Reflective photonic nanojets generated from cylindrical concave micro-mirrors
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Reflective photonic nanojets generated from cylindrical concave micro‑mirrors Wen Yang1 · Rong Gao1 · Yimin Wang1 · Song Zhou1 · Juchen Zhang1 Received: 12 June 2020 / Accepted: 13 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Reflective photonic nanojets (r-PNJs) produced by cylindrical concave micro-mirrors are numerically investigated. Simulation shows that the full-width at half-maximum (FWHM) of the r-PNJ is associated with the angle of the cylindrical concave mirror. It is found that the FWHM of the r-PNJ can achieve 0.37 of the wavelength, when the mirror with the angle θ = 130° is put in air. For the concave mirror immersed in water with θ = 100°, the FWHM of the r-PNJ can reach about 0.3λ. Two symmetric vortexes of Poynting vectors are close to the r-PNJ, which leads to the narrowed r-PNJ. Through combining a dielectric micro-cylinder with the concave mirror immersed in water, the waist of the r-PNJ can achieve 0.27 of the incident wavelength. Keywords Photonic nanojets · Reflection mode · Concave mirror
1 Introduction It is known that both lenses and concave mirrors can converge light to the focal spot [1]. Lenses adopt the transmission mode, while concave mirrors utilize the reflection mode. However, the limit imposed by the diffraction of optical waves restricts generation of sub-wavelength focal spots [2]. The capability of generating photonic nanojets (PNJs) that allows overcoming the diffraction limited resolution [3–5] has been proposed and demonstrated experimentally using microspheres [6–9] and micro-cylinders [10–12]. PNJs formed by microspheres and micro-cylinders illuminated under a plane wave are distributed at the shadow of the micro-particles. These PNJs are produced in transmission mode. Focusing of a dielectric particle on a silicon substrate with a nano-gap was investigated in [13]. However, the calculation of the intensity distribution inside the sphere was not mentioned. Later, PNJs generated in reflection mode with wide-angle performance were produced by * Song Zhou [email protected] 1
Jiangsu Key Laboratory of Advanced Manufacturing Technology, Faculty of Mechanical and Material Engineering, Huaiyin Institute of Technology, Huai’an 223003, China
hemispherical particles on metallic substrates in [14] and three-dimensional dielectric cuboids on metallic substrates in [15–17], demonstrating the capability to make r-PNJs localized out of the particles. Recently, it was experimentally found that a thin rectangular dielectric–metal structure could have a function of a flat focusing mirror based on photonic jet effect in reflection mode in [18]. In [19], a new method to produce r-PNJs using high refractive index contrast between a near-unity-index microsphere lens and a high-index dielectric substrate was successfully produced. The FWHM of the r-PNJ could reach 0.48 of wavelength, which localizes out of the microsphere. A specific regime of near-field focusing, called s-PNJ, supported by a dielectric micro-cylinder located near a metal mirror wa
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