Dong Jun Kang

Simple fabrication of diffraction gratings by two-beam interference method in highly photosensitive hybrid sol-gel films

Sol-gel hybrid materials containing a large quantity of photoactive molecules exhibited large changes in both refractive index and volume by UV exposure. The materials were used for the fabrication of diffraction gratings using the two-beam interference method. With this technique, we could simply fabricate the diffraction gratings and easily control the grating periods. The diffraction effects and efficiencies of gratings rely heavily on the UV doses and the fabricated diffraction gratings showed a good diffraction performance.

Single-step photopatterning of diffraction gratings in highly photosensitive hybrid sol-gel films

Inorganic-organic hybrid materials prepared by a sol-gel process showed a large photoinduced negative refractive-index change (2x10-2) and a volume change. Diffraction gratings were made directly by irradiation with a KrF excimer laser (248 nm) through a phase mask without any etching process. The dependence of diffraction efficiency on irradiation conditions was investigated. The maximum diffraction efficiency, measured by the Littrow configuration with a He-Ne laser (633 nm), was 3.4% in reflection mode.

Direct photofabrication of refractive-index-modulated multimode optical waveguide using photosensitive sol-gel hybrid materials

Photosensitive sol-gel hybrid materials (hybrimers) exhibited a thick film property, an efficient refractive index tunability through the control of their compositions, and a high photosensitivity upon UV exposure. The materials were used for the direct photofabrication of the multimode optical waveguide (MOW) with a large core structure. Due to the outstanding optical properties of these materials, problems and complexities associated with the fabrication of MOW could be overcome, and a MOW with good propagation performance could be easily fabricated. Importantly, a propagation loss of as low as 0.13dBcm−1 at 850nm could be obtained.

Fabrication and Characteristics of Sol-Gel Derived Fluorinated Hybrid Material Films

Fluorinated inorganic-organic hybrid materials (HYBRIMER) were successfully prepared from fluoroalkylsilanes (FASs) containing fluoro-alkyl functions and methacryloxypropyltrimethoxysilane (MPTMS) through a sol-gel process. The influence of concentration and fluoro-alkyl chain lengths of FASs on the physical characteristics of the fluorinated HYBRIMER films was examined. Larger fluorine contents lowered the refractive index within a range, which was closely dependent on the fluoro-alkyl chain lengths of FASs. Thermo-optic coefficients (TOC) were negative values, and the values grew with increasing fluorine contents and fluoro-alkyl chain lengths. Also, the thermal stability is enhanced by addition of fluorine in the HYBRIMER.

Direct photofabrication of focal-length-controlled microlens array using photoinduced migration mechanisms of photosensitive sol-gel hybrid materials.

Photosensitive sol-gel hybrid (SGH) materials exhibited the peculiar photoinduced migration behavior of unreacted molecules from unexposed areas to exposed areas by selective UV exposure. Using the photoinduced migration mechanism of the photosensitive SGH materials, the microlens array (MLA) with a smooth surface was directly photofabricated, and the focal length was controlled by changing the photoinduced migration parameters. The higher photoactive monomer content and the thicker film creating a higher curvature produced a smaller focal length of the MLA. Thus, a simple fabrication and easy control of the focal length can be applicable to a fabrication of an efficient MLA.

Fabrication of Thermally Durable Sub-wavelength Periodic Structures upon Inorganic–Organic Hybrid Materials by Nano-imprinting

We fabricated sub-wavelength periodic structures by thermal nano-imprinting using our original functionally modified inorganic–organic hybrid materials. The fabricated structures exhibited excellent uniformity and surface smoothness over a large imprinted area. In addition, optical transmittance is more than 90% within visible wavelength regions after heat treatment for 2 h at 300 °C. These nano-imprinted periodic structures showed high thermal durability without any structural changes after heating for 2 h at 200 °C, which is superior to commercially available organic imprinting materials. These nano-imprinted sub-wavelength periodic structures using our original inorganic–organic hybrid materials offer great potential for several applications in microphotonic and nanophotonic devices.

Fabrication of high-efficiency Fresnel-type lenses by pinhole diffraction imaging of sol-gel hybrid materials

Sol-gel hybrid materials containing a large quantity of photoactive molecules exhibited large changes in both refractive index and volume on UV exposure. The materials were used for fabrication of Fresnel-type lenses using a simple method: pinhole diffraction imaging. With this technique, problems associated with the contact method could be overcome and Fresnel-type lenses with good focusing performance could be fabricated easily. Importantly, a high diffraction efficiency approaching 85% could be obtained.

Fabrication of micro-optical devices by holographic interference of high photosensitive inorganic-organic hybrid materials (Photo-HYBIRMER)

Sol-gel derived photosensitive inorganic-organic hybrid materials (Photo-HYBRIMER) containing a large quantity of photoactive molecules exhibit the large changes in both refractive index (over 10) and volume (over 30%) on UV exposure. The materials could be used for direct fabrication of micro-optical devices using holographic interference. With the change of the beam number for holographic interference (1-beam, 2-beam, 3-beam and 4-beam interference), various typed micro-optical devices (Fresnel-type lens, 1Dand 2D-typed diffraction gratings) could be easily fabricated. Importantly, the fabricated micro-optical devices exhibited the very homogeneous surface structures and good optical performance.

Fabrication of thermally durable subwavelength periodic structures upon hybrid materials by nano-imprinting method

Micro- and nano-sized periodic structures are important components for wavelength dispersion, conversion, signal processing and modulation of optical signals. Thus, many micromachining technologies for the micro- and nano-sized patterns have been recently studied. Especially, nano-imprinting lithography has been rapidly developed as an attractive, low-cost alternative to photolithography and other lithographic techniques. Meanwhile, newly optimized materials for the specific nano-imprinting process have been continually required. Recently, the functionally modified inorganic-organic hybrid materials were found to have a highly efficient thermal curability, a high optical transparency in the visible and near infrared wavelength regions, and excellent mechanical and thermal properties once fully cured. In this study, we could successfully fabricate the micro- and nano-sized periodic structures by thermal nanoimprinting process using our original functionally modified hybrid materials. The fabricated structures exhibited the excellent uniformity and surface smoothness through a large imprinted area. In addition, the optical transparency is more than 90% within visible and near infrared wavelength regions. These nano-imprinted periodic structures showed the highly thermal durability without any structural changes for 2 hours at 300°C, which is much better than commercially available organic imprinting materials. Thus, these nano-imprinted sub-wavelength periodic structures using our original hybrid materials have great potentials for several applications to micro- and nano-photonic devices.