Structured Light Beams: Science and Applications

Coherent structured light beams are developed by producing non-uniform phases and inducing polarization distribution in the cross-section of laser beams. In the early years of laser development, various kinds of structured modes were observed in laser beams [1]; however, the fundamental Gaussian mode saw tremendous interest compared to higher modes, owing to its closed-packed intensity distribution along the beam axis. Later, developments in structured light fields highlighted their importance in laser-based applications. In the recent past, rigorous research has been conducted regarding structured light fields and various kinds of structured modes have been generated. The most well-known structured beams are the Laguerre-Gaussian (LG), Hermite-Gaussian, Ince-Gaussian, Bessel, Airy, and Bottle beams [2,3,4,5,6]. In addition to these scalar modes, various kinds of vector modes have been fabricated in laser beams by systematically modulating the polarization distribution in the transverse beam cross-section. Based on the polarization distribution, vector beams can be broadly classified into V-point and C-point singularity vector beams [7,8,9]. The structured modes can be generated either directly from the laser cavities, or by utilizing external diffractive optical elements [10,11,12,13].

One of the major parameters in structured light fields is their wavelength, which plays a pivotal role in the search for their applications. Laser sources can provide only a limited number of wavelengths on the electromagnetic spectrum, and these lasing wavelengths have been predominantly constrained by the properties of the laser gain media [14]. This barrier has been subsequently broken down by developments in nonlinear optics [15]. The structured light field wavelength can tune in most of the electromagnetic spectrum, from deep ultra-violet to submillimeter wavelengths, through nonlinear wave-mixing [16,17,18]. The combination of structured beam optics and nonlinear optics has produced wavelength-variant structured laser sources for on-demand application.

Structured light fields have seen a wide range of applications, the most popular of which are listed below. The doughnut intensity distribution of the first-order LG beam, with an orbital angular momentum (OAM) number l = 1 and a radial index p = 0, has been successfully utilized in imaging with a resolution below the diffraction limit in both the stimulated emission depletion microscope and the super-resolution fluorescence depletion microscope [19,20]. The superposition of multiple LG modes with a nonzero OAM number and a zero radial index have been successfully utilized in high-density optical data transfer [21]. The self-healing and non-diffraction properties of Bessel beams have been used for simultaneous trapping in the multiple planes perpendicular to a beam’s propagation [22]. The helical wavefront of the optical vortices in the LG and Bessel modes was successfully utilized in the micro/nanofabrication of materials [23].

Further, the precise focusing of scalar and vector beams through high numerical apertures offers exciting possibilities for the creation of three-dimensional (3D) optical structures. Remarkable progress has been made in this field, particularly in the development and control of 3D nanostructures exhibiting optical chirality. This innovative approach enables the effective transfer of light helicity or angular momentum to these carefully crafted 3D nanostructures, paving the way for future applications in advanced optical technologies [24,25]. By employing radially and azimuthally polarized laser beams with arbitrary shapes, one can effectively achieve high-aspect-ratio material processing in silicon, facilitating the generation of intricate 3D optical structures [26]. In this context, a single multifocal metasurface with a high numerical aperture has demonstrated the ability to simultaneously generate distinct 3D optical polarization structures along the light propagation path. This advancement presents significant potential for utilization in various applications in the field of photonics [27]. Recent work has unequivocally shown that high optical chirality can be achieved in silica through the imprinting of twisted nanogratings by 3D polarization structuring using a Bessel beam [28], and, in particular, through nanofabrication in silicon with spatially shaped laser pulses and anisotropic seeding [29]. Such work has provided useful insights into the applications of laser-written 3D structures in the fabrication of chiral devices, nanophotonic systems, electronic–photonic systems, and light–matter interactions.

The motivation of the present Special Issue is to explore the most recent generations, characterizations, and applications of structured beams. Fifteen manuscripts were submitted to this Special Issue and eleven manuscripts have been published. Among these eleven articles, seven are original, while four are reviews. The published articles do not overlap in topic, and cover most kinds of structured beams. These articles discuss the generation and characterization of various kinds of scalar- and vector-structured beams. The wavelength tuning of structured modes by nonlinear wave mixing is demonstrated to obtain structured modes at the desired wavelength for any application. The propagation properties of the structured beams under tight focusing conditions, as well as their properties within the materials, are investigated. Further, many of the applications of structured beams are discussed. The published articles are listed below.