Topological beaming of light: proof-of-concept experiment

Abstract

Beam shaping in nanophotonic systems remains a challenge due to the reliance on complex heuristic optimization procedures. In this work, we experimentally demonstrate a novel approach to topological beam shaping using Jackiw-Rebbi states in metasurfaces. By fabricating thin-film dielectric structures with engineered Dirac-mass distributions, we create domain walls that allow precise control over beam profiles. We observe the emergence of Jackiw-Rebbi states and confirm their localized characteristics. Notably, we achieve a flat-top beam profile by carefully tailoring the Dirac-mass distribution, highlighting the potential of this method for customized beam shaping. This experimental realization establishes our approach as a new mechanism for beam control, rooted in topological physics, and offers an efficient strategy for nanophotonic design.

Fig. 1

Principles of topological beam shaping and Jackiw-Rebbi state engineering in metasurfaces. a Schematic representation of leakage-radiation beam shaping with engineered Dirac mass distribution. The guided-mode resonance (GMR) with a tailored standing-wave envelope emits a desired beam profile through first-order diffraction. b Characteristic bi-exponential envelope profile f(x) of the Jackiw-Rebbi (JR) state at a topological junction (gray dotted curve). c Corresponding spatially-varying Dirac mass distribution m(x) that generates the JR state profile in (b)

Fig. 2

Experimental realization of topological Jackiw-Rebbi state. a Focused ion beam image of the fabricated topological junction device from plan (scale bar, 5 μm), bird-eye and cross-sectional (scale bar, 500 nm) views. b Schematic of the angle-resolved confocal microscopy setup. Obj, objective lens; OSA, optical spectrum analyzer. Inset images display the sample (obtained from visible camera) and radiation pattern (from IR camera), with white dashed lines marking the topological interface

Fig. 3

Angle-resolved transmission spectra demonstrating topological phase transition and emergence of the Jackiw-Rebbi state. a Topological phase, F = 0.3, a = 880 nm. b Critical phase, F = 0.44, a = 860 nm. c Trivial phase, F = 0.6, a = 840 nm. d Topological junction sample, revealing a localized JR state within the bandgap

Fig. 4

Spatial characteristics of the Jackiw-Rebbi state. a Visible camera images of the sample and measurement area. b Angle-resolved transmission spectrum of the junction structure. c Local resonance-spectrum under normal incidence with inset showing reflectance profile at the JR resonance wavelength (red dashed)

Fig. 5

Experimental realization of flat-top beam shaping. a Schematic overview of the structure designed for flat-top beam profile generation, illustrating trivial (blue), critical (green), and topological (red) phases. Scale bar, 2 μm. b, c Local transmission spectra for the junction structure with flat-top regions of widths 85 μm and 127.5 μm, respectively. Insets display the reflectance spectra at the JR state resonance wavelength (red dashed line, 1.3 μm), demonstrating the achieved flat-top beam profiles

Fig. 6

Measured leakage-radiation distributions U_LR(x, z) for various Dirac mass configurations. a–c Topological junction structure with critical phase widths w of 0, 85 μm, 127.5 μm, respectively. d Conventional GMR structure with constant Dirac mass (w = 500 μm). Left panels show 2D distributions over |x| ≤ 200 μm and 0 ≤ z ≤ 200 μm. Right graphs compare experimental beam profiles (black lines) with FEM simulations (gray lines). Bottom panels illustrate corresponding Dirac mass distributions m(x) over the measurement region