Ultraviolet Beam Research Articles

Directly nanopatternable nanoporous titania – Application to cell growth engineering

Titanium dioxide (TiO2) is a well-known biocompatible material with important applications in many fields of biomedicine. Here we show our results on the development of a new TiO2 based sol-gel system that can act as negative tone resist for both ultraviolet (UV) and electron beam (EB) lithographies. The system allows the direct patterning of high resolution structures (down to 40nm). Moreover, it allows the preparation of nanopatterned nanoporous TiO2 simply by changing the conditions of resist development. The presented system can be used as functional tool for the systematic investigation of cell behavior with respect to a complex topographic texture of the substrate (cell guidance). TiO2 structures of different dimensions, geometries and nanoporosity, whose size ranges from a few hundred to a few tens nanometer, are obtained after UV and/or EB lithography. The adhesion of different cell lines, i.e., human osteosarcoma (HOS) and neuroblastoma (N2A), on the patterns is assessed and our results show that cells have a preference for TiO2 patterns with respect to glass.

Materials Properties and Solvated Electron Dynamics of Isolated Nanoparticles and Nanodroplets Probed with Ultrafast Extreme Ultraviolet Beams

We present ultrafast photoemission measurements of isolated nanoparticles in vacuum using extreme ultraviolet (EUV) light produced through high harmonic generation. Surface-selective static EUV photoemission measurements were performed on nanoparticles with a wide array of compositions, ranging from ionic crystals to nanodroplets of organic material. We find that the total photoelectron yield varies greatly with nanoparticle composition and provides insight into material properties such as the electron mean free path and effective mass. Additionally, we conduct time-resolved photoelectron yield measurements of isolated oleylamine nanodroplets, observing that EUV photons can create solvated electrons in liquid nanodroplets. Using photoemission from a time-delayed 790 nm pulse, we observe that a solvated electron is produced in an excited state and subsequently relaxes to its ground state with a lifetime of 151 ± 31 fs. This work demonstrates that femotosecond EUV photoemission is a versatile surface-sensitive probe of the properties and ultrafast dynamics of isolated nanoparticles.

Fabrication of circular sawtooth gratings using focused UV lithography

This paper presents a novel micro-fabrication method using focused ultraviolet (UV) light to manufacture three-dimensional sawtooth structures in ultra-thick negative photoresist to fabricate a novel multi-prism x-ray lens. The method uses a lens to shape the UV beam instead of the photomask conventionally used in UV lithography. Benefits of this method include the ability to manufacture sawtooth structures in free form, for example in circular shapes as well as arrays of these shapes, and in resist that is up to 76 μm thick.To verify the method, initially a simple simulation based on Fourier optics was done to predict the exposure energy distribution in the photoresist. Furthermore, circular sawtooth gratings were manufactured in a 76 μm SU-8 resist. The UV lens was fabricated using electron beam lithography and then used to expose the SU-8 with UV light. This paper details the complete developed process, including pre-exposure with an e-beam and cold development, which creates stable sawtooth structures. The measured profile was compared to the ideal sawtooth and the simulation. The main discrepancy was in the smallest feature size, the sawtooth tips, which were wider than the desired structures, as would be expected by simulation.

Destruction of a spherical polystyrene microparticle in pulsed ultraviolet beam for micromanipulation purpose

The modelling of heating a spherical polystyrene microparticle by the laser beam having the wavelength 355 nm and varying spot size is described. The simulation results are presented for two different values of the beam spot radius. Nature experiments are reported aimed at measuring the velocity of a particle with the radius 5µm accelerated by microexplosion of the neighbouring particle.

Additional chain-branching pathways in the low-temperature oxidation of branched alkanes

Chain-branching reactions represent a general motif in chemistry, encountered in atmospheric chemistry, combustion, polymerization, and photochemistry; the nature and amount of radicals generated by chain-branching are decisive for the reaction progress, its energy signature, and the time towards its completion. In this study, experimental evidence for two new types of chain-branching reactions is presented, based upon detection of highly oxidized multifunctional molecules (HOM) formed during the gas-phase low-temperature oxidation of a branched alkane under conditions relevant to combustion. The oxidation of 2,5-dimethylhexane (DMH) in a jet-stirred reactor (JSR) was studied using synchrotron vacuum ultra-violet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). Specifically, species with four and five oxygen atoms were probed, having molecular formulas of C8H14O4 (e.g., diketo-hydroperoxide/keto-hydroperoxy cyclic ether) and C8H16O5 (e.g., keto-dihydroperoxide/dihydroperoxy cyclic ether), respectively. The formation of C8H16O5 species involves alternative isomerization of OOQOOH radicals via intramolecular H-atom migration, followed by third O2 addition, intramolecular isomerization, and OH release; C8H14O4 species are proposed to result from subsequent reactions of C8H16O5 species. The mechanistic pathways involving these species are related to those proposed as a source of low-volatility highly oxygenated species in Earth's troposphere. At the higher temperatures relevant to auto-ignition, they can result in a net increase of hydroxyl radical production, so these are additional radical chain-branching pathways for ignition. The results presented herein extend the conceptual basis of reaction mechanisms used to predict the reaction behavior of ignition, and have implications on atmospheric gas-phase chemistry and the oxidative stability of organic substances.

