laser irradiation

We report on picosecond coherent anti-Stokes Raman scattering (CARS) probing of the Si optical phonon spectrum transformation during picosecond pulsed-laser irradiation. CARS spectra were obtained in reflection at various laser fluences up to melting. The observed spectrum broadening is theoretically explaned in terms of phonon heating and laser-induced mechanical stress build-up on a time scale of 10ps.     

Wave vector conservation in the first order resonant Raman scattering

The dependence of the shape and the intensity of the first order resonant Raman line 1LO on the direction and magnitude of phonon wave vector has been investigated in CdS crystals. Comparison of RRS in different orientations shows that in the most pure samples the phonon wave vector is determined, to a great extent, by the momentum conservation law. In Ni doped crystals one can observe violation of momentum conservation, resulting in a sharp increase of 1 LO intensity. The shape of the 1 LO line in such sample does not depend on the experimental orientation. For the first time the dependence of the Raman intensity on the scattering angle has been observed by the comparison of forward and backward scattering spectra. Observation of this dependence shows that the free excitons are the dominating intermediate states in the resonant Raman scattering in A₂B₆ compounds.     

Infrared and Raman spectra of GeSe2

Far infrared room temperature reflectivity spectra have been measured with polarized light for single crystal GeSe₂ in the range 40–400 cm^-1 for all three principal directions. Altogether 15 “Reststrahlen” peaks were observed, seven for E∥a, six for E∥b and only two for E∥c. Optical parameters were calculated using both Kramers-Krönig integration and a fitting procedure. A nonpolarized Raman spectrum of GeSe₂ was also measured. 17 Raman active lines were observed. The strongest one was at 213cm^-1.     

Light scattering from magnons in ferromagnetic semiconductors

The effect of the electron-magnon interaction on the Raman scattering by magnons is discussed. It is shown that in addition to Loudon's spin-orbit coupling mechanism there is another, carrier-mediated, contribution to the Raman line. The frequency dependence of the resonant scattering is discussed and application to CdCr₂Se₄ is made.     

Phonon-assisted combined resonance in magneto-photoconductivity of n-InSb

A combined cyclotron-spin resonance assisted by an emission of LO phonon has been observed in the longitudinal and transverse magneto-photoconductivity of n-InSb. The theoretical analysis predicts a resonance in magneto-optical absorption for light polarization e 6 H and a weakly resonant behavior for e ⊥ H. This agrees with the observations.     

Photoluminescence and resonant Raman scattering in N-doped ZnO thin films

A combination of studies on photoluminescence and resonant Raman scattering in N-doped ZnO thin films were carried out at room temperature. In the photoluminescence spectra, a transformation of radiative recombination mechanism from free-exciton to donor-acceptor-pair transition was observed. An enhancement of resonant Raman scattering processes as well as longitudinal optical (LO) phonon overtones up to the sixth order were observed in the Raman spectra. Also, the nature of the 1LO phonon underwent a transformation from a pure A₁(LO) mode to a quasimode with mixed A₁ and E₁ symmetry. The underlying mechanisms accounting for the influences of N doping on the optical properties of ZnO were related to the incorporation of extrinsic defects in the crystal lattice.