The spectroscopy of wide-bandgap fluoride materials doped with divalent ytterbium is presented. The structure of impurity-trapped excitons is explored, vacuum ultraviolet excitation is used to investigate the transfer processes between excitations, and the effect of confinement on self-trapped excitons is studied. The excited-state structure of impurity-trapped excitons is measured in the multisite system NaMgF₃:Yb²⁺. A two-colour ultraviolet-infrared pulsed photoluminescence enhancement technique is employed to probe the interlevel transitions and dynamics of impurity-trapped excitons in doped insulating phosphor materials. NaMgF₃:Yb²⁺ exhibits emission from two charge-compensation centres with peaks at 22 300 cm⁻¹ (448 nm) and 24 000 cm⁻¹ (417 nm). The observed photoluminescence enhancement is caused by a combination of intra-excitonic excitation and electron trap liberation. The electron traps are inferred to have a depth of approximately 800 cm⁻¹. Time-resolved VUV spectroscopic studies of emission and excitation spectra of CaF₂:Yb, NaMgF₃Yb and MgF₂:Yb are presented to investigate excitation and relaxation mechanisms of both impurity-trapped excitons and intrinsic excitons in each fluoride host. Host-to-impurity energy transfer mechanisms leading to formation of impurity-trapped excitons are discussed. The 4f¹⁴ → 4f¹³5d CaF₂:Yb²⁺ absorption bands are successfully modeled with a semi-empirical effective Hamiltonian calculation for NaMgF₃:Yb²⁺ and MgF₂:Yb²⁺. The excitation and emission spectra of all studied materials are compared. Results on VUV spectroscopy of 3 and 5 monolayer CdF₂–CaF₂ superlattices show the change in optical behaviour of the self-trapped exciton in CdF₂ when it is confined and give an indication of the radius of the exciton. The decay of the emission is modeled with three components, corresponding to three self-trapped exciton states. Results on the VUV spectroscopy of CdF₂–CaF₂ superlattices show that the confinement effect seems to equally influence the energy of excitonic and bandgap absorption in 3 and 5 monolayer superlattices. At the same time, as the self-trapped exciton is more confined, the emission is blue-shifted by 1600 cm⁻¹ indicating that the effective excitonic radius is about three monolayers.

Within this thesis, photo-induced mechanisms of the light-matter interaction are investigated in tailored and photofunctional materials that differ significantly in their optical and structural properties. The individual coupling mechanisms in congruently melted, nominally undoped or iron doped lithium niobate crystals as well as in structurally disordered photoswitchable molecules embedded into a solid state polymer are examined in particular by the principle of holographic grating recording and transient absorption spectroscopy which provide new insight into a variety of material response properties. In case of photoswitchable ruthenium sulfoxide compounds, the underlying mechanism can be unambiguously assigned to a photochromic material response evoked by a photochemical reaction, i.e., a non-instantaneous, local ligand isomerisation. Comparable results are obtained for iron-doped, oxidized lithium niobate where holographic grating recording is related to the photophysical generation of transient excitonic states whose photochromic properties are characterized by targeted ns-pump, supercontinuum probe spectroscopy. In the event of nominally undoped lithium niobate, the holographic amplification of two sub-picosecond pulses is attached to the phenomenon of two-beam coupling on a self-induced dynamic grating. By correlating the individually obtained mechanisms of the light-matter interaction and the light-induced material response, generally accepted conclusions on a microscopic level can be achieved. A major influence of the internal structure and orientation of the excited states, i.e., an appropriate threedimensional structural arrangement, is deduced as a prerequisite for the formation of light-induced, macroscopic refractive index changes while absorption and microscopic refractive index alterations linked via the Kramers-Kronig relation are unaffected. In systems featuring a random distribution of excited states, an orientational order might be achieved as a consequence of linear polarized light, i.e., by polarization structuring. Moreover, if the photorefractive effect can be ruled out, the material response in lithium niobate can be solely assigned to a local alteration of the transient electronic states, i.e., to the photochromic properties of polarons and/or excitonic states, which is in particular comparable to the linkage isomerism of molecular photoswitchable molecules. In addition, the influence of structural parameters on the light-matter/surface interaction is studied on the μm-scale by analyzing the diffraction phenomenon arising from a relief grating. A considerable impact on the surface grating assisted coupling is determined by the transition from cw-lasers to ultrashort laser pulses which enables interference quenching. However, this phenomenon is of no consequence in case of selfinduced holographic gratings.

