Programmable nanophotonics with the plasmonic phase-change material In₃SbTe₂

RWTH Publications (RWTH Aachen)

Abstract

Conventional infrared optics compared to visible optics are bulky, expensive and mostly lack customization for specific application areas. In recent years, metasurfaces consisting of sophisticatedly arranged metallic or dielectric nanoantennas emerged as a convenient platform for miniaturization of optical components and even realization of customized functionalities based on abrupt changes in amplitude, phase or polarization of the scattered light. However, the fabrication of such optical metasurfaces requires cumbersome and time-consuming techniques. Phase-change materials (PCMs) in combination with direct laser writing promise a convenient alternative for metasurface fabrication. PCMs were originally employed for non-volatile data storage technology due to two different meta-stable phases governed with a strong refractive index contrast. % In combination with metasurfaces, these PCMs allow for tuning the resonances of nanoantennas based on a change in refractive index, but have been also utilized for direct laser writing of metasurfaces. Recently, the plasmonic PCM In$_3$SbTe$_2$ (IST) has been introduced into nanophotonics due to its ability to switch reversibly from a dielectric amorphous phase to a metallic crystalline phase in the entire infrared spectral range. This behavior has been exploited for non-volatile resonance tuning of nanoantennas by reconfiguring the size and shape of the antennas themselves with precise laser pulses. In this thesis, the plasmonic PCM IST is established as a versatile nanophotonic platform, facilitating fabrication and investigation of functional metasurfaces for thermal emission and beam-shaping, as well as rapid prototyping of polariton optics for launching and confinement of surface waves. Polaritons are light coupled to charge oscillations in matter, enabling strong field confinements far below the diffraction limit. First, emissivity-shaping metasurfaces consisting of optically programmed crystalline IST grating bars are demonstrated. The resonance wavelength of the gratings is shifted from 6 to 14 µm by increasing the bar width. Infrared images are investigated with a hyperspectral detector and the polarization dependency is exploited to encode two different images within the same metasurface. Additionally, a narrowband diffuse thermal emitter based on a Fabry Perot resonator with IST on top and a spectral bandwidth of 1 µm is shown. The switchable IST layer enabled switching off the emissivity of the device. Afterwards, infrared beam-shaping is studied with geometric phase metasurfaces consisting of rotated IST rod antennas in combination with circularly polarized light. A multitude of functionalities, such as beam steering, lensing, optical vortex beams and holography is showcased. Finally, IST is combined with the surface phonon polariton hosting polar crystal SiC to launch and confine polaritons. Via optical programming, circular polariton resonators are fabricated and reconfigured to retrieve a field confinement of $\lambda/36$. The surface plasmon polariton (SPP) confinement of doped semiconductors is increased with thin PCM layers enabling the detection of infrared SPPs on bulk materials which has not been shown previously. The ability to directly program arbitrary nanostructures with IST could replace previously established fabrication techniques and will boost the research area of active nanophotonics by rapid-prototyping of functional metasurfaces and polariton optics.

Authors 1

1. Lukas Conrads corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen