J. Parravicini

Observation of an intrinsic nonlinearity in the electro-optic response of freezing relaxors ferroelectrics

We demonstrate an electro-optic response that is linear in the amplitude but independent of the sign of the applied electric field. The symmetry-preserving linear electro-optic effect emerges at low applied electric fields in freezing nanodisordered KNTN above the dielectric peak temperature, deep into the nominal paraelectric phase. Strong temperature dependence allows us to attribute the phenomenon to an anomalously reduced thermal agitation in the reorientational response of the underlying polar-nanoregions.

Equalizing disordered ferroelectrics for diffraction cancellation

We show how the cross-over effect of dipolar glasses can be used to observe diffraction cancellation in composite ferroelectric samples independently of composition. We are able to selectively frustrate the dielectric anomaly of different compositionally disordered photorefractive ferroelectrics to achieve scale-free optical propagation at one same temperature.

Photorefractive light needles in glassy nanodisordered KNTN

We study the formation of 2D self-trapped beams in nanodisordered potassium-sodium-tantalate-niobate (KNTN) cooled below the dynamic glass transition. Supercooling is shown to accelerate the photorefractive response and enhance steady-state anisotropy. Effects in the excited state are attributed to the anomalous slim-loop polarization curve typical of relaxors dominated by non-interacting polar-nano-regions.

Programming scale-free optics in disordered ferroelectrics

Using the history dependence of a dipolar glass hosted in a compositionally disordered lithium-enriched potassium tantalate niobate (KTN:Li) crystal, we demonstrate scale-free optical propagation at tunable temperatures. The operating equilibration temperature is determined by previous crystal spiralling in the temperature/cooling-rate phase space.

Rejuvenation in scale-free optics and enhanced diffraction cancellation life-time

We demonstrate rejuvenation in scale-free optical propagation. The phenomenon is caused by the non-ergodic relaxation of the dipolar glass that mediates the photorefractive nonlinearity in compositionally-disordered lithium-enriched potassium-tantalate-niobate (KTN:Li). We implement rejuvenation to halt aging in the dipolar glass and extend the duration of beam diffraction cancellation.

Aging solitons in photorefractive dipolar glasses

We study experimentally the aging of optical spatial solitons in a dipolar glass hosted by a nanodisordered sample of photorefractive potassium-sodium-tantalate-niobate (KNTN). As the system ages, the waves erratically explore varying strengths of the nonlinear response, causing them to break up and scatter. We show that this process can still lead to solitons, but in a generalized form for which the changing response is compensated by changing the normalized wave size and intensity so as to maintain fixed the optical waveform.

Funnel-based biomimetic volume optics

We demonstrate the use of three-dimensional funnel index of refraction patterns analogous to those of retinal Muller cells as support for tunable and multi-functional volume optical component miniaturization and integration. Our experiments in paraelectric photorefractive crystals show how a single funnel can act both as a waveguide and a tunable focusing/defocusing micro-lens. Pairing multiple funnel patterns, we are also able to demonstrate ultra-compact tunable beam-splitting, with distinct guided output modes in under 1mm of propagation.

Anti-diffracting beams through the diffusive optical nonlinearity

Anti-diffraction is a theoretically predicted nonlinear optical phenomenon that occurs when a light beam spontaneously focalizes independently of its intensity. We observe anti-diffracting beams supported by the peak-intensity-independent diffusive nonlinearity that are able to shrink below their diffraction-limited size in photorefractive lithium-enriched potassium-tantalate-niobate (KTN:Li).

Photorefractive solitons in an out-of-equilibrium disordered ferroelectric

We study two-dimensional soliton beams in disordered ferroelectrics. Super-cooling accelerates photorefractive response and changes steady-state anisotropy. Effects are attributed to the anomalous polarization response of polar-nano-regions.

Artificial retinal glial-like waveguides for biomimetic volume optics

Summary form only given. In the vertebrate eye light must be funnelled through a mangled mass of scattering tissue by Müller cells [1]. In distinction to conventional waveguides, that are essentially tubular, these cells have a double-funnel shape, and can efficiently focus, collect, transfer, and outcouple light without the strong mode selectivity of waveguides. Here we explore the use of biologically inspired funnel index of refraction patterns that mimic retinal Müller cells as versatile volume blueprints for multiple optical functions [2]. Compared to tube-like patterns typical of soliton-based waveguides, funnels are fully three-dimensional structures (illustrated in Fig. 1(LEFT)) that achieve either focusing, guiding, and defocusing: the key ingredient is the changing shape along the propagation direction (say the z axis) that can, depending on circumstances, act as a lens (the cellular ”end-feet”), or as a fiber (the cellular ”body”), thus forming a basic blueprint to multifunctional optics.Funnel patterns are achieved through photorefraction. During the pattern writing phase, light propagates with a polarization orthogonal to the external bias so as to allow for an efficient build-up of the space-charge field Esc without it affecting the beam: the beam diffracts and produces the funnel-like Esc. In the second ”readout phase”, the exposed region in the crystal is used to affect the propagation of light polarized along the x direction, parallel to E0. In this case, the electro-optic response is maximized, and Esc is kept fixed using a low intensity visible beam. The functionality of the pattern is fixed by the duration te of the writing phase (this determines the saturation parameter a in Fig. 1(CENTER) that flags the different response curves).Experiments are carried out in a sample of Cu-doped paraelectric KLTN. During the exposure phase, a 800 nW TEM00 beam of λ = 543 nm polarized along they direction is focused down to w0 6.5 μm at the minimum waist plane at zc. Bias fields range from E0 = -4.6kV/cm to +4.6 kV/cm and the amplitude of the index modulation is Δn ~ 3 χ 10-5, a value insufficient to alter diffraction. In the readout phase, intensity reduced to 10 nW and the optical polarization is rotated in the x direction, and Δn0 ~ 4 10-4 (see Fig. 1 (RIGHT)). We find conditions for tunable lensing, waveguiding, and use the quadratic electro-optic response to rapidly switch functionality, passing, for example, from a focusing to a defocusing regime, and explore the rich variety of multi-funnel patterns that ultimately allow for a remarkably compact optical components, such as switches and splitters. Results can form the basis for more elaborate fully three-dimensional optical circuitry for light control in highly miniaturized environments that are dominated by strong diffraction.