Divide and structure: generating and interswitching orthogonal eigenstates of complementary petal beams using a π-shifted Sagnac interferometer.

One of the many facets of structured light are Ferris wheel/petal beams that can be generated by the addition/superposition of two beams with opposite vorticity/orbital angular momentum (OAM). We demonstrate a simple scheme employing a -shifted Sagnac interferometer (SI) containing a spiral phase plate (SPP) that divides and structures an incoming beam into two azimuthally complementary petal beams representing orthogonal eigenstates. The half-wave plate in the SI can interswitch/route these intensity patterns between the two outputs of the interferometer.

Divide and focus: generating novel focal polarization modalities by symmetry-based phase tailoring in one dimension.

Symmetry-based tailoring of photonic systems recently heralded the advent of novel concepts, such as photonic topological insulators and bound states in the continuum. In optical microscopy systems, similar tailoring was shown to result in tighter focusing, spawning the field of phase- and polarization-tailored light. Here, we show that even in the fundamental case of 1D focusing using a cylindrical lens, symmetry-based phase tailoring of the input field can result in novel features.

Vectorial spin Hall effect of light upon tight focusing.

The spin Hall effect of light is a manifestation of angular momentum conservation in the process of spin-orbit interaction of light. This optical Hall effect is exhibited in tight focusing of a circularly polarized asymmetric input beam as a shift of the center of gravity of the focal spot in the transverse plane, perpendicular to the direction/axis of symmetry breaking. It is commonly established that the direction of this shift depends on the sign of the spin.

Enlightening Arago-Poisson spot using structured light.

We show that structured light can amplify the intensity of an Arago-Poisson bright spot, the cornerstone proof of the wave nature of light, by several orders of magnitude. Specifically, we use a thin annular beam produced by either an axicon-lens combination or two axicons to illuminate an opaque circular obstacle. Experimental results confirm the numerical calculations.

Breaking the symmetry to structure light.

We show that by breaking the symmetry of a beam subjected to tight focusing, namely by obscuring half of it or, equivalently, shifting the beam away from the lens axis, it is possible to obtain novel light properties in the focal spot which, to the best of our knowledge, have not been observed before. For example, a linearly polarized beam half-obstructed or shifted from the axis generates longitudinal and transverse electrical field components, both of which peak on-axis. The ratio of the intensities of these two components can be tuned by changing the shift distance, the size, and the azimuthal location of the displaced incoming beam.

Tuning the resolution and depth of field of a lens using an adjustable ring beam illumination.

A pair of axicons with an adjustable separation between them is used to generate a variable diameter ring beam with high efficiency. This beam illuminates a lens to produce quasi-diffraction-free beams with a tunable spot size and depth of field. We studied the generated beam characteristics while changing either the ring diameter or its thickness.

Comparison of different schemes for stimulated Brillouin scattering enhancement.

A comparison of different schemes to enhance stimulated Brillouin Scattering (SBS) in single-mode optical fiber is performed. Specifically, we evaluated SBS generation efficiency in a 3 km length fiber with and without power recycling versus the same fiber placed in a ring resonator and in a ring resonator in a recycling configuration. For the latter case, a large number of both odd and even higher-order Stokes and anti-Stokes harmonics is generated.

Tighter focus for ultrashort pulse vector light beams: change of the relative contribution of different field components to the focal spot upon pulse shortening.

We investigate the focusing of Poisson-spectrum few cycle pulsed light beams for linear, circular, azimuthal, and radial input polarizations with and without a first-order vortex. It is shown that, for all the considered cases, the focal spot is tighter when compared to long pulses due to the increased blue frequency content in the ultrashort pulses spectrum. More significantly, we show, for what we believe is the first time, that upon pulse shortening different focused beam vector components associated with different Bessel functions J and J undergo a change in the relative weight of their respective contribution to the focal spot size.

Separating the Siamese twins: using a π-shifted Sagnac interferometer to control the relative weight/influence of circular and linear birefringence on the loop transmission facilitating their measurement.

We demonstrate a novel method to measure circular birefringence (CB) and linear birefringence (LB) present simultaneously in the device under study. By using a π-shifted Sagnac interferometer, the scheme eliminates the dependence on incoming polarization and on the orientation angle of the linear birefringence. Moreover, due to different handedness symmetry/response of CB and LB to counter-propagating waves, the technique allows us to control the relative influence of the two birefringences leading to a requirement of only two measurements to determine both of them.

Time behavior of focused vector beams.

We elucidate the pecularities of time behavior of focused vector optical fields. In particular, for linear or radial incident polarizations, we demonstrate explicitly the π/2 phase delay between transverse and longitudinal components of the field generated at the focus, i.e.

Ultrafast rotating dipole or propeller-shaped patterns: subwavelength shaping of a beam of light on a femtosecond time scale.

We report on a remarkable property of azimuthally (radially) polarized light beams containing a vortex or an orbital angular momentum: upon tight focusing of a first-order vortex beam, the subwavelength spot has a shape of an electric (magnetic) dipole rotating at an optical frequency. For beams with a vortex of order m, the generated pattern is propeller-shaped and rotates at a 1/m fraction of the optical frequency. The applications include petahertz control of electrical or optical conductance between two electrodes or waveguides of two-terminal junctions.

Toward the optical "magic carpet": reducing the divergence of a light sheet below the diffraction limit.

In 3D, diffraction-free or Bessel beams are well known and have found applications in diverse fields. An analog in 2D, or pseudonondiffracting (PND) beams, is a nontrivial problem, and existing methods suffer from deficiencies. For example, Airy beams are not highly localized, some PND beams have significant side lobes, and a cosine beam has to be truncated by a very narrow aperture thus discarding most of the energy.

