Characteristics Of Material Surfaces And Properties Of Organic Materials

A femtosecond laser source and two-photon photoemission are used to study the surfaces of Cu(111) and Cu(775). We analyze the surface polarization tensor and volume polarization tensor near the surface when the laser is reflected from the centrosymmetric crystals of the classes m3m and 432. All-optical switch based on azo dye ethyl red (ER) doped methyl methacrylate (PMMA) polymer films are studied.1. Two-photon photoemission method is used to study the two model systems of flat and vicinal Cu(111) over the photon energy range of 3.4 to 5 eV. Since excitation time of the fetosecond laser is comparable to the lifetime of the image state electrons, it is possible to probe the photon energy over which the electron distribution curve exhibits an excitation resonance. We explore the range of energies and intensities over which this approach is limited. Due to the strong excitation of the electronic manifold in this case also allows us to report a previously unnoticed intermediate-state band on the stepped Cu(775) surface.2. We demonstrate angle-resolved, tunable, two-photon photoemission (2PPE) to map a bulk unoccupied band, viz. the Cu sp-band 0 to 1 eV below the vacuum level. in the vicinity of the L point. This short-lived bulk band is seen due to the strong optical pump rate, and the observed transition energies and their dispersion with photon energy hω, are in excellent agreement with tight-binding band-structure calculations. The variation of the 2PPE final-state energy with hωhas a measured slope of～1.64, in contrast with values 1 or 2 observed for 2PPE from two-dimensional (2D) states.3. When laser is incident on the nonlinear media, there will be second harmonic generation in the reflected beam. Systematic analysis of the surface polarization tensor and volume polarization tensor near the surface of centrosymmetric crystals of the classes m3m and 432 is given, and then the possibility of separation out a contribution to the reflected second harmonic light (SH) from a transition layer near the surface is discussed, this contribution arises from the loss of inversion symmetry. Longitudinal laser induces polarization tensor at the surface is investigated for the first time.4. The absorption spectra of the ER doped PMMA film were measured under the illumination of He-Cd laser (441.6 nm, CW) and semiconductor laser (532 nm, CW) and the change of the refractive index of the ER doped PMMA thin film induced by 532nm laser was simulated using the Kramers-Kronig relation. Additionally, the transmission intensity mutual modulations of the two beams were studied. Our results suggest that ethyl red (ER) has potential applications to all-optical devices.5. The all-optical switching characteristics of the ER doped PMMA film samples were investigated experimentally at different illumination intensities and modulation frequencies of the chopped pump beam (532 nm, CW) and the probe beam (632.8 nm, CW). The influence of the doping concentrations and the solvents on the all-optical switching effect of the films was discussed. At low pump power (8 mW), the modulation depth of 50% was obtained and the response time was near 1 ms at room temperature. Furthermore, the maximal modulation depth reaches 72% when the modulation period is 0.66 ms. An excellent switching performance could be obtained by choosing the proper doping concentrations (from 2 wt% to 6 wt%) of the samples, and the switching characteristics of the samples can be improved using cyclohexanone solvent. The experimental results show that ER-doped polymer has potential application to optical switching.ER doped PMMA film samples and two pumping beams were used to enhance remarkably the modulation depth of all-optical switches to almost to 100% in which two linearly polarization beams which one beam is 45 degree to the probe beam as the pumping beam, the other is perpendicular to the probe beam as the erasing beam. Furthermore, maximum-to-minimum output intensity ratio 2000:1 was achieved in our experiment, which is very useful and important for optical storage and image display.