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(a) Schematic representation of the experiment. The 266 or 400 nm laser pump pulses create photocarriers in the sample and at a time later the terahertz probe pulse travels through the sample, resulting in a transmitted signal . (b) Scans of the transmission [blue (bottom) trace] and modulation [red (top) trace]; average over 20 scans and . These measurements were performed at 20 K using 400 nm pump pulses to photoinject charge carriers.
(a) Real (in red) and imaginary (in blue) components of the conductivity as a function of frequency at 20 K after exciting pentacene with 400 nm pulses with a laser fluence of (all traces represent an average of 20 scans). The spectra are shown for different times after excitation, ranging from 0.7 to 2.1 ps (with intervals of 0.2 ps). For the conductivity at , the original data points are shown. The dashed-dotted line shows a fit function consisting of a frequency independent part (which represents the response from the free carriers) (Refs. 6 and 7) and a Lorentzian, showing that the conductivity data for contain a resonant absorption feature at . (b) Real and imaginary components of the conductivity at [performed under the same conditions as Fig. 2(a) but on a different sample and averaged over 50 scans]. (c) Real and imaginary components of the conductivity at room temperature when exciting at 266 nm with a fluence of for and , averaged over 30 scans.
The spectrally integrated modulation as a function of delay time for pentacene optically excited at 400 nm with a laser fluence of 20 (black trace, averaged over 20 scans), 13 (red trace, 10 scans), 8.6 (blue trace, 10 scans) and (gray trace, 15 scans). The traces are normalized by their laser fluence.
The Fourier transform of the autocorrelation function for both the coupling between the HOMO levels (a, in red) and LUMO levels (b, in blue). For simplicity, we show an average of the spectral density obtained for all the inequivalent pairs of nearest neighbors in the plane since the difference between them is negligible when the broadening is introduced. The dotted traces show the original curves, whereas the solid ones represent the case of adding a broadening of 0.3 THz to the original spectra. The black traces and show the conductivity (depicted on the right axis) at for 20 K when optically exciting at 400 nm (c) and at room temperature when optically exciting at 266 nm (d).
The spectrally integrated modulation as a function of delay time for pentacene when exciting with 400 nm pulses with a laser fluence of . The traces are recorded at different temperatures: 20 K (black), 100 K (red), and 200 K (blue). For , each modulation trace is fit by a function indicated by the gray dotted traces. The inset shows the parameters (the size of the CAT signal) and (the size of the free carrier contribution) as a function of temperature.
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