Light Waves in Two Dimensions

In 1884, a schoolmaster and theo­logian named Edwin Abbott wrote a novella called Flatland, which tells the story of a world populated by sentient two-dimensional shapes. While intended as a satire of rigid Victorian social norms, Flatland has long fascinated mathe­maticians and physicists and served as the setting for many a thought experiment. When a wave of light is confined on a two-dimen­sional plane by certain materials, it becomes something known as a polariton – a particle that blurs the distinc­tion between light and matter. Polari­tons have exciting implications for the future of optical circuits because, unlike electronic integrated circuits, inte­grated optics is difficult to minia­turize with commonly used materials. Polari­tons allow light to be tightly confined to the nanoscale, even potentially to the thickness of a few atoms.

2D refractive optical elements such as lenses, prisms, and metalenses allow for polariton wavefront engineering and sub-wavelength focusing. (Source: Harvard SEAS)

The challenge is, all of the ways we currently have to control light – lenses, waveguides, prisms – are three-dimen­sional. “The ability to control and confine light with fully repro­grammable optical circuits is vital for future highly-integrated nano­photonic devices,” said Michele Tamagnone, a postdoctoral fellow in Applied Physics at the Harvard John A. Paulson School of Engi­neering and Applied Sciences SEAS. Now, Tamagnone and a team of researchers have developed rewritable optical components for surface light waves.

In previous research, the team, led by Federico Capasso, the Robert L. Wallace Professor of Applied Physics, demons­trated a technique to create and control polaritons by trapping light in a flake of hexagonal boron nitride. In this study, the researchers put those flakes on the surface of GeSbTe (GST) – the same materials used on the surface of rewritable CDs and Blu-ray discs. “The rewritable property of GST using simple laser pulses allows for the recording, erasing and rewriting of infor­mation bits. Using that principle, we created lenses, prisms and wave­guides by directly writing them into the material layer,” said postdoc Xinghui Yin.

The lenses and prisms on this material are not three-dimen­sional objects as in our world, but rather two-dimen­sional shapes, as they would be in Flatland. Instead of having a semispherical lens, the polari­tons on the Flatland-esc material pass through a flat semi­circle of refracting material that act as a lens. Instead of traveling through a prism, they travel through a triangle and instead of optical fibers, the polari­tons move through a simple line, which guides the waves along a predefined path.

Using near-field micro­scopy, which allows the imaging of features much smaller than the wavelength of light, the researchers were able to see these components at work. They also demons­­trated for the first time that it is possible to erase and rewrite the optical components that they created. “This research could lead to new chips for appli­cations such as single molecule chemical sensing, since the polaritons in our rewritable devices correspond to frequencies in the region of spectrum where molecules have their telltale absorption fingerprints,” said Capasso. (Source: Harvard SEAS)

Reference: K. Chaudhary et al.: Polariton nanophotonics using phase-change materials, Nat. Commun. 10, 4487 (2019); DOI: 10.1038/s41467-019-12439-4

Link: Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, USA

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