Shin Wook Yi

Ultraprecise microreproduction of a three-dimensional artistic sculpture by multipath scanning method in two-photon photopolymerization

Ultraprecise fabrication of three-dimensional (3D) microstructures comes up to be one of the most important issues in two-photon induced photopolymerization. To date, it has been difficult to fabricate 3D microstructures without any deformation due to the surface tension between a rinsing material and solidified microstructures during the developing process: In general, the surface tension significantly affects the precision of the resulting 3D microstructures. To overcome this problem, the authors propose a simple and effective laser scanning method to reinforce the strength of 3D microstructures without loss of precision. Overall, a complex 3D artistic sculpture such as “The Thinker” was reproduced in the controlled ultraprecise form, which shows that the proposed method enables the fabrication of 3D patterns with dramatically improved precision and stability.

Three-dimensional microfabrication using two-photon absorption by femtosecond laser

As a femtosecond laser has recently been developed, both of high power and high photon density are easily obtained. The high photon density results in photopolymerization of urethane acrylate resin whose absorption spectrum is shorter than that of the femtosecond laser. The stereo-lithography using the two-photon absorption (TPA) makes micro structures with great resolution. We used this phenomenon to make micron-sized structures with sub-micron resolution. Before fabricating 3-D structures, precise 2-D structures were preceded. The TPA photopolymerization was applied of poly-dimethyl siloxane (PDMS) molding. In this paper, we report the recent progress and application of this technology in our laboratory.

Corrected Article: “Ultraprecise microreproduction of a three-dimensional artistic sculpture by multipath scanning method in two-photon photopolymerization” [Appl. Phys. Lett 90, 013113 (2007)]

Ultraprecise fabrication of three-dimensional (3D) microstructures comes up to be one of the most important issues in two-photon induced photopolymerization. To date, it has been difficult to fabricate 3D microstructures without any deformation due to the surface tension between a rinsing material and solidified microstructures during the developing process: In general, the surface tension significantly affects the precision of the resulting 3D microstructures. To overcome this problem, the authors propose a simple and effective laser scanning method to reinforce the strength of 3D microstructures without loss of precision. Overall, a complex 3D artistic sculpture such as “The Thinker” was reproduced in the controlled ultraprecise form, which shows that the proposed method enables the fabrication of 3D patterns with dramatically improved precision and stability.

DIRECT NANO-PATTERNING METHODS USING NONLINEAR ABSORPTION IN PHOTOPOLYMERIZATION INDUCED BY A FEMTOSECOND LASER

Investigation into the methods of direct fabrication for submicron-detailed patterns without the use of any photomask was carried out using two-photon polymerization (TPP). Some patterns can be fabricated easily in the range of several micrometers by solidifying inside a photopolymerizable resin by a scanning process using a volume-pixel (voxel) matrix that is transformed from a bitmap figure file. The voxels are generated consecutively to merge into adjoining voxels in the process of fabricating a pattern. The resolution of a fabricated pattern can be obtained under the diffraction limit of a laser beam due to the nonlinear absorption phenomenon. In this paper, a top-down reverse building technique was introduced to fabricate a polymeric pattern on a metal layer. Also, a contour offset algorithm (COA) and a beam expanding method have been attempted to make precise patterns and to enlarge the working area. Through this work, several patterns for various applications on nano- and microtechnology were fabricated on a glass plate and a thin metal layer to demonstrate the usefulness of the developed algorithm.

Fabrication of 3D microstructure and analysis of voxel generation by ultrafast-laser-induced two-photon absorption

Multi-photon absorption phenomena induced by ultra fast laser have been considered for many applications of microfabrications such as metal ablation, glass etching and photopolymerization. Among the applications, the photopolymerization by two-photon absorption (TPA) has been regarded as a new microfabricating method. It is possible to be used in photo mask correcting, diffractive optical element and micro machining. The TPA photopolymerization is made possible to fabricate a complicated three dimensional (3D) micro-structure which the conventional photomask technology has not been able to make. In fact, the shape of the voxel (volume pixel: a unit structure of TPA fabrication) is an important factor which could affect the microfabrication process. In this paper, we have reported that 3D micro-structures were fabricated and the generation of voxel shape was analyzed for various optical conditions.

Fabrication of PDMS (poly-dimethyl siloxane) molding and 3D structure by two-photon absorption induced by an ultrafast laser

Multi-photon absorption phenomena induced by ultra fast laser have been considered for many applications of microfabrications such as metal ablation, glass etching and photopolymerization. Among the applications, the photopolymerization by two-photon absorption (TPA) has been regarded as a new microfabricating method. It is possible to be used in photo mask correcting, diffractive optical element and micro machining. The TPA photopolymerization is made possible to fabricate a complicated three dimensional structure which the conventional photomask technology has not been able to make. Furthermore the TPA photopolymerization process applied to a two dimensional structure fabrication may take shorter time than the old process since the absence of etching and deposition processes. Recently we have made a simple 3D structure and applied the technique to PDMS(poly-dimethyl siloxane) molding.

Contour Offset Algorithm (COA) in nano Replication Printing (nRP) for fabricating nano-precision features

A Contour Offset Algorithm (COA) has been developed to fabricate nano-precision figures or patterns in the range of several microns by a nano-Replication Printing (nRP) process. In the nRP process, a femtosecond laser illuminated on photosensitive monomer resin to induce polymerization of the liquid monomer according to a volume pixel (voxel) matrix which is transformed from a two-tone (black and white) bitmap file. After two-photon absorbed photo-polymerization (TPP), a droplet of ethanol is dropt on a glass plate to remove the unnecessary remaining liquid resin, leaving only polymerized patterns on the glass plate. In the nRP process, the replicated patterns do not precisely coincide with the initial designs due to an essential shortage of nRP process. Fabricated patterns by means of the nRP process become larger than the design in the amount of the voxel radius. In this work, an outer contour matrix of an initial design was constructed and reduced according to an offset-ratio calculated by the COA in order to obtain more precise patterns. Both the effectiveness and the accuracy of the proposed algorithm were demonstrated through chosen example.