Huaiyi Ding

Broadband absorption enhancement achieved by optical layer mediated plasmonic solar cell

We propose a novel thin solar cell design, integrating plasmonic component with optical layer, for conspicuous performance improvement in organic (P3HT: PCBM) thin film solar cell. Despite the relatively simple structure, the designed solar cell can get strikingly high spectral performance with the short circuit current density (J(sc)) enhancement up to 67%; and a nicely large J(sc) enhancement over 50% can be easily obtained spanning rather a broad geometric parametric range. The mechanisms responsible for this significant and broadband absorption enhancement as well as the effects of intercalating a plasmonic nanoparticles (NPs) array and an optical layer are theoretically and systematically investigated by finite-difference time-domain calculations (FDTD). The origin of the dramatically increased absorption is believed to be the synergistic effect between 1) the enhanced electric field and forward scattering upon excitation of localized surface plasmon resonance (LSPR) of the NPs, and 2) the favorable redistributions of light field in the device due to the beneficial interference effect mediated by the optical layer. Such a design concept is quite versatile and can be easily extended to other thin film solar cell systems.

Tailoring the coupling between localized and propagating surface plasmons: realizing Fano-like interference and high-performance sensor

Surface plasmon modes originated from various metallic nanostructures possess unique features of strong nanoscale light confinement and enhancement with tunable energy, which make them attractive and promising for a variety of applications such as sensing, solar cell, and lasing. Here, we have investigated the interaction between localized and propagating surface plasmons in a structure consisting of a gold nanobar array and a thick gold film, separated by a silica dielectric spacer layer. It is found that the reflection spectrum of the designed plasmonic structure can be readily tailored by changing the gold nanobar size, array period and the spacer layer thickness. Moreover, the strong coupling between the localized and propagating modes can result in an anticrossing behavior and even induce a Fano-like asymmetric lineshape. Importantly, the sensitivity and the figure of merit (FoM) of this plasmonic system can reach as high as 936 nm/RIU and 112, respectively. Our study offers a new, simple, efficient and controllable way to design the plasmonic systems with desired modes coupling and spectral lineshapes for different applications.

Realizing full visible spectrum metamaterial half-wave plates with patterned metal nanoarray/insulator/metal film structure.

Abrupt phase shift introduced by plasmonic resonances has been frequently used to design subwavelength wave plates for optical integration. Here, with the sandwich structure consisting of a top periodic patterned silver nanopatch, an in-between insulator layer and a bottom thick Au film, we realize a broadband half-wave plate which is capable to cover entire visible light spectrum ranging from 400 to 780 nm. Moreover, when the top layer is replaced with a periodic array of composite super unit cell comprised of two nanopatches with different sizes, the operation bandwidth can be further improved to exceed an octave (400-830 nm). In particular, we demonstrate that the designed half-wave plate can be used efficiently to rotate the polarization state of an ultra-fast light pulse with reserved pulse width. Our result offers a new strategy to design and construct broadband high efficiency phase-response based optical components using patterned metal nanoarray/insulator/metal structure.

Maximizing Integrated Optical and Electrical Properties of a Single ZnO Nanowire through Native Interfacial Doping

A native interfacial doping layer introduced in core-shell type ZnO nano-wires by a simple vapor phase re-growth procedure endows the produced nano-wires with both excellent electrical and optical performances compared to conventional homogeneous ZnO nanowires. The unique Zn-rich interfacial structure in the core-shell nanowires plays a crucial role in the outstanding performances.

The Raman enhancement effect on a thin GaSe flake and its thickness dependence

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 SiO2/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.

Remarkable enhancement of photovoltaic performance of ZnO/CdTe core–shell nanorod array solar cells through interface passivation with a TiO2 layer

All-inorganic solid-state ZnO/CdTe core–shell nanorod array solar cells (NRASCs) have been fabricated by a simple low-temperature and low-cost chemical solution method. A thin TiO2 layer with different thickness was introduced at the ZnO/CdTe interface using atomic layer deposition and its effect on the photovoltaic performance of the NRASCs was investigated. It is found that the overall power conversion efficiency of the ZnO/TiO2 (4 nm)/CdTe NRASC can reach up to 1.44% under AM 1.5G illumination (100 mW cm−2), which is about 6 times of the NRASC without TiO2 layer. By further systematic characterizations, we find that the thin TiO2 layer, serving as an efficient passivation and blocking layer at the interface of ZnO/CdTe nanorod, can remarkably suppress the charge recombination at the interface but negligibly affect the light absorption and the charge separation efficiency, thus leading to significant increases of the carrier lifetime and the open-circuit voltage of the NRASCs. This result expands the knowledge and opportunities for low-cost, high-performance NRASCs through simple interface engineering.

