Optofluidic tunable lenses using laser-induced thermal gradient

This paper reports a new design of optofluidic tunable lens using a laser-induced thermal gradient. It makes use of two straight chromium strips at the bottom of the microfluidic chamber to absorb the continuous pump laser to heat up the moving benzyl alcohol solution, creating a 2D refractive index gradient in the entrance part between the two hot strips. This design can be regarded as a cascade of a series of refractive lenses, and is distinctively different from the reported liquid lenses that mimic the refractive lens design and the 1D gradient index lens design. CFD simulation shows that a stable thermal lens can be built up within 200 ms. Experiments were conducted to demonstrate the continuous tuning of focal length from initially infinite to the minimum 1.3 mm, as well as the off-axis focusing by offsetting the pump laser spot. Data analyses show the empirical dependences of the focal length on the pump laser intensity and the flow velocity. Compared with previous studies, this tunable lens design enjoys many merits, such as fast tuning speed, aberration-free focusing, remote control, and enabling the use of homogeneous fluids for easy integration with other optofluidic systems.