Research On The Novel Photonic Devices Based On Liquid Crystal And Photonic Crystal

As the evolution of the advanced material, more and more photonic devices are proposed. In this thesis, we design and fabricate some novel photonic devices based on the liquid crystal and photonic crystal. And some technical issues about the liquid crystal and photonic crystal devices have been also solved.Generally, a waveplate only works for a single wavelength. It is highly desirable to develop an achromatic waveplate over a wide range of wavelengths. The conventional achromatic waveplates were realized with birefringent crystal. However, the fabrication of these crystal achromatic waveplates is very complicated and result in a high cost. In Chapter 2.3, we describe an optimal design for achromatic quarter-wave plate based on two twist nematic liquid crystal (TNLC) cells. As the maturity of the liquid crystal process technology, the structure of the waveplate we develop is very simple and can be integrated easily. Using this liquid crystal waveplate, the linearly polarized light can be turned into circularly polarized light within wavelength range 1200nm～1650nm.The liquid crystal variable optical attenuator (VOA) is always realized based on the TNLC cells. And it is a polarization-dependent device. We can use polymer dispersed liquid crystal (PDLC) to fabricate a polarization-independent VOA. However, The PDLC based VOAs reported in the literature have a conflict in performance between the polarization dependent loss (PDL) and the attenuation range. In Chapter 2.4, we analyzed and optimized the electro-optical properties of the PDLC films, both theoretically and experimentally. Then we fabricated a PDLC based VOA with the threshold voltage 8 V. The device has a typical attenuation range of 26 dB at wavelength 1550 nm. The insertion loss is 1.8 dB and the corresponding PDL is smaller than 1 dB, which is better than the ones reported before.As one of the most important novel photonics devices, the photonic crystal (PhC) properties are determined once and for all when the PhC is fabricated. Some tunable PhCs infilled with liquid crystal have been reported. However, the responding time of the liquid crystal is very slow, only about some msec. In Chapter 3.3, we consider tuning of PhC by choosing electromagnetically induced transparency (EIT). The respond time of the EIT based on semiconductor material is very fast, about some psec. As an example of this mechanism, we will consider a PhC-slab which displays negative refraction. By turning the control field on and off, this structure can shift the wavelength interval (between 1550nm and 1548.7nm) in which negative refraction occurs.Polymer photonic crystals may seem unlikely to yield useful photonic structures, because the refractive indices of such materials are substantially less than those of the more traditional used solid-state materials. However, as Chapter 3.4 shows, polymer PhC based polarization beam splitter (PBS) is presented. Computer simulations show that more than 99.9% of TM-polarized light is reflected by the PBS, whereas 98.9% of TE-polarized light propagates through the PBS over the C and L bands for optical communication with good angular insensitivity of about±5°. We can use multi-beam interference method to fabricate the polymer PhCs. The proposed PBS has several advantages when compared with other types of high refractive index PhC based PBSs, both in terms of performance and fabrication as the fabrication process is relatively simple, rapid and cheap.The extraction efficiency of the general light emitting diode (LED) is not high enough. In Chapter 4, we use PhCs to enhance the LEDs optical performance, including extraction efficiency and far field pattern. We develop software use to evaluate the LEDs performance. Using this software, we analyze the relation between the LEDs optical properties and the photonic crystal lattice, depth of the air holes and the LEDs structure. The extraction efficiency of PhCs LED we developed is more than 50%.