Hyperbolic media have attracted much attention in the photonics community due to their ability to confine light to arbitrarily small volumes and their potential applications to super-resolution technologies. The two-dimensional counterparts of these media can be achieved with hyperbolic metasurfaces that support in-plane hyperbolic guided modes upon nanopatterning, which, however, poses notable fabrication challenges and limits the achievable confinement. We show that thin flakes of a van der Waals crystal, α-MoO3, can support naturally in-plane hyperbolic polariton guided modes at mid-infrared frequencies without the need for patterning. This is possible because α-MoO3 is a biaxial hyperbolic crystal with three different Reststrahlen bands, each corresponding to a different crystalline axis. These findings can pave the way toward a new paradigm to manipulate and confine light in planar photonic devices.

Controlling the twist angle between two stacked van der Waals (vdW) crystals is a powerful approach for tuning their electronic and photonic properties. Hyperbolic media have recently attracted much attention due to their ability to tailor electromagnetic waves at the subwavelength-scale which, however, usually requires complex patterning procedures. Here, we demonstrate a lithography-free approach for manipulating the hyperbolicity by harnessing the twist-dependent coupling of phonon polaritons in double-layers of vdW α-MoO3, a naturally biaxial hyperbolic crystal. The polariton isofrequency contours can be modified due to this interlayer coupling, allowing for controlling the polaritonic characteristics by adjusting the orientation angles between the two layers. Our findings provide opportunities for control of nanoscale light flow with twisted stacks of vdW crystals.

Two-dimensional van der Waals (vdW) crystals can sustain various types of polaritons with strong electromagnetic confinements, making them highly attractive for the nanoscale photonic and optoelectronic applications. While extensive experimental...

Nanostructured all-inorganic metal halide perovskites have attracted considerable attention due to their outstanding photonic and optoelectronic properties. Particularly, they can exhibit room-temperature exciton−polaritons (EPs) capable of confining electromagnetic fields down to the subwavelength scale, enabling efficient light harvesting and guiding. However, a real-space nanoimaging study of the EPs in perovskite crystals is still absent. Additionally, few studies focused on the ambient-pressure and reliable fabrication of large-area CsPbBr 3 microsheets. Here, CsPbBr 3 orthorhombic microsheet single crystals were successfully synthesized under ambient pressure. Their EPs were examined using a real-space nanoimaging technique, which reveal EP waveguide modes spanning the visible to near-infrared spectral region. The EPs exhibit a sufficient long propagation length of over 16 μm and a very low propagation loss of less than 0.072 dB•μm −1 . These results demonstrate the potential applications of CsPbBr 3 microsheets as subwavelength waveguides in integrated optics.

Polar van der Waals (vdW) crystals that support phonon polaritons have recently attracted much attention because they can confine infrared and terahertz (THz) light to deeply subwavelength dimensions, allowing for the guiding and manipulation of light at the nanoscale. The practical applications of these crystals in devices rely strongly on deterministic engineering of their spatially localized electromagnetic field distributions, which has remained challenging. The polariton interference can be enhanced and tailored by patterning the vdW crystal -MoO 3 into microstructures that support highly in-plane anisotropic phonon polaritons. The orientation of the polaritonic in-plane isofrequency curve relative to the microstructure edges is a critical parameter governing the polariton interference, rendering the configuration of infrared electromagnetic field localizations by enabling the tuning of the microstructure size and shape and the excitation frequency. Thus, the study presents an effective rationale for engineering infrared light flow in planar photonic devices.

Specific geometric morphology and improved crystalline properties are of great significance for the development of materials in micro–nano scale. However, for high-melting molybdenum (Mo), it is difficult to get high-quality structures exhibiting a single-crystalline nature and preconceived morphology simultaneously. In this paper, a pyramid-shaped single-crystalline Mo nanostructure was prepared through a thermal evaporation technique, as well as a series of experimental controls. Based on detailed characterizations, the growth mechanism was demonstrated to follow a sequential process that includes MoO2 decomposition and Mo deposition, single-crystalline islands formation, layered nucleation, and competitive growth. Furthermore, the product was measured to show excellent physical properties. The prepared nanostructures exhibited strong nano–indentation hardness, elastic modulus, and tensile strength in mechanical measurements, which are much higher than those of the Mo bulks. In the measurement of electronic characteristics, the individual structures indicated very good electrical transport properties, with a conductance of ∼0.16 S. The prepared film with an area of 0.02 cm2 showed large-current electron emission properties with a maximum current of 33.6 mA and a current density of 1.68 A cm–2. Optical properties of the structures were measured to show obvious electromagnetic field localization and enhancement, which enabled it to have good surface enhanced Raman scattering (SERS) activity as a substrate material. The corresponding structure–response relationships were further discussed. The reported physical properties profit from the basic features of the Mo nanostructures, including the micro–nano scale, the single-crystalline nature in each grain, as well as the pyramid-shaped top morphology. The findings may provide a potential material for the research and application of micro–nano electrons and photons.

Highly confined and low‐loss hyperbolic phonon polaritons (HPhPs) sustained in van der Waals crystals exhibit outstanding potential to concentrate the long‐wave electromagnetic fields deep into the subwavelength region. However, precise tuning on the HPhP propagation characteristics is a critical challenge to facilitate its practical applications in nanophotonic devices and circuits. This study, by taking advantage of the varying air gaps in a suspended van der Waals α‐MoO3 crystal, shows the feasibility to tune wavelength and damping rate of the HPhPs propagating inside the α‐MoO3. The results indicate that the dependence of polariton wavelength on the gap distance for HPhPs in lower and upper Reststrahlen bands contradict each other. Most interestingly, the tuning range of the polariton wavelengths for HPhPs in the lower band, which exhibits in‐plane hyperbolicity, is wider than that for HPhPs in the upper band which exhibits out‐of‐plane hyperbolicity. A polariton wavelength elongation of up to 160% and a reduction in the damping rate up to 35% is recorded. These findings can not only provide fundamental insight into the nanoscale manipulation of light by polaritonic crystals, but also open up new opportunities for tunable nanophotonic applications.

One of the main bottlenecks in the development of terahertz (THz) and long-wave infrared (LWIR) technologies is the limited intrinsic response of traditional materials. Hyperbolic phonon polaritons (HPhPs) of van der Waals semiconductors couple strongly with THz and LWIR radiation. However, the mismatch of photon − polariton momentum makes far-field excitation of HPhPs challenging. Here, we propose an In-Plane Hyperbolic Polariton Tuner that is based on patterning van der Waals semiconductors, here α-MoO3, into ribbon arrays. We demonstrate that such tuners respond directly to far-field excitation and give rise to LWIR and THz resonances with high quality factors up to 300, which are strongly dependent on in-plane hyperbolic polariton of the patterned α-MoO3. We further show that with this tuner, intensity regulation of reflected and transmitted electromagnetic waves, as well as their wavelength and polarization selection can be achieved. Our results can help the development of THz and LWIR miniaturized devices.