Free-Form Optical Fiber with a Square Mode and Top-Hat Intensity Distribution.

The development of bend-induced effectively single-mode fiber with a square cross-section and flat top-hat intensity distribution is reported using core topology nanostructuring dedicated to femtosecond laser ablation systems. The fiber's core comprises 5419 silica and germanium-doped silica nanorods with a diameter of 430 nm each arranged into a hexagonal lattice. The distribution of the rods is calculated using in-house developed code based on the Monte Carlo algorithm to obtain a target shape of mode and intensity distribution. As a proof-of-concept, a silica nanostructured fiber with a 24 µm core is developed and verified against the purity of mode guidance, bending, and guiding losses. It is shown that for a wavelength of 1030 nm, the fiber is effectively single-mode with 96% mode purity when bending with a radius of 20 cm is applied. The fiber has a measured mode area of 360 µm2, numerical aperture of 0.03, and total losses of 0.07 dB m-1.

Direct generation of multicolor Bessel beams from a Pr3+: WPFG fiber laser.

Multicolor visible high-order Bessel (Bessel-vortex) beams which have a helical wavefront and a long confocal length have garnered significant interest for applications in materials processing and biomedical technologies. In this paper, we demonstrate the direct generation of multicolor (523, 605 and 637 nm) Bessel-vortex beams from a Pr3+-doped water-proof fluoro-aluminate glass (Pr3+: WPFG) fiber laser with an intracavity lens which induces chromatic and spherical aberration. The handedness of the generated Bessel-vortex beam is selectively controlled through lateral displacement of the intra-cavity lens.

Direct generation of lower-order cylindrical vector vortex modes from an end-pumped Pr3+:LiYF4 laser.

We report the direct generation of vector vortex laser modes at 640 nm from a compact, diode end-pumped continuous-wave P r 3+:L i Y F 4 (YLF) laser that utilizes an intracavity lens and diaphragm. On-axis displacement of the intracavity lens, combined with appropriate choice of the intracavity diaphragm, enables selective generation of a desired radial and azimuthal vector laser mode. Such compact, vector vortex laser sources based on P r 3+:Y L F in the visible wavelength region are a significant enabling technology for a wide range of applications.

Broadband optical vortex beam generation using flat-surface nanostructured gradient index vortex phase masks.

We developed a new kind of compact flat-surface nanostructured gradient index vortex phase mask, for the effective generation of optical vortex beams in broadband infrared wavelength range. A low-cost nanotechnological material method was employed for this work. The binary structure component consists of 17,557 nano-sized rods made of two lead-bismuth-gallium silicate glasses which were developed in-house. Those small rods are spatially arranged in such a way that, according to effective medium theory, the refractive index of this internal structure is constant in the radial direction and linearly changes following azimuthal angle. Numerical results demonstrated that a nanostructured vortex phase mask with a thickness of 19 µm can convert Gaussian beams into fundamental optical vortices over 290 nm wavelength bandwidth from 1275 to 1565 nm. This has been confirmed in experiments using three diode laser sources operating at 1310, 1550, and 1565 nm. The generation of vortex beams is verified through their uniform doughnut-like intensity distributions, clear astigmatic transformation patterns, and spiral as well as fork-like interferograms. This new flat-surface component can be directly mounted to an optical fiber tip for simplifying vortex generator systems as well as easier manipulation of the generated OVB in three-dimensional space.

Creation of galaxy-shaped vortex relief structures in azo-polymers with petal-like beams.

We demonstrate the formation of surface relief structures in azo-polymers which exhibit multiple spiral arms, through irradiation of a rotating petal-like beam formed by the coherent superposition of Laguerre-Gaussian modes with opposite handedness. Intriguingly, the fabricated relief structures reflect full geometric parameters of the irradiated petal beam, such as handedness, topological charge, initial azimuthal phase and even ellipticity, corresponding to azimuthal and polar angles along equator and meridian planes of an orbital Poincaré sphere. The handedness, or direction of rotation, of the fabricated structures with multiple spiral arms could be controlled via the rotation and polarization directions of the irradiating laser field. This effect highlights an exotic coupling between the optical intensity gradient induced mass transport of the irradiated material and the spin angular momentum characteristics of the irradiating optical field. The azimuthal orientation of the surface relief structures could also be tuned by altering the initial relative phase between the coherently superposed Laguerre-Gaussian modes with opposite handedness, constituting the irradiating petal laser field. This work offers new insights into fundamental interactions which occur between light and matter, and we believe, will pave the way towards advanced technologies, such as ultrahigh density optical data storage.

Red, orange, and dual wavelength vortex emission from Pr:WPFGF fiber laser using a microscope slide output coupler.

Visible vortex beams have a large array of applications; however, the sources are often large or complex. Here, we present a compact vortex source with red, orange, and dual wavelength emission. This Pr:Waterproof Fluoro-Aluminate Glass fiber laser uses a standard microscope slide as an interferometric output coupler, yielding high quality first order vortex modes in a compact setup. We further demonstrate the broad (∼5 nm) emission bands in the orange (610 nm), red (637 nm) and near-infrared regions (698 nm), with the potential for green (530 nm) and cyan (485 nm) emission. This is a low-cost, compact and accessible device giving high quality modes for visible vortex applications.

