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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SGPCM (Switching graphene-plasmon with phase-change materials)

Teaser

Summary

Propagating graphene plasmon polaritons are electromagnetic waves originating from collective oscillations of graphene electrons coupled to photons, which enable strong confinement and the electric control of electromagnetic fields at extreme subwavelength-scale dimensions. For that reason, they have great application potential in the fields of infrared bio-sensing and optical signal processing at deep subwavelength scale.

The project â€œSGPCMâ€ aimed to image, control and switch the propagation of graphene plasmon polaritons with the help of controlling the structural phase of a phase change material (PCM), particularly chalcogenide the GeSbTe (GST) alloy, which could provide the basic understanding for developing non-volatile switchable, ultracompact plasmonic devices. Moreover, the project aimed on exploring polaritons and their potential applications in other two dimensional (2D) materials and nanostructures made out of them, such as phonon polaritons in hexagonal boron nitride (hBN).

Work performed

The researcher used scattering-type scanning near-field optical microscope (s-SNOM) to image plasmons in CVD-grown graphene on thermally switched GST substrates. It was observed that the plasmon wavelength changed by switching the PCM. The plasmon wavelength on c-PCM reduces by a factor of two as compared to the case on the a-PCM, which can be explained by the c-PCMÂ´s higher refraction index. However, the graphene plasmons propagate only very short distances in both cases, which can be explained by the low mobility of the CVD-grown graphene. Further, losses in the PCM itself introduces additional plasmon damping. In order to study the influence of PCM switching on polaritons in 2D materials with longer propagation length, the researcher applied s-SNOM to image hBN phonon polaritons. It was found that hBN polaritons propagate much longer on the PCM than graphene plasmons, which is due to their much lower damping. The wavelength of hBN phonon polaritons were also controlled by the switching of the PCM. However, the wavelength change was much smaller than that of graphene plasmons on switched PCM. This observation is explained by the fact that polariton polaritons in hBN slabs are volume-confine modes, which are less sensitive to changes of the environment.

Within the project, the researcher developed the very first hyperbolic mid-infrared metasurface by nanostructuring a thin layer of hBN that supports deep subwavelength-scale phonon polaritons that propagate with in-plane hyperbolic dispersion. He designed the metasurface and used s-SNOM to visualize the concave (anomalous) wavefronts of a diverging polariton beam, which represent a landmark feature of hyperbolic polaritons. The results of this work illustrate how near-field microscopy can be applied to reveal the exotic wavefronts of polaritons in anisotropic materials and demonstrate that nanostructured van der Waals materials can form a highly variable and compact platform for hyperbolic infrared metasurface devices and circuits. (Science 359, 892 (2018))

Furthermore, the researcher has also contributed to three other high-impact publications that are related to polariton manipulation (although not via PCM). First, he has participated a work that leads to the discovery of anisotropic phonon polariton propagation along the surface of Î±-MoO3, a natural van der Waals material. This work verified phonon polaritons with elliptic and hyperbolic in-plane dispersion, as well as with ultra-low losses, which could enable directional and strong lightâ€“matter interactions, nanoscale directional energy transfer and integrated flat optics in applications ranging from bio-sensing to quantum nanophotonics. (Nature, 562, 557â€“562 (2018))

Second, the researcher has participated in a study on resonating surface phonon polaritons in linear hBN antennas. He contributed to the understanding that the antenna resonances are based on waveguide modes originating from the hybridization of hyperbolic surface phonon polaritons that propagate along the edges of the hBN waveguide. This work establishes the basis for the understanding and design of linear waveguides, resonators, sensors and metasurface elements based on hyperbolic 2D materials and metamaterials. (Nature Communications, 8, 15624 (2017))

Third, the researcher also contributed in the first demonstration of surface-enhanced infrared absorption (SEIRA) spectroscopy exploiting well-defined, grating of hBN nanoribbon supporting phonon polaritons. He performed numerical simulations to interpret the measured data and understand the underlying physics. The realization of â€œphononic SEIRAâ€ could become a highly interesting platform for molecular sensing, and even more exciting, for fundamental studies of vibrational strong coupling at the nanoscale. The results open the door for many infrared photonic applications, such as ultra-small high-quality infrared resonators, for example, for field-enhanced infrared spectroscopy. (Light-Science

Final results

The project studied the fundamental optical properties of graphene and other 2D materials. The most important achievement is the first realization of a mid-infrared hyperbolic metasurface based on nanostructured 2D materials (Science 359, 892 (2018). On such artificial surfaces, the optical waves emitted from a point source propagate only in certain directions and with open (concave) wavefronts. These unusual waves are called hyperbolic surface polaritons. Because they propagate only in certain directions, and with wavelengths that are much smaller than that of light in free space, they could help to miniaturize optical devices for sensing and signal processing. The work further demonstrates how s-SNOM, a rather new imaging technology commercialized by a German company, can be applied to unveil exotic optical phenomena in anisotropic materials and for verifying new metasurface design principles.