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Hyperbolic phonon polaritons (HPhPs) in orthorhombic-phase molybdenum trioxide ($alpha$-MoO3) show in-plane hyperbolicity, great wavelength compression and ultra-long lifetime, therefore holding great potential in nanophotonic applications. However, its polaritonic response in the far-infrared (FIR) range has long remained unexplored due to challenges in experimental characterization. Here, using monochromated electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), we probe HPhPs in $alpha$-MoO3 in both mid-infrared (MIR) and FIR frequencies and correlate their behaviors with microstructures and orientations. We find that low-structural symmetry leads to various phonon modes and multiple Reststrahlen bands (RBs) over a broad spectral range (over 70 meV) and in different directions (55-63 meV and 119-125 meV along b axis, 68-106 meV along c axis, 101-121 meV along a axis). These HPhPs can be selectively excited by controlling the direction of swift electrons. These findings provide new opportunities in nanophotonic and optoelectronic applications such as directed light propagation, hyperlenses and heat transfer.
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The exploitation of phonon-polaritons in nanostructured materials offers a pathway to manipulate infrared (IR) light for nanophotonic applications. Notably, hyperbolic phonons polaritons (HP2) in polar bidimensional crystals have been used to demonstrate strong electromagnetic field confinement, ultraslow group velocities, and long lifetimes (~ up to 8 ps). Here we present nanobelts of {alpha}-phase molybdenum trioxide ({alpha}-MoO3) as a low-dimensional medium supporting HP2 modes in the mid- and far-IR ranges. By real-space nanoimaging, with IR illuminations provided by synchrotron and tunable lasers, we observe that such HP2 response happens via formation of Fabry-Perot resonances. We remark an anisotropic propagation which critically depends on the frequency range. Our findings are supported by the convergence of experiment, theory, and numerical simulations. Our work shows that the low dimensionality of natural nanostructured crystals, like {alpha}-MoO3 nanobelts, provides an attractive platform to study polaritonic light-matter interactions and offer appealing cavity properties that could be harnessed in future designs of compact nanophotonic devices.
Surface phonon polaritons (SPhPs) in polar dielectrics offer new opportunities for infrared nanophotonics due to sub-diffraction confinement with low optical losses. Though the polaritonic field confinement can be significantly improved by modifying the dielectric environment, it is challenging to break the fundamental limits in photon confinement and propagation behavior of SPhP modes. In particular, as SPhPs inherently propagate isotropically in these bulk polar dielectrics, how to collectively realize ultra-large field confinement, in-plane hyperbolicity and unidirectional propagation remains elusive. Here, we report an approach to solve the aforementioned issues of bulk polar dielectrics SPhPs at one go by constructing a heterostructural interface between biaxial van der Waals material (e.g., MoO3) and bulk polar dielectric (e.g., SiC, AlN, and GaN). Due to anisotropy-oriented mode couplings at the interface, the hybridized SPhPs with a large confinement factor (>100) show in-plane hyperbolicity that has been switched to the orthogonal direction as compared to that in natural MoO3. More interestingly, this proof of concept allows steerable, angle-dependent and unidirectional polariton excitation by suspending MoO3 on patterned SiC air cavities. Our finding exemplifies a generalizable framework to manipulate the flow of nano-light and engineer unusual polaritonic responses in many other hybrid systems consisting of van der Waals materials and bulk polar dielectrics.
Hyperbolic phonon polaritons (HPhPs) sustained in van der Waals (vdW) materials exhibit extraordinary capabilities of confining long-wave electromagnetic fields to the deep subwavelength scale. In stark contrast to the uniaxial vdW hyperbolic materials such as hexagonal boron nitride (h-BN), the recently emerging biaxial hyperbolic materials such as {alpha}-MoO3 and {alpha}-V2O5 further bring new degree of freedoms in controlling light at the flatland, due to their distinctive in-plane hyperbolic dispersion. However, the controlling and focusing of such in-plane HPhPs are to date remain elusive. Here, we propose a versatile technique for launching, controlling and focusing of in-plane HPhPs in {alpha}-MoO3 with geometrically designed plasmonic antennas. By utilizing high resolution near-field optical imaging technique, we directly excited and mapped the HPhPs wavefronts in real space. We find that subwavelength manipulating and focusing behavior are strongly dependent on the curvature of antenna extremity. This strategy operates effectively in a broadband spectral region. These findings can not only provide fundamental insights into manipulation of light by biaxial hyperbolic crystals at nanoscale, but also open up new opportunities for planar nanophotonic applications.
Highly confined and low-loss hyperbolic phonon polaritons (HPhPs) sustained in van der Waals crystals exhibit outstanding capabilities of concentrating long-wave electromagnetic fields deep to the subwavelength region. Precise tuning on the HPhP propagation characteristics remains a great challenge for practical applications such as nanophotonic devices and circuits. Here, we show that by taking advantage of the varying air gaps in a van der Waals {alpha}-MoO3 crystal suspended gradiently, it is able to tune the wavelengths and dampings of the HPhPs propagating inside the {alpha}-MoO3. The results indicate that the dependences of polariton wavelength on gap distance for HPhPs in lower and upper Reststrahlen bands are opposite to each other. Most interestingly, the tuning range of the polariton wavelengths for HPhPs in the lower band, which exhibit in-plane hyperbolicities, is wider than that for the HPhPs in the upper band of out-of-plane hyperbolicities. A polariton wavelength elongation up to 160% and a reduction of damping rate up to 35% are obtained. These findings can not only provide fundamental insights into manipulation of light by polaritonic crystals at nanoscale, but also open up new opportunities for tunable nanophotonic applications.
Moire engineering as a configuration method to twist van der Waals materials has delivered a series of advances in electronics, magnetics and optics. Yet these advances stem from peculiar moire superlattices which form at small specific twisting angles. Here we report the configuration of nanoscale light-matter waves-the polaritons-by twisting stacked $alpha$-phase molybdenum trioxide $alpha$-MoO3 slabs in the broad range of 0$^o$ to 90$^o$. Our combined experimental and theoretical results reveal a variety of polariton wavefront geometry and topological transitions via the twisting. The polariton twisting configuration is attributed to the electromagnetic interaction of highly anisotropic hyperbolic polaritons in stacked $alpha$-MoO3 slabs. The nano-polaritons demonstrated in twisted $alpha$-MoO3 hold the promise as tailored nano-light for on-demand nanophotonic functionalities.
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