VCSEL-Based Optical Trapping Systems for Microfluidic Applications

Optical trapping and manipulation by laser beams offers the unique possibility to handle single micrometer-sized particles such as living cells without any mechanical contact, damage or contamination. A second hot topic in biology is microfluidics, where the examination of biological samples in channel structures with widths below 100 µm reduces the used sample volume significantly. While the combination of both techniques results in attractive lab-on-a-chip structures for particle sorting and analysis, the commonly bulky trapping setup is contradictory to the miniaturized concept. Here, the use of vertical-cavity surface-emitting lasers (VCSELs) as light sources in optical trapping systems allows a strong reduction of the setup complexity owing to the small dimensions, low cost and high beam quality of these devices. This thesis gives a detailed study on optical manipulation systems based on vertically emitting laser diodes. A standard optical tweezers setup as well as a novel, miniaturized system, the so-called integrated optical trap are investigated. The latter aims for particle separation and sorting in microfluidics resulting in low-cost, portable modules. A classical optical tweezers system based on a high numerical aperture objective in combination with a VCSEL light source is investigated. Standard multi-mode as well as single-mode surface relief VCSELs are used as laser source. With both kinds of VCSELs, optical trapping of polystyrene particles of sizes ranging from 4 to 15µm is demonstrated with some milliwatts of optical power at the sample stage. A maximum trapping force of 4.4 pN for 15 µm particles is achieved with the multi-mode laser, proving the suitability of multi-mode lasers for optical manipulation despite their inferior beam profile. By using two-dimensional VCSEL arrays instead of solitary lasers, the system is extended to a multiple optical tweezers setup in a straightforward manner. To avoid any additional optics, densely packed VCSEL arrangements with a device spacing of less than 25 µm are used, where a novel fabrication process allows the seamless integration of the inverted surface relief technique for enhanced beam quality. By electrical switching between individual devices of the array, non-mechanical particle translation with velocities of up to 12 µm/s is achieved. With a tilted linear VCSEL array, an optical lattice is generated in the optical tweezers setup, and continuous deflection of particles is realized. By substituting the sample stage in the optical tweezers setup with a microfluidic chip fabricated from polydimethylsiloxane (PDMS), particle redirection at a channel junction is realized using a solitary VCSEL source as well as a tilted linear VCSEL array. For the latter, the particles are deflected when passing the optical lattice, thus, the position of the lasers is fixed and no moving parts are necessary, which further reduces the setup complexity. To achieve a drastic miniaturization of the trapping setup, namely the integrated optical trap, the laser source is placed directly underneath the sample chamber. A weakly focused laser beam is generated in the particle solution by integrating an additional microlens on the VCSEL output facet. To determine appropriate lens geometries, the beam propagation inside the integrated trap structure is calculated and the thermal reflow process for lens fabrication is studied in detail concerning lens diameter, reflow temperature and substrate material. By combining the microlens with the inverted relief technique, the quality of the focused beam is strongly improved with respect to divergence, transverse beam profile and beam diameter, where a minimum of 7 µm is measured at the focal point. With first solitary integrated optical traps, deflection, levitation and transverse trapping of 10 µm polystyrene particles is demonstrated for optical powers of 5mW. In a next step, integrated optical trap arrays are realized based on closely spaced twodimensional arrangements of lensed relief VCSELs. To transfer the continuous deflection scheme demonstrated in the classical tweezers setup to the integrated trap, linear arrays of parallel working VCSELs are investigated. To support the design of the multiple integrated trap structure, a simulation of the optical deflection process is performed. Here, a dependence on the geometric and material properties of the particles is predicted, so applications in microfluidic particle sorting are intended. Compact and portable modules are obtained by integrating the laser chip with the microfluidic chip using flip-chip bonding. Although the finished modules show strong heating of the VCSEL chip resulting in a significant reduction of the device performance, simultaneous trapping as well as continuous particle deflection was successfully demonstrated with a total optical power of just 5mW. The results presented in this work demonstrate the potential of VCSELs as laser sources for optical trapping and microparticle manipulation. In conventional optical tweezers setups, the use of VCSELs reduces the setup complexity significantly, while first prototypes of ultra-compact integrated optical traps based on VCSELs confirm the feasibility of portable and inexpensive microfluidic sorting systems.