No description available.

No description available.

The work presented in this thesis describes an investigation into the properties and behaviour of nonlinear optical guest molecules doped into polymeric matrices. The interactions of the guest molecule 2-(N, N dimethylamino)-5-nitroacetanilide (DAN) with a polycarbonate and polymethyl methacrylate (PMMA) host are compared. A detailed characterisation of the two systems is described employing infra red spectroscopy and analysis of the chromophore alignment during electric field poling. The study reveals that hydrogen bond formation between the guest and the polar polycarbonate backbone accounts for the unusually good alignment stability previously reported in the polycarbonate system. The molecular hyperpolarisibility of DAN in PMMA is also measured and the apparent enhancement compared with solution measurements is accounted for by the more polar nature of the polymer environment. A new technique allowing the measurement of the dipole moment of polar molecules doped into thin polymer films is also presented. The technique is demonstrated on a series of zwitterionic chromophores whose measured dipole moments range from 30 to 40 D. Electrochroism measurements are performed to account for aggregation of the monomer species which then permits the first hyperpolarisibility of the molecules to be calculated. The values of dipole moment and hyperpolarisibility are found to be very sensitive to the choice of dielectric cavity shape used when deriving the local field correction factors. The measured values are therefore compared with theoretical calculations and a preferred cavity shape is proposed.

No description available.

Master of Science / Department of Physics / Brian Washburn / Zenghu Chang / Attosecond pulse generation is a powerful tool to study electron dynamics in atoms and molecules. However, application of attosecond pulses is limited by the low photon flux of attosecond sources. Theoretical models predict that the harmonic efficiency scales as λ[lambda]-6 in the plateau region of the HHG spectrum, where λ [lambda] is the wavelength of the driving laser. This indicates the possibility of generating more intense attosecond pulses using short wavelength driving lasers. The purpose of this work is to find a method to generate intense single attosecond pulses using a 400 nm driving laser. In our experiments, 400 nm femtosecond laser pulses are used to generate high harmonics. First, the dependence of the high harmonic generation yield on the ellipticity of 400 nm driving laser pulse is studied experimentally, and it is compared with that of 800 nm driving lasers. A semi-classical theory is developed to explain the ellipticity dependence where the theoretical calculations match experiment results very well. Next, 400 nm short pulses (sub-10 fs) are produced with a hollow core fiber and chirped mirrors. Finally, we propose a scheme to extract single attosecond pulses with the Generalized Double Optical Gating (GDOG) method.

Hui Yuen Yung. / Thesis submitted in: August 1998. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (leaves 70-73). / Text in English; abstract also in Chinese. / Hui Yuen Yung. / Acknowledgements --- p.vi / Abstract --- p.vii / Chapter 1 --- Introduction --- p.1 / Chapter 2 --- Second harmonic generation --- p.5 / Chapter 2.1 --- Introduction --- p.5 / Chapter 2.2 --- Maxwell's equations in dielectric media --- p.6 / Chapter 2.3 --- Second harmonic generation --- p.7 / Chapter 2.4 --- Phase matching --- p.8 / Chapter 2.5 --- Quasi phase matching --- p.10 / Chapter 2.6 --- Inversion symmetry --- p.11 / Chapter 3 --- Third order nonlinear optical processes in optical fibers --- p.14 / Chapter 3.1 --- Introduction --- p.14 / Chapter 3.2 --- Optical fibers --- p.15 / Chapter 3.3 --- Third order interaction between waves of same frequency --- p.16 / Chapter 3.4 --- Third order interaction between waves of different frequencies --- p.19 / Chapter 4 --- Multiphoton ionization interference effect --- p.23 / Chapter 4.1 --- Historical development --- p.23 / Chapter 4.2 --- Multiphoton ionization interference effect --- p.26 / Chapter 4.3 --- Periodic ionization --- p.27 / Chapter 4.4 --- Periodic electric field --- p.28 / Chapter 4.5 --- Physical interpretation --- p.29 / Chapter 5 --- Experimental setup --- p.32 / Chapter 5.1 --- Introduction --- p.32 / Chapter 5.2 --- Laser system --- p.32 / Chapter 5.3 --- Optical fibers --- p.33 / Chapter 5.4 --- Coupling light into fibers --- p.34 / Chapter 5.5 --- Detection system --- p.36 / Chapter 5.6 --- Optical layout --- p.36 / Chapter 6 --- Second harmonic generation in optical fibers --- p.40 / Chapter 6.1 --- Introduction --- p.40 / Chapter 6.2 --- Self-preparation in optical fibers --- p.41 / Chapter 6.3 --- Polarization dependence --- p.42 / Chapter 6.4 --- Seeding optical fibers --- p.42 / Chapter 6.5 --- Seeding by varying green light intensity --- p.45 / Chapter 6.6 --- Square dependence of second harmonic generation in optical fibers --- p.46 / Chapter 7 --- Erasure of x(2) grating in optical fibers --- p.56 / Chapter 7.1 --- Introduction --- p.56 / Chapter 7.2 --- Experiment --- p.58 / Chapter 7.3 --- Results --- p.59 / Chapter 7.3.1 --- Erasure by different propagating mode --- p.59 / Chapter 7.3.2 --- Erasure in germanium-doped fiber --- p.60 / Chapter 7.3.3 --- Erasure in erbium-doped fiber --- p.61 / Chapter 7.4 --- Discussion --- p.61 / Chapter 8 --- Conclusion --- p.68 / Chapter 8.1 --- Summary of our work --- p.68 / Chapter 8.2 --- Outlook --- p.69 / Chapter 8.2.1 --- Multiphoton ionization in polymer --- p.69 / Chapter 8.2.2 --- Erasure by blue light --- p.69 / Bibliography --- p.70

