| In the end of the 20th century, the appearance of Internet indicates that people have entered a new great era of information in which both the amount of information and the requirement of it increase rapidly. Under such conditions, the optical fiber communication system (OFCS) appeared to be the first choice of the high-speed and wide-band networks due to its merits such as super large capability, good transparency, wavelength route choice characteristic, excellent compatibility and expansibility. However, in the present OFCS, there are many electronic components with some disadvantages such as slow switching speed, clock displacement, serious crosstalk and high power loss which can easily lead to the "bottleneck" of the OFCS. Currently, to break the "bottleneck", people have put forward the concept of "all-optical networks" as an improvement of the existing OFCS. It discards the electronic devices of the existing OFCS and realizes the transmission process from the source point to the aim point completely performed in light domain while information exchange at points adopts all-optical networks exchange technique. The realization of all-optical networks cannot be separated from the development of various nonlinear ultra-fast photonic devices among which an all-optical switch (AOS) is one of the key components. In general, AOS are based on such nonlinear optical effects as nonlinear refraction, nonlinear absorption, nonlinear frequency conversion and nonlinear coupling and so on. Taking advantage of the third-order nonlinear refraction of materials, AOS' work principle can be described as follows: With a control beam inducing the change of refractive index of materials, the other signal beam transmitting in the material will produce an additional phase difference to realize the function "On" or "Off' of optical switches. AOS can break through the transmission speed limits of electro-optical, acousto-optical, thermo-optical and micro-electro-mechanical switches and has been a current research focus. To make nonlinear refraction type AOS, the nonlinear optical (NLO) materials should possess large third-order nonlinear refraction, low linear and nonlinear absorption and fast response speed. If materials own large third-order nonlinear refractive index, the power density of control light need not be very high lest the devices be damaged. If the linear and nonlinear absorption are very low, the information transmission loss can be lowered accordingly, which weakens the thermal effect, increases the switching speed and then enhances the reliability and stability of the system.Besides, materials with proper third-order nonlinear optical properties can also be used for personnel and facility protection in the war. The laser technology is more and more widely used in military applications due to its high energy and good collimation since it was invented in the sixties of the last century. In recent years, research projects on laser weapons attracted much investment and experienced fast development. These weapons choose eyes and sensors as targets and can directly disable or destroy them by using the high energy laser beam irradiating in a certain direction. With the unceasing development of these weapons, the threat people confront becomes more and more serious. Therefore, how to develop the effective devices to avoid these injuries turns out to be a problem needing to be solved exigently. The new generation optical limiting (OL) devices mainly use the NLO effects of materials such as reverse saturable absorption, two-photon absorption, nonlinear refraction, nonlinear reflection and scattering, etc.. Compared with the early laser defenders, they own many advantages like fast response, wide protected waveband, low OL threshold, large damage threshold and high linear transmission, etc.. The third-order NLO properties of materials can also be used in the compression (mode-locking and Q-switching) and shaping of laser pulses, optical bistability and femtosecond lasers' Kerr Lens Mode-locking technique in which the refractive index of materials varies with optical intensity, making the gain of lasers' peak pulses higher than that of sequential lasers' background and thus producing ultra-short laser pulses.Currently, the investigations of NLO devices, such as AOS and OL devices, include not only the improvement and innovation of technologies and structures but also the exploration of new action mechanism and novel materials. However, people still don't find such materials whose performances are comprehensively suitable to the development of these NLO devices until now. Therefore, it seems to be particularly important to explore and synthesize novel optical materials possessing excellent third-order NLO properties and good factors of merits. In the same way, establishment and optimization of the measurement techniques on the optical nonlinearity are also very important in the process of exploring the novel NLO materials, which can not only measure the NLO parameters but also reveal the ultimate origin of NLO properties. This can supply important instruction for the synthesis of materials with more excellent performances as well as provide reliable data for further device designing. For this purpose, the dissertation mainly comprises the following aspects:Firstly, four series and nearly twenty kinds of metal-organic DMIT complexes whose central metal ions are Ni, Cu, Co and Au were explored and synthesized, and MeAu, MeCu, EtCu, PrCu, O-CtCu and CtNi were reported for the first time as new optical materials. Recently, the investigations and characterizations of the third-order NLO materials are one of the key focuses in NLO materials research field. Mass materials are reported and can be divided into two categories, namely inorganic and organic optical materials. The former mainly include semiconductors, titanium salts, metal oxides and sulfide glass and the latter cover one-dimensional long-chain polymer, small molecule compounds, fullerence molecules and their derivatives, and transition metal organic complexes, etc.. Compared with inorganic materials, organic materials are attracting great interest because of their relatively low cost, ease of fabrication and integration into devices, tailorability which allows one to finely tune the chemical structures and properties for a given NLO process, high laser damage thresholds, low dielectric constants, fast NLO response times, and large off-resonance NLO susceptibilities. Moreover, the electron delocalization will be enlarged when transition metal ions are introduced into the organic conjugated