Active controllable optical phase shifters employing the localized particle plasmon resonance in the near infrared of n-doped metal oxide nanoparticles

• Applicants: Professor Dr. Markus Haase; Professor Dr. Egbert Oesterschulze
• Subject Area: Experimental Condensed Matter Physics ; Synthesis and Properties of Functional Materials
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 492328754

Conventional redoxactive electrochromic (EC) materials have the capability to control the optical absorption of an EC film by means of an electrochemical potential. Although the Kramers-Kronig relations predict that this effect is accompanied by an optical phase shift for a wave passing through the film, it is hard to detect the phase shift over a broad spectral range because of the strong absorption mentioned. Therefore, the goals of this project are the investigation and active control of the optical phase shift employing inorganic n-doped metal oxide nanoparticles (NP) with capacitive EC effect. They exhibit a localized particle plasmon resonance (LSP) with strong absorption in a narrow band in the near infrared accompanied by an optical phase shift over almost the complete optical range with low absorption. This basic idea of our phase shifting concept was successfully proven in theory by evaluating the refractive index and absorption coefficient of a TiO2 NP film using a simplifying effective media approach (Bruggeman model). In the experiment the free electron density in the EC film will be varied by means of an external voltage allowing a continuous variation of the phase shift. This interdisciplinary project demands on one hand the expertise of the chemical synthesis of metal oxide NP including the control of size and doping level and the preparation of pastes for stencil printing. On the other hand, it requires the expertise to integrate these NP materials as thin film into microfabricated EC electrodes. This also involves the theoretical modeling, optoelectrochemical characterization and the application and proof of concept. The complementary expertise of the applicants has been successfully demonstrated in a recent BMBF project. In the project we will determine the spectral range of the phase shifting capability and its dependence on the chemical material composition, doping level and NP size. For the characterization of the LSP resonance in the near infrared and the optical response, we will perform UV-Vis-NIR spectroscopy and spectral ellipsometry. The phase shifting capability will be investigated for a discrete set of wavelengths with a spatially resolving Michelson interferometer. These experimental data are further used for a comprehensive theoretical modelling of materials and devices.The phase shifting capability of the metal oxide NP electrodes will be proven in two different sets of experiments: an optical phase modulator based on a microstructured phase grating (discrete set of diffraction angles and controllable intensity) and an innovative continuous beam stearing system controlling the polar and azimuthal angle. This requires an extensive FEM simulation of the potential distribution and the calculation of the spatially resolved resultant material-related phase change.

• DFG Programme: Research Grants