Develop Azobenzene Optical Material And Research On Its Third-Order Optical Nonlinearity

Three requirements are suggested for developing azobenzene materials of nonlinear optical switching according to theoretical researches: (1) The number ofdouble bonds of the polymer is about 10 so that the higher %^ is expected; (2)There should be non-degenerate ground state in the molecules so that the photo-induced polaron or bipolaron are formed; (3) The probability of the charge transfer (CT) state should be high when the material molecules are excited under appropriate optical pulses.On the basis of theoretical analysis of the relation between organic material structure and its optical nonlinearity, a series of six samples of azobenzene were produced with conventional organic chemistry method, and the organic thin film MANA / PMMA ^ CANA / PMMA with the host, polymethylmethacrylate (PMMA), and the guest, push-pull azobenzene compounds, 4-N-methylamino-4'-nitroazobenzene(MANA) and 4'-(2",4",6"-trichlorophenazoamino)-phenazo-4-nitrobenzene (CANA) , were also developed. Absorption spectrta, fluorescence spectra , linear absorption coefficient, linear refractive index, molecule concentration of liquid samples and thickness of thin films were measured. Geometrical structure of the sample was studied with quantum chemistry approaches, which resulted in getting 111 energy states with their parameters including transition energy, spin quantum number, state dipole, oscillation strength and transition dipole components. Both of the 3rd order hyperpolarizability of the sample and the bulk susceptibility were calculated.The bulk 3rd order susceptibility (^(3'~10"9esu), time responce (~10ps) and figure of merit (W~10) of the doped films were measured with Z-scan techniqueand phase conjugated forward degenerate four wave mixing(DFWM). Two photon absorption coefficient (bata- 20xlO~8cm/W) and its related figure of merit (T-0.3) were also obtained. On the basis of analysing experimental data, it can be seen that the optical nonlinearity of the sample with chlorine substitution is better than the one with iodine or bromine substitution. The effect of transient grating on the X value has been discussed.For improving the x(3) value of the samples, the 355nm laser was used to pump the sample to reach the excited state and then the exited state enhanced X(3) value was measured with the experiment of double wave coupling using the 532 nm laser. Results showed it was 1 order of magnitude higher than that in the ground state. But the responce time in both cases were the same. The phase-spacefilling of excitons was used to explain x(3)value enhanced by the excited state.The signals in the experiment featured both a fast and a slow processes, which were discussed theoretically.Since optical bistability is an important condition to realize optical logic and full optical switching with the material, the dispersive optical bistability of the sample was researched. The sample thin film was doped on a prism to form a Fabry-Pelot quasi-waveguide structure. When the device was detuned from resonance by more than one linewidth and the incident power exceeded the critical power, the input-output characteristics exhabited a bistable behavior. The result shows this kind of organic azobenzene has its potential application in photonic devices due to its higher x(3) value , faster response time and optical bistability.