Surface wettability depends on both physical surface structure and chemical material. In this report, we demonstrate super-hydrophobic surface of cast polymethyl methacrylate (PMMA) sheet by femtosecond laser fabrication. Twodimensional micro-array structures of square-typed pillars with various heights, widths, and intervals were fabricated on the PMMA surface by femtosecond laser irradiation and chemical etching. The Yb:KGW femtosecond laser processing system (λ=1030 nm) delivering 250 fs pulses at a repetition rate 100 kHz was employed for fabrication. The contact angle of PMMA changed 64° (hydrophilic plane) to 150° (super-hydrophobic structure). We also improved superhydrophobicity up to 170° contact angle by spin-coating PMMA surface with PDMS and fabricating regular microstructures including irregular nano-structures. We also coated the structured PMMA surface with a car ash spray material to use another combination of surface morphology and chemistry. All the experimental results were compared with those expected values by Cassie-Baxter model.
In this work, we report three-dimensional memory by recording optical bits with irradiation of near-infrared femtosecond laser pulses and by reading photoluminescence change in the blue due to permanent reduction of Eu3+ to Eu2+ in sodium borate glasses. We produced a multilayered micro-bit pattern which was read out by detecting the blue emission from the 405 nm excitation with a high S/N ratio. The readout was performed by using a scanning reflection-type confocal microscope.
We report ultraviolet-induced transient and stable absorption and two-color holographic recording at 514, 633, and 830 nm in thulium doped stoichiometric LiNbO3 crystal. The gating light at 355 nm increased the diffraction efficiency and sensitivity. We found the optimum gating-recording intensity ratio for maximum diffraction efficiency and weak nonvolatile holographic storage with gating.
From the ultraviolet induced absorption, we have found a broad blue band and little involvement of Pr ions in ultraviolet illumination. Ultraviolet excited photoluminescence and the excitation spectra for visible emission were also measured in Pr:LiNbO3. We estimated photovoltaic constant and photoconductivity by measuring photovoltaic current and voltage with different boundary conditions. With ultraviolet gated short-circuit holographic recording, we have measured enhanced sensitivity and diffraction efficiency.
By rotating the reference beam instead of a photopolymer, we combined rotational multiplexing with angular or shift multiplexing for high capacity hologram storage in DuPont's HRF-150-100 photopolymer material. We used the spherical reference and signal waves for angular and rotational as well as shift and rotation multiplexing. To get uniform diffraction efficiency, we used the time-scheduled exposure during recording.
The broad photoluminescent emission from Ce:Sr0.6Ba0.4Nb2O6 has been observed at 550 nm with excitation in the blue. Photoluminescent excitation spectrum has revealed a trivalent cerium absorption band that is assumed to be a 4f-5d transition. Temperature dependence of the photoluminescence spectra and its lifetimes in 15 - 365 K showed the existence of two different trap centers in the material. We have also investigated the photoluminescence during two-wave mixing experiment to study trap centers.
We report enhanced photorefractive sensitivity in Mn, Ce doped LiNbO3 at 514 nm and 633 nm under ultraviolet illumination at 365 nm. Without ultraviolet sensitization, the crystal showed very weak diffraction efficiency and self-erased behavior. The accumulation of charged states by ultraviolet light is observed even after the hologram is erased because of the asymmetric process of recording and reading. The concentrations of doped ions and the intensity ratio of ultraviolet and recording light are important factors to improve sensitivity in this material.
We have determined the effective density of deep photorefractive centers and the effective electro-optic coefficient by measuring two-wave mixing gains in a Ce:Pb0.5Ba0.5Nb2O6 photorefractive crystal and compared with those of PBN. We have also investigated the light-induced absorption through pump-probe measurements at relatively low optical power densities. The density of shallow trap centers and their properties are studied from the buildup and decay of the light-induced absorption and its dependence on light intensity and temperature. Two- center model is applied to explain the experimental results.
We have observed the red to blue upconversion in 2%Tm3+∶LaF3 and 2%Tm3+∶YAlO3 crystals. The 3F2 level of Tm ions was excited and the dynamics of the ultraviolet and blue emission from the 1D2 and 1G4 was investigated.