Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy has been recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS3 and h-BN crystals. This issue inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Waals interaction. To do this, we made correlative far- and near-field characterizations validated by first-principle calculations that reveal a huge birefringence of 1.5 in the infrared and 3 in the visible light for MoS2. Our findings demonstrate that this remarkable anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics.

We report a comprehensive experimental study of optical and electrical properties of thin polycrystalline gold films in a wide range of film thicknesses (from 20 to 200 nm). Our experimental results are supported by theoretical calculations based on the measured morphology of the fabricated gold films. We demonstrate that the dielectric function of the metal is determined by its structural morphology. Although the fabrication process can be absolutely the same for different films, the dielectric function can strongly depend on the film thickness. Our studies show that the imaginary part of the dielectric function of gold, which is responsible for optical losses, rapidly increases as the film thickness decreases for thicknesses below 80 nm. At the same time, we do not observe a noticeable dependence of optical constants on the film thickness for thicker samples. These findings establish design rules for thin-film plasmonic and nanophotonic devices.

Layered semiconductors such as transition metal dichalcogenides (TMDs) offer endless possibilities for designing modern photonic and optoelectronic components. However, their optical engineering is still a challenging task owing to multiple obstacles, including the absence of a rapid, contactless, and the reliable method to obtain their dielectric function as well as to evaluate in situ the changes in optical constants and exciton binding energies. Here, we present an advanced approach based on ellipsometry measurements for retrieval of dielectric functions and the excitonic properties of both monolayer and bulk TMDs. Using this method, we conduct a detailed study of monolayer MoS 2 and its bulk crystal in the broad spectral range (290-3300 nm). In the near-and midinfrared ranges, both configurations appear to have no optical absorption and possess an extremely high dielectric permittivity making them favorable for lossless subwavelength photonics. In addition, the proposed approach opens a possibility to observe a previously unreported peak in the dielectric function of monolayer MoS 2 induced by the use of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) seeding promoters for MoS 2 synthesis and thus enables its applications in chemical and biological sensing. Therefore, this technique as a whole offers a state-of-the-art metrological tool for next-generation TMD-based devices.

films are also the key element of plasmonic waveguides [10,11] and hyperbolic metamaterials. [12,13] Growth of continuous and ultrathin gold films on different substrates, such as glass, silicon oxide, silicon nitride, graphene etc. is notoriously difficult due to the poor wetting of gold to these substrates. [14,15] The growth kinetics of metal films is generally determined by the adsorption and diffusion behavior of metal adatoms on the substrate. A small ratio of the adsorption energy of metal adatoms on the substrate to the bulk cohesive energy of the metal and low diffusion barrier for an adatom favor the 3D island growth behavior also known as the Volmer-Weber growth mode. [16] Within the framework of this growth model, the formation of a metal film is associated with the following stages: nucleation of islands, island growth, island impingement and coalescence, percolation, and channel filling to finally form a continuous thin film. To reduce the percolation threshold of ultrathin gold films, adhesion or seed layers of Ti, Cr, Ni, Pt, or Ge are commonly used. However, these adhesion layers significantly affect the optical and electrical properties of ultrathin metal nanostructures. [17][18][19][20][21][22] Recently, the organosilane-based adhesion layers (mercaptosilanes and aminosilanes) were used for the deposition of sub-10 nm thick continuous Au films on silicon and glass surfaces. [23][24][25][26][27] However, organosilanes are not compatible with nonoxidized silicon surfaces and poorly compatible with standard lift-off procedures, that imposes severe limitations to their use as adhesion layers. [21] Adhesion layers based on organosilanes are also inefficient for the deposition of atomically thin metal films [14] and does not move us closer to the deposition of 2D layers from bulk plasmonic metals. Actually, the latter seems now as impossible as the deposition of atomically thin carbon films had been considered before 2004. [28] In the present paper, we propose the use of MoS 2 monolayer as an entirely new type of "universal" (i.e., it can be transferred to any arbitrary substrate) [29][30][31] adhesion layer for ultrathin (<10 nm) high-quality continuous gold films. To test the feasibility of this idea, we deposited ultrathin gold films of different thicknesses onto monolayer MoS 2 , grown on silicon dioxide substrates (Figure 1a), and studied their structural and optical properties.An electron beam evaporator Nano Master NEE-4000 was used to deposit Au films on top of atmospheric pressure CVD (APCVD)-grown full area coverage MoS 2 monolayers on silicon wafers with a 285 nm thick SiO 2 coating (from 2D semiconductors Inc.). The deposition was performed at Sub-10 nm continuous metal films are promising candidates for flexible and transparent nanophotonics and optoelectronic applications. In this article, it is demonstrated that monolayer MoS 2 is a perspective adhesion layer for the deposition of continuous conductive gold films with a thickness of only 3-4 nm. Optical properties of continuous ultrath...

Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy was recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS3 and h-BN crystals. This inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Walls interaction. To do this, we carried out a correlative far- and near-field characterization validated by first-principle calculations that reveals an unprecedented birefringence of 1.5 in the infrared and 3 in the visible light for MoS2. Our findings demonstrate that this outstanding anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics.