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3D Internal Hollow Microstructures Manufacturing Technology By Femto-second Laser

Posted on:2017-05-25Degree:DoctorType:Dissertation
Country:ChinaCandidate:C ZhengFull Text:PDF
GTID:1108330503492420Subject:Optical Engineering
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Fabrication of hollow structure inside a certain transparent dielectric substrate with the help of a femtosecond laser which possesses intensive peak power density and a very short pulse duration can effectively avoid the drawbacks when fabricate such structurers by using traditional surficial structuring methods, such as a low machining speed, cumbersome fabrication procedures, low success rate, difficult in structuring a complicated 3D spacial microchannel networks, etc.. It thus possess a very broad applications and a bright future in the applications of integrated microchip systems. This thesis provide a systematically study about the physical mechanisms and machining features when fabricating continuous or discrete hollow cavitary structures inside a polymer dielectric polymethyl methacrylate(PMMA) from both theoretical and experimental aspects. The main contents including: the analysis of the photon-induced decomposition processes of PMMA when focuse femtosecond laser beam inside the PMMA substrate with the help of Raman spectroscopy; three dimensional hollow microchannel fabrication inside PMMA with low repetition rate Ti:sapphire femtosecond laser; embedded microball lenses fabricated inside PMMA with a high repetition rate femtosecond fiber laser.The main works and achievements are as follows:1. The mechanism of femtosecond laser internal manufacturing of cavitary structures was studied. When laser pulses with high power density generated from a femtosecond laser are tightly focused inside a transparent dielectric, plasma will generated through nonlinear ionization and avalanche ionization. Complicate relaxation processes such as the formation of color center and microexplosion will happen subsequently, thus modifications or ablations will occur in the focusing region. The relaxation mechanisms varies with the repetition rate of the lasers. Pulses interact independently with material when using a low repetition rate femtosecond laser, while if the object is irradiated with a high-repetition-rate femtosecond laser, the heat accumulation effect will in turn govern the whole relaxing process.The difference in the decomposition process of PMMA when machining on the surface and inside a PMMA substrate with the femtosecond laser irradiation were studied. When machining on the surface, the major fission procedue is the side-chain scission and small gas molecules such as CO2 and CH4 are thus released. If PMMA is internally manufactured, both the main-chain scission and side-chain scission will taken place since the processing volume is highly restrained inside the metarial. As a result, small gas molecules and free residuals are decomposed; free residuals will then combined together from crosslinking reaction and forms crosslinked polymer molecules with complicated 3D spatial networks.2. Continuous hollow microchannel can be directly written with the help of Ti:sapphire femtosecond laser inside PMMA. Without using any assisting setups, we use bottom-up manufacturing pattern together with the subsequential procedues such as reparation and enlargement of the entrance and exit, a three-dimensional microchannel with long distances, large channel diameters, and complicated spatial structures can be rapidly fabricated. The obtained microchannel is proved to have excellent connectivity, optical transparency and hydrophilicity that can be utilized in various microfluidic applications.The influences of laser power, scanning speed, manufacturing depth, scanning cycles and the total deposited energy on the resulting channel properties are detailedly studied. Among them, the variation of the laser power will induce three kinds of variations where self-focusing effect and filamentation due to nonlinear effect are the main reason that the morphologies of microchannel changes with the average power. With the scanning speed increased, the diameters of the cross section will firstly raise to a maximum value and then gradually decrease. The reflection and scattering caused by the dense plasma is believed have a great influence when a low scanning speed is applied. Increase of the manufacturing depth will cause a linear decrease of the cross-section diameters. The increase of scanning cycles will induce linear increase for the diameters along the horizontal and vertical direction. Meanwhile, the increase of diameter along the vertical direction is mainly towarding the inverse direction of the laser propagation direction where the generation of laser-supported absorption wave is believed should be taken into account. When microchannels are machined at the same power and processing time, the diameter of the cross-section will largely affected by the increase of scanning cycles; but when the laser energy deposited in a unit length are same, the laser power plays a more important role in the manufacturing process.3. Embedded microball lenses are fabricated in PMMA with high repetition rate femtosecond fiber laser. The lenses are divided as convex microball lens(VMBL) and concave microball lens(CMBL). Among them, the increase of the refractive index of VMBL is due to the ‘melting-resolidify’ process of the heat accumulation effect caused by the irradiation with high repetition rate femtosecond laser pulses. The formation of CMBL is mainly due to the scission of gas molecules caused by heat accumulation effect and nonlinear photoionization. The gas molecules will flow to the irradiation center(laser focus) with the Marangoni convection and forms a gas cavity after the input of laser pulses is ceased. The cavity is observed surrounded with a densified shell.The influences on the machining properties taken by laser powers, focusing lenses, processing times and repetition rates are detailedly studied. The study shows, the processing window is relatively narrow for VMBL fabrication- the certain value is around 90 m W when focused with a 50 times objective lens; but CMBL can be fabricated with a broader band of average power from 400 m W to 1.5W. The cavity diameter grows rapidly at the early stage of the laser irradiation and growing relatively smoothly and slower from then on. The results shows the diameter of the lenses won’t have distinct change when using deferent repetition rates with a same average power.The evolution of the structural changes of both lenses with the growing time are also included in this study. We observed and analyzed the structural developments of CMBL during the laser irradiation. Meanwhile, the increase of the VMBL diameters with cooling time and the decrease of the lenses’ refractive index are also observed.By utilizing embedded microball lenses, we successfully structured an invert telescopic imaging system. The micro-telescope with a VMBL as a front lens can clearly image a remote object, while if the front lens is changed to CMBL, light sources located in a 4π space can be imaged at a same time, illustrating the telescopic system with a CMBL has a wide-angle imaging ability.The control strategies of optical properties and geometrical shape of the CMBL are proposed. The propagation of the stress wavefront is observed so we proposed an annealing method to bake the sample at the temperature of 90°C for 20 min to cure boundary deformations caused by stress wavefront. Finally, by using the optimized fabrication parameters, we successfully fabricate VMBL and CMBL arrays and imaging objects with both lens arrays.
Keywords/Search Tags:femtosecond laser, internal manufacturing, hollow structure, microfluidic chip, embedded microball lens
PDF Full Text Request
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