We report on electrically modulating and switching the wavy properties of acoustic phonons in nanoscale piezoelectric heterostructures which are strained both from the pseudomorphic growth at the interfaces as well as through external electric fields. In symmetry planes of such structures, the generation and detection of the transverse acoustic modes are forbidden, and only longitudinal acoustic phonons are generated by ultrafast displacive screening of strains. We show that the combined application of lateral and vertical electric fields can not only turn on and off various modes but they can also modulate the amplitudes and frequencies of the modes [1-3]. The role of the electrical controllability of phonons was further demonstrated as changes to the propagation velocities; the electrically polarized TA waves; and the geometrically varying optical sensitivities of phonons. The capability to manipulate the phononic functionalities with electric fields is analogous to that for manipulating photons and electrons in major technological devices and can be a practical route for integrated phononic circuitry.
[1] C. S. Kim et al., Appl. Phys. Lett. 100, 101105 (2012).
[2] H. Jeong et al., Phys. Rev. Lett. 114, 043603 (2015).
[3] H. Jeong et al., Phys. Rev. B (to appear).
The combined applications of vertical and lateral electric fields give rise to a novel degree of freedom in controlling transverse acoustic (TA) and longitudinal acoustic (LA) phonons. The coherent acoustic pulses were generated in GaN-based quantum wells (QWs) along polar c-axis under the ultrafast optical screening of the field-dependent strains and measured via the dynamic interference between reflected optical beams off the surface and the acoustic pulses. We electrically turned on the otherwise forbidden TA mode in the laterally biased region. The frequencies of the modes could be also modified via the field-induced changes in the propagation velocities. Finally, we experimentally investigated a novel method of controlling the phase of LA mode, generated from oppositely strained quantum wells and barriers under the vertical electric field.
We report on measurements and calculations of the ultrafast exciton relaxation dynamics in ZnO. Time-resolved
differential reflectivity measurements of bulk ZnO were performed as a function of excitation wavelength. Bi-exponential
decays of the A and B exciton states are observed with a fast (~2-5 ps scale) and a slower (~50-100
ps scale) component, which depend strongly on excitation wavelength. Theoretical calculations based on a
multi-state, coupled rate equation model were directly compared with the experiments to account for the rapid
scattering between the A and B valence bands. Results show that the inter-valence band scattering is most
likely not responsible for the fast initial relaxation. Instead our results show that carrier diffusion can play an
important role in explaining the initial fast relaxation.
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