Coupling of light from an optical fiber taper into silver nanowires

We report the coupling of photons from an optical fiber taper to surface plasmon modes of silver nanowires. The launch of propagating plasmons can be realized not only at ends of the nanowires, but also at the midsection. The degree of the coupling can be controlled by adjusting the light polarization. In addition, we present the coupling of light into multiple nanowires from a single optical fiber taper simultaneously. Our demonstration offers a novel method for optimizing plasmon coupling into nanoscale metallic waveguides and promotes the realization of highly integrated plasmonic devices.Comment: 5 pages, 4 figure

Reconfigurable Photonic Crystal Cavities

Photonic crystals are optical structures that contain a periodic modulation of their refractive index, allowing them to control light in recent years of an unprecedented capacity. Photonic crystals may take on a variety of configurations, in particular the photonic crystal cavity, which may “hold” light in small volumes comparable to the light’s wavelength. This capability to spatially confine light opens up countless possibilities to explore for research in telecommunications, quantum electrodynamics experiments and high-resolution sensor applications. However, the vast functionality potentially made available by photonic crystal cavities is limited due to the difficulty in redefining photonic crystal components once they are formed in their (typically) solid material. The work presented in this thesis investigates several approaches to overcome this issue by reconfiguring photonic crystal cavities

Reconfigurable Photonic Crystal Cavities

Photonic crystals are optical structures that contain a periodic modulation of their refractive index, allowing them to control light in recent years of an unprecedented capacity. Photonic crystals may take on a variety of configurations, in particular the photonic crystal cavity, which may “hold” light in small volumes comparable to the light’s wavelength. This capability to spatially confine light opens up countless possibilities to explore for research in telecommunications, quantum electrodynamics experiments and high-resolution sensor applications. However, the vast functionality potentially made available by photonic crystal cavities is limited due to the difficulty in redefining photonic crystal components once they are formed in their (typically) solid material. The work presented in this thesis investigates several approaches to overcome this issue by reconfiguring photonic crystal cavities

Controlled Lasing in Gallium Nitride Nanowires

There is considerable interest in ultra-small coherent light sources. A strong candidate is a semiconductor-nanowire laser, where a single, monolithic nanowire functions simultaneously as an optical microcavity and active medium, leading to an extremely compact and robust laser. Recent advances in nanowire synthesis have enabled realization of optically pumped nanowire lasers in different material systems, including III-V, III-nitride, and II-VI semiconductors. However, due to the limited lasing control techniques, most of the nanowire lasers operate in naturally-occurring multi-mode and randomly polarized states. Lasing control in nanowire lasers is strongly desired for many practical applications. For instance, specifically polarized lasing is desired for atom trapping and biological detection, and single-mode lasing is crucial for applications needing high beam quality and spectral purity such as nanolithography and on-chip communications. Motivated by these practical requirements, this dissertation concentrates on the study of fundamental lasing characteristics and their control in gallium nitride (GaN) nanowire lasers. GaN nanowire lasers typically operate in a combined multi-longitudinal and multi-transverse mode state. Two schemes are introduced here for controlling the optical mode and achieving single-mode operation of the nanowire lasers. The first method involves placing two nanowires side-by-side in contact to form a coupled cavity. The coupled cavity can generate a Vernier effect, which is able to suppress both multi-longitudinal and multi-transverse mode operation, giving rise to the single-mode lasing in these nanowire lasers. For the second method, single-mode lasing is achieved by placing individual GaN nanowires onto gold substrates. The nanowire-gold contact generates a mode-dependent loss, which can strongly attenuate high-order guiding modes and ensure single-mode operation. Additionally, polarization properties of the gallium nitride nanowire lasers are studied experimentally by direct analysis of light emission from the nanowire end-facets. Linearly and elliptically polarized emissions are both obtained from a single nanowire at different pump strength, and a clear switching of the polarization states is observed with the change of optical excitation. This polarization change is attributed to a switching of transverse modes due to their difference in cavity losses. Finally, lasing polarization control is allowed by the coupling of the GaN nanowire lasers to an underlying gold substrate. The gold substrate breaks the symmetry of the nanowire geometry and generates an inherent polarization-sensitive loss. These effects allow us to demonstrate linearly polarized emission of GaN nanowire lasers, with a large extinction ratio and a fixed polarization orientation parallel to the substrate surface

