The polarization of light plays an important role in optical science and engineering. While most textbook treatments of light assume beams of spatially homogeneous polarization, there is an increasing interest in beams with spatially inhomogeneous state of polarizations. New effects and phenomena have been predicted and observed for light beams with these unconventional polarization states. To capture the latest development in this important emerging field of optics, it is our pleasure to introduce you the Optics Express Focus Issue on the Unconventional Polarization States of Light, with special attention to innovative work being done with radial polarizations, azimuthal polarizations, and other types of polarization vortices and spatially engineered polarizations.

The recent interest in these states can be traced back to the studies of laser cavities and Bessel-Gauss azimuthal modes (Erdogan and Hall, J. Appl. Phys. 68, 1435 (1990)), and optical imaging such as optimal concentration of electromagnetic radiation in the focal region (Sheppard and Larkin, J. Mod. Opt. 41, 1495, (1994); Optik,107, 79, (1997)). Overwhelming attention was given to high numerical aperture focusing of those so-called cylindrical vector (CV) beams named and studied by Brown et al. (Opt. Express 7, 77 (2000)). Sharper focusing with radial polarization was experimentally demonstrated by Leuchs et al. (Phys. Rev. Lett. 91, 233901 (2003)). Zhan and Leger explored focal field engineering with CV beams (Opt. Express 10, 324 (2002)). Many techniques of generating and manipulating CV beams and their applications have been reported in the past decade. These earlier developments were reviewed in a recent article by Zhan (Adv. Opt. Photon., 1, 1, 2009).

Their peculiar physics and broad applications have drawn increasing interests in CV beams and other more general unconventional polarization states of light. In this Focus Issue you will find contributions from scientists around the world who are active in this field. Researchers continue to come up new concepts for unconventionally polarized beams and are developing techniques to generate and characterize them. A so-called Full Poincaré beam that has local polarization states span the entire surface of the Poincaré sphere is introduced in a paper by Beckley et al. The propagation properties of this type of beams are investigated and a generation method using stressed window is demonstrated. Also based on the Poincaré sphere, Wang et al. propose and demonstrate hybridly polarized vector fields that have non-zero gradient of State of Polarizations across the beam. A paper by Visser et al. reports a new experimental setup that can observe the geometric phase that accompanies non-cyclic polarization changes. A real-time space-variant polarization characterization method is presented by Fridman et al.

Interests in the focusing properties of unconventional polarizations remain strong. The properties of highly focused linearly and radially polarized beams are studied in detail and a method of producing several special three dimensional polarization distributions within the focal region have been proposed by Gu et al. Pu et al. study the properties of a femtosecond vortex light pulse focused by a high numerical aperture objective lens and report interesting findings in the velocity variation and the orbital angular momentum within the focal volume. Application of the focusing properties of these unconventional polarizations in optical trapping is reported by Kozawa et al.

For fifty years, the laser has been a driving force for myriad scientific explorations and commercial applications. Fiber lasers attracted keen interest recently due to their compactness, high efficiency and robustness. In this Focus Issue, Xu et al. report an all-fiber laser that produces CV beam output. Another paper demonstrates a fiber laser that produces CV modes and other more general vectorial output modes using intracavity axial birefringence (Zhan et al.). Some of the observed behaviours of this fiber laser may be connected to the findings reported in a timely paper that studies the optical vortices in uniaxial crystals (Kivshar et al.).

The intrinsic strong polarization dependence of subwavelength nanostructures naturally connects the interests in these unconventional polarization states with nanophotonic applications. Focusing that employs plasmonic nanostructures under unconventionally polarized illumination has been reported by several groups (Yuan et al.; Levy et al.; and Agio et al.). Numerical studies on the responses from nanoscatterers with different sizes indicate potential benefits of radial polarization in near-field CARS microscopy (Huang et al.). Enhanced transmission up to four times through a single sub-wavelength coaxial aperture illuminated with a strongly focused radial polarization is demonstrated experimentally (Banzer et al.). The same group (Branzer et al.) also presented a versatile measurement technique that can be used to study optical frequency electric and magnetic response from a single nanostructure (such as a split-ring resonator). These findings provide detailed insights into the nanoscale physics that may have important applications from sensing, nanoscale imaging, quantum nano-optics, to metrology and nanolithography.

There has indeed been rapid growth in the number of scientists and engineers actively looking to understand and exploit unconventional polarizations in optics. While this collection of papers is representative of the international and interdisciplinary scope of interest, there are no doubt other research groups and many more applications than those represented in this issue. We therefore encourage our colleagues who have an ongoing interest in this subject to submit their future work in this very promising area of optics to Optics Express.

Sincerely,

Thomas Brown and Qiwen Zhan