Zone edge phonons in CdS1−xSex

Zone edge phonons of mixed CdS_1?xSe_x have been studied by mean of infrared absorption and Raman scattering techniques. In the A point of the Brillouin zone, it has been shown that transverse acoustical phonons have a one mode behaviour, and that optical phonons have a two modes behaviour. CdS and CdSe zone center phonons can combined and give a LO(CdS) + LO(CdSe) Raman peak in addition to the 2LO(CdS) and 2LO(CdSe) peaks; this is not the case for phonons from the edge of the Brillouin-zone where no CdS + CdSe combination can take place.     

Pressure dependence of the impurity soft modes in Li and Na : KTaO3

Low energy Raman scattering measurements on samples of K_0.76Na_0.24TaO₃, K_0.9Na_0.1TaO₃ and K_0.75Li_0.25TaO₃ under hydrostatic pressures of up to 60 kbar supply confirming evidence to the hypothesis that the Na⁺ ions in these crystals are displaced from the centers of the sites they occupy. The pressure Raman experiments also reveal another second order Raman line interpreted as Raman scattering by two TA phonons at M. A line induced by the substitution of Na for K is found to have an energy about equal to the energy of this new second order Raman line. It is shown that if this is a resonant mode it must be an even mode.     

Exciton-phonon interaction in mixed silver halides: Resonant Raman scattering vs photoluminescence

An investigation of resonant Raman scattering in mixed crystals of AgBr:Cl at 1.8 K shows that the zero-phonon and LO phonon-assisted exciton luminescence excited in the free indirect exciton absorption, exhibits an anomalous dependence on the exciton photon energy E_L. Close to the exciton gap, the bands show a Raman-like behaviour with their peaks at constant energetic distance from E_L. As E_L is tuned further into the absorption, the bands gradually develop into normal photoluminescence. The effect is explained by taking into account exciton relaxation via scattering by long-wavelength acoustic phonons, a process which is strongly energy dependent. In addition, resonant Raman scattering observed for excitation in the zero-phonon absorption suggests study for the first time of the mode behaviour of certain off-zone center phonons in this system.     

Vibrational spectra of a GaS single crystal

The Raman and infrared active long wavelength phonons of a GaS single crystal were studied at different temperatures in the 10–600 cm^?1 range. Properly polarized Raman spectra could be obtained with the 4880 Å exciting line and the previous assignment of the E_1g modes controversed recently could be confirmed. Infrared spectra were recorded in the 30–600 cm^?1 region. The vibrational frequencies of the crystal were also calculated using a method developed by Wieting and six new frequencies corresponding to infrared and Raman inactive modes have been proposed.We have observed that the degree of leakage of scattered intensity in unallowed polarizations increases when the wavelength of the exciting line moves off the exciton absorption front. The phonon at 74 cm^?1 was particularly sensitive and the question of the antiresonant behaviour of this compound is raised.     

Surface enhanced Raman scattering by CdS quantum dots

Surface enhanced Raman scattering is studied in nanostructures with CdS quantum dots formed using the Langmuir-Blodgett technology. Features due to quantum dot longitudinal optical phonons are observed in the Raman spectra of both free CdS quantum dots and such dots distributed in an organic matrix. The surface enhanced Raman scattering by nanostructures with CdS quantum dots covered by an Ag cluster film is observed experimentally. Applying Ag clusters onto the nanostructure surfaces results in a sharp (40-fold) increase in the intensity of Raman scattering by optical phonons in the quantum dots. It is shown that the dependence of surface enhanced Raman scattering on the excitation energy is resonant with a maximum at the energy corresponding to the maximum absorption coefficient of Ag clusters.     

Raman scattering in low-dimensional HfTe5

Nine long-wavelength phonons have been observed by Raman scattering in the quasi-one-dimensional crystal HfTe₅. The assignment of these phonon modes is made on the basis of symmetry arguments using the correlation method. The similarity between the HfTe₅ and the ZrTe₃ chain-like crystal structures instigates the attribution of the most characteristic modes like the high-frequency chalcogen diatomic-mode and the low frequency rigid-chain mode. Although no new line appears even at 14 K, the irregular temperature dependence of the spectra suggests the possibility of a structural instability connected with a large peak in the electrical resistivity and with the X-ray diffraction anomalies.     

Acoustical phonon assisted two-photon excitation of excitonic molecules and resonant raman scattering in evaporated CuCl films

In evaporated CuCl films, Chase et al. have concluded the Bose-Einstein condensation of excitonic molecules for the origin of a sharp emission line which has been ascribed so far to the resonant Raman line in single crystals. A trace of the similar works on evaporated samples confirms that the sharp line shifts its energy with the change of the excitation photon energy; indicating the Raman character. When the excitation photon energy exceeds the resonant energy the Raman line shows broadening, and a new emission band appears which can be ascribed to the radiative annihilation of excitonic molecules generated by the two-photon absorption and one acoustical phonon emission.     

