Tunable Liquid Gradient Refractive Index (L-GRIN) lens with two degrees of freedom

TLDR

The authors introduce the Liquid Gradient Refractive Index (L‑GRIN) lens, a tunable optofluidic microlens that focuses light inside a microfluidic device and offers a new strategy for integrative tunable microlenses in lab‑on‑a‑chip applications. The L‑GRIN lens achieves focusing by creating a hyperbolic‑secant refractive‑index profile through CaCl₂ diffusion between co‑injected laminar flows, allowing the focal distance and output light direction to be tuned by adjusting flow conditions. The device demonstrates tunable focal distance and output direction, low fluid consumption, competitive focusing performance, and high compatibility with existing microfluidic systems.

Abstract

We report a tunable optofluidic microlens configuration named the Liquid Gradient Refractive Index (L-GRIN) lens for focusing light within a microfluidic device. The focusing of light was achieved through the gradient refractive index (GRIN) within the liquid medium, rather than via curved refractive lens surfaces. The diffusion of solute (CaCl2) between side-by-side co-injected microfluidic laminar flows was utilized to establish a hyperbolic secant (HS) refractive index profile to focus light. Tailoring the refractive index profile by adjusting the flow conditions enables not only tuning of the focal distance (translation mode), but also shifting of the output light direction (swing mode), a second degree of freedom that to our knowledge has yet to be accomplished for in-plane tunable microlenses. Advantages of the L-GRIN lens also include a low fluid consumption rate, competitive focusing performance, and high compatibility with existing microfluidic devices. This work provides a new strategy for developing integrative tunable microlenses for a variety of lab-on-a-chip applications.