The terahertz part of the electromagnetic spectrum possesses numerous promising applications such as high-speed wireless communication, security screening, chemical identification, and nondestructive biosensing. [1] However, the terahertz spectral region has remained technologically uncharted, due to the lack of efficient devices to generate, manipulate, and detect terahertz waves. In the past decade, there has been a significant progress in active control of terahertz waves using metamaterials integratedThe strikingly contrasting optical properties of various phases of chalcogenide phase change materials (PCM) has recently led to the development of novel photonic devices such as all-optical non-von Neumann memory, nanopixel displays, color rendering, and reconfigurable nanoplasmonics. However, the exploration of chalcogenide photonics is currently limited to optical and infrared frequencies. Here, a phase change material integrated terahertz metamaterial for multilevel nonvolatile resonance switching with spatial and temporal selectivity is demonstrated. By controlling the crystalline proportion of the PCM film, multilevel, non-volatile, terahertz resonance switching states with long retention time at zero hold power are realized. Spatially selective reconfiguration at submetamaterial scale is shown by delivering electrical stimulus locally through designer interconnect architecture. The PCM metamaterial also features ultrafast optical modulation of terahertz resonances with tunable switching speed based on the crystalline order of the PCM film. The multilevel nonvolatile, spatially selective, and temporally tunable PCM metamaterial will provide a pathway toward development of novel and disruptive terahertz technologies including spatio-temporal terahertz modulators for high speed wireless communication, neuromorphic photonics, and machine-learning metamaterials.

No abstract

Phase change materials provide unique reconfigurable properties for photonic applications that mainly arise from their exotic characteristic to reversibly switch between the amorphous and crystalline nonvolatile phases. Optical pulse based reversible switching of nonvolatile phases is exploited in various nanophotonic devices. However, large area reversible switching is extremely challenging and has hindered its translation into a technologically significant terahertz spectral domain. Here, this limitation is circumvented by exploiting the semiconducting nature of germanium antimony telluride (GST) to achieve dynamic terahertz control at picosecond timescales. It is also shown that the ultrafast response can be actively altered by changing the crystallographic phase of GST. The ease of fabrication of phase change materials allows for the realization of a variable ultrafast terahertz modulator on a flexible platform. The rich properties of phase change materials combined with the diverse functionalities of metamaterials and all-optical ultrafast control enables an ideal platform for design of efficient terahertz communication devices, terahertz neuromorphic photonics, and smart sensor systems.

Spatiotemporal manipulation of electromagnetic waves has recently enabled a plethora of exotic optical functionalities, such as non‐reciprocity, dynamic wavefront control, unidirectional transmission, linear frequency conversion, and electromagnetic Doppler cloak. Here, an additional dimension is introduced for advanced manipulation of terahertz waves in the space‐time, and frequency domains through integration of spatially reconfigurable microelectromechanical systems and photoresponsive material into metamaterials. A large and continuous frequency agility is achieved through movable microcantilevers. The ultrafast resonance modulation occurs upon photoexcitation of ion‐irradiated silicon substrate that hosts the microcantilever metamaterial. The fabricated metamaterial switches in 400 ps and provides large spectral tunability of 250 GHz with 100% resonance modulation at each frequency. The integration of perfectly complementing technologies of microelectromechanical systems, femtosecond optical control and ion‐irradiated silicon provides unprecedented concurrent control over space, time, and frequency response of metamaterial for designing frequency‐agile spatiotemporal modulators, active beamforming, and low‐power frequency converters for the next generation terahertz wireless communications.

Antiferromagnetic insulators are a ubiquitous class of magnetic materials, holding the promise of low-dissipation spin-based computing devices that can display ultra-fast switching and are robust against stray fields. However, their imperviousness to magnetic fields also makes them difficult to control in a reversible and scalable manner. Here we demonstrate a novel proof-of-principle ionic approach to control the spin reorientation (Morin) transition reversibly in the common antiferromagnetic insulator α-Fe2O3 (haematite) – now an emerging spintronic material that hosts topological antiferromagnetic spin-textures and long magnon-diffusion lengths. We use a low-temperature catalytic-spillover process involving the post-growth incorporation or removal of hydrogen from α-Fe2O3 thin films. Hydrogenation drives pronounced changes in its magnetic anisotropy, Néel vector orientation and canted magnetism via electron injection and local distortions. We explain these effects with a detailed magnetic anisotropy model and first-principles calculations. Tailoring our work for future applications, we demonstrate reversible control of the room-temperature spin-state by doping/expelling hydrogen in Rh-substituted α-Fe2O3.

