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Now showing 1 - 10 of 12
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    Effects of dichroic dye in UV-cured polymer dispersed liquid crystal films
    (1996-05-01)
    Wu, JJ
    ;
    Wang, CM
    ;
    Chen, SH
    ;
    光電工程研究所
    ;
    Institute of EO Enginerring
    We study the effects of dichroic dye in ''ultraviolet'' polymer dispersed liquid crystal (PDLC) films. Experimental data show that for a dye-doped PDLC film the absorption coefficient for He-Ne laser light is different from that for UV light. Transmittance of a PDLC film changes when the UV curing intensity or dye concentration is varied. It is found that under UV the illumination with a constant intensity the dye concentration can be optimized to obtain minimum transmittance and to improve the contrast ratio. Correspondingly, for a fixed dye concentration the UV curing intensity can be optimized to obtain a minimum transmittance. The experimental results can be described by a theory based on Beer's law. Besides, the experimental results show that the curing temperature is an important factor that affects the nematic liquid crystal droplet size in the UV curing process.
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    Electrooptical properties of aligned dye-doped polymer dispersed liquid crystal films
    (1998-12-01)
    Wu, JJ
    ;
    Wang, CM
    ;
    Li, WY
    ;
    Chen, SH
    ;
    光電工程學系
    We investigate how plane-parallel and twisted alignment treatments influence the electrooptical properties of dye-doped polymer dispersed liquid crystal (PDLC) films. These films are transparent in the electric field-on state and opaque in the field-off state. The relationship between field-off state transmittance of such dye-doped PDLC films and polarization of the incident light is discussed. Experimental data confirm that, even for unpolarized : Incident light, the contrast ratio of these aligned films surpasses that of a nonaligned dye-doped PDLC film.
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    Effect of thermal annealing on band edge absorption spectrum
    (2001-11-01)
    Lin, GR
    ;
    Pan, CL
    ;
    光電工程學系
    ;
    Department of Photonics
    We report the effects of annealing temperature on the near-band-gap transmittance, the absorption coefficient. and the evolution of shallow-level defects of arsenic-ion-implanted GaAs (hereafter referred to as GaAs:As+) using Fourier-transform infrared spectroscopy. The maximum above-band-gap transmittance and absorption coefficient are determined to be 0.4 and 6 x 10(3) cm(-1), respectively. An anomalous absorption peak at the photon energy of 1.37-1.4eV for the GaAs:As+ samples annealed at lower than 500 degreesC is observed. By either fitting the absorption curve, the band-gap energy of rapid-thermal-annealed GaAs:As+ is evaluated. which blue shifts from 1.36 eV to 1.41 eV as the annealing temperature increases from 300 degreesC to 800 degreesC. The slightly rising absorption spectra at the near-band-gap region suggest the existence of defects and scattering centers. The activation energies and formatting mechanisms of the dense defects in the Rapid-thermal-annealed GaAs:As+ samples were identified, By characterizing the differential absorption spectra, the eVolution of some defect-related absorption bands during the high-temperature annealing process were observed and the correlated structural phenomena were explained.
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    Optical constants of two typical liquid crystals 5CB and PCH5 in the THz frequency range
    (2003)
    Pan, RP
    ;
    Tsai, TR
    ;
    Chen, CY
    ;
    Pan, CL
    ;
    電子物理學系
    The complex refractive indices of two benchmark nematic liquid crystal, 4'-n-pentyl-4-cyanobiphenyl (5CB) and 4-(trans-4'pentylcyclohexyl)-benzonitrile (PCH5) have been determined in the frequency range from 0.2 to 0.8 THz. The technique of coherent THz time-domain spectroscopy (THz-TDS) was used. We show that the birefringence of 5CB is in the range of 0.15 to 0.21, while that of PCH is from 0.01 to 0.08. Both liquid crystals exhibit relatively small absorption in this frequency range. The large birefringence of 5CB indicates possible applications of liquid-crystal-based devices for modulation and polarization control of electromagnetic radiation in the THz frequency range.
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    UV stability of high birefirngence liquid crystals
    (2004)
    Lin, PT
    ;
    Wu, ST
    ;
    Chang, CY
    ;
    Hsu, CS
    ;
    應用化學系
    The ultraviolet stability of several high birefringence liquid crystal structures was studied. Compounds containing carbon-carbon double bond or triple bond are less stable than those containing merely phenyl rings. In addition to the structural effect, absorption wavelength also plays an important role to the degradation mechanisms.
