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Two-dimensional (2D) tin(II) sulfide (SnS) crystals belong to a class of orthorhombic semiconducting materials that are lately attracting significant interest, given their remarkable properties, such as in-plane anisotropic optical and electronic response, multiferroic nature and lack of inversion symmetry. The 2D SnS crystals exhibit anisotropic response along the in-plane armchair (AC) and zigzag (ZZ) crystallographic directions, offering an additional degree of freedom in manipulating their behavior. Therefore, calculating the AC/ZZ directions is important in characterizing the 2D SnS crystals. In this work, we take advantage of the lack of inversion symmetry of the 2D SnS crystal, that produces second harmonic generation (SHG), to perform polarization-resolved SHG (P-SHG) nonlinear imaging of the in-plane anisotropy. We fit the P-SHG experimental data with a nonlinear optics model, that allows us to calculate the AC/ZZ orientation from every point of the 2D crystal and to map with high-resolution the AC/ZZ direction of several 2D SnS flakes belonging in the same field of view. It is found that the P-SHG intensity polar patterns are associated with the crystallographic axes of the flakes and with the relative strength of the second order nonlinear susceptibility tensor in different directions. Therefore, our method provides quantitative information of the optical in-plane anisotropy of orthorhombic 2D crystals, offering great promise for performance characterization during device operation in the emerging optoelectronic applications of such crystals.

Atomically thin two-dimensional (2D) materials can be vertically stacked with van der Waals bonds, which enable interlayer coupling. In the particular case of transition metal dichalcogenide (TMD) bilayers, the relative direction between the two monolayers, coined as twist-angle, modifies the crystal symmetry and creates a superlattice with exciting properties. Here, we demonstrate an all-optical method for pixel-by-pixel mapping of the twist-angle with resolution of 0.23 degrees, via polarization-resolved second harmonic generation (P-SHG) microscopy and we compare it with four-dimensional scanning transmission electron microscopy (4D-STEM). It is found that the twist-angle imaging of WS2 bilayers, using the P-SHG technique is in excellent agreement with that obtained using electron diffraction. The main advantages of the optical approach are that the characterization is performed on the same substrate that the device is created on and that it is three orders of magnitude faster than the 4D-STEM. We envisage that the optical P-SHG imaging could become the gold standard for the quality examination of TMD superlattice-based devices.

Layered materials (LMs) are at the centre of an ever increasing research effort due to their potential use in a variety of applications. The presence of imperfections, such as bi- or multilayer areas, holes, grain boundaries, isotropic and anisotropic deformations, etc. are detrimental for most (opto)electronic applications. Here, we present a set-up able to transform a conventional scanning electron microscope into a tool for structural analysis of a wide range of LMs. An hybrid pixel electron detector below the sample makes it possible to record two dimensional (2d) diffraction patterns for every probe position on the sample surface (2d), in transmission mode, thus performing a 2d+2d=4d STEM (scanning transmission electron microscopy) analysis. This offers a field of view up to 2 mm2, while providing spatial resolution in the nm range, enabling the collection of statistical data on grain size, relative orientation angle, bilayer stacking, strain, etc. which can be mined through automated open-source data analysis software. We demonstrate this approach by analyzing a variety of LMs, such as mono- and multi-layer graphene, graphene oxide and MoS2, showing the ability of this method to characterize them in the tens of nm to mm scale. This wide field of view range and the resulting statistical information are key for large scale applications of LMs., Comment: A. Orekhov and D. Jannis contributed equally to this work. J. Verbeeck is corresponding author

We present a theoretical investigation of the yet unexplored dynamics of the produced excited carriers upon irradiation of hexagonal Silicon Carbide (6H-SiC) with femtosecond laser pulses. To describe the ultrafast behaviour of laser induced out-of-equilibrium carriers, a real time simulation based on Density Functional Theory (DFT) methodology is used to compute both the hot carrier dynamics and transient change of the optical properties. A Two-Temperature model (TTM) is also employed to derive the relaxation processes for laser pulses of wavelength 401 nm, duration 50 fs at normal incidence irradiation which indicate that surface damage on the material occurs for fluence ~1.88 Jcm-2. This approach of linking, for the first time, real time calculations, transient optical properties and TTM modelling, has strong implications for understanding both the ultrafast dynamics and relaxation processes and providing a precise investigation of the impact of hot carrier population in surface damage mechanisms in solids., Comment: To appear in Physical Review B

The simultaneous ordering of different degrees of freedom in complex materials undergoing spontaneous symmetry-breaking transitions often involves intricate couplings that have remained elusive in phenomena as wide ranging as stripe formation, unconventional superconductivity or colossal magnetoresistance. Ultrafast optical, x-ray and electron pulses can elucidate the microscopic interplay between these orders by probing the electronic and lattice dynamics separately, but a simultaneous direct observation of multiple orders on the femtosecond scale has been challenging. Here we show that ultrabroadband terahertz pulses can simultaneously trace the ultrafast evolution of coexisting lattice and electronic orders. For the example of a charge-density-wave (CDW) in 1T-TiSe2, we demonstrate that two components of the CDW order parameter - excitonic correlations and a periodic lattice distortion (PLD) - respond very differently to 12-fs optical excitation. Even when the excitonic order of the CDW is quenched, the PLD can persist in a coherently excited state. This observation proves that excitonic correlations are not the sole driving force of the CDW transition in 1T-TiSe2, and exemplifies the sort of profound insight that disentangling strongly coupled components of order parameters in the time domain may provide for the understanding of a broad class of phase transitions.

