Taking advantage of imperfections to focus light in photonic crystals inside the stop gap

[R. Uppu et al., arxiv, 2007.11104v1 (2020)]

Photonic crystals have the ability to forbid the entrance of light for certain ranges of frequencies, that is even the usual reason why we build them. Within the spatial frequency range for which the stop band of a given photonic crystal exists, modulation of the input wavefront should not dramatically modify the penetration of light, which exhibits an exponential decay. In this paper, R. Uppu and collaborators from the University of Twente demonstrate that it is indeed possible to drastically change the penetration properties of light inside a photonic crystal by optimizing the wavefront taking advantage of unavoidable sources of disorder. They experimentally show the focusing of light inside such crystal beyond the expected maximal penetration length - the Bragg length.

Postdoctoral position at the Langevin Institute

Wavefront shaping and study of light propagation in disordered multimode fibers

We are recruiting a postdoc for 1+ year(s) to work on the study of light propagation in multimode fibers for telecommunication applications using wavefront shaping. Join un in Paris!

See our most recent publication: [Learning and avoiding disorder in multimode fibers, arxiv, 2020]

Contact: Sébastien Popoff - sebastien.popoff(at)espci.fr

More information here.

Unscrambling entanglement through a complex medium

[N. H. Valencia, S. Goel, W. McCutcheon, H. Defienne and M. Malik, Nature Physics (2020)]

Quantum properties of light may enable unconditionally secure optical communications. In this respect, high-dimensional entangled states offer a way of exceeding the limitations of current approaches to quantum communication (e.g. larger information capacity and increased noise resilience). For example, the orbital angular momentum of photons was first used to establish high-dimensional quantum key distribution (HD-QKD) protocols in free-space, but with a limited range due to diffraction and the presence of atmospheric turbulence. Alternatively, multimode optical fibers (MMF) can be used to transport information encoded in parallel across many modes over large distances, and with limited losses. However, the complex mode mixing process occurring during light propagation through the fiber scrambles the encoded information, making it unusable by the receiver. In their work, N. H. Valencia and co-workers demonstrate the transport of six-dimensional spatial-mode entanglement through a 2m-long commercial MMF, by compensating the random mode mixing effect using a transmission matrix-based wavefront-shaping technique. Such an ability to certify the presence of high-dimensional entanglement between two parties (Alice and Bob) is an essential step towards the implementation of practical HD-QKD protocols in optical fibers.

Parallelized STED microscopy using tailored speckles

[N. Bender et al., arxiv, 2007.15491 (2020)]

Super-resolution fluorescence microscopy techniques, such as stimulated emission depletion (STED), rely on depleting fluorescence around a region smaller than the limit of diffraction. This can be achieved with a doughnut-shaped beam that is then scanned to produce an image. Such a process is time-consuming. Structured illumination techniques were proposed to parallelize the process by having multiple zeros of the field in the same image, for example with an array of doughnut beams. However, it typically limits optical sectioning as the field conserves its shape for quite large distances along the axial direction. One way to overcome this limitation is to use speckle patterns. Speckle exhibits numerous singularities, allowing parallelization of the technique, and they rapidly and non-repeatably change along the axial direction, guarantying the optical sectioning while being robust to aberrations. The issue is that speckle singularities (optical vortices) are not isotropic, leading to distortions of the image. In the present paper, N. Bender and co-authors use wavefront shaping to design ideal speckle patterns for non-linear microscopy to achieve isotropic and uniform super-resolution.