Research On Laser Fabrication Technologies And Performance Characterization Of Micro-Optical Components

Femtosecond laser micro-fabrication technology (FLMFT) is a laser directwriting technology. It features high accuracy, simple process, high tolerance tocomplexity of geometry, and therefore is the technology of choice for the fabricationof micro-objects, such as micro-optics, micro-mechanics, etc. With the developmentof optical communication and optical computation, optical micro-processing systemsdemonstrate a growing demand for a variety of micro-optics. Micro-optics, as withtheir macroscale equivalents, have generally particular surfaces to modulate light, sothey need a high-accuracy micro-fabrication technology which is, at the same time,insensitive to the complexity of geometry. In addition, light driven mciro-rotor, whichcombines micro-optics and micro-mechanics, has significant value in bio-technology,medical analysis, etc. due to flexibility of manipulation that is free of mechanicalcontact. However, current micro-fabrication technologies such as photolithography,imprinting, self-assembly, face a challenge of fabricating high-efficiency micro-rotordriven by light. Confronted by the above issues, this dissertation covers the study ofthe combination of FLMFT and micro-optics as well as the design and analysis of thefabrication products. The major achievements are:1) we fabricated micro Dammanngratings, which had higher diffraction efficiency than those facricated by other laserfabrication technologies, based on the improvement on the spatial resolution of andefficiency of laser micro-fabrication through the study on the process ofphoto-polymerization of polymer material induced by femtosecond laser. In addition,we invented a technique for rapid production of micro-lenses, which utilizesbuoyancy and surface tension.2) we realized refractive micro logarithmic axicon,which has special optical properties such as large focal depth, and whichdemonstrated fine imaging function that focal depth without aberration is significantlyenlarged compared to common micro-optics.3) we designed and fabricated a light driven micro-rotor with helicoidal blades, the size of which, as large as50μm, is oneorder of magnitude larger than that of rotors ever reported, and which could rotate asfast as500rpm. The average transfer of angular momentum from photons, whichrepresents the efficiency of the rotor converting light to mechanical energy, attains ashigh as34.55/photon, much higher than that ever reported.The study at the very beginning aimed at the fabrication of aquasi-three-dimensional phase-type diffractive optical element—micro Dammanngrating. A Dammann grating is a diffractive element with a complicated periodstructure. Combined with a Fourier lens, it can split a beam into an array of coherentbeamlets, regardless of the coherency of the incidence. Due to this feature, it findsapplications in array illumination, multiple imaging, star couplers. We employedFLMFT in the fabrication of phase-type micro-Dammann gratings, whichdemonstrated good optical performance when combined with the Fourier lensproduced via a technique that we had invented. Its diffraction efficiency was higherthan those reported produced in glass while fabrication accuracy was greatlyimproved and process time was largely shortened. This work not only attests thefeasibility of miniaturizing Dammann gratings, but also shows the advantage ofFLMFT in fabrication of Dammann gratings, that is, free of expensive mask, flexibledesign, and shorter fabrication period.The following study turned to the design and fabrication of three-dimensionalrefractive micro-optics with more complex surface—aspherical micro-lenses. Amicrolens is a small lens generally with a diameter less than a millimeter. They have awide range of applications in modern optical devices. Therefore, numeroustechnologies have been developed for their manufacture in the past decades. However,much less studies have referred to the fabrication employing FLMFT, whoseadvantage embodies in the fabrication of complex surfaces. We designed a hyperboliclens, which, under the regime of geometric optics, can focus parallel light into aperfect point theoretically. Had optimized the fabrication parameters and strategies,we fabricated this hyperbolic micro-lens, which showed good focusing performance. Further study into the fabrication of complex surfaces led to the fabrication ofRMLAs, which is characterized by the capability of focusing parallel light into asegment of the optical axis with evenly distributed intensity. Compared with acommon lens, it has a much larger focal depth, which means the image formed by thiselement keeps almost the same in a wide range and thus may be a useful character forimaging systems that can be used to improve the stability of imaging and depth offield. In optical tweezers, a large focal depth is also useful for capturing smallparticles. We studied the fabrication of RMLAs, employing FLMFT, examined theoptical performance of fabricated RMLAs, which showed their particular opticalproperties and a potential of further improvement for the performance of imagingsystems.Besides the fabrication of micro-optics, we also studied light driven micro-rotors,which are a combination of micro-optics and micro-mechanics. In the past twodecades, much research has focused on how to rotate a rotor rather than on theconversion of light energy into mechanical energy, due to lack of technology thatallows complex structure for high conversion efficiency being produced. In our work,we designed a light-driven micro-rotor with a complex three-dimensional structureand investigated its rotation. The highest rotation speed was over500rpm. We alsoanalyzed the kinetic mechanism of light driving theoretically and experimentally. Inaddition, the light-to-mechanical energy conversion efficiency, measured by averagetransfer of angular momentum from photons, was studied as well, and experimentallydetermined to be as high as34.55/photon, much higher than that of those everreported, and agreed to the theoretical estimation in order of magnitude. Moreover,this three-dimensional micro-rotor is one order of magnitude larger than those everreported, and may find applications where a high-efficiency micro-device with largeactuating range for conducting, pumping, agitating micro-fluid is needed.