Signal processing by opto-optical interactions between self-localized and free propagating beams in liquid crystals

Pasquazi, Alessia; Alberucci, Alessandro; Peccianti, Marco; Assanto, Gaetano

doi:10.1063/1.2158026

Cited by 79 publications

(48 citation statements)

References 19 publications

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“…The latter response provides a low-power mechanism for nonlinear optics [19] and light localization into self-confined lightbeams, the so-called "nematicons" [20]. Nematicons are bright spatial solitons (solitary waves) which are stable in two transverse dimensions due to the nonlocal response associated with the elastic intermolecular links in the liquid state [21,22]; they support graded-index waveguides able to confine additional (incoherent) signals/beams of different wavelengths as well as powers and profiles [23][24][25][26][27][28][29][30], are robust against refractive index perturbations [31][32][33][34][35] and collisional interactions [36][37][38]. Aided by nematicons, reorientational and electronic nonlinear responses, characterized by distinct time-and power-scales, can synergystically be combined [39,40].…”

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confidence: 99%

Soliton-assisted random lasing in optically-pumped liquid crystals

Perumbilavil

Piccardi

Buchnev

et al. 2016

Applied Physics Letters

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We demonstrate a novel random laser configuration by exploiting the coexistence of optical gain and light self-localization in a reorientational nonlinear medium. A spatial soliton launched by a near-infrared beam in dye-doped nematic liquid crystals enhances and confines stimulated emission of visible light in the optically-pumped gain-medium, yielding random lasing with enhanced features.In random lasers, a disordered distribution of scattering centers provides the required feedback for oscillations in optically amplifying media. In recent years they have attracted a great deal of attention, mainly due to the versatility stemming from cavity-less geometries and the ease of realization [1][2][3][4][5][6][7][8][9]. In liquid crystals, suitable dopants can provide the gain action through optical pumping, while optical birefringence in conjunction with intense fluctuations of the dielectric tensor yield the required recurrent multiple scattering for random resonances to occur [10][11][12][13][14][15][16][17]. In the nematic phase, moreover, liquid crystals are positive uniaxial materials subject to optic axis reorientation under the action of electric fields, either at low or optical frequencies [18]. The latter response provides a low-power mechanism for nonlinear optics [19] and light localization into self-confined lightbeams, the so-called "nematicons" [20]. Nematicons are bright spatial solitons (solitary waves) which are stable in two transverse dimensions due to the nonlocal response associated with the elastic intermolecular links in the liquid state [21,22]; they support graded-index waveguides able to confine additional (incoherent) signals/beams of different wavelengths as well as powers and profiles [23][24][25][26][27][28][29][30], are robust against refractive index perturbations [31-35] and collisional interactions [36][37][38]. Aided by nematicons, reorientational and electronic nonlinear responses, characterized by distinct time-and power-scales, can synergystically be combined [39,40]. Owing to their large numerical aperture [41], nematicon waveguides solitons have also been employed in experiments involving incoherent light generation by fluorescence [42] or amplified spontaneous emission [43], offering a means to better collect and couple the emitted light into optical fibers.In this Letter we demonstrate a novel example of synergy between diverse nonlinear responses: the combination of spatial solitons and random lasing into a "nematicon random laser", whereby a low-power self-confined beam provides a guided-wave landscape for the stimu- * assanto@uniroma3.it lated emission induced by collinear optical pumping of dye-doped nematic liquid crystals (NLC).Several benefits can be expected from adopting such light-induced guided-wave configuration for random lasing. At variance with standard thin film geometries, the thick NLC cell provides an extended volume where optical pumping can produce fluorescence and, in turn, stimulated emission and lasing action via random scattering and feedback. The la...