Femtosecond (fs)-resolved simultaneous measurements of charge and spin dynamics reveal the coexistence of two different quasi-particle excitations in colossal magneto-resistive (CMR) manganites, with {\em fs} and {\em ps} relaxation times respectively. Their populations reverse size above a {\em photoexcitation-intensity-threshold} coinciding with a "sudden" antiferro-to-ferromagnetic switching during $<$100 fs laser pulses. We present evidence that fast, metallic, mobile quasi--electrons dressed by {\em quantum spin fluctuations} coexist with slow, localized, polaronic charge carriers in non-equilibrium phases. This may be central to CMR transition and leads to a laser-driven charge reorganization simultaneously with quantum fs magnetism via an emergent quantum-spin/charge/lattice transient coupling., Comment: main text plus supplementary

An accurate calculation of the exciton ground and excited states in AlGaAs and InGaAs coupled quantum wells (CQWs) in an external electric field is presented. An efficient and straightforward algorithm of solving the Schrodinger equation in real space has been developed and exciton binding energies, oscillator strengths, lifetimes, and absorption spectra are calculated for applied electric fields up to 100 kV/cm. It is found that in symmetric 8-4-8 nm GaAs/Al(0.33)Ga(0.67)As CQW structure, the ground state of the system switches from direct to indirect exciton at approximately 5 kV/cm with dramatic changes of its binding energy and oscillator strength while the bright excited direct-exciton state remains almost unaffected. It is shown that the excitonic lifetime is dominated either by the radiative recombination or by tunneling processes at small/large values of the electric field, respectively. The calculated lifetime of the exciton ground state as a function of the bias voltage is in a quantitative agreement with low-temperature photoluminescence measurements. We have also made freely available a numerical code for calculation of the optical properties of direct and indirect excitons in CQWs in an electric field., Comment: 12 pages, 11 figures

Periodic incorporation of quantum wells inside a one--dimensional Bragg structure is shown to enhance coherent coupling of excitons to the electromagnetic Bloch waves. We demonstrate strong coupling of quantum well excitons to photonic crystal Bragg modes at the edge of the photonic bandgap, which gives rise to mixed Bragg polariton eigenstates. The resulting Bragg polariton branches are in good agreement with the theory and allow demonstration of Bragg polariton parametric amplification., Comment: 4 pages, 4 figures

This is a supplementary material to our Comment arXiv:0909.4265 on M. Stern, V. Garmider, E. Segre, M. Rappaport, V. Umansky, Y. Levinson, and I. Bar-Joseph, Phys. Rev. Lett. 101, 257402 (2008).

This is a comment on M. Stern, V. Garmider, E. Segre, M. Rappaport, V. Umansky, Y. Levinson, and I. Bar-Joseph, Phys. Rev. Lett. 101, 257402 (2008).

We report on the kinetics of the inner ring in the exciton emission pattern. The formation time of the inner ring following the onset of the laser excitation is found to be about 30 ns. The inner ring was also found to disappear within 4 ns after the laser termination. The latter process is accompanied by a jump in the photoluminescence (PL) intensity. The spatial dependence of the PL-jump indicates that the excitons outside of the region of laser excitation, including the inner ring region, are efficiently cooled to the lattice temperature even during the laser excitation. The ring formation and disappearance are explained in terms of exciton transport and cooling., Comment: 19 pages, 6 figures

We propose and analyze a k-filtering effect which gives rise to the drastic difference between the actual spatial coherence length of quasi-two-dimensional (quasi-2D) excitons or microcavity (MC) polaritons in planar nanostructures and that inferred from far-field optical measurements. The effect originates from the conservation of in-plane wavevector k in the optical decay of the particles in outgoing bulk photons. The k-filtering effect explains the large coherence lengths recently observed for indirect excitons in coupled quantum wells (QWs), but is less pronounced for MC polaritons at low temperatures, T<10K.

We report on the kinetics of a low-temperature gas of indirect excitons in the optically-induced exciton trap. The excitons in the region of laser excitation are found to rapidly -- within 4 ns -- cool to the lattice temperature T = 1.4 K, while the excitons at the trap center are found to be cold -- essentially at the lattice temperature -- even during the excitation pulse. The loading time of excitons to the trap center is found to be about 40 ns, longer than the cooling time yet shorter than the lifetime of the indirect excitons. The observed time hierarchy is favorable for creating a dense and cold exciton gas in optically-induced traps and for in situ control of the gas by varying the excitation profile in space and time before the excitons recombine., Comment: 4 pages, 3 figures