Responsive Polyelectrolyte Multilayer Cladding for Reversibly Programmable Photonics

Polyelectrolyte multilayers (PEMs) are versatile, easily applied thin films, which respond to various triggers such as salt concentration or pH, leading to morphological changes in the polymer network. Aiming to use PEMs to program photonic circuits, we prepare the multilayers on model substrates by alternatingly depositing poly(allylamine hydrochloride) and poly(acrylic acid). These PAH/PAA PEMs reversibly swell upon treatment with acidic (pH = 2) and near-neutral (pH = 5.5) solutions. Interestingly, drying from these discrete pH values generates PEMs with low or high porosity and thus, contrasting refractive indices. Next, we demonstrate that the refractive index change between the distinct states of the PEM alters the response of the underlying photonic devices. We present two approaches to control this shift of the device output. The first requires a thin layer of silica, applied between the device and the PEM, while the second method involves area-selective deposition of the PEM on parts of the device. Each of these allows us to reversibly program the device response, without requiring continuous input, paving the way for the much desired programmability of photonic integrated circuits.