Controlled