Showing papers by "Y. R. Shen published in 1974"

Study of pretransitional behavior of laser-field-induced molecular alignment in isotropic nematic substances

Abstract: We have measured the optical Kerr effect and the intensity-dependent ellipse rotation in order to study the pretransitional behavior of field-induced molecular alignment in the isotropic nematic substances $p$-methoxy-benzylidene $p\ensuremath{-}n$-butylaniline (MBBA) and $p$-ethoxy-benzylidene-$p$-butylaniline (EBBA). The results agree well with predictions of the Landau---de Gennes model. Both the order-parameter relaxation time and the steady-state field-induced birefringence show critical divergence towards the isotropic \ensuremath{\rightarrow} nematic transition with a ${(T\ensuremath{-}{T}^{*})}^{\ensuremath{-}1}$ temperature dependence. In the case of MBBA, our results are also consistent with the results from light scattering, but the method we use is perhaps more straightforward and accurate. The nonlinear refractive indices and other relevant parameters of the materials are derived from the experiment.

Study of Dispersive Raman Modes in Cu 2 O by Resonant Raman Scattering

Abstract: We have observed shifts in energy of a number of Raman modes of Cu2o as a function of incident photon energy. We explain such energy shifts as due to dispersion in the phonons involved and obtain quantitative agreement between theory and experiment. From our data we have also determined the effective mass of the ls yellow exciton in Cu2o to be (3.0 + 0.2)\"m. ' 0 -r Research supported by the Atomic Energy Commission. · .. · .. · . ·. ;. 2. -2LBL-2577 It is generally accept~d(l) that o~e-phonon Raman modes are characterized by zone-center phonons while multiphonon Raman modes reflect the phonon density-of-states near critical points. In either case for a given crystal orientation, the Raman frequency shift is independent of the exciting photon energy. In this Letter we report for the first time .the observation of some Raman modes in Cu2o whose frequencies.vary with the exciting lase:r frequency. We have developed a theory to explain our results quantitatively and sho\\m that it is possible t9 obtG.in phonon dispersion and effective mass of the ls exciton band in Cu2o by resonance Raman scattering (RRS). Recently Yu et al. (2) have shown that RRS of the 2r~2-phonon mode in CuiO(J) is dominated by the scattering process shown in Figure l(a). For a fixed. incident photon energy wi , the momentum q of the. ·. r 12 phonon emitted is determined uniquely by the resonance condition: 2 nq /2M+ w 0 (q) (1) where. w1 and w0 are respectively;the energies of the ls yellow exciton at q = 0 and the r12 phonon and M is the effective mass.of the ls exciton band which is assumed to be spherical. Thus by measuring the energy of the 2r~2 mode as a function of one can in principle measure the dispersion curve . 7 -1 of the r12 phonon for q up to 2 x 10 em .or about 1/4 of the B.Z. However no such shift in the energy of the 2r~2 mode was observed. This ·is because the ri2 mode happens to be almost dispersionless{ 4) .. But there are other . phonons in cu2o more dispersive than the r 12 phonon. .This prompted us to measure the position of other Raman peaks of Cu2o as a function of wi. Our measurements were performed at \"' 2°K using a conventional Raman spectrometer and a CW dye laser(S) on single crystals of cu2o grown by oxidation of pure copper and from melt. Results from both kinds of samples are essentially '\\• ' .. ·. 3. LBL-2577 . ..... . :·::.·· identical. Samples grown from melt have much stronger luminescence background which tends to obscure the weaker Raman lines. We therefore presented here only results obtained from samples grown by oxidation of Cu. Figures 2(a) and (b) show two Raman spectrf.of Cu2o obtained with two different wi. We notice that the peaks labelled y and z have' clearly shifted in energy with w •• ~ In Figure 3 we have plotted the peak position of all the R:aman modes cf cu2o between 190 and 400 cm-l as a function of In this paper w~ shall consider·only the modes X, Y, Z and r12 + and leave the rest .to a more detailed future publication • . RAMA.J.'i MODE: Y . We consider the peak Y first because of its comparatively larger cross-section. We notice in' Figure 3 that this peak starts by splitting off from the 2r~2 mode (220 cm1) and shifts to larger values of wi-ws as wi increases above the phonon-assisted excitonic absorption edge (w1 + w0 = 16514 em -l). · This suggests that this mode is . due to scattering of two r 12 phonons plus a low energy dispersive phonon which is presumably a longitudinal acoustic (LA) phonon [LA phonons couple more strongly to excitons than TA · phonons]. There is also good reason to expect such anacoustic-phonon_sideband of the 2 mod~. In Reference 2 it.was shown that the damping y of the ls yellow exciton with a non-zero momentum q is dominated .by the damping y · due to emission of an acoustic phonon. From the 'cascade' theory of ac Martin an:d Van:1a (G), we expect the ratio of the ·intensity of this sideband to the intensity of the 2r~2 ~ode to be Yac/. y ~ 1.' ·The above interpretation is supported by the follow.ing quantitative analysis. Applying perturbation· theory to the scat.tering process shown in Figure l(b), we can show that the Raman cross~secti.on of the 2r;-2 +LA (w 1 ) ... a<

Calculated and measured reflectivity of ZnGeP/sub 2/

Abstract: Reflectivity spectra of the chalcopyrite ternary compound ZnGe${\mathrm{P}}_{2}$ are studied experimentally and theoretically. The measurements of the reflectivity in both the complete parallel and perpendicular polarizations are made at 5\ifmmode^\circ\else\textdegree\fi{}K. A full zone energy-band structure, the reflectivity and imaginary part of the frequency-dependent dielectric function are calculated using the empirical pseudopotential method. Comparison is made with the measured reflectivity and modulated reflectivity and prominent features in the experimental spectra are identified and associated with interband transitions. In addition, spin-orbit interactions are included at a few points of the Brillouin zone.

Absorption, Photoluminescence, and Resonant Raman Scattering in Bil3

Abstract: Absorption, photoluminescence, and resonant Raman scattering have been measured in BiI3 at low temperatures with a tunable dye laser. The results do not support the “bielectron” model of Gross et al. Absorption, Photolumineszenz und der resonante Ramaneffekt wurden an BiJ3 bei tiefen Temperaturen mit Hilfe eines durchstimmbaren Farbstofflasers gemessen. Die Ergebnisse konnen nicht im Rahmen des „Zweielektronenmodells” von Gross et al. erklart werden.

Exciton Enhanced Raman Scattering of LO Phonons in Trigonal Se

Abstract: Resonance enhancements in the Raman cross-section of longitudinal optical (LO) phonons in trigonal Se have been measured around its excitonic absorption edge at low temperatures. Our results are explained in terms of resonant scattering of LO phonons via the Frohlich interaction.