激光诱导多孔硅晶格畸变的Raman光谱和光致发光谱研究

研究了掺钛水热法制备多孔硅的Raman光谱和光致发光谱.实验发现,当激光功率较低时,多孔硅的Raman光谱在略低于520cm^-1附近表现为一锐的单峰,和晶体硅的Raman光谱类似.随激光功率增大,该单峰向低波数移动,Raman和光致发光峰的强度与激光强度的一次方成正比.当激光功率增大到一定值时,该单峰分裂成两个Raman峰,光致发光谱的强度突然增大,与激光强度之间不再满足一次方的关系,位于低波数一侧的Raman峰随激光功率增大进一步向低波数移动.多孔硅Raman光谱随激光功率的变化是 关键词： 多孔硅 Raman光谱 光致发光     

Electric field modulated Raman scattering in CdS

We have observed electric field modulated Raman scattering by A₁ LO phonons in CdS. The field induced scattering is observed with a geometry in which Raman scattering by A₁ LO phonons is normally allowed. The interference of the field induced and allowed terms in the transition susceptibility leads to a modulated Raman scattering intensity proportional to the applied field. This is contrasted with data previously reported on field induced Raman scattering by E₁ LO phonons in a configuration in which the Raman scattering is normally forbidden and in which there is no interference between linear wavevector dependent and field induced terms in the transition susceptibility. Electric field effects on Raman scattering by TO phonons and by 2 LO phonons is also discussed.     

Phonon anomalies and structural transition in spin ice Dy2Ti2O7: a simultaneous pressure‐dependent and temperature‐dependent Raman study

We revisit the assignment of Raman phonons of rare‐earth titanates by performing Raman measurements on single crystals of O¹⁸ isotope‐rich spin ice and nonmagnetic pyrochlores and compare the results with their O¹⁶ counterparts. We show that the low‐wavenumber Raman modes below 250 cm⁻¹ are not due to oxygen vibrations. A mode near 200 cm⁻¹, commonly assigned as F_2g phonon, which shows highly anomalous temperature dependence, is now assigned to a disorder‐induced Raman active mode involving Ti⁴⁺ vibrations. Moreover, we address here the origin of the ‘new’ Raman mode, observed below T_C ~ 110 K in Dy₂Ti₂O₇, through a simultaneous pressure‐dependent and temperature‐dependent Raman study. Our study confirms the ‘new’ mode to be a phonon mode. We find that dT_C/dP = + 5.9 K/GPa. Temperature dependence of other phonons has also been studied at various pressures up to ~8 GPa. We find that pressure suppresses the anomalous temperature dependence. The role of the inherent vacant sites present in the pyrochlore structure in the anomalous temperature dependence is also discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Spin-flip Raman laser at magnetic fields up to 14 T

We extended the range of operation of the CO₂ laser pumped spin-flip Raman laser in n-InSb up to magnetic fields of 14 T. The laser at high fields is strongly influenced by optical phonons which interact resonantly with the electron spin system, and by intraband absorption of the Raman scattered light in the scattering medium.     

Raman scattering from crystalline and amorphous (GaSb)1-xGex semiconducting films

The first reported Raman scattering experiments have been performed on single crystal and amorphous films of the metastable alloy (GaSb)_1-xGe_x with compositions across the pseudobinary phase diagram. In crystalline films, the optical phonons exhibit a “one-two” type mode behavior. Broadening and softening of the Raman peaks with increasing alloy concentrations are attributed to a relaxation of q-vector selection rules due to substitutional disorder on both the cation and anion sublattices. Additionally, disorder induced scattering from the zone-edge acoustic phonons was observed. In amorphous alloy films, the reduced Raman spectra were compared to the one-phonon densities of states of the end-member crystals. The resulting apparent lack of polarization dependence indicated that the amorphous films were of the random network type.     

A high pressure raman study of ThO2 to 40 GPa and pressure-induced phase transition from fluorite structure

The pressure dependence of the first-order Raman peak and two second-order Raman features of ThO₂ crystallizing in the fluorite-type structure is investigated using a diamond anvil cell, up to 40GPa. A phase transition from the fluorite phase is observed near 30 GPa as evidenced by the appearance of seven new Raman peaks. The high pressure phases of ThO₂ and CeO₂ exhibit similar Raman features and from this it is believed that the two structures are the same, and have the PbCl₂-type structure. The pressure dependence dω/dP of the observed phonons and their mode Grüneisen parameters are similar to the isostructural CeO₂. The observed second-order Raman features are also identified from the calculated phonon dispersion curves for ThO₂.     

