Anisotropic materials are characterized by a unique optical response, which is highly polarization-dependent. Of particular interest are layered materials formed by the stacking of two-dimensional (2D) crystals that are naturally anisotropic in the direction perpendicular to the 2D planes. Black phosphorus (BP) is a stack of 2D phosphorene crystals and a highly anisotropic semiconductor with a direct band gap. We show that the angular dependence of polarized Raman spectra of BP is rather unusual and can be explained only by considering complex values for the Raman tensor elements. This result can be traced back to the electron-photon and electron-phonon interactions in this material.

Characterization of the second-and third-order nonlinear optical susceptibilities of monolayer MoS 2 ... Supplementary Information forCharacterization of the second-and third-order nonlinear optical susceptibilities of monolayer MoS 2 using multiphoton microscopy

In this article, we review the current status of Raman spectroscopy in orthorhombic black phosphorus (BP) (for simplicity, henceforth, referred to as BP). BP is a layered semiconductor crystal that recently regained interest because it can be exfoliated down to the single layer, thus exhibiting 2-D properties. First, we briefly review the crystalline structure and the phonon dispersion relations in BP. Then, the symmetries of the Raman-active modes are discussed in the light of group theory, and the scattering configurations for observing the different phonon modes are presented. Polarized Raman spectroscopy results are discussed and reveal unusual angular dependence features, which can be ascribed to the linear dichroism and to the complex nature of the electron-phonon matrix elements. Edge phonon effects originated from rearrangements of the atomic terminations are also discussed. Subsequently, Raman modes that emerge from interlayer interaction and that are only visible in the few-layer regime are presented and discussed. Finally, we outline new perspectives to BP Raman spectroscopy in directions that remain partially or totally unexplored and that can provide potentially important outcomes to problems such as defects, oxidation, doping, strain, stacking order and other BP-like 2-D materials.

We investigate the effects of two-dimensional confinement on the lasing properties of a classical random laser system operating in the incoherent feedback (diffusive) regime. A suspension of 250 nm rutile (TiO2) particles in a rhodamine 6G solution was inserted into the hollow core of a photonic crystal fiber generating the first random fiber laser and a novel quasi-one-dimensional random laser geometry. A comparison with similar systems in bulk format shows that the random fiber laser presents an efficiency that is at least 2 orders of magnitude higher.

Black phosphorus has recently emerged as a new layered crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and behaviour of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements.