In this study, nanoparticles (NPs) of various types and sizes are arranged to enhance both the omnidirectional light harvesting of solar cells and the light extraction of light emitting diodes (LEDs). A graded‐refractive‐index NP stack can minimize reflectance, not only over a broad range of wavelengths but also at different incident angles; the photocurrent of silicon‐based solar cells an also be significantly improved omnidirectionally. In addition, the optical gradient of an NP stack can also enhance the light‐extraction efficiency of LEDs, due to both the graded refractive index and the moderate surface roughness. Large particles having sizes on the same order of the wavelength of the incident light roughen the LED surfaces further and extract light from beyond the critical angle, as supported by three‐dimensional finite‐difference time‐domain simulations. Using this approach, the photoluminescence intensity can be increased by up to sevenfold. The developed technique: arranging sequences of different NPs in graded‐refractive‐index stacks, and considering their ability to scatter light due to their sizes and optical constants, may also significantly improve the performance of various optoelectronic devices.

In this paper, we describe a high-reflectance liquid mirror prepared from densely packed silver nanoparticles (AgNPs) of two different sizes. We controlled the particle size during the synthetic process by controlling the temperature. Varying the concentration of the ligand also allowed us to optimize the arrangement of the AgNPs to achieve liquid mirrors exhibiting high specular reflectance. Scanning electron microscopy and atomic force microscopy confirmed that the particles of the liquid mirror were well-packed with an interparticle distance of merely 2 nm; thus, the interstices and surface roughness of the NPs were effectively minimized. As a result of decreased scattering loss, the reflectance in the shorter wavelength regime was increased effectively. The AgNP film was also sufficiently thick to reflect the light in the longer wavelength regime. In addition, we used three-dimensional finite-difference time domain simulations and experimental measurements to investigate the relationship between the localized surface plasmon resonance (LSPR) and the specular reflection of the liquid mirrors. By changing the packing density of the AgNPs, we found that the LSPR effect could yield either a specular reflection peak or dip at the LSPR wavelengths in the reflection spectra of the liquid mirrors. Relative to previously reported liquid mirrors, the reflectance of our system is obviously much greater, especially in the shorter wavelength regime. The average reflectance in the range from 400 to 1000 nm could reach 77%, comparable with that of mercury-based liquid mirrors.

In this study, we developed a reliable method to analyze the interference-enhanced Raman scattering (IERS) effect on graphene by considering the surface electric field (E-field), which can be calculated precisely by measuring the optical admittance of the thin-film assembly. Through accurate tuning of the optical properties of one-dimensional photonic crystals (1D-PhCs), the strong and controllable interference effect allowed the surface E-field to be maximized and, thereby, to optimize the enhancement factors of the Raman scattering signals of graphene. Using this approach, we could enhance both the G and the 2D bands of graphene largely, uniformly, and equally, by about 180 times relative to those obtained on a silicon substrate. Under certain conditions, the Raman peak of graphene could even be enhanced by over 400 times. After transferring single-layer graphene (SLG) and few-layer graphene (FLG) onto various substrates, we found that the Raman spectra of both SLG and FLG on the 1D-PhCs substrate were enhanced without changing the band-to-band ratio or the peak positions of the main Raman bands of graphene. Without inducing any additional signal disturbance, this enhancement technique allowed us to maintain the accurate and precise informational features from the Raman spectra. The experimental enhancement factors in the coenhanced Raman spectra of graphene were higher than those previously obtained using the IERS effect. Moreover, the surface E-field of 1D-PhCs could be modulated by changing the incident angle of the excited light source, thereby allowing fine-tuning of the working wavelength. Thus, by controlling only the surface E-field, the Raman signals of graphene could be enhanced dramatically without any distortion on spectra. Accordingly, using 1D-PhCs and the optimized IERS effect is very helpful for fine structural characterization of graphene through conventional Raman spectroscopy.

In this study we developed a method, using a simple two-layer nanocavity structure, to significantly enhance light outcoupling from two-dimensional (2D) materials. Because the surface electric fields (E-fields) of the nanocavities were enhanced greatly over ultrabroadband regimes, the excitation of various 2D materials with laser light and their Raman and photoluminescence (PL) light emissions were all enhanced dramatically while maintaining band-to-band ratios and peak positions precisely. At the same time, the optical visibility of the 2D materials was also enhanced significantly over a broad spectral regime. Using a single type of Ag/SiO2 nanocavity structure, we obtained a 475-fold, equal enhancement in the intensities of the main Raman peaks of single-layer graphene (SLG) and more than a 350-fold increase in the intensities of both the Raman and PL signals of single-layer tungsten disulfide (WS2). Notably, the light outcouplings of these 2D materials were enhanced dramatically without any spectral distortion generated by the nanocavity. Moreover, a nanocavity structure prepared from a nonplasmonic metal reflector also enhanced the light outcoupling from 2D materials by over 200-fold. Combined with Raman and PL spectroscopy, such simple nanocavity structures appear to have great applicability for precise and reliable investigations, providing abundant structural information, of a variety of 2D materials.

Plasmonic nanolasers provide a valuable opportunity for expanding sub‐wavelength applications. Due to the potential of on‐chip integration, semiconductor nanowire (NW)‐based plasmonic nanolasers that support the waveguide mode attract a high level of interest. To date, perovskite quantum dots (QDs) based plasmonic lasers, especially nanolasers that support plasmonic‐waveguide mode, are still a challenge and remain unexplored. Here, metallic NW coupled CsPbBr3 QDs plasmonic‐waveguide lasers are reported. By embedding Ag NWs in QDs film, an evolution from amplified spontaneous emission with a full width at half maximum (FWHM) of 6.6 nm to localized surface plasmon resonance (LSPR) supported random lasing is observed. When the pump light is focused on a single Ag NW, a QD‐NW coupled plasmonic‐waveguide laser with a much narrower emission peak (FWHM = 0.4 nm) is realized on a single Ag NW with the uniform polyvinylpyrrolidone layer. The QDs serve as the gain medium while the Ag NW serves as a resonant cavity and propagating plasmonic lasing modes. Furthermore, by pumping two Ag NWs with different directions, a dual‐wavelength lasing switch is realized. The demonstration of metallic NW coupled QDs plasmonic nanolaser would provide an alternative approach for ultrasmall light sources as well as fundamental studies of light matter interactions.