We propose the stabilization of the output beam of Broad Area Semiconductor (BAS) amplifiers through the introduction of a spatially periodic modulated potential. We show that a periodic modulation of the pump profile in transverse and longitudinal directions, under certain ‘resonance’ condition, can solve two serious problems of BAS amplifiers (and possibly lasers), which are (i) the lack of an intrinsic spatial mode selection mechanism in linear amplification regimes and (ii) the modulation instability (also called Bespalov-Talanov instability) in nonlinear regimes. The elimination of these two drawbacks can significantly improve the spatial quality of the emitted beam in BAS amplifiers.
Spontaneous pattern formation in a variety of spatially extended nonlinear systems always occurs through a modulation instability, sometimes called Turing instability: the homogeneous state of the system becomes unstable with respect to growing modulation modes. Therefore, the manipulation of the modulation instability is of primary importance in controlling and manipulating the character of spatial patterns initiated by that instability. We show that a spatio-temporal periodic modulation of the potential of spatially extended systems results in a modification of its pattern forming instability. Depending on the modulation character the instability can be partially suppressed, can change its spectrum (for instance the long wave instability can transform into short wave instability), can split into two, or can be completely eliminated. The latter result is of special practical interest, as it can be used to stabilize the intrinsically unstable system. The result bears general character, as it is shown here on a universal model of the Complex Ginzburg-Landau equation in one and two spatial dimensions (and time). The physical mechanism of the instability suppression can be applied to a variety of intrinsically unstable dissipative systems, like self-focusing lasers, reaction-diffusion systems, as well as in unstable conservative systems, like attractive Bose Einstein condensates.
We report the propagation of high-intensity sound beams in a sonic crystal, under self-collimation or reduced-divergence conditions. The medium is a fluid with elastic quadratic nonlinearity, where the dominating nonlinear effect is harmonic generation. The conditions for the efficient generation of narrow, nondiverging beam of second harmonic are discussed. Numerical simulations are in agreement with the analytical predictions made, based on the linear dispersion characteristics in modulated media and the nonlinear interaction in a quadratic medium under phase matching conditions.
Radziunas, M.; Herrero, R.; Botey, M.; Staliunas, K. Journal of the Optical Society of America B. Optical physics Vol. 32, num. 5, p. 993-1000 DOI: 10.1364/JOSAB.32.000993 Date of publication: 2015-05-01 Journal article
We perform a detailed theoretical analysis of the far-field narrowing in broad-area edge-emitting semiconductor amplifiers that are electrically injected through contacts periodically modulated in both longitudinal and transverse directions. The beam propagation properties within the semiconductor amplifier are explored by a (1 + 2)-dimensional traveling wave model and its coupled-mode approximation. Assuming a weak field regime, we analyze the impact of different parameters and modulation geometry on the narrowing of the principal far-field component.
En este trabajo hemos demostrado numérica y experimentalmente la posibilidad de obtener focalización en reflexión mediante el uso de espejos planos en diferentes configuraciones como en estructuras dieléctricas de multicapas o redes con periodicidades por debajo de la longitud de onda (subwavelength gratings SWG). Una de las propiedades importantes de estos espejos es que mantienen una invariancia transversal lo que favorece su posible uso en elementos focalizadores sub-micrométricos como dispositivos en circuitos fotónicos integrados.El principio físico en que se basan estos dispositivos es el de difracción anómala, similar a la idea de lentes planas o super-lentes. En la primera estructura propuesta, espejo con modulación espacial del periodo (chirped), demostramos el principio básico de funcionamiento y la posibilidad de obtener focalización en reflexión con superficies planas. Para aumentar la estabilidad y optimizar su funcionamiento hemos propuesto un dispositivo optimizado que conjuga espejos chirped con defectos en la cavidad, obteniendo distancias focales más largas. Finalmente hemos demostrado numérica y experimentalmente un nuevo diseño con el que podemos focalizar en reflexión a distancias más largas.En esta tesis también se proponen redes de difracción con periodicidades por debajo de la longitud de onda. La focalización en reflexión en incidencia normal puede obtenerse cuando se tiene en cuenta el retardo de fase entre las imágenes reflejadas procedentes de los procesos de dispersión primario y secundario en la red. Dicha configuración puede extenderse a estructuras con simetría 2D cuadrada y estructuras 2D cuasi-simétricas permitiendo obtener focalización en reflexión para más configuraciones. El uso de dos SWG paralelas permite obtener, además de la focalización en reflexión, un desplazamiento negativo del haz (efecto Goos-Hanchen). Este efecto es debido a la superposición de los desplazamientos laterales originados en dos soluciones contra-propagantes que dan como resultado una pendiente del desplazamiento lateral negativa con respecto al ángulo de incidencia, lo que da lugar a la focalización por reflexión.Todos las estructuras propuestas en esta tesis carecen de eje óptico, lo cual representa una innovación importante y abre la posibilidad de utilizarlas para fabricación de dispositivos ultra-compactos. Una de las aplicaciones que podría tener esta estructura es la de propagar ( a modo de guía) y focalizar haces localizados espacialmente sin dispersión espacial.
