Chemical enhancement is one of the important mechanisms in surface-enhanced Raman spectroscopy, however, its origin is still under debate. Recently, a two dimensional (2D) layered material has been thought to be a strong candidate to investigate the chemical mechanism of Raman enhancement because it has a flat surface, a well defined structure and is without the interference of electromagnetic enhancement. Herein we report systematic studies of the Raman enhancement effect on a gallium selenide (GaSe) flake by using a copper phthalocyanine (CuPc) molecule as a probe. It is found that the Raman signal of CuPc on the monolayer GaSe can be significantly increased by one order of magnitude compared to that on a SiO 2 /Si substrate. Moreover, the enhancement effect is found to decrease with increasing thickness of the GaSe flake. The origin of the Raman enhancement is attributed to the chemical mechanism resulting from the charge transfer between the GaSe flake and the detected molecules. The supposition is further verified by the investigation of the quenching photoluminescence of GaSe as well as the Raman enhancement effect of CuPc with different thicknesses on the GaSe flake.Our work will shed more light on the understanding of the chemical mechanism for Raman enhancement and expand more practical applications of GaSe.

Unidirectional reflectionless phenomenon is reported in periodic ternary layered material (PTLM). The unit of the material is composed of two real dielectric layers and a complex medium (loss or gain) layer. The model is analyzed by coupled mode theory. Because of the asymmetric coupling between the forward and backward propagating modes, the left- and right-side reflectivities of this PTLM are generally unequal. The necessary and sufficient (NS) condition for unidirectional reflectionless phenomenon is presented in a concise formulation. And the underlying physical mechanism of the unidirectional reflectionless phenomenon in this material is revealed by numerical simulations. Both unidirectional reflectionless and symmetric reflection phenomena can be realized by judicious choice of the structural and optical parameters.

Metallic nanogap is very important for a verity of applications in plasmonics. Although several fabrication techniques have been proposed in the last decades, it is still a challenge to produce uniform nanogaps with a few nanometers gap distance and high throughput. Here we present a simple, yet robust method based on the atomic layer deposition (ALD) and lift-off technique for patterning ultranarrow nanogaps array. The ability to accurately control the thickness of the ALD spacer layer enables us to precisely define the gap size, down to sub-5 nm scale. Moreover, this new method allows to fabricate uniform nanogaps array along different directions densely arranged on the wafer-scale substrate. It is demonstrated that the fabricated array can be used as an excellent substrate for surface enhanced Raman scatting (SERS) measurements of molecules, even on flexible substrates. This uniform nanogaps array would also find its applications for the trace detection and biosensors.