Ran Hee Kim

Improvement of Spatial Resolution in Nano-Stereolithography Using Radical Quencher

The improvement of spatial resolution is a fundamental issue in the two-photon, polymerization-based, laser writing. In this study, a voxel tuning method using a radical quencher was proposed to increase the resolution, and the quenching effect according to the amount of radical quencher was experimentally investigated. Employing the proposed method, the lateral resolution of the line patterns was improved almost to 100 nm. However, a shortcoming of the quenching effect was the low mechanical strength of polymerized structures due to their short chain lengths. Nano-indentation tests were conducted to evaluate quantitatively the relationship between mechanical strength and the mixture ratio of the radical quencher into the resins. The elastic modulus was dramatically reduced from an average value of 3.015 to 2.078 GPa when 5 wt% of radical quencher was mixed into the resin. Threedimensional woodpile structures were fabricated to compare the strength between the resin containing radical quencher and the original resin.

TWO-PHOTON STEREOLITHOGRAPHY

Two-photon stereolithography based on photopolymerization provides the ability to fabricate real three-dimensional (3D) microstructures beyond the resolution of focal size. In this paper, our recent research focusing on improvement of spatial resolution in two-photon stereolithography is reviewed. The influence of system and fabrication conditions in relation to the spatial resolution is discussed. For small and low aspect ratio voxels, a minimum power and minimum exposure time (MPMT) scheme is introduced. During the two-photon process, an ascending technique, wherein the truncation amount of volumetric pixels is controlled, can be applied to improve the resolution of two-dimensional patterns. 3D Microfabrication with less than 100 nm resolution can be realized by using the radical quenching effect. After the two-photon process, the resolution of fabricated patterns can be refined to 60 nm by post-processing of plasma ashing.

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.

The impact of charge defects and resonance enhancement on the two- photon absorption activity of spirofluorene and ladder-type pentaphenylene derivatives

Novel two-photon absorption (TPA) chromophores that contain spirofluorene and ladder-type pentaphenylenes (LPPs) as π-centres with diarylamine groups as electron donors (D) in the D–π–D arrangement are prepared. The spirofluorenes with different donors at the both terminals exhibit TPA activity increase as the donor strength increases (from N-ethylcarbazoyl 1, triphenylamino 2, to N,N-dibutylaminophenyl 3). Compared to the spirofluorene derivatives, the LPP derivatives show the larger TPA efficiency due to better coplanarity throughout the molecule. As the LPP unit added from single (4), double (5), to triple (6), the increment of the TPA cross-section exceeds the one-fold of a LPP unit, which can be attributed to the cooperative enhancement of the TPA cross-section (σ2) between the LPP units. We have also explored the effect of introducing charge defects and examined the resonance enhancement by chemical oxidation on the TPA properties of LPP derivatives 4 and 5. Nearly an order of magnitude enhancement was found in the TPA cross-section of dye 4, which was interpreted as the role of charge transfer from LPP π-centre to the cationic diarylamine end groups.

Highly biocompatible amphiphilic perylenediimide derivative for bioimaging

We report the synthesis and biological studies of a fluorescence dye with an oligoethylene glycol substituted (OEG) perylene centered dye N,N’-(2,6-diisopropylphenyl)-1-[oligo(ethylene glycol)methyl ether]-1,6,7,12-trichloroperylene-3,4:9,10-tetracarboxdiimide (PDI-OEG). The activity of the dye is juxtaposed with a precursor molecule without the OEG substitution. The OEG substitution contributes to the increased biocompatibility of PDI-OEG. Cell viability studies lead to the survival of more than 80% of the PDI-OEG cultured cells endorsing its biocompatibility. Fluorescence imaging studies were carried out using multiple cell lines. Ex-vivo studies involving nude mice were used to establish liver and lung specific organ targeting of PDI-OEG. This fluorophore is an excellent example of a stable and biocompatible red emitting small molecule for bioimaging.

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.

Liquid crystal dimers having vary oxyethylene flexible spacers

ABSTRACT In this article, we are prepared that the liquid crystal dimers have aromatic-ester type mesogenic units or aromatic-Schiff base type mesogonic units and confirmed by 1H-NMR spectrometry. The mesomorphic and optical properties of the resultant dimers were studied by differential scanning calorimetry and polarizing optical microscopy.

TWO-PHOTON ABSORBING PHENYLENEVINYLENE DERIVATIVE HAVING SILYLOXY MOIETIES IN DONOR UNITS

Two-photon absorbing phenylenevinylene derivative having silyloxy moieties in electron donor units (EHSEA) was prepared. The two-photon absorption (TPA) cross-section value of this chromophore was found to be 1.93×10-48cm4sec/photon at 740 nm by using TPA-induced fluorescence method with 80 fs pulse laser. The nonlinear measurement with ns scale pulses has indicated that EHSEA exhibits an efficient optical power limiting activity. We have obtained micropatterns with high resolution by TPA-induced polymerization using EHSEA as a photosensitizer.

Multibranched and dendritic organic materials with high two-photon absorption activity

A three-dimensional micro-object was fabricated by two-photon initiated photopolymerization (TPIP) of soft-resin with the efficient two-photon absorbing multi-branched chromophore, EPS-3arm, which possesses high two-photon absorption cross-section of 5.21×10-47 cm4s/photons at 700 nm. The structural stability of the fabricated micro-objects could be achieved with aid of sol-gel process. The two-photon absorbing stilbazolium-cored dendrimer was also synthesized. The stillbazolium chromophore was covalently linked with the dendrimer chains so as to restrict free conformation motion. The hindered molecular twisting motion of the stilbazolium dye within the dendrimer is responsible for 5 to 16 times of enhancement in fluorescence emission along with the increased emission lifetime. The steady-state and time-resolved fluorescence emission study of the solid-state film indicate an efficient site-isolation and restriction of conformation motion of the active two-photon chromophore.

Improvement of Spatial Resolution in Two-Photon Stereolithography

Two-photon stereolithography based on photopolymerization provides the ability to fabricate real three- dimensional (3D) microstructures beyond the resolution of focal size. In this paper, our recent research focusing on improvement of spatial resolution in two-photon stereolithography is reviewed. The influence of system and fabrication conditions in relation to the spatial resolution is discussed. For small and low aspect ratio voxels, a minimum power and minimum exposure time (MPMT) scheme is introduced. During the two-photon process, an ascending technique, wherein the truncation amount of volumetric pixels is controlled, can be applied to improve the resolution of two-dimensional patterns. 3D microfabrication with less than 100 nm resolution can be realized by using the radical quenching effect. After the two-photon process, the resolution of fabricated patterns can be refined to 60 nm by post-processing of plasma ashing.