Raman scattering from GaSe1−xTex

The long-wavelength optical phonons of the layer GaSe_1?xTe_x have been investigated at room temperature by means of Raman scattering spectroscopy. The spectra of the Bridgman grown crystals were excited with the 1,06 μm line of the continuously operated YAG:Nd³⁺ laser. Detailed study of the Raman spectra of GaSe_1?xTe_x solid solutions showed that there is an abrupt change in the frequency-composition dependences for all observed modes. It is shown, that a phase transition from hexagonal ?-GaSe to monoclinic GaTe in GaSe_1?xTe_x solid solutions takes place in the composition range 0.27 ? × ? 0.72. Only one mode behaviour of the optical phonons was observed in GaSe_1?xTe_x system.     

Thermal Behavior of the Fluorescence Lines of Ho3+ in Cr,Tm,Ho:YAG

The fluorescence spectra of Cr,Tm,Ho:YAG crystal have been detected at the temperature from 65 to 295 K with interval of 10 K. The width and the shift of Ho3+ lines at 4959.6 cm-1 (at 65 K) have been investigated and interpreted in terms of the interactions between lattice vibration (phonons) and ions. The results showed that the line broadening with the temperature was mainly due to the Raman scattering of phonons,and the line shift with temperature was mainly due to the emission and absorption of virtual phonons.     

Correlation of size and oxygen bonding at the interface of Si nanocrystal in Si–SiO2 nanocomposite: A Raman mapping study

Raman spectroscopy/mapping is used to investigate the variation of Si phonon wavenumbers, i.e., lower wavenumber (LW ~ 495–510 cm⁻¹) and higher wavenumber (HW ~ 515–519 cm⁻¹) phonons, observed in Si–SiO₂ multilayer nanocomposite (NCp) grown using pulsed laser deposition. Sensitivity of Raman spectroscopy as a local probe to surface/interface is effectively used to show that LW and HW phonons originate at surface (Si–SiO₂ interface) and core of Si nanocrystals, respectively. The consistent picture of this understanding is developed using Raman spectroscopy monitored laser heating/annealing and cooling experiment at the site of the desired wavenumber, chosen with the help of Raman mapping. Raman spectra calculations for Si₄₁ cluster with oxygen and hydrogen termination show strong mode at 512 cm⁻¹ for oxygen terminated cluster corresponding to the vibration of surface Si atoms. This supports our attribution of LW phonons to be originating at the Si–SiO₂ surface/interface. These results along with XPS show that nature of interface (oxygen bonding) in turn depends on the size of nanocrystals and LW phonons originate at the surface of smaller Si nanocrystals. The understanding developed can conclude the ongoing debate on large variation in Si phonon wavenumbers of Si–SiO₂ NCps in the literature. Copyright © 2015 John Wiley & Sons, Ltd.     

Optical investigation of coherent and thermal phonons in high-T c superconductors

Low-energy phonons in NdBa₂Cu₃O_7?x and Bi₂Sr₂CaCu₂O_8+x superconducting single crystals have been studied by Raman scattering and reflectivity measurements with femtosecond-scale resolution. Raman scattering provides information on equilibrium thermal phonons, whereas information about a coherent state of the phonon system is obtained by measuring in the time domain when the phonon system is pumped by an ultrashort pulse and subsequently probed by a properly gated second pulse. It is shown that both methods yield similar results for the phonon mode energies, while the energy relaxation and dephasing rates exhibit a tendency to disagreement.     

Pressure and temperature dependence of Raman scattering and optical absorption in crystalline S4N4

Raman scattering and optical absorption in crystalline S₄N₄ have been measured both as a function of pressure at 295 K and low temperatures. Polarized single crystal Raman data were also obtained as an aid in the assignment of the Raman active phonons. The pressure coefficients of the Raman active external and S-S stretching modes show a discontinuity near 7 kbar indicative of a second order phase change. The optical absorption edge at about 2.5 eV of a sublimed film of S₄N₄ shows red shifts of 1.3 × 10^?5 eV bar^?1 and 6.3 × 10^?4 eV K^?1 with pressure and temperature respectively. In the light of these results, the electronic, vibrational and structural properties of the crystal are discussed.     

Infrared absorption by zone edge lattice vibrations in mixed gallium indium phosphide

The combinations of zone edge phonons of mixed Ga_1?xIn_xP have been studied by means of infrared absorption, these crystals exhibit a one mode behavior for both the zone center and the zone edge phonons.     

Raman spectra of IVB and VIB transition metal disulfides using laser energies near the absorption edges

In this paper, we present Raman spectra of ZrS₂, HfS₂, MoS₂ and WS₂ using laser energies near the energies of the absorption edges. The Raman spectra probe the properties of the first-excited electronic state and the nature of the electron-phonon coupling. The spectra of the IVB disulfides are independent of the laser excitation energy, suggesting weak electron-phonon interaction. In contrast, additional Raman bands appear in the spectra of the VIB disulfides as the laser energy approaches the band gap energy. The new modes in the spectra of MoS₂ and WS₂ cannot be assigned as first-order processes nor as combination bands of the phonons with zero momentum. The resonance Raman scattering of MoS₂ is analyzed in terms of second-order scattering due to the coupling of phonon modes of nonzero momentum with an electronic transition associated with excitonic states.