We propose a simple realistic two-dimensional complex parity-time-symmetric photonic structure that is described by a non-Hermitian potential but possesses real-valued eigenvalues. The concept is developed from basic physical considerations to provide asymmetric coupling between harmonic wave components of the electromagnetic field. The structure results in a nonreciprocal chirality and asymmetric transmission between in- and out-coupling channels into the structure. The analytical results are supported by a numerical study of the Bloch-like mode formations and calculations of a realistic planar semiconductor structure.
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.
An approach to switching between different patterns of light beams transmitted through the woodpile photonic crystals filled with liquid crystals is proposed. The phase transition between the nematic and isotropic liquid crystal states leads to an observable variation of the spatial pattern transmitted through the photonic structure. The transmission profiles in the nematic phase also show polarization sensibility due to refractive index dependence on the field polarization. The experimental results are consistent with a numerical calculation by Finite Difference Time Domain method.
The enhancement of sound inside a two dimensional exponentially chirped crystal during the soft reflections of waves is experimentally and theoretically explored in this work. The control of this enhancement is achieved by a gradual variation of the dispersion in the system by means of a chirp of the lattice constant. The sound enhancement is produced at some planes of the crystal in which the wave is softly reflected due to a progressive slowing down of the sound wave. We find that the character of the sound enhancement depends on the function of the variation of dispersion, i.e., on the function of the chirp. A simple coupled mode theory is proposed to find the analytical solutions of the sound wave enhancement in the exponentially chirped crystal. Harmonic and time domain numerical simulations are performed to interpret the concept of the soft reflections, and to check the analytically calculated field distributions both in good agreement with experiments. Specially we obtain stronger sound enhancement than in linearly chirped crystals. This sound enhancement could motivate applications in energy harvesting, e.g., to increase the efficiency of detectors and absorbers. (C) 2014 Author(s).
We study the formation of gap solitons in the presence of a parametric pump. It is shown that a parametric pump can stabilize stationary solitons continuously emitting dispersive waves. The resonant interactions of the radiation and the solitons are studied and it is shown that the solitons can be effectively controlled by the radiation. In particular it is shown that the solitons can collide or get pinned to inhomogeneities due to the interactions mediated by the resonant radiation.
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.
We experimentally demonstrate the recently predicted effect of near-field focusing for light beams from flat dielectric subwavelength gratings (SWGs). This SWGs were designed for visible light 532 nm and fabricated by direct laser writing in a negative photoresist, with the refractive index n = 1.5 and the period d = 314 nm. The laterally invariant gratings can focus light beams without any optical axis to achieve the transversal invariance. We show that focal distances can be obtained up to 13 mu m at normal reflection for TE polarization.
