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Employing several nematic liquid crystal mixtures, we investigate how the thermo-optic response of nonlinear birefringent soft-matter affects the propagation of light beams and the features of self-induced waveguides. We address the formation of optical spatial solitons and the control of their trajectories versus temperature, comparing the measurements with the expectations based on a simplified model, showing an excellent agreement. Moreover, in a guest&ndash, host mixture with an absorbing dye dopant, we study the competition between reorientational and thermal nonlinearities, demonstrating that the two processes can be adjusted independently in order to tune the soliton properties, i.e., trajectory and confinement strength. Our results are an important contribution to better comprehend the role played by material properties on linear and nonlinear beam propagation, as well as their exploitation for signal processing and addressing.

Using an external low-frequency electric field applied to dye-doped nematic liquid crystals, we demonstrate that random lasing obtained by optical pumping can be steered in angular direction by routing an all-optical waveguide able to collect the emitted light. By varying the applied voltage from 0 to 2 V, we reduce the walk-off and sweep the random laser guided beam over 7 degrees. Full Text: PDF References V. S. Letokhov, "Generation of light by a scattering medium with negative resonance absorption," Sov. Phys. JETP 26 (4), 835 (1968). DirectLink H. Cao, J. Y. Xu, D. Z. Zhang, S.-H. Chang, S. T. Ho, E. W. Seelig, X. Liu, and R. P. H. Chang, "Spatial Confinement of Laser Light in Active Random Media," Phys. Rev. Lett. 84 (24), 5584 (2000). CrossRef D. S. Wiersma, "The physics and applications of random lasers," Nature Phys. 4 (5) 359-367 (2008). CrossRef D. Wiersma and S. Cavalieri, "A temperature-tunable random laser," Nature 414, 708-709 (2001). CrossRef G. Strangi, S. Ferjani, V. Barna, A. De Luca, N. Scaramuzza, C. Versace, C. Umeton, and R. Bartolino, "Random lasing and weak localization of light in dye-doped nematic liquid crystals," Opt. Express 14 (17), 7737 (2006). CrossRef G. Strangi, S. Ferjani, V. Barna, A. De Luca, C. Versace, N. Scaramuzza, and R. Bartolino, "Random lasing in dye doped nematic liquid crystals: the role of confinement geometry," SPIE 6587, 65870P (2007) doi: 10.1117/12.722887 CrossRef S. Ferjani, V. Barna, A. De Luca, C. Versace, and G. Strangi, "Random lasing in freely suspended dye-doped nematic liquid crystals," Opt. Lett. 33(6), 557-559 (2008). CrossRef S. Ferjani, L-V. Sorriso, V. Barna, A. De Luca, R. De Marco, and G. Strangi, "Statistical analysis of random lasing emission properties in nematic liquid crystals," Phys. Rev. E 78 (1) 011707 (2008). CrossRef H. Bian, F. Yao, H. Liu, F. Huang, Y. Pei, C. Hou, and X. Sun, "Optically controlled random lasing based on photothermal effect in dye-doped nematic liquid crystals," Liq. 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Umeton, "Routing of Anisotropic Spatial Solitons and Modulational Instability in liquid crystals," Nature 432, 733-737 (2004). CrossRef J. Beeckman, K. Neyts and M. Haeltermann, "Patterned electrode steering of nematicons," J. Opt. A - Pure Appl. Opt. 8 (2), 214-220 (2006). CrossRef A. Piccardi, M. Peccianti, G. Assanto, A. Dyadyusha and M. Kaczmarek, "Voltage-driven in-plane steering of nematicons," Appl. Phys. Lett. 94, 091106 (2009). CrossRef R. Barboza, A. Alberucci, and G. Assanto, "Large electro-optic beam steering with Nematicons", Opt. Lett. 36 (14), 2611–2613 (2011). CrossRef A. Piccardi, A. Alberucci, R. Barboza, O. Buchnev, M. Kaczmarek, and G. Assanto, "In-plane steering of nematicon waveguides across an electrically adjusted interface", Appl. Phys. Lett. 100 (25), 251107 (2012). CrossRef Y. V. Izdebskaya, "Routing of spatial solitons by interaction with rod microelectrodes," Opt. Lett. 39(6), 1681-1684 (2014). CrossRef A. Pasquazi, A. Alberucci, M. Peccianti, and G. Assanto, "Signal processing by opto-optical interactions between self-localized and free propagating beams in liquid crystals," Appl. Phys. Lett. 87, 261104 (2005). CrossRef S. V. Serak, N. V. Tabiryan, M. Peccianti and G. Assanto, "Spatial Soliton All-Optical Logic Gates", IEEE Photon. Technol. Lett. 18 (12), 1287-1289 (2006). CrossRef M. Peccianti, C. Conti, G. Assanto, A. De Luca and C. Umeton, "All Optical Switching and Logic Gating with Spatial Solitons in Liquid Crystals," Appl. Phys. Lett. 81(18), 3335-3337 (2002). CrossRef A. Fratalocchi, A. Piccardi, M. Peccianti and G. Assanto, "Nonlinearly controlled angular momentum of soliton clusters," Opt. Lett. 32(11), 1447-1449 (2007). CrossRef Y. Izdebskaya, V. Shvedov, G. Assanto, and W. Krolikowski, Nat. Comm. 8, 14452 (2017). CrossRef M. Peccianti and G. Assanto, "Nematicons," Phys. Rep. 516, 147-208 (2012). CrossRef Y. Izdebskaya, A. Desyatnikov, G. Assanto and Y. 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Combining a reorientational spatial optical soliton with optical gain and scattering in dye-doped liquid crystals, we demonstrate that a random laser can emit a smooth laser beam in a well-defined direction.

Spatial solitons can affect and enhance random lasing in optically-pumped dye-doped nematic liquid crystals. Upon launching two collinear beams in the sample, the first to pump the fluorescent guest molecules and the second to induce a reorientational soliton, strikingly the second beam not only guides the emitted photons in the soliton waveguide, but also enhances the lasing efficiency and modulates its spectral width. By altering the scattering paths of the emitted photons, the soliton also contributes to the selection of the lasing modes, as further confirmed by the observed kinks in the input/output characteristics. These experimental results demonstrate that random lasing can be efficiently controlled by a light beam which does not interact with the gain molecules, opening a route towards light-controlled random lasers. publishedVersion

Light-controlling-light is one of the most advanced paradigms in optical signal processing, including light-induced waveguides as well as all-optical switching and routing. Other fundamental aspects of all-optical processing are optical memories and sequential elements, which require responses depending on the evolution history of the system, such as the hysteresis stemming from optical multistability. Hereby, we report on optical bistability and hysteresis in cavityless geometries and in the presence of self-localized beams, i.e., spatial optical solitons, exploiting the nonlocal reorientational nonlinearity of nematic liquid crystals (NLC). When the optic axis of NLC is initially orthogonal to the applied electric field, the molecular dipoles start to rotate above a threshold named after the Fréedericksz transition, the latter being usually of the second order. Here, we show that such transition become of the first order via the intrinsic feedback provided by self-focusing, in turn leading to the appearance of a hysteresis loop between diffracting and self-confined beams. We report on hysteresis of the beam size versus input power, as well as hysteresis versus applied voltage at a fixed beam power. Our findings introduce a novel kind of cavity-less optical bistability with propagating light beams and disclose a novel approach to information storage based on light self-localization.

Thanks to their unique properties, nematic liquid crystals feature a variety of mechanisms for light-matter interactions. For continuous-wave optical excitations, the two dominant contributions stem from reorientational and thermal nonlinearities. We thoroughly analyze the competing roles of these two nonlinear responses with reference to self-focusing/defocusing and, eventually, the formation of nonlinear diffraction-free wavepackets, the so-called spatial optical solitons. To this extent we refer to dye-doped nematic liquid crystals in planar cells and continuous-wave beams at two distinct wavelengths in order to adjust the relative weights of the two responses. The theoretical analysis is complemented by numerical simulations in the highly nonlocal approximation and compared to experimental results., 13 pages, 12 figures, submitted to PRE

We report on nematicon self-steering in two settings: pure nematic liquid crystals, where the steering stems directly from optical reorientation, and dye-doped liquid crystals, where strong confinement owing to a guest-host interaction allows the appearance of a transverse force acting on the beam.

