Yu. Loiko

Wave-vector and polarization dependence of conical refraction

We experimentally address the wave-vector and polarization dependence of the internal conical refraction phenomenon by demonstrating that an input light beam of elliptical transverse profile refracts into two beams after passing along one of the optic axes of a biaxial crystal, i.e. it exhibits double refraction instead of refracting conically. Such double refraction is investigated by the independent rotation of a linear polarizer and a cylindrical lens. Expressions to describe the position and the intensity pattern of the refracted beams are presented and applied to predict the intensity pattern for an axicon beam propagating along the optic axis of a biaxial crystal.

Blue-detuned optical ring trap for Bose-Einstein condensates based on conical refraction

We present a novel approach for the optical manipulation of neutral atoms in annular light structures produced by the phenomenon of conical refraction occurring in biaxial optical crystals. For a beam focused to a plane behind the crystal, the focal plane exhibits two concentric bright rings enclosing a ring of null intensity called the Poggendorff ring. We demonstrate both theoretically and experimentally that the Poggendorff dark ring of conical refraction is confined in three dimensions by regions of higher intensity. We derive the positions of the confining intensity maxima and minima and discuss the application of the Poggendorff ring for trapping ultra-cold atoms using the repulsive dipole force of blue-detuned light. We give analytical expressions for the trapping frequencies and potential depths along both the radial and the axial directions. Finally, we present realistic numerical simulations of the dynamics of a 87Rb Bose-Einstein condensate trapped inside the Poggendorff ring which are in good agreement with corresponding experimental results.

Polarization tailored novel vector beams based on conical refraction

Coherent vector beams with involved states of polarization (SOP) are widespread in the literature, having applications in laser processing, super-resolution imaging and particle trapping. We report novel vector beams obtained by transforming a Gaussian beam passing through a biaxial crystal, by means of the conical refraction phenomenon. We analyze both experimentally and theoretically the SOP of the different vector beams generated and demonstrate that the SOP of the input beam can be used to control both the shape and the SOP of the transformed beam. We also identify polarization singularities of such beams for the first time and demonstrate their control by the SOP of the input beam.

Generating a three-dimensional dark focus from a single conically refracted light beam

We report here the generation of a three-dimensional (3D) dark focus from a single focused monochromatic Gaussian beam that undergoes conical refraction when it propagates along one of the optic axes of a biaxial crystal. We study the resulting ring intensity pattern behind the crystal as a function of the ratio between the ring radius and the beam waist and derive the particular parameter values for which a 3D dark focus with null intensity at the ring center is formed. We have performed experiments with a KGd(WO(4))(2) biaxial crystal, reporting the generation of a bottle beam in full agreement with our theoretical investigations.When a light beam passes through a cascade of biaxial crystals with aligned optic axes, the resulting transverse intensity pattern consists of multiple concentric rings. We provide a simple formulation for the pattern formation for both circularly and linearly polarized input beams, that could be applied for a cascade of an arbitrary number of biaxial crystals. We have experimentally investigated multiple ring formation with up to three cascade biaxial crystals, showing that the theoretical formulation is in full agreement with the experimental results.

Super-Gaussian conical refraction beam

We demonstrate the transformation of Gaussian input beams into super-Gaussian beams with a quasi flat-top transverse profile by means of the conical refraction phenomenon by adjusting the ratio between the ring radius and the waist radius of the input beam to 0.445. We discuss the beam propagation of the super-Gaussian beam and show that it has a confocal parameter three times larger than the one that would be obtained from a Gaussian beam. The experiments performed with a KGd(WO4)2 biaxial crystal are in good agreement with the theoretical predictions.

High-directional wave propagation in periodic loss modulated materials

Abstract Amplification/attenuation of light waves in artificial materials can become sensitive to the propagation direction by spatially modulating the gain/loss response of the medium on the wavelength scale. We give a numerical proof of the high anisotropy of the gain/loss in two dimensional periodic structures with square and rhombic lattice symmetry by solving the full set of Maxwells equations using the finite difference time domain method. Anisotropy of amplification/attenuation leads to the narrowing of the angular spectrum of propagating radiation with wavevectors close to the edges of the first Brillouin Zone. The effect provides a novel and useful method to filter out high spatial harmonics from noisy beams.

Doppler-free adiabatic self-induced transparency

The ability to render a medium transparent to a resonant laser field opens a wide range of applications, from slow light and dark state polariton physics to light-matter interfaces. In atomic vapours at room temperature, Doppler broadening plays, in general, a negative role tending to reduce transparency. In a two-level (2L) system Doppler-free transparency can be achieved by means of self-induced transparency (SIT) (Fig.1(a,d)), which consists in preparing an optical pulse propagating in the medium such that an integer number of Rabi oscillations are performed [1]. Therefore, SIT requires a very precise temporal control of the Rabi frequency, i.e. the pulse area being an integer multiple of 2π. In three- (3L) and/or multi- level atomic systems, resonant transparency could be achieved by using induced atomic coherences, e.g., by means of the coherent population trapping (CPT) [2] and the electromagnetically induced transparency (EIT) phenomena [3] or via a double-STIRAP process [4]. In all these cases, the required two-photon (or Raman) resonance condition severely restricts the applications of the previous approaches to laser fields with nearly identical frequencies.

The effect of the vectorial degree of freedom on the dynamics of class-A lasers

Abstract A rigorous treatment of the vectorial problem in lasers leads to new characteristic constants: the decay rate of the electric field polarization and the frequency of the polarization relaxation oscillations. The physical factor distinguishing them in lasers is the resonator anisotropy. The presence of the polarization characteristic constants provides a new vectorial mechanism for the laser instabilities. The effect of these constants on the behavior of class A lasers is considered in details. The problem is handled in terms of Jones matrices and vectors although a comparison between the present and the Lamb model is given as well. Co-dimension one (saddle node, pitchfork, and Hopf) and two (Takens–Boganov) bifurcations are found analytically in lasers with no magnetic field operating on transitions favoring both linearly ( j → j ′ +1) and circularly ( j >0) polarized emission. A numerical analysis of the lasers subject to the action of the magnetic field reveals that their periodic behaviors can be also attributed to the effect of the novel characteristic constants.

Optical quantum memory for polarization qubits with V-type three-level atoms

We investigate an optical quantum memory scheme with V-type three-level atoms based on the controlled reversible inhomogeneous broadening technique. We theoretically show the possibility of storing and retrieving a weak light pulse interacting with the two optical transitions of the system. This scheme implements a quantum memory for a polarization qubit—a single photon in an arbitrary polarization state—without the need of two spatially separated two-level media, thus offering the advantage of experimental compactness overcoming the limitations due to mismatching and unequal efficiencies that can arise in spatially separated memories. The effects of a relative phase change between the atomic levels, as well as of phase noise due to, for example, the presence of spurious electric and magnetic fields are analysed.

Subdiffractive pulses in photonic crystals

Photonic crystals are used to modify the properties of light propagation by introducing photonic band-gaps in radiation spectra (i.e. converting light conductors into insulators) and also by modification of the diffraction of the light. At the crossover point between positive and negative diffraction the zero diffraction point can be expected, giving rise to the effect of self-collimation. We study the subdiffractive pulse propagation by developing the normal form description in the vicinity of the zero diffraction point.