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The polar nature of calcite results in lattice vibrations that can be stimulated through gratings and nanostructures to design spatially and spectrally coherent thermal radiation patterns. In order to obtain optimal design control over such patterned materials, it is first necessary to understand the fundamental emissivity properties of the lattice vibrations themselves. Because calcite is a uniaxial material, when the optic axis (OA) is tilted with respect to the crystal surface, the surface wave solutions to Maxwell’s equations and vibrational modes that are permitted will change due to the crystal’s structural anisotropy. This implies that the OA orientation can play a critical role in dictating which modes can be harnessed when designing a narrowband or angular thermal emitter. Here we explore the angle and polarization dependence of the bulk far-field emissivity of unpatterned calcite with tilted OA. We show that by manipulating the OA orientation via crystallographic off-cut, polarization, and sample rotation, the emissivity at a given frequency can vary by as much as 0.8. These results suggest that, in addition to serving as a basis for modifying the behavior of the relevant phonon polaritons, OA orientation can be used to alter the thermal emission pattern without the need for complex lithographic patterning.

Polaritons are hybrid excitations of matter and photons. In recent years, polaritons in van der Waals nanomaterials—known as van der Waals polaritons—have shown great promise to guide the flow of light at the nanoscale over spectral regions ranging from the visible to the terahertz. A vibrant research field based on manipulating strong light–matter interactions in the form of polaritons, supported by these atomically thin van der Waals nanomaterials, is emerging for advanced nanophotonic and opto-electronic applications. Here we provide an overview of the state of the art of exploiting interface optics—such as refractive optics, meta-optics and moire engineering—for the control of van der Waals polaritons. This enhanced control over van der Waals polaritons at the nanoscale has not only unveiled many new phenomena, but has also inspired valuable applications—including new avenues for nano-imaging, sensing, on-chip optical circuitry, and potentially many others in the years to come. This Review discusses the state of the art of interface optics—including refractive optics, meta-optics and moire engineering—for the control of van der Waals polaritons.

Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale1-10. So far these polaritons have been limited to two classes: bulk polaritons, which propagate inside a material, and surface polaritons, which decay exponentially away from an interface. Here we report a near-field observation of ghost phonon polaritons, which propagate with in-plane hyperbolic dispersion on the surface of a polar uniaxial crystal and, at the same time, exhibit oblique wavefronts in the bulk. Ghost polaritons are an atypical non-uniform surface wave solution of Maxwell's equations, arising at the surface of uniaxial materials in which the optic axis is slanted with respect to the interface. They exhibit an unusual bi-state nature, being both propagating (phase-progressing) and evanescent (decaying) within the crystal bulk, in contrast to conventional surface waves that are purely evanescent away from the interface. Our real-space near-field imaging experiments reveal long-distance (over 20 micrometres), ray-like propagation of deeply subwavelength ghost polaritons across the surface, verifying long-range, directional and diffraction-less polariton propagation. At the same time, we show that control of the out-of-plane angle of the optic axis enables hyperbolic-to-elliptic topological transitions at fixed frequency, providing a route to tailor the band diagram topology of surface polariton waves. Our results demonstrate a polaritonic wave phenomenon with unique opportunities to tailor nanoscale light in natural anisotropic crystals.

Perhaps the most significant progress in the field of infrared optics and nanophotonics has been made through the real space realisation of polaritons in two-dimensional materials that provide maximum light confinement functionalities. The recent breakthrough discovery of in-plane hyperbolicity in natural van der Waals materials has revealed a most exciting optical property that enables in-plane anisotropic dispersion. Yet, the most intriguing feature of in-plane anisotropic dispersion is the manipulation of polaritons at the nanoscale. This development has opened a new window of opportunity in order to develop unique nanophotonic devices with unprecedented control. This chapter will cover these developments with focus on fundamental understandings and progress of real space visualisation of in-plane anisotropic polaritons in the near-field range. The last section will conclude with the future prospects of this rapidly emerging area.

