Colloidal self assembly is an efficient method for making 3-D ordered nanostructures suitable for materials such as photonic crystals and macroscopic solids for catalysis and sensor applications. Colloidal crystals grown by convective methods exhibit defects on two different scales. Macro defects such as cracks and void bands originate from the dynamics of meniscus motion during colloidal crystal growth while micro defects like vacancies, dislocation and stacking faults are indigenous to the colloidal crystalline structure. This paper analyses the crystallography and energetics of the microscopic defects from the point of view of classical thermodynamics and discusses the strategy for the control of the macroscopic defects through optimization of the liquid-vapor interface. / Singapore-MIT Alliance (SMA)

This thesis concerns some experimental and theoretical issues in fiber optics. In particular, properties and devices based on photonic crystal fibers (PCFs) are investigated. The work can be grouped into three parts. In the first part we use sound to control light in PCFs. The lowest order flexural acoustic mode of various PCFs is excited using an acoustic horn. The acoustic wave acts as a traveling long-period grating. This is utilized to couple light from the lowest order to the first higher order optical modes of the PCFs. Factors affecting the acoustooptic coupling bandwidth are also investigated. In particular, the effect of axial variations in acoustooptic phase-mismatch coefficient are studied. In the second part of the thesis we use an electric field to control transmission properties of PCFs. Tunable photonic bandgap guidance is obtained by filling the holes of an initially index-guiding PCF with a nematic liquid crystal and applying an electric field. The electric field introduces a polarization-dependent change of transmission properties above a certain threshold field. By turning the applied field on/off, an electrically tunable optical switch is demonstrated. The third part consists of two theoretical works. In the first work, we use relativistic causality, i.e. that signals cannot propagate faster than the vacuum velocity of light, to show that Kramers-Kronig relations exist for waveguides, even when material absorption is negligible in the frequency range of interest. It turns out that evanescent modes enter into the Kramers-Kronig relations as an effective loss term. The Kramers-Kronig relations are particularly simple in weakly guiding waveguides as the evanescent modes of these waveguides can be approximated by the evanescent modes of free space. In the second work we investigate dispersion properties of planar Bragg waveguides with advanced cladding structures. It is pointed out that Bragg waveguides with chirped claddings do not give dispersion characteristics significantly different from Bragg waveguides with periodic claddings.

Pillar-array based optical cavities have unique properties, e.g., having a large and connected low dielectric index space (normally air space), exhibiting a large band gap for transverse magnetic modes, having a large percent of electric field energy in air and standing on a substrate. These properties make them well suitable for applications such as optical sensing and terahertz quantum cascade lasers. However there has been rare research in it due to the common belief that pillar arrays have excessive leakage to the substrate. With careful design, we provided several methods to reduce such a leakage and experimentally proved a high quality factor (Q) pillar-array based cavity is practical. We also explored the usage of such a cavity for optical sensing. Numerical methods such as finite-difference time-domain and plane-wave expansion were used in the design of the cavity. Then in microwave spectrum, cavities consisting of dielectric rods were used to test the validity of the theory. Additionally, we observed that a high-Q cavity for modes above light line is feasible and it is very suitable to measure the optical absorption of materials introduce inside the mode volume. Finally in the optical domain, pillar arrays were fabricated in Si/SiO2 material system and measured. Q as high as 27,600 was shown and when applying accurate refractive indices, for every delta n = 0.01, the peak wavelength shifted as large as 3.5 nm, testifying the ultra sensitivity of the cavity to the environmental dielectric change.

Pillar-array based optical cavities have unique properties, e.g., having a large and connected low dielectric index space (normally air space), exhibiting a large band gap for transverse magnetic modes, having a large percent of electric field energy in air and standing on a substrate. These properties make them well suitable for applications such as optical sensing and terahertz quantum cascade lasers. However there has been rare research in it due to the common belief that pillar arrays have excessive leakage to the substrate. With careful design, we provided several methods to reduce such a leakage and experimentally proved a high quality factor (Q) pillar-array based cavity is practical. We also explored the usage of such a cavity for optical sensing. Numerical methods such as finite-difference time-domain and plane-wave expansion were used in the design of the cavity. Then in microwave spectrum, cavities consisting of dielectric rods were used to test the validity of the theory. Additionally, we observed that a high-Q cavity for modes above light line is feasible and it is very suitable to measure the optical absorption of materials introduce inside the mode volume. Finally in the optical domain, pillar arrays were fabricated in Si/SiO2 material system and measured. Q as high as 27,600 was shown and when applying accurate refractive indices, for every delta n = 0.01, the peak wavelength shifted as large as 3.5 nm, testifying the ultra sensitivity of the cavity to the environmental dielectric change.

In this study, we study three kinds of surface treatments in the blue phase lasers. Three kinds of surface treatments include no surface treatment, homogeneous alignment ,HA, and vertical alignment ,VA. Blue Phase liquid crystal is a three-dimensional photonic crystal, and it can be used to be a laser cavity. When the blue phase doped with a laser dye, a laser emission can be observed under appropriate pumping energy. In the first part, we fabricate the blue phase lasers with three kinds of surface, there are different surface treatments to study the surface effect of the optoelectronic properties. It is found that the threshold pumping power is significantly decreased under a surface treatment on glass substrate. In the second part, we change cooling rate in the formation of the blue phase liquid crystals, and study the optoelectronic properties of blue phase lasers. The experiment results reveal that the slower cooling rate leads to a order blue phase substrate, resulting in a lower threshold pumping energy of the blue phase laser.

Blue phases have been known to exist in chiral liquid crystals between the cholesteric and isotropic phases. A blue phase as a self-assembled three-dimensional cubic structure with lattice periods of several hundred nanometers exhibits not only selective Bragg reflections of light in the visible wavelength but optically isotropy owning to its highly symmetric molecular structure. Locally, blue phases still exhibit local anisotropic physical properties because of anisotropic structure of the nematic liquid crystal molecules, which make it possible to be easily controlled by an external field. This dissertation studies the effects in blue phases under various external fields, including electrical field, optical field, and temperature. Firstly, we investigated the bistable effect under the influence of an electric field and transition mechanism between various lattice orientations in the negative liquid crystal blue phase. The blue phase exists over a wide temperature range ~16oC, and three lattices (110), (112) and (200) of BPI are confirmed with Kossel diagrams. The red platelet (110) lattice and blue platelet (200) lattice can be stabilized and switched to each other by particular pulse voltages. We also studied the behavior that an electric field induced planar state and electro-hydrodynamatic effect in the blue phase. Additionally, the reflected color of the (200) lattice can be adjusted from 455nm to 545 nm by temperature induced lattice distortions and provided with reversibility. Secondly, we presented an optically switchable band gap of a 3D photonic crystal that is based on an azobenzene-doped liquid crystal blue phase. Two kinds of azobenzene, M12C and 4MAB, were utilized to switch photonic band gap of blue phases and to change the phase transition temperature of blue phase, respectively. For M12C- doped liquid crystal blue phase, the trans-cis photoisomerization of M12C induced by irradiation using 473nm light caused the deformation of the cubic unit cell of the blue phase and a shift in the photonic band gap. The fast back-isomerization of azobenzene was induced by irradiation with 532nm light. The crystalline structure was verified using a Kossel diffraction diagram. Moreover, we also demonstrated an optically addressable blue phase display, based on Bragg reflection from the photonic band gap. For 4MAB- doped liquid crystal blue phase, the trans-cis photoisomerization of 4MAB destabilizes cubic unit cell of the blue phase and reduces the phase transition temperature. We observed the phase sequences of the 4MAB-doped blue phase as a function of the time of UV irradiation. Various distinct phases can be switched to another specific phase by controlling irradiated time and temperature of the sample. Therefore, the corresponding bandgap can be switched on and off between blue phase and isotropic phase, or varied from 3D to 1D between blue phase and cholesteric phase. Finally, we investigated the thermal hysteresis in the phase transition between the cholesteric liquid crystal and the blue phase of liquid crystal. The thermal hysteresis of such a chiral doped nematic liquid crystal occurs over 6oC. Both the CLC phase and the blue phase can stably exist at room temperature and be switched to each other using temperature-controlled processes. Further, we demonstrated two sets of bistable conditions using various surface treatments. In a homogeneous aligned sample, two stable states, CLC with a planar alignment and blue phase with a uniform lattice distribution, reflect light of wavelengths 480-510nm and 630nm, respectively, as determined by the corresponding Bragg¡¦s reflection conditions. In the untreated sample, the CLC phase with a focal conic texture can scatter light and the blue phase with a non-uniform lattice distribution provides high isotropic optical transparency.

The cavity quantum electrodynamics has been applied to investigate the strong coupling interaction dynamics process between the microcavity field and the atom. The high quality cavity is a key to the realization of cavity quantum electrodynamics. Photonic crystal nanocavities are with small mode volumes and large quality factors. Lights are confined within the nanocavity. They can be used for cavity QED experiments of Fabry-Perot cavity. We have provided a realization of a quantum entanglement method for quantum information processing. In this paper, we discuss the entanglement swapping in the strong coupling process between two level atoms interacting with the photonic crystal microcavities fields of coherent states. We investigate the atomic level population and the entanglement degree of the system. We have found that the atomic maximal entangled state can be transformed into the photonic crystal microcavity maximal coherent entangled state cavity field, whereas the photonic crystal microcavity maximal coherent entangled state cavity field can be transformed into the atomic maximal entangled state.

Based on a full-wave formulation, a vector boundary element method (VBEM) is proposed to model the photonic crystal fibers (PCFs) (microstructured fibers). The accuracy and efficiency of the approach are confirmed by comparing the results calculated with those in previous literatures. With employing the VBEM, the guiding characteristics, including the effective indexes, vector mode patterns, and the polarization properties of the PCFs are investigated. There polarization characteristics of the PCFs with elliptical air holes (EPCFs) and the one ring air-hole EPCF embedded in the step-index core are studied and discussed. In addition, based on the VBEM formulations, a novel and efficient numerical approach to calculate the dispersion parameters of the PCFs is also proposed. The effect of the PCF geometrical structure on the group velocity dispersion property is reviewed, and then the one-ring defect and two-ring defect PCFs are studied and designed for the ultra-flattened dispersion applications. As an example, a four-ring (two-ring defect) PCF with flattened dispersion of ¡Ó0.25 ps/km/nm from 1.295£gm to 1.725£gm wavelength is numerically demonstrated.

Abstract A projected fringe profilomertry ¡]PFP¡^ using a supercontinuum light illumination for micro-scale measurement is proposed. The supercontinuum light is generated by launching ultra short laser into a highly nonlinear photonic crystal fibers. The supercontinuum light with the following advantage¡G ¡]1¡^ Depth of the field is very large in the projected system. ¡]2¡^No speckle noise in the illumination system. Experiment results has shown that using supercontinuum light is superior to other illumination system This study indicates that the proposed measurement scheme could be applied to 3D shape measurements with large depth variation, especially for semi-conductor devices¡Bmicro electro-mechanical devices and biomedical species.

Near-field light confinement on scanning probe is the backbone technology for near-field imaging with subwavelength resolution that overcomes the diffraction limit by exploiting the properties of evanescent waves. The fusion of the photonics and the latest nanofabrication technology creates emerging frontier for near-field light confinement research with new design approach. The propagation of light can now be controlled by periodical structure at subwavelength scale with low loss in the artificially synthesized dielectric material. New light propagation patterns can now be implemented in subwavelength structure, such as directional free space light focus grating coupler, photonic bandgap material like photonic crystal by permitting light propagation at certain wavelength while prohibiting light outside of bandgap, and nano-slot light resonator for increased light-matter interaction at nanometer scale. Advances in this research area will have tremendous impact on electromagnetic modeling and biomedical technology for probe based subwavelength optical detection. My doctoral research focused on investigating highly efficient, nanofabrication compatible directional light coupling structure and near-field subwavelength light focus through photonic crystal material. The distinct significance of this research was placed on exploitation of the embedded metallic grating coupler of high free space directivity and subwavelength light processing circuit of enhanced near-field transmission rate, the two most dominating basic elements of the scanning optical imaging system. First, I designed a compact elliptical grating coupler based on embedded noble metal such as gold or silver that efficiently interconnects free space with dielectric rectangular waveguide. The dense system integration capability shows the application potential for on-chip interfacing subwavelength light processing circuits and near-field fluorescent biosensors with far-field detection of superb radiation directivity and coupling efficiency. Second, a novel all-dielectric light confinement probe designed by slotted photonic crystal waveguide provides a light confinement mechanism on the lateral plane. The resonating nano-cavities and the λ/4 nano-slot are used to enlarge the light throughput while as the nano-slot waveguide provides single subwavelength center lobe. The impetus of this research is the growing interests by near-field imaging researchers to obtain a low loss visible light confinement probe designs through mass production. / text