The recent discovery of natural biaxial hyperbolicity in van der Waals crystals, such as α ‐MoO ₃ , has opened up new avenues for mid‐IR nanophotonics due to their deep subwavelength phonon polaritons. However, a significant challenge is the lack of active tunability of these hyperbolic phonon polaritons. In this work, we investigate heterostructures of graphene and α ‐MoO ₃ for actively tunable hybrid plasmon phonon polariton modes via electrostatic gating in the mid‐infrared spectral region. We observe a unique propagation direction dependent hybridization of graphene plasmon polaritons with hyperbolic phonon polaritons for experimentally feasible values of graphene chemical potential. We further report an application to tunable valley quantum interference in this system with a broad operational bandwidth due to the formation of these hybrid modes. This work presents a lithography‐free alternative for actively tunable, anisotropic spontaneous emission enhancement using a sub‐wavelength thick naturally biaxial hyperbolic material.

The vast repository of van der Waals (vdW) materials supporting polaritons offers numerous possibilities to tailor electromagnetic waves at the nanoscale. The development of twistoptics—the modulation of the optical properties by twisting stacks of vdW materials—enables directional propagation of phonon polaritons (PhPs) along a single spatial direction, known as canalization. Here we demonstrate a complementary type of directional propagation of polaritons by reporting the visualization of unidirectional ray polaritons (URPs). They arise naturally in twisted hyperbolic stacks with very different thicknesses of their constituents, demonstrated for homostructures of $$\alpha$$α -MoO₃ and heterostructures of $$\alpha$$α -MoO₃ and $$\beta$$β -Ga₂O₃. Importantly, their ray-like propagation, characterized by large momenta and constant phase, is tunable by both the twist angle and the illumination frequency. Apart from their fundamental importance, our findings introduce twisted asymmetric stacks as efficient platforms for nanoscale directional polariton propagation, opening the door for applications in nanoimaging, (bio)-sensing, or polaritonic thermal management.

Mid-infrared (IR) spectral region is of immense importance for astronomy, medical diagnosis, security and imaging due to the existence of the vibrational modes of many important molecules in this spectral range. Therefore, there is a particular interest in miniaturization and integration of IR optical components. To this end, 2D van der Waals (vdW) crystals have shown great potential owing to their ease of integration with other optoelectronic platforms and room temperature operation. Recently, 2D vdW crystals of $$\alpha$$α - $$\hbox {MoO}_{3}$$MoO 3 and $$\alpha$$α - $$\hbox {V}_2 \hbox {O}_5$$V 2 O 5 have been shown to possess the unique phenomenon of natural in-plane biaxial hyperbolicity in the mid-infrared frequency regime at room temperature. Here, we report a unique application of this in-plane hyperbolicity for designing highly efficient, lithography free and extremely subwavelength mid-IR photonic devices for polarization engineering. In particular, we show the possibility of a significant reduction in the device footprint while maintaining an enormous extinction ratio from $$\alpha$$α - $$\hbox {MoO}_{3}$$MoO 3 and $$\alpha$$α - $$\hbox {V}_2$$V 2 $$\hbox {O}_5$$O 5 based mid-IR polarizers. Furthermore, we investigate the application of sub-wavelength thin films of these vdW crystals towards engineering the polarization state of incident mid-IR light via precise control of polarization rotation, ellipticity and relative phase. We explain our results using natural in-plane hyperbolic anisotropy of $$\alpha$$α - $$\hbox {MoO}_{3}$$MoO 3 and $$\alpha$$α - $$\hbox {V}_2$$V 2 $$\hbox {O}_5$$O 5 via both analytical and full-wave electromagnetic simulations. This work provides a lithography free alternative for miniaturized mid-infrared photonic devices using the hyperbolic anisotropy of $$\alpha$$α - $$\hbox {MoO}_{3}$$MoO 3 and $$\alpha$$α - $$\hbox {V}_2$$V 2 $$\hbox {O}_5$$O 5 .

Lonsdaleite silicon has exhibited a wealth of fascinating properties and is known to have photoluminescence at room temperature. Several researchers have reported the limitations of diamond cubic silicon in the area of optoelectronic devices due to its indirect band gap. Therefore, different phases of silicon are investigated worldwide for the substitute of diamond silicon to overcome its limitation. Recently, it is suggested that lonsdaleite silicon nanowires (SiNWs) can be used as a potential material for optoelectronic applications. Therefore, the optical properties of lonsdaleite silicon nanowires are investigated here by Raman spectroscopy, UV-vis absorption spectroscopy and photoluminescence spectroscopy. Phonon dispersion curve, which has been computed using density functional calculations, is utilized to study the Raman modes of lonsdaleite silicon nanowires. The absorption coefficient of lonsdaleite silicon nanowires shows a remarkable enhancement in comparison with that of diamond structured SiNWs. Furthermore, greenish-yellow photoluminescence is also observed here from lonsdaleite silicon nanowires.