Photodetectors based on intrinsic graphene can operate over a broad wavelength range with ultrafast response, but their responsivity is much lower than commercial silicon photodiodes. The combination of graphene with two-dimensional (2D) semiconductors may enhance the light absorption, but there is still a cutoff wavelength originating from the bandgap of semiconductors. Here, we report a highly responsive broadband photodetector based on the heterostructure of graphene and transition metal carbides (TMCs, more specifically Mo2C). The graphene–Mo2C heterostructure enhanced light absorption over a broad wavelength range from ultraviolet to infrared. In addition, there is very small resistance for photoexcited carriers in both graphene and Mo2C. Consequently, photodetectors based on the graphene–Mo2C heterostructure deliver a very high responsivity from visible to infrared telecommunication wavelengths.

Polaritons, originating from the interactions between photons and material excitations, have attracted attention because of their strong field compression and deeply subdiffractional scales. For practical applications, it is crucial to manipulate polaritons efficiently, but doing so has remained challenging because of the relatively poor tunability of traditional polaritonic media. Fortunately, in the past decade, polaritons hosted by van der Waals (vdW) materials have allowed new opportunities to tackle this difficulty. We review the state of the art in the manipulation of polaritons at the extreme scale in vdW materials. Benefiting from the large and expanding catalogue of vdW materials and associated architectures and techniques, more accessible manipulation strategies are expected, not only offering control of light at the nanoscale with new degrees of freedom, but also offering insight into nanophotonics, meta-optics, topological physics and quantum materials., This work was supported by the Australian Research Council (ARC, CE170100039 and DE220100154) and Shenzhen Nanshan District Pilotage Team Program (LHTD20170006). P.L. acknowledges support from the National Natural Science Foundation of China (grant no. 62075070) and the Innovation Fund of WNLO. P.A.-G. acknowledges support from the European Research Council under starting grant no. 715496, 2DNANOPTICA and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation grant no. PID2019-111156GB-I00). J.D.C. acknowledges support from the Office of Naval Research, grant no. N000142212035.

Photodetectors based on intrinsic graphene can operate over a broad wavelength range with ultrafast response, but their responsivity is much lower than commercial silicon photodiodes. The combination of graphene with two-dimensional (2D) semiconductors may enhance the light absorption, but there is still a cutoff wavelength originating from the bandgap of semiconductors. Here, we report a highly responsive broadband photodetector based on the heterostructure of graphene and transition metal carbides (TMCs, more specifically Mo

Owing to their low lattice thermal conductivity, many compounds of the n(PbTe)-m(Bi2Te3) homologous series have been reported in the literature with thermoelectric (TE) properties that still need improvement. For this purpose, in this work, we have implemented the band engineering approach by applying biaxial tensile and compressive strains using the density functional theory (DFT) on various compounds of this series, namely Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5. All the fully relaxed Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5 compounds are narrow band-gap semiconductors. When applying strains, a semiconductor-to-metal transition occurs for all the compounds. Within the range of open-gap, the electrical conductivity decreases as the compressive strain increases. We also found that compressive strains cause larger Seebeck coefficients than tensile ones, with the maximum Seebeck coefficient being located at −2%, −6%, −3% and 0% strain for p-type Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5, respectively. The use of the quantum theory of atoms in molecules (QTAIM) as a complementary tool has shown that the van der Waals interactions located between the structure slabs evolve with strains as well as the topological properties of Bi2Te3 and PbBi2Te4. This study shows that the TE performance of the n(PbTe)-m(Bi2Te3) compounds is modified under strains.