High-power, high-repetition-rate performance characteristics of β-BaB₂O₄ for single-pass picosecond ultraviolet generation at 266 nm.

We report a systematic study on the performance characteristics of a high-power, high-repetition-rate, picosecond ultraviolet (UV) source at 266 nm based on β-BaB2O4 (BBO). The source, based on single-pass fourth harmonic generation (FHG) of a compact Yb-fiber laser in a two-crystal spatial walk-off compensation scheme, generates up to 2.9 W of average power at 266 nm at a pulse repetition rate of ~80 MHz with a single-pass FHG efficiency of 35% from the green to UV. Detrimental issues such as thermal effects have been studied and confirmed by performing relevant measurements. Angular and temperature acceptance bandwidths in BBO for FHG to 266 nm are experimentally determined, indicating that the effective interaction length is limited by spatial walk-off and thermal gradients under high-power operation. The origin of dynamic color center formation due to two-photon absorption in BBO is investigated by measurements of intensity-dependent transmission at 266 nm. Using a suitable theoretical model, two-photon absorption coefficients as well as the color center densities have been estimated at different temperatures. The measurements show that the two-photon absorption coefficient in BBO at 266 nm is ~3.5 times lower at 200°C compared to that at room temperature. The long-term power stability as well as beam pointing stability is analyzed at different output power levels and focusing conditions. Using cylindrical optics, we have circularized the generated elliptic UV beam to a circularity of >90%. To our knowledge, this is the first time such high average powers and temperature-dependent two-photon absorption measurements at 266 nm are reported at repetition rates as high as ~80 MHz.

Enhancement of ultraviolet-beam generation through a double walk-off compensation

We propose a novel walk-off-compensation method for ultraviolet-light-source development. A walk-off occurring in the frequency mixing of infrared and green beams is doubly compensated for using an external walk-off compensator and a nonlinear-material set with internal walk-off-compensation arrangement. We theoretically and experimentally verified that our method can improve the power and beam shape of the output ultraviolet beam.

Proposal of Integrated Sensitized Solar Cell Films Based on Sputtered ZnO Thin Films

One of the issues in dye-sensitized solar cells (DSCs) is the internal resistivity. To solve this, we proposed the waveguide-type sensitized solar cell based on thin-film semiconductors, where, tapered vertical waveguides and planar waveguides collect light beams and guide them to the solar cells. In the present work, fabrication and evaluation of the tapered vertical waveguide films, and sensitization of ZnO thin films are investigated. The tapered vertical waveguides are fabricated by the built-in mask method using a photopolymer exposed to tilted ultra-violet beams to form beveled walls after development. The tapered vertical waveguide with a taper angle of 30o is found to increase the light beam collecting efficiency by 2~3 time. For sensitization of ZnO thin films, we attempt to apply the sputtering for ZnO thin film growth, and two-dye stacked structures of p-type dye/n-type dye or ZnO/Cr2O3 multi-layer structures for sensitization.

Directly patternable high refractive index ferroelectric sol–gel resist

The development of a ferroelectric negative tone sol–gel resist for Ultraviolet (UV) and Electron Beam (EB) lithography is presented. A new system based on Lead Zirconate Titanate (PZT, with formula PbZr0.52Ti0.48O3) was synthesized by sol–gel method. The lithographic performances were investigated and several structures spanning from the micron range down to less than 50 nm have been achieved by UV and EB lithography. The system interaction with UV light and Electron beam was thoroughly characterized by FT-IT spectroscopy. The exposed PZT was annealed at high temperatures in order to study the crystalline phase evolution, the optical constants values and stability of patterned structures. After exposure and annealing, the refractive index of the material can vary from 1.68 up to 2.33 (@400 nm), while the ferroelectric behaviour seems to be maintained after high temperature annealing. These results suggest a possible application of PZT resist not only as ferroelectric but also as nanopatternable high refractive index material. Moreover, direct nanopatterning by means of Focused Ion Beam (FIB) lithography was verified and the potentiality for the preparation of high aspect ratio hollow nanostructures will be presented.

