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Tailoring Optical Vector Fields with Optical Antenna

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Tutor: MingHai
School: University of Science and Technology of China
Course: Optics
Keywords: surface plasmon polariton,cylindrical vector beam,optical antenna,Bessel beam,ne
CLC: O431.2
Type: PhD thesis
Year:  2013
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Abstract:
In the field of integrated optics and nanophotonics, due to the diffraction limit, the high conversion efficiency between propagating field and localized field is an important and fundamental physical problem. Based on the theory of microwave antenna, optical antenna is designed to efficiently convert free-propagating optical radiation to localized energy, and vice versa. Subwavelength metallic structures become increasingly accessible with the rapid development of modern nanofabrication techniques, assisting the rapid expansion of the research in optical antenna. Optics antenna is regarded as one of the major trend in the modern optics development, and it has great potential applications in supper resolution, high resolution spectrum technology, high efficiency solar cell and photolithography. The aim of the dissertation is studying the design and application of optical antenna, hi this dissertation, we mainly study the applications of disc-shaped, bull eye and spiral optical antennas in the field of near-field optics in theory and experiment. Through the interaction between optical vector field and optical antenna with different structure, the properties of optical field can be engineered according to specific applications.The main scientific problems solved and academic achievements achieved in this dissertation have been summarized in the list below:Problem1:Recently, a nanoaperture surrounded by five concentric circular corrugations has been demonstrated for significant enhancement of the fluorescence count rates per molecule and high emission directivity. Besides, the nanoscale spin photon source have high relevance for the application in quantum optical information processing and integrated photonic circuits. However, the modulation of angular momentum carried by the radiated field remains unsolved.Academic idea and solution:Through controlling the optical resonance in the vicinity of emitters, the properties of emitted photons from the emitters can be engineered. The conservation of spin and angular momentum are independent. However, when the incident spin photons encounter a metallic structure with anisotropic inhomogeneous boundaries, a spin-dependent behavior of plasmonic field could be observed. This phenomenon is due to spin-orbit interaction that is manifested by a geometric Berry’s phase. We analytically, numerically and experimentally study the emission properties of a nanoscale emitter coupled to a plasmonic spiral structure. The spiral antenna would couple the emission into circularly polarized unidirectional emission in the far field, the spin carried by the emitted photons is determined by the handedness of the spiral antenna. Increasing number of turns of the spiral leads to narrower angular width of the emission pattern in the far field. By increasing the spiral pitch in the units of surface plasmon wavelength, these circularly polarized photons also gain orbital angular momentum with different topological charges. In the experiment, quantum dots are adopted as the nano-emitters. For a five-turn Archimedes’spiral antenna, field intensity increase up to70-fold simultaneously with antenna directivity of11.7dB has been measured in the experiment, and a circular polarization extinction ratio of10is obtainable. In addition, if the nanoscale emitter is displaced from the geometrical center of the spiral antenna, the emission peak will shift from the normal direction accordingly. The steering angle depends on the displacement of the nanoscale emitter. This steering phenomenon has been confirmed experimentally. For a3-turn Archimedes’spiral antenna, experimental results reveal that steering angles of3°and7°are obtainable when the excited quantum dots are moved horizontally from the center with a displacement of200nm and500nm, respectively.Problem2:Nanoscale lithography with complex patterns can be fabricated through point by-point scanning in principle. Efficient plasmonic excitation and focusing can be achieved through matching the axially symmetric dielectric/metal plasmonic lens structure to the polarization symmetry of radially polarized illumination. However, the singularity center of the radially polarized beam needs to be aligned to the center of the plasmonic lens structure. This necessitates a scanning mechanism that leads to slow writing speed and limit the realistic size of lithography area.Academic idea and solution:The plasmonic field distribution in the center relies on the handedness of incident circular polarization. It has been shown that RHC beam will be focused into a solid spot by a LHS structure, while the field distribution of LHC beam focused by a LHS structure is a doughnut with a dark center.We study a high efficiency plasmonic near-field probe that integrates a spiral plasmonic lens and a sharp conical tip under circular polarized illumination. To achieve high field enhancement, two layers of spiral plasmonic lens and a composite tip design are adopted. The plasmonic probe exhibits optical spin dependence due to the use of spiral plasmonic lens. Under633nm wavelength excitation, an electric field enhancement factor of366and circular polarization extinction ratio of81can be achieved. Such a spin dependence enables the hot spot at the tip apex to be switched on and off by modulating the polarization handedness. The probe can be made in an array format that is suitable for large area parallel near-field optics applications such as lithography and microscopy.Problem3:There are many axisymmetric optical