In this work the transient non-instantaneous polarization, i.e., laser-pulse injected small polarons and self-trapped excitons, is studied in the perovskite-like ferroelectric lithium niobate. The investigations span a time scale from femtoseconds to several hours. It is shown that the established small polaron picture is not able to describe transient absorption and photoluminescence of lithium niobate consistently. Several strong indications are presented demonstrating that the photoluminescence cannot be caused by geminate small polaron annihilation. Instead, the idea of radiatively decaying self-trapped excitons at the origin of the blue-green photoluminescence is revived. Excitons pinned on defect sites are proposed to lead to the already observed long-lived transient absorption in the blue spectral range in Mg- and Fe-doped crystals. Excitons pinned on iron-defects are studied in more detail. Their spectral fingerprint and absorption cross section is determined. Furthermore, it is shown that the occurrence of these pinned STEs can be tailored by chemical treatment of the samples and the experimental parameters such as the pump pulse intensity and photon energy. Based on the new experimental results and reviewing data published in literature, an atomistic picture of hopping and pinning of self-trapped excitons in lithium niobate is proposed. The question is addressed whether small polarons and self-trapped excitons in lithium niobate are coupled species in the sense that oppositely-charged polarons may merge into self-trapped excitons or STEs break into small polaron pairs. Decay kinetics of transient absorption and luminescence assigned to free small polarons and STEs indicate that this is not the case. For a more complete picture the ultrafast time scale is investigated as well. The formation times of small polarons and STEs are determined, which both lie in the range of 200 fs. No indications are found on the (sub)picosecond time scale indicating a coupling of both quasi-particle species either. In order to gain access to the formation of self-trapped excitons a custom-built femtosecond broadband fluorescence upconversion spectrometer is installed. Based on an already existing scheme, it is adapted to the inspection of weakly luminescent solid samples by changing to an all reflective geometry for luminescence collection. To avoid the necessity for an experimentally determined photometric correction of the used setup, an already established calculation method is extended considering the finite spectral bandwidth of the gate pulses. The findings presented here are important not only as fundamental research, but also regarding the technical application of lithium niobate and other similar nonlinear optical crystals. The simultaneous occurrence of both small polarons and self-trapped excitons is a rather rarely described phenomenon. Usually, the optical response of wide band gap oxide dielectrics is associated with only one of these quasi-particle species. This work may therefore be a stimulus to review the existing microscopic models for transient phenomena in other oxide dielectrics, which may help to improve their application in nonlinear optical and electro-optical devices. In this context the ultrafast transient photoluminescence spectroscopy established here for weakly luminescing solid samples may again provide valuable insight. With respect to lithium niobate, the results do not only resolve inconsistencies between the microscopic pictures described in literature, but also provide information regarding the extends to which the propagation of ultrashort laser pulses may be affected by (pinned-)STE absorption. It is shown that tailoring of the long-lived absorption center in the blue spectral range is possible, which may be used to avoid optical damage when high repetition rates are applied. It is important to emphasize that the microscopic model proposed in this work is mainly based on experimental indications. It is the task of further detailed theoretical investigations, e.g., via time-dependent density functional theory, to test whether the proposed model is justified. From an experimental perspective the important question remains whether (pinned-)STEs contribute to a photorefractive effect. In the experimentally easily accessible spectral range no absorption feature of mobile STEs is observed. As a complementary experimental technique, ultrafast holographic spectroscopy may reveal an excitonic contribution to photorefraction and provide further insight to STE transport and pinning phenomena.

Quasi-particles formed in lithium niobate after pulse exposure were investigated by transient absorption and photoluminescence spectroscopy as well as numerical simulations. This includes the formation process, the transport through the crystal, interim pinning on defects during the relaxation process, and the final recombination with deep centers. It was shown that the charge-transport through the crystal can be described by a hopping transport including different types of hops between regular or defective lattice sites, i.e., the transport includes a mixture of free and bound small polarons. Furthermore, the different types of hops connected with varying activation energies and their distribution are responsible for an altered temporal decay curve when changing the crystal composition or temperature. Additionally, it was shown that the hitherto accepted recombination model is insufficient to describe all transient absorption and luminescence effects in lithium niobate under certain experimental conditions, i.e., long-living absorption dynamics in the blue/UV spectral range do not follow the typical polaron dynamics and cannot be described under the assumption of charge compensation. However, similar decay characteristics between self-trapped excitons known from photoluminescence spectroscopy and the unexpected behavior of the transient absorption were found leading to a revised model. This includes, besides the known polaron relaxation and recombination branch, a significant role of self-trapped excitons and their pinning on defects (pinned STEs). Since the consideration of further absorption centers in the relaxation path after pulse exposure might result in misinterpretations of previously determined polaron absorption cross-sections and shapes, the necessity to perform a review became apparent. Therefore, a supercontinuum pump-probe experiment was designed and all measurements applied under the same experimental conditions (temperature, polarization) so that one can extract the absorption amplitudes of the single quasi-particles in a spectral range of 0.7-3.0eV. The detailed knowledge might be used to deconvolve the absorption spectra and transform them to number densities of the involved centers which enables one to obtain an easier insight into recombination and decay dynamics of small polarons and self-trapped excitons. As the hopping transport of quasi-particles and the concept of pinned STEs might be fundamental processes, a thorough understanding opens up the possibility of their exploitation in various materials. In particular, results presented herein are not only limited to lithium niobate and its applications; an extension to a wide range of further strongly polar crystals in both their microscopic processes and their use in industry can be considered.