Dye-doped nematic liquid crystals support random lasing under optical pumping, as well as reorientational optical spatial solitons acting as all-optical waveguides. By synergistically combining these two responses in a collinear pump-soliton geometry, the resulting soliton-enhanced random laser exhibits higher conversion efficiency and better directional properties. After a short account on random lasers and solitons in nematic liquid crystals–nematicons–we describe our experimental results on nematicon-molded random lasers.

We demonstrate thermo-optic control on the propagation of optical spatial solitons in nematic liquid crystals. By varying the sample temperature, both linear and nonlinear optical properties of the reorientational material are modulated by acting on the refractive indices, the birefringence, and the elastic response. As a result, both the trajectory and transverse confinement of spatial solitons can be adjusted, demonstrating an effective means to tune and readdress self-induced optical waveguides.

We show that reorientational nematic liquid crystals are an ideal workbench for the investigation of non-perturbative nonlinear optical effects and report light self-steering, power-controlled negative refraction and spontaneous symmetry breaking.

We demonstrate a guided-wave 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.See also erratum at:Appl. Phys. Lett. 110, 019902 (2017); https://doi.org/10.1063/1.4973864

Spontaneous symmetry breaking (SSB) occurs when noise triggers an initially symmetric system to evolve toward one of its nonsymmetric states. Topological and optical SSB involve material reconfiguration/transition and light propagation/distribution in time or space, respectively. In anisotropic optical media, light beam propagation and distribution of the optic axis can be linked, thereby connecting topological and optical SSB. Using nonlinear soft matter, namely uniaxial liquid crystals, we report on simultaneous topological and optical SSB, showing that spatial solitons enhance the noise-driven transition of the medium from a symmetric to an asymmetric configuration, while acquiring a power-dependent transverse velocity in either of two specular directions with respect to the initial wavevector. Solitons enhance SSB by further distorting the optic axis distribution through nonlinear reorientation, resulting in power-tunable walk-off as well as hysteresis in beam refraction versus angle of incidence.

We demonstrate that light refraction at a straight interface between an isotropic dielectric and a nematic liquid crystal can change from positive to negative depending on power. The phenomenon relies on the reorientational response and the all-optical rotation of the optic axis, causing in turn variations in walk-off and beam self-steering.

Refraction at a dielectric interface can take an anomalous character in anisotropic crystals, when light is negatively refracted with incident and refracted beams emerging on the same side of the interface normal. In soft matter subject to reorientation, such as nematic liquid crystals, the nonlinear interaction with light allows tuning of the optical properties. We demonstrate that in such material a beam of light can experience either positive or negative refraction depending on input power, as it can alter the spatial distribution of the optic axis and, in turn, the direction of the energy flow when traveling across an interface. Moreover, the nonlinear optical response yields beam self-focusing and spatial localization into a self-confined solitary wave through the formation of a graded-index waveguide, linking the refractive transition to power-driven readdressing of copolarized guided-wave signals, with a number of output ports not limited by diffraction.

The great interest arisen in the past decades in optical spatial solitons, i.e. self-confined waves supported by the balance between diffraction and nonlinear self-focusing, stems from their character of light-induced waveguides, thus allowing versatile and reconfigurable schemes for optical signal handling and processing [1]. Nematic Liquid Crystals (NLC) are excellent materials for the generation, propagation and the control of spatial solitons, also known as Nematicons [2]: NLC exhibit large birefringence and transparency, coupled with a high reorientational nonlinearity owing to (mean) molecular rotation induced by quasi-static and/or optical frequency electric fields. In addition, the high nonlocal character of their response stabilizes two-dimensional selfconfinement of beams into solitons [3,4]. Hereby we illustrate planar NLC cell configurations whereby nematicons can be electrically routed by propagating in a non-uniform dielectric with transverse and longitudinal variations of the refractive index and/or the orientation of the optic axis, i.e. by undergoing voltage-driven modulation of linear and nonlinear properties. The numerical models and numerical results are in excellent agreement with the experimental data.

