R. Keil

Wave localization at the boundary of disordered photonic lattices

We report on the experimental observation of reduced light-energy transport and disorder-induced localization close to a boundary of a truncated 1D disordered photonic lattice. Our observations uncover that a higher level of disorder near the boundary is required to obtain similar localization than in the bulk.

Geometric Potential and Transport in Photonic Topological Crystals

We report on the experimental realization of an optical analogue of a quantum geometric potential for light wave packets constrained on thin dielectric guiding layers fabricated in silica by the femtosecond laser writing technology. We further demonstrate the optical version of a topological crystal, with the observation of Bloch oscillations and Zener tunneling of a purely geometric nature.

Adiabatic transfer of light via a continuum in optical waveguides.

The optical analog of the stimulated Raman adiabatic passage via a continuum is experimentally demonstrated for photonic tunneling in femtosecond laser written waveguides. The results clearly show that the mechanism of light transfer relies on destructive interference and on the existence of a photonic dark state.

Polychromatic beam splitting by fractional stimulated Raman adiabatic passage

We propose and demonstrate a femtosecond laser inscribed micro-optical device for broadband beam splitting based on the interruption of the stimulated Raman adiabatic passage. For the spectral characterization waveguide fluorescence microscopy is applied by exciting nonbridging oxygen holes and exciton defects at several wavelengths. Additionally, spectrally resolved nearfield imaging shows octave spanning 50:50 beam splitting.

Klein tunneling of light in waveguide superlattices

We present an experimental observation of Klein tunneling of light waves in lattices of evanescently coupled waveguides with a superimposed potential step. The incident wave packet mass which is a characteristic feature of Klein tunneling is generated by a minigap in the band structure of the lattice. We studied different masses and measured the tunneling rates across the potential step.

Nonlinearity-induced broadening of resonances in dynamically modulated couplers

We report the observation of nonlinearity-induced broadening of resonances in dynamically modulated directional couplers. When the refractive index of the guiding channels in the coupler is harmonically modulated along the propagation direction and is out-of-phase in two channels, coupling can be completely inhibited at resonant modulation frequencies. We observe that nonlinearity broadens such resonances and that localization can be achieved even in detuned systems at power levels well below those required in unmodulated couplers.

Perfect imaging through a disordered waveguide lattice

It is experimentally demonstrated that perfect imaging is possible in disordered wave guiding media, provided that the disorder is off-diagonal, i.e., that only the spacing varies randomly between the otherwise identical lattice sites. On-diagonal disorder or Kerr nonlinearity destroys the imaging.

Observation of two-dimensional superlattice solitons

We observe experimentally two-dimensional solitons in superlattices comprising alternating deep and shallow waveguides fabricated via the femtosecond-laser direct writing technique. We find that the symmetry of linear diffraction patterns as well as soliton shapes and threshold powers largely differ for excitations centered on deep and shallow sites. Thus, bulk and surface solitons centered on deep waveguides require much lower powers than their counterparts on shallow sites.

Observation of discrete solitons in lattices with second-order interaction

We present what we believe to be the first experimental demonstration of discrete solitons under the influence of first- and second-order lattice site interaction in zigzag femtosecond laser-written waveguide arrays. We show how the formation of these entities is affected by different ratios of the interaction strength. Increasing the second-order coupling between the lattices sites yields an almost linear increase of the input power required for soliton formation.

Negative coupling between defects in waveguide arrays

We report on the experimental demonstration of negative coupling constants between defect guides in a waveguide lattice. We find that coupling can only be negative if the defects are negative and an odd number of lattice sites is between the defect guides.