| The photonic nanojet is a high-intensity, extremely narrow sub-wavelength electromagnetic field region, which is generated by the focusing effect of Mie scattering from dielectric microspheres or micro-columns on the electromagnetic field. Because of these characteristics, it is widely used in laser processing, nanolithography, optical high-density storage, and super-resolution microscopy. From the perspective of radially polarized light, a donut-shaped dielectric micro-body is used to focus the beam. Since radially polarized light can produce a strong longitudinal field in the focusing region, by optimizing the donut size, refractive indexes, and the relative position between the donut and the focal point of the objective lens, a longitudinal photonic nanojet with more than 90% of the beam quality can be obtained, and its intensity is increased by about an order of magnitude compared to that without the donut. Moreover, the full width at half-maximum of focal spot is below diffraction limit under high refractive index. Therefore, this structure is expected to have applications in particle acceleration, optical tweezers, and Raman spectroscopy. |
