Switchable self-trapping and remote interactions of multiple beams in a photoresponsive hydrogel Fariha Mahmood 1 , Derek R. Morim 1 , Amos Meeks 2 , Ankita Shastri 3 , Andy Tran 1 , Anna V. Shneidman 2,4 , Victor V. Yashin 5 , Anna C. Balasz 5 , Joanna Aizenberg* 2,3,4 , and Kalaichelvi Saravanamuttu* 1 1 Department of Chemistry and Chemical Biology, McMaster University, Hamilton, ON, Canada 2 John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA 3 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA 4 Kavli Institute for Bionano Science and Technology, Harvard University, Cambridge, MA, USA 5 Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA, USA We study the behaviour of laser beams as they propagate through a spiropyran-doped hydrogel material. The chromophore, which is initially in its merocyanine form, is converted to spiropyran upon illumination, prompting a light-induced contraction of the gel. This causes a local increase in refractive index, which influences the propagation of the incident beam, giving rise to nonlinear phenomena such as self-trapping. The initially broad and divergent beam narrows, eventually maintaining its entrance face profile throughout the path length of the hydrogel. This self-trapping process is characterised by an increase in beam power coupled with a decrease in beam diameter. When the light is removed, spiropyran spontaneously relaxes back to merocyanine, reversing the contraction and returning the gel to its original state. Due to the crosslinked nature of the hydrogel, contraction in one region inhibits further contraction elsewhere. Thus, when two beams are launched into the material, they do not narrow to the same extent. However, by blocking one beam, the other can be permitted to fully self-trap. In this way, one beam may be “switched” or controlled by another beam, without the need for optical overlap, through the medium of the hydrogel. In the current work, we describe additional studies of three and four collinear beams in this hydrogel medium.
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