Investigation of micro-optofluidic components and their applications.
Date of Issue2011
School of Mechanical and Aerospace Engineering
The field of optofluidics is a combination of optics and microfluidics to generate new functionalities and to improve the integration level of optical components. Liquid replacing, liquid refractive index control and interface control are the three main methods to make reconfigurable optofluidic components. This thesis focuses on the manipulation of the interface between different flows through hydrodynamic spreading to develop tunable in-plane optofluidic components. A circular chamber is used to hydrodynamically develop the optofluidic lenses with liquid core liquid cladding (L2) structure. The two-dimensional dipole flow theory and the hydrodynamic spreading theory are applied to describe single-fluid and two-fluid models respectively. The model described here provides a method to configure optofluidic lenses with a mathematically pre-defined curvature, whose interface is atomically smooth. Furthermore, the curvature of the interface as well as the focal length can be tuned by adjusting the flow rate ratio. An optofluidic bi-concave lens, which can both focus and diverge the light, is first proposed and demonstrated. In the focusing mode of the optofluidic concave lens, a liquid with low refractive index is used as the core stream and a liquid with higher refractive index serves as the cladding stream. An auxiliary cladding liquid with a very low flow rate and a refractive index matching that of polydimethylsiloxane (PDMS) is introduced to prevent the light scattering. The light emitted from an optical fibre can be focused and the focal length can be tuned by adjusting the flow rate ratio between the core and the cladding streams. If the core inlet is blocked, the liquid from cladding inlets will converge into a new core stream and the liquid from the auxiliary cladding inlet will serve as cladding streams. Therefore, the device can seamlessly switch from the focusing mode to diverging mode. In the diverging mode, a liquid with high refractive index works as the core stream, and a liquid with refractive index matching PDMS is used as cladding streams. Owing to the higher refractive index of the core stream and the tuneable lens interface, the divergence of the light beam can be expanded and adjusted. Combining the performance of focusing and diverging, the device can greatly enhance the tunability of the focal length of an optofluidic lens. Based on the L2 structure, an optofluidic prism is developed to manipulate the in-plane light beam. This tunable micro optofluidic prism is hydrodynamically formed by one core and two cladding streams inside a sector-shape chamber. Liquid with a higher refractive index is employed as the core stream, which forms the geometry of a triangular prism. Liquid with a lower refractive index serves as cladding streams. Due to the higher refractive index of the core stream, this configuration acts as a prism. The apex angle of this optofluidic prism can be tuned by adjusting the flow rate ratio between core and cladding streams, and therefore the deviation angle of an incident light beam can be changed accordingly. Since the propagation direction of refracted light beam can be accurately controlled by choosing a proper flow rate ratio, this tunable prism can continuously scan the light beam, and therefore can be engaged in the alignment of optical path, or in the development of optical switches. Aperture stop or entrance pupil is an important optical component in imaging and detection systems. The component determines the amount of light passing through the optical system as well as the angular aperture on the object side. A tunable optofluidic aperture stop, which can be dynamically re-configured according to the flow condition, is proposed and demonstrated. A liquid core liquid cladding structure was used to form this aperture. The core liquid is optically transparent allowing light rays to pass through, while the ink with negligible optical transmittance (~ 0%) works as the cladding liquid blocking the propagation of light. When the aperture is tuned with a given flow rate ratio, the amount of light which reaches the image space as well as the angular aperture on the image side can be adjusted accordingly. Due to the solid-based lens interfaces, previous optofluidic flow cytometer designs suffer from light scattering, which degrades the quality of the focused light beam. Divergence or scattering of the light beam results in a larger beam width. One obvious disadvantage resulting from the large beam width is that the cytometer would miscount if multiple particles enter the large illumination area because the multiple particles would give only one burst in the collected signal. Normally this burst owing to multiple particles has a larger peak value than that of one particle. Thus the other consequent problem is that this burst of the signal would be misleading as it would be wrongly interpreted as a larger particle. A new design of a microfluidic flow cytometer with an optofluidic lens formed in a circular chamber is proposed and tested. The optofluidic lens has a mathematically predictable focal length, and is immune to light scattering. A well-focused light beam is achieved with a much smaller beam width of 23 um compared to all previous flow cytometers. This flow cytometer presents a good performance on particle counting, and the signal intensity shows small aberrations and strong dependence on the particle size.