Recently, long-period fiber gratings (LPFGs) based on PCFs have been demonstrated by using heating or a mechanically pressure to induce periodic index variations along the fibers. However, LPFGs fabricated by these two methods suffer the structure damage. In this thesis we propose novel optically controllable LPFGs based on the photoresponsive photonic liquid crystal fibers (PLCFs) and no structure damage occurs during the fabrication process. The photoresponsive PLCF was filled with a LC mixture consisting of the nematic LC E7 and the photoresponsive 4MAB. The properties of the photoresponsive PLCF can be modulated by using laser irradiation. In addition, the transmission bands of the photoresponsive PLCF can also be tuned by controlling the 4MAB concentration or operation temperature. An optically controllable LPFG was fabricated based on the photoresponsive PLCF by using blue-laser irradiation through a mask with 700-£gm grating period. The measured resonant wavelength appeared at 1539 nm with the FWHM was 27 nm, and the maximum dip depth was about −15 dB with a 6.5-dB insertion loss. The LPFG was shown to be erasable by using a green laser. In addition, we have also investigated the effects of the number of grating period, 4MAB concentrations, operation temperatures, thermal recovery properties, and irradiation intensity on the LPFGs. Our proposed optically controllable LPFGs possess reversible property and are quite useful to be applied in tunable optical devices.

Fabry-Pérot (FP) filters are widely used in telecommunications, lasers and spectroscopy to measure the wavelengths of light. The properties of a FP filter depend on the wavelength and incident angle of the light source, the thickness of the etalon and the refractive index of the material between the reflecting surfaces. In previous studies, the nematic liquid crystal (NLC) is employed as the medium of FP filters because of its simple structure and ease of modulation. The directors of the NLC could be rotated by applying an electric field. Due to the birefringence of the NLC, the optical characteristics of the device are polarization dependent. Blue phase liquid crystal (BPLC) is the phase between cholesteric phase and isotropic phase. It¡¦s an optically isotropic material can function as an active index-tuning material adopted in a FP filter, and the characteristics of the BPLC-based FP filter are polarization independent. By applying an electric field, the Kerr effect can be induced due to the local reorientation of liquid crystals in BP structure, leading to the effective index change of BPLC and the transmission peak shifts. Furthermore, the effective index of BPLC approaches the ordinary index of host LCs under increasing electric fields. In addition, the BPLC using polymer network construction can be stabilized in room temperature and improves the convenience of the device. According to the experimental results, the tunability of the BPLC-based FP filter is about 1nm/V. The measured response time of the BPLC-based FP filter is 1ms.

In this research, the polarization converters were manufactured by impinging intensity-gradient-distribution laser on DDLC (dye-doped liquid crystal) samples, which the gradient distributed light intensity was produced by applying linear variable ND filter. The performances of these polarization converters were demonstrated, which can convert a linear polarized incident light into a circular polarized light or an elliptic polarized light. The influence of cell gap and beam size of probe beam on the DDLC polarization converter has been discussed.

This thesis utilized pulse shaping and Gerberg-Saxton algorithm to develop high power tunable narrowband terahertz radiation and applications. By the way of using Freezing algorithm and Gerberg-Saxton, pulse width, pulse duration, pulse position and pulse number can be controlled. The pulse train can be pumping source of high power tunable narrowband terahertz radiation, by avoid the saturations and different pulse duration. The simulation shown the result can have 5GHz narrowband width closing to commercial product. Besides, we demonstrated the THz multi-wavelength interference method by using THz-TDS (terahertz time domain spectroscopy) to measure the liquid crystal cell gap. We have also used THz-TDS to investigate the complex optical constants and birefringence of cholesteric liquid crystal mixture BL006. The extraordinary (ne) and ordinary (no) indices of refraction at are 1.784-1.817 and 1.516-1.555 giving rise to a birefringence of 0.25¡V0.29 in 0.2THz~1.6THz.

