Ionic-Driven Optical Tuning: Unlocking Dynamic Refractive Index Control with PEDOT:PSS

Graded index (GRIN) materials have gained significant interest for their potential in photonics, offering control over light propagation without the need for complex structuring. In this context, we explore a novel approach using a three-terminal organic electrochemical transistor (OECT) configuration to create a spatially tunable refractive index profile in a PEDOT:PSS film. This method harnesses ionic modulation driven by gate and drain voltages to precisely manipulate the charge distribution within the polymer matrix, resulting in controlled spatial gradients of the refractive index. Specifically, by injecting ions into the PEDOT:PSS film through careful voltage application [see Fig. 1(a)], we achieve a spatial variation of charge concentration along its length that results in transmission modulation (relative transmittance variation <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta\mathrm{T}/\mathrm{T}$</tex>: with respect to un-biased state) across different points of the channel [see Fig. 1(b)]. The interaction between bulk ionic and electronic charges allows for a gradient index that we characterize using transmission measurements. By employing a model based on Drude and Lorentz oscillators, we extract spatially resolved parameters of the complex refractive index, including plasma frequencies, damping, and resonance frequencies, across the film. Our findings demonstrate that different voltage configurations yield distinct refractive index modulation patterns, offering a broad design space for tunable optical properties in PEDOT:PSS [1]. The voltage-programmable nature of this approach presents a powerful solution for overcoming the limitations of traditional GRIN materials. Unlike conventional methods that rely on nanoscale patterning, our technique enables continuous, controllable refractive index profiles without blazed grating effects. This flexibility facilitates the realization of axial and radial refractive index distributions with isoindex surfaces modulated through simple electrical control. Consequently, this platform opens the door to a new class of flat, ultra-compact, and broadband optical devices. In summary, the demonstrated ionic modulation of PEDOT:PSS offers unprecedented spatio-optical tunability, paving the way for applications in telecommunications, medical devices, astronomy, defense, and smart windows. Future innovations could include adaptable optical interconnections, tunable focal-length devices, and low-frequency displays, all enabled by this versatile, voltage-driven photonic platform.