Choy Kwok Wai = 利用全光極化在本體聚合物中產生準相匹配二次諧波 / 蔡國偉. / Thesis submitted in: August 2005. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references (leaves 132-133). / Text in English; abstracts in English and Chinese. / Choy Kwok Wai = Li yong quan guang ji hua zai ben ti ju he wu zhong chan sheng zhun xiang pi pei er ci xie bo / Cai Guowei. / Acknowledgements --- p.ii / Abstract --- p.iii / Chinese Abstract --- p.V / Table of Contents --- p.vi / List of Figures --- p.ix / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Principle and origin of nonlinear optics --- p.2 / Chapter 1.2 --- Tensor properties and Inversion symmetry --- p.3 / Chapter 1.3 --- Relationship between microscopic and macroscopic nonlinear susceptibility for a poled polymer --- p.5 / Chapter 1.4 --- Second Harmonic Generation by nonlinear polarization --- p.7 / Chapter 1.5 --- Outline of the thesis --- p.9 / Figures --- p.10 / Chapter Chapter 2 --- Poling and relaxation of nonlinear optical polymeric systems --- p.11 / Chapter 2.1 --- Guest-Host nonlinear optical polymeric systems --- p.11 / Chapter 2.2 --- Poling techniques --- p.13 / Chapter 2.2.1 --- Thermal-assisted Electric Poling --- p.13 / Chapter 2.2.2 --- Photo-assisted Electric Poling --- p.15 / Chapter 2.2.3 --- All-optical Poling --- p.17 / Chapter 2.3 --- Quasi-phase-matching in bulk polymers by All-optical Poling --- p.20 / Chapter 2.4 --- Photorefractive effect in polymers --- p.22 / Chapter 2.5 --- Relaxation study of NLOP system --- p.23 / Figures --- p.27 / Chapter Chapter 3 --- Experimental Method --- p.33 / Chapter 3.1 --- Experimental Setup --- p.33 / Chapter 3.1.1 --- Laser system --- p.33 / Chapter 3.1.2 --- Data acquisition system --- p.34 / Chapter 3.1.3 --- Reference arm --- p.34 / Chapter 3.1.4 --- Optical Path --- p.35 / Chapter 3.2 --- Sample Preparation --- p.36 / Chapter 3.2.1 --- Polymer Films Preparation --- p.36 / Chapter 3.2.2 --- Bulk Polymers Preparation --- p.38 / Chapter 3.3 --- Poling and Thermal history of samples --- p.41 / Figures --- p.43 / Chapter Chapter 4 --- Growth and decay of optical nonlinearity in all-optically poled polymers --- p.53 / Chapter 4.1 --- Poling profiles for DR-1 and DANS doped bulk polymer by All-optical poling --- p.53 / Chapter 4.1.1 --- Poling profiles for DR-1 doped bulk polymer and polymer films by All-optical poling --- p.53 / Chapter 4.1.2 --- Possibilities of degradation of the samples during the experiment --- p.56 / Chapter 4.1.3 --- Effects of probing beam intensity and the duration of probing on the growth and decay of X(2) --- p.57 / Chapter 4.1.4 --- Poling profiles for DANS doped bulk polymer by All-optical poling --- p.57 / Chapter 4.2 --- Dependence of the poling profiles on various parameters in all-optical poling --- p.59 / Chapter 4.2.1 --- Dependence on the temperature --- p.59 / Chapter 4.2.2 --- Dependence on the relative intensities between the two seeding beams --- p.60 / Chapter 4.2.3 --- Dependence on the relative phase between the two seeding beams --- p.64 / Chapter 4.2.4 --- Tensorial properties of X(2) --- p.67 / Chapter 4.2.5 --- Dependence on the thickness of the sample --- p.68 / Chapter 4.3 --- Comparison of different relaxation models --- p.69 / Figures --- p.71 / Chapter Chapter 5 --- Growth and decay of optical nonlinearity in poled polymers with charge-generating additives --- p.89 / Chapter 5.1 --- Orientational enhancement in NLOP systems --- p.89 / Chapter 5.2 --- Space-charge field by charge-transporting molecules and sensitizer --- p.89 / Chapter 5.3 --- Poling profile of the bulk DR-1 polymer with charge-generating ingredients --- p.91 / Chapter 5.4 --- Poling profile of the bulk DANS polymer with the presence of charge-transporting molecules and sensitizers --- p.100 / Figures --- p.107 / Chapter Chapter 6 --- Growth and decay of optical nonlinearity in a nano-structured composite system --- p.113 / Chapter 6.1 --- Historical background --- p.113 / Chapter 6.2 --- Preparation of the composites films --- p.116 / Chapter 6.2.1 --- Preparation of the silver nano-particles --- p.116 / Chapter 6.2.2 --- Preparation of the composite films --- p.117 / Chapter 6.3 --- Poling profiles of polymer films with silver nano-particles --- p.118 / Figures --- p.124 / Chapter Chapter 7 --- Conclusions --- p.129 / References --- p.132