We investigate, both theoretically and experimentally, spatial soliton interaction with dielectric interfaces in a strongly anisotropic medium with non-locality, such as nematic liquid crystals. We throw light on the role of refractive index gradients as well as optic axis variations in both voltage- and self-driven angular steering of non-local solitons. We specifically address and then employ in experiments a suitably designed electrode geometry in a liquid crystalline cell in order to define and tune a graded dielectric interface by exploiting the electro-optic response of the material through the in-plane reorientation of the optic axis in two distinct regions. We study both refraction and total internal reflection as well as voltage controlled steering of spatial solitons.

We study the angular steering of spatial solitons in nematic liquid crystals, i.e. nematicons, exploiting their electro-optic response. Approaches to electro-optic steering include acting on the optic axis and walk-off, propagating the soliton through a voltage-adjusted refractive interface or getting it totally internally reflected. Using comb electrodes in a planar cell, maximum angular deflections of 70° are expected with the standard nematic liquid crystal mixture E7.

We investigate the excitation and propagation of self-confined beams and the electric steering of soliton-based optical waveguides in nematic liquid crystalline planar cells. Controlled addressing of a signal is achieved in space by exploiting the electro-optic response of the uniaxial material, employing either uniform or patterned electrodes to alter either the Poynting vector or the wavevector, respectively.

We demonstrate the generation and propagation of dark spatial solitons in dye-doped nematic liquid crystals, exploiting a defocusing nonlinearity stemming from a reduced order parameter. Good agreement is obtained between the experimental measurements and a simplified model. Moreover, a probe beam is used to confirm the guiding properties of the soliton.

We study the interaction of a spatial soliton waveguide with a voltage defined and electrically tuned interface in nematic liquid crystals, whereby the optic axis is reoriented through the use of patterned electrodes. We investigate refraction and total internal reflection of nematicon wavepackets, disclosing the role of anisotropy and obtaining a remarkable in-plane steering as large as 55°.

We report on power-dependent self-transverse acceleration of optical spatial solitons in two settings: pure nematic liquid crystals, where the self-steering is a consequence of all-optical reorientation through nonlinear changes in birefringent walk-off, and dye-doped nematic liquid crystals, where the guest-host interaction amplifies reorientation and enhances the transverse force acting on the soliton, thus changing its trajectory.

Using external beams on the photoconductive layer of a liquid crystal light valve we demonstrate all-optical control of soliton induced waveguides in nematics. Using this approach we implement a half-adder and a 3-bit demultiplexer.

Using a photoconductive light valve with nematic liquid crystals, we introduce a versatile platform for the excitation and routing of spatial optical solitons, with external beams controlling the whereabouts of the underlying all-optically induced waveguides and their spatial dynamics. Using this all-optical control of soliton trajectory, we demonstrate a NOR gate, an XNOR, and a Boolean half-adder.

Blue light self-confinement in active dye-doped nematic liquid crystals is reported, demonstrating trapping of the emitted red light in dissipative spatial solitons formed by all-optical reorientation.

Soliton induced waveguides can be all-optically readdressed in planar cells by modifying the molecular anchoring with an external light source. Using an elliptically shaped control beam we demonstrate refraction and total internal reflection of spatial solitons in dye-doped nematic liquid crystals. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3009658]

We investigate, both theoretically and experimentally, self-trapping of light beams in nematic liquid crystals arranged so as to exhibit the optical Fréedericksz transition in planar cells. The resulting threshold in the nonlinear reorientational response supports a bistable behavior between diffracting and self-localized beam states, leading to the appearance of a hysteretic loop versus input excitation. Our results confirm the role of nematic liquid crystals in the study of non-perturbative nonlinear photonics.

We investigate the attractive interaction between spatial solitons in nematic liquid crystals with a tunable nonlinearity and a constant nonlocality. The experimental study, carried out by controlling the orientation of the optic axis via the electro-optic response, shows how the interactions depend on reorientation, in excellent agreement with a model accounting for the anisotropic nature of the dielectric.

Using an external voltage and interdigitated electrodes in a liquid crystalline cell, we demonstrate tunable steering of light beams and spatial solitons without the detrimental walk-off out of the plane of propagation. The results agree well with a simple model describing birefringence and reorientation in uniaxial nematic liquid crystals.

We demonstrate random lasing control exploiting optical gain and light self-localization in a reorientational medium. A spatial soliton in dye-doped nematic liquid crystals modulates stimulated emission of visible light, yielding random lasing with enhanced features.