Second Harmonic Generation: Probing Photochromic Interfaces and Ultrafast Charge Transfer Processes

by Michael Schulze

Institution: Freie Universität Berlin
Department: FB Physik
Degree: PhD
Year: 2015
Record ID: 1098390
Full text PDF: http://edocs.fu-berlin.de/diss/receive/FUDISS_thesis_000000098841


The inherent interface sensitivity of the second harmonic generation (SHG) setup developed and realized in the framework of this thesis is successfully utilized to investigate functionalized surfaces and functional thin layers, viz. photochromic azobenzene and fulgimide self-assembled monolayers (SAMs) and the electron donor/acceptor (D/A) heterojunction C60/P3HT. Applying light as an external stimulus and observing the induced changes in the interfacial non-linear optical (NLO) response, the switching state of the photochromic interfaces is read out via the isomerization-correlated static charge reallocation. Moreover, the ultrafast charge transfer (CT) across the D/A interface is monitored with time-resolved SHG on a femtosecond timescale. En route towards innovative photonic and optoelectronic data processing and transmitting microelectronic components and data storage devices, photochromic interfaces exhibiting reversibly controllable NLO properties inhere intriguing potential. The strategy of anchoring fulgimide and azobenzene chromophores via SAMs to metallic and semiconducting substrates, respectively, results in substantial NLO contrasts. Polarization-resolved SHG measurements enable to identify a correlation of these NLO responses to conjugation and orientation of the π-electron system and hence reveal information on the chromophore alignment. In case of the fulgimide system, density functional theory as well as correlated wavefunction method calculations confirm these results. In addition, the NLO contrast depends on a precise balance of chromophore-functionalized and unmodified spacer chains. A progressing inability to photoisomerize sets in at intermediate azobenzene packing densities. The successful chromophore-substrate decoupling is furthermore reflected in large isomerization cross-sections and results in thermal stability at ambient conditions. The C 60 /P3HT heterojunction is investigated regarding the dynamics and energetics of the ultrafast charge transfer across the D/A interface. Reducing the complexity of this prototypal system to a single, well-defined D/A interface enables a maximized exploitation of the interface sensitivity of the SHG probe and hence an unambiguous understanding of the fundamental CT mechanisms. The formation and subsequent thermalization of 'hot excitons' in the donor occurs on an ultrafast timescale. However, in the presence of the acceptor the charge transfer is identified as the dominating relaxation mechanism. Most remarkably, the yield of the CT process is revealed to correlate with the excitation wavelength. It increases with the amount of excess energy. Im Rahmen dieser Arbeit wurde ein experimenteller Aufbau entwickelt und realisiert, welcher die intrinsische Grenzflächensensitivität des nichtlinear optischen (NLO) Effekts der Frequenzverdopplung (engl. second harmonic generation, SHG) ausnutzt, um funktionalisierte Oberflächen und funktionale Dünnschichten zu untersuchen. Im Speziellen wurde der Schaltzustand selbst-organisierter Monolagen (engl.…