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Publisher: London : Nature Publishing Group × Subject: Probes ×

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    Compressively sampling the optical transmission matrix of a multimode fibre
    (London : Nature Publishing Group, 2021) Li, Shuhui; Saunders, Charles; Lum, Daniel J.; Murray-Bruce, John; Goyal, Vivek K.; Čižmár, Tomáš; Phillips, David B.
    The measurement of the optical transmission matrix (TM) of an opaque material is an advanced form of space-variant aberration correction. Beyond imaging, TM-based methods are emerging in a range of fields, including optical communications, micro-manipulation, and computing. In many cases, the TM is very sensitive to perturbations in the configuration of the scattering medium it represents. Therefore, applications often require an up-to-the-minute characterisation of the fragile TM, typically entailing hundreds to thousands of probe measurements. Here, we explore how these measurement requirements can be relaxed using the framework of compressive sensing, in which the incorporation of prior information enables accurate estimation from fewer measurements than the dimensionality of the TM we aim to reconstruct. Examples of such priors include knowledge of a memory effect linking the input and output fields, an approximate model of the optical system, or a recent but degraded TM measurement. We demonstrate this concept by reconstructing the full-size TM of a multimode fibre supporting 754 modes at compression ratios down to ∼5% with good fidelity. We show that in this case, imaging is still possible using TMs reconstructed at compression ratios down to ∼1% (eight probe measurements). This compressive TM sampling strategy is quite general and may be applied to a variety of other scattering samples, including diffusers, thin layers of tissue, fibre optics of any refractive profile, and reflections from opaque walls. These approaches offer a route towards the measurement of high-dimensional TMs either quickly or with access to limited numbers of measurements.
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    Nonlocal dielectric function and nested dark excitons in MoS2
    (London : Nature Publishing Group, 2019) Koitzsch, A.; Pawlik, A.-S.; Habenicht, C.; Klaproth, T.; Schuster, R.; Büchner, B.; Knupfer, M.
    Their exceptional optical properties are a driving force for the persistent interest in atomically thin transition metal dichalcogenides such as MoS2. The optical response is dominated by excitons. Apart from the bright excitons, which directly couple to light, it has been realized that dark excitons, where photon absorption or emission is inhibited by the spin state or momentum mismatch, are decisive for many optical properties. However, in particular the momentum dependence is difficult to assess experimentally and often remains elusive or is investigated by indirect means. Here we study the momentum dependent electronic structure experimentally and theoretically. We use angle-resolved photoemission as a one-particle probe of the occupied valence band structure and electron energy loss spectroscopy as a two-particle probe of electronic transitions across the gap to benchmark a single-particle model of the dielectric function ϵ(q, ω) against momentum dependent experimental measurements. This ansatz captures key aspects of the data surprisingly well. In particular, the energy region where substantial nesting occurs, which is at the origin of the strong light–matter interaction of thin transition metal dichalcogenides and crucial for the prominent C-exciton, is described well and spans a more complex exciton landscape than previously anticipated. Its local maxima in (q≠0,ω) space can be considered as dark excitons and might be relevant for higher order optical processes. Our study may lead to a more complete understanding of the optical properties of atomically thin transition metal dichalcogenides.