Optical Needle Beam Generated by a Mesoscale Dielectric Microtoroid

Achieving an optical needle beam (ONB) is a hotspot due to its critical role in many applications In this paper, we demonstrate a long ONB with subwavelength width generated by a mesoscale dielectric microtoroid structure. The key properties of the ONB, including the propagation distance, and the full-width at half maximum (FWHM), are studied in detail using finite-difference time-domain (FDTD) simulation. Result shows that an ONB with propagation distance of $24.97 \lambda$ is achieved. Moreover, the properties of ONB can be modified flexibly by changing the refractive index of the microtoroid. Compare with the traditional methods, our dielectric microtoroid has advantages of compact structure, easy for fabrication, and low cost. Such an ONB may have applications in many fields, including microscopy, materials processing, data storage, biomedicine, nonlinear optics and so on.