Pinzheng Zhang

Bio: Pinzheng Zhang is an academic researcher from National University of Singapore. The author has contributed to research in topics: DNA damage & Apoptosis. The author has an hindex of 1, co-authored 2 publications receiving 5 citations.

Lanthanide-doped nanoparticles in photovoltaics – more than just upconversion

Abstract: Development of photon conversion nanomaterials could principally leverage unutilized portions of the solar spectrum to address the increasing demand for renewable energy. However, improving photovoltaic performance using lanthanide-doped, spectral-converting nanomaterials remains a challenge. For photon upconversion, the most significant issues lie in their low quantum efficiencies and the need for high-power laser excitation. Despite these constraints, lanthanide-doped upconversion nanomaterials hold great promise to enhance the light-harvesting capacity and the conversion efficiency of existing solar cell modules. In this review, we highlight recent advances in developing high-efficiency upconversion nanoparticles for photovoltaic application. Special attention will be paid to fundamental energy transfer mechanisms, the survey of strategies for nanoparticle synthesis and surface modification, and various schemes of nanoparticle integration into photovoltaic devices. We also discuss future research directions and practical challenges in coupling upconversion nanomaterials with existing photovoltaic technologies.

The phase transition in VO2 probed using x-ray, visible and infrared radiations

Abstract: Vanadium dioxide (VO2) is a model system that has been used to understand closely-occurring multiband electronic (Mott) and structural (Peierls) transitions for over half a century due to continued scientific and technological interests. Among the many techniques used to study VO2, the most frequently used involve electromagnetic radiation as a probe. Understanding of the distinct physical information provided by different probing radiations is incomplete, mostly owing to the complicated nature of the phase transitions. Here we use transmission of spatially averaged infrared ({\lambda}=1500 nm) and visible ({\lambda}=500 nm) radiations followed by spectroscopy and nanoscale imaging using x-rays ({\lambda}=2.25-2.38 nm) to probe the same VO2 sample while controlling the ambient temperature across its hysteretic phase transitions and monitoring its electrical resistance. We directly observed nanoscale puddles of distinct electronic and structural compositions during the transition. The two main results are that, during both heating and cooling, the transition of infrared and visible transmission occur at significantly lower temperatures than the Mott transition; and the electronic (Mott) transition occurs before the structural (Peierls) transition in temperature. We use our data to provide insights into possible microphysical origins of the different transition characteristics. We highlight that it is important to understand these effects because small changes in the nature of the probe can yield quantitatively, and even qualitatively, different results when applied to a non-trivial multiband phase transition. Our results guide more judicious use of probe type and interpretation of the resulting data.

Structural Colored Fabrics with Brilliant Colors, Low Angle Dependence, and High Color Fastness Based on the Mie Scattering of Cu2O Spheres.

Abstract: Compared with conventional textile coloring with dyes and pigments, structural colored fabrics have attracted broad attention due to the advantages of eco-friendliness, brilliant colors, and anti-fading properties. The most investigated structural color on fabrics is originated from a band gap of multilayered photonic crystals or amorphous photonic structures. However, limited by the nature of the color generation mechanism and a multilayered structure, it is challenging to achieve structural colored fabrics with brilliant noniridescent colors and high fastness. Here, we propose an alternative strategy for coloring a fabric based on the scattering of Cu2O single-crystal spheres. The disordered Cu2O thin layers (<0.6 μm) on the surface of fabrics were prepared by a spraying method, which can generate vivid noniridescent structural color due to the strong Mie scattering of Cu2O single-crystal spheres. Importantly, the great mechanical stability of the structural color was realized by firmly binding Cu2O spheres to the fabric using a commercial binder. The structural color can be tuned by changing the diameter of Cu2O spheres. Furthermore, complex patterns can be easily obtained by spray coating Cu2O spheres with different particle sizes using a mask. According to color fastness test standards, the dry rubbing, wet rubbing, and light fastness of the structural color on fabric can reach level 5, level 4, and level 6, respectively, which is sufficient to resist rubbing, photobleaching, washing, rinsing, kneading, stretching, and other external mechanical forces. This coloring method may carve a practical avenue in textile coloring and has potentials in other practical applications of structural color.

Nonlinear Exciton‐Mie Coupling in Transition Metal Dichalcogenide Nanoresonators

Abstract: Thanks to a high refractive index, giant optical anisotropy, and pronounced excitonic response, bulk transition metal dichalcogenides (TMDCs) have recently been discovered to be an ideal foundation for post-silicon photonics. The inversion symmetry of bulk TMDCs, on the other hand, prevents their use in nonlinear-optical processes such as second-harmonic generation (SHG). To overcome this obstacle and broaden the application scope of TMDCs, MoS2 nanodisks are engineered to couple Mie resonances with C-excitons. As a result, their alliance produces 23-fold enhancement of SHG intensity with respect to the resonant SHG from a high-quality exfoliated MoS2 monolayer under C-exciton excitation. Furthermore, SHG demonstrates a strongly anisotropic response typical of a MoS2 monolayer due to the single-crystal structure of the fabricated nanodisks, providing a polarization degree of freedom to manipulate SHG. Hence, these results significantly improve the potential of bulk TMDCs enabling an avenue for next-generation nonlinear photonics.

Tuning magnetic Mie-exciton interaction from the intermediate to strong coupling regime in a WSe2 monolayer coupled with dielectric-metal nanoresonators

Abstract: Manipulation of coherent light-matter interactions at the nanoscale with the suppression of the incoherent damping process is of great importance for both fundamental research and future applications of quantum information devices. Here, we propose a low-loss dielectric nanoparticle-on-mirror (NPoM) system for flexible control of the coherent interaction between magnetic Mie resonances and the excitonic mode in a monolayer of $\mathrm{W}{\mathrm{Se}}_{2}$. We demonstrate that the dielectric NPoM system simultaneously enables a large field enhancement and low absorption in the dielectric nanoresonator, thus greatly facilitating the coherent magnetic Mie-exciton coupling. Importantly, the efficiency of radiating the magnetic mode to the far field can be readily manipulated by changing the thickness of the dielectric spacer and the particle size, therefore offering flexible tuning of the interaction from the intermediate to strong coupling regime. Moreover, the highly directional emission at the Mie resonance dramatically suppresses the incoherent damping process between the two individual systems via the continuum reservoir. Such a NPoM-based hybrid system is expected to offer not only the possibility to manipulate light-matter interactions but also new avenues for low-loss, high-efficiency coherent light control at the deep nanoscale.

Functional meta-optics and nanophotonics govern by Mie resonances

Abstract: Scattering of electromagnetic waves by subwavelength objects is accompanied by the excitation of electric and magnetic Mie resonances, that may modify substantially the scattering intensity and radiation pattern. Scattered fields can be decomposed into electric and magnetic multipoles, and the magnetic multipoles define magnetic response of structured materials underpinning the new field of all-dielectric resonant meta-optics. Here we review the recent developments in meta-optics and nanophotonics, and demonstrate that the Mie resonances can play a crucial role offering novel ways for the enhancement of many optical effects near magnetic and electric multipolar resonances, as well as driving a variety of interference phenomena which govern recently discovered novel effects in nanophotonics. We further discuss the frontiers of all-dielectric meta-optics for flexible and advanced control of light with full phase and amplitude engineering, including nonlinear nanophotonics, anapole nanolasers, quantum tomography, and topological photonics.