High-contrast imaging of magnetic domains were performed using magneto-optical Kerr effect enhanced by the plasmon filter. The plasmon filter was prepared by annealing Au thin films deposited on a SiO2 substrate. Au nanoparticles were formed by deforming the Au thin film. The filter exhibited an intense absorption peak at wavelength around 570 nm. The images were obtained setting the filter onto a magnet surface using a magneto-optical Kerr effect microscopy. The image contrast was evaluated by comparing Coefficient of Variations (CV) which exhibited the variation of the data in luminance histograms of the observed images. As the value of CV increases, the range of the luminance value expressing the image of magnetic domains is increased. The value of CV of the observation image with the plasmon filter was larger than only substrate, indicating that the contrast of the observation image was improved. High-contrast image of magnetic domains was obtained using the plasmon filter.
Cu-based thermal detectors were directly fabricated on poly(ethylene) naphthalate (PEN) films using femtosecond laserinduced reduction metal oxide nanoparticles. Both CuO and CuO/NiO mixed nanoparticle solution films on PEN films were reduced and sintered to form Cu-based micropatterns. Femtosecond laser system operating with a pulse duration of 120 fs, wavelength of 780 nm, and repetition frequency of 80 MHz, was used for direct writing in air. P-type Cu2O/NiO composite micropatterns were obtained only on the glass substrates, even though n-type Cu-Ni micropatterns were formed on the both glass and flexible PEN substrates. These results indicate that metal oxide nanoparticles were well-reduced on the polymer substrates because of their low thermal conductivities. In addition, the re-oxidization of the reduced metals were prevented on the polymer substrates due to their contribution as reductants. Both thermistor-type Cu-rich thermoelectric-type and Cu-Ni-rich thermal detectors were fabricated by reducing CuO nanoparticles and CuO/NiO mixed nanoparticles, respectively. The sensitivity of the detector was 5.3 μV/K. The devices can be used as various sensors in internet of things.
2D and 3D metal microstructures are directly written using femtosecond laser reductive sintering of metal oxide nanoparticles. CuO nanoparticle solutions including CuO nanoparticles, a reductant agent, and a dispersant, were coated on glass substrates. Then, focused femtosecond laser pulses were irradiated onto the solution film to write the microstructures in air. Finally, non-irradiated CuO nanoparticles were removed. Cu-rich and Cu2O-rich microstructures were selectively fabricated by controlling the laser irradiation conditions. We demonstrated direct-writing of 2D and 3D Cu-based microstructures using femtosecond laser-induced reduction of CuO NPs. Using respective appropriate femtosecond laser conditions, 3D Cu-rich microstructures were obtained by a combined process of the dispensing coating and laser irradiation. The Cu-rich hot-film flow sensor which had microbridge structure, were successfully produced. This direct-writing technique is useful for fabricating various sensors on arbitral shaped substrates in air.
Cu-based fine patterns were directly fabricated using femtosecond laser reduction of Cu2O nanospheres (NSs) via nonlinear optical absorption. Cu2O NS solution films, containing Cu2O NSs, polyvinylpyrrolidone (PVP), and 2-propanol, were prepared by spin-coating of the Cu2O NS solution on glass substrates or Cu-coated glass substrates. Finer line patterns were formed by scanning the focused femtosecond laser pulses. The absorption of the Cu2O NS solution film at wavelength of the femtosecond laser pulses, 780 nm, was low, whereas the intense absorption at wavelength of 390 nm was observed. Finer patterns were obtained on the Cu-coated glass substrates than on the glass substrates. The minimum line width of 0.6 μm was obtained on the Cu-sputtered film, which was smaller than the focal spot diameter of 1.3 μm. The heat accumulation is lower on the Cu-sputtered films due to their high thermal conductivity, resulting that the line width with the sub diffraction limit was achieved. The electrical conductivity of the patterns on the glass substrates was evaluated to be 4.1×106 S/m at scanning speed of 200 μm/s and pulse energy of 0.312 nJ, which is close to that of bulk copper.
Copper (Cu)-based micropatterns were fabricated on polymer substrates using femtosecond laser reduction of copper (II) oxide (CuO) nanoparticles. CuO nanoparticle solution, which consisted of CuO nanoparticles, ethylene glycol as a reductant agent, and polyvinylpyrrolidone as a dispersant, was spin-coated on poly(dimethylsiloxane) (PDMS) substrates and was irradiated by focused femtosecond laser pulses to fabricate Cu-based micropatterns. When the laser pulses were raster-scanned onto the solution, CuO nanoparticles were reduced and sintered. Cu-rich and copper (I)-oxide (Cu2O)-rich micropatterns were formed at laser scanning speeds of 15 mm/s and 0.5 mm/s, respectively, and at a pulse energy of 0.54 nJ. Cu-rich electrically conductive micropatterns were obtained without significant damages on the substrates. On the other hand, Cu2O-rich micropatterns exhibited no electrical conductivity, indicating that microcracks were generated on the micropatterns by thermal expansion and shrinking of the substrates. We demonstrated a direct-writing of Cu-rich micro-temperature sensors on PDMS substrates using the foregoing laser irradiation condition. The resistance of the fabricated sensors increased with increasing temperature, which is consistent with that of Cu. This direct-writing technique is useful for fabricating Cu-polymer composite microstructures.
