Thesis defense of Jan Niklas Mundry

This thesis presents fundamental research and concepts for active photonic elements operating in the telecom wavelength regime. The aim of the study is to determine the characteristics of the investigated nanostructures and to evaluate the implementation of the proposed materials in photonic research or in potential optical devices.
In the first part of this thesis the optical properties as well as the photonic application of vanadium dioxide (VO2) nanocrystals (NCs) are studied. VO2 exhibits an easily accessible insulator-to-metal phase transition (IMT) near ambient temperatures. Upon excitation it undergoes an atomic rearrangement that is accompanied by a substantial modification of the complex dielectric function. Specifically, VO2 NCs are characterized by near-infrared plasmonic resonances and show a wide IMT-hysteresis featuring a supercooled metallic state in the cooling-down process. When VO2 undergoes the IMT, the near-infrared transmission peaks of a moderate-finesse etalon containing a sub-wavelength layer of VO2 NCs are found to markedly shift in their spectral position and peak transmissivity. These modifications are related to the substantial change of the dielectric function connected to the structural phase transition. Both heat deposition and ultrafast optical excitation permit to actively control the etalon’s functionality. Transfer matrix simulations of the layered structure including the actual dielectric properties of VO2 NCs validate the experimental findings qualitatively. Much less is known about the nonlinear optical properties of VO2 beyond the established IMT. To this end the nonlinear optical response of a thin film of VO2 NCs is investigated with open aperture z-scans involving femtosecond near-infrared pulses. A pronounced saturable absorption on the short-wave side of the resonance as well as a marked reverse saturable absorption in the telecom window are observed. These nonlinearities can be attributed to a transient red-shift of the plasmonic resonance of the NCs, in line with the temperature dependence of the linear absorption and the theoretical expectation for electronic heating. The results hold promise for the use of VO2 nanocrystals as a saturable absorber, e.g., to mode-locked near-infrared lasers.
In the second part a semiconductor heterostructures based on hexagonal ultranarrow GaN/AlN multi-quantum wells (MQWs) is investigated. The tailored inter-miniband (IMB) transition is characterized in terms of its linear and ultrafast nonlinear optical properties. In particular, the central energy and exceptionally large energetic width of the IMB transition are analyzed and the time-scales of the electron relaxation are examined using the established pump-probe scheme. In line with theoretical predictions for LO-phonon scattering, a fast relaxation is found for resonant IMB excitation. In stark contrast, significantly larger relaxation times are observed for photon energies addressing the above barrier continuum. The last section reports on a new type of nonlinear metasurface taking advantage of these telecom-range IMB transitions. The heterostructure is functionalized with an array of plasmonic antennas featuring cross-polarized resonances at these near-infrared wavelengths and their second harmonic. This kind of nonlinear metasurface allows for substantial second harmonic generation at normal incidence which is completely absent for an antenna array without the heterostructure underneath. While the second harmonic is originally radiated only into the plane of the quantum wells, a proper geometrical arrangement of the plasmonic elements permits to redirect the second harmonic to free-space radiation, which is eventually emitted perpendicular to the surface.

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