Gaetano Assanto

Route to Nonlocality and Observation of Accessible Solitons

We develop a general theory of spatial solitons in a liquid crystalline medium exhibiting a nonlinearity with an arbitrary degree of effective nonlocality. The model accounts the observability of accessible solitons and establishes an important link with parametric solitons.

Observation of Optical Spatial Solitons in a Highly Nonlocal Medium

We report on the observation and quantitative assessment of self-trapped pulsating beams in a highly nonlocal nonlinear regime. The experiments were conducted in nematic liquid crystals and allow a meaningful comparison with the prediction of a scalar theory in the perturbative limit, while addressing the need for beyond-paraxial analytical treatments.

All-optical diode in a periodically poled lithium niobate waveguide

We have demonstrated a guided-wave all-optical diode based on engineered quasiphase Matching in a LiNbO3 channel waveguide. For input peak powers beyond 1.5 W at 1.55 μm, the device exhibited a spatially nonreciprocal response, leading to optical isolation with contrasts as high as 90% at 3.1 W, in agreement with theoretical predictions.

Electrically assisted self-confinement and waveguiding in planar nematic liquid crystal cells

We report on spatial soliton formation and self/cross waveguiding in planar cells containing a nematic liquid crystal in the presence of an externally applied voltage. Self-confinement and cross-induced guidance are demonstrated with an Argon ion laser (514 nm) and a helium–neon probe (633 nm), respectively, over millimeter lengths and with milliwatt pump powers.

Routing of anisotropic spatial solitons and modulational instability in liquid crystals

In certain materials, the spontaneous spreading of a laser beam (owing to diffraction) can be compensated for by the interplay of optical intensity and material nonlinearity. The resulting non-diffracting beams are called ‘spatial solitons’ (refs 1–3), and they have been observed in various bulk media. In nematic liquid crystals, solitons can be produced at milliwatt power levels and have been investigated for both practical applications and as a means of exploring fundamental aspects of light interactions with soft matter. Spatial solitons effectively operate as waveguides, and so can be considered as a means of channelling optical information along the self-sustaining filament. But actual steering of these solitons within the medium has proved more problematic, being limited to tilts of just a fraction of a degree. Here we report the results of an experimental and theoretical investigation of voltage-controlled ‘walk-off’ and steering of self-localized light in nematic liquid crystals. We find not only that the propagation direction of individual spatial solitons can be tuned by several degrees, but also that an array of direction-tunable solitons can be generated by modulation instability. Such control capabilities might find application in reconfigurable optical interconnects, optical tweezers and optical surgical techniques.

Nonlocal spatial soliton interactions in nematic liquid crystals

The interaction between spatial optical solitons in undoped nematic liquid crystals is governed by reorientational nonlinearity with a significant nonlocality. We demonstrate several cases of interactions, including crossing, interlacing, and merging of spatial solitons in voltage-biased liquid-crystal cells. A scalar model accounts for the main features that were experimentally observed.

Large nonlinear phase shifts in second-order nonlinear-optical processes

We show that processes such as second-harmonic generation and subsequent downconversion, and parametric mixing in general, can lead to large field-dependent phase shifts for the input beams under a variety of conditions.

Efficient high-speed near-infrared Ge photodetectors integrated on Si substrates

We have fabricated Ge/Si heterojunction photodetectors with high responsivities of 550 mA/W at 1.32 μm and 250 mA/W at 1.55 μm and time responses shorter than 850 ps. High quality Ge was epitaxially grown on Si using ultrahigh vacuum/chemical vapor deposition followed by cyclic thermal annealing. The beneficial effect of the post-growth thermal annealing on the electrical properties of Ge epilayers, due to the reduction of threading-dislocation densities, is confirmed by the dramatic enhancement of the performance of the photodetectors.

Metal–semiconductor–metal near-infrared light detector based on epitaxial Ge/Si

In this letter we report on a metal–semiconductor–metal photodetector based on thick relaxed Ge layers, epitaxially grown on silicon after insertion of a low-temperature-grown Ge buffer layer. The detector shows a good responsivity at normal incidence at both 1.3 and 1.55 μm, with a maximum responsivity of 0.24 A/W at 1.3 μm under a 1 V bias. A response time of about 2 ns has been measured.

All-optical switching and logic gating with spatial solitons in liquid crystals

Using mW light beams to generate spatial solitons in nematic liquid crystals, all-optical switching/logic can be performed on a signal launched in the soliton-induced waveguides. Through the collisional behavior of solitons in a nonlocal medium, the signal can be steered in angle and output position. A power-dependent X junction, AND, and NOR gates are demonstrated.