devices in the optical system, such as bull-eye structure and near-field scanning optical microscope probe. Surface plasmon polaritons only could be excited in the direction of polarization when linearly polarized beam is used as excitation, which limit the intensity of localized field at the focus and the optical resolution.Academic idea and solution:The local electrical field of radially polarized beam is linearly polarized along the radial directions. For the plasmonic structure with rotational symmetry, surface plasmons excited by the radial polarization at all azimuthal directions interfere constructively. Therefore, a tightly focused plasmonic field with strong field enhancement is obtained at the focus. According to this feature, we improve the performance of extraordinary optical transmission structure and near-field probe structure.We numerically study the extraordinary optical transmission of a plasmonic structure that combines a circular nanoantenna and a vertical annular nanoslit etched into a gold film under radially polarized illumination. The nanoantenna collects the incident field and localized it in a horizontal Fabry-Perot cavity over the gold film. Due to the symmetry matching between the structure and the illumination polarization, surface plasmons can be excited effectively and enhanced the transmission. Through optimizing the structure parameters, the transmission efficiency can be greatly enhanced by251times. This axisymmetric extraordinary optical transmission setup may be fabricated on the facet of an optical fiber for optical sensing applications.We numerically study a plasmonic near-field probe design that integrates a sharp metallic conical tip at the center of a multiple concentric rings plasmonic lens under radially polarized illumination. Due to the symmetry match between the plasmonic structure and the illumination polarization, surface plasmon waves can be efficiently excited and focused by the annular rings structure towards the conical tip at the center. The metallic tip further localizes and enhances the plasmonic field at the tip apex. With a5μm tip height and5nm tip radius, spatial resolution with the full-width-at-half-maximum of5.97nm and electric energy enhancement of7.29×104can be achieved with632.8nm optical excitation. The enhancement factor of this probe design does not strongly depend on the tip cone angle and the excitation wavelength. The strong local field enhancement at the end of the tip and its less stringent fabrication requirements make this probe design very attractive for a broad range of application in near-field optical imaging.Problem4:Evanescent Bessel beam has great potential applications in non-linear optics, atom optics and optical trapping. J1evanescent Bessel beam could be generated by highly focus the radially polarized beam onto the surface of silver film. However, in the application of optical trapping, various orders of evanescent Bessel beam are supposed to be generated which apply to optical trapping of different kinds of particles.Academic idea and solution:Photonic crystal has controllable dispersion relation and transmission properties. By choosing the appropriate parameters, only optical wave with specific wavelength, incident angle and polarization could transmit inside the photonic crystal.We propose simple setups for generating evanescent Bessel beams using one-dimensional photonic crystal, which can be a defect mode photonic crystal or a normal one. The angular selectivity provided by the multilayer structure mimics the role of an axicon for Bessel beam generation. When an azimuthally polarized beam is strongly focused onto the last interface of the1D photonic crystal, an evanescent Bessel beam of the first order is produced, while an evanescent Bessel beam of the zeroth order will be created under a radially polarized beam illumination. Switching between a donut shape and a solid focal distribution can be easily realized by controlling the polarization of the illumination.Highlights of the dissertation:1. We propose an Archimedes’spiral transmitting antenna which shows the capability of holistic controlling of photons radiated from nano-emitters, through coupling the emitters to a miniature plasmonic spiral antenna. This technique enables the engineering of photon emission in terms of the intensity, directivity, direction, polarization and angular momentum. The theoretical predictions have been experimentally confirmed. For a five-turn Archimedes’ spiral antenna, field intensity increase up to70-fold simultaneously with antenna directivity of11.7dB has been measured in the experiment, and a circular polarization extinction ratio of10is obtainable.2. Based on the unique focusing properties of Archimedes’ spiral antenna for a circularly polarized illumination, we propose a high efficiency plasmonic near-field probe that integrates a spiral plasmonic lens and a sharp conical tip under circular polarized illumination. To achieve high field enhancement, two layers of spiral plasmonic lens and a composite tip design are adopted. It can be used in photolithography with an array format. Under633nm wavelength excitation, an electric field enhancement factor of366and circular polarization extinction ratio of81can be achieved. By modulating the handedness of the incident photon, the hot spot at the tip apex can be switched on and off.3. Based on the unique focusing properties of radially and azimuthally polarized beam and the spatial filter fuction of photonic crystal, we propose the setups for generating evanescent J0and J1type Bessel beam by the use of one-dimensional photonic crystal. In the application of optical trapping of golden particle, the gradient forces provided by the generated zeroth-order evanescent Bessel beam are much larger compared to the cases using different methods, which leads to a stable three dimensional optical trapping.
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