Engineering the Spectrum of Near-Field Thermal Radiation

Thermal emission observed at sub-wavelength distances from the thermal source is referred to as near-field thermal radiation. Thermal radiation in the near-field regime can exceed Planck’s blackbody limit by orders of magnitude and be quasi-monochromatic. Due to these unique properties, near-field thermal radiation is very promising for several thermal management and energy harvesting applications. Many of these applications, such as nanogap thermophotovoltaics and thermal rectification, require near-field spectra that are not found among natural materials. Artificial metamaterials, which are engineered at the sub-wavelength scale, have been theoretically proposed for tuning the spectrum of near-field thermal radiation. However, engineering the near-field spectra using metamaterials has not been experimentally demonstrated mostly due to the complexities associated with guiding the near-field evanescent waves to an FTIR spectrometer located in the far zone. Additionally, the possibility of tuning the near-field spectra by engineering materials at length scales much smaller than the thermal wavelength, i.e., atomic length scales, has not been explored theoretically or experimentally. In this dissertation, a new technique is proposed and implemented for measuring the near-field thermal spectra. The proposed technique is verified against the theoretical predictions of near-field thermal radiation from natural materials. This technique is then utilized for measuring the near-field spectra thermally emitted by metamaterials made of silicon carbide nanopillars, and the tunability of the near-field thermal spectra by changing the dimensions of the nanopillars at the sub-wavelength scale is demonstrated. Using numerically-exact simulations, it is shown that the effective medium theory, commonly used for theoretical study of the near-field thermal spectra of nanopillar metamaterials, is not valid in the near-field regime. Additionally, the tunability of near-field spectra by using spherically-shaped sub-wavelength particles is theoretically investigated by developing analytical expressions for predicting the energy density emitted by spherical particles. Lastly, near-field thermal radiation from quantum dots, which have a length scale comparable to the atomic scales, is theoretically studied for the first time. It is shown that the near-field thermal radiation is highly impacted by the size-dependent quantum confinement effect that arises at the atomic length scales, thus providing a new mechanism for tuning the near-field thermal emission spectra

Plasmonic Sensors beyond the Phase Matching Condition: A Simplified Approach

The conventional approach to optimising plasmonic sensors is typically based entirely on ensuring phase matching between the excitation wave and the surface plasmon supported by the metallic structure. However, this leads to suboptimal performance, even in the simplest sensor configuration based on the Otto geometry. We present a simplified coupled mode theory approach for evaluating and optimizing the sensing properties of plasmonic waveguide refractive index sensors. It only requires the calculation of propagation constants, without the need for calculating mode overlap integrals. We apply our method by evaluating the wavelength-, device length- and refractive index-dependent transmission spectra for an example silicon-on-insulator-based sensor of finite length. This reveals all salient spectral features which are consistent with full-field finite element calculations. This work provides a rapid and convenient framework for designing dielectric-plasmonic sensor prototypes-its applicability to the case of fibre plasmonic sensors is also discussed

Optical Nanofibers: a new platform for quantum optics

The development of optical nanofibers (ONF) and the study and control of their optical properties when coupling atoms to their electromagnetic modes has opened new possibilities for their use in quantum optics and quantum information science. These ONFs offer tight optical mode confinement (less than the wavelength of light) and diffraction-free propagation. The small cross section of the transverse field allows probing of linear and non-linear spectroscopic features of atoms with exquisitely low power. The cooperativity -- the figure of merit in many quantum optics and quantum information systems -- tends to be large even for a single atom in the mode of an ONF, as it is proportional to the ratio of the atomic cross section to the electromagnetic mode cross section. ONFs offer a natural bus for information and for inter-atomic coupling through the tightly-confined modes, which opens the possibility of one-dimensional many-body physics and interesting quantum interconnection applications. The presence of the ONF modifies the vacuum field, affecting the spontaneous emission rates of atoms in its vicinity. The high gradients in the radial intensity naturally provide the potential for trapping atoms around the ONF, allowing the creation of one-dimensional arrays of atoms. The same radial gradient in the transverse direction of the field is responsible for the existence of a large longitudinal component that introduces the possibility of spin-orbit coupling of the light and the atom, enabling the exploration of chiral quantum optics.Comment: 65 pages, to appear in Advances in Atomic, Molecular and Optical Physic

Probing extraordinary nanoscale energy transfer using bimaterial microcantilevers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references.Nanostructured materials have recently drawn a great deal of attention in the field of energy research such as for solar photovoltaic, thermophotovoltaic and thermoelectric applications. The energy transport properties of nanostructures can differ greatly from their bulk counterparts because the characteristic dimensions of nanostructures are often comparable with the wavelength or the mean free path of energy carriers such as photons, phonons and electrons. Due to the small dimensions, probing energy transfer at the nanoscale is extremely challenging. By developing new experimental techniques based on the bi-material microcantilevers used in Atomic Force Microscopes (AFM), this thesis has studied several extraordinary energy transfer phenomena at the nanoscale including near-field radiation beyond Planck's law, high thermal conductivity polymers and the optical absorption of micro/nanostructures. First, surface phonon polaritons, which is one type of electromagnetic surface waves, are demonstrated to enhance the thermal radiation between two surfaces at small gaps by measuring radiation heat transfer between a microsphere and a flat surface down to a 30 nm separation. The corresponding heat transfer coefficients at nanoscale gaps are three orders of magnitude larger than that of the Planck's blackbody radiation limit. This work will have practical impacts in areas such as thermophotovoltaic energy conversion, radiative cooling, and magnetic data recording. Next, a new technique is developed to fabricate ultra-drawn polyethylene nanofibers. We demonstrated that these ultradrawn nanofibers can have a thermal conductivity as high as ~ 100 W/m.K, which is about a 3 orders of magnitude enhancement compared to that of bulk polymers. Such high thermal conductivity polymers can potentially provide a cheaper alternative to conventional metal-based heat transfer materials. Finally, an experimental setup is presented to directly measure the spectral absorption of individual micro/nanostructures in applications to solar photovoltaics. Further refinement on experimental technique and characterization using the platform will guide the optimization of dimension, shape, and materials selections of nanostructures in order to maximize the efficiencies of solar cells.by Sheng Shen.Ph.D