Ion beam modification of phase-change materials for optical applications

Active optical metasurfaces with dynamic switchable, tunable, and recongurable optical functionalities are an emerging eld in photonics and optoelectronics. Especially, chalcogenide-based phase-change materials, such as Ge2Sb2Te5 (GST), can be fast and repeatedly switched by external stimuli between crystalline and amorphous states, typically accompanied by a tremendous difference of its electrical and optical properties. This thesis introduces ion irradiation of GST thin films in order to design purely GST-based metasurfaces in different ways: First, ion irradiation can directly be utilized to generate lattice defects, structural disorder, and ultimately amorphize GST to trigger the phase-change from crystalline to amorphous. Several structural and electronic transitions were observed during the ion bombardment of initially crystalline (rock-salt or hexagonal) GST films. As the amount of damage can be precisely controlled by adjusting the ion fluence, ion irradiated GST posseses gradually tailorable optical properties. Furthermore, a focused ion beam system was utilized to locally tailor disorder within crystalline GST films in sub-wavelength dimensions in order to design reconfigurable metasurfaces. Second, the phase-change behavior of GST itself can be manipulated by ion beam impurity doping. The incorporation of e.g. group V elements shifts the crystallization temperature. By controlling the doping level in highly confined regions, switchable active metasurfaces can be fabricated that are either triggered thermally or by applying various intense laser pulses. Both approaches show that ion irradiation is a powerful tool to transform phase-change materials in active, inherently planar, non-volatile, and reconfigurable optical devices.