Abstract
The practical application of Bloch oscillations in semiconductor superlattices as THz emitters is hampered by the low emission power and the rapid dephasing of the coherent signal.1 At higher excitation densities the quadratic scaling of the THz emission power saturates due to the stronger dephasing of the signal. We adopt an approach previously employed for the investigation of the dephasing of interband dynamics in four wave mixing2 to study the dephasing of the intraband dynamics responsible for the THz emission. We demonstrate that for higher carrier densities the dominant dephasing process at low temperatures is carrier-carrier scattering. In addition, the presence of incoherent relaxed background carriers generated by a prepulse lead to a stronger dephasing than in the case where the same amount of coherent scatterers are created. The experiments are performed on a superlattice consisting of 9.7-nm GaAs wells separated by 1.7-nm Al0 3Ga0 7as barriers with a calculated width of the first electron miniband of 18 meV. In order to apply an electric field in the growth direction the structure is embedded in the intrinsic region of a pin-diode. The sample is held at 20 K in a dosed-cycle cryostat. The coherent dynamics is probed by THz emission spectroscopy. A prepulse 20 ps before the exciting pulse creates a cold background plasma. The long delay makes sure that the coherent motion of the carriers is completely dephased. Figure 1 displays THz transients for different intensities of the pre-pulse. Clearly, a higher carrier density leads to a stronger damping of the Bloch oscillation. A single exponentially damped cosine function fitted to the transients allows to determine the dephasing time t, which corresponds to a homogenous line-width Γ = h/πT. The fit procedure is restricted to longer time delays to avoid instantanous contributions. The Iinewidth shown in Fig. 2 (solid circles) reveals a linear dependency on the excitation density according to Γ(n) = F(0) + γnexc. Γ(0) = 1.25 meV accounts for the density-independent Iinewidth, i.e., due to interface roughness as well as inhomogenous broadening. The slope γincoh = 1.5*10−11 cm2 well is a measure for the influence of carrier-carrier scattering on the dephasing. The domi-nance of this process is obvious as for densities above l*1010 cm−2 well−1 the homogenous linewidth exceeds Γ(0). We compare γincoh from the pre-pulse experiments with a variation of the excitation pulse power leading to a higher density of coherent scatterers (open squares in Fig. 2). In this case the interaction parameter γcoh = 0.9*10−11 cm*2 well is lower than for the pre-pulse experiment. This indicates that the incoherent carrier background leads to a stronger dephasing of the intraband dynamics by about a factor of 2, While theoretical work investigated the influence of the coherent density on the dephasing of Bloch oscillations3 the differences between coherent and incoherent scatterers has not been addressed yet.
© 1998 Optical Society of America
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