Abstract
Engineered microsphere possesses the advantage of strong light manipulation at sub-wavelength scale and emerges as a promising candidate to shrink the focal spot size. Here we demonstrated a center-covered engineered microsphere which can adjust the transverse component of the incident beam and achieve a sharp photonic nanojet. Modification of the beam width and working distance of the photonic nanojet were achieved by tuning the cover ratio of the engineered microsphere, leading to a sharp spot size which exceeded the optical diffraction limit. At a wavelength of 633 nm, a focal spot of 245 nm (0.387 λ) was achieved experimentally under plane wave illumination. Strong localized field with Bessel-like distribution was demonstrated by employing the linearly polarized beam and a center-covered mask being engineered on the microsphere.
Highlights
On dielectric microsphere surface and prevented the multiple reflection at the gap between the microsphere top surface and metal boundary
The opaque cover on the engineered microsphere surface functions as a filter which removes the beams propagating near the optical axis
Modification of the Ex field leads to generation of different photonic nanojets with tunable beam sizes and working distances
Summary
On dielectric microsphere surface and prevented the multiple reflection at the gap between the microsphere top surface and metal boundary. The designed mask, which blocks the beam propagating in the vicinity of the optical axis, is important to the formation of a sharp focal spot. A photonic nanojet with FWHM = 245 nm (0.387 λ, λ = 633 nm) is demonstrated in both simulation and experiment by combining the properties of the linear polarization illumination, center-covered mask, and the dielectric microsphere. We achieve a sharp photonic nanojet using a linearly polarized beam and center-covered mask created on the microsphere. Unlike conventional large scale mask coated on an objective lens, the cover mask we employed is designed and fabricated directly onto the microsphere surface. 3D finite-difference time-domain (Lumerical FDTD) is used for the theoretical analysis and the experimental verifications are carried out using a high resolution optical microscope under the illumination of linear polarization beam
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