In recent years, the ever-increasing demand for high-capacity transmission systems has driven remarkable advances in technologies that encode information on an optical signal. Mode-division multiplexing makes use of individual modes supported by an optical waveguide as mutually orthogonal channels. The key requirement in this approach is the capability to selectively populate and extract specific modes. Optical supersymmetry (SUSY) has recently been proposed as a particularly elegant way to resolve this design challenge in a manner that is inherently scalable, and at the same time maintains compatibility with existing multiplexing strategies.<p> </p> Supersymmetric partners of multimode waveguides are characterized by the fact that they share all of their effective indices with the original waveguide. The crucial exception is the fundamental mode, which is absent from the spectrum of the partner waveguide. Here, we demonstrate experimentally how this global phase-matching property can be exploited for efficient mode conversion. Multimode structures and their superpartners are experimentally realized in coupled networks of femtosecond laser-written waveguides, and the corresponding light dynamics are directly observed by means of fluorescence microscopy. We show that SUSY transformations can readily facilitate the removal of the fundamental mode from multimode optical structures. In turn, hierarchical sequences of such SUSY partners naturally implement the conversion between modes of adjacent order. Our experiments illustrate just one of the many possibilities of how SUSY may serve as a building block for integrated mode-division multiplexing arrangements. Supersymmetric notions may enrich and expand integrated photonics by versatile optical components and desirable, yet previously unattainable, functionalities.
Sub-wavelength structures are a crucial ingredient for modern optics. A class of ultrashort laser pulse induced, selforganized modifications in bulk transparent materials have attracted particular interest in recent years. Despite the multitude of potential applications of these so-called “nanogratings”, their underlying structure on the nanometer scale has been the subject of intensive debate throughout the decade since their discovery: Are they merely continuous modulation patterns of the material density, or do they consist of a substructure of hollow cavities? As nanogratings are embedded within the bulk material the conventional visualization technique relies on polishing and subsequent etching to excavate the modifications. However, such invasive sample preparation effectively erases sub-100 nm features. Moreover, they only provide access to two-dimensional cross sections. To overcome these limitations, we employed small angle X-ray scattering (SAXS), focused ion beam (FIB) milling and scanning electron microscopy (SEM) to reveal the underlying three-dimensional structure of nanogratings. Our results show that small cavities are the primary constituents of the nanogratings. These cavities grow predominantly during the first 100 laser pulses and reach a final size of about 30x200x300 nm<sup>3</sup>. Prolonged exposure to laser pulses increases the absolute number of cavities. Their threedimensional arrangement forms characteristic periodic planes of nanogratings.
To gain insight into the processes mediating the cumulative action of subsequent laser pulses which gives rise to the
formation of nanogratings, we performed double pulse experiments with femtosecond laser pulses with a delay time
ranging from 0.5 ps to 1 ns. We determined the polarisation contrast intensity of the inscribed lines as a measure for the
birefringent strength of the nanogratings. Our experiments show an enhanced nanograting formation for pulse
separations below 500 ps. We attribute this to the presence of self trapped excitons serving as transient material memory
enhancing the impact of the second pulse. In contrast, nanograting formation at pulse separation times up to several
seconds is being mediated by dangling bond type defects as evidenced by spectrally resolved absorption measurements.
We report on the impact of topological defects on the formation of discrete spatial solitons in waveguide arrays.
The influence of defects, i.e. waveguides with detuned effective refractive index, is well understood within such
systems. They have been shown to support linear bound states and thus influence the formation of spatial
solitons in the surrounding sites. We show numerically and demonstrate experimentally how the presence of
topological defects caused by junctions within the otherwise periodical system similarly has a strong influence
on the surrounding sites.
We report on the fabrication of birefringent optical components based on so-called nanogratings. These selforganized
nanostructures with sub-wavelength periodicity are formed during femtosecond laser processing of
transparent materials, resulting in characteristic birefringent modifications. Nanogratings provide the means for
the direct inscription of customized birefringent elements with position-dependent retardation. We present our
investigations on the formation process of nanogratings in fused silica and the influence of fabrication parameters,
thereby identifying ways to systematically control the structural properties of the gratings. Consequently, we
were able to fabricate nanograting-based birefringent elements with specific retardations in bulk fused silica.
Self-imaging in integrated optical devices is interesting for many applications including image transmission,
optical collimation and even reshaping of ultrashort laser pulses. However, in general this relies on boundary-free
light propagation, since interaction with boundaries results in a considerable distortion of the self-imaging
effect. This problem can be overcome in waveguide arrays by segmentation of particular lattice sites, yielding
phase shifts which result in image reconstruction in one- as well as two-dimensional configurations. Here, we
demonstrate the first experimental realization of this concept. For the fabrication of the segmented waveguide
arrays we used the femtosecond laser direct-writing technique. The total length of the arrays is 50mm with a
waveguide spacing of 16 μm and 20μm in the one- and two-dimensional case, respectively. The length of the
segmented area was 2.6mm, while the segmentation period was chosen to be 16 μm. This results in a complete
inversion of the global phase of the travelling field inside the array, so that the evolution dynamics are reversed
and the input field is imaged onto the sample output facet. Accordingly, segmented integrated optical devices
provide a new and attractive opportunity for image transmission in finite systems.
We report the realization of an evanescently coupled laser-written type II array in χ-cut Lithium niobate. Certain
processing parameters allow evanescent fields to extend beyond the regions of damage, while still increasing the
index sufficiently to guide light. An array consisting of eleven coupled waveguides was fabricated. Coupling
was evaluated by observing discrete diffraction patterns of single waveguide excitations at various array sites.
Homogeneous coupling was verified within the array, while the outermost guides are slightly detuned due to
being formed by just one damage structure.
The evanescent coupling of femtosecond laser written waveguides with elliptical and circular shape is investigated
in detail. Elliptical waveguides are used to investigate directional tuning of the coupling properties in a square
array by tilting the elliptical waveguides. This allows to specifically pronounce diagonal coupling. In contrast,
directional insensitive coupling is demonstrated in a circular waveguide array based on circular waveguides.