Electrical manipulation of crystal symmetry for switching transverse acoustic phonons.

We experimentally explore the use of a novel device where lateral electric fields can be applied to break the translational symmetry within the isotropic plane and hence change the selection rules to allow normally forbidden transverse acoustic (TA) phonon generations. The ultrafast screening of the lateral electric field by the photocarriers relieves shear strain in the structure and switches on the propagating TA waves. The amplitude and on-state time of the TA mode can be modulated by the external field strength and size of the laterally biased region. The observed frequency shift with an external bias as well as the strong geometrical dependence confirm the role of the asymmetric potential distribution in electrically manipulating the crystal symmetry to control modal behavior of acoustic phonons.

Temperature dependence of the radiative recombination time in ZnO nanorods under an external magnetic field of 6 T.

The Temperature dependence of the exciton radiative decay time in ZnO nanorods has been investigated, which is associated with the density of states for the intra-relaxation of thermally excited excitons. The photoluminescence decay time was calibrated by using the photoluminescence intensity in order to obtain the radiative decay time. In the absence of an external magnetic field, we have confirmed that the radiative decay time increased with temperature in a similar manner to that seen in bulk material (∼ T1.5). Under an external magnetic field of 6 T parallel to the c-axis, we found that the power coefficient of the radiative decay time with temperature decreased (∼ T1.3) when compared to that in the absence of a magnetic field. This result can be attributed to an enhancement of the effective mass perpendicular to the magnetic field and a redshift of the center-of-mass exciton as a consequence of perturbation effects in the weak-field regime.

Stimulated emission features of bound excitons in ZnO nanotubes.

We have investigated the detailed features of photoluminescence (PL) in vertically aligned ZnO nanotube (NT) arrays as a function of temperature, pumping power, and experimental geometries. In samples with different wall thickness (15 or 60 nm), the temperature-dependent PL energy followed the Varshni's formula whose fitting parameters were found to be rather close to zero-dimensional case in the 15 nm-thick NTs with much larger intensity. In reflective geometry with circular excitation beam shape, the emission gradually evolved from spontaneous to stimulated regime, inferred from amplitude and line-width variation. On the other hand, in the edge-emission geometry with needle-like excitation shape, the interaction length dependence was directly traced by using an adjustable slit.

Cooperative recombination of a quantized high-density electron-hole plasma in semiconductor quantum wells.

We investigate photoluminescence from a high-density electron-hole plasma in semiconductor quantum wells created via intense femtosecond excitation in a strong perpendicular magnetic field, a fully quantized and tunable system. At a critical magnetic field strength and excitation fluence, we observe a clear transition in the band-edge photoluminescence from omnidirectional output to a randomly directed but highly collimated beam. In addition, changes in the linewidth, carrier density, and magnetic field scaling of the photoluminescence spectral features correlate precisely with the onset of random directionality, indicative of cooperative recombination from a high-density population of free carriers in a semiconductor environment.