Turduev, M.; Botey, M.; Herrero, R.; Kurt, H.; Staliunas, K.; Giden, I. IEEE Photonics Conference p. 164-165 DOI: 10.1109/IPCon.2014.6995263 Presentation's date: 2014-10-15 Presentation of work at congresses
We propose for the first time a simple realization of a two-dimensional Parity-Time symmetric hexagonal shaped photonic structure composed of honeycomb lattice. The structure has a symmetric periodic modulation of the refractive index on the wavelength scale, which is combined with an anti-symmetric gain/loss distribution on the same scale. That leads to non-reciprocal light coupling at resonant frequencies. The design of the realistic structure is based on a simple physical concept: alternating low index cylinders with gain and loss in a honeycomb configuration, embedded in a higher index dielectric background.
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.
VALCARCEL, G.; Martínez-Quesada, M.; Staliunas, K. Philosophical transactions of the Royal Society A. Mathematical physical and engineering sciences Vol. 372, num. 2027 DOI: 10.1098/rsta.2014.0008 Date of publication: 2014-09-22 Journal article
We present a review, together with new results, of a universal forcing of oscillatory systems, termed 'rocking', which leads to the emergence of a phase bistability and to the kind of pattern formation associated with it, characterized by the presence of phase domains, phase spatial solitons and phase-bistable extended patterns. The effects of rocking are thus similar to those observed in the classic 2 : 1 resonance (the parametric resonance) of spatially extended systems of oscillators, which occurs under a spatially uniform, time-periodic forcing at twice the oscillations' frequency. The rocking, however, has a frequency close to that of the oscillations (it is a 1 : 1 resonant forcing) and hence is a good alternative to the parametric forcing when the latter is inefficient (e. g. in optics). The key ingredient is that the rocking amplitude is modulated either in time or in space, such that its sign alternates (exhibits p-phase jumps). We present new results concerning a paradigmatic nonlinear optical system (the two-level laser) and show that phase domains and dark-ring (phase) solitons replace the ubiquitous vortices that characterize the emission of free-running, broad area lasers.
The control of spatial propagation properties of narrow light beams such as divergence, focusing or imaging are main objectives in optics and photonics. In this letter, we propose and demonstrate experimentally a flat focusing mirror, based on an especially designed dielectric structure without any optical axis. More generally, it also enables imaging any light pattern in reflection. The flat focusing mirror with a transversal invariance can largely increase the applicability of structured photonic materials for light beam propagation control in small-dimension photonic circuits.
Herrero, R.; Kumar, S.; Radziunas, M.; Botey, M.; Staliunas, K. International Conference on Transparent Optical Networks p. 1-3 DOI: 10.1109/ICTON.2014.6876275 Presentation's date: 2014-09-08 Presentation of work at congresses
The spatial modulation of pump in broad emission area semiconductor amplifiers has two important advantages
in this kind of devices. A 2-dimensional periodic modulation of the pump profile introduces by one side, a filtering effect and an improvem ent of the beam quality. Moreover, the spatial modulation changes the effective diffraction inside the material and reduces the modulation instability generated by the self-focusing effects and the material nonlinearity.
A negative Goos-Hänchen (GH) shift of light beam reflecting from planar microphotonic structures consisting of two parallel dielectric subwavelength diffraction gratings is predicted. The phenomenon occurs because the energy flow of the mode guided in the subwavelength structure is predominantly directed in the opposite direction to the energy flow of the incident beam. The numerical calculations show that the negative GH shift can reach approximately six times that of the wavelength in realistic structures, so that the phenomenon can be experimentally observed. In addition to the negative lateral shift, the focusing of the beam in reflection from the grating is also observed as a secondary effect.
We explore, theoretically and experimentally, the spatial (angular) filtering of two-dimensional light beams by longitudinally chirped axisymmetric photonic microstructures. The structures comprise a set of planes of concentric rings with the separation between the plates smoothly varying along the propagation direction. Axisymmetric structures were recorded in a bulk of glass, in which the refractive index has been modulated using tightly focused femtosecond laser pulses. We show that the spatial filtering recently shown in nonchirped axisymmetric structures can be substantially improved by the chirp: the angular range of filtering was increased approximately two times. The numerical study reveals that the filtering efficiency can be strongly increased using the longer and larger index contrast axisymmetric photonic structures.
