One characteristic of the coherent light generated from lasers is that it is polarized, that is, all the emitted photons have the same state of polarization. Polarization relates to the spatial orientation of the oscillation of electromagnetic waves. The capability to manipulate the polarization of light has enabled numerous applications, in fields ranging from microscopy through high-speed optical communications to liquid-crystal display technology and even 3D video. Laser engineers usually concern themselves with linear polarization (where the orientation of the oscillations remains constant) or elliptical polarization (where the orientation rotates as the wave propagates in space). However, more obscure and difficult to harness forms of polarization are also possible. These include radial and azimuthal polarization, which are usually met in hollow (doughnut-shaped) beams, and imply that the orientation of the wave either is aligned or is perpendicular to the radial direction of the propagating beam, respectively. Generation and control of these particular modes of polarization is actually of technological importance, since cylindrically polarized beams can focus on tighter spots as compared with the more conventional linear or elliptical polarizations. This has significant implications for such fields as optical trapping, nonlinear optics, or material processing.

The paper by Euser et al. studies an optical fiber that not only supports and maintains the state of polarization of cylindrically polarized beams but also exhibits distinctly different refractive index and group-velocity dispersion for the radial and the azimuthal modes. These properties are achieved by introducing a nanometer-scale hole (a “nanobore”) in the otherwise solid core of a photonic crystal fiber, which is responsible for a high concentration of energy in its vicinity. In experiments that are supported both by quasi-analytical and numerical calculations, it is shown that the nanobore is also responsible for the distinct waveguiding properties of the radial and the azimuthal modes in the fiber. Properties, such as the cylindrical birefringence and the dispersion of the two modes, can be controlled by adjusting the structural parameters of the fiber. Moreover, the dispersion of the two polarization modes is affected differently by the exact size of the nanobore, thereby allowing great flexibility in the control that can be achieved.

The paper comes from an authoring group that is likely to make a strong impact on the field and that combines an established track record in photonic crystal fiber technology and a long experience in the investigation of cylindrically polarized light. Indicative of this is that the reported fiber has already been investigated for its applications in quantum optics from the same group of researchers. Developments in fiber technology like those reported in this paper will become of fundamental significance for the implementation of practical fiber devices that will enable exploitation of these new modes of propagation of optical fields.

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