system, which can further enhance the optical nonlinearities. Our group selected DMIT complexes as our research objectives, through analysis of the relationships between material structures and nonlinear optical properties, during the exploration of new materials with good NLO performance. The anion part richly containing sulphur in the molecule can be abbreviated as Metal(dmit)2. The "dmit" represents l,3-dithiol-2- thione-4,5-dithiolate and the central metal atoms can be Ni, Cu, Pd and Pt, etc.. Their properties can be changed through substituting different metal ions. Early, people mainly studied their synthesis and electrical conductivity. However, people found that these materials own largerπelectron conjugated systems and then began to pay more attention to their optical nonlinearities. Our group explored and synthesized systematically four series and nearly twenty kinds of DMIT materials by introducing different central metal ions through theory analysis and chemical synthesis. Some complexes such as MeAu, MeCu, EtCu, PrCu, O-CtCu and CtNi etc., are novel optical materials and their single crystal structures, linear absorption and some other properties are reported for the first time. Depending on the studies of material performance, we picked out several nonlinear optical materials, such as MeAu and EtCu, which maybe have important applications in the field of nonlinear optics.Secondly, a set of Z-scan experimental apparatus was established by ourselves to characterize the third-order optical nonlinearities of DMIT materials, and the automations of the equipment control and data collection were perfected. There are many techniques to characterize the third-order optical nonlinearities of materials including nonlinear interferometry, three-wave mixing, degenerate four-wave mixing, ellipse rotation and beam distortion measurements, etc.. These traditional methods have some disadvantages such as their complicated experimental apparatus, poor controllability, and indistinguishability of the real and imaginary parts of nonlinear susceptibility, namely the third-order nonlinear refraction and nonlinear absorption, respectively. On the contrary, accurate determination of these NLO parameters is very important and can provide us the instruction in practical applications. In 1989, a new and simple method, Z-scan technique, aiming to measure both the nonlinear refraction and nonlinear absorption, was proposed by M. Sheik-Bahae et al. By using time-resolved two-color Z-scan technique we can study the nonlinear response time of materials and further distinguish the various NLO effects with different response time. The dissertation analyzed Z-scan method roundly and gave the detailed theory analysis progress and commonly used formulas to obtain their performance parameters by experiment. Meantime, we established a set of Z-scan experimental apparatus in our laboratory by ourselves and developed a program to realize the automation of experimental data collection and displacement device control using Labview software, which harmonized its work procedure, saved a lot of time and increased the precision of results.Thirdly, the third-order NLO properties of DMIT materials were systematically studied using Z-scan technique and time-resolved Optical Kerr Gate method. In this dissertation, we investigated systematically the optical nonlinearities of four series of DMIT materials in the near-infrared waveband with different pulse widths. First of all, with 40 ps laser pulses at 1064 nm, we obtained a lot of NLO parameters such as the third-order nonlinear susceptibility, third-order nonlinear refractive index, nonlinear absorption coefficient, molecular second-order hyperpolarizability, absorption cross section of ground and excited states, and two-photon absorption cross section, etc.. The values of molecular second-order hyperpolarizability lie between 10-32 and 1030 esu according to the change of central metal ions. Various factors which maybe affect the third-order NLO properties were also investigated as follows: 1. We studied the influence of molecular structures on linear absorption and third-order optical nonlinearities and compared our results with those reported in the literatures. 2. We synthesized the new complex O-CtCu by air-oxidation method and studied the optical nonlinearities of O-CtCu and the pre-oxidation complex CtCu. 3. The third-order optical nonlinearities of the new complex MeAu were investigated at 532 nm and 1064 nm with picosecond pulses and the difference between the results was explained in view of the linear absorption spectrum. We found that MeAu possesses high non-resonant third-order optical nonlinearities which satisfy the requirements of AOS for materials' figures of merit, namely W = n2I/α0λand T =βλln2 (|W|＞＞1 and |T|＜＜1).Then we studied the nonlinear absorption properties of DMIT materials in nanosecond regime (pulses widths of 1 ns and 15 ns) at a wavelength of 1053 nm. It can be seen from Z-scan experiments that the same material can present different phenomena when irradiated by laser pulses with different widths. The five-level model was used to analyze the mechanism responsible for the difference and revealed that the nonlinear absorption was dominated by the singlet-singlet excited state transitions for 40 ps and 1 ns laser pulses, while for 15 ns pulses it can be mainly attributed to the triplet-triplet excited state transitions. The effective nonlinear absorption coefficients and excited states absorption cross sections in nanosecond regime were calculated. We estimated the time of intersystem crossing according to our data and the relevant literatures. In addition, we measured the nonlinear transmission curves and estimated the ratio of the effective excited states absorption cross section of triplet states to that of ground states using a simple model. Finally, the dissertation studied the nonlinear response time properties of DMIT materials with femtosecond resolved Optical Kerr Effect method and obtained the third-order nonlinear susceptibility, molecular second-order hyperpolarizability and the response time. The data revealed fast response times of DMIT materials are about 200 fs.All the outcomes suggest that DMIT materials possess good performance and have potential applications in the NLO field. The work is supported by high technology 863 National Plan (Grant No.2002AA313070), National Natural Science Foundations (Grant Nos. 60377016 and 60476020), and the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (Grant No. 200539). |
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