We determine the strongly layer-dependent exciton binding energies in few-layer black phosphorus by infrared spectroscopy.

A hyperbolic plasmonic surface supports highly directional propagating polaritons with extremely large density of states. Such plasmon polaritons have been realized in artificially structured metasurfaces. However, the upper bound of the achievable plasmon wave vector is limited by the structure size, which calls for a natural hyperbolic surface without any structuring. Here, we experimentally demonstrate a natural hyperbolic plasmonic surface based on thin films of WTe2 in the light wavelength range of 16 to 23 microns by far infrared absorption spectroscopy. The topological transition from the elliptic to the hyperbolic regime is further manifested by mapping the iso-frequency contours of the plasmon. Moreover, the anisotropy character and plasmon frequency exhibit prominent temperature dependence. Our study demonstrates the first natural platform to host 2D hyperbolic plasmons, which opens exotic avenues for the manipulation of plasmon propagation, light-matter interaction and light emission in planar photonics.

Interlayer interactions in 2D materials, also known as van der Waals (vdWs) interactions, play a critical role in the physical properties of layered materials. It is fascinating to manipulate the vdWs interaction, and hence to “redefine” the material properties. Here, we demonstrate that in-plane biaxial strain can effectively tune the vdWs interaction of few-layer black phosphorus with thickness of 2-10 layers, using infrared spectroscopy. Surprisingly, our results reveal that in-plane tensile strain efficiently weakens the interlayer coupling, even though the sample shrinks in the vertical direction due to the Poisson effect, in sharp contrast to one’s intuition. Moreover, density functional theory (DFT) calculations further confirm our observations and indicate a dominant role of the puckered lattice structure. Our study highlights the important role played by vdWs interactions in 2D materials during external physical perturbations.

Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast time scales is of central importance in understanding many-body interactions and nonequilibrium quasiparticle dynamics. Here, we applied temporally, spectrally, and spatially resolved pump-probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorus, with layer numbers ranging from 2 to 9. We have observed layer-dependent resonant transient absorption spectra with both photobleaching and red-shifted photoinduced absorption features, which could be attributed to band gap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron-phonon couplings in photoexcited black phosphorus, providing important insights into the ultrafast optical, nanomechanical, and optoelectronic properties of this novel two-dimensional material.

The vibrational and electronic properties of 2-dimensinal (2D) materials can be efficiently tuned by external strain due to their good stretchability. Resonant Raman spectroscopy is a versatile tool to study the physics of phonons, electrons and their interactions simultaneously, which is particularly useful for the investigation of strain effect on 2D materials. Here, for the first time, we report the resonant Raman study of strained few-layer InSe (γ-phase). Under ~ 1% of uniaxial tensile strain, one order of magnitude enhancement of Raman intensity for longitudinal optical (LO) phonon is observed, while other modes exhibit only modest change. Further analysis demonstrates that it arises from the intraband electron-phonon scattering channel for LO phonon in resonance. The large enhancement of Raman intensity provides us a sensitive and novel method to characterize the strain effect and a mapping of the strain distribution in a wrinkled sample is demonstrated. In addition, we observed sizable redshifts of firstorder optical phonon modes. The shift rate exhibits phonon mode dependence, in excellent agreement with density functional perturbation theory (DFPT) calculations.Our study paves the way for sensitive strain quantification in few-layer InSe and its application in flexible electronic and optoelectronic devices. 3 / 41 I. INTRODUCTION Mechanical cleavage of graphene [1] by K. S. Novoselov et al. arouses tremendous research interest in 2D materials. A variety of 2D semimetals and semiconductors have been discovered ever since, such as transition metal dichalcogenides (TMDCs) [2], silicone [3], stanine [4] and black phosphorus [5,6]. Atomically thin indium selenide (γphase) joins the family lately with unique electronic properties [7-9]. Quantum Hall effect was observed in the high quality few-layer InSe electronic devices [9]. Strong quantum confinement in the out-of-plane direction gives rise to layer-dependent bandgap [7], covering a large range of visible and near infrared regions. Few-layer InSe has promised great application potentials in electronics and optoelectronics [10-12]. The mechanical stretchability of 2D materials opens the door for straining, to continuously and reversibly tune their lattice constants and electronic properties [13]. Raman spectroscopy is a crucial diagnostic tool to evaluate the strain effect. Phonon softening and splitting are commonly observed in 2D materials under uniaxial tensile strain, such as graphene [14,15], TMDCs [15,16] and black phosphorus [18,19], indicating the weakening of the bond strength and the symmetry-breaking. The band structure and electronic properties of 2D materials can be engineered efficiently via strain as well. For example, prominent strain-induced shift of the band gap and indirectto-direct bandgap transition were observed in multilayer TMDCs [20]. Owing to the small Young's modulus ( ~ 45 N/m) [21], the bandgap of few-layer InSe can be easily tuned by uniaxial tensile strain with shift rate up to 90-150meV/% [22,23]. Therefore, 5 / 41 II. EXPERI...

The strength of light-matter interaction is of central importance in photonics and optoelectronics. For many widely studied two-dimensional semiconductors, such as MoS 2 , the optical absorption due to exciton resonances increases with thickness. However, here we will show, few-layer black phosphorus exhibits an opposite trend. We determine the optical conductivity of few-layer black phosphorus with thickness down to bilayer by infrared spectroscopy. On the contrary to our expectations, the frequency-integrated exciton absorption is found to be enhanced in thinner samples. Moreover, the continuum absorption near the band edge is almost a constant, independent of the thickness. We will show such scenario is related to the quanta of the universal optical conductivity of graphene (σ 0 = e 2 /4ħ), with a prefactor originating from the band anisotropy.

Hyperbolic polaritons exhibit large photonic density of states and can be collimated in certain propagation directions. The majority of hyperbolic polaritons are sustained in man-made metamaterials. However, natural-occurring hyperbolic materials also exist. Particularly, natural in-plane hyperbolic polaritons in layered materials have been demonstrated in MoO3 and WTe2, which are based on phonon and plasmon resonances respectively. Here, by determining the anisotropic optical conductivity (dielectric function) through optical spectroscopy, we predict that monolayer black phosphorus naturally hosts hyperbolic exciton-polaritons due to the pronounced in-plane anisotropy and strong exciton resonances. We simultaneously observe a strong and sharp ground state exciton peak and weaker excited states in high quality monolayer samples in the reflection spectrum, which enables us to determine the exciton binding energy of ~452 meV. Our work provides another appealing platform for the in-plane natural hyperbolic polaritons, which is based on excitons rather than phonons or plasmons.

In 2D materials, the quantum confinement and van der Waals‐type interlayer interactions largely govern the fundamental electronic and optical properties, and the dielectric screening plays a dominant role in the excitonic properties. This suggests strongly layer‐dependent properties and a central topic is to characterize and control the interlayer interactions in 2D materials and heterostructures. Black phosphorus is an emerging 2D semiconductor with unusually strong interlayer interactions and widely tunable direct bandgaps from the monolayer to the bulk, offering an ideal platform to probe the layer‐dependent properties and the crossover from 2D to 3D (i.e., the scaling effects). In this review, a comprehensive and thorough summary of the fundamental physical properties of black phosphorus is presented, with a special focus on the layer‐dependence character, including the electronic band structures, optical absorption and photoluminescence, and excitonic properties, as well as the band structure engineering by means of electrical gating, strain, and electrochemical intercalation. Finally, an outlook is given for the future research.