InGaP top cell often limits the efficiency of multi-junction solar cells. Its efficiency is expected to increase by introducing strain-balanced In<sub>1-x</sub>Ga<sub>x</sub>P/In<sub>1-y</sub>Ga<sub>y</sub>P multiple-quantum-wells since the concentration of photoexcited carriers into the wells enhances radiative recombination. To keep carrier collection efficiency to be similar to the case of a bulk InGaP cell, effective mobilities of carriers through the cascaded quantum wells are evaluated. The critical mobilities for maintaining good carrier transport as a photovoltaic device are discussed, which provides a general design principle for the multiplequantum- well architecture.
Research on chiral metamaterials has drawn much attention in recent years. By virtue of chirality, for example, it has been shown that chiral metamaterials can achieve negative refractive index without great energy dissipation. In this report, we applied effective parameter retrieval technique to study the material properties of helical metamaterial. The retrieval procedure yields electromagnetic parameters through employing finite-difference time-domain (FDTD) method under periodic boundary condition. We numerically obtain several electromagnetic parameters of the structure and show that the resonance properties and the index of refraction of the helical metamaterial have strong relationship with its circular dichroism. The optical properties of the structure are also discussed, which provides general design guidelines for engineering functional chiral metamaterials.
Helix photonic metamaterials are attractive to many applications due to the unique properties of strong circular dichroism
and gyrotropy. In this study, the optical properties of metallic helix metamaterial were systematically investigated. Such
metamaterial is composed of three-dimensional metallic helical nanowires arranged in a two-dimensional array. 3D
finite-difference time-domain (FDTD) method was adopted for simulating the spectral response under the excitation of
circularly polarized light. We show that the spectral responses were correlated to the dimensions of the helix structures.
Generally, the resonance wavelengths as well as optical properties were determined by the geometrical parameters and
the composed materials of the structures. When the dimension scaled down, electromagnetic interactions between helices
are pronounced, which consequently affect the optical responses of the structures. The dependency between structure
dimension and the corresponding optical properties were discussed and presented in this report.