International audience; The Pb2Bi2Te5 compound has been reported in the literature with two stacking sequences –Te–Pb–Te–Bi–Te–Bi–Te–Pb–Te– and –Te–Bi–Te–Pb–Te–Pb–Te–Bi–Te– labelled in this work as A and B, respectively. The electronic and the thermoelectric properties of the Pb2Bi2Te5 compound with the 2 different stacking sequences have been determined from a series of first principles calculations using density functional theory (DFT). The related compounds PbTe and Bi2Te3 have also been investigated for comparison. Different exchange–correlation functionals have been tested, without spin–orbit coupling, which has been found to have important effects. The elastic moduli, dielectric constants, Born effective charges, and phonon dispersion within the quasi-harmonic approximation have also been calculated and based on these calculations results, the thermal conductivity has been determined by solving the Boltzmann transport equation. Additionally, the QTAIM theory was employed to explain the differences in the properties of the 2 stackings. The most interesting compound for thermoelectric applications has been found to be Pb2Bi2Te5 with the stacking B sequence. The highest zT values have been found to be 4.02 in the a-axis direction and 2.26 in the c-axis one.

The Pb2Bi2Te5 compound has been reported in the literature with two stacking sequences -Te-Pb-Te-Bi-Te-Bi-Te-Pb-Te- and -Te-Bi-Te-Pb-Te-Pb-Te-Bi-Te- labelled in this work as A and B, respectively. The electronic and the thermoelectric properties of the Pb2Bi2Te5 compound with the 2 different stacking sequences have been determined from a series of first principles calculations using density functional theory (DFT). The related compounds PbTe and Bi2Te3 have also been investigated for comparison. Different exchange-correlation functionals have been tested, without spin-orbit coupling, which has been found to have important effects. The elastic moduli, dielectric constants, Born effective charges, and phonon dispersion within the quasi-harmonic approximation have also been calculated and based on these calculations results, the thermal conductivity has been determined by solving the Boltzmann transport equation. Additionally, the QTAIM theory was employed to explain the differences in the properties of the 2 stackings. The most interesting compound for thermoelectric applications has been found to be Pb2Bi2Te5 with the stacking B sequence. The highest zT values have been found to be 4.02 in the a-axis direction and 2.26 in the c-axis one.

Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale

Phonon polaritons (PhPs)—light coupled to lattice vibrations—with in-plane hyperbolic dispersion exhibit ray-like propagation with large wave vectors and enhanced density of optical states along certain directions on a surface. As such, they have raised a surge of interest, promising unprecedented manipulation of infrared light at the nanoscale in a planar circuitry. Here, we demonstrate focusing of in-plane hyperbolic PhPs propagating along thin slabs of α-MoO3. To that end, we developed metallic nanoantennas of convex geometries for both efficient launching and focusing of the polaritons. The foci obtained exhibit enhanced near-field confinement and absorption compared to foci produced by in-plane isotropic PhPs. Foci sizes as small as λp/4.5 = λ0/50 were achieved (λp is the polariton wavelength and λ0 is the photon wavelength). Focusing of in-plane hyperbolic polaritons introduces a first and most basic building block developing planar polariton optics using in-plane anisotropic van der Waals materials., J.M.-S. acknowledges financial support from the Ramón y Cajal Program of the Government of Spain and FSE (RYC2018-026196-I) and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation grant number PID2019-110308GA-I00). P.A.-G. acknowledges support from the European Research Council under starting grant no. 715496, 2DNANOPTICA, and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation grant number PID2019-111156GB-I00). J.T.-G. acknowledges support through the Severo Ochoa Program from the Government of the Principality of Asturias (PA-18-PF-BP17-126). G.A.-P. acknowledges support through the Severo Ochoa Program from the Government of the Principality of Asturias (PA-20-PF-BP19-053). K.V.V. and V.S.V. acknowledge the financial support from the Ministry of Science and Higher Education of the Russian Federation (agreement no. 075-15-2021-606). A.Y.N. acknowledges the Spanish Ministry of Science, Innovation, and Universities (national projects MAT2017-88358-C3-3-R and PID2020-115221GB-C42) and the Basque Department of Education (PIBA-2020-1-0014). R.H. acknowledges financial support from the Spanish Ministry of Science, Innovation, and Universities (national project number RTI2018-094830-B-100 and project number MDM-2016-0618 of the Marie de Maeztu Units of Excellence Program) and the Basque Government (grant number IT1164-19).

