{'fraction_non_alphanumeric': 0.020520915548539857, 'fraction_numerical': 0.0, 'mean_word_length': 5.5, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Two-photon single molecule spectroscopy

The field of optical probing of individual molecules has been rapidly growing and diversifying during the last decade and has attracted the interest of researchers from various disciplines. Several techniques have been applied to detect single molecules in different environments. Spectroscopic investigations on individual absorbers remove the ensemble averaging that occurs when a whole population is probed simultaneously, and can directly provide the distribution and time trajectory of an observable. Of particular interest in this regard are optical resonance frequency fluctuations of single dopants in a solid environment, which occur due to guesthost interactions. This thesis is concerned with two-photon excitation spectroscopy of single dopant molecules in a solid matrix. Two-photon absorption is one of the most basic non-linear optical processes and its study on the level of a single quantum system is of fundamental interest. Further, two-photon confocal microscopy is a technique featuring a reduced background if compared with conventional confocal microscopy. Its use for single-molecule imaging opens many possibilities to investigate new materials. One subject of this work is the demonstration of two-photon excitation as an additional technique for single-molecule spectroscopy, and the study of the molecular response to effects which are particularly related to two-photon excitation. Two-photon spectroscopy of single diphenyloctatetraene molecules in a n-telradecane matrix was accomplished at cryogenic temperatures. The experiment was made possible by means of a confocal optical design providing a low background and a high photon collection efficiency. Further, the right choice of chromophore-matrix pair was of crucial importance. By comparing oneand twophoton spectra of the same electronic transition, interactions between the individual molecules and the solid matrix were investigated, with the main focus on TR-induced spectral dynamics in twophoton experiments. Studies of spectral features as a function of temperature and laser power showed the importance of four mechanisms for the analysis of two-photon spectra: (i) the ac-Stark shift at intense laser irradiation, (ii) the activation of local phonons by a laser-induced heating in the excitation volume, (iii) the interaction with non-equilibrium phonons, and (iv) dynamics of tunneling two-level systems that are accelerated by the IR irradiation and the non-equilibrium conditions in the sample.