Ultrafast photochemistry and photophysics in the solid state
Yu-Hsiang Cheng and Frank Gao
Many structural and chemical dynamics in the solid state occur irreversibly and with a build-up of reaction products. Irreversible or long-lived solid-state dynamics cannot be studied using conventional ultrafast spectroscopy, in which repeated measurements are made on a sample that returns to its initial state after each laser shot. We have developed a novel method for real-time measurement of ultrafast dynamical events in a single laser shot. An excitation laser pulse is followed by 400 probe pulses, which arrive at the sample at different times and at slightly different angles. Figure 1 below illustrates the generation of a time-structured probe pulse. The crossed echelons transform the input pulse into a 2D array of spatially and temporally resolved pulses. Each probe pulse, after transmission through or reflection off the sample, arrives at a different region of a CCD camera. Thus a single laser shot yields a complete time-dependent record of the sample response.
Figure 1:Crossed-echelon method
Figure 2 illustrates how our setup differs from the conventional pump-probe setup. The pump-probe setup (a) employs a delay line to achieve time-resolution, where the pump and probe pulses are separated by a varying time delay. In the single-shot setup (b) there is no delay line. The time resolution, set by the echelon thickness, is 25 femtoseconds. This technique provides a real-time picture of chemical and structural dynamics in a single shot.
Figure 2: Comparison of conventional pump-probe experiment and single-shot experiment
Photochemistry in Organic Crystals
We employed single-shot femtosecond spectroscopy to study the effect of the surrounding lattice structure on the photodissociation and recombination of the triiodide ion (I3-). The panels on the left of Figure 3 illustrate the structures of three different organic crystals (tetrabutylammonium triiodide: TBAT, tetraethylammonium triiodide: TEAT, and tetraphenylphosphonium triiodide: TPPT). The different crystal structures provide different circumstances to the reactant. The panels on the right of Figure 3 show the transient absorption spectra of photofragment I2- after photolysis of triiodide in the corresponding crystals. As shown, the dynamics of dissociation and recombination differ dramatically depending on the environment. This example demonstrates the significance of lattice structure on solid state photochemistry as well as the potential of single-shot femtosecond spectroscopy as a tool for the study of irreversible solid state dynamics. For more details, see reference . Recently, we have looked into the response of the lattice to photodissociation of triiodide. Raman scattering of lattice excitations following photodissociation may be visible with the single-shot instrument, and would give a more complete picture of how the surrounding environment mediates photochemistry.
Transient absorption spectra
Figure 3: Transient absorption spectra of three triiodide crystals.
Photoinduced Phase Transitions in Semimetals
Recently, we investigated the semiconductors Bi, Sb, Te, and Ge2Sb2Te5 (GST) under highly non-equilibrium conditions. Past studies have shown that large laser-induced changes in carrier occupation in metals and semiconductors can lead to substantial band structure renormalization and even to structural phase transitions. Our experiments are aimed at researching the fundamental limits of the interaction of light with matter and the behavior of matter under extreme carrier densities.
By monitoring the coherent phonon response of these materials upon laser excitation, we observe different types of behavior. Bi, Sb, and Te support coherent excitation of fully symmetrical phonon modes at low excitation intensities. At high excitation intensities, these materials undergo non-thermal melting, as indicated by the absence of the phonon modes which are present in the low-intensity regime. Non-thermal melting is a recently understood effect, which only occurs upon the interaction of semiconductors with femtosecond laser pulses.
In order to further investigate the mechanism of nonthermal melting, we have looked at the photoinduced phase transition in Bi and Te thin films. In a thin film, the electrons are confined to two dimensions, so the degree carrier diffusion away from the probe spot is variable. By varying the thickness of the film, we can alter the electronic temperature of the semimetal and reveal the role hot carriers and lattice vibrations play in the phase transition.
Below is a comparison of bulk and thin film response to strong optical excitation. We pump the sample with a high intensity (11.0 and 6.6 mJ/cm2, respectively) pulse, and wait some time to see if the low-fluence pump-probe response recovers. The bulk recovers more quickly because carrier diffusion is not limited by the thin film.
GST exhibits similar properties under high density excitation. A reversible transition between amorphous and crystalline phases exists at high fluence excitation. However, few studies have been able to investigate the short-time dynamics of this phase transition. Single-shot measurements will help reveal how this phase transition occurs.
Figure 4: Double-Pump Measurements of Bi bulk (left) and 25 nm thin film (right).In bulk, Bi oscillations recover more quickly than in the thin film, indicating faster return to an ordered crystal structure.
1. "Dual Echelon Femtosecond Single-Shot Spectroscopy," T. Shin, J.W. Wolfson, S.W. Teitelbaum, M. Kandyla, and K.A. Nelson, Rev. Sci. Inst. 85, 083115 (2014)
2. "Irreversible Organic Crystalline Chemistry Monitored in Real Time," P.R. Poulin and K.A. Nelson, Science 313, 1756-1760 (2006)
3. "Single-shot transient absorption of I3- in solutions and glasses," P.R. Poulin and K.A. Nelson, in Ultrafast Phenomena XIV, T. Kobayashi, T. Okada, T. Kobayashi, K.A. Nelson, and S. De Silvestri , eds. (Springer-Verlag, Berlin 2005), pp. 529-531
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5. "Dual-echelon single-shot femtosecond spectroscopy," G.P. Wakeham and K.A. Nelson, Opt. Lett. 25, 357-363 (2000)