Polaritons are hybrid excitations of matter and photons. In recent years, polaritons in van der Waals nanomaterials-known as van der Waals polaritons-have shown great promise to guide the flow of light at the nanoscale over spectral regions ranging from the visible to the terahertz. A vibrant research field based on manipulating strong light-matter interactions in the form of polaritons, supported by these atomically thin van der Waals nanomaterials, is emerging for advanced nanophotonic and opto-electronic applications. Here we provide an overview of the state of the art of exploiting interface optics-such as refractive optics, meta-optics and moiré engineering-for the control of van der Waals polaritons. This enhanced control over van der Waals polaritons at the nanoscale has not only unveiled many new phenomena, but has also inspired valuable applications-including new avenues for nano-imaging, sensing, on-chip optical circuitry, and potentially many others in the years to come.

Hyperbolic phonon polaritons (HPhPs) in α-phase molybdenum trioxide (α-MoO3) have recently attracted significant attention. They propagate anisotropically along the flake of α-MoO3 and show anomalously concave wavefronts when being excited by a pointlike source. Such anisotropic propagation is governed by the anisotropic wavevectors of HPhPs, which have been studied in different works. However, extracting the complex-valued wavevectors of all HPhP modes directly from the observed anomalous wavefront remains elusive. Here, we theoretically and experimentally demonstrate that the complex-valued HPhP wavevectors can be accurately quantified by fitting the concave wavefront profiles with a modified damped sine-wave function in all allowed directions. To that end, HPhPs are launched by an infrared antenna on a thin flake of α-MoO3 and are imaged in real space by using scanning near-field optical microscope. From the recorded concave wavefronts of HPhPs, we have experimentally retraced both the real part and the imaginary part of polariton wavevectors along different propagation directions. Our results are of fundamental importance for analyzing the optical properties of HPhPs in α-MoO3, which can also be generic to other anisotropic optical/polaritonic systems.

The fabrication of van der Waals heterostructures mainly extends to two-dimensional (2D) materials that are exfoliated from their bulk counterparts, which is greatly limited by high-volume manufacturing. Here, we demonstrate multilayered PtS2/PtSe2 heterojunctions covering a large area on the SiO2/Si substrate with a maximum size of 2″ in diameter, offering throughputs that can meet the practical application demand. Theoretical simulation was carried out to understand the electronic properties of the PtS2/PtSe2 heterojunctions. Zero-bias photoresponse in the heterojunctions is observed under laser illumination of different wavelengths (405–2200 nm). The PtS2/PtSe2 heterojunctions exhibit broad band photoresponse and high quantum efficiency at infrared wavelengths with lower bounds for the external quantum efficiencies being 1.2% at 1064 nm, 0.2% at 1550 nm, and 0.05% at 2200 nm, and also relatively fast response time at the dozens of millisecond level. The large area, broad band 2D heterojunction photodet...

Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides, and black phosphorus, have attracted intense interest for applications in ultrafast pulsed laser generation, owing to their strong light–matter interactions and large optical nonlinearities. However, due to the mismatch of the bandgap, many of these 2D materials are not suitable for applications at near-infrared (NIR) waveband. Here, we report nonlinear optical properties of 2D α-Mo2C crystals and the usage of 2D α-Mo2C as a new broadband saturable absorber for pulsed laser generation. It was found that 2D α-Mo2C crystals have excellent saturable absorption properties in terms of largely tunable modulation depth and very low saturation intensity. In addition, ultrafast carrier dynamic results of 2D α-Mo2C reveal an ultrashort intraband carrier recovery time of 0.48 ps at 1.55 μm. By incorporating 2D α-Mo2C saturable absorber into either an Er-doped or Yb-doped fiber laser, we are able to generate ultrashort pulses with ver...

The presence of a direct band gap and high carrier mobility in few-layer black phosphorus (BP) offers opportunities for using this material for infrared (IR) light detection. However, the poor air stability of BP and its large contact resistance with metals pose significant challenges to the fabrication of highly efficient IR photodetectors with long lifetimes. In this work, we demonstrate a graphene-BP heterostructure photodetector with ultrahigh responsivity and long-term stability at IR wavelengths. In our device architecture, the top layer of graphene functions not only as an encapsulation layer but also as a highly efficient transport layer. Under illumination, photoexcited electron-hole pairs generated in BP are separated and injected into graphene, significantly reducing the Schottky barrier between BP and the metal electrodes and leading to efficient photocurrent extraction. The graphene-BP heterostructure phototransistor exhibits a long-term photoresponse at near-infrared wavelength (1550 nm) with an ultrahigh photoresponsivity (up to 3.3 × 10

Surface plasmons make a high degree of localization of electromagnetic fields achievable at the vicinity of metal surfaces. Topological insulators (TIs) are a family of materials which are insulating in the bulk but have metallic surfaces caused by the strong spin–orbit coupling. Surface plasmons supported by the surface state on topological insulators have attracted incredible interests from ultraviolet to mid-infrared frequencies. In this work, we experimentally investigate the near-field properties of Bi2Te3 nanosheets using scattering-type scanning near-field optical microscopy (s-SNOM). The s-SNOM tip enables to detect significantly enhanced intensity in its near field at precisely controlled positions with regards to Bi2Te3 structure. With the help of highly position-selective excitation and high-pixel real-space mapping, we discover near-field patterns of bright outside fringes which are associated with its surface-metallic, plasmonic behavior at mid-infrared frequency. Thereby, we experimentally d...

Phonon polaritons—light coupled to lattice vibrations—in polar van der Waals crystals are promising candidates for controlling the flow of energy on the nanoscale due to their strong field confinement, anisotropic propagation and ultra-long lifetime in the picosecond range1,2,3,4,5. However, the lack of tunability of their narrow and material-specific spectral range—the Reststrahlen band—severely limits their technological implementation. Here, we demonstrate that intercalation of Na atoms in the van der Waals semiconductor α-V2O5 enables a broad spectral shift of Reststrahlen bands, and that the phonon polaritons excited show ultra-low losses (lifetime of 4 ± 1 ps), similar to phonon polaritons in a non-intercalated crystal (lifetime of 6 ± 1 ps). We expect our intercalation method to be applicable to other van der Waals crystals, opening the door for the use of phonon polaritons in broad spectral bands in the mid-infrared domain., J.T.-G. and G.Á.-P. acknowledge support through the Severo Ochoa Program from the Government of the Principality of Asturias (nos. PA-18-PF-BP17-126 and PA-20-PF-BP19-053, respectively). J.M.-S. acknowledges finantial support from the Clarín Programme from the Government of the Principality of Asturias and a Marie Curie-COFUND grant (PA-18-ACB17-29) and the Ramón y Cajal Program from the Government of Spain (RYC2018-026196-I). K.C., X.P.A.G., H.V. and M.H.B. acknowledge the Air Force Office of Scientific Research (AFOSR) grant no. FA 9550-18-1-0030 for funding support. I.E. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (grant no. FIS2016-76617-P). A.Y.N. acknowledges the Spanish Ministry of Science, Innovation and Universities (national project no. MAT2017-88358-C3-3-R) and the Basque Government (grant no. IT1164-19). Q.B. acknowledges the support from Australian Research Council (grant nos. FT150100450, IH150100006 and CE170100039). R.H. acknowledges support from the Spanish Ministry of Economy, Industry, and Competitiveness (national project RTI2018-094830-B-100 and the Project MDM-2016-0618 of the María de Maeztu Units of Excellence Program) and the Basque Goverment (grant no. IT1164-19). P.A.-G. acknowledges support from the European Research Council under starting grant no. 715496, 2DNANOPTICA.

