Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-6-9178
Timestamp: 2019-04-25 04:14:29+00:00

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A photonic nanojet (PNJ) is a tightly focused beam that emerges from the shadow surface of microparticles. Due to its high peak intensity and subwavelength beam waist, the PNJ has increasingly attracted attention, with potential applications in optical imaging, nanolithography, and nanoparticle sensing. A variety of ways have been demonstrated to further shrink the beam waist of PNJs, such as engineering the microparticle geometry and optimizing a multilayer structure. In this simulation work, we report the realization of an ultranarrow PNJ, which is formed by an engineered two-layer microcylinder of high refractive-index materials. Finite element analysis shows that under 632.8 nm illumination, the full width at half maximum of the beam waist can reach 87 nm (~λ/7.3). As far as we know, this is the narrowest PNJ ever reported. Using the backscattering intensity as a contrast mechanism, we also demonstrated the imaging resolution and capability of the ultranarrow PNJ through numerical simulations. We anticipate that this ultranarrow PNJ will open new possibilities in a variety of research areas, including nanoparticle detection, biomedical imaging, and nanolithography.
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Fig. 1 Schematic of the cylindrical structure used in numerical simulations.
Fig. 2 (a) Poynting vectors (small blue arrows) and streamlines (red solid lines) for a one-layer microcylinder of a high refractive-index material. R = 5λ and n = 3. The position of the focus is at d = 1.45 μm away from the center. (b) Poynting vectors and streamlines for the engineered one-layer microcylinder after splitting at d = 0.95 μm. R = 5λ and n = 3.5 (c) Poynting vectors and streamlines for the engineered two-layer microcylinder. Rs = 5λ, ns = 1.4, Rc = 4.55λ, and nc = 3.5. The splitting occurs at d = 1.0 μm. (d) Simulated intensity map of the PNJ formed by the engineered two-layer microcylinder. The PNJ outside the shadow surface is shown enlarged in the inset.
Fig. 3 (a)-(c) Transverse intensity profiles (along the y axis) of the PNJ generated by the engineered two-layer microcylinder, quantified at different positions along the x axis. (d)-(e) Transverse intensity profiles of the PNJ generated by the engineered one-layer microcylinder, quantified at different positions along the x axis. All the intensities are normalized by the intensity of the incident light. Note that the vertical axes do not all have the same range.
Fig. 4 Evolution of the FWHMs of transverse intensity profiles along the x axis for both the engineered two-layer microcylinder (red dots) and the engineered one-layer microcylinder (black squares).
Fig. 5 (a) An illustration of the imaging process. A typical bar pattern, with the same line width (LW) and line spacing (LS), is scanned along the negative y direction. (b)-(e) Images reconstructed from scanning a series of bar patterns with LWs of 180 nm (b), 150 nm (c), 120 nm (d) and 110 nm (e), respectively. (f) Measured LW as a function of exact LW. Absolute values of the relative errors between these two variables are also plotted.
Fig. 6 (a) Illustration of the imaging process. A micrometer-long target, with three different defects embedded, is scanned along the negative y direction. (b) Reconstructed images of the long target. For comparison, the profile of the refractive index of the sample is also plotted as a red dashed line.

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