Special Issue on Polarized Light and Optical Systems

Polarization is often measured to study the interaction of light and matter, so the description of the polarization of light beams is of both practical and fundamental interest. The polarization of light plays an important role in optical transmission, photonics, nanoplasmonics, high-power lasers and amplifiers, optical coherence tomography and imaging systems, and fiber optics.

The purpose of this Special Issue is to highlight recent progress in polarization optics and to introduce new polarization effects in light propagation, including tightly focused light beams, in free space, subwavelength structures, and waveguides, as well as the development of new optical elements and devices for controlling, processing, and transmitting information. The details are given below. Olga Korotkova and Yalcin Ata formulated and derived the propagation laws for the 4 × 4 matrix consisting of the scintillation indices [1]. The results can be of interest for building robust communication and sensing systems, operating in the presence of atmospheric fluctuations for signal transmission through underwater turbulent environments or in soft, homogeneous, and isotropic biological tissues. Jose Gil describes the geometric representation of general polarization states [2]. The approach presented solves the problem of representing geometrically, in a simple and meaningful manner, all polarization states (three-dimensional, in general). The geometric features of the polarization density object are determined by the intrinsic Stokes parameters, which allows for a complete and systematic classification of polarization states. In [3], Jose Gil et al. analyzed the physical significance of the determinant of a Mueller matrix in terms of its intrinsic fundamental properties. Mueller polarimetry is nowadays a well-known and useful optical characterization technique, providing substantial information on a huge variety of materials and structures from many areas of science and engineering. Colin Sheppard et al. give a table of Mueller matrices M, coherency matrices C, and coherency matrix factors F for different polarization components and systems [4]. The Mueller matrix can be used to describe polarization effects of a depolarizing material. Vipin Tiwari and Nandan Bisht experimentally demonstrated the complete polarimetric characterization of a twisted nematic liquid crystal spatial light modulator (TNLC-SLM) for its entire grayscale range using the combined Jones–Stokes polarimetry [5]. The results of this study can be applied for the utilization of the SLM in beam shaping, tissue polarimetry, and structured light applications.

Depolarization has been found to be a useful contrast mechanism in biological and medical imaging. Colin Sheppard et al. presented a historical review of relevant polarization algebra, measures of depolarization, and purity spaces [6]. Nikolai Petrov presented a theoretical analysis of the depolarization of vector light beams on propagation in free space [7]. Hybrid vector Bessel beams with polarization-OAM (orbital angular momentum) entanglement, which are the modal solutions of the Maxwell equations, are proposed to study the evolution of vector beams in free space. Polarization-invariant vector Bessel beams with phase and polarization singularities, which combine spin and OAM, are proposed. The change in the polarization state and the degree of polarization on propagation is shown for an arbitrary incident beam that is not a modal solution of the Maxwell equations. The close connection of the degree of polarization with the quantum-mechanical purity parameter is emphasized.

Juan Carlos de Sande et al. considered a wide class of nonuniformly, totally polarized beams that exhibit a propagation-invariant intensity profile and polarization pattern during paraxial propagation [8]. Beams of this type are of interest in polarimetric techniques that use a single input beam for the determination of the Mueller matrix of a homogeneous sample. Dezhi Su et al. investigated the DoLP (degree of linear polarization) of an object in the marine background modelled by considering the RCE (radiation coupling effect) [9]. The influence of the RCE on the infrared polarization characteristics of the object is analyzed, and a long-wave infrared (LWIR) polarization measurement system is built to measure the DoLP of the object at different temperatures and observation angles. These results are of great significance to the research of polarization detection and identification of infrared objects in the marine background.

In recent years, structured laser beams for shaping inverse energy flow regions, regions with a direction of energy flow opposite to the propagation direction of a laser beam, have been actively studied. Svetlana Khonina et al. [10] investigated the possibility of controlling inverse energy flow distributions by using the generalization of well-known cylindrical vector beams with special polarization symmetry—vector Lissajous beams (VLBs)—defined by two polarization orders (p, q). Such optical fields with negative energy flow generated by VLBs are useful for controlling light on a nano- and microscale to form a given distribution of forces acting on optically trapped particles.

Various new vector light field and optical elements exploiting the polarized light are proposed. Andrey Ustinov et al. demonstrated the use of conventional and so-called generalized spiral phase plates for the formation of light fields with an inverse energy flux when they are illuminated with linearly polarized radiation [11]. Svetlana Khonina et al. proposed binary diffractive optical elements, combining several axicons of different types (axis-symmetrical and spiral), for the generation of a 3D intensity distribution in the form of multiple vector optical ‘bottle’ beams, which can be tailored by a change in the polarization state of the illumination radiation [12]. Sergey Degtyarev et al. proposed a new element for producing an azimuthally polarized beam with a vortex phase dependence [13]. Authors propose a refractive bi-conic axicon for the transformation of a circular-polarized beams into an azimuthally polarized ring form vortex beam. Grigoriy Greisukh et al. consider only two-layer sawtooth relief-phase microstructures instead of the usual multilayer sawtooth microstructures to achieve the required level of light intensity homogeneity both in the required spectral and angular ranges [14]. The possibility of the effective use of these diffractive elements in wide-angle optical systems is emphasized. Svetlana Eliseeva et al. obtained the transmission spectra of a microresonator (MCR) structure with Bragg mirrors, the working cavity of which is filled with a magnetically active finely layered ferrite-semiconductor structure with material parameters controlled by an external magnetic field [15]. The polarization characteristics of radiation passing through a MCR and the possibility of polarization control using an external magnetic field are studied. Victor Kotlyar et al. proposed a new vector light field, E_nV, with inhomogeneous linear polarization and high order n that depends on a real parameter [16].

Sharp focusing of light is also considered. Sergey Stafeev et al. considered the tight focusing of light with linear polarization. Using the Richards–Wolf formalism, it is shown that before and after the focal plane, there are regions in which the polarization is circular (elliptical) [17]. This result allows the use of linearly polarized light to rotate microparticles (the size of the circularly polarized region is about 0.3 µm by 0.3 µm) around its center of mass. Victor Kotlyar et al., using the Richard–Wolf formulae, derived analytical relationships to describe projections of the Poynting vector (the energy flow) and the SAM vector when tightly focusing a linearly polarized optical vortex with a topological charge of 2 [18]. Victor Kotlyar et al. theoretically and numerically studied a new type of nth order hybrid vector light field that is tightly focused with an aplanatic system [19].

Xuan Zhang et al. study the scattering properties of the partially coherent vector beams with the deterministic media having the classic symmetric and parity-time (PT) symmetric scattering potential functions [20]. Their findings may find applications, e.g., in controlling the directionality in light scattering.

In summary, the new vector light beams possessing of unique characteristics, propagation invariant polarized optical beams, and new optical elements exploiting the polarized light can be of interest for building robust communication and sensing systems operating in free space and in the presence of atmospheric fluctuations.

The results presented in this Special Issue may be useful in free-space optical communications, singular optics, photonics, optical coherence tomography, imaging, and may have implications for future quantum networks.

I hope that this Special Issue can serve as a source for both obtaining useful information about modern polarization optics and for choosing new interesting research directions.