create mixed dimensional heterostructures [5] of 2D materials and functional substrates, which provide an even wider opportunity to tailor diverse material functionalities and interface phenomena. In this regard, a very promising substrate is SrTiO 3 (STO), a transition metal oxide with an ABO 3 -type perovskite crystal structure, that has attracted significant attention due to both its unusual bulk properties like high dielectric constant, [6] quantum paraelectricity, [7] and several interface phenomena such as the observation of 2D electron gas at interfaces with oxide insulators [8] and enhancement of superconducting T c at the interfaces with 2D FeSe. [9] The speciality of STO lies in its rich phonon spectrum centered on its AFD structural phase transition [10] (from cubic to tetragonal). The uniqueness of the phonons in STO, specifically the low energy soft phonons when compared to conventional phonons present in most materials lies in the following: i) they arise at a phase transition temperature and their frequency (energy) softens to zero near the transition ii) their intensity increases at low temperatures whereas conventionally phonon intensity decreases at lower temperatures The reduced electrical screening in 2D materials provides an ideal platform for realization of exotic quasiparticles, that are robust and whose functionalities can be exploited for future electronic, optoelectronic, and valleytronic applications. Recent examples include an interlayer exciton, where an electron from one layer binds with a hole from another, and a Holstein polaron, formed by an electron dressed by a sea of phonons. Here, a new quasiparticle is reported, "polaronic trion" in a heterostructure of MoS 2 /SrTiO 3 (STO). This emerges as the Fröhlich bound state of the trion in the atomically thin monolayer of MoS 2 and the very unique low energy soft phonon mode (≤7 meV, which is temperature and field tunable) in the quantum paraelectric substrate STO, arising below its structural antiferrodistortive (AFD) phase transition temperature. This dressing of the trion with soft phonons manifests in an anomalous temperature dependence of photoluminescence emission leading to a huge enhancement of the trion binding energy (≈70 meV). The soft phonons in STO are sensitive to electric field, which enables field control of the interfacial trion-phonon coupling and resultant polaronic trion binding energy. Polaronic trions could provide a platform to realize quasiparticle-based tunable optoelectronic applications driven by many body effects.The last few years have witnessed several extraordinary quantum functionalities emerging at interfaces of van der Waals heterostructures. [1][2][3][4] Recently, there has been a growing interest to

In this manuscript, we demonstrate a method based on atomic force microscopy which enables local probing of surface wettability. The maximum pull-off force, obtained from force spectroscopy shows a remarkable correlation with the macroscopically observed water contact angle, measured over a wide variety of surfaces starting from hydrophilic, all the way through to hydrophobic ones. This relationship, consequently, facilitates the establishment of a universal behaviour. The adhesion forces scale with the polar component of surface energy. However, no such relation could be established with the dispersive component. Hence, we postulate that the force(s) which enable us to correlate the force spectroscopy data measured on the nanoscale to the macroscopic contact angle are primarily arising from electrostatic-dipole-dipole interactions at the solid-liquid interface. London forces play less of a role. This effect in is line with density functional theory (DFT) calculations suggesting a higher degree of hydroxylation of hydrophilic surfaces. This result shows that molecular simulations and measurements on an atomic scale can be extrapolated to macroscopic surface wetting problems.

Herein, we report a systematic study of water contact angle (WCA) of rare-earth oxide thin-films. These ultra-smooth and epitaxial thin-films were grown using pulsed laser deposition and then characterized using X-Ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), and atomic force microscopy (AFM). Through both the traditional sessile drop and the novel f-d method, we found that the films were intrinsically hydrophilic (WCA < 10°) just after being removed from the growth chamber, but their WCAs evolved with an exposure to the atmosphere with time to reach their eventual saturation values near 90° (but always stay 'technically' hydrophilic). X-Ray photoelectron spectroscopy analysis was used to further investigate qualitatively the nature of hydrocarbon contamination on the freshly prepared as well as the environmentally exposed REO thin-film samples as a function of the exposure time after they were removed from the deposition chamber. A clear correlation between the carbon coverage of the surface and the increase in WCA was observed for all of the rare-earth films, indicating the extrinsic nature of the surface wetting properties of these films and having no relation to the electronic configuration of the rare-earth atoms as proposed by Azimi et al.