Nanocavity-Enhanced Giant Stimulated Raman Scattering in Si Nanowires in the Visible Light Region.

Silicon photonics has been a very active area of research especially in the past two decades in order to meet the ever-increasing demand for more computational power and faster device speeds and their natural compatibility with complementary metal-oxide semiconductor. In order to develop Si as a useful photonics material, essential photonic components such as light sources, waveguides, wavelength convertors, modulators, and detectors need to be developed and integrated. However, due to the indirect electronic bandgap of Si, conventional light emission devices such as light-emitting diodes and lasers cannot be built. Therefore, there has been considerable interest in developing Si-based Raman lasers, which are nonlinear devices and require large stimulated Raman scattering (SRS) in an optical cavity. However, due to the low quantum yield of SRS in Si, Raman lasers have very large device footprints and high lasing threshold, making them unsuitable for faster, smaller, and energy-efficient devices. Here, we report strong SRS and extremely high Raman gain in Si nanowire optical cavities in the visible region with measured SRS threshold as low as 30 kW/cm2. At cavity mode resonance, light is confined into a low mode volume and high intensity electromagnetic mode inside the Si nanowire due to its high refractive index, which leads to strong SRS at low pump intensities. Electromagnetic calculations reveal greater than 6 orders of magnitude increase in Raman gain coefficient at 532 nm pump wavelength, compared to the gain value at 1.55 µm wavelength reported in literature, despite the 108 higher losses at 532 nm. Because of the high gain in such small structures, we believe that this is a significant first step in realizing a monolithically integrable nanoscale low-powered Si Raman laser.

Lateral epitaxial growth of two-dimensional layered semiconductor heterojunctions.

Two-dimensional layered semiconductors such as MoS2 and WSe2 have attracted considerable interest in recent times. Exploring the full potential of these layered materials requires precise spatial modulation of their chemical composition and electronic properties to create well-defined heterostructures. Here, we report the growth of compositionally modulated MoS2-MoSe2 and WS2-WSe2 lateral heterostructures by in situ modulation of the vapour-phase reactants during growth of these two-dimensional crystals. Raman and photoluminescence mapping studies demonstrate that the resulting heterostructure nanosheets exhibit clear structural and optical modulation. Transmission electron microscopy and elemental mapping studies reveal a single crystalline structure with opposite modulation of sulphur and selenium distributions across the heterostructure interface. Electrical transport studies demonstrate that the WSe2-WS2 heterojunctions form lateral p-n diodes and photodiodes, and can be used to create complementary inverters with high voltage gain. Our study is an important advance in the development of layered semiconductor heterostructures, an essential step towards achieving functional electronics and optoelectronics.

Synthesis and optical characterizations of chain-like Si@SiSe2 nanowire heterostructures.

A new type of chain-like Si@SiSe(2) nanowire heterostructures has been successfully synthesized via a one-step Au-catalyzed chemical vapor deposition (CVD) route. The composition and microstructure of the achieved structures were investigated by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM) and micro-Raman spectroscopy. Optical characterization was investigated using a confocal photoluminescent system with an Ar(+) laser (488 nm) as the excitation source. The results reveal that these chain-like structures emit pronounced and broadband red light, even visible with the naked eye at room temperature. A self-organization model was proposed to illustrate the formation of these heterostructures, and the photoluminescent properties were discussed in detail. These new Si-based nanostructures may be helpful for further study of the basic physical properties of SiSe(2) and will find interesting applications in nanophotonic technologies and devices.

Color-changeable optical transport through Se-doped CdS 1D nanostructures.

The optical-transport properties of 1D Se-doped CdS nanostructures with different doping contents and/or crystallization degrees are reported. The locally excited photoluminescence shows a significant redshift during the transport along the long axis of the 1D structures and can leave enough PL intensity for detection. The magnitude of the redshift is found to be highly dependent on the content of doping and the crystallization degree. The experimental results are compared with theoretical calculations based on the fundamental absorption rule of the semiconductor, which demonstrates that the redshift is related to the optical reabsorption effects induced by the local structural disorder in the semiconductors. Such optical properties of 1D semiconductor structures might be of interest for potential applications in color-tunable nanosized light-emitting and/or frequency-converting devices.