Wettability modification of human tooth surface by water and UV and electron-beam radiation.

The wettability of the human tooth enamel and dentin was analyzed by measuring the contact angles of a drop of distilled water deposited on the surface. The samples were cut along the transverse and longitudinal directions, and their surfaces were subjected to metallographic mirror-finish polishing. Some samples were also acid etched until their microstructure became exposed. Wettability measurements of the samples were done in dry and wet conditions and after ultraviolet (UV) and electron beam (EB) irradiations. The results indicate that water by itself was able to increase the hydrophobicity of these materials. The UV irradiation momentarily reduced the contact angle values, but they recovered after a short time. EB irradiation raised the contact angle and maintained it for a long time. Both enamel and dentin surfaces showed a wide range of contact angles, from approximately 10° (hydrophilic) to 90° (hydrophobic), although the contact angle showed more variability on enamel than on dentin surfaces. Whether the sample's surface had been polished or etched did not influence the contact angle value in wet conditions.

Proposal of Integrated Sensitized Solar Cell Films Based on Sputtered ZnO Thin Films

The dye-sensitized solar cell (DSC) is expected to be the next-generation device for energy conversion.1 One of the issues in the DSC is the internal resistivity. In conventional DSCs, as shown in Fig. 1(a), porous semiconductors are used in order to increase the number of adsorbed dye molecules on the semiconductor surface. In this case, although sufficient light absorption is available, the porous structure degrades the crystalline quality, suppressing electron mobility, and makes the carrier path in the semiconductor narrow, increasing the internal resistivity of DSCs. To solve this problem, we proposed the waveguide-type sensitized solar cell2based on thin-film semiconductors shown in Fig. 1(b). In this cell, guided lights in the thin films are used to generate photocurrents instead of normally-incident lights. In such “guided light” configuration, the lights pass through a substantial number of dye molecules to enhance light absorption, consequently, enhance photocurrents. A schematic illustration of the proposed integrated sensitized solar cell film is shown in Fig. 2. The waveguide-type sensitized solar cells are embedded in a light beam collecting film consisting of two stacked films; a film with tapered vertical waveguides and a film with planar waveguides having vertical mirrors. Incident lights are collected by the tapered vertical waveguides into the planar waveguides through the vertical mirrors, and are guided to the waveguide-type sensitized solar cells. Following advantages are expected in the integrated sensitized solar cell film. -Reduction of semiconductor material consumption. -Availability of optical circuit functions. -Light-weight and flexible characteristics. In the present work, fabrication and evaluation of the tapered vertical waveguide films, and sensitization for ZnO thin films were investigated. A fabrication process of the tapered vertical waveguide films by the built-in mask method3is shown in Fig. 3. A photopolymer layer on an Al built-in mask is exposed to tilted ultra-violet (UV) beams from different directions to make cured regions having beveled walls. After development, by removing the removable layer, a tapered vertical waveguide film is obtained. In Fig. 4(a), a photograph for a top view of a tapered vertical waveguide film is presented. Light beam collecting efficiency of the tapered vertical waveguide is shown in Fig. 4(b). The efficiency was evaluated by introducing white light of a halogen lamp from the upper side and measuring the power of the light that transmits through the Al built-in mask window. The tapered vertical waveguide with a taper angle of 30othat is placed in front of the Al built-in mask increases the light beam collecting efficiency by 2~3 times, confirming the light beam collecting capability of the tapered vertical waveguides. The efficiency will be improved by optimizing the structure and the fabrication process. For sensitization of ZnO thin films, to date, we have investigated photocurrents generated in vacuum-evaporated ZnO thin films with two-dye stacked structures2 grown on the surface by the liquid-phase molecular layer deposition (LP-MLD)2,4,5, and demonstrated that guided lights propagated in the vacuum-evaporated ZnO thin films enhance the dye sensitization capability by 4~10 times compared to the normally-incident lights. One concern about the vacuum-evaporated ZnO thin films is that the film quality is not high, which limits the solar energy conversion efficiency. To improve the film quality of ZnO, we attempt to apply ZnO thin films deposited by the sputtering to the semiconductor instead of the vacuum-evaporated ZnO. Figs. 5(a) and (b) show two proposed sensitization mechanisms for the sputtered ZnO thin films. In Fig. 5(a), a two-dye stacked structure is grown using LP-MLD by sequentially providing p-type dye (rose bengal: RB) and n-type dye (crystal violet: CV) on sputtered ZnO thin film surfaces. This structure is expected to widen the photocurrent spectra as shown in Fig. 5(c) because electrons excided in the p-type dye are injected into ZnO directly, and electrons excided in the n-type dye are injected into ZnO through the p-type dye. In the model shown in Fig. 5(b), the two-dye stacked structure is replaced with a ZnO/Cr2O3 multi-layer, which is regarded as a multiple quantum well consisting of Cr2O3quantum wells with different widths. This structure is expected to widen the photocurrent spectra, too. Experiments on these attempts are under way. References 1 B. O’Regan and M. Gratzel, Nature 353, 737 (1991). 2 T. Yoshimura, H. Watanabe, and C. Yoshino, J. Electrochem. Soc., 58, 51 (2011). 3 T. Yoshimura, M. Ojima, Y. Arai, and K. Asama, IEEE J. Select. Topics in Quantum Electron., 9, 49 (2003). 4T. Yoshimura, Japanese Patent, Tokukai Hei3-60487 (1991) [in Japanese]. 5 T. Yoshimura, Japanese Patent, Tokukai 2009-60487 (2009) [in Japanese]. Figure 1