Botey, M.; Herrero, R.; Turduev, M.; Zhao, D.; Giden, I.; Kurt, H.; Staliunas, K. International Conference on Transparent Optical Networks p. 1-4 DOI: 10.1109/ICTON.2014.6876621 Presentation's date: 2014-07-09 Presentation of work at congresses
We propose a simple realization of a 2-dimensonal (2D) PT-symmetric photonic structure: a honeycomb arrangement of alternating cylinders with gain or loss in a dielectric background. The structure provides a periodic modulation of the refractive index on the wave length scale, which combined with an anti-symmetric gain/loss distribution on the same scale, leads to non-reciprocal light coupling at resonance frequencies. Numerical simulations show asymmetric right-left transmission of light beams.
La recerca desenvolupada en el marc d'aquesta tesi doctoral és un estudi teòric, numèric i experimental de la modificació de la forma de feixos de llum (filtratge espacial, col·limació i focalització) en el rang visible de freqüències utilitzant estructures de cristall fotònic. Els cristalls fotònics (CFs) són materials amb una modulació periòdica de l'índex de refracció en l'escala de la longitud d'ona, i són principalment coneguts per les seves propietats relacionades amb la dispersió temporal. Tot i això, la dispersió espacial també pot ser modificada mitjançant CFs, fet que permet controlar les propietats espacials de feixos monocromàtics de llum.En la primera part de la tesi, mostrem experimentalment el fet que certes modificacions de la dispersió espacial en CFs poden donar lloc a filtratge espacial (angular) de feixos de llum. L'estudi es focalitza en la millora de l'eficiència del filtratge espacial mitjançant la introducció de "chirp" (la variació del període longitudinal de l'estructura) en el CF. A més, per tal d'incrementar l'efecte considerem diferents estructures i materials. El treball presentat en aquesta tesi doctoral acosta a la realitat la creació d'una nova generació de filtres espacials per a circuits micro-fotònics i micro-dispositius.La segona part d'aquest estudi se centra en l'anàlisi teòric, numèric i experimental de la formació de dispersió espacial negativa en CFs, la gual dóna lloc a efectes de col·limació i focalització un cop travessat el CF. Les idees desenvolupades en aquesta tesi doctoral també són aplicables a sistemes amb pèrdues, en particular a CFs metàl·lics. Els resultats de les simulacions mostren l'existència d'ambdós efectes, filtratge espacial i focalització, en CFs metàl·lics.
The research developed in the framework of this PhD thesis is a theoretical, numerical and experimental study of light beam shaping (spatial filtering, beam collimation and focusing) in the visible frequency range using photonic crystal structures. Photonic crystals (PhCs) are materials with periodic, spatially modulated refractive index on the wavelength scale. They are primarily known for their chromatic dispersion properties. However, they can also modify the spatial dispersion, which allows managing the spatial properties of the monochromatic light beams.
In the first part of my thesis we experimentally show that particular spatial dispersion modification in PhCs can lead to spatial (angular) filtering of light beams. The study is focused on the spatial filtering efficiency improvement by introducing chirp (the variation of longitudinal period of the structure) in the crystal structure. Additionally, to enhance the effect, we consider different geometries and materials. The work presented in this PhD thesis brings closer to reality the creation of a new generation spatial filters for micro-photonic circuits and micro-devices.
The second part of the study is devoted to the theoretical, numerical and experimental analysis of the formation of negative spatial dispersion in PhCs, which gives rise to collimation and focusing effects behind the PhCs. The ideas developed in my PhD also work in lossy systems, in particular in metallic PhCs. The simulation results for metallic PhCs are presented, in which both effects- spatial filtering and beam focusing, are shown.
