Dielectrophoresis-actuated in-plane optofluidic lens with tunability of focal length from negative to positive

Abstract

This paper reports a tunable in-plane optofluidic lens by continuously tuning a silicone oil-air interface from concave to convex using the dielectrophoresis (DEP) force. Two parallel glasses are bonded firmly on two sides by NOA 81(Norland Optical Adhesive 81) spacers, forming an open microfluidic channel. An ITO (indium tin oxide) strip and another unpatterned ITO layer are deposited on two glasses as the top and bottom electrodes. Initially, a capillary concave liquid-air interface is formed at the end of the open channel. Then the DEP force is enabled to continuously deform the interface (lens) from concave to convex. In the experiment, the focal length gradually decreases from about -1 mm to infinite and then from infinite to around + 1 mm when the driving voltage is increased from 0 V to 260 V. Particularly, the longitudinal spherical aberration (LSA) is effectively suppressed to have LSA < 0.04 when the lens is operated in the focusing state. This work is the first study of in-plane tunable lenses using the DEP force and possesses special merits as compared to the other reported tunable lenses that are formed by pumping different liquids or by temperature gradient, such as wide tunability, no need for continuous supply of liquids, low power consumption (~81 nJ per switching) due to the capacitor-type driving, and the use of only one type of liquid. Besides, its low aberration makes it favorable for light manipulation in microfluidic networks.

Biomimetic optics: liquid-based optical elements imitating the eye functionality

“…The methods for controlling the shape of the refracting liquid interface are based on applying external stimuli such as electric fields, causing electrowetting [19][20][25][26][27][28][29][30][31][32][33][34][35] and dielectrophoresis [36][37][38][39][40][41] effects, thermal fields [42][43][44], pneumatic and/or hydraulic pressure [45][46][47], pH change [48], optical irradiation [49] and acoustic pressure [50,51]. All these methods have advantages and disadvantages associated with features of the optical element design, implementation of optical characteristics control, quality of obtained images and the response time of an optical system to applied external stimuli [10,11].…”

“…Electrical effects such as electrowetting and dielectrophoresis [19][20][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] are the most widely used for tuning optical parameters of individual optical elements such as liquid lenses and apertures. A reversible change in the contact angle is an effective way to control the shape and position of a liquid droplet on a dielectric substrate or an interface between two immiscible conductive and non-conductive liquids in contact with the dielectric wall of a cell.…”

“…The dielectrophoresis is used as a driving force to tune the optical parameters of systems consisting of two immiscible dielectric liquids having different values of the dielectric constants and the refractive indices [36][37][38][39][40]. Contrary to the electrowetting effect, the dielectrophoresis results in the volume forces and does not require the use of electroconductive liquids and providing the contact of the electrode with the liquid.…”

“…The response time of the solutocapillary lenses takes values from 5 to 30 s depending on the power of the laser and the concentration components [56]. This response is slower in comparison with that of the liquid lens tuned by the laser-induced thermocapillary forces [54] and the electrically actuated lenses [28,39]. However, this drawback can be overcome by decreasing the initial size of the droplet and selecting liquids with low viscosity and with a larger difference in surface tension between them.…”

Tunable-Focus Liquid Lens through Charge Injection

“…Apart from the above methods, the electrically driven method is the most common technique used for manipulating liquid lenses [27][28][29]. The electrically driven mechanism mainly consists of electrowetting [27,28] and dielectrophoresis [29]. Owing to high accuracy and flexibility, the electrically driven method has been widely investigated for many applications, such as adaptive optics, optical switching, and displays.…”