We report the generation of coherent optical and acoustic phonons in mixed valent manganite La 1/4 Pr 3/8 Ca 3/8 MnO 3 using femtosecond infrared pump-probe spectroscopy. Temperature-dependent measurements of the time-resolved optical reflectance, obtained over a range of 5-300 K, revealed that the energy of the photoexcited electrons dissipated during relaxation to acoustic phonons, in the high-temperature paramagnetic phase, and to optical phonons, in the low-temperature charge-ordering phase. Analysis of the temperaturedependent behavior reveals that the modal amplitudes of the coherent phonons appear strongly correlated with the charge-ordering phase.
The influence of Cd molar fraction on the terahertz generation from Zn 1-x Cd x Te has been studied. The optimal Cd compositions have been obtained as x=0.10 at λ=820nm and x=0.65 at λ=850nm.
We observe coherent phonons of 4.0 THz and 45 GHz oscillation frequencies in the magnetically disordered phase (T>T N ) of multiferroic LuMnO 3 using femtosecond IR spectroscopy. We attribute the phonons to the displacement of polyhedron MnO 5 .Recently hexagonal manganites are getting renewed interests because of their multiferroic behavior which potentially enables electrical and optical controls of magnetic properties. Ultrafast spectroscopy provides useful tools to study the competition dynamics among and relaxations of spin, charge, and lattice degrees of freedom [1]. The mutiferroic LuMnO 3 has three temperature-dependent phases separated by the ferroelectric (FE) transition at ~900 K and the antiferromagnetic (AFM) transition at T N ~ 90 K. Here we report on the observation and the characteristics of coherent phonons in LuMnO 3 .We have performed an optical pump-probe experiment on LuMnO 3 single crystals. In our experiment, a Ti:sapphire cavity-dumping laser was used to produce 50-fs pulses at ~790 nm. The sample was pumped and then probed by laser pulses at 400-kHz repetition. The pump and probe pulses were cross-polarized with an angle of 8 degree to the sample. The laser fluence of the pump was 150 μJ/ cm 2 , being 5 times larger than the probe fluence. The induced reflectance ΔR/R was measured by the weak probe pulse as a function of time-delay generated by a raster scanning shaker. The temperature of the sample was varied from 10 to 300 K in a cryostat filled with liquid He. Figure 1 shows the time dependence of the photo-induced reflectivity ΔR/R of LuMnO 3 in the vicinity of T N . At high T> T N , both in the short and long time-scales, we observe the reflectance periodically changing in time. In the measurement of the short time scale shown in Figs. 1(a) and (b), the differential reflectance, ΔR/R, oscillates fast in time due to the lattice vibration, or the coherent optical phonon is generated. The measured Fig 1 Transient reflectance ΔR/R of LuMnO3. Coherent optical phonon mode (4.00 THz) measured at 190 K in (a) and at various temperatures in (b). Coherent acoustic phonon mode (45 GHz) measured at 190 K in (c) and at various temperatures in (d).a2742_1.pdf JTuD107.pdf
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