Near-infrared quarter-waveplate with near-unity polarization conversion efficiency based on silicon nanowire array

Metasurfaces made of subwavelength resonators can modify the wave front of light within the thickness much less than free space wavelength, showing great promises in integrated optics. In this paper, we theoretically show that electric and magnetic resonances supported simultaneously by a subwavelength nanowire with high refractive-index can be utilized to design metasurfaces with near-unity transmittance. Taking silicon nanowire for instance, we design numerically a near-infrared quarter-waveplate with high transmittance using a subwavelength nanowire array. The operation bandwidth of the waveplate is 0.14 μm around the center wavelength of 1.71 μm. The waveplate can convert a 45° linearly polarized incident light to circularly polarized light with conversion efficiency ranging from 94% to 98% over the operation band. The performance of quarter waveplate can in principle be tuned and improved through optimizing the parameters of nanowire arrays. Its compatibility to microelectronic technologies opens up a distinct possibility to integrate nanophotonics into the current silicon-based electronic devices.

Revealing anisotropy and thickness dependence of Raman spectra for SnS flakes

In this work, we have successfully synthesized SnS flakes with different thicknesses and systematically investigated their polarization-dependent Raman properties. It is found that the different Raman mode of SnS shows distinctly anisotropic thickness dependence. For B3g mode, the polar plot of Raman intensities is insensitive to the flake thickness. However, the behavior of Ag mode is entirely different. Under parallel polarization configuration, with decreasing the flake thickness, the maximum Raman intensity of Ag mode changes from the armchair direction to the zigzag direction with 514.5 nm excitation. The results can be understood by the complex Raman tensor owing to the large absorption of SnS. Moreover, under the perpendicular polarization configuration, the Raman intensity of Ag mode along 45° direction becomes apparently different from that along 135° direction. Our finding not only deepens the understanding of anisotropic Raman properties of SnS but also provides inspiration for further studies on the other 2D IV–VI materials.

Great Disparity in Photoluminesence Quantum Yields of Colloidal CsPbBr3 Nanocrystals with Varied Shape: The Effect of Crystal Lattice Strain

Understanding the big discrepancy in the photoluminesence quantum yields (PLQYs) of nanoscale colloidal materials with varied morphologies is of great significance to its property optimization and functional application. Using different shaped CsPbBr3 nanocrystals with the same fabrication processes as model, quantitative synchrotron radiation X-ray diffraction analysis reveals the increasing trend in lattice strain values of the nanocrystals: nanocube, nanoplate, nanowire. Furthermore, transient spectroscopic measurements reveal the same trend in the defect quantities of these nanocrystals. These experimental results unambiguously point out that large lattice strain existing in CsPbBr3 nanoparticles induces more crystal defects and thus decreases the PLQY, implying that lattice strain is a key factor other than the surface defect to dominate the PLQY of colloidal photoluminesence materials.

The role of a few-layer TiOx surfactant: remarkably-enhanced succeeding radial growth and properties of ZnO nanowires

Mastery over the radial growth rate and geometrical size of a ZnO nanowire (NW) while preserving its excellent luminescent and electrical properties can not only offer a fascinating potential for future applications, but also represent a grand challenge. The NW radial growth remains very slow due to the preferred growth direction along its c-axis (less than 1 A s−1), which hinders its applications in NW photonics such as waveguide lasing and resonant cavities. In this study, coating a few layers of TiOx on the NW matrix, through atomic layer deposition (ALD), was initiated intentionally before the re-growth. We found that this atomic layer TiOx could simply but surprisingly speed up the succeeding radial growth up to two orders of magnitude, with the epitaxial single-crystal wurtzite structure being well preserved. Unlike most other impurities, Ti element tends to “float” on the outer surface of the re-grown NWs, thus naturally guaranteeing the superior intrinsic electrical and luminescent properties. The ability to thrive the radial growth of ZnO NWs yet endow them with optimal integrated performance is exceptionally desirable for the design and construction of NW photonic devices.