Raman spectroscopy is established as a valuable tool to study and characterize two-dimensional (2D) systems, but it exhibits two drawbacks: a relatively weak signal response and a limited spatial resolution....

In this work we probe the third-order non-linear optical property of graphene, hexagonal boron nitride and their heterostructure by the use of coherent anti-Stokes Raman Spectroscopy. When the energy difference of the two input fields match the phonon energy, the anti-Stokes emission intensity is enhanced in h-BN, as usually expected while for graphene a anomalous decrease is observed. This behaviour can be understood in terms of q coupling between the electronic continuum and a discrete phonon state. We have also measured a graphene/h-BN heterostructure and demonstrate that the anomalous effect in graphene dominates the heterostructure optical response.Two-dimensional materials like graphene, hexagonal boron nitride (h-BN) and heterostructures exibit novel physical properties and promisses different applications in electronics and photonics [1][2][3][4]. Different non-linear optical phenomena like second-, third-harmonic generation and four wave mixing (FWM) [5][6][7] can be quite strong in these materials [8][9][10][11][12][13][14][15]. However, the interpretation of the nonlinear optical response is strongly affected by electronic and phonon resonances [16][17][18][19], therefore the knowledge of the interplay between these resonances is desirable.Here we measured the third order optical emission by the degenerated four wave mixing emission of graphene, h-BN and their heterostructure near phonon resonances. We show that while the FWM signal in h-BN shows the expected enhancement, in graphene the signal is decreased exactly at the phonon resonance. These results are explained in terms of interference effects between the electronic continuum and discrete phonon states for these two different materials. We also show that this unusual effect in graphene dominates the optical response in the graphene/h-BN heterostructure.Four wave mixing is a third-order non-linear optical phenomena, where three frequencies are combined to generate a forth [7]. In this work we are restricted to the case of degenerate FWM, where two photons of frequency ω 1 combines with a photon of ω 2 at the material and generate the emission of another photon with frequency ω 4 . The energy conservation in this case is given by ω 4 = 2 ω 1 − ω 2 . Hendry et al. [8] have measured the FWM intensity in graphene as a function of the pump laser energy and its third order optical non linear optical property was characterized. Also different theoretical works have calculated the third-order optical conductivity of graphene [20][21][22][23], showing the importance of different physical quantities like Fermi energy or temperature. However these works did not treat the problem of the third order optical nonlinearity near phonon resonances. The so-called coherent anti-Stokes Raman spectroscopy (CARS) is a special case of FWM when the energy difference between ω 1 and ω 2 matches a phonon energy ( ω ph ), then ω 4 corresponds exactly to the anti-Stokes frequency in Raman scattering. In general, when the energy condition ω 1 − ω 2 = ω ph is satisfied, the ω ...

Semiconducting transition metal dichalcogenides have significant nonlinear optical effects. In this work we have used second-harmonic generation and the four-wave mixing spectroscopy in resonance with the excitons in MoS2, MoSe2, and WS2 monolayers to characterize the nonlinear optical properties of these materials. We show that trions and excitons are responsible for enhancing the nonlinear optical response and determine the exciton and trion energies by comparing with the photoluminescence spectra. Moreover, we extract the second- and third-order optical sheet susceptibility (χ (2) and χ (3)) across exciton energies and compare with values found in the literature. We also demonstrate the ability to generate different nonlinear effects in a wide spectral range in the visible region for monolayer MoS2, opening the possibility of using two-dimensional materials for nonlinear optoelectronic and photonic applications.

We have found different Raman signatures of AB fibrils and in brain tissues from unmixed analysis, providing a detailed image of amyloid plaques in the brain, with the potential to be used as biomarkers.

The deposition of amyloid plaques is considered one of the main microscopic features of Alzheimer’s disease (AD). Since plaque formation can precede extensive neurodegeneration and the main clinical manifestations of...