Holographic binary grating liquid crystal cells fabricated by one-step exposure of photocrosslinkable polymer liquid crystalline alignment substrates to a polarization interference ultraviolet beam.

Holographic binary grating liquid crystal (LC) cells, in which the optical anisotropy was rectangularly modulated even as the grating was fabricated using holographic exposure, were fabricated by one-step polarization holographic exposure of an empty glass cell, the interior of which was coated with a photocrosslinkable polymer LC (PCLC). The present study is of great significance in that three types of holographic binary grating LC cells containing twisted alignments can be fabricated by simultaneous exposure of two PCLC substrates to the UV interference beams, which are sinusoidally modulated. The polarization conversion properties of the diffracted beams are explained well by theoretical analysis based on Jones calculus.

High-quality ultraviolet beam generation in multimode photonic crystal fiber through nondegenerate four-wave mixing at 532 nm

All-fiber ultraviolet (UV) light sources are of great practical interest for a multitude of applications spanned across different sectors, from industrial processes such as nonthermal, high-resolution materials processing, to biomedical applications such as eye surgery, to name a few. However, production of UV light sources with high beam quality has been a problem to this day as the fiber designs required to reach UV wavelengths by four-wave mixing with widely available pumps (i.e., 532nm) are challenging because of their small size and increased risk of material damage. In this Letter, a specific pumping scheme is presented that allows the conversion of two pump photons in different modes to UV light in the fundamental mode and the corresponding idler in a higher order mode. The process has also been shown to work experimentally, and UV light at 390.5nm in the fundamental mode was successfully generated.

Ultraviolet Beam Focusing in Gallium Arsenide by Direct Excitation of Surface Plasmon Polaritons

This paper proposed that ultraviolet beam could be focused by gallium arsenide (GaAs) through direct excitation of surface plasmon polaritons. Both theoretical analysis and computer simulation showed that GaAs could be a reasonably good plasmonic material in the air in the deep ultraviolet waveband. With a properly designed bull’s eye structure etched in GaAs, the ultraviolet electric field could be enhanced to as high as 20 times the incident value, and the full‐width‐half‐maximum (FWHM) of the light beam could be shrunk from ~48° to ~6°. As a plasmonic material, GaAs was compared to Ag and Al. Within the studied ultraviolet waveband, the field enhancement in GaAs was much stronger than Ag but not as high as Al.

Novel Molecular Resist for EUV and Electron Beam Lithography

A novel molecular resist molecule was prepared by incorporation of 1,8-diazabicycloundece-7-ene into a tert-butyloxycarbonyl protected phenol malonate group. The resist shows high-resolution capability in both extreme ultraviolet (EUV) and electron beam lithography.