La recerca desenvolupada en el marc d'aquesta tesi doctoral és un estudi teòric, numèric i experimental de la modificació de la forma de feixos de llum (filtratge espacial, col·limació i focalització) en el rang visible de freqüències utilitzant estructures de cristall fotònic. Els cristalls fotònics (CFs) són materials amb una modulació periòdica de l'índex de refracció en l'escala de la longitud d'ona, i són principalment coneguts per les seves propietats relacionades amb la dispersió temporal. Tot i això, la dispersió espacial també pot ser modificada mitjançant CFs, fet que permet controlar les propietats espacials de feixos monocromàtics de llum. En la primera part de la tesi, mostrem experimentalment el fet que certes modificacions de la dispersió espacial en CFs poden donar lloc a filtratge espacial (angular) de feixos de llum. L'estudi es focalitza en la millora de l'eficiència del filtratge espacial mitjançant la introducció de "chirp" (la variació del període longitudinal de l'estructura) en el CF. A més, per tal d'incrementar l'efecte considerem diferents estructures i materials. El treball presentat en aquesta tesi doctoral acosta a la realitat la creació d'una nova generació de filtres espacials per a circuits micro-fotònics i micro-dispositius. La segona part d'aquest estudi se centra en l'anàlisi teòric, numèric i experimental de la formació de dispersió espacial negativa en CFs, la gual dóna lloc a efectes de col·limació i focalització un cop travessat el CF. Les idees desenvolupades en aquesta tesi doctoral també són aplicables a sistemes amb pèrdues, en particular a CFs metàl·lics. Els resultats de les simulacions mostren l'existència d'ambdós efectes, filtratge espacial i focalització, en CFs metàl·lics.
Recently the beam focusing in reflection from chirped dielectric mirror has been proposed and demonstrated, where the negative (anomalous) diffraction is responsible for this flat mirror focusing. For a strong focusing performance (large focal distance), a wide angular range of strong (negative) angular dispersion is required. We show that a defect layer in the dielectric mirror (one layer is of a double size), can increase the angular dispersion, and thus improve the focusing performance. By introducing a defect layer in the chirped mirror, the focal distances can be increase from 12 µm up to 22 µm in a specific, calculated, structure, as our numerical integration show.
The study of spatial dispersion of two-dimensional complex crystals, with periodic modulations of both gain-loss and the refractive index, reveals simultaneous nondiffractive-nondiffusive light propagation. Narrow light beams and light patterns propagate without dispersion while amplified through the complex material. We determine and explore nondiffractive-nondiffusive regimes for collinear and noncollinear propagation.
Cheng, Y.; Peckus, M.; Kicas, S.; Trull, J.; Cojocaru, C.; Vilaseca, R.; Drazdys, R.; Staliunas, K. Conference on Lasers and Electro-Optics: Science & Innovations p. 1-2 DOI: 10.1364/CLEO_SI.2014.SW1M.7 Presentation's date: 2014-06-08 Presentation of work at congresses
A novel application of chirped dielectric mirrors for narrow beam focalization is proposed and demonstrated numerically and experimentally. Analogy to temporal dispersion compensation by chirped dielectric mirrors is discussed.
We propose and experimentally show the mechanism of beam super-collimation by axisymmetric photonic crystals, specifically by periodic (in propagation direction) structure of layers of concentric rings. The physical mechanism behind the effect is an inverse scattering cascade of diffracted wave components back into on-and near-axis angular field components, resulting in substantial enhancement of intensity of these components. We explore the super-collimation by numerical calculations and prove it experimentally. We demonstrate experimentally the axial field enhancement up to 7 times in terms of field intensity.
We report Bessel-like beam formation of acoustic waves by means of an axisymmetric grating of rigid tori. The results show that the generated beam pattern is similar to that of Bessel beams, characterized by elongated non-diffracting focal spots. A multiple foci structure is observed, due to the finite size of the lens. The dependence of the focal distance on the frequency is also discussed, on the basis of an extended grating theory. Experimental validation of acoustic Bessel-like beam formation is also reported for sound waves. The results can be generalized to wave beams of different nature, as optical or matter waves.
