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.

Figure 1

(a) Schematic of a structure with asymmetry potential distribution; blue cones denote laser excitation spots moving along the $x$ axis (from $p$ contact to $n$ contact). (b) PL peak energy as a function of excitation position in the lateral ($x$) direction under external bias ($V_{\text{ext}}$). (c) Conduction band profile near the $\mathrm{MQW}/n$-GaN interface calculated from the PL peak energy variations at 4 V. Spatial distributions of (d) normal strain ${ϵ}_{zz}$ and (e) shear strain ${ϵ}_{zx}$ deduced from the conduction band profile. Lattice structures distorted by (f) ${ϵ}_{zz}$ and (g) ${ϵ}_{zx}$, where $\nu$ is the Poisson’s ratio.

Figure 2

(a) Measurement schematic for DRS due to AC phonons. (b) Transient DRS as a function of time delay under different $V_{\text{ext}}$ from 0 to 5.5 V. (c) Oscillatory parts of DRS due to acoustic phonons. (d) Fast Fourier transform (FFT) spectra of the acoustic phonon signals. (e) Band-pass-filtered signals under 4 V around the LA (108–144 GHz) and TA (52–88 GHz) modes. Insets outline the epicenters and propagation of the AC modes. (f) On/off amplitude ratio of the TA wave and on-state time ${\tau }_{\text{TA}}$ as a function of $V_{\text{ext}}$.

Figure 3

(a) Modal spectra for ascending waves as a function of $V_{\text{ext}}$. (b) AC mode amplitudes with increasing $V_{\text{ext}}$. (c) Peak frequency for each AC mode under $V_{\text{ext}}$. (d) $V_{\text{ext}}$-induced variation in dielectric constant. (e) $V_{\text{ext}}$-induced elastic constant variation for each mode.

Figure 4

(a) Modal spectra for ascending waves at different probe polarizations $\varphi$ under $V_{\text{ext}}$ of 5.5 V. (b) AC mode amplitudes with $\varphi$ from 0° to 90°. (c) Peak frequency for each AC mode as a function of $\varphi$. (d) Refractive index ellipsoids for perturbed (at 5.5 V) and unperturbed (at 0 V) structures.