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Optical singularities, which are positions within an electromagnetic field where certain field parameters become undefined, hold significant potential for applications in areas such as super-resolution microscopy, sensing, and communication. This potential stems from their high field confinement and characteristic rapidly-changing field distributions. Although the systematic characterization of the first singularities dates back many decades, recent advancements in sub-wavelength wavefront control at optical frequencies have led to a renewed interest in the field, and have substantially expanded the range of known optical singularities and singular structures. However, the diversity in descriptions, mathematical formulations, and naming conventions can create confusion and impede accessibility to the field. This review aims to clarify the nomenclature by demonstrating that any singular field can be conceptualized as a collection of a finite set of principal, 'generic' singularities. These singularities are robust against small perturbations due to their topological nature. We underscore that the control over the principal properties of those singularities, namely, their protection against perturbations and their dimension, utilizes a consistent mathematical framework. Additionally, we provide an overview of current design techniques for both stable and approximate singularities and discuss their applications across various disciplines.
Lim et al. (Sun,) studied this question.