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We study coupling of leaky Berreman modes in polar dielectric films (SiO2) through a thin metallic layer (gold) and show the familiar signatures of normal mode splitting. Due to very large negative real part of the dielectric function of gold, the splitting shows up only for extremely thin coupling layers. In contrast, coupling of Berreman modes through a dielectric spacer layer reveals novel possibilities of having bound states in continuum, albeit in the limit of vanishing losses. It is shown that the corresponding dispersion branches of the symmetric and antisymmetric modes can cross. BIC is shown to occur on one of these branches which is characterized by lower loss. In fact the BIC corresponds to the point where the radiative losses are minimized. For thicker layers (both spacer and the polar dielectric) BIC is shown to occur on the higher order dispersion branches. The origin of BIC is traced to the Fabry–Perot type mechanism due of the excitation of the leaky guided modes in the central layer.

All-dielectric metamaterials conforming to an optical reflectionless potential (ORP) offer broadband, omni-directional suppression of reflection. Though they are predicted to possess broadband negative group velocity dispersion (GVD), ultrashort pulse propagation through such materials has not been studied so far, to the best of our knowledge. In this work, we demonstrate negative GVD and group delay dispersion over broadband covering visible to near-infrared wavelengths. We investigate the role of ORP in supercontinuum generation (SC), which is observed to be polarization independent. The negative GVD in ORPs is interesting for pulse compression, phase compensation, dispersion engineering, and controlled SC generation.

We investigated the angular spectrum of bound state in the continuum (BIC) associated with topological charge and present an empirical tool to characterize BIC. While near-field confirms Bessel-Gaussian distribution, far-field is a perfect vortex.

A new property of the trapped mode (bound state in the continuum, BIC) supported by a dielectric resonant metasurface, which changes its lattice symmetry, is uncovered. The transformation of a metasurface composed of identical nanodisk resonators into a “diatomic” structure when one half of the nanodisks change their diameters is studied. The resulting folding of the Brillouin zone in the k‐space transforms the trapped (BIC) mode to quasi‐BIC resonances manifested in the polarization‐independent response. This novel feature is verified experimentally in the transmission of the metasurfaces illuminated by light with both linear and circular polarizations.

The orbital-angular-momentum (OAM) modes in optical fibers have polarization mode dispersion (PMD) properties similar to those of single-mode fibers (SMFs). The + l and − l order OAM modes supported by the same fiber vector modes undergo random cross coupling and exhibit a frequency-dependent time delay. We name this effect “OAM-PMD” and extend the formalism developed for PMD in SMFs to describe OAM-PMD. The characteristics of the modal beat lengths, birefringence correlation lengths, and the mean value of OAM-PMD are investigated. A fixed-analyzer technique is proposed and demonstrated to characterize this phenomenon in OAM fibers. Two different types of OAM fiber are examined. The measured results are compared with the theoretical calculations.

We numerically and experimentally show that all photons generated by spontaneous parametric down-conversion (SPDC) follow a transverse amplitude similar to that of the pump. This amplitude transfer from pump to SPDC is revealed in the Fourier image plane of the down-converted photons restricted by an aperture. We also observe a considerable shift of the image plane from the actual Fourier plane, when size of the aperture is gradually increased. The shift of the Fourier image of down-converted photons affects the quality of spatial mode-based projection in various quantum correlation experiments with parametric down-converted photon pairs. The results may be useful in applications of down-converted photons for quantum imaging and quantum communication., Comment: 5 pages, 6 figures

The stability of higher charge optical vortices generated with a spatial light modulator is investigated. It is observed that higher charge vortices split into an array of unit charge vortices with the tilting of a spatial light modulator. An interferometric method is used to show the exact location of split unit charge vortices. The rotation dynamics of generated unit charge vortices is visualized from the recorded images. A symmetric high charge vortex can be produced with a normal incidence of input laser beam to the spatial light modulator.

