The Potential of Combining Thermal Scanning Probes and Phase‐Change Materials for Tunable Metasurfaces

Abstract

ultra-thin equivalents, but may also introduce new functionalities (phase discontinuities, anomalous reflection, and refraction, etc.). [2,3] However, the shape of the wavefront is defined by the metasurface design, including material selection and geometry, and is fixed after fabrication. Active postfabrication control requires the tunability of the optical response of each metasurface element. [4,5] One common approach for active metasurfaces is to capitalize on the change of the material polarizabilityeither of the scatterer or its surroundings. This can be achieved, for example, by modulation of the charge density in doped semiconductors (e.g., GaAs) or graphene, [6] by changing the state of liquid crystals adjacent to metallic antennas, [7] or by including phase-transition (e.g., vanadium dioxide VO 2) [8-10] or phase-change [e.g., germanium antimony telluride (GeTe) x (Sb 2 Te 3) 1−x ] [11] materials in the metasurface. Mechanical tuning with stretchable substrates or actuators, [12] chemical reactions at the scatterers, [13] or enhanced optical nonlinearities [14] are other possible approaches for post-fabrication tunability. Phase-change materials (PCMs) are among the best-suited materials to provide tunability of metasurfaces due to the property contrast between their amorphous (A) and crystalline (C) phase. [15] In contrast to phase-transition materials (e.g., VO 2), PCMs feature non-volatile states and thus, no energy is needed to maintain the material properties. The structural change is accompanied with a refractive index change Δn = |n C − n A | Metasurfaces allow for the spatiotemporal variation of amplitude, phase, and polarization of optical wavefronts. Implementation of active tunability of metasurfaces promises compact flat optics capable of reconfigurable wavefront shaping. Phase-change materials (PCMs) are a prominent material class enabling reconfigurable metasurfaces due to their large refractive index change upon structural transition. However, commonly employed laser-induced switching of PCMs limits the achievable feature sizes and restricts device miniaturization. Thermal scanning-probe-induced local switching of the PCM germanium telluride is proposed to realize near-infrared metasurfaces with feature sizes far below what is achievable with diffraction-limited optical switching. The design is based on a planar multilayer and does not require fabrication of protruding resonators as commonly applied in the literature. Instead, it is numerically demonstrated that a broad-band tuning of perfect absorption can be realized by the localized tip-induced crystallization of the PCM. The spectral response of the metasurface is explained using resonance mode analysis and numerical simulations. To facilitate experimental realization, a theoretical description of the tip-induced crystallization employing multiphysics simulations is provided to demonstrate the great potential for fabricating compact reconfigurable metasurfaces. The concept can be applied not only for plasmonic sensing and spatial freq...

Phase Change Metasurfaces by Continuous or Quasi-Continuous Atoms for Active Optoelectronic Integration

“…Other than the frequently used nano-antennas, SRRs or crosses for a GST-integrated metasurface hybrid framework, other atoms such as nano-holes, squares and rings were also employed for active tuning, e.g., the tunable extraordinary transmission (EOT) of visible and near-IR light by both electrically and optically induced GST phase transition [ 113 , 114 ], the mid-IR transmissive filter [ 115 ] and the lately reported mid-wave spectral filter [ 116 ], etc. In addition, due to the appreciable visible and UV loss of GST, most GST-based devices were demonstrated in the middle-IR range except a few that covered UV, visible and near-IR range [ 117 , 118 , 119 ].…”

Design of Phase‐Change Memory Using Apertureless Scanning Near‐Field Optical Microscopy in the Near‐Infrared Region

“…Most recently a concept of combining thermal scanning probe with PCMs to achieve a reconfigurable metasurface was proposed. [ 31 ] According to this idea, a thermal scanning probe heated to 1223 K was in contact with the sample having amorphous GeTe layer capped with SiO 2 media. The thermal energy released from the heated probe enabled the phase‐transformation inside the GeTe layer, and an ultrasmall feature size can be obtained by carefully shrinking the probe diameter.…”

Ultra‐Thin Switchable Absorbers Based on Lossy Phase‐Change Materials

“…To achieve more versatile absorbers, research has been directed towards realizing tunable absorption functionality. [8,[14][15][16][17][18][19][20][21][22] Phase-change materials (PCMs) are especially interesting for this because they offer non-volatile tuning (in contrast to the volatile tuning with transparent conducting oxides or phasetransition materials) by switching between their amorphous and crystalline structural phases whose optical properties differ significantly, because of a unique bonding mechanism, referred to as metavalent bonding. [23][24][25][26] They can be switched between their phases by thermal, [27] electrical, [28] or optical heating [29] on time-scales down to a few nanoseconds [30] and they have already been successfully commercialized in products like re-writable DVDs [29] and PC-RAM.…”

“…In addition, these PCMs feature low losses in the infrared, which makes them suitable for many nanophotonics applications [34][35][36] like waveguides, [37,38] photonic memory, [39,40] polaritonics, [41][42][43][44] tunable metasurfaces, [45][46][47][48][49][50][51][52][53] and tunable metasurface absorbers. [8,14,18,[54][55][56][57] But the same low-loss optical properties become disadvantageous for constructing ultra-thin absorbers with thicknesses well beyond the quarterwavelength limit. While their use is still viable in the visible and near-infrared spectral range [58][59][60] where they have significant optical losses, they are no longer suitable at infrared frequencies.…”