Muriel Botey

Beam shaping in spatially modulated broad-area semiconductor amplifiers

Summary form only given. Broad area semiconductor (BAS) laser are relevant high conversion light sources despite that the spatial and temporal quality of the emitted beam is relatively low due to the absence of a natural transverse mode selection mechanism [1]. To overcome this serious drawback different solutions have been incorporated in the design, such as external gratings, external injection spatially modulated injection or distributed feedback. In addition, in BAS lasers a modulation instability (or Bespalov-Talanov instability) can occur due to nonlinear focussing, leading to filamentation effects and deteriorating the spatial quality of the laser emission. In absence of cavity mirrors, planar semiconductor structures can act as light amplifiers undergoing however analogue disadvantages.In the present work, we study a simple and effective new mechanism to improve the spatial beam quality of a planar semiconductor amplifier configuration. We consider a two-dimensional modulation, which can feasibly be realized by a periodical grid of electrodes for the electrical pump of the semiconductor, as illustrated in Fig. 1.a. The main result obtained is that such a micro-modulation of the spatial pump profile on a spatial scale of the order of several wavelengths, can indeed lead to a substantial improvement of the spatial quality of the amplified beam on a large spatial scale, see Fig. 1.b. The quantitative analysis of the spatial filtering is performed by numerical integration of a paraxial propagation model derived from [2,3], and on analytical estimations. Previous studies of wave propagation in media with spatially modulated Gain/Loss (GL) profiles show that a periodic modulation of GL on a wavelength scale can lead to particular beam propagation effects, such as self-collimation, spatial (angular) filtering, or beam focalisation [4]. In those works a purely GL modulation has been considered; however, in semiconductor media due to the linewidth enhancement factor, hfactor, a periodical spatial pump distribution causes a combined Gain and refraction Index Modulation (GIM). Hence, the aim of the present paper, performed under realistic parameters and conditions, is to demonstrate how the angular spectrum of the radiation from a spatially modulated GIM BAS amplifier becomes narrower while propagating and being amplified. The study predicts that the normalized beam quality factor - M 2 factor- can reduce almost to unity indicating that the BAS amplifier output becomes perfectly Gaussian for even strongly random initial input beam profiles, for a single-pass propagation length on the order of millimeters, see Fig. 1.c. Beyond the present report, this new technique could be implemented to improve the spatial quality of emission of BAS lasers.

Double fitting of Maker fringes to characterize near-surface and bulk second-order nonlinearities in poled silica

An experimental analysis of the distribution and thickness of the bulk nonlinearity induced in poled silica is reported. The second-order susceptibility decreases exponentially from the anodic interface. Maker fringe patterns showing a double structure are interpreted in relation to the presence of two nonlinear profiles, one concentrated near the anodic surface and another extending into the bulk of the sample. The Maker fringe theory is properly generalized and a double fitting technique reproducing well the experimental results is used to characterize the induced nonlinearities. The dependence of the second-harmonic signal on the poling temperature is given, which is different from that of sol-gel silica.

Suppression of modulation instability in broad area semiconductor amplifiers

We show that a two-dimensional periodic modulation of the pump profile (modulation both along and perpendicular to the optical axis) can suppress the modulation instability in broad emission area semiconductor amplifiers. In the case of a realistic finite-width amplifier the modulation instability can be completely eliminated.

Flat lensing by periodic loss-modulated materials

We propose a flat lensing effect using a periodic loss-modulated material. In particular, we consider a two-dimensional square and rhombic arrangement of lossy cylinders embedded in a host media with the same refractive index. The effect is predicted by the dispersion curves obtained by a coupled mode expansion of Maxwell equations and by numerical beam propagation experiments. From both analytical and numerical studies, we show that, for a range of frequencies, light beams undergo negative diffraction on propagation through the loss-modulated medium, providing a window of high transmission. The phase shifts accumulated by negative diffraction within the structure are then compensated by normal diffraction, leading to substantial focalization beyond it.

Beam shaping in two-dimensional metallic photonic crystals

We study light beam propagation in periodic metallic nanostructures—metallic photonic crystals (MPhCs). In particular, we consider a two-dimensional rhombic array of metallic cylinders embedded in air and explore its ability to tailor spatial propagation of light beams. We show that the structure supports self-collimated propagation and negative (anomalous) diffraction. In this later case, flat lensing is observed, leading to the focusing of beams behind the MPhCs. Moreover, the anisotropic attenuation of light provided by the structure enables spatial filtering of noisy beams.

Light generation at the anomalous dispersion high energy range of a nonlinear opal film

We study experimentally and theoretically light propagation and generation at the high energy range of a close-packed fcc photonic crystal of polystyrene spheres coated with a nonlinear material. We observe an enhancement of the second harmonic generation of light that may be explained on the basis of amplification effects arising from propagation at anomalous group velocities. Theoretical calculations are performed to support this assumption. The vector KKR method we use allows us to determine, from the linear response of the crystal, the behavior of the group velocity in our finite photonic structures when losses introduced by absorption or scattering by defects are taken into account assuming a nonzero imaginary part for the dielectric constant. In such structures, we predict large variations of the group velocity for wavelengths on the order or smaller than the lattice constant of the structure, where an anomalous group velocity behavior is associated with the flat bands of the photonic band structure. We find that a direct relation may be established between the group velocity reduction and the enhancement of a light generation processes such as the second harmonic generation we consider. However, frequencies for which the enhancement is found, in the finite photonic crystals we use, do not necessarily coincide with the frequencies of flat high energy bands.

Numerical and experimental demonstration of a wavelength demultiplexer design by point-defect cavity coupled to a tapered photonic crystal waveguide

We propose and experimentally demonstrate a demultiplexer with point-defect resonators and a reflection feedback mechanism in a photonic crystal waveguide (PCW). A tapered PCW has been chosen as the necessary reflector, which enhances the drop efficiency. Due to the variation of the single-mode waveguide width of the tapered PCW, spatial alteration of the effective refractive index can be achieved. This phenomenon is used to reflect back the forward propagating wave which is then coupled again to the drop channels via the resonators. High transmission efficiency to the dropout channels is numerically predicted by calculations, either in two- and three-dimensional models, and analytically described by a coupled-mode theory. Moreover, an experimental realization in the microwave regime provides confirmation that the targeted wavelengths can be properly transmitted at the drop channels with low crosstalk and relatively high efficiencies.

Acoustically penetrable sonic crystals based on fluid-like scatterers

We propose a periodic structure that behaves as a fluid–fluid composite for sound waves, where the building blocks are clusters of rigid scatterers. Such building-blocks are penetrable for acoustic waves, and their properties can be tuned by selecting the filling fraction. The equivalence with a fluid–fluid system of such a doubly periodic composite is tested analytical and experimentally. Because of the fluid-like character of the scatterers, sound structure interaction is negligible, and the propagation can be described by scalar models, analogous to those used in electromagnetics. As an example, the case of focusing of evanescent waves and the guided propagation of acoustic waves along an array of penetrable elements is discussed in detail. The proposed structure may be a real alternative to design a low contrast and acoustically penetrable medium where new properties as those shown in this work could be experimentally realized.

Enhanced transmission band in periodic media with loss modulation

We study the propagation of waves in a periodic array of absorbing layers. We report an anomalous increase of wave transmission through the structure related to a decrease of the absorption around the Bragg frequencies. The effect is first discussed in terms of a generic coupled wave model extended to include losses, and its predictions can be applied to different types of waves propagating in media with periodic modulation of the losses at the wavelength scale. The particular case of sound waves in an array of porous layers embedded in air is considered. An experiment designed to test the predictions demonstrates the existence of the enhanced transmission band.

Unlocked evanescent waves in periodic structures

We predict the existence of evanescent modes with unlocked phases in two-dimensional (2D) dielectric periodic structures. Contrary to what is known for one-dimensional structures, where evanescent fields lock to the host modulation, we show that in 2D systems a new class of evanescent modes exists with an unlocked real part of the wave vector. Hence, beams constructed from such unlocked evanescent waves can exhibit spatial effects. A significant focalization of a beam propagating within the band gap of a flat photonic crystal slab is also shown. The predicted phenomenon is expected to be generic for spatially modulated materials.