Optical coherence is of fundamental importance for both classical and quantum applications. This motivates the development of approaches for increasing the degree of coherence, which can be quantified by a measure of purity. The purity is preserved in linear conservative systems, and accordingly the manipulation of coherence was realized with specially introduced loss in bulk optical setups or diffraction on metal films involving optical absorption and plasmon coupling. Here we suggest and show experimentally for the first time that manipulation and measurement of optical coherence and state purification can be efficiently realized in integrated non-Hermitian parity-time (PT) symmetric photonic structures composed of elements with different loss or gain. Specifically, we design and fabricate laser-written waveguide directional couplers that contain two sections. The first section realizes a PT-like coupler, where one of the two waveguides features extra radiative losses via modulation. The second section consists of straight coupled waveguides with specially detuned propagation constants, which are optimized to enable a full reconstruction of the purity and optical coherence by measuring the interference pattern in both waveguides through fluorescence imaging. In PT symmetric regime, we observe that the purity of an initially fully incoherent (mixed) state is increased followed by a revival of the input state. This constitutes an important experimental evidence of reversible manipulation of light coherence in PT coupled waveguides. We anticipate that this method can facilitate a wide range of applications from classical to quantum optics, including filtering out noise and optimizing the visibility of interferometric measurements.
Significant interest has been devoted to tailoring optical fields that transversely accelerate during propagation in the form of Airy, Weber and Mathieu beams. In this work, we introduce a new type of optical field that exhibits controlled angular acceleration during propagation which is achieved by superpositions of Bessel beams with non-canonical phase functions. We demonstrate these angular accelerating fields by modulating the phase and amplitude of a supercontinuum source with the use of a phase-only spatial light modulator (SLM). We illustrate that by considering only the first diffraction order when the SLM is encoded with a blazed grating, the SLM is capable of tailoring the spatial profile of broadband sources without any wavelength dependence. By digitally simulating free-space propagation on the SLM, we compare the effects of real and digital propagation on the angular rotation rates of the resulting optical fields for various wavelengths. The development of controlled angular accelerating optical fields will be useful in areas such as particle manipulation, plasma control, material processing and non-linear optics.
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.
Topological insulators are a new phase of matter, with the striking property that conduction of electrons occurs
only on the surface. In two dimensions, surface electrons in topological insulators do not scatter despite defects
and disorder, providing robustness akin to superconductors. Topological insulators are predicted to have wideranging
applications in fault-tolerant quantum computing and spintronics. Recently, large theoretical efforts were
directed towards achieving topological insulation for electromagnetic waves. One-dimensional systems with
topological edge states have been demonstrated, but these states are zero-dimensional, and therefore exhibit no
transport properties. Topological protection of microwaves has been observed using a mechanism similar to
the quantum Hall effect, by placing a gyromagnetic photonic crystal in an external magnetic field. However,
since magnetic effects are very weak at optical frequencies, realizing photonic topological insulators with scatterfree
edge states requires a fundamentally different mechanism - one that is free of magnetic fields. Recently, a
number of proposals for photonic topological transport have been put forward. Specifically, one suggested
temporally modulating a photonic crystal, thus breaking time-reversal symmetry and inducing one-way edge
states. This is in the spirit of the proposed Floquet topological insulators, where temporal variations in solidstate
systems induce topological edge states. Here, we propose and experimentally demonstrate the first external
field-free photonic topological insulator with scatter-free edge transport: a photonic lattice exhibiting topologically
protected transport of visible light on the lattice edges. Our system is composed of an array of evanescently coupled
helical waveguides arranged in a graphene-like honeycomb lattice. Paraxial diffraction of light is described by
a Schrödinger equation where the propagation coordinate acts as ‘time’. Thus the waveguides' helicity breaks zreversal
symmetry in the sense akin to Floquet Topological Insulators. This structure results in scatter-free, oneway
edge states that are topologically protected from scattering.
Three dimensional Light Bullets (3D-LBs) are the most symmetric solitary waves, being nonlinear optical
wavepackets propagating without diffraction nor dispersion. Since their theoretical prediction, 3D-LB's have
constituted a challenge in nonlinear science, due to the impossibility to avoid catastrophic collapse in conventional
homogeneous nonlinear media. We have recently observed stable 3D-LBs in media with periodically
modulated transverse refractive index profile. We found that higher order linear and nonlinear effects force the
3D-LBs to evolve along their propagation path and eventually decay. The evolution and decay mechanism entails
spatiotemporal effects, which under certain conditions, leads to superluminally propagating wavepackets.
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 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.
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.
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.
For various applications it is interesting to directly visualize the propagation of light in waveguides. For this
purpose, we used special fused silica glasses with a high content of OH. This leads to the formation of color
centers when waveguides are written with fs laser pulses. When light is launched into the waveguides the color
centers are excited and the fluorescence can be directly observed. This is especially interesting in waveguide
arrays for the visualization of the evanescent coupling, since the discrete light evolution exhibits many features
which are in strong contrast to propagation in common isotropic media. As an example for the visualization
we will discuss here the possibility to excite a completely incoherent propagation within the waveguide array
although the sources are fully coherent. When multiple waveguides are excited, the light evolution in the array
can be described as a superposition of the single propagating amplitudes. The formula for the resulting intensity
contains an interference term. One can explicitly show that this interference term vanishes for certain excitation
patterns. When for instance two adjacent waveguides are excited the light propagates as there was no interference
term, which is equivalent to the simple sum of the two intensities of the single amplitudes. This suggests the term
"quasi-incoherent" for this new kind of propagation effect. In contrast a coherent superposition including the
interference term is obtained for an excitation of two waveguides when there is one waveguide located between
the two excited ones.
We report on the observation of a two-dimensional discrete soliton in a cubic 5 × 5 fs laser written waveguide array for the first time. In addition to the localization the sharp defined edges of the array allow to study the influence of the array's boundaries. The results are in excellent agreement with the theoretical predictions. These results provide the basis for a variety of future applications for nonlinear two-dimensional integrated optical devices.
We report on the investigation of the nonlinear refractive index in femtosecond laser written waveguide arrays in fused silica. The nonlinear refractive index is significantly reduced compared to the unmodified material. Due to the dependence of the processing parameters the effective nonlinearity in such waveguide structures can be tuned. This offers additional flexibility in the design of nonlinear devices.