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We report on the first observation of the emission of electromagnetic radiation from coherent lattice vibrations in a semiconductor. Coherent optical phonons are excited by ultrashort laser pulses in singlecrystal tellurium. THz frequency radiation emitted by the Dember-field-driven phonons is detected by time-resolved THz-emission spectroscopy. The measurements are complemented by optical pump-probe experiments with a polarization-sensitive detection scheme utilizing the symmetry of the Raman tensor. 42.65.Re, 78.47.+p The investigation of eoherent phonons in solid-state materials exeited with ultrashort laser pulses is a subjeet of great eurrent interest. In reeent years, eoherent phonons have been observed in the transient optical refleeti vity and in the transmission of several semieonduetors [1][2][3]. For a eertain elass of materials including Te, only eoherent optieal phonons of Al symmetry have been deteeted in femtosecond time-resolved experiments [2], although modes of different symmetry have been identified in ew-infrared (IR) reflectivity and Raman seattering experiments. The seleetive excitation of Al modes has been explained by a displaeive exeitation mechanism, in whieh a strong interband exeitation leads to an ultrafast displacement of the atomie equilibrium position [2,4]. We present a detailed study of eoherent phonons in Te. In addition to the Al mode, phonons of E symmetry are observed in time-resolved measurements of the anisotropie refleetivity ehanges (RC), obeying selection rules given by the symmetry of the Raman tensor. Sinee all phonons in Te of symmetry different from Al are IR aetive, their eoherent excitation is expeeted to be aceompanied by the emission of eleetromagnetie radiation. In this Letter, we report on the first time-resolved observation of THz emission that originates from the maeroseopie polarization associated with coherent longitudinal optieal (LO) phonons. The radiation is detected with THz-emission speetroseopy that allows the direet measurement of the amplitude and phase of the eoherent eleetromagnetie radiation emitted after pulsed optieal exeitation [5].The IR aetivity of phonons in the single-element semieonduetor Te arises from the strong eleetronie polarizability and the laek of inversion symmetry [6]. Te erystallizes in a hexagonal lattiee (spaee group Dj or D~) eonsisting of three atoms per unit eell arranged in a helix along the c axis. The lattiee vibrations eonsist of a fully symmetrieal, only Raman-aetive Al mode (3.6 THz), two degenerate Raman-and IR-aetive E modes (E~O/LO: 2.76/3.09 THz, E~O/LO: 4.22/4.26 THz), and one only IR-aetive A 2 mode (A 2 ,To/Lo: 2.6/2.82 THz) [7]. The internal polarization is either perpendicular (E modes) or parallel (A 2 mode) to the c axis. All experiments are performed on two different Te single erystals with the c axis perpendieular (c~) and parallel (e 11) to the surfaee.To prove the excitation of eoherent phonons different from the Al mode, we perform optieal pump-probe measurements and deteet the anisotropie RC. ...

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Porosity superlattices have been investigated by transmission electron microscopy, photoluminescence and reflectance spectroscopy. The superlattices were formed on p-type doped Si using two different techniques. Firstly, for homogeneously doped substrates we have periodically varied the formation current density and thereby the porosity. Secondly, the current density was kept constant while etching was performed on periodically doped Si layers. For the first type of superlattices the layer thicknesses were determined by transmission electron microscopy. The results are in good agreement with the values calculated from the etching rate and time. For both types of superlattices, reflectance and photoluminescence spectra show strong modulation due to the periodicity of the superlattice.