Photon-induced effects allow for the fundamental understanding of ferroelectric semiconductors, both by analyzing the photon excitation and emission, but equally inspecting the nanoscopic and mesoscopic length scales of these systems [Seidel & Eng, Current Applied Physics 14 (2014) 1083]. I will introduce here into a multitude of novel approaches and effects when applying 'photonic probes' for researching wide-bandgap ferroelectrics and charged domain walls.

Firstly, applying simple photoluminescence (PL) analysis to LiNbO3 (LNO) [Reichenbach et al., APL 105 (2014) 122906] allows us to easily differentiate between the various stages of domain switching, i.e. between natural and singly-switched domains. Analyzing the PL spectra reveals direct fingerprints of those electron [Reichenbach et al., JAP 115 (2014) 213509] and hole [Kaempfe et al., PRB 93 (2016) 174116] polarons that are involved in this ferroelectric switching. 

Secondly, I will introduce into the upstream topic of charged domain walls (CDWs) found these days in a variaty of bulk single crystals (sc) (including LNO) and thin-film ferroelectrics. CDWs constitute a two-dimensional electron gas that fully penetrates the bulk sc [M. Schoeder et al., Adv. Funct. Mater. 22 (2012) 3936]. Photon-assisted ac/dc electron transport at variable temperatures was carried out along such CDWs [Schroeder et al., Mater. Res. Express 1 (2014) 035012] observing electron hopping to be the relevant transport mechanism in these 2D topological conductors, with a very low activation energy of ~100 meV, only. Using a pulsed photon excitation at IR wavelengths instead, suddenly allows us to map the full 3D topology of such CDWs in real time and real space [Kaempfe et al., APL 107 (2015) 152905], making use of Cerenkov second harmonic generation (CSHG) at domain walls [Kaempfe et al., PRB 89 (2014) 035314]. I will delineate how CSHG helps in improving the electronic transport properties in our CDWs.

Thirdly, pushing the photon energy even to lower values at NIR to THz wavelengths provides now access to phonon polaritons. We applied IR-scattering near-field optical microscopy (IR-sSNOM) operated between a 4 - 250 µm wavelength [Kehr et al., ACS Photonics 3 (2015) 20] to a broad palette of such ferroelectric semiconductors, i.e. LNO [Kehr et al., PRL (2008) 256403], BaTiO3 [Doering et al., APL 105 (2014) 053109] and GaV4S8 [Keszmarki et al., Nature Mater. 14 (2015) 1116]. The spectral analysis and imaging of the near-field response by IR-sSNOM down to a 5 nm lateral resolution provides a clear view of the dynamics and forces present in such nanosystems upon phase transitions and when applying external stimuli.