Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, surface contributions to transport have so-far only been singled out indirectly via quantum oscillations [1, 2], or for devices based on gated and doped topological insulator thin films, a situation in which the surface carrier mobility could be limited by defect and interface scattering [3][4][5][6]. Here we present the first direct measurement of surface-dominated conduction on an atomically clean surface of bulk-insulating Bi 2 Te 2 Se.Using nano-scale four point setups with variable contact distance, we show that the transport at 30 K is two-dimensional rather than three-dimensional and by combining these measurements with angle-resolved photoemission results from the same crystals, we find a surface state mobility of 390(30) cm 2 V −1 s −1 at 30 K at a carrier concentration of 8.71(7)×10 12 cm −2 .

Metasurfaces have seen a great evolution over the last few years, demonstrating a high degree of control over the amplitude, phase, polarization, and spectral properties of reflected or transmitted electromagnetic waves. Nevertheless, the inherent limitation of static metasurface realizations, which cannot be controlled after their fabrication, engages an ongoing pursuit for reconfigurable metasurfaces with profound tunability. In this paper, we mitigate this grand challenge by demonstrating a new method for free-space rapid optical tunability and modulation, utilizing a planar aluminum nanodisk metasurface coated with indium tin oxide (ITO) on a thin film of lithium niobate (LiNbO) with a chromium/gold (Cr/Au) substrate. Resonance coupling gives rise to an enhanced, confined electromagnetic field residing in the thin film, leading to a narrow and high contrast dip in reflectance of around 1.55 μm. The precise spectral position of this resonance is tuned using the electro-optic Pockels effect by applying an electric bias voltage across the thin film of LiNbO. By doing so, we show that we can likewise modulate the optical reflectance from the metasurface around a wavelength of 1.54 μm. Following that, we experimentally demonstrate a free-space, planar optical modulator with a modulation depth of 40%. The device paves the way for the integration of metasurfaces in applications requiring tunable optical components such as tunable displays, spatial light modulators for advanced imaging, free-space communication, beam scanning LIDARs with no moving parts, and more.

Strongly enhanced and confined electromagnetic fields generated in metal nanostructures upon illumination are exploited in many emerging technologies by either fabricating sophisticated nanostructures or synthesizing colloid nanoparticles. Here we study effects driven by field enhancement in vanishingly small gaps between gold islands in thin films near the electrically determined percolation threshold. Optical explorations using two-photon luminescence (TPL) and near-field microscopies reveals super-cubic TPL power dependencies with white-light spectra, establishing unequivocally that the strongest TPL signals are generated with close to the percolation threshold films, and occurrence of extremely confined (~ 30 nm) and strongly enhanced (~ 100 times) fields at the illumination wavelength. For linearly polarized and sufficiently powerful light, we observe pronounced optical damage with TPL images being sensitive to both wavelength and polarization of illuminating light. We relate these effects to thermally induced morphological changes observed with scanning electron microscopy images. Fascinating physics involved in light interaction with near-percolation metal films along with their straightforward and scalable one-step fabrication procedure promises a wide range of fascinating developments and technological applications within diverse areas of modern nanotechnology, from bio-molecule optical sensing to ultra-dense optical data storage.

Organic–inorganic hybrid perovskites have emerged in recent years as a promising alternative to silicon solar cells and other optoelectronic devices, mostly due to their high photon yields, long carrier lifetime, adjustable bandgap, and other merits. While patterning photonic nanostructures onto their inorganic counterparts is well established to augment their capabilities, lack of compatibility with conventional lithography techniques hinders the implementation of those principles with perovskites. Hereby, the fabrication of MAPbI3 nanophotonic structures such as nanoscale metasurfaces is demonstrated via soft lithography, a method in which the patterning is done when the perovskite is not fully crystallized, allowing for crystallization within the mold with the end result of facile and unharmful imprinting of sub‐micron features onto perovskite thin films, over large areas and with the potential to scale up in a seamless way. By doing so, a substantial increase in light absorption as well as twofold photoluminescence enhancement from the perovskite thin film is shown. These results are supported by spectral and lifetime measurements. This method is pertinent to many device configurations and can assist in realizing the future of high‐efficiency perovskite‐based devices, including solar cells, LEDs, lasers, and more.

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