Person: Tang, Yiqiao
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Tang
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Yiqiao
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Tang, Yiqiao
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Publication Chiroptical Hot Spots in Twisted Nanowire Plasmonic Oscillators(AIP Publishing, 2013) Tang, Yiqiao; Sun, Li; Cohen, AdamWe image the chiroptical response of crossed nanowire junctions as a function of the vertical offset between the nanowires and the incident wavelength. These samples show chiroptical “hot spots” with strongly localized circular differential scattering at the nanowire crossing points. These chiroptical hot spots are not apparent in the spatially averaged spectra. The sign and magnitude of the chiroptical enhancement show a complex dependence on sample geometry, which we reproduce with a fully retarded analytical scattering model. These results suggest strategies for engineering devices with enhanced chiral light-matter interactions.Publication Chirality of Light and Its Interaction with Chiral Matter(2013-03-18) Tang, Yiqiao; Cohen, Adam Ezra; Walsworth, Ronald; Nelson, DavidThis thesis conducts a systematic study on the chirality of light and its interaction with chiral matter. In the theory section, we introduce a measure of local density of chirality, applying to arbitrary electromagnetic fields. This optical chirality suggests the existence of superchiral modes, which are more selective than circularly polarized light (CPL) in preferentially exciting single enantiomers in certain regions of space. Experimentally, we demonstrate an 11-fold enhancement over CPL in discriminating chiral fluorophores of single handedness in a precisely sculpted superchiral field. This result agrees to within 15% with theoretical predictions. Any chiral configuration of point charges is beyond the scope of our theory on optical chirality. To address chiroptical excitations at nanoscale, we develop a model of twisted dipolar oscillators. We design a simple tunable chiral nanostructure and observe localized chiroptical “hot spots” with dramatically enhanced circular differential scattering. Our work on superchiral light and 3D chiral metamaterials establishes optical chirality as a fundamental and tunable property of light, with implications ranging from plasmonic sensors, absolute asymmetric synthesis to new strategies for fabricating three-dimensional chiral metamaterials. This thesis is organized as such: Chapter 1 provides a background on previous studies of chiroptical phenomena, and recent efforts in preparing chiral metamaterials. Chapter 2 derives theory on optical chirality, superchiral modes and coupled-dipolar oscillators at nanoscale. Chapter 3 introduces material, apparatus, and pitfalls in chiroptical experiments. Chapter 4 is an overview of the experimental procedure and results on generating and observing superchiral enhancement. Chapter 5 describes the experiments on using spectroscopic polarization microscopy to study chiral 3D chiral metamaterials. Finally in Chapter 6, I discuss quantization of optical chirality and perspectives on future directions.Publication Optical Chirality and Its Interaction with Matter(American Physical Society, 2010) Tang, Yiqiao; Cohen, AdamWe introduce a measure of the local density of chirality of the electromagnetic field. This optical chirality determines the asymmetry in the rates of excitation between a small chiral molecule and its mirror image, and applies to molecules in electromagnetic fields with arbitrary spatial dependence. A continuity equation for optical chirality in the presence of material currents describes the flow of chirality, in a manner analogous to the Poynting theorem for electromagnetic energy. “Superchiral” solutions to Maxwell’s equations show larger chiral asymmetry, in some regions of space, than is found in circularly polarized plane waves.Publication Limits on Fluorescence Detected Circular Dichroism of Single Helicene Molecules(American Chemical Society, 2009) Tang, Yiqiao; Cook, Timothy A.; Cohen, AdamFluorescent imaging of single helicene molecules is applied to study the optical activity of chiral fluorophores. In contrast to the previous report by Hassey et al. (Science 2006, 314, 1437), the dissymmetry factors of single chiral fluorophores are found not to differ significantly from the bulk value of |g| < 10^{−4} at 457 nm. Linear dichroism and birefringence of the dichroic mirror inside the fluorescence microscope change the polarization state of the incoming laser beam significantly; i.e., circular polarized light sent into the microscope becomes highly elliptically polarized after reflection from the dichroic mirror. Compensation for this effect should be made to avoid artifacts brought by linear dichroism in single immobilized molecules.Publication Spectroscopy in Sculpted Fields(Elsevier, 2009) Yang, Nan; Tang, Yiqiao; Cohen, AdamMetallic and magnetic nanostructures set electromagnetic boundary conditions which can lead to highly contorted fields in their immediate vicinity. While much attention has been devoted to enhancements in electric field strength, we argue that equally interesting phenomena arise from enhancements in magnetic and electric field gradients. Nonuniform fields near nanostructures can induce molecular transitions that are forbidden by electric dipole selection rules. We illustrate this claim with two examples. ‘‘Superhelical’’ electromagnetic fields are predicted to show enhanced asymmetry in their interaction with chiral molecules, far greater than that due to circularly polarized light. Such fields could be used to induce chiral photochemistry with large enantiomeric excess. Steeply varying DC magnetic fields are predicted to enhance the rate of intersystem crossing in molecular bi-radicals. Such fields could provide a route to new nanomagnetic catalysts and to magnetic control of chemical reactions.