Design Of Laser Microbeam System Combined Laser Scissors With Optical Tweezers And Fabrication Of Related Components

Laser microbeam is a novel bio-photonic microscopic manipulating technology, which includes optical tweezers and laser scissors two different kinds of biological micro-manipulating tools. Optical tweezers utilizes gradient force derived from photon momentum transfer to trap particles, such as cells, bacteria and other biological macro-molecules. Laser scissors, relying on photo-ablation interaction between high influence, short pulse ultraviolet laser irradiation and biotissues, can conduct precise laser microsurgery on cell walls, chromosomes, et al. Laser microbeam, combining optical tweezers and laser scissors, has become a necessary multi-functional tool in the life science studies.Considering the basic function requirements of optical tweezers and laser scissors, theoretical design and experiments concerning laser microbeam system have been studied in the thesis. The study is on the basis of the LOTII optical tweezers apparatus and near-field optics researches of our group. The thesis mainly includes two aspects: the one part is the designing and experiments of laser microbeam system, which includes experiments of laser trapping, design of coupling beam path of laser scissors, corresponding experiments of laser scissors. The other part is concerning the fabrication techniques of micro- and nano- fiber components, which includes near-field optical fiber nano-probes and optical fiber micro-lens fabrication. Micro- & nano- fiber components can serve and support for the studies of near-field or fiber tweezers and scissors, highly efficient coupling between laser output and optical fibers. Fiber components with appropriate configuration and dimension are necessary for the above studies.In the front two chapters, the history, theories and current studies of optical tweezers were reviewed. The principles and characteristics of ultraviolet laser scissors were introduced. The history and prospect of laser microbeam were reviewed. The advantage of near-field optical trapping was introduced. The recent advances and main difficulties were also discussed. The fabrication techniques of near-field optical fiber probe by chemical etching method were introduced, deduced and discussed in detail. All these laid background for further studies.Based on the LOTII tweezers system, yeast cells trapping was conducted using 40×and 100×microscopic objectives. Through comparison of different experimental results, effects of trapping accuracy, operation stability and trapping power depended on numerical aperture (NA) of focus objective was analyzed either. Optical trapping of larger mammary cancer cells and hippocampal neuron cells were conduct. The experimental results were analyzed. The study scheme of the combination of optical tweezers and patch clamp to study electrophysiological characteristics of suspended cells was proposed. The operability of the scheme was experimentally verified too.Coupling beam path of laser scissors was introduced into the LOTII tweezers system by inserting a dichroic mirror. The influence of tweezers beam path, translating control of scissors focus by steering mirror, and chromatic abbreviation of different wavelengths after focusing were considered and calculated. The beam parameter product of the FTSS 355-50 pulsed UV laser and coupling lens design parameters meeting experimental requirement were also computed. The drilling of onion epidermal cells was demonstrated by UV laser scissors.Near field optical fiber probes with large taper angle and high throughput were fabricated by hybrid arc heating-micropulling-melting and static etching. Pure silica-core ultraviolet multimode fiber probes with micron dimension were fabricated by tube etching. UV fiber microlens with micron dimension were produced by hybrid etching and melting method for the study of optical fiber scissors.According to the preparation of tapered fiber hemispherical microlens by etching and melting method, a geometric calculation model for fabricating hemispherical micro-lensed optical fiber by melting tapered flat fiber end into hemispherical fiber end has been developed. If the taper diameter and taper angle are known, the radius of hemispherical micro-lens of the fiber taper after melting can be calculated. According to the linear relationship between etching fiber diameter and etching time, the etching velocity can be determined by the sampling and analysis of the etched fiber. Thus, the etching time can be calculated according to the desired designing requirement. Through precisely controlling the etching time the hemi-spherical micro-lensed fiber with different radius can be produced by etching-cutting-melting method. The radiuses of microlenses fabricated by this method made a good coincidence with the designed radius values.The study results of this thesis show that1) The numerical aperture (NA) of the trapping focusing lens highly influences the trapping stability and trapping accuracy. The greater NA, the higher beam convergence and greater gradient force, thus leads to a higher trapping stability. Multiple particles axial trapping was demonstrated using less NA objective, which reduces axial trapping accuracy. The above phenomenon was not found when using greater NA objective. A higher NA of focusing lens results in a higher trapping accuracy. The trapping efficiency of the optical tweezers was partly determined by the beam quality of the trapping laser. The better the beam quality is, the higher the trapping efficiency. Moreover, manipulation of cells with several times larger than the laser focus is possible. The study scheme of the combination of optical tweezers and patch clamp to study electrophysiological characteristics of suspended cells is feasible experimentally.2) Laser scissor light beam can be coupled into optical tweezers system by inserting a dichroic mirror which is well designed for NIR transmission and UV reflection, which makes nearly no difference on trapping and manipulation of optical tweezers. The cutting function of laser scissors can be fulfilled by controlling laser spot via rotating the dichroic mirror. The beam parameters of FTSS 355-50 type UV laser can meet the requirements of fiber coupling. High efficiency fiber coupling can be achieved by designing special coupling lens. The drilling of onion epidermal cells by laser scissors shows that energy profile of beam determines the shape of drilling hole, the dimension of the hole is associated with the numbers of laser pulses, both the photoablation and photothermal effects exist in the drilling process.3) Large taper angle and high efficiency optical fiber nano-probes for near-field imaging were fabricated by a method combining heated micro-pulling and static chemical etching. The finished probes have the taper region which fiber core unchanged while cladding is tapered. The light transmittance of the probe is higher than that of the probe produced by heating and pulling, while the taper angle is larger than probes fabricated by static chemical etching, which helps to increasing light throughput of the probe. The volume of the fiber cladding reducing by heated micro-pulling decreasing the etching time, which minimizes the uncertainties of the etching process and thus improves the yield of the usable fiber probes.4) Hemispherical micro-lensed optical fiber with various curvature radiuses can be fabricated by etching-cutting-melting method. According to the linear relationship between etching fiber diameter and etching time, as well as the characteristic of fiber volume maintaining constant before and after electric arc melting, the relationship between the curvature radius of fiber hemispherical micro-lens and etching time was deduced. Through controlling the etching time precisely the hemi-spherical micro-lenses with different radius at the fiber ends can be produced.