We show that periodic distributions of gain or losses on the wavelength scale allow managing spatial diffraction of light beams, with no index contrast. It has been recently predicted that such artificial periodic structures, analogous to Photonic Crystals (PhCs), would also hold the novel spatial beam propagation effects reported for PhCs such as subdiffraction propagation, self-collimation, spatial filtering or beam focusing by a lens with flat interfaces. In particular, we consider an ideal periodic 2-dimensional (2D) arrangement of lossy cylinders embedded in air. We analytically show that this loss distribution affects diffraction. Indeed, a significant focusing behind a thin flat-flat crystal slab is observed, following the estimation of anomalous spatial dispersion for specific frequency ranges. Besides, close to the edges of the first Brillouin Zone, the light intensity map of a Gaussian beam exiting the lossy structure exhibits a high transmission windows instead of the transmission stop band expected for PhCs. This results from the strong anisotropic attenuation provided by the loss periodicity. Finally, we also consider a more realistic system with combined modulations of refractive index and losses: a 2D metallic photonic crystal (MPhC). We demonstrate that MPhCs also support selfcollimation and focusing, being such effects associated to zero and negative diffraction respectively. Finally, due to the anisotropic attenuation of light, the structure is also able to spatially filter noisy beams.
We simulate and analyze how beam quality improves while being amplified in edge emitting broad area semiconductor amplifiers with a periodic structuring of the electrical contacts, in both longitudinal and lateral directions. A spatio-temporal traveling wave model is used for simulations of the dynamics and nonlinear interactions of the optical fields, induced polarizations and carrier density. In the case of small beam amplification, the optical field can be expanded into few Bloch modes, so that the system is described by a set of ODEs for the evolution of the mode amplitudes. The analysis of such model provides a deep understanding of the impact of the different parameters on amplification and on spatial (angular) filtering of the beam. It is shown that under realistic parameters the two-dimensional modulation of the current can lead not only to a significant reduction of the emission divergence, but also to an additional amplification of the emitted field
We predict that narrow beams, reflecting from flat subwavelength diffraction gratings, can focus. The effect is shown for the beams of electromagnetic radiation; however, it should be observable for beams of waves of arbitrary nature (microwaves, surface plasmons, and acoustic and mechanical waves). We present analytical estimations of the focusing performance obtained by multiple scattering calculations and demonstrate the focusing effect numerically for an optical system (reflections from an array of dielectric cylinders), using the finite-difference time-domain calculations.
Kumar, N.P.; Maigyte, L.; Botey, M.; Herrero, R.; Staliunas, K. Journal of the Optical Society of America B. Optical physics Vol. 31, num. 4, p. 686-690 DOI: 10.1364/JOSAB.31.000686 Date of publication: 2014-03-05 Journal article
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.
We investigate radiation of the solitary waves in the first band gap of the waveguide array with a defocusing nonlinearities of different types (Kerr nonlinearity and saturating nonlinearity). We confirm recent findings that gap solitons (GSs) are unstable for their eigenfrequencies around the middle of the band gap for Kerr nonlinearity. The instability is mediated by four-wave mixing process and appears in the form of radiation of solitons into mode continua of the upper and lower bands. We find that this soliton radiation is reduced (and even suppressed completely) in case of a saturating nonlinearity, resulting in substantial stabilization of the GSs.
Purlys, V.; Maigyte, L.; Gailevicius, D.; Peckus, M.; Malinauskas, M.; Gadonas, R.; Staliunas, K. Optics letters Vol. 39, num. 4, p. 929-932 DOI: 10.1364/OL.39.000929 Date of publication: 2014-02-15 Journal article
We propose and show experimentally axisymmetric spatial (angular) filtering of two-dimensional light beams by axisymmetric photonic microstructures. Such three-dimensional microstructures (similar to photonic crystals), in gapless configuration, were recorded in bulk of glass, where the refractive index has been point-by-point modulated using tightly focused femtosecond laser pulses. Axisymmetric angular filtering of approximately 25 mrad is demonstrated experimentally. (C) 2014 Optical Society of America
Murad, M.; Sorel, M.; Herrero, R.; Botey, M.; Marsh, J.; Staliunas, K. High Power Diode Lasers and Systems Conference p. 8-9 DOI: 10.1109/HPD.2013.6706592 Presentation's date: 2013-10-16 Presentation of work at congresses
It is proposed to control the beam quality of broad-area semiconductor devices by two-dimensional structuring enforcing phase coherence across the device. First results based on MMI couplers are presented.
