Simultaneous ultrafast probing of intramolecular vibrations and photoinduced charge carriers in rubrene using broadband time-domain THz spectroscopy

We determine the ultrafast frequency- and time-resolved complex dielectric responses of photoexcited, single-crystal rubrene in the frequency range of $10–30\mathrm{THz}$ $(330–1000{\mathrm{cm}}^{-1})$ using ultrafast broadband THz spectroscopy. In this frequency range, we observe the response of both photogenerated mobile charges and intramolecular vibrational modes simultaneously, both of which vary with time after excitation. The data in conjunction with a theoretical model indicate a dynamic blueshift of the $15.5\mathrm{THz}$ phonon.

Figure 1

(a) THz probe pulse in the frequency and time domain (inset). (b) Linear IR absorption spectrum of the rubrene crystal measured with an IR spectrometer (solid line) and extracted from the TD-THz experiment (dotted line, offset vertically, see text). Note the saturation of some of the peaks in the IR spectrum—the actual absorbance may be higher for some of the peaks. The molecular structure (Ref. 28) of rubrene is shown as an inset in (b).

Figure 2

(a) Photoinduced changes in THz transmission through rubrene as a function of delay time after photoexcitation, measured with a HgCdTe detector (grey line). Black solid line is the result of the calculation described in the text. (b) Dynamics on short time scales. The line marks the HgCdTe detector result; the marked traces represent dynamics inferred from time- and frequency-resolved experiments: the signal integrated over all frequencies (circles) and the purely resonant signal (resonant minus nonresonant) at $15.5\mathrm{THz}$ (integrated from $15\text{to}16\mathrm{THz}$, triangles). (c) First $800\mathrm{fs}$ of the real part of the differential signal $\Delta E∕E$; the horizontal frequency axis corresponds with that of the bottom figure. Linear gray scale from 0.2 (white) to $-0.05$ (black). (d) The real (full line) and imaginary (dotted line) parts of $\Delta E∕E$ at the maximum of the signal (integrated from $200\text{to}400\mathrm{fs}$).

Figure 3

Comparison of the experimental $\Delta E∕E$ data around $15.5\mathrm{THz}$ (a) with the modeled data at the same frequency range (b). Scale as in Fig. 2, but inverted for clarity, (see text for details). Panel (c) depicts the central frequency of both the experimental (squares) and modeled (solid line) data obtained from Lorentzian fits to horizontal cuts through panels (a) and (b).