Research On The Key Performance Of Organic Electro-optic Materials For Terahertz Applications

The important value of terahertz(THz) technology has been increasingly recognized in a variety of research areas, such as communication, radar, security inspection, radio astronomy, and biomedical sciences. In this new research field, the ability of THz generation and detection is a key factor to promote the development of THz technology. The optical rectification method to generate terahertz and electro-optic(EO) sampling technology to detect terahertz based on the utilization of EO materials has become the common method to realize the broadband THz generation and detection. Compared with conventional crystalline EO materials, organic EO materials lack phonon absorption and are capable of much higher EO performance and much faster response times. These materials are also highly flexible and processable, allowing required performance improvement by means of molecular design and modification. Therefore, organic EO materials are widespread concerned in the THz community. Especially, high number-density monolithic organic EO materials have rapidly become the research focus of EO materials in recent years due to its great potential in EO performance. However, in order to meet the application requirements of THz generation and detection, the key performance of this type of material which includes EO activity, thermal stability and absorption loss must be simultaneously optimized. In this dissertation, the conduction mechanism during the re-orientation of the chromophores induced by electric field poling is analyzed. Solid-state device engineering concepts to eliminate leakage current during the poling process are presented with experimental evidence to support the efficacy of enhancing the EO coefficient(r33) of monolithic organic EO materials. The method to evaluate the intrinsic poling efficiency of monolithic organic EO materials is established. The structure-function relationships of monolithic organic chromophores are systematically studied, expanding the knowledge system of the relationship between molecular structure and the key performance of the material. A fast method to characterize the refractive index and absorption coefficient of organic EO materials in the THz frequency region is exploringly studied. The main contents of this dissertation are concluded as follows:1. Benzocyclobutene barrier layer for suppressing conductance in nonlinear optical devices during electric field poling. The EO performance of monolithic organic EO materials has greatly improved due to the increase of the number density of the chromophores in the material system, which also causes the increase in the material’s conductivity. High conductivity of the EO material during the poling process increases leakage current, which in turn limits the amount of acentric order that is induced, becoming the main challenge of the realization of large r33. After understanding the nature of the interface-limited current and analyzing the conduction mechanism, we propose a benzocyclobutene(BCB) barrier layer between the electrode and the EO material to prevent charge injection. The optimum thickness of the BCB layer is determined through the study of the impacts on EO activity with a BCB layer at different thicknesses. The presence of the BCB layer is very effective in suppressing the extent of leakage current or the conductance of the device, keeping the effective poling voltage nearly identical to the applied voltage, resulted in a factor of 2 increase in the maximum observed r33 compared to the device without a barrier layer. The suppression of the leakage current in combination with a new chromophore JRD1 enabled the construction of EO devices that had r33 values greater than the previous plateau value of ~ 400 pm/V in several measurements with poling fields ≥ 85 V/μm. An ultra-high principal electro-optic coefficient of 556 pm/V is observed, which we believe is a record high value in the published literature. The BCB barrier layer is shown to reduce current during poling by one or two orders of magnitude relative to the well-studied titanium dioxide barrier layer of current utilization.2. The evaluation criteria for intrinsic poling efficiency of monolithic organic EO materials. The high conductance of devices fabricated from monolithic materials during poling is significant, causing a notable drop in voltage across the EO layer. So, the use of the applied electric field as the poling field in the calculation of poling efficiency previously is not capable to accurately calculate poling efficiencies of these materials. In this paper, the average electric field during poling is used in the calculation for a more accurate estimation of intrinsic poling efficiency and it is estimated from the fitting of the voltage curve in different poling stages. This modified algorithm of poling efficiency could accurately evaluate the actual EO performance of monolithic organic EO chromophores from conventional devices for EO characterization taking into full account the impact on the realization of EO coefficient caused by the decrease of electric field strength during poling process.3. Structure-function relationship in monolithic organic EO chromophores. There are two potential disadvantages of monolithic organic EO materials in real applications: first, some of the monolithic organic EO chromophores readily form crystals, causing phase separation; second, the thermal stability(mainly reflected by the glass transition temperature) of this type of materials is generally lower than desirable, which reduces the reliability and lifetime of the working devices. Our research focuses on the structure-function relationship of monolithic organic EO chromophores to solve these problems, exploring the impact on the key performance of the EO materials(e.g. maximum EO coefficient, poling efficiency, glass transition temperature, and conductivity) caused by molecular modification based on the gradual change of the existing molecular structure. In a library of EO molecules with varied bridge segments, molecular modification of the donor with bis(tertbutyldiphenylsilyl) groups led to improvement in formation of amorphous films and led to enhanced poling efficiency. Further modification to include a carbazole site-isolation group on the bridge effectively reduced intermolecular dipole-dipole interactions, led to a material with poling efficiency of approximately 3(nm/V)2, and an increased glass transition temperature to 20-40 ?C higher than similar reported monolithic materials.4. The optical properties of ultra-thin polymer films in the THz region. The application of organic EO materials in THz generation and detection requires knowledge of the THz refractive index to address phase matching requirements and the THz absorption to guide the selection of appropriate material composition. Conventional THz characterization requires thick-films, which can be problematic in the aspects associated with thick-film fabrication and processing. We propose a new method for characterizing the refractive index and absorption coefficient of EO materials from the thin-film device utilizing time-domain attenuated total reflection spectroscopy in the THz frequency region. A new device architecture is developed accordingly, with the multi-layer model established for analysis and Matlab program to implement the analysis. The optical properties of the test material showed good agreement with the results from pellets measurements by conventional THz-TDS system. In this way, we are able to circumvent the need for thick films and characterize thin films(around a few microns) both before and after the poling process, enabling more rapid characterization of the key properties.