We investigate beam shaping in broad area semiconductor amplifiers induced by a 2-dimensional (longitudinal and lateral) periodic modulation of the pump on a scale of several microns. The study is performed by solving numerically a (2¿+¿1)-dimensional model for the semiconductor amplifier. We show that, under realistic conditions, the anisotropic gain induced by the pump periodicity can show narrow angular profile of enhanced gain of less than 1°, providing an intrinsic filtering mechanism and eventually improving the spatial beam quality.
Kumar, N.P.; Herrero, R.; Botey, M.; Staliunas, K. Journal of the Optical Society of America B. Optical physics Vol. 30, num. 10, p. 2684-2688 DOI: 10.1364/JOSAB.30.002684 Date of publication: 2013-09-23 Journal article
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
Missatge de l'editor: This paper was published in Journal of the Optical Society of America B and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/josab/abstract.cfm?uri=josab-30-10-2684. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.
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.
We experimentally demonstrate full two-dimensional focalization of light beams at visible frequencies by a three-dimensional woodpile photonic crystal. The focalization (the flat lensing) with focal distances of the order of 50–70 µm is experimentally demonstrated. Experimental results are compared with numerical calculations and interpreted by harmonic expansion studies.
Missatge de l'editorial: "This paper was published in Optical letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/ol/fulltext.cfm?uri=ol-38-14-2376&id=258441. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law."
Botey, M.; Kumar, N.P.; Herrero, R.; Maigyte, L.; Pico, R.; Staliunas, K. International Conference on Transparent Optical Networks p. 1-4 DOI: 10.1109/ICTON.2013.6602994 Presentation's date: 2013-06-26 Presentation of work at congresses
We show how a periodic modulation of loss on the wavelength scale provides negative diffraction propagation. In particular, we consider a 2D square arrangement of lossy cylinders embedded in air, with no index contrast. The phase-shifts accumulated in propagation trough such a structure, with flat-flat interfaces, are then compensated by normal diffraction in air leading to a substantial focalization of a light beam behind the crystal. The transmitted light intensity map exhibits a window of high transmission as a result of anisotropic loss provided by the periodicity. The effect is accounted buy the dispersion curves obtained by a coupled mode expansion of Maxwell equations and numerical experiments. The predicted phenomenon is expected to be generic for spatially modulated materials, and indeed we show that the results can be extended to acoustics.
Maigyte, L.; Cojocaru, C.; Purlys, V.; Trull, J.; Gailevicius, D.; Peckus, M.; Malinauskas, M.; Staliunas, K. International Conference on Transparent Optical Networks Presentation's date: 2013-06-25 Presentation of work at congresses
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.
A novel use of chirped mirrors for narrow beam focalization is proposed. Focusing in reflection from flat chirped mirror has been demonstrated numerically and experimentally. Analogy to temporal dispersion mirrors is discussed.
Maigyte, L.; Cojocaru, C.; Purlys, V.; Trull, J.; Gailevicius, D.; Peckus, M.; Malinauskas, M.; Staliunas, K. International Conference on Transparent Optical Networks p. We.C6.1-1-We.C6.1-4 DOI: 10.1109/ICTON.2013.6602990 Presentation's date: 2013-06 Presentation of work at congresses
We investigate spatial propagation effect behind the Photonic Crystal under particular dispersion conditions, which lead to a near field focusing. We analyze and experimentally demonstrate full two-dimensional near field focusing of light beams at visible frequencies behind the flat three-dimensional woodpile photonic crystal. Experimental results correspond well with numerical FDTD calculations as well as with the mode expansion studies. The focusing distance of 50 - 70 µm behind the crystal is obtained.