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confidence: 99%

Soliton-assisted random lasing in optically-pumped liquid crystals

Perumbilavil

Piccardi

Buchnev

et al. 2016

Applied Physics Letters

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“…Once created, nematicons are stable and robust, with the ability to survive defects and perturbations [3][4][5][6][7]. An approach to introducing a defect into nematic liquid crystals (NLC) is to alter the response of the medium to light by introducing a small amount of dye molecules, a technique known as doping [8].…”

Section: Abstract-mentioning

confidence: 99%

Optical path control of solitary waves in dye-doped nematic liquid crystals

Assanto

Skuse

Smyth³

2009

Photon. Lett. Poland

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Abstract-The light-induced steering of spatial optical solitary waves in dye-doped nematic liquid crystals is analyzed. Shining a control beam through the cell results in a boundary mediated change in the orientation of the molecular director, thereby a perturbation in the refractive index of the medium. This re-orientational effect can optically steer spatial solitary waves propagating in an orthogonal plane within the liquid crystal layer. The equations governing this steering are solved using a Lagrangian formulation, resulting in modulation equations for the self-trapped beam path. Very good agreement is obtained between solutions of the modulation equations and numerical solutions of the full governing equations.Nematicons-spatial optical solitary waves in nematic liquid crystals-result from a balance between linear diffraction and nonlinear self-focusing of an input beam in the presence of nonlocality, an effect brought about by the thermal or reorientational response of the material to light [1][2]. Once created, nematicons are stable and robust, with the ability to survive defects and perturbations [3][4][5][6][7]. An approach to introducing a defect into nematic liquid crystals (NLC) is to alter the response of the medium to light by introducing a small amount of dye molecules, a technique known as doping [8]. Dyedoping causes a change in the absorption of the NLC. In specific spectral intervals, an external illumination, i.e. a control beam, impinging onto the medium can perturb the molecular reorientation according to the control beam intensity and/or polarisation [9][10][11]. Whilst nematicons can survive the interaction with such defects, their evolution will be somehow affected, with a change in trajectory. Piccardi, Assanto, Lucchetti and Simoni [11] experimentally observed nematicons propagating in a dye-doped cell where control beams induced refractive defects; they demonstrated solitary wave steering dependent on power, shape and relative position of the external perturbation with respect to the nematicon. They also reported solitary wave refraction and total internal reflection [11]. In this Letter we investigate theoretically the steering of a nematicon by an optically driven refractive index change. Modulation equations describing * E-mail: N.Smyth@ed.ac.uk the beam path are derived and solutions of these equations are found to be in excellent agreement with numerical solutions of the governing equations.Let us consider a linearly polarised, coherent light beam propagating through a cell filled with a dye-doped nematic liquid crystal (DD-NLC), as illustrated in Fig. 1. The input beam is launched so that it initially propagates at an arbitrary angle with respect to the direction z down the cell. The coordinates (x,y) are orthogonal to the z direction. The input beam is polarised with its electric field along x, i.e. it is an extraordinary wave in the NLC equivalent uniaxial. In the bulk NLC the molecules are pre-tilted at an angle θ 0 in the plane (x,z) by an external electric field (voltage)...

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“…Since this reorientational response is nonresonant, refractive index changes can also be produced by voltage(s) applied across the NLC or by finite light beams travelling along y orthogonally to the solitary beam. Localized variations of the refractive index, in either one (interface) or two (defect) dimensions, can cause the beams propagating within the cell to refract, reflect, bend [9,[22][23][24][25][26][27][28][29][30][31]. In the paraxial slowly varying envelope approximation, neglecting birefringent walkoff, the non-dimensional equations governing the evolution of extraordinarily polarised light beams of amplitudes u and v through the NLC are nonlinear Schrödinger (NLS)-type envelopes of the electric fields of the soliton (nematicon [9]) and vortex beams, respectively; F(x,z) indicates a y-uniform refractive index variation across the cell; the parameter Γ measures the elastic response of the medium and in the usual experimental regimes is large, 0(100) [9,33,34]; the parameter q is proportional to the magnitude of the external pre-tilting voltage (bias).…”

Section: Introductionmentioning

confidence: 99%