Cu-based micro-temperature sensors were directly fabricated on poly(dimethylsiloxane) (PDMS) blood vessel models in EVE using a combined process of spray coating and femtosecond laser reduction of CuO nanoparticles. CuO nanoparticle solution coated on a PDMS blood vessel model are thermally reduced and sintered by focused femtosecond laser pulses in atmosphere to write the sensors. After removing the non-irradiated CuO nanoparticles, Cu-based microtemperature sensors are formed. The sensors are thermistor-type ones whose temperature dependences of the resistance are used for measuring temperature inside the blood vessel model. This fabrication technique is useful for direct-writing of Cu-based microsensors and actuators on arbitrary nonplanar substrates.
Refractive indices of a chemically amplified photoresist were changed by femtosecond (fs) laser irradiation without post exposure bake (PEB) treatment. We have proposed a combined process of fs nonlinear lithography and plasma etching for the fabrication of functional photonic devices with 3-D surfaces of inorganic materials. In this study, we report the nonlinear lithographic properties using fs laser pulses focused by a low-NA objective lens. Diffraction gratings were written directly inside the pre-baked resists by fs laser nonlinear absorption. From diffraction efficiencies using He-Ne laser light, refractive indices were changed by 8 × 10-3 without PEB treatment which was required for cross-linking reaction. In contrast, no changes of refractive index were observed in the case of ultraviolet light exposure (i-line). Considering this large refractive index change and the threshold intensity of nonlinear absorption of the resist, self-guiding of fs laser pulses can occur due to the optical confinement in the radial direction. In fact, filamentary patterns which were optical-axially asymmetric and longer than the focal depth could be obtained without translating the focal spot using this lithographic property.
SiO2-based variable microfluidic lenses were fabricated by femtosecond laser lithography-assisted micromachining
(FLAM). Optofluidic devices have attracted much interest because the adaptive nature of liquids in microfluidics enables
unique optical performance that is not achievable within all solid state devices. SiO2-based microfluidic devices are,
particularly, attractive due to high transparency, physical and chemical stabilities. However, it is generally rather difficult
to form the microstructures in microchannels because photolithography process is limited to planar substrates. In our
study, we fabricated SiO2-based variable microfluidic lenses, which had micro-Fresnel lenses inside the channels, by
using FLAM, which was a combined process of nonlinear lithography and plasma etching. The resist patterns of the
Fresnel lenses were directly written inside chemically amplified negative-tone photoresist on SiO2-based microchannels
of 250 μm width and 6 μm depth using femtosecond laser-induced nonlinear optical absorption. Following that, the
patterns were transferred to the bottom of the channels by using CHF3 and O2 mixed plasma. SiO2-based Fresnel lenses
with smooth surface were formed on the bottoms. When the channel was filled with the air, the focal spot was observed
2020 μm from the lens surface. By injecting silicone oil into the channel, the incident light was switched to the
dispersed.
SiO2-based diffractive/refractive hybrid microlenses were fabricated by using femtosecond laser-induced nonlinear
optical processes. Recently, hybrid devices have received much attention as important components for optical pickup
systems and integrated sensors. SiO2-based devices are particularly promising because of high transparency, physical and
chemical stabilities. For these devices, microfabrication upon nonplanar substrates such as convex lenses, which is
difficult for the semiconductor processes, is required. In this study, microFresnel lens patterns were directly written
inside positive-tone resists upon convex microlenses of 240 μm diameters by using femtosecond laser-induced nonlinear
absorption. The spot diameters are primarily determined at any position inside the resist by the region volume at which
the nonlinear absorption occurs. Therefore, the precise patterns could be formed even upon the nonplanar substrates.
After post-exposure-bake and development treatment, the patterns were transferred onto underlying lenses by CHF3
plasma. Here, the etching depth was 1 μm. Consequently, SiO2-based hybrid lenses with smooth surfaces were obtained. When He-Ne laser of 632.8 nm wavelength was coupled to this hybrid lens, the focal spot was 630 μm from the lens
surfaces. This focal length agreed with theoretical value of 618 μm. More functional optical devices would be realized
by improvement of fabrication processes.
Silica-based nonplanar surface structures were fabricated by use of femtosecond laser lithography-assisted
micromachining (FLAM), which is a combined process of femtosecond laser lithography and plasma etching. Diffractive
optical elements (DOEs) are widely used for photonic applications such as optical pickup, interconnection and so forth.
Most DOEs have been produced by semiconductor fabrication process. Although this process is useful to form
complicated fine structures, there exist two problems. First, it is rather difficult to fabricate nonplanar surfaces including
slopes and curves, which is effective to enhance diffraction efficiencies of DOEs. Second, microstructures cannot be
fabricated onto nonplanar substrates. In the FLAM, nonplanar patterns are directly written inside resists by use of
femtosecond laser-induced nonlinear optical absorption. Then, the patterns are transferred to underlying silica glasses by
CHF3 plasma. By use of FLAM, we successfully fabricated silica-based microFresnel lenses including curves and slopes
on planar substrates and cross-grating structures onto a convex microlens. In particular, as for the latter, uniform grating
structures with smooth surfaces were observed even at top and curved regions of the lenses. The FLAM is expected to be
useful for the fabrication of highly functional DOEs such as diffractive/refractive hybrid microlenses.
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