Formation of polarization-dependent optical vortex beams via an engineered microsphere

Inverse Design of Whispering-Gallery Nanolasers with Tailored Beam Shape and Polarization

Control over the shape and polarization of the beam emitted by a laser source is important in applications such as optical communications, optical manipulation and high-resolution optical imaging. In this paper, we present the inverse design of monolithic whispering-gallery nanolasers which emit along their axial direction with a tailored laser beam shape and polarization. We design and experimentally verify three types of submicron cavities, each one emitting into a different laser radiation mode: an azimuthally polarized doughnut beam, a radially polarized doughnut beam and a linearly polarized Gaussian-like beam. The measured output laser beams yield a field overlap with respect to the target mode of 92%, 96%, and 85% for the azimuthal, radial, and linearly polarized cases, respectively, thereby demonstrating the generality of the method in the design of ultracompact lasers with tailored beams.

Whispering-gallery modes promote enhanced optical backflow in a perforated dielectric microsphere

Optical energy flow inside a dielectric microsphere is usually co-directed with the optical wave vector. At the same time, if the optical field in a microsphere is in resonance with one of the high-quality spatial eigenmodes (whispering-gallery modes- WGMs), a region of reverse energy flow emerges in the shadow hemisphere. This area is of considerable practical interest due to increased optical trapping potential. In this Letter, we consider a perforated microsphere with an air-filled pinhole fabricated along the particle diameter and numerically analyze the peculiarities of WGM excitation in a nanostructured microsphere. A pinhole isolates the energy backflow region of a resonant mode and changes a perforated microsphere into an efficient optical tweezer. For the first time, to the best of our knowledge, a multiple enhancement of the energy backflow intensity in the pinhole at a WGM resonance is revealed and we discuss the ways for its manipulation.

Focusing characteristics of cylindrical vector beams through a multi-focal all-dielectric grating lens

A novel, to the best of our knowledge, type of multi-focal all-dielectric grating lens is proposed in this work, and focusing characteristics of cylindrical vector beams through the lens are investigated in detail. Based on the negative refraction mechanism of negative-first-order diffraction and Fermat's principle, a multi-focal lens is designed. By analyzing the diffraction effect of the grating, the essential factor that affects the focus quality is found. Through a two-step optimization process, secondary foci and the focal displacement of primary foci caused by high-order diffractions are overcome, and the quality of the focal field is significantly improved. This work provides a reference for micro-lens design for focus modulation, and the research results also have potential applications in the fields of light-field manipulation and optical tweezers.

High-quality longitudinally polarized photonic nanojet created by a microdisk

The Letter reports the generation of a high-quality longitudinally polarized photonic nanojet by illuminating a dielectric microdisk with a focused radially polarized light. High-quality longitudinally polarized beams can be generated using a microdisk with a wide range of refractive indices. By optimizing the shape and refractive index of the microdisk, the radial component can be effectively suppressed, and a maximum beam quality of 90% and a field enhancement factor of up to two orders can be achieved with a sub-diffraction-limited spot size. Moreover, the focusing performance of the microdisk is observed to be stable within a wide range of focusing numerical aperture values of the incident light.