Investigations On Properties Of Dmit Polymeric Complex Films Which Show Third-order Nonlinear Optical Effects Of Suitable For All-optical Switching Applications

All-optical switching can break through the bottleneck of single-channel transmission velocity of other switching such as electro-optical switching, acousto-optical switching, thermal-optical switching and mechanical-optical switching and make the data transmit from source node to aim node within optics field. Therefore, research of key materials and devices which can achieve all-optical switching is a frontier issue of the development of international photoelectric science and technology in today and to be paid high attention by people.There are various type of all-optical switchings, in which the most representative Mach-Zehnder interferometric all-optical switching makes use of the third-order nonlinear optical (NLO) effect of materials. A controlled beam is irradiated into material resulting in variety of refractive index. When a signal beam is irradiated to the material, its phase changes, then achieve the "on" and "off" of switching. The change of phase is proportional with 2πn2IL/λ, where I is intensity of irradiance, L is the interaction length of light and n2 is the nonlinear refractive index. The properties of all-optical switching, such as switch velocity, energy loss, sensitivity for polarization, insert loss and so on, are all determined by third-order NLO materials which are used in fabrication of devices.For a long time, the research about third-order NLO materials both at home and abroad are almost focused on ferroelectric liquid crystal basing on molecular orientation, semiconductor based on band gap resonance absorption, metallic oxide, chalcogenide glass, organic dye, fullerene derivative and polymers such as PDA, PA, PPV and the like. Among above-mentioned materials, the response velocity of liquid crystal is too slow, the nonlinear refractive indices of organic dye and polymers are too small, the nonlinear absorption of semiconductor is too large because of the action of resonance absorption in band gap as well as its slower response velocity. In a word, all above-mentioned materials are not suitable for the demand of all-optical switching device.In the dissertation, basing on the demand of all-optical switching application, we devote ourselves to explore a series of complexes which have properties of larger third-order NLO effects and higher mobility. Especially when metal ion is introducted into the conjugated system of complex, the charge transfer between metal and organic system makes the electron delocalization property of total conjugated system stronger, then further enhances the third-order NLO response and electron mobility, and induce off-resonant third-order NLO effects with ultrafast response time. In the earlier works, the conductive properties of 4,5-dithiolato-1,3-dithiole-2-thione (abbreviated as dmit) complexes were widely concerned. Recently, It has been gradually found that the molecules of these materials have excellentπelectron conjugated system, and thus the beginning of their third-order NLO properties. In this dissertation, we will make investigations focused on these classes of organic/inorganic complex semiconductors and study their third-order NLO properties as well as other relative characteristics.In previous works, our group has done in-depth studies on the third-order NLO properties of dmit complex in solution and had selected several materials which have larger nonlinear refraction and smaller nonlinear absorption. In this paper, we aimed at several typical dmit complex and did some systemic studies for their third-order NLO properties in PMMA film. The factors that impact the NLO properties of materials were discussed from their inherent mechanism. The thermal-induced loss and light transmission loss which can affect the NLO response and optical quality of the film were also explored preliminarily. The purpose is to get NLO materials which are applicable for all-optical switching device and ideal methods that can enhance nonlinear refraction and weaken nonlinear absorption, and provide experimental and theoretical evidence for the final device of the materials. The works of the dissertation are mainly reflected in the following aspects:1. Investigations of third-order NLO properties of dmit/PMMA polymeric complex films. Screened out several candidate materials which have properties of larger nonlinear refraction and smaller nonlinear absorption and can be used in all-optical switching device. Discovered the laws which the third-order NLO properties are closely related to central metal ion and structure.All-optical switching device demands materials have properties of not only larger nonlinear refraction but also smaller linear and nonlinear absorption. For the parameters obtained by Z-scan method such as nonlinear refractive indices, nonlinear absorptive coefficients and so on, two figures of merit W=n2I/α0λ和T=βλ/n2 were used to analyze the suitability of a material for application in all-optical switching devices. For the application purpose, it is necessary to achieve (?)＞＞1 and (?)＜＜1.For the research of the dmit comploexes with the metal ions are Au and Cu we found the two types of materials both show larger nonlinear refraction. Cu(dmit)2 compounds show larger nonlinear absorption compared with Au(dmit)2 compounds. The obtained figures of merit of two Au(dmit)2 compounds: [C16H33(CH3)3N][Au(dmit)2] (CTAu) and [(CH3)4N][Au(dmit)2] (MeAu) both satisfy the demands of (?)＞＞1and (?)