Superfocusing Performances Of Planar Lenses Based On Metallic Nanowaveguides

As the key part for optical focusing and imaging,lenses and optical systems have been widely used in many technologies such as microscopic imaging,deep-space exploration and micro-nano manufacturing.And they are also the core devices in many fields,such as the national major special project for earth observation.However,traditional refractive lenses are not suitable for advanced imaging systems with miniaturization,lightweight,integration and high resolution,as a result of their defects such as large size,heavy weight,difficulty in surface processing and the diffraction limit.The use of micro-/nanostructures to modulate light has become an international cutting-edge technology.Ultra-thin planar lenses composed of micro-/nanostructures provide a new scheme to solve the above problems.In this dissertation,new principles and methods for realizing superfocusing are explored with planar lenses formed by metallic nanoslits,aiming to solve the existing technological drawbacks of the reported superfocusing metallic lenses such as near-field operation,no working distance,and small depth of focus.The main research work and results are as follows:(1)The quasi-far-field superfocusing with precisely optical manipulation is proposed.The coupling mechanism of optical field in adjacent nanoslits and phase delay characteristics of coupled nanoslits are uncovered.An optimization design method for a metallic planar TM-lens formed by coupled nanoslits is achieved.Numerical simulation results demonstrate that the realized focal length of an optimally designed TM-lens agrees excellently with the design value,thus solving the problem of focal shift in the previous research.And a superfocusing focal point is also obtained in the quasi-far field by the TM-lens,which overcomes the shortcomings of the previously reported superlenses such as no working distance and small depth of focus.Meanwhile,the factors influencing the superfocusing performances of such TM-lens are explored.Moreover,the focusing performances of a TM-lens demonstrate good robustness.(2)The quasi-far-field superfocusing over a broad wavelength range is proposed.The extraordinary transmission characteristics of a TE polarized light in a metallic nanoslit are analyzed.The phase delay of a nanoslit increases with the increase of the nanoslit width.And the width of a nanoslit modulating the phase delay is much larger than that of TM lenses,which avoids the difficulty in the preparation of TM lenses.Metallic planar lenses with TE-polarized design formed by nanoslits are constructed.Numerical simulation results show that the TE-polarized design approach for metallic nanoslit lenses can be employed to focus light covering a broad wavelength range from visible to ultraviolet,solving the problem of extremely low transmittance of TM-lenses in near ultraviolet.Finally,an oil-immersed TE-lens formed by nanoslits for focusing light at 405 nm with designed focal length of 1.5 ?m is fabricated using focused-ion-beam milling.The measurements on the fabricated lens show excellent agreement with numerical simulations of the design,which testifies the design and numerical calculations of the TE-lenses.(3)A broadband deep subwavelength optical focusing based on coupled nanoslits is proposed.The equation for a metallic graded-index(MGRIN)lens design is constructed.The physical mechanism of a MGRIN lens focusing is uncovered.Furthermore,the design procedure of a MGRIN lens is accomplished and a MGRIN lens formed by coupled nanoslits is ultimately designed.Numerical simulation results indicate that deep subwavelength focusing with focal size of 8 nm(?/125)is achieved by the MGRIN lens designed at the operating wavelength of ?=1 ?m.Moreover,the nanofocusing capability of the MGRIN lens without redesigning the structure can be well kept when the incident wavelength changes over a broad range from 0.65 ?m to 4 ?m.Particularly,for the incident wavelength of ?=3 ?m,the full width at half maximum(FWHM)of the focal point is ?/375,much smaller than that of?/100 achieved in previous research.Meanwhile,when the incident wavelength increases from 0.65 ?m to 4 ?m,the focal length of the MGRIN lens increases correspondingly and the focal depth also increases with larger than one incident wavelength.The intensity of focus is the highest at the design wavelength of ?=1 ?m,while it gradually decreases as the incident wavelength is away from the design one.