Self-Suppression of the Giant Coherent Anti-Stokes Raman Scattering Background for Detection of Buried Interfaces with Submonolayer Sensitivity.

Despite its success in many fields, the implementation of coherent anti-Stokes Raman spectroscopy (CARS) in tackling the problems at interfaces was hindered by the enormous resonant and nonresonant background from the bulk. In this work, we have developed a novel CARS scheme that can probe a buried interface via ≥105-fold suppression of the nonresonant and resonant bulk contribution. The method utilizes self-destructive interference between the forward and backward CARS generated in the bulk near the Brewster angle. As a result, we can resolve the vibrational spectrum of submonolayer interfacial polar/apolar species immersed in the surrounding medium with huge CARS responses. We expect that our approach opens up the opportunity to interrogate the interfaces involving apolar molecules and benefits other nonlinear optical spectroscopic techniques, e.g., sum-frequency spectroscopy and four-wave mixing spectroscopy in general, to promote the signal-to-background noise ratio.

Study of Thermal Expansion Coefficient of Graphene via Raman Micro-Spectroscopy: Revisited.

The thermal expansion coefficient (TEC) of a 2D material is a fundamental parameter for both material property and applications. A joint study is hereby reported, using Raman microspectroscopy and molecular dynamics (MD) simulations, of the substrate effects on thermal properties of graphene. It is found that besides the lateral strain induced by the substrate, out-of-plane coupling strongly affects the temperature-dependent vibrational modes and TEC of graphene. MD simulation shows significant reduction of the density of states for longer wavelength out-of-plane vibrations when the graphene is supported on an alkane substrate. The negative TEC of freestanding graphene becomes smaller when out-of-plane rippling is suppressed. In order to measure TEC of 2D materials with the out-of-plane coupling being taken into consideration, a Raman microspectroscopic scheme to separate the contributions of lateral strain and out-of-plane coupling to TEC is developed. The TEC of graphene on octadecyltrichlorosilane substrate is found to be (-0.6 ± 0.5) × 10-6 K-1 at room temperature, which is fundamentally smaller than that of freestanding graphene. These results shed light on the fundamental understanding of the interaction between 2D material and substrate, and offer a general recipe for studying separately in-plane and out-of-plane couplings on supported materials.

Enhanced light-matter interactions in graphene-covered dielectric magnetic mirrors.

Enhanced interactions of light with graphene on the surface of a lossless dielectric magnetic mirror (DMM) are studied theoretically and experimentally in the visible range, where the DMM is composed of truncated dielectric photonic crystals (PCs). The absorption of graphene on the DMM was enhanced by about 4-fold for the spectral range within the forbidden gap of PCs over a wide range of incidence angles for both transverse electric and transverse magnetic polarizations compared with that of free-standing graphene. Moreover, the enhanced local electric field on the DMM surface led to much better detection efficiencies of the photocurrent, Raman spectroscopy and enhanced third-harmonic generation of graphene. These results offer a new way to achieve an enhanced interaction of light with graphene and develop new compact graphene-based devices.