Generation of V-point polarization singularity using single phase encoding with a spatial light modulator.

A liquid crystal Spatial Light Modulator (SLM) can be used in various ways to produce vector-vortices. Superposition of scalar vortices with orthogonal polarization is a common approach, while a more recent technique is to use dual-phase modulation. These approaches require modulation of at least two phase patterns with a SLM or multiple SLMs. In this paper, we propose a novel technique to produce vector-vortices by modulating orthogonal light components through a single phase pattern with a SLM. It does not require interferometric setups, and simplifies the generation of light beams with V-point polarization singularities. Because of compact and robustness of our experimental setup, it can be easily integrated to any device for applications of vector-vortices.

Focus movement distance per pulse dependence of electrical conductivity and diameter of diamond internal modification induced by picosecond laser.

Internal and local modifications via ultrashort pulsed laser illumination to diamond are promising for manufacturing diamond electronic devices. The relationship between the diameter/electrical conductivity of modified regions and the laser fluence distribution was investigated. Picosecond laser illumination without scanning the laser focus fabricated short modified regions in diamond. As a result, the calculated laser fluence distribution matches the distribution of the modified regions. Wire-shaped modified regions were fabricated via laser illumination with scanning of the laser focus, and the corresponding diameter and electrical conductivity were investigated by controlling the laser focus movement distance per pulse (Vf). The modified regions fabricated with varying Vf were divided into three categories depending on the trend of the relationship between the diameter and electrical conductivity. The diameters of the modified regions were constant at the maximum values when Vf was sufficiently small, decreased with increasing Vf, and reached a minimum when Vf was sufficiently large. The modified regions became more electrically conductive with increasing Vf, even when the deposited energy per unit length decreased. Moreover, the electrical conductivity decreased significantly when the diameter became constant at the minimum value. Finally, the relationship between the diameter/electrical conductivity of the modified regions and the laser fluence distribution was elucidated.

Self-referenced interferometry for single-shot detection of vector-vortex beams.

Vector-vortex (VV) beams are of significant interest for various applications. There have been substantial efforts toward developing a fast and efficient method for the characterization of generated VV beams which is crucial for their usage. Polarimetric approaches are commonly used to identify unknown VV beams but require multiple intensity recordings. This paper demonstrates a technique to detect VV beams and identify their parameters using the concept of self-referenced interferometry. The approach uses a single recorded interferogram to determine the beam parameters that allow rapid detection. The method even enables detection of VV beams having high-order optical vortices.

Nanoneedle formation via doughnut beam-induced Marangoni effects.

Recently, nanosecond pulsed optical vortices enables the production of a unique chiral and sharp needle-like nanostructure (nano-needle). However, the formation process of these structures has been unsolved although mass transport by angular momentum would contribute to the chirality. Here, we reveal that another key factor in the formation of a sharp nano-needle is the Marangoni effect during the melting condition at high temperature. Remarkably, the thickness and height of the nano-needle can be precisely controlled within 200 nm, corresponding to 1/25 of beam radius (5 µm) beyond the diffraction limit by ring-shaped inhomogeneous temperature rise. Our finding will facilitate the development of advanced nano-processing with a variety of structured light beams.

Direct generation of 523†nm orbital Poincaré mode from a diode-pumped Pr3+:LiYF4 laser with an off-axis optical needle pumping geometry.

We demonstrate an ultra-compact (8 mm-long) green orbital Poincaré Pr3+:LiYF4 (YLF) laser, in which the intracavity lasing modes are represented as eigenmodes on an equivalent orbital Poincaré sphere (eOPS). This is achieved through the simple application of an 'optical-needle' pumping geometry in conjunction with off-axis laser mode-manipulation. Optical vortices that are produced and mapped on north and south poles of the eOPS exhibit a maximum output power of 304 mW with a pump-to-laser-output slope efficiency of 12%. In addition, higher-order structured modes, including Ince Gaussian (IG), helical IG (HIG), Hermite-Laguerre Gaussian (HLG) and vortex lattices, can be directly generated from the laser cavity.

Laguerre-Gaussian beam generation via enhanced intracavity spherical aberration.

We demonstrate an end-pumped Laguerre-Gaussian (LG) mode-selectable Nd:YVO4 laser utilizing enhanced intracavity spherical aberration. The cavity was designed to exploit strong spherical aberration generated by an expanded beam, incident on a short-focal-length lens, which enabled oscillation of cavity modes of different order. This compact-cavity laser could operate efficiently with high-order LG mode, with the order of this mode being selectively changed by simply adjusting the distance between the short-focal-length lens and the output coupler. Scalar LG modes from LG0,±10 to up to LG0,±33 were observed in the experiment. The output power of the LG0, ±33 mode was 1.87 W under an absorbed pump power of 6.6 W.

Near and mid-infrared optical vortex parametric oscillator based on KTA.