In this study we use PDMS thin films to fabricate of multi-domain surface. PDMS gets the characteristic of good physical properties and chemical, not only low surface free energy, flexible, also has low toxicity, low cost and can protect our environment. We used the DI water that was dropped on the PDMS thin film surface and became stable to explore the wettability of the surface, and we had a discussion about when liquid crystal dropped on the PDMS thin film surface. Then we also observe the phenomenon that was liquid crystal will shift a little distance and we used the optical image to analyze our device.

Mechanical Rubbing still is an useful alignment method in this time. In this study, we investigated the surface wettability of rubbed polyimide(PI) thin film. By using contact angle measurements, we found the surface free energy have strong connection with surface roughness in different rubbing times. We also discussed the wettability of rubbed PI thin film in parallel and anti-parallel of rubbing direction. We found that if we rubbed PI thin film 3 times in 0.3mm depth then the structure of SSFLC is more uniform than the alignment effect of other rubbing times and we found that the values of polar energy in parallel and anti-parallel of rubbing direction are very close. In the other part of study, we degenerated the polyimide before coating. We found that the reflection anisotropic spectrum(RAS) have a blueshift and the rubbing strength will be more influential in the wettability of PI thin film than the film with no degenerate process.

This study is based on the Owen-Wendt-Rabel and Kaelble method¡Aand we using several substrates which's surface energy is already known to determind the surface eneygy of liquid. We measure the contact angle of the liquid on the substract to calculate the dispers part and the polar part of the surface enerfy. And then we compare the system with different sets of substrates and choose one proper set to be the standard substrates.Finally we test the system by several common liquid which¡¦s surface energy is already known,and measure the surface energy of liquid crystal. In this study,we found that the surface energy of the liquid crystal on an alignment layer is anisotropy,and discuss requirements of the standard substrate.

Polymer liquid crystal possesses advantages of polymer in chemical industry and liquid crystal in display industry,so it is attracted more attention in science and technology. Diacrylate is a polymer liquid crystal with photosensitive property, so ,it can be applied to optical storage . He-Ne laser induced polymerization in diacrylate mesogen RM257 and RM82 had been verified in previous study. Furthermore, holography pattern can be recorded in RM257 and RM82 by controlling both the temperature of sample and the time of exposing. In this study, we consider the study of nonlinear optical properties of diacrylate using Z-SCAN techeique.¡¨Z-SCAN¡¨ is a simply yet highly sensitive single-beam experimential technique ,it can be used to measure both nonlinear absorption and nonlinear refraction.In this study ,we measured effect of absorption of diacrylate in irradiation of He-Ne laser using Z-SCAN technique.To investigate the reason that He-Ne laser induced polymerization in both RM257 and RM82.

In this thesis, we propose and demonstrate a simple and precise method for measuring mm scaled cell gap by using terahertz time domain spectroscopy (THz-TDS) system. This method allows us to measure the cell gap from 15mm to 1.5mm. In addition, the accuracy of measured thickness for the proposed method is also discussed and analyzed. Meanwhile, a nematic liquid crystal BL006 with birefringence as high as 0.27 in THz frequency range and its optical properties of cholesteric liquid crystal (CLC) as mixing chiral materials are investigated and reported. The ordinary refractive index and average effective refractive index at 20oC are from 1.52 to 1.56 and from 1.61 to 1.64, respectively, in THz frequency ranging from 0.2 THz to 1.4THz. In addition, we also demonstrate that cell filled with CLC is with polarization independent property for THz radiation. Through the 5mm cell filled CLC with diluted concentration of the dopant chiral material for decreasing the critical voltage, an electric controlled polarization independent phase shifter with the modulation depth exceeding 2pi is demonstrated. Furthermore, we also investigate the driving field dependence of phase retardation and discuss the reliability.

We take the electron deficient heterocyclic hexaazatriphenylene (HAT) as our central core and readily synthesized by the condensation of hexaketo- cyclohexane with the respective 1,2-bis-alk-oxy-4,5-diaminobenzene. We use the polarised optical microscopy (POM) and differential scanning calorimetry (DSC) to study the mesophase range.These mesophases are identified as columnar phases by diffraction (XRD).