Optical programming of infrared phase-change material metasurfaces

  • Optisches Programmieren von infraroten Phasenwechselmaterial-Meta-Oberflächen

Heßler, Andreas; Taubner, Thomas (Thesis advisor); Wuttig, Matthias (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

Infrared light is tightly interwoven with modern technology, from telecommunications over thermal imaging to medical diagnostics. Infrared optical instruments, like mirror objectives, are bulky and only have one functionality while compact optical components with multiple integrated, reconfigurable functionalities are needed, for example, for mobile phone technology or autonomous driving. Nanometer-thick active metasurfaces based on phase-change materials (PCMs) have emerged as a promising way for realizing such devices. They consist of periodically arranged subwavelength-sized antennas ("meta-atoms"). The PCM in or around the antennas can be switched non-volatilely between its amorphous and crystalline phases. The corresponding drastic change in the refractive index of the PCM tunes the antennas' resonance. Commonly, multiple operation states are achieved by switching the whole PCM on the metasurface equally. For programmable metasurfaces with freely reconfigurable functionality, however, the ability to selectively change the light amplitude and phase of each individual meta-atom is required. In this thesis, therefore, concepts for the programming of infrared PCM metasurfaces by local optical switching with a focused laser are developed and realized experimentally. First, the local optical addressing of individual meta-atoms is introduced and demonstrated at aluminum nanorod antenna arrays covered with a 75 nm-thick layer of the PCM Ge3Sb2Te 6 (GST). Simultaneous control of size, position and crystallization depth of the switched PCM volume is used to tune the resonances of the antennas by more than one resonance width (FWHM) from about 5 µm to 6 µm. Next, this technology is applied to dielectric infrared Huygens' metasurfaces where each meta-atom consists of a disk with a germanium core sandwiched by two 70 nm-thick GST layers. The antenna resonances of individual disks are tuned by up to 360 nm (1.8 FWHM), leading to a change in the light phase of up to 0.8*2π at an average transmittance of 50%. Different spatial light phase distributions are optically encoded onto the metasurface. Finally, the next-generation plasmonic PCM In3SbTe2 (IST) is introduced. Its ability to switch between dielectric and metallic optical properties in the infrared spectral range enables completely new resonance tuning mechanisms. Direct optical writing, erasing and reconfiguring of plasmonic antennas in a 50 nm-thick IST film are demonstrated. Electric dipole resonances of rod antennas are thus tuned by more than 4 µm and magnetic dipole resonances of split-ring resonators are shifted by more than 1.6 µm. Finally, a tunable mid-infrared absorber with nearly 90% absorptance as well as nanoscale selective screening of Al-slit antennas and soldering of Au-dimers are presented. The developed technology of optical programming of PCM metasurfaces may pave the way towards the ideal of a "universal" metasurface which can freely manipulate incident light. This could lead to highly efficient, ultracompact active optical elements like tunable lenses, dynamic holograms, and spatial light modulators.

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