Ultrafast THz photonics and phononics of strained piezoelectric InGaN/GaN quantum wells
Prof. Dr. Dmitry Turchinovich
Wurtzite InGaN/GaN quantum wells are characterized by the very strong the built-in electric field on the order of MV/cm, as a result of large lattice
mismatch (up to 2%) and large material piezoelectricity. This “frozen charge” and “frozen strain” make InGaN/GaN QWs an system of choice for
demonstration of various kind of ultrafast dynamics.
Such, femtosecond optical excitation of electron-hole pairs in the QW in the presence of built-in electric field leads to instantaneous creation of a dipole. These dipoles can screen, partially or even completely, the built-in electric field. The polarization dynamics, resulting from such a dynamical screening, leads to a free-space THz emission. The THz emission saturates at the point, when the QW reaches complete screening (observed as the vanish of Stark shift in the PL spectra).
Once the built-in electric field is removed by the dynamical screening, the strain in the QWs is not compensated by the electric field anymore. This results in ultrafast lattice dynamics, with the phonon frequencies in the THz range. The acoustic and electromagnetic THz emissions in InGaN/GaN QWs are directly correlated, confirming the common origin in ultrafast dynamical screening of the built-in field.
|Fig. 1||Origin of the built-in piezoelectric field in wurtzite InGaN/GaN QWs. (b) Principle of dynamical screening in biased QWs.|
|Fig. 2||(a) THz electromagnetic emission from the optically excited InGaN/GaN QWs at various excitation strength. (b) Dependency of the THz amplitude on the pump fluence. (c) Time-integrated photoluminescence spectra of InGaN/GaN QWs at various excitation strength. (d) Spectral position of PL maximum on the pump fluence, demonstrating the vanish of Stark shift, which is the confirmation of complete screening of the built-in field in the QW.|
|Fig.3||(a) THz acoustic emission pulse from the multi-QW sample, containing 10 QWs of 1.8 nm thickness. (b) A correlation plot between the THz electromagnetic and acoustic emission, for lower optical pump fluences.|
D. Turchinovich, P. Uhd Jepsen, B. S. Monozon, M. Koch, S. Lahmann, U. Rossow, and A. Hangleiter
"Ultrafast polarization dynamics in biased quantum wells under strong femtosecond optical excitation"
Phys. Rev. B 68, 241307(R) (2003)
D. Turchinovich, B. S. Monozon, and P. Uhd Jepsen
"Role of dynamical screening in excitation kinetics of biased quantum wells: Nonlinear absorption and ultrabroadband terahertz emission"
J. Appl. Phys. 99, 013510 (2006)
P. J. S. van Capel, D. Turchinovich, H. P. Porte, S. Lahmann, U. Rossow, A. Hangleiter, and J. I. Dijkhuis
"Correlated terahertz acoustic and electromagnetic emission in dynamically screened InGaN/GaN quantum wells"
Phys. Rev. B 84, 085317 (2011)