Propagation invariant laser beams for optical metrology applications

Propagation invariant laser beams (PILBs) represent a class of coherent structured field distributions possessing several unique properties. PILB shapes have been traditionally produced as the output mode field distributions of stable laser resonators. To expand the diversity of possible PILB shapes, additional techniques have been developed, including PILB transformations with the aid of anamorphic optical systems. The unique properties of PILBs make them an attractive choice in several optical metrology applications, including superresolution microscopy. In this paper, we apply novel PILB shapes produced with anamorphic optical systems to optical metrology of sub-wavelength sized phase objects. The anamorphic transformation technique is based on a single propagating laser field, and is therefore simpler to implement than interferometric beam transformations based on superposition of optical fields. We explore the interactions between PILBs and nanoscale phase objects, representative of sub-wavelength lithographic patterns and nano-particles. We observe that the sensitivity of PILBs to phase perturbations is dependent on the beam shape. Analysis of the far field diffraction patterns produced can provide information about the location and shape of the nanoscale phase structures. Results of our study can be applied to high-resolution detection of small phase objects in a variety of fields within optical metrology.