Transfer characteristic of GaAs/AlGaAs HEMT measured at V SD = 100 mV. Left inset shows experimental set-up and sketch of the device with contact pads S (source), D (drain), and G (gate). In optical experiments, transistors were irradiated by elliptically, circularly, or linearly polarized radiation at normal incidence. Right inset shows a sketch of the transistor cross section.
(a) Gate voltage dependence of the photovoltage induced in GaAs/AlGaAs HEMT by linearly polarized radiation. The data for frequencies f = 1.1, 2.0, 2.5 THz are multiplied by factors 5, 2, 10, respectively. Note that the signals are read out after the inverting amplifier with a voltage amplification of 20 dB. (b) Gate voltage dependence calculated after Eq. (7) and assuming . Inset shows fitting parameters and as a function of the radiation frequency.
Photovoltage as a function of the azimuth angle α. Solid line shows the fit to Eq. (1). The dashed and dotted-dashed lines display individual contributions proportional to and , respectively. The leftinset shows the polar plot of measured for f = 0.1 THz, U g = –400 mV (squares) and f = 1.1 THz, U g = –500 mV (open circles) togetherwith the corresponding fits to Eq. (1). Note that the maxima signals for f = 0.1 THz and 1.1 THz are normalized to 1 and 0.5, respectively. The right inset shows the picture of the transistor with bonding wires and defines the angle α. On top the polarization direction corresponding to various azimuth angles is plotted. For zero angle α the radiation electric field vector is parallel to the y-axis.
(a) Photoresponse as a function of the gate voltage, measured for right-() and left-() circularly polarized radiation with f = 1.1 THz. Inset shows the data for f = 0.8 THz. These data are normalized by the maximum signal in response to radiation. (b) Helicity dependence of the photovoltage measured for radiation with f = 0.8 THz. Solid line shows the fit to Eq.(2). The dashed and dotted-dashed lines display individual contributions proportional to the parameters and , respectively. The ellipses on topillustrate the polarization states for various . The inset shows the photosignal pulse traces measured for and radiation.
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