We investigated high energy, near and mid-infrared optical vortex lasers formed by a 1 µm optical vortex-pumped KTiOAsO4 (KTA) optical parametric oscillator. The orbital angular momentum (OAM) of the pump beam can be selectively transferred to the signal or idler output by changing the reflectivity of the output coupler. With this system, 1.535 µm vortex signal output with an energy of 2.04 mJ and 3.468 µm vortex idler output with an energy of 1.75 mJ were obtained with a maximum pump energy of 21 mJ, corresponding to slope efficiencies of 14% and 10%, respectively. The spectral bandwidth (full width at half maximum, FWHM) of the signal and idler vortex outputs were measured to be Δλs ~ 1.3 nm (~ 5.5 cm-1) and Δλi ~ 1.7 nm (~ 1.4 cm-1), respectively.

Chirogenesis and Amplification of Molecular Chirality Using Optical Vortices.

The study of chirogenesis of organic molecules is important to elucidate the origin of the homochirality of biomolecules on Earth. Here, we have accomplished chiral symmetry breaking from a racemate using optical vortices with orbital angular momentum and a helical wavefront. We propose a new methodology of asymmetric transformation by the combination of enantioselective crystal nucleation by irradiation with optical vortices and crystallization-induced dynamic optical resolution of conglomerate crystals. Chiral green vortices generated using a spiral phase plate (SPP) with a 532 nm CW-laser were used to irradiate a supersaturated solution of a racemic isoindolinone, leading to crystal nucleation. The handedness of the crystals were controlled by the winding direction of the chiral optical vortices. The molecular chirality of the isoindolinone was then amplified by dynamic crystallization.

Cascaded vector vortex mode generation from a solid-state Raman laser.

Cascaded vector vortex mode generation from a Ba(NO3)2 Raman laser cavity pumped by a vector LG0,2 green laser was demonstrated for the first time to our knowledge. The generated Stokes outputs exhibited a second-order vector vortex beam as evidenced by linearly polarized flower-shaped spatial forms with four petals. The achieved optical conversion efficiencies of the first, second, and third Stokes outputs were 6.59%, 4.22%, and 0.11%, respectively, at a maximum pump energy of 3.6 mJ (360 mW).

Ultraviolet intracavity frequency-doubled Pr3+:LiYF4 orbital Poincaré laser.

Here we demonstrate intracavity frequency-doubling of an ultra-compact (cavity length < 20 mm) Pr3+:LiYF4 (YLF) orbital Poincaré laser, in which the fundamental modes are represented on an equivalent orbital Poincaré sphere (eOPS) and a singularities hybrid evolution nature sphere (SHENS). The generated ultraviolet (UV, 320 nm) output carries orbital angular momentum (OAM), and it typically exhibits an optical bottle beam with a 3-dimensional dark core, formed of a coherent superposition of eigen Laguerre-Gaussian (LG) modes. Such ultraviolet structured light beams with OAM offer many advanced applications from microscopy to materials processing.

Broadband high-resolution terahertz single-pixel imaging.

We report a simple single-pixel imaging system with a low mean squared error in the entire terahertz frequency region (3-13 THz) that employs a thin metallic ring with a series of directly perforated random masks and a subpixel mask digitization technique. This imaging system produces high pixel resolution reconstructed images, up to 1200 × 1200 pixels, and imaging area of 32 × 32 mm2. It can be extended to develop advanced imaging systems in the near-ultraviolet to terahertz region.

Direct generation of 1108†nm and 1173†nm Laguerre-Gaussian modes from a self-Raman Nd:GdVO4 laser.

We demonstrate a continuous-wave self-Raman Nd:GdVO4 Laguerre-Gaussian (LG) mode laser based on different Raman shifts of 382 cm-1 and 882 cm-1 by shaping the pumping beam with the use of an axicon lens and a focusing lens. Selective generation of LG mode beams at 1108 nm or 1173 nm, or simultaneously 1108 nm and 1173 nm, was achieved by carefully adjusting the alignment of the laser cavity. The maximum Raman LG mode output powers at the wavelengths of 1108 nm (the first-Stokes emission of the 382 cm-1 Raman shift) and 1173 nm (the first-Stokes emission of the 882 cm-1 Raman shift) were measured to be 49.8 mW and 133.4 mW at the absorbed pump power of 5.69 W, respectively. The generated LG modes, formed via the incoherent superposition of two LG mode beams with positive and negative topological charges, carry zero orbital angular momentum. Such LG mode laser sources have the potential to fill in the wavelength gap of lasers in the visible and infrared regions.

Photopolymerization with high-order Bessel light beams.

We study photopolymerization with high-order Bessel light beams with phase singularities on-axis. Self-trapping and self-focusing of propagation-invariant light beams in a photopolymer allow the fabrication of extended helical microfibers with a length scale of a centimeter, which is more than an order of magnitude larger than the propagation distance of the Bessel light beams. We show the evolution of microfibers rotating at a rate proportional to the incident optical power, while the periodicity of the helical structures remains constant, irrespective of the laser power. This suggests that optical momentum transfer plays a predominant role in the growth and rotation of such fiber structures.