Enhancing Surface-Enhanced Raman Scattering Intensity through Light Diffuse Reflection on Ag/ZnO Nanowire Arrays

In this study, we present a new strategy to enhance the intensity of surface-enhanced Raman scattering (SERS) signals by using three-dimensional aggregated silver nanoparticles (AgNPs) on the zinc oxide nanowire arrays (Ag/ZnO NWA). The ZnO NWA acts as a light-scattering substrate, providing multidirectional incident light and reflecting a portion of the Raman scattered light, resulting in an improved SERS signal intensity. The composite nanowire structure of the Ag/ZnO NWA was successfully fabricated, and 400 nm ZnO nanowires in the Ag/ZnO NWA exhibit optimal diffuse light reflection, resulting in a significant improvement of the SERS signal intensity. The SERS enhancement factor (EF) for the Ag/ZnO NWA was determined to be EF (ISERS/NSERS)/(IRaman/NRaman) = 3.5 × 106. This substrate generated a ∼7.9-fold increased SERS signal compared to the preaggregated AgNPs in the absence of the ZnO NWA. The detection limit of the Ag/ZnO NWA was estimated by measuring SERS spectra of rhodamine B, enabling discrimination down to 10–15 M. Our approach inducing multidirectional diffuse reflection light to SERS hot spots provides a simple and efficient strategy to enhance SERS signal intensity, with broad implications for other spectroscopic applications

Enhancement of Light Absorption in Silicon Nanowire Photovoltaic Devices with Dielectric and Metallic Grating Structures

We report the enhancement of light absorption in Si nanowire photovoltaic devices with one-dimensional dielectric or metallic gratings that are fabricated by a damage-free, precisely aligning, polymer-assisted transfer method. Incorporation of a Si₃N₄ grating with a Si nanowire effectively enhances the photocurrents for transverse-electric polarized light. The wavelength at which a maximum photocurrent is generated is readily tuned by adjusting the grating pitch. Moreover, the electrical properties of the nanowire devices are preserved before and after transferring the Si₃N₄ gratings onto Si nanowires, ensuring that the quality of pristine nanowires is not degraded during the transfer. Furthermore, we demonstrate Si nanowire photovoltaic devices with Ag gratings using the same transfer method. Measurements on the fabricated devices reveal approximately 27.1% enhancement in light absorption compared to that of the same devices without the Ag gratings without any degradation of electrical properties. We believe that our polymer-assisted transfer method is not limited to the fabrication of grating-incorporated nanowire photovoltaic devices but can also be generically applied for the implementation of complex nanoscale structures toward the development of multifunctional optoelectronic devices

Switching of Photonic Crystal Lasers by Graphene

Unique features of graphene have motivated the development of graphene-integrated photonic devices. In particular, the electrical tunability of graphene loss enables high-speed modulation of light and tuning of cavity resonances in graphene-integrated waveguides and cavities. However, efficient control of light emission such as lasing, using graphene, remains a challenge. In this work, we demonstrate on/off switching of single- and double-cavity photonic crystal lasers by electrical gating of a monolayer graphene sheet on top of photonic crystal cavities. The optical loss of graphene was controlled by varying the gate voltage <i>V</i>_g, with the ion gel atop the graphene sheet. First, the fundamental properties of graphene were investigated through the transmittance measurement and numerical simulations. Next, optically pumped lasing was demonstrated for a graphene-integrated single photonic crystal cavity at <i>V</i>_g below −0.6 V, exhibiting a low lasing threshold of ∼480 μW, whereas lasing was not observed at <i>V</i>_g above −0.6 V owing to the intrinsic optical loss of graphene. Changing quality factor of the graphene-integrated photonic crystal cavity enables or disables the lasing operation. Moreover, in the double-cavity photonic crystal lasers with graphene, switching of individual cavities with separate graphene sheets was achieved, and these two lasing actions were controlled independently despite the close distance of ∼2.2 μm between adjacent cavities. We believe that our simple and practical approach for switching in graphene-integrated active photonic devices will pave the way toward designing high-contrast and ultracompact photonic integrated circuits