Creating order with the help of randomness: generating transversely random, longitudinally invariant vector optical fields.

We show that it is possible to generate transversely random, diffraction-free/longitudinally invariant vector optical fields. The randomness in transverse polarization distribution complements a previously studied one in intensity of scalar Bessel-type beams, adding another degree of freedom to control these beams. Moreover, we show that the relative transversely random phase distribution is also conserved along the optical axis.

Development of spot size and lateral intensity distribution generated by exponential, logarithmic, and linear axicons.

Axicons are known to produce a nearly Bessel beam transverse intensity distribution. It has been previously recognized that linear axicons and logarithmic axicons with a predefined distant depth of field (DOF) and no blocking present a development region characterized by an enlarged central spot size. In this paper, we aim to obtain a better insight on the formation of lateral light distribution in this development region.

Characterization of a refractive linear axicon with distant depth of field and no central blocking.

We report the manufacturing and characterization of a refractive linear axicon producing a linearly increasing axial intensity Bessel-type beam over a predetermined range starting away from the axicon and without central blocking when illuminated by a plane wave. This is in contrast to a classical axicon that generates a diffraction-free beam starting from the axicon tip and extending to a range limited by the input beam aperture. The measured characteristics of the beam produced by the linear axicon, including its intensity distribution and spot size, are in good agreement with the theoretical predictions.

Engineering the smallest 3D symmetrical bright and dark focal spots.

A simple roadmap is established for the construction of the smallest three-dimensional (3D) isotropic focal spots. It is achieved in a 4Pi configuration by imposing a restriction/condition of equal transverse and longitudinal spot sizes to determine the position of an annular aperture and then optimize its size. The calculations were performed for cylindrically symmetric radial, azimuthal, and circular polarizations for the cases of in-phase and out-of-phase counter-propagating beams as well as when a vortex was added to the beams.

Righting the handedness' dichotomy: schemes of Sagnac interferometer containing chiral elements for suppression of dependence on circular birefringence and circular dichroism.

We present two configurations (one with and another without a half-wave plate) of a Sagnac interferometer (SI) containing chiral optical elements where either the Sagnac loop mirror's (SLM) reflectance is circular birefringence (CB) independent or polarization dependence/circular dichroism (CD) is canceled in both reflection and transmission. These schemes allow use of chiral components as feedback elements/filters in SLM of a laser and in switches/modulators and sensors requiring compensation of chiral media CD, as well as allowing calibration of CD measurements. We also compare/show the differences between SI containing devices with either CB or linear birefringence (LB).

Removing left-right asymmetry in a Sagnac interferometer applied to cancel its reflectance dependence on birefringence.

We present a left-right symmetry restoring method, which removes the detrimental birefringence in the single-mode fiber Sagnac interferometer, achieved with the aid of a half waveplate oriented at a specific angle. We show theoretically and demonstrate experimentally that adding a π-shift between clockwise and counterclockwise propagating, horizontally (in fiber loop plane) polarized field components, the Sagnac loop mirror's reflection becomes independent on birefringence of an element placed in the loop.

How low can STED go? Comparison of different write-erase beam combinations for stimulated emission depletion microscopy.

We compare different beam combinations for stimulated emission depletion microscopy. The four considered copolarized, mutually symmetric, but complementary write + erase beam combinations are circularly polarized beam + circularly polarized vortex with charge +1 or -1, azimuthally polarized with a vortex + azimuthally polarized, and radially polarized beam + radially polarized with a vortex. The resulting fluorescent spot was calculated for plane incident pump and erase beams, for plane waves with added high NA annular ring apertures, and when both incident beams were optimized with amplitude-phase masks.

Enlightening darkness to diffraction limit and beyond: comparison and optimization of different polarizations for dark spot generation.

We compare generation of a dark spot using focusing of beams with azimuthal polarizion, radial polarization with a vortex, and a circular polarization with either a first or second order vortex. By optimization of the amplitude-phase pupil, it is ascertained that azimuthal polarization is the most suitable one to obtain the diffraction bounded dark spot per se whose scalar approximation limit has FWHM=0.29λ.

The taming of absorption: generating a constant intensity beam in a lossy medium.

We report the first observation (to our best knowledge) of a constant intensity, quasi-Bessel/nondiffracting beam in an absorbing medium generated by a novel optical element, "exicon," or exponential intensity axicon. Such absorption-compensated and diffraction-resistant beams can find applications in illumination, remote sensing, free-space communications, imaging in biological tissues, nonlinear optics, and other situations where absorption and diffraction hinder light propagation.

Optimization of focusing of linearly polarized light.

We show that, by adding a π-phase shift to one-half of a linearly polarized beam, the roles of the transversal and longitudinal field components of the focused beam are interchanged, resulting in better focusing of the longitudinal component in the direction perpendicular to the phase jump line. For this component the scheme produces a spot with FWHM >15% smaller than a spot generated with either linearly or radially polarized light for any NA. The scheme has a similar advantage when applied to circularly polarized light, and it holds for both a plane wave and a realistic case of a Gaussian incident beam.

Demonstration of a Fresnel axicon.

We design and manufacture a Fresnel axicon (fraxicon) that generates a quasi-diffraction-free/Bessel beam with a large depth of field. The novel optical element is characterized with both coherent and incoherent light, and its behavior is compared with that of a classical axicon. While the fraxicon exhibits a strong interference pattern in the on-axis intensity distribution, it can be smoothed out when using broadband light, partial spatial coherence light, or by period randomization.