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    Synthesis and Characterization of Two-Photon Chromophores Based on a Tetrasubstituted Tetraethynylethylene Scaffold
    (2014-06-01)
    Lin, Tzu-Chau
    ;
    Liu, Yi-You
    ;
    Li, May-Hui
    ;
    Liu, Che-Yu
    ;
    Tseng, Sheng-Yang
    A new series of model dye molecules composed of three multibranched analogues based on the tetrasubstituted tetraethynylethylene structural motif have been synthesized and experimentally shown to possess strong and widely dispersed two-photon absorption (2PA) in the near-IR region. It was found that the spectral position of the major 2PA band could be tuned by the electronic nature of the selected substitution units. The studied model fluorophores also exhibited fairly low photodegradation of their fluorescence intensity even under prolonged UV-light irradiation, which is beneficial for the development of fluorescence probes that are needed for long-term light exposure. Furthermore, representative chromophores were selected to demonstrate the power-control properties within the femtosecond and nanosecond time domains.
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    Ultralow Reflection from a-Si Nanograss/Si Nanofrustum Double Layers
    (2013-03-25)
    Ravipati, Srikanth
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    Shieh, Jiann
    ;
    Ko, Fu-Hsiang
    ;
    Yu, Chen-Chieh
    ;
    Chen, Hsuen-Li
    A double-layer nanostructure comprising amorphous Si nanograss on top of Si nanofrustums (NFs) with a total height of 680 nm exhibits ultralow reflection. Almost near-unity absorption and near-zero reflectance result in this layered nanostructure, over a broad range of wavelengths and a wide range of angles of incidence, due to the low packing density of a-Si and the smooth transition of the refractive index from the air to the Si substrate across both the nanograss and NF layers.
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    Light-induced absorption of Ru-doped Bi12TiO20 crystals
    (2003)
    Piquette, S
    ;
    Marinova, V
    ;
    Lin, SH
    ;
    Hsu, KY
    ;
    光電工程學系
    The effect of ruthenium doping on optical properties of Bi12TiO20 (BTO) single crystals is studied. A low concentration of ruthenium do not change absorption behavior of BTO, however higher ruthenium addition significantly shifts the optical absorption to the near infrared spectral region with an absorption shoulder appearing around 1.8 eV. The absorption coefficient increases with increasing the Ru concentration. Light-induced absorption changes are observed and the dynamics of build-up and dark decay processes are investigated. Results show that concentration of shallow levels increase with Ru content. Evolution of the build -up and dark decay of light induced absorption consists of fast and slow components: the first process occurs within few seconds, while the second one needs several hours (or days). It is established that light-induced absorption can be modified by preliminary illumination and thermal treatments thus multilevel lead to the transient and persistent parts of light-induced absorption. A model, assuming one deep and two shallow levels is proposed to explain the bi-exponential behavior of the build-up and the dark decay of light- induced absorption when the crystals are preliminary annealed in oxygen atmosphere.
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    Photolysis Cross-Section of Ozone Dimer
    (2011-11-04)
    Chen, I-Cheng
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    Chen, Andrew F.
    ;
    Huang, Wen-Tsung
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    Takahashi, Kaito
    ;
    Lin, Jim J.
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    The Role of the n pi* (1)A(u) State in the Photoabsorption and Relaxation of Pyrazine
    (2011-11-04)
    Lin, Chih-Kai
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    Niu, Yingli
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    Zhu, Chaoyuan
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    Shuai, Zhigang
    ;
    Lin, Sheng Hsien
    The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD-DFT, CASSCF, CASPT2, and related quantum chemical calculations. The second singlet n pi* state, (1)A(u), which is conventionally regarded dark due to the dipole-forbidden (1)A(u) <- (1)A(g) transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring n pi* (1)B(3u) and pi pi* (1)B(2u) states through vibronic coupling. The scans on potential-energy surfaces further indicated that the (1)A(u) state intersects with the (1)B(2u) states near the equilibrium of the latter, thus implying its participation in the ultrafast relaxation process.