Controlling light with light

 

 

PAAD3

Twisted LC cell. Linear polarised light causes PAAD molecules to align perpendicular of the light’s polarisation.

We study the intrinsic and photoinduced properties of thin films of azobenzene complex dyes (PAAD) in the visible and in the THz range.

In the visible range we have successfully demonstrated strong, birefringent phase gratings light in thin 15-35 nm PAAD films. The gratings were found not to be associated with any periodic surface relief, a typical feature in thicker azobenzene layers.  We have also shown that the photo-induced refractive index change in PAADs is anisotropic with largest refractive index modulation in the direction parallel to the polarization of the writing beams [1].  The origin of such refractive index modulation cannot be fully attributed to a molecular reorientation effect. This intriguing mechanism of interaction of thin PAAD films with light will be studied further, in particular by considering the interaction with the substrate surface and its role in the formation of the gratings.

Azo PAAD films were also used as photoalignment layer in LC cells. Optically controlled, rewritable modulators and phase shifters are developed, based on a twisted nematic liquid crystal cells. The modulators are bistable, with switching between states controlled by one-step illumination with visible light. The photo-alignment properties of PAAD facilitated reversible switching between two perpendicular alignment states at the cell surface, resulting in controllable polarization manipulation. The resulting modulation in transmission for different visible and NIR wavelengths were demonnstrated with with an optical contrast of up to 90-100%. This result paves the way to LC modulators and phase shifters which can be controlled remotely using only external light sources [2]

We are also working on exploiting the anisotropy of azobenzene layers in THz region of spectrum and investigate, through indirect measurements, the shift of plasmonic resonance in matamaterials designed for an efficient, active control of THz radiation.

 

1. E. Mavrona, S. Mailis, N. Podoliak, G. D’Alessandro, N. Tabiryan, M. Trapatseli, J.-F. Blach, M. Kaczmarek, and V. Apostolopoulos, “Intrinsic and photo-induced properties of high refractive index azobenzene based thin films,” Opt. Mater. Express 8 (2), 420-430 (2018)
https://doi.org/10.1364/OME.8.000420


2. E. Perivolari, J.R. Gill, N. Podoliak, V. Apostolopoulos, T.J. Sluckin, G. D’Alessandro and M. Kaczmarek, Optically controlled bistable waveplates, J. Mol. Liq. (2017)
https://doi.org/10.1016/j.molliq.2017.12.119