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.

The recent discovery of natural biaxial hyperbolicity in van der Waals crystals, such as α-MoO3, 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-MoO3 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.

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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 .

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anisotropy is invoked by lithographic patterning, natural hyperbolicity of these vdW materials is attributed to structural anisotropy of crystal unit cell. In particular, hexagonal boron nitride (h-BN), [3] α-phase molybdenum trioxide (α-MoO 3 ) [1,6] and α-phase vanadium pentoxide (α-V 2 O 5 ) [2] are natural hyperbolic materials (NHMs) which exhibit Reststrahlen Bands (RBs)the spectral region between longitudinal optical (LO) and transverse optical (TO) phonons-in the mid-IR spectral region and show hyperbolicity due to interaction of optical phonons with photons (lightmatter interaction). Phonons have a relatively long lifetime compared to plasmons resulting in lower optical losses than their analogous plasmonic-based metamaterials [7][8][9] in which photons are coupled with plasmons. Many NHMs [1][2][3] exhibit hyperbolic anisotropy in mid-IR spectral region (3-30 µm) which has diverse applications like polarized IR imaging, [10] molecular sensing, [11,12] free space communication, [13] and quantum interference. [14] Unlike h-BN, which possesses uniaxial hyperbolic anisotropy (i.e., ε xx = ε yy ≠ ε zz ), α-MoO 3 exhibits in-plane hyperbolic anisotropy (i.e., ε xx ≠ ε yy ≠ ε zz ) which is particularly beneficial for planar mid-IR optical devices. [15,16] With this motivation, there has been recent interest in developing flat optics based on vdW layered materials which can be integrated with chip-scale platforms using vdW integration, operational at room temperature, and does not involve complex lithographic fabrication techniques. [17][18][19][20] We present an application of α-MoO 3 in the mid-IR spectral region, that is, from 545-1000 cm −1 (around 10-18 µm), as a thin film polarizer that reflects the light with one state of polarization while transmitting the light with its orthogonal state of polarization. Here, single-crystal α-MoO 3 thin films are synthesized using physical vapor deposition (schematically shown in Figure 1a) and are transferred on top of potassium bromide (KBr) window, purchased from Edmund Optics, using mechanical exfoliation technique. In-plane anisotropy of the synthesized α-MoO 3 thin film is confirmed using polarizationresolved Raman spectroscopy. We optimize the mid-IR optical responses of α-MoO 3 , mainly transmittance and reflectance, as a function of the thickness. Optimum thickness of α −MoO 3 based IR polarizer is found to lie in the range of 2.5-3.5 µm for which the extinction ratio (ER) is obtained more than 7.5Integration of conventional mid to long-wavelength infrared (IR) polarizers with chip-scale platforms is restricted by their bulky size and complex fabrication. Van der Waals materials based polarizer can address these challenges due to its nonlithographic fabrication, ease of integration with chip-scale platforms, and room temperature operation. In the present work, mid-IR optical response of the sub-wavelength thin films of α-phase molybdenum trioxide (α-MoO 3 ) is investigated for application toward high temperature mid-IR transmission and reflection type thin film...

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