| Terahertz time-domain spectroscopy (THz-TDS) technology is widelyemployed in industrial and scientific in the last decade. Current THz technologiesare limited in their frequency response because of phonon absorbance and poorphase matching in crystalline emitters and detectors. In contrast, amorphouselectro-optic (EO) polymer composite materials have the potential for broad-bandwith, spectral gap-free THz emission and detection while requiring arelatively low pump laser power. EO polymer family have become an alternate tocrystalline EO materials have shown higher EO coefficient, greater coherencelength and broader bandwidth free of spectral gaps compared with theircrystalline counterparts. With better EO polymers being developed continuously,it can be envisioned that EO polymers will take a larger role in the near future.Although the EO polymers have a wide range of advantages, but it lacks a kindof material which has excellent comprehensive performance, e.g. higher EOcoefficient, good stability of chromophores orientation degree and low dispersionin the THz frequency because of it is limited in nature of the material itself,poling geometries and processing techniques. Therefore it has importantscientific meaning and practical value to design and synthesis a high qualityfactor EO polymer in order to widely use in THz-TDS system.In this dissertation, side chain EO polymer as the main body and doped in aproportion of small EO molecule is prepared into double EO molecule polymersystem. With the introduction of the strong polarity molecules, the polarity of theside chain EO polymer as the main body can be improved, so the chromophoresin the side chain molecules can be easier to orientation under the external electricfield. Increase the number of chromophores polymer system because of the Because of the addition of small EO molecule, the macroscopic nonlinear opticaleffect will be enhanced. Through the EO coefficient and chromophore of stabilitytesting, the EO coefficient of EDB(6wt%)/2,4-dinitraniline-carbazole-plyphosphazenes(94wt%) is33pm/V and its stability of chromophore keeps inthe initial value of69%after600hours at room temperature.Up to now, using an EO polymer sensor to detect THz radiation, a polymerfilm is sandwiched between two poling electrodes, which consist of eitherevaporated gold or a transparent conducting oxide (TCO), such as indiumtinoxide (ITO) or TCO-TCO. This thin-film device is easy fabrication, but thedetection efficiency and sensitivity are limited in their structure due to theabsorbability and high reflectivity of metal in the THz regime. On the other side,the poling direction of the device is normal to the polymer plane, therefore to beused it needs to consider the incident angle. In this dissertation, we adopted lasermicromachining technique to deposite interdigitated coplanar electrodes on thealuminum evaporated glass substrate and preparated the thin film device withnterdigitated coplanar electrodes. Because of the electrodes are in plane, on theone hand, to avoid the influence of electrodes for detection sensitivity, on theother hand, this devices allows for optimal overlap of the THz electric field withthe poling direction of the polymer flim without an external slant angle. Underthe same film thickness, the ratio of the THz amplitude from sensor withinterdigitated coplanar electrodes and sensor with sandwhich structure is1.3,thereby increasing the sensing efficiency by30%.THz-TDS technology is employed in material testing, main objective is toobtain information on the complex refractive index, dielectric constant, andconductivity of the sample in the THz regime. In this dissertation, we selectLDPE and its nanocomposite which widely used in the dielectric engineeringfield, as the research object. To used the Terahertz time-domain spectroscopysystem based on the double EO molecules polymer for Terahertz sensing studyon the THz spectroscopy of the LDPE, LDPE with different humidity, and LDPEnanocomposite. |
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