The biaxial van der Waals semiconductor α-phase molybdenum trioxide (α-MoO

Phonon polaritons-light coupled to lattice vibrations-in polar van der Waals crystals are promising candidates for controlling the flow of energy on the nanoscale due to their strong field confinement, anisotropic propagation and ultra-long lifetime in the picosecond range

Phonon polaritons (PhPs) have attracted significant interest in the nano-optics communities because of their nanoscale confinement and long lifetimes. Although PhP modification by changing the local dielectric environment has been reported, controlled manipulation of PhPs by direct modification of the polaritonic material itself has remained elusive. Here, chemical switching of PhPs in α-MoO3 is achieved by engineering the α-MoO3 crystal through hydrogen intercalation. The intercalation process is non-volatile and recoverable, allowing reversible switching of PhPs while maintaining the long lifetimes. Precise control of the intercalation parameters enables analysis of the intermediate states, in which the needle-like hydrogenated nanostructures functioning as in-plane antennas effectively reflect and launch PhPs and form well-aligned cavities. We further achieve spatially controlled switching of PhPs in selective regions, leading to in-plane heterostructures with various geometries. The intercalation strategy introduced here opens a relatively non-destructive avenue connecting infrared nanophotonics, reconfigurable flat metasurfaces and van der Waals crystals., Phonon polaritons hold promises for nanophotonic applications but external control of phonon polaritons remains challenging. Here, the authors achieve reversible and non-volatile switching of phonon polariton by modifying crystal structure and lattice vibrations via hydrogenation.

The fabrication of ultrasmall nanogaps (sub-1 nm) with high density is of significant interest and importance in physics, chemistry, life science, materials science, surface science, nanotechnology, and environmental engineering. However, it remains a challenge to generate uncovered and clean sub-1-nm gaps with high density and uniform reproducibility. Here, a facile and low-cost approach is demonstrated for the fabrication of high-density sub-1-nm gaps from Au nanoparticle monolayers as reproducible surface-enhanced Raman scattering (SERS) substrates. Au nanoparticles with larger diameters possess lower surface charge, thus the obtained large-area nanoparticle monolayer generates a high-density of sub-1-nm gaps. In addition, a remarkable SERS performance with a 1011 magnitude for the Raman enhancement is achieved for 120 nm Au nanoparticle monolayers due to the dramatic increase in the electromagnetic field enhancement when the obtained gap is smaller than 0.5 nm. The Au nanoparticle monolayer is also transferred onto a stretchable PDMS substrate and the structural stability and reproducibility of the high-density sub-1-nm gaps in Au monolayer films are illustrated. The resultant Au nanoparticle monolayer substrates with an increasing particle diameter exhibit tunable plasmonic properties, which control the plasmon-enhanced photocatalytic efficiency for the dimerization of p-aminothiophenol. The findings reported here offer a new opportunity for expanding the SERS application.

High-performance infrared (IR) photodetectors made by low dimensional materials promise a wide range of applications in communication, security and biomedicine. Moreover, light-harvesting effects based on novel plasmonic materials and their combinations with two-dimensional (2D) materials have raised tremendous interest in recent years, as they may potentially help the device complement or surpass currently commercialized IR photodetectors. Graphene is a particularly attractive plasmonic material because graphene plasmons are electrically tunable with a high degree of electromagnetic confinement in the mid-infrared (mid-IR) to terahertz regime and the field concentration can be further enhanced by forming nanostructures. Here, we report an efficient mid-IR room-temperature photodetector enhanced by plasmonic effect in graphene nanoresonators (GNRs)/graphene heterostructure. The plasmon polaritons in GNRs are size-dependent with strong field localization. Considering that the size and density of GNRs are controllable by chemical vapor deposition method, our work opens a cost-effective and scalable pathway to fabricate efficient IR optoelectronic devices with wavelength tunability.

Spanish Ministry of Economy, Industry and Competitiveness (national projects MAT2015-65525-R, FIS2014-60195-JIN, MAT2017-88358-C3-3-R, MAT2014-53432-C5-4-R, and the project MDM2016-0618 of the Maria de Maeztu Units of Excellence Programme). P.A.-G. acknowledges support from the European Research Council under Starting Grant 715496, 2DNANOPTICA. J.M.-S. acknowledges support through the Clarín Programme from the Government of the Principality of Asturias and a Marie Curie-COFUND grant (PA-18-ACB17-29). P.L. acknowledges support from a Marie Sklodowska-Cuire individual fellowship (SGPCM-705960), Ma, W., Alonso-González, P., Li, S., Nikitin, A.Y., Yuan, J., Martín-Sánchez, J., Taboada-Gutiérrez, J., Amenabar, I., Li, P., Vélez, S., Tollan, C., Dai, Z., Zhang, Y., Sriram, S., Kalantar-Zadeh, K., Lee, S.-T., Hillenbrand, R., Bao, Q.

