5. Frequency-dependent response of suspended carbon nanotubes
100
FELIX pulse
I (nA)
5 mV bias
50
2 mV bias
0 1000
0 mV bias
1020
1040
1060
1080
1100
t (μs) Figure 5.4. Averaged current through the nanotube for three bias voltages, as the pulse arrives. The time is measured from the electrical trigger signal. The duration of the pulse is indicated in green. The rise and decay times of the response are limited by the measurement electronics.
average values of the current taken in the time-domain from 0 to 200 μs after the electronic trigger signal (800 μs before the terahertz pulse), which we define as I0 . The green dots represent the current-voltage characteristics measured in a conventional way without any applied radiation, these measurements were performed with the windows blocked when the free-electron laser was switched off. The agreement between the black and green dots clearly shows that in the pulsed measurements we measure the ’dark’ current outside the pulse duration. The measured peak current value, IP , is not the final equilibrium value for the given incident power. Since the rise-time in the response is known to be due to the filter at the output of the measurement electronics, with a time-constant of 10 μs, we can estimate that the measured peak current is approximately ∼ 85% of the steady state value at the given incident power. The current-voltage characteristics of the peak current, IP (red dots in Fig. 5.5), show an increase in conductance and an offset voltage compared to the current measured outside the FELIX pulses, I0 (black dots). The currentvoltage characteristics are similar to the DC current-voltage characteristics measured under continuous 108 GHz radiation in Chapter 4. We conclude that within 10 μs the heating process has already been established. In addition, we 64