Trapping light by mimicking gravitational lensing

Journal name:
Nature Photonics
Volume:
7,
Pages:
902–906
Year published:
DOI:
doi:10.1038/nphoton.2013.247
Received
Accepted
Published online

Abstract

One of the most fascinating predictions of the theory of general relativity is the effect of gravitational lensing, the bending of light in close proximity to massive stellar objects. Recently, artificial optical materials have been proposed to study the various aspects of curved spacetimes, including light trapping and Hawking radiation. However, the development of experimental ‘toy’ models that simulate gravitational lensing in curved spacetimes remains a challenge, especially for visible light. Here, by utilizing a microstructured optical waveguide around a microsphere, we propose to mimic curved spacetimes caused by gravity, with high precision. We experimentally demonstrate both far-field gravitational lensing effects and the critical phenomenon in close proximity to the photon sphere of astrophysical objects under hydrostatic equilibrium. The proposed microstructured waveguide can be used as an omnidirectional absorber, with potential light harvesting and microcavity applications.

At a glance

Figures

  1. Analogue of light deflection in a gravitational field and microstructured optical waveguide.
    Figure 1: Analogue of light deflection in a gravitational field and microstructured optical waveguide.

    a, Depiction of light deflection by the gravitational field of a massive stellar object. b, Schematic view of the microstructured optical waveguide formed around a microsphere and used to emulate the deflection of light by a gravitational field. In the experimental set-up, a grating is drilled across a 50-nm-thick silver layer, which is then used to couple the incident laser light into the waveguide. Red arrows denote the incident laser beam.

  2. Structural and optical measurements of the sample.
    Figure 2: Structural and optical measurements of the sample.

    a, Interference pattern around the microsphere illuminated by white (top) and blue (bottom) light. b, Surface profile of the PMMA layer measured with AFM. c, The effective refractive index of the microstructured waveguide is extracted, showing a strong power-law dependence with radial distance from the microsphere. d, A particular example of light bending in close proximity to the microsphere. The incident beam is coupled into the waveguide using a diffraction grating drilled into the metal layer.

  3. Scattered field intensity around the microsphere.
    Figure 3: Scattered field intensity around the microsphere.

    a,b, Scattered field intensity observed in the experiment (a) and calculated using a full-wave finite-difference frequency-domain (FDFD) electromagnetic code (b). In the calculations, the effective refractive index is extracted from the experimental data. From top to bottom, the incident beam impact parameter is gradually decreased, resulting in a strong increase in beam deflection from the original path. At critical impact parameters (bottom two images), the light rays approach the photon sphere with a fraction of the incident energy scattered away from the microsphere, while the rest is captured around the microsphere.

  4. Deflection angles.
    Figure 4: Deflection angles.

    Deflection angles measured in the experiment (symbols) and calculated (red and blue lines) based on equation (3). Because of the final width of the incident light beam, two deflection angles θ1 and θ2, corresponding to the edges of the beam (at 1/e intensity), can be extracted unambiguously. The beam envelope is then represented by two points of closest approach rt1 and rt2, corresponding to the two deflection angles shown in the inset and calculated using equation (2). For the purpose of presentation we depict the deflection angles as a function of the geometrical average between the two turning distances: . Error bars due to the experiment are also included. The experimental data closely match the theory for all measured cases. A singularity in the deflection angle is observed for rt2  a = 28.5 µm, corresponding to the photon sphere of our system.

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Affiliations

  1. National Laboratory of Solid State Microstructures & Department of Physics, National Center of Microstructures and Quantum Manipulation, Nanjing University, Nanjing 210093, China

    • C. Sheng,
    • H. Liu,
    • Y. Wang &
    • S. N. Zhu
  2. College of Engineering and Science, Louisiana Tech University, Ruston, Louisiana 71270, USA

    • D. A. Genov

Contributions

C.S., H.L., Y.W. and S.N.Z. proposed and carried out the experiment. D.A.G. contributed to the experimental characterization and interpretation, and proposed and developed the theory. D.A.G., C.S. and H.L. co-wrote the manuscript.

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The authors declare no competing financial interests.

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