Airy beam, a non-diffracting waveform, has peculiar properties of self-healing and self-acceleration. Due to such unique properties, the Airy beam finds many applications including curved plasma wave-guiding, micro-particle manipulation, optically mediated particle clearing, long distance communication and nonlinear frequency conversion. However, many of these applications including laser machining of curved structures, generation of curved plasma channels, guiding of electric discharges in a curved path, study of nonlinear propagation dynamics and nonlinear interaction demand Airy beam with high power, energy and wavelength tunability. Till date, none of the Airy beam sources have all these features in a single device. Here, we report a new class of coherent sources based on cubic phase modulation of a singly-resonant optical parametric oscillator (OPO), producing high-power, continuous-wave (cw), tunable radiation in 2-D Airy intensity profile existing over a length >2 m. Based on a MgO-doped periodically poled LiNbO3 crystal pumped at 1064 nm, the Airy beam OPO produces output power more than 8 W and wavelength tunability across 1.51–1.97 μm. This demonstration gives new direction for the development of sources of arbitrary structured beams at any wavelength, power and energy in all time scales (cw to femtosecond).

Vector vortex beams are classified into four types depending upon spatial variation in their polarization vector. We have generated all four of these types of vector vortex beams by using a modified polarization Sagnac interferometer with a vortex lens. Further, we have studied the non-coaxial superposition of two vector vortex beams. It is observed that the superposition of two vector vortex beams with same polarization singularity leads to a beam with another kind of polarization singularity in their interaction region. The results may be of importance in ultrahigh security of the polarization-encrypted data that utilizes vector vortex beams and multiple optical trapping with non-coaxial superposition of vector vortex beams. We verified our experimental results with theory.

We demonstrate the mode division multiplexing system using two orbital angular momentum modes over 1.4 km RCF fiber. We also use a new mode demultiplexing scheme enabling us to do electrical polarization demultiplexing.

We demonstrate a technique for making copies of optical vortices with the same topological charge using a single spatial light modulator. It has been shown that by using suitable diffractive optical elements, several copies of optical vortices with the same topological charge could be created. We have devised diffractive optical elements using a single spatial light modulator which could be used to make a vortex and its copies without inverting the charge. The nature of the topological charge was investigated with the interferometric technique. The experimental results are verified with the theoretical analysis. We anticipate that our results may find applications ranging from optical manipulation to quantum information.

The diffraction of an optical vortex through an iris diaphragm which is a close approximation to a circular aperture has been investigated. The results are compared with those obtained from the diffraction of a Gaussian beam through the same aperture. In our findings the diffraction of an optical vortex and a Gaussian beam produce ball bearing sort of structure of darkness and brightness. The singularity of the vortex beam is found to be consistent even after the diffraction through the aperture. The presence of singularity at the centre of diffraction pattern of an optical vortex has been confirmed by interferometry. There is a good agreement between the experimental and numerical results. We propose that these results may find various applications in optical trapping experiments.

We report on a simple experimental scheme to generate and control the orbital angular momentum (OAM) spectrum of the asymmetric vortex beams in a nonlinear frequency conversion process. Using a spiral phase plate (SPP) and adjusting the transverse shift of the SPP with respect to the incident Gaussian beam axis, we have transformed the symmetric (intensity distribution) optical vortex of order l into an asymmetric vortex beam of measured broad spectrum of OAM modes of orders l, l − 1, l − 2, …, 0 (Gaussian mode). While the position of the SPP determines the distribution of the OAM modes, we have also observed that the modal distribution of the vortex beam changes with the shift of the SPP of all orders and finally results in a Gaussian beam (l = 0). Using single-pass frequency doubling of the asymmetric vortices, we have transferred the pump OAM spectra, l, l − 1, l − 2, …, 0, into the broad spectra of higher order OAM modes, 2l, 2l − 1, 2l − 2, …, 0 at green wavelength, owing to OAM conservation in nonlinear processes. We also observed an increase in single-pass conversion efficiency with the increase in asymmetry of the pump vortices producing a higher power vortex beam of mixed OAM modes at a new wavelength than that of the pure OAM mode.

An optical vortex is a light beam having a helical wavefront and a zero intensity region, a dark core, in the centre when viewed in the direction of the propagation of light beam. They can be created using computer generated holographic technique. We have studied the effect of the resolution of the computer generated hologram on the intensity profile of the vortex beam. The simulation results as well as the experimental results were in good agreement. We also give the significance of the obtained result for future research in this field.