The fabrication of van der Waals heterostructures mainly extends to two-dimensional (2D) materials that are exfoliated from their bulk counterparts, which is greatly limited by high-volume manufacturing. Here, we demonstrate multilayered PtS

2D transition metal dichalcogenides (TMDCs) have attracted considerable attention due to their impressively high performance in optoelectronic devices. However, efficient infrared (IR) photodetection has been significantly hampered because the absorption wavelength range of most TMDCs lies in the visible spectrum. In this regard, semiconducting 2D MoTe

The study on the pollination ecology of cultivated Ficus pumila var. awkeotsang showed that this plant had two apparent blossoming periods, i.e., in spring and autumn, which were relatively longer than those of the wild one. Cultivated Ficus pumila var. awkeotsang couldnt rely on the wasps living in the variant syconia to pollinate or lay eggs. Under the circumstances the wasps were separated from wild F. pumila var. awkeotsang, the pollination system was damaged because of the absence of the wasps. The pollination wasps could be introduced artificially to cultivated F. pumila var. awkeotsang, but the pollination system was restored relatively slowly. The rate of syconia hanging on trees could only reach 51.11% after four years, with an average production rate of the female flowers being 64.37%, and an average gall tuming rate of gall flowers being 64.35%. The growing syconia degree of female and male flowering phases was low, and the collective way of the wasps' flying out and the small number of the wasps entering the syconia were the causes of low rate co-existing breeding.

The biaxial van der Waals semiconductor α‐phase molybdenum trioxide (α‐MoO3) has recently received significant attention due to its ability to support highly anisotropic phonon polaritons (PhPs)—infrared (IR) light coupled to lattice vibrations—offering an unprecedented platform for controlling the flow of energy at the nanoscale. However, to fully exploit the extraordinary IR response of this material, an accurate dielectric function is required. Here, the accurate IR dielectric function of α‐MoO3 is reported by modeling far‐field polarized IR reflectance spectra acquired on a single thick flake of this material. Unique to this work, the far‐field model is refined by contrasting the experimental dispersion and damping of PhPs, revealed by polariton interferometry using scattering‐type scanning near‐field optical microscopy (s‐SNOM) on thin flakes of α‐MoO3, with analytical and transfer‐matrix calculations, as well as full‐wave simulations. Through these correlative efforts, exceptional quantitative agreement is attained to both far‐ and near‐field properties for multiple flakes, thus providing strong verification of the accuracy of this model, while offering a novel approach to extracting dielectric functions of nanomaterials. In addition, by employing density functional theory (DFT), insights into the various vibrational states dictating the dielectric function model and the intriguing optical properties of α‐MoO3 are provided., G.Á-P. and J.T.-G. acknowledge support through the Severo Ochoa Program from the I of the Principality of Asturias (grants No. PA-20-PF-BP19-053 and PA-18-PF-BP17-126, respectively). I.E. acknowledges support from the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P). J.M.-S. acknowledges support through a Clarín Marie Curie-COFUND grant from the Government of the Principality of Asturias and the EU (PA18-ACB17-29), and the Ramón y Cajal Program (RYC2018-026196-I) from the Government of Spain. Q. B. acknowledges support from Australian Research Council (ARC, FT150100450, IH150100006 and CE170100039). J.D.C. was supported by the National Science Foundation (U.S.A.) under grant number U0048926. A.Y.N. acknowledges the Spanish Ministry of Science, Innovation and Universities (national project MAT2017-88358-C3-3-R) and Basque Government (grant No. IT1164-19). P.A-G. acknowledges support from the European Research Council under Starting Grant 715496, 2DNANOPTICA.