Elongated Photonic Nanojet from Truncated Cylindrical Zone Plate

Previously (Chen et al., 2004), it was shown that dielectric cylinder can form focal spots with small diameters and long depth. This type of focal spot was called photonic nanojet. In this paper, it was shown that dielectric cylinder of radius 595？nm (1.12 of wavelength) forms near the surface a photonic nanojet with diameter equal to 0.31 of wavelength and depth of focus equal to 0.57 of wavelength. Adding truncated concentric rings with radiuses equal to radiuses of zone plate to the cylinder increases the depth of focus to 1.18 of the wavelength. The diameter and intensity of focal spot near the cylinder surface remain unchanged. 1. Introduction One of the promising areas of research in nanophotonics is tight focus that is formed using dielectric cylinders or spheres with diameters about wavelength. This type of focal spot was called “photonic nanojet” [1]. Unique features of photonic nanojets are small diameter and low divergence of the beam. The name “photonic nanojet” was for the first time used in [1]. In [2], photonic nanojet was formed using dielectric spheres with different diameters illuminated by plane wave . Also in [2], it was shown that the spheres with diameters of 1？μm and 2？μm formed photonic nanojets with diameters in the plane perpendicular to the direction of incident polarization equal to and . In [3], photonic nanojet with the maximum intensity 150 times larger than the intensity of the incident beam was obtained. In [4] photonic nanojet was obtained experimentally using scanning confocal microscope. It was shown that latex sphere with diameter equals to 3？μm forms focal spot with a diameter FWHM = 270？nm (the wavelength equals to 520？nm). The influence of incident light polarization was investigated in [5]. It was shown that radially polarized light formed photonic nanojet with effective focal volume 10 times smaller than the circularly polarized light. Photonic nanojet could be formed not only by spheres but by finite or infinite cylinders. For instance, in [6] photonic nanojet formed by discs from silica nitride with width 400？nm and diameters from 1 to 10？μm was obtained experimentally. The diffraction of light by multilayers cylinders was considered in [7]. Dielectric sphere with properly graded refractive index was used in [8] to increase the length of photonic nanojet. The length of a photonic nanojet formed by the graded-index microsphere can be increased to approximately 20 wavelengths. 2. Numerical Modeling The diffraction of the TE-polarized plane wave was simulated using FDTD method [9] implemented in FullWAVE [10].