Browsing by Subject "nonlinear optics"
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Item Investigations of amino acid-based surfactants at liquid interfaces(Texas A&M University, 2005-11-01) Yang, DengliangHerein are presented collective studies of amino acid-based surfactants, also known as lipoamino acids, at liquid interfaces. Chapter III describes an investigation of domain morphology of N-Stearoylglutamic acid (N-SGA) Langmuir monolayers at the air/water interface by epifluorescence microscopy. Anisotropic feather-like domains were observed in L-enantiomeric monolayers while symmetric circular domains were found in racemic N-SGA monolayers. At a surface pressure of 30 mN/m the enantiomeric domains melted at 31 ??C while the racemic domains melted at 27 ??C. This result is exactly opposite to the behavior found in bulk crystals where the racemate melts at a higher temperature. These results were explained in terms of different molecular packing and hydrogen bonding between bulk crystals and two-dimensional thin films for enantiomeric and racemic compounds. Chapter IV summarizes the investigations of hydrogen bonding in N-acyl amino acid monolayers by vibrational sum-frequency spectroscopy (VSFS). The intermolecular hydrogen bonding interaction between the adjacent molecules through amide-amide groups in N-stearoylalanine (N-SA) is characterized by an NH stretch peak at 3311 cm-1. This is the first time that the amide NH stretching signals have been detected with the VSFS technique. A similar peak was detected at 3341 cm-1on N-SGA monolayer. The higher frequency indicates that the H-bond strength is weaker due to the larger size of the glutamic acid residue. The NH stretch mode can thus be used as a fingerprint of hydrogen bonding among amide-amide groups. A peak at 3050 cm-1 due to hydrogen bonding among carboxyl groups was also resolved from the VSFS spectra. Molecular models of intermolecular hydrogen bonding were proposed.Item Linear and Nonlinear Optics in a System of Massless Dirac Fermions(2014-08-10) Yao, XianghanGraphene electrons possess linear energy dispersion relation, and thus behave as two-dimensional (2D) Dirac fermions. Consequently, compared with the conventional 2D electron gas systems (2DEG) found in MOSFETs and quantum wells, graphene exhibits a variety of electronic and optoelectronic properties that are characteristic of 2D Dirac fermions. Similar 2D Dirac fermions are found at the surface layer of 3D topological insulator, and they are topological protected from backscattering due to spin-orbital coupling and time reversal symmetry. We here calculate the linear and nonlinear optical response of graphene in strong magnetic and optical fields, using a quantum-mechanical density-matrix formalism. We show that graphene in a magnetic field possesses a giant mid- or far-infrared optical nonlinearity, perhaps the highest among known materials. The high nonlinearity originates from the unique electronic properties and selection rules near the Dirac point. As a result, even one monolayer of graphene gives rise to an appreciable nonlinear frequency conversion efficiency for incident infrared radiation. Inspired by the highly efficient four-wave mixing process in the 2D Dirac fermion systems, we further propose a new mechanism of generating polarization-entangled photons based on the parametric generation process in the third section of this dissertation. Unique properties of quantized electron states in a magnetized graphene and optical selection rules near the Dirac point give rise to a giant optical nonlinearity and a high rate of photon production in the mid- or far-infrared range. A similar mechanism of photon entanglement may exist in topological insulators where the surface states have a Dirac-cone dispersion and demonstrate similar properties of magneto-optical absorption. In the absence of a magnetic field, the surface plasmon resonance provides an alternative method to enhance nonlinear frequency conversion efficiency. In the forth section of this dissertation, a graphene-based difference frequency generation (DFG) of terahertz plasmons is proposed as an example to study nonlinear photonplasmon interaction in 2D Dirac fermion systems. Our results demonstrate strong enhancement of the DFG efficiency near the plasmon resonance and the feasibility of phase-matched nonlinear generation of plasmons over a broad range of frequencies. Considering graphene plasmonics' superiorities in wave confinement, dissipation and tunability, a graphene-based nonlinear terahertz plasmon generation process promises applications in terahertz sources and sensors, as well as integrated photonic circuits.Item Nonlinear magneto-optic effects in optically dense Rb vapor(Texas A&M University, 2004-09-30) Novikova, Irina BorisovnaNonlinear magneto-optical effects, originated from atomic coherence, are studied both theoretically and experimentally in thermal Rb vapor. The analytical description of the fundamental properties of coherent media are based on the simplified three- and four-level systems, and then verified using numerical simulations and experimental measurements. In particular, we analyze the modification of the long-lived atomic coherence due to various physical effects, such as reabsorption of spontaneous radiation, collisions with a buffer gas atoms, etc. We also discuss the importance of the high-order nonlinearities in the description of the polarization rotation for the elliptically polarized light. The effect of self-rotation of the elliptical polarization is also analyzed. Practical applications of nonlinear magneto-optical effects are considered in precision metrology and magnetometery, and for the generation of non-classical states of electromagnetic field.Item Novel Nonlinear Optics and Quantum Optics Approaches for Ultrasound-Modulated Optical Tomography in Soft Biological Tissue(2012-02-14) Zhang, HuiliangOptical imaging of soft biological tissue is highly desirable since it is nonionizing and provides sensitive contrast information which enables the detection of physiological functions and abnormalities, including potentially early cancer detection. However, due to the diffusive nature of light in soft biological tissue, it is difficult to achieve simultaneously good spatial resolution and good imaging depth with pure optical imaging modalities. This work focuses on the ultrasound-modulated optical tomography (UOT): a hybrid technique which combines the advantages of ultrasonic resolution and optical contrast. In this technique, focused ultrasound and optical radiation of high temporal coherence are simultaneously applied to soft biological tissue. The intensity of the sideband, or ultrasound ?tagged? photons depends on the optical absorption in the region of interest where the ultrasound is focused. Demodulation of the optical speckle pattern yields the intensity of tagged photons for each location of the ultrasonic focal spot. Thus UOT yields an image with spatial resolution of the focused ultrasound ? typically submillimeter ? whose contrast is related to local optical absorption and the diffusive properties of light in the organ. Thus it extends all the advantages of optical imaging deep into highly scattering tissue. However lack of efficient tagged light detection techniques has so far prevented ultrasound-modulated optical tomography from achieving maturity. The signal-to-noise ratio (SNR) and imaging speed are two of the most important figures of merit and need further improvement for UOT to become widely applicable. In the first part of this work, nonlinear optics detection methods have been implemented to demodulate the ?tagged? photons. The most common of these is photorefractive (PR) two wave mixing (TWM) interferometry, which is a time-domain filtering technique. When used for UOT, it is found that this approach extracts not only optical properties but also mechanical properties for the area of interest. To improve on TWM, PR four wave mixing (FWM) experiments were performed to read out only the modulated light and at the same time strongly suppressing the ?untagged? light. Spectral-hole burning (SHB) in a rare-earth-ion-doped crystal has been developed for UOT more recently. Experiments in Tm3 :Y3Al5O12 (Tm:YAG) show the outstanding features of SHB: large angle acceptance (etendue), light speckle processing in parallel (insensitive to the diffusive light nature) and real-time signal collection (immune to light speckle decorrelation). With the help of advanced laser stabilization techniques, two orders of magnitude improvement of SNR have been achieved in a persistent SHB material (Pr^3 :Y2SiO5) compared to Tm:YAG. Also slow light with PSHB further reduces noise in Pr:YSO UOT that is caused by polarization leakage by performing time-domain filtering.