Terahertz-Induced Optical Emission of Photoexcited Undoped GaAs Quantum Wells

Intense terahertz (THz) pulse induces photoluminescence (PL) flash from undoped high-quality $\mathrm{GaAs}/\mathrm{AlGaAs}$ quantum wells under continuous wave laser excitation. The number of excitons increases 10 000-fold from that of the steady state under only laser excitation. The THz electric field dependence and the relaxation dynamics of the PL flash intensity suggest that the strong electric field of the THz pulse ionizes impurity states during the 1 ps period of the THz pulse and release carriers from a giant reservoir containing impurity states in the AlGaAs layers.

Figure 1

Schematic experimental setup for photoluminescence flash induced by intense THz pulses. Generated THz pulses are focused onto the GaAs QW sample, and the luminescence is detected by a Hamamatsu streak camera coupled to a spectrometer with a resolution of about 1 meV. The visible laser beam is directed through a small hole in the parabolic mirror used to focus the THz pulse.

Figure 2

(a) Typical streak camera image of the photoluminescence flash from the QWs excited at 10 K by both the visible cw laser and THz pulse. The visible laser intensity is $0.6\mathrm{W}/{\mathrm{cm}}^2$ and the peak electric field of THz pulse is $0.77\mathrm{MV}/\mathrm{cm}$. (b) Spectrally integrated transients of the photoluminescence flash with the THz pulse in (a) (red solid line) and without the THz pulse (blue dashed line). The dashed arrow indicates the arrival time of the THz pulse. (c) Spectra of the time-integrated PL flash shown in (a) (red solid line) and the PL emission excited by only the visible cw laser with an intensity of $0.6\mathrm{W}/{\mathrm{cm}}^2$ (blue dashed line).

Figure 3

(a) Visible cw laser power dependence of the flash intensity induced by a $0.77\mathrm{\text{−}}\mathrm{MV}/\mathrm{cm}$ THz pulse (solid circle). The solid line to guide the eye represents the 0.2-power-law intensity dependence. (b) THz electric field dependence of the PL flash intensity (solid circle). The solid curve is a theoretical calculation assuming a field-assisted direct-tunneling process described by Eqs. (1, 2). The inset shows temperature dependence of PL excited by a 2.33 eV cw-laser and PL flash.

Figure 4

Photoluminescence flash intensity induced by an intense THz pulse as a function of delay time between the visible and THz pulses. The experimental conditions are described in the body of the Letter. The solid line is the fitting curve of a biexponential function.