Optical frequency conversion via the nonlinear effect of third harmonic generation is shown to be resonantly enhanced in few-layer black phosphorus. This feature is believed to be a consequence of exciton-related resonance, as the enhancement is strongly correlated with the observation of exciton-recombination photoluminescence. Few-layer thicknesses are obtained both via mechanical exfoliation and laser thinning.

We report the first detailed characterization of the sheet third-harmonic optical susceptibility, χ(3)s, of tungsten diselenide (WSe2). With a home-built multiphoton microscope setup developed to study harmonics generation, we map the second and third-harmonic intensities as a function of position in the sample, pump power and polarization angle, for single- and few-layers flakes of WSe2. We register a value of |χ(3)s| ≈ 0.9 × 10−28 m3 V−2 at a fundamental excitation frequency of ℏω = 0.8 eV, which is comparable in magnitude to the third-harmonic susceptibility of other group-VI transition metal dichalcogenides. The simultaneously recorded sheet second-harmonic susceptibility is found to be |χ(2)s| ≈ 0.7 × 10−19 m2 V−1 in very good agreement on the order of magnitude with recent reports for WSe2, which asserts the robustness of our values for |χ(3)s|.

Reports on micromachining, [11,12] atomic healing, [13] nanoparticles decoration, [14,15] lateral heterostructures, [16,17] and heterocrystals [18] have demonstrated the feasibility of defects manipulation and postprocessing techniques. All those results have been summarized in important recent reviews on graphene [19] and TMDs. [20,21] One very important aspect is how the defect-engineered and postprocessed materials are characterized. The most widely used characterization techniques are scanning transmission electron microscopy (STEM) [2] for atomically resolved analysis of crystalline structure and different material phases, and PL emission [3,5] for micrometric-resolved linear optical characterization. However, to the best of our knowledge, no report has yet explored the potential of second-harmonic generation (SHG) as a characterization tool for point defects in TMDs. SHG has been extensively applied in studies about nonlinear optical properties of 2D materials, [22][23][24][25][26] mostly related to crystal symmetry investigation, grain boundaries characterization, [27] and in van der Waals heterostructures properties. [28] Particularly for monolayer TMDs, which lack inversion symmetry, changes in SHG signal can be used to track the density of defects in the material as compared to the pristine sample, with spatial resolutions typically at the order of sub-micrometer size for diffraction-limited laser beams.Lateral homojunctions in as-grown chemical transport deposition monolayer tungsten disulfide (WS 2 ) have been recently reported by Liu et al. [29] These homojunctions, as observed by fluorescence microscopy, may give rise to PL concentric patterns of alternating bright and dark regions. Further energydispersive X-ray spectroscopy (EDX) analysis and density functional theory (DFT) calculation revealed that this phenomenon is due to chemical heterogeneity: bright PL areas have less sulfur (S) vacancies, and since it is dominated by excitons, the PL emission is strong; dark PL areas have more S vacancies, increasing the density of mid-gap states, changing the bandgap from direct to indirect, and making PL emission weaker.The modulation in the density of defects should also impact the nonlinear optical properties of monolayer WS 2 crystal, since the second-order susceptibility of the material may experience an improvement in the infrared due to the presence of midgap states related to defects. In this work, we report SHG as Defects engineering in transition metal dichalcogenides is a topic of intense research recently, since crystal properties can be controlled and tailored during and after fabrication. In this context, defects characterization is key to understand the material structure and enable specific applications. In this work, second-harmonic generation (SHG) spectroscopy is used to map concentric triangular defective regions in as-grown monolayer tungsten disulfide, demonstrating that SHG can be used for defects observation and characterization in layered noncentrosymmetric nanomaterials. In monola...

A population imbalance at different valleys of an electronic system lowers its effective rotational symmetry. We introduce a technique to measure such imbalance (a valley polarization), which exploits the unique fingerprints of this symmetry reduction in the polarization-dependent secondharmonic generation (SHG). We present the principle and detection scheme in the context of hexagonal two-dimensional crystals, which include graphene-based systems and the family of transition metal dichalcogenides, and provide a direct experimental demonstration using a molybdenum diselenide monolayer with 2H polytype at room temperature. We deliberately use the simplest possible setup, where a single pulsed laser beam simultaneously controls the valley imbalance and tracks the SHG process. We further developed a model of the transient population dynamics, which analytically describes the valley-induced SHG rotation in very good agreement with the experimental data. In addition to providing the first experimental demonstration of the effect, this work establishes a conceptually simple, compact, and transferable way of measuring instantaneous valley polarization, with direct applicability in the nascent field of valleytronics.

