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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 71: Organic Electronics and Photovoltaics - OLEDs and Molecular Excitations
CPP 71.11: Talk
Thursday, March 15, 2018, 17:45–18:00, C 243
Nonlinear optical response of chromophores from real-time time-dependent DFT — •Caterina Cocchi1, Stefano Pittalis2, and Carlo A. Rozzi2 — 1Physics Department and IRIS Adlershof, Humboldt-Universität zu Berlin, Germany — 2CNR Istituto Nanoscienze, Modena, Italy
The impulsive excitation scheme for time propagation in time-dependent density-functional theory [1] can be efficiently adopted to study the optical response also beyond the linear regime, mimicking a broadband laser beam illuminating the sample. We demonstrate the potential of this approach by investigating the nonlinear optical response of carbon-conjugated chromophores, e.g., phthalocyanines and thiophenes, that absorb visible light of specific frequencies while being transparent to others. At increasing intensity of the applied field, the sharp peak giving the absorption edge in the linear regime loses weight, while the oscillator strength concomitantly grows in the transparent windows. We clarify that this behavior, related to a third-order nonlinearity, is driven by reverse saturable absorption and is responsible for optical limiting in these compounds [2]. Upon strong incoming fields, incrementally populated dipole- and/or spin-forbidden excitations enhance the absorption in their frequency range while allowed transitions in the linear regime are partially quenched. Our results contribute to the understanding of the fundamental physical mechanisms behind nonlinear light absorption of chromophores.
[1] K. Yabana and G. F. Bertsch, PRB 54, 4484 (1996). [2] C. Cocchi et al., PRL 112, 198303 (2014).