We demonstrate a new class of optical parametric oscillators producing high-power, output beam in 2-D Airy intensity profile. The Airy beam has output power >8 W, longest ever acceleration length (>2 m), and wavelength tunability across 1.51–1.971 μm.

We demonstrate, analytically and experimentally, a simple, but effective method to determine the topological charge of an optical vortex by using a spherical bi-convex lens, a ubiquitous optical element found in any optics laboratory. Just by tilting the lens and recording the intensity distribution of a propagating vortex at a predicted position past the lens, we have been able to measure both the sign and the magnitude of the topological charge m up to m = ± 14 . Our experimental results are in excellent agreement with analytical predictions.

The reflectogram of a fiber grating is used to characterize vector modes of an optical fiber supporting orbital angular momentum states. All modes, with a minimal effective index separation around 10(-4), are simultaneously measured. OAM states are reflected by the FBG, along with a charge inversion, at the center wavelength of the Bragg reflection peak of the corresponding fiber vector mode.

The reflectogram of a fiber grating is used to characterize vector modes of an optical fiber supporting orbital angular momentum states. All modes, with a minimal effective index separation around 10e-4, are successfully measured.

We generate perfect optical vortex (POV) beams, whose intensity distribution is independent of the order, and scatter them through a rough surface. We show that the size of produced speckles is independent of the order of the POV and their Fourier transform gives the random non-diffracting fields. The invariant size of speckles over the free space propagation verifies their non-diffracting or non-diverging nature. The size of speckles can be easily controlled by changing the axicon parameter, used to generate the Bessel–Gauss beams whose Fourier transform provides the POV. These results may be useful in applications of POV for authentication in cryptography.

We show that determination of the orbital angular momentum (OAM) of an optical vortex can be possible using simple convex lens, an ubiquitous optical element found in any optics laboratory.

Topological aberrations of optical vortices introduced by a spatial light modulator are discussed. In the presence of such an aberration, a high charge vortex breaks into equal number of unit charge vortices.

We introduce an asymmetry in the core of a high charge optical vortex by using an appropriate computer generated hologram. The splitting of a high charge optical vortex core into unit charge vortices has been found to depend on the extent of the asymmetry. For a second order vortex, the trajectories of the split unit charged vortices and their separation have been recorded as a function of change in the asymmetry of the core. We find a good agreement between the experimentally obtained and numerically calculated results.

The experimental spatial coherence function of the one dimensional projection of optical vortices is compared with that of their two dimensional counterpart and also used to derive the information entropy and the Wigner distribution function.

We generate two dimensional Airy beams using a cubic phase mask and study its propagation through photorefractive material experimentally. The diffraction free nature as well as the acceleration of Airy beams is observed. Using beam propagation method (BPM), we confirm the experimental results. We also study the effect of an applied electric field on the propagation of Airy beams through photorefractive materials numerically. It is observed that the applied field leads to interaction between the lobes of Airy beams.

We derive a mathematical description of a perfect vortex beam as the Fourier transformation of a Bessel beam. Building on this development, we experimentally generate Bessel-Gauss beams of different orders and Fourier transform them to form perfect vortex beams. By controlling the radial wave vector of a Bessel-Gauss beam, we can control the ring radius of the generated beam. Our theoretical predictions match with the experimental results and also provide an explanation for previous published works. We find the perfect vortex resembles that of an orbital angular momentum (OAM) mode supported in annular profiled waveguides. Our prefect vortex beam generation method can be used to excite OAM modes in an annular core fiber.

We present an optical fiber supporting 36 information bearing orbital angular momentum (OAM) states spanning 9 OAM orders. We introduce design techniques to maximize the number of OAM modes supported in the fiber; while avoiding LP mode excitation. We fabricate such a fiber with an air core and an annular index profile using the MCVD process. We introduce a new technique for shaping OAM beams in free-space to obtain better coupling efficiency with fiber with annular index profiles. We excite 9 orders of OAM in the fiber, using interferometry to verify the OAM state on exiting the fiber. Using polarization multiplexing and both signs for the topological charge, we confirm support of 36 states, exploiting to our knowledge the highest number of OAM modes ever transmitted in optical fiber.

A novel type of few-mode fiber, characterized by an inverse-parabolic graded-index profile, is proposed for the robust transmission of cylindrical vector modes as well as modes carrying quantized orbital angular momentum (OAM). Large effective index separations between vector modes (2.1 × 10(-4)) are numerically calculated and experimentally confirmed in this fiber over the whole C-band, enabling transmission of OAM(+/-1,1) modes for distances up to 1.1 km. Simple design rules are provided for the optimization of the fiber parameters.

We observe phase singularities in the superposed field of two Gaussian beams. It is seen that the formation of these singularities depends on the tilt between two Gaussian beams and the separation of their beam axes. By reversing the angle or the position of the beams, one can change the sign of the vortex. We have shown the formation of single as well as multiple vortices by changing the tilt angle and the position of two Gaussian beams. The experimental results are verified with theoretical analysis. We also observe that such a vortex structure can be formed through superposition of two backreflected Gaussian beams from any optical element with two flat surfaces, as illustrated through a beam splitter and a neutral density filter. This technique is very useful for generation of vortices with high-power lasers where one cannot use a spatial light modulator.

We have investigated scattering of optical vortices via beam fanning as they propagate in photorefractive SBN crystal. The measurements show that the power decay of the vortex through the crystal depends on the order of the vortex. This decay for optical vortices is compared with that of the Gaussian laser beam and it is found to be faster than the vortices. The experimental results are explained using the dependence of the two-wave mixing or beam coupling in the photorefractive materials on the degree of coherence. Furthermore, the coherence dependence of the beam fanning has been verified with partially coherent light.

We have produced beams with dark rings and studied their propagation through photovoltaic photorefractive media. We observe that dark rings, instead of forming dark ring solitons, break into vortex–antivortex pairs, forming quadrupoles. The experimental results could be taken as a consequence of modulational instability during beam propagation that is revealed through our numerical analysis.

We make optical vortices of different topological charge and diffract them through a quadratic phase mask using the same spatial light modulator. This phase mask shows the diffraction in which the positive diffracted order has different dynamics than the negative diffracted order. The diffraction pattern and its orientation depend on the charge of the vortex as well as its sign. The experimental results are verified with exact analytical results.

We generate experimentally optical ring lattice structures which are the superposition of two coaxial Laguerre-Gaussian modes with common waist position and waist parameter. Although these structures are not diffraction-free, they show self-healing property. This self-reconstruction of the ring lattice can be understood by looking into the transverse energy flow at different z planes. The experimental results are verified by the numerical simulations.

We study, experimentally as well as theoretically, the spatial coherence function and the Wigner distribution function for one-dimensional projections of optical vortices of different orders. The information entropy derived from the spatial coherence functions has been used to quantify the information content of the vortices and compared with those obtained for the Gaussian beam. The experimental results verify the theoretical findings of Agarwal and Banerji [Opt. Lett. 27, 800 (2002)].

Mode division multiplexing (MDM) systems using orbital angular momentum (OAM) modes can recover the data in D different modes without recourse to full (2 D × 2 D ) multiple input multiple output (MIMO) processing. One of the biggest challenges in OAM-MDM systems is the mode instability following fiber propagation. Previously, MIMO-free OAM-MDM data transmission with two modes over 1.1 km of vortex fiber was demonstrated, where optical polarization demultiplexing was employed in the setup. We demonstrate MDM data transmission using two OAM modes over 1.4 km of a specially designed ring core fiber without using full MIMO processing or optical polarization demultiplexing. We demonstrate reception with electrical polarization demultiplexing, i.e., minimal 2 × 2 MIMO, showing the compatibility of OAM-MDM with current polarization demultiplexing receivers.

We demonstrate a new class of tunable Airy beam source based on OPO, producing high-power, continuous-wave, single-frequency Airy beam radiation. The source provides total tunability of 460nm and maximum of 8.1W in 2D-Airy beam structure.

We propose and fabricate a novel few-mode optical fiber for transmission of cylindrical vector modes. Effective index separations larger than 2.1×10-4, even with strong fiber bends, are achievable in the C-band.

We present the design, simulation, and experimental characterization of a family of ring-core fibers suitable for OAM transmission. The simplicity of design and easily fabricated multiplicity of fibers facilitates analysis and modeling as compared to other OAM fibers.