We report the development of a monolithic, mechanically tunable waveguide platform based on the flexible polymer polydimethyl siloxane (PDMS). Such devices preserve single mode guiding across a wide range of linear geometric distortions. This enables the realization of directional couplers with tunable splitting ratios via elastic deformation of the host chip. We fabricated several devices of this type, and verified their operation over a range of wavelengths, with access to the full range of input/output ratios. The low cost and relative ease of fabrication of these devices via a modified imprint lithographic technique make them an attractive platform for investigation of large scale optical random walks and related optical phenomena.

We present a method to map the evolution of photonic random walks that is compatible with nonclassical input light. Our approach leverages a newly developed flexible waveguide platform to tune the jumping rate between spatial modes, allowing the observation of a range of evolution times in a chip of fixed length. In a proof-of-principle demonstration we reconstruct the evolution of photons through a uniform array of coupled waveguides by monitoring the end-face alone. This approach enables direct observation of mode occupancy at arbitrary resolution, extending the utility of photonic random walks for quantum simulations and related applications.Quantum walks have been studied extensively in the context of quantum computing and simulation [1][2][3][4]. Systems built around a quantum walk have been proposed as physical simulators for a wide variety of quantum [5] and classical [6] phenomena. When random walks are leveraged for physical simulation the dynamics and evolution of the random walker as it traverses the graph are often of primary interest.In the experimental domain, quantum walks have been demonstrated across a range of physical systems, employing trapped particles [7-9] and photons [10][11][12][13][14][15]. The field of photonic quantum walks is particularly developed, owing to the ease of access to long coherence times and the ability to perform high fidelity manipulation of single particles using relatively low cost devices. Within this class, devices comprising waveguide arrays have proved popular due to their favourable scaling properties [16].When simulating a physical system, the evolution of the quantum walker is commonly inferred by monitoring fluorescence in the host device [11,[17][18][19]. Unfortunately, this signal is able to capture only the intensity of the propagating optical modes, and so is unsuitable for following the evolution of more complicated inputs, for example multi-photon entangled states. Systems which leverage these inputs are only able to obtain a snapshot of the random walk at a fixed propagation length corresponding to the output plane [20]. Even where the evolution of the walker is not of primary interest, access to this information may be desired as a diagnostic or calibration tool.We have designed and implemented an optical circuit in which the evolution of a photonic quantum walk can be observed in a chip of constant length. In our system, a sequence of observations at the end face combined with appropriate tuning of the device parameters exposes the evolution of the input state in a manner that is compatible with single photon intensities, and extensible to multiphoton states. We demonstrate this technique by implementing the well-studied [12] one dimensional, continuous-time random walk on a uniform graph.A continuous-time, discrete-space quantum walk on a one-dimensional graph comprising a set of vertices connected with edges (as depicted in Fig 1, middle row) can be realized physically by injecting photons into an array of identical, continuously coupled waveguides....

A polymorph of glycyl-L-alanine HI.H2O is synthesized from chiral cyclo-glycyl-L-alanine dipeptide. The dipeptide is known to show molecular flexibility in different environments, which leads to polymorphism. The crystal structure of the glycyl-L-alanine HI.H2O polymorph is determined at room temperature and indicates that the space group is polar (P21), with two molecules per unit cell and unit cell parameters a = 7.747 Å, b = 6.435 Å, c = 10.941 Å, α = 90°, β = 107.53(3)°, γ = 90° and V = 520.1(7) Å3. Crystallization in the polar point group 2, with one polar axis parallel to the b axis, allows pyroelectricity and optical second harmonic generation. Thermal melting of the glycyl-L-alanine HI.H2O polymorph starts at 533 K, close to the melting temperature reported for cyclo-glycyl-L-alanine (531 K) and 32 K lower than that reported for linear glycyl-L-alanine dipeptide (563 K), suggesting that although the dipeptide, when crystallized in the polymorphic form, is not anymore in its cyclic form, it keeps a memory of its initial closed chain and therefore shows a thermal memory effect. Here, we report a pyroelectric coefficient as high as 45 µC/m2K occurring at 345 K, one order of magnitude smaller than that of semi-organic ferroelectric triglycine sulphate (TGS) crystal. Moreover, the glycyl-L-alanine HI.H2O polymorph displays a nonlinear optical effective coefficient of 0.14 pm/V, around 14 times smaller than the value from a phase-matched inorganic barium borate (BBO) single crystal. The new polymorph displays an effective piezoelectric coefficient equal to deff=280 pCN−1, when embedded into electrospun polymer fibers, indicating its suitability as an active system for energy harvesting.

On page 10693, M. J. L. F. Rodrigues, C. J. S. de Matos and co-workers show that optical frequency conversion via the nonlinear effect of third harmonic generation is resonantly enhanced in few-layer black phosphorus. This enhancement is believed to be related to exciton resonances, as it is strongly correlated with exciton-recombination photoluminescence. Few-layer thicknesses are obtained both via mechanical exfoliation and laser thinning.