|Title:||Optofluidic tunable lenses for in-plane light manipulation|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Nanofluids -- Optical properties
|Department:||Department of Applied Physics|
|Pages:||xxvi, 132 pages : color illustrations|
|Abstract:||Optofluidics seamlessly integrates optics and microfluidics in a single chip to make full use of the advantages of both. This doctoral study focuses on developing optofluidic lenses for in-plane focusing and diverging of light and presents three new designs of tunable liquid lenses: the laser-induced thermal gradient lenses, the dielectrophoresis (DEP) actuated lenses using single liquid-air interface, and the DEP-actuated lens using two liquid-air interfaces. The first design is a tunable lens using laser-induced thermal gradient effect. Two metal strips are used to absorb the pump laser and to heat up the flowing solution, creating a 2D refractive index (RI) gradient for beam reshaping. Raytracing simulation and experiments are 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. Compared with previous designs, this thermal lens enjoys unique merits such as fast response (200 ms), using only one liquid, remote control, freely relocation of the lens, etc. In the second design, the DEP effect is used to deform one liquid-air interface that is sandwiched between two parallel plates. The initially concave liquid-air interface can be continuously deformed into a convex one, tuning the lens from the divergent state (f = -1 mm) to the convergent state (f = +1 mm). Due to the capacitor-type driving, it consumes only 81 nJ per switching circle. In addition, the longitudinal spherical aberration (LSA) is effectively suppressed using the edge pinning effect. The third design is an in-plane tunable lens using two DEP-actuated liquid-air interfaces. By increasing the applied voltage, the focal length can be tuned from -0.5 mm (0V) to infinite (175V) and then to +0.5 mm (250V). It achieves an f number of 0.91 while consumes only 6.7 nJ per switching circle. In the last two DEP-actuated lenses, large RI difference at liquid-air interface results in small focal length, and the uses of static flow (do not require continuous liquid supply) and electrical actuation make them easy to operate and simple to integrate with other lab-on-a-chip devices/systems. In summary, three new designs of optofluidic tunable lenses have been presented for in-plane manipulation of light. Theoretical simulations and experiments have been conducted to examine the performance of the optofluidic devices. The thermal lens is new in term of working principle and facilitates the noncontact tuning of focusing state, while the DEP liquid lenses enable continuous tuning from sharp divergence to tight focusing without the requirement of continuous flow. As compared to the reported hydrodynamic-pressure or diffusion based liquid lenses that require continuous supply of two or more types of liquids and are tuned by adjusting the flow rates, our three types of lenses are unique and superior since they all allow the use of a single type of liquid and the two DEP-actuated lenses work at static flow and can be easily tuned by electrical voltage. These optofluidic lenses may find potential applications in lab-on-a-chip systems that require the functions such as optical switching, particle trapping, beam reshaping, optical in/out coupling, and so on.|
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