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(a) Experimental diagram: The femtosecond pulse is used to trigger the electronics for the QCL and to sample the intensity of the THz pulse. Orthogonal polarizers before and after result in a signal on the photodiode proportional to the THz intensity. (b) Timing schematic for the detection; note that the probe pulse always samples the same point on the RT modulation due to the integer relationship.
Light-current-voltage characteristics of the device with the quasi-dc current (modulated at 10 kHz, 20% duty cycle) applied. Optical power without the gain-switching pulse (blue) was measured with a pyroelectric detector. Optical power with the gain-switching pulse applied (red) is measured using the method shown in Figure 1. Circular marks indicate the currents 170 mA, 220 mA, 270 mA, and 385 mA when the gain-switching pulse is on and squares mark currents of 450 mA and 595 mA when the gain-switching pulse is off.
Sampled THz intensity from the QCL as the device is switched on. For each scan, the dc current is 270 mA corresponding to the circular mark in Figure 2. (a) No RT modulation is applied. A modulation at 13.23 GHz is applied with powers of (b) 4 dBm, (c) 14 dBm, and (d) 21 dBm.
Sampled intensity of the QCL with the gain-switching pulse for dc currents of (a) 385 mA, (b) 270 mA, (c) 220 mA, and (d) 170 mA. A modulation of 13.23 GHz and 18 dBm is applied. The inset shows the results of fitting several gaussian peaks to the recorded emission. The laser appears to start at later times for lower currents due to the finite rise-time of the gain-switching pulse.
Pulses generated by the QCL and sampled by the femtosecond pulse. The device is biased at 450 mA, indicated by the black square in Fig. 2, and has modulation of 13.23 GHz and 21 dBm applied. No gain-switching pulse is present. (a) The sampled pulses (black) with a gaussian curve fitted to each peak (red). (b) The FWHM of each of these fitted peaks. The transform limited width of 19 ps (calculated from Fig. 6(c)) is shown as a dashed line. The dependence of the pulse amplitude on the RT frequency is shown as an inset, the value of 13.23 GHz is indicated with a dashed line.
Spectra from the device with and without RT modulation applied. (a) and (c) QCL current is 450 mA. (b) and (d) QCL current is 595 mA (squares in Figure 2). There is a modulation of 21 dBm at 13.23 GHz applied in (c) and (d). These spectra were taken using an intra-cavity mixing technique using near-IR radiation at 811.83 nm, more details can be found in Ref. 20.
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