＜＜1 . However, the values T of Cu(dmit)2 compounds are much larger than 1. We can conclude from those results that the two materials: CTAu and MeAu both satisfy the demands of all-optical switching devices for the NLO materials and have a promising prospect in the field of all-optical switching device. While for Cu(dmit)2 compounds are not suitable for use in all-optical switching applications.By increasing the doping concentration of nonlinear elements into the BFDT/PMMA polymeric complex films, the nonlinear responses of the films were enhanced obviously. With the nonlinear refraction enhanced, the change of nonlinear absorption was tiny. Otherwise, besides the larger nonlinear refraction effect, the BFDT/PMMA films showed tiny nonlinear absorption so that it can be ignored. The values that W were about at between 1.05～3.87 and T≈0) is up to the demand of (?)＞＞1 and (?)＜＜1. Therefore, BFDT is also a better candidate material which may be applied in fabrication of all-optical switching device in the future.2. Investigations of ultrafast time response properties of dmit complexes.Using femtosecond optical Kerr technology, the response time of OKE signal of dmit materials were all about 200 fs. Which were faster than that of current popular electro-optic switching about 4～6 orders of magnitude. The response time of some materials were first reported in this dissertation. Besides the third-order NLO effect induced by largerπ-conjugated structure, the ultrafast response velocity properties of dmit materials compared with other materials is the main reason that we select the type of materials as our study subject.3. Investigations of thermal effects of dmit/PMMA polymeric complex films.Obtained the law that, with the increase of the nonlinear elements in the film system, the influence of thermal effect is accordingly enhanced as well as the NLO effect. The thermo-optic coefficients of dmit/PMMA films were calculated to be at 10-5/℃magnitude by using a prism-coupler system attached temperature-controlled apparatus. The change of refractive index induced by thermal effect is the major factor that disturb the implementation of switch and response velocity. The influence of thermal effect can be depressed effectively by reducing the linear and nonlinear absorption of film systems.4. Investigations of light transmission loss of BFDT/PMMA polymeric complex films.Reducing the light transmission loss of waveguide film can improve the optical stability and practicality, then optimize the switching performance of waveguide device. By comparing the light transmission loss coefficients of BFDT/PMMA films at different doping concentrations, we obtained the tendency that the transmission loss of the film increases approximate linearly with the increase of doping concentration. Improving the fabrication technique and surface treatment technology of film and reduce scattering points in film is one of the effective methods to reduce the light transmission loss.Dmit/PMMA polymeric complex films were prepared using polymer coated method. We used surfactant as the coating agent. The films were observed using a scanning probe microscopy (SPM) and it was found that nanocrystalline particles separated out from PMMA matrix were distributed uniformly on the surface of the films. The particle scales were about within 20～50 nm. Dmit complex were doped into the polymeric PMMA matrix uniformly. The light transmission properties of BFDT/PMMA polymeric complex film which is prepared by polymer coated method were improved obviously.5. Investigations of optical limiting properties of Cu(dmit)2 compounds at the excitation of nanosecond pulse laser.Analyzed using a five-level mode, the conclusion that Cu(dmit)2 compounds shows the property of reverse saturable absorption at the excitation of 18 ns pulse was obtained. Then the optical limiting induced by reverse saturable absorption was studied in detail. We obtained the limiting thresholds of [(C2H5)4N]2[Cu(dmit)2] (EtCu) and [(C4H9)4N]2[Cu(dmit)2] (BuCu) in acetone solution in three different thicknesses by 18 ns pulse excitation. The larger linear transmittance and lower nonlinear transmittance of Cu(dmit)2 complexes indicated their promising prospects in the field of optical limiting such as laser protection. The difference of the two materials was expatiated from the perspective of different cations in molecular structure. It was confirmed that the smaller cation radius is the reason that the optical limiting effect of EtCu is stronger than that of BuCu.In conclusion, in the dissertation we analyze the third-order NLO and time response properties. The NLO responses of dmit materials which have metal ion and no metal ion, have same metal ion but different cation, have different metal ion were investigated at conditions of different pulse width and wavelength. The influence of cations and metal ions for NLO response of materials and relationship between nonlinear absorption and different pulse width were also indicated in the dissertation. In addition, by investigation on thermal effect and light transmission loss of dmit/PMMA polymeric complex films with different doping concentrations, we indicated the influence of doping concentration of nonlinear elements for the thermal-induced loss and light transmission loss. By screening materials, we obtained some NLO materials which would be promising materials at fields of all-optical switching or optical limiting. Basing on exploration and characterization of current materials, through indicating the internal relationship between molecular structures, external factors and functional features of materials, we found some effective methods of solving problems, and what is important, pointed the way for the further in-depth investigation in the future. The works of the dissertation will provide important experimental foundations for the further fabrication and investigation of waveguide devices.