Lithium niobate (LiNbO3) and Lithium tantalate (LiTaO3) are high-performance nonlinear optical materials that have been popularly used in electro-optical modulation and nonlinear optics. Second-harmonic generation (SHG) microscope is applied to get SH images, as compared to etching method. SH image method has the advantage of faster and non-invasive inspection. Our system can rapid and non-destructive mapping of the detailed features of the crystal fibers in three dimensions. By simulation, we can explain that in the interface of +Z and ¡VZ domain, because the Li ions can stay in positive and negative two stable energy valleys. It may induce stronger SH intensity by QPM than +Z and -Z domain.

Optical second harmonic generation (SHG) is a nonlinear process that converts two photons of frequency w to a single photon of frequency 2w. In the electricdipole approximation, this process requires a noncentrosymmetric medium. For thin films present at the interface between two centrosymmetric media¡Ðsuch as the interface between two liquids, the interface between a liquid and air, or for the surface of a variety of insulators and semiconductors¡Ðonly the molecules which participate in the asymmetry of the interface contribute to the SHG. Surface SGH from molecules which exhibit a large nonlinear optical response at a specific wavelength can be used to measure the relative surface coverage of molecules at the interface as a function of the external electrochemical parameters. The polarization dependence of the surface SHG can be used to estimate the average molecular orientation of chromophores at the liquid/liquid electrochemical interface. The potential dependence of the SHG from the interface provides important information on the position of the adsorbed chromophores with respect to the interface.

Silicon nanocrystals (NCs), especially Si NCs embedded in SiO₂, have been studied intensely for decades for their potential application in silicon photonics, especially as efficient room temperature light emitters. Despite progress in fabricating photonic devices from Si NCs, the origin of the efficient photoluminescence (PL), the electronic and microscopic structure of the nanocrystals, and the structure of the elusive NC/SiO₂ interfaces for the oxide-embedded nanocrystals, remain controversial. Optical spectroscopy provides a powerful noninvasive tool for probing the structure of the Si NCs, including the active buried NC/SiO₂ interfaces of embedded particles. In this thesis work, oxide-embedded and free-standing alkyl-passivated silicon nanocrystals, prepared by different techniques, have been studied by linear and nonlinear optical spectroscopies. Cross-polarized 2-beam second-harmonic and sum-frequency generation (XP2-SHG/SFG) has been applied spectroscopically to study oxide embedded Si NCs of different sizes (3 to 5 nm diameter) and interface chemistries. The SHG/SFG spectra of silicon nanocrystals (Si NCs) prepared by implanting Si ions uniformly into silica substrates, then annealing, are compared and contrasted to their spectroscopic ellipsometric (SE) and photoluminescence excitation (PLE) spectra. Three resonances--two close in energy to E₁ (3.4 eV) and E2 (4.27 eV) critical-point resonances of crystalline silicon (c-Si), and a broad resonance intermediate in energy between E₁ and E₂--are observed in all three types of spectra. These features are observed in conjunction with a sharp 520 cm⁻¹ Raman peak characteristic of c-Si and an a-Si tail in the Raman spectra. The appearance of bulk-like CP resonances in the parallel PLE, SE and SHG/SFG spectra from Si NCs suggests the basic electronic structure of the bulk c-Si is preserved in nano-particles as small as 3 nm in diameter, albeit with significant size-dependent modification. At the same time, the prominence of a non-bulk-like resonance intermediate in energy between E₁ and E₂ CPs in all three types of spectra demonstrates the important contribution of nano-interfaces to the electronic structure.We also applied Raman spectroscopy to study oxide-embedded and oxide-free alkyl-passivated Si NCs with diameters ranging from 3 nm to greater than 10 nm synthesized by thermal decomposition of hydrogen silsesquioxane (HSQ). While oxide matrix complicates the size-dependence of the Raman peak shift for oxide-embedded nanocrystals, the Raman peak of the free-standing alkyl-passivated Si NCs shifts monotonically with NC size. / text