Study on resist performance of chemically amplified molecular resists based on cyclic oligomers

Novel resist materials are required for lithographic processing with ionization radiation such as extreme ultraviolet (EUV) and electron beam (EB) exposure tools. In this study, we synthesized positive-tone chemically amplified molecular resist materials with pendant adamantyl ester (AD) and cyclohexyl 2-propyl ether moieties based on cyclic oligomers such as noria, calixarene dimer, cyclodextrin, and pillar[5]arene, and we examined the lithographic performances of sensitivity, etching durability, and patterning under EUV and EB exposure. We clarified that the sensitivity of the resist materials was consistent with the structure of the cyclic oligomers, i.e., the hole size of the molecular structure might be an important factor relevant to high-resolution resist materials. We found that chemically amplified molecular resists based on cyclic oligomers such as noria, calixarene dimer, cyclodextrin, and pillar[5]arene are promising candidates for higher-resolution resist materials.

Comparative study of UV radiation hardness of n+p and p+n duo-lateral position sensitive detectors

We report experimental results on the degree of radiation damage in two duo-lateral position sensitive detectors (LPSDs) exposed to 193 nm and 253 nm ultraviolet (UV) beam. One of the detectors was an in-house fabricated n+p LPSD and the other was a commercially available p+n LPSD. We report that at both wavelengths, the degradation damage from the UV photons absorption caused a much more significant deterioration in responsivity in the p+n LPSD than in the n+p LPSD. By employing a simple method, we were able to visualize the radiation damage on the active area of the LPSDs using 3-dimensional graphs. We were also able to characterize the impact of radiation damage on the linearity and position error of the detectors.

Well-Defined LaBO<sub>3</sub>:Tb<sup>3+</sup> Hierarchical Architectures: Facile Synthesis and Luminescence Properties

Well-defined LaBO3 hierarchical architectures have been successfully synthesized by a designed hydrothermal conversion method. La (OH)3 nanowires were prepared through a simple hydrothermal process. Subsequently, uniform 3D LaBO3 architectures were synthesized at the expense of the La (OH)3 nanowires during a hydrothermal process. The whole process was carried out under aqueous conditions without the use of any organic solvent, surfactant, or catalyst. The as-obtained LaBO3:Tb3+ product shows strong characteristic green emission under ultraviolet or low-voltage electron beam excitation, which might find potential applications in fields of fluorescent lamps, display systems, and optoelectronic devices.

The Production of Green Polymer - Hyperbranched Curable Palm Oil Oleic Acid

Palm oil is one of the main raw materials available in the country. Hence, the diversification ofpalm oil derivatives into new products has been given priority. The development of new radiation curable materials for applications in radiation ultraviolet (UV) or electron beam (EB) curing technology is an important research area. The hyperbranched curable polyurethane acrylate (HBPUA) from oleic acid of palm oil has been synthesized through a three-step reaction. Dipentaerythritol was initially reacted with 2, 2-bis (hydroxymethyl) propionic acid in a solution medium aided by p-toluene sulfonic acid as a catalyst to form hyperbranched polyol polyester first generation (HBP-1). HBP-1 then used as the core and reacted with palm oil oleic acid to form the hyperbranched polyol polyester second generation (HBP-2). HBPUA was prepared by reacting HBP-2 resin with diisocyanate and hydroxyl-containing acrylate monomer with the presence of 0.1-2 wt% dibutyltin dilaurate as a catalyst. The reaction was confirmed by several analytical data namely hydroxyl value (OHV), fourier transform infrared (FTIR), gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) analyses. The HBPUA was easily curable when subjected to EB radiation. The thermal decomposition of HBPUA showed good thermal stability up to 450 oC.

A comparison study of in-column and on-column detection for electrochromatography

Duplex capillary columns, the standard for electrochromatography using optical detection, consist of a packed and an open section. Normally, optical detection is performed in an on-column manner, i.e. at a point right after the packed section. It was deemed that band broadening may take place when an analyte band travels from the packed bed, through the frit and down to the open section. In this study, without using any sintering steps for fritting or window creation, robust packed capillary columns were prepared using transparent capillaries based on single particle fritting technology. The detection point could be easily shifted by simply sliding the transparent column against the ultraviolet (UV) beam. In this way, the band broadening effect was directly evaluated as a function of the detection point, which was positioned before or after the end frit. The consistent van Deemter curves recorded indicate that there was no efficiency difference between the positions investigated. The result proved that the significant band broadening effect previously observed via on-column detection should be caused by the sintered frit used, while the single particle frit made through a purely physical process did not lead to efficiency degradation. The conservative separation performance recorded at different positions around the column's end also suggests the applicability of on-line tandem detection strategy, e.g. UV followed by mass spectrometry (MS), on the same capillary column, which should be a promising approach to mining multiplex detection information from a single microseparation process.