Maigyte, L.; Cojocaru, C.; Purlys, V.; Trull, J.; Gailevicius, D.; Peckus, M.; Malinauskas, M.; Staliunas, K. European Conference on Lasers and Electro-Optics - International Quantum Electronics Conference Presentation's date: 2013-05-15 Presentation of work at congresses
In this paper we investigate spatial propagation effect behind the Photonic Crystal (PhC) with nontrivial dispersion properties, which leads to a near field focusing. We analyze and experimentally demonstrate full two-dimensional focus formation of light beams at visible frequencies behind the three-dimensional woodpile photonic crystal. Experimental results correspond well with numerical FDTD calculations as well as with the mode expansion studies. The focusing distance of 60-80 micrometers behind the crystal is obtained.
Maigyte, L.; Purlys, V.; Gailevicius, D.; Peckus, M.; Malinauskas, M.; Staliunas, K. European Conference on Lasers and Electro-Optics - International Quantum Electronics Conference Presentation's date: 2013-05-15 Presentation of work at congresses
We show, theoretically and experimentally that chirped photonic crystals (longitudinal period varies along the propagation direction) can provide a substantial spatial (angular) filtering of the light beams and can reach efficiencies up to 50%.
Cheng, Y.; Kicas, S.; Peckus, M.; Trull, J.; Cojocaru, C.; Drazdys, R.; Vilaseca, R.; Staliunas, K. Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference Presentation's date: 2013-05-13 Presentation of work at congresses
We predict generally that narrow beams can focus in reflection from the flat interface photonic structures, and we demonstrate the effect experimentally in particular realization,i.e. in reflection from one-dimensional chirped mirror with flat surface.
Radziunas, M.; Herrero, R.; Botey, M.; Staliunas, K. European Conference on Lasers and Electro-Optics - International Quantum Electronics Conference DOI: 10.1109/CLEOE-IQEC.2013.6800767 Presentation's date: 2013-05-12 Presentation of work at congresses
Broad area (BA) lasers and amplifiers are robust, compact and highly efficient devices for generation of high power beams. These beams, however, suffer from a poor spatial and temporal beam quality. The stabilization and shaping of the optical beams in BA devices can be achieved, for example, by injection of the optical beam at some angle to the longitudinal axis , by the external optical feedback from the corresponding off-axis mirror , or by special geometry of the device .
Herrero, R.; Botey, M.; Radziunas, M.; Staliunas, K. European Conference on Lasers and Electro-Optics - International Quantum Electronics Conference DOI: 10.1109/CLEOE-IQEC.2013.6801816 Presentation's date: 2013-05-12 Presentation of work at congresses
We propose and analyze a beam-shaping mechanism that in broad-area semiconductor amplifiers occurs due to spatial pump modulation on a micrometer scale. The study, performed under realistic parameters and conditions, predicts a spatial (angular) filtering of the radiation, which leads to a substantial improvement of the spatial quality of the beam during amplification. Quantitative analysis of spatial filtering performance is presented based on numerical integration of the paraxial propagation model and on analytical estimations.
Staliunas, K. European Conference on Lasers and Electro-Optics - International Quantum Electronics Conference p. 1 DOI: 10.1109/CLEOE-IQEC.2013.6801835 Presentation's date: 2013-05 Presentation of work at congresses
Modulation Instability (MI) is at the basis of spontaneous pattern formation in many nonlinear spatially extended systems in Nature, technologies, and in everyday live. In spite of variety of spatial patterns in different systems, the very onset of a spatio-temporal dynamics, the breaking of initial spatial and temporal symmetry, is initiated by MI. The said is valid for dissipative nonlinear systems, where dissipative patterns set in, but also for conservative systems. The examples in latter case ranges from the filamentation of light in Kerr-nonlinear media, instabilities of Bose condensates with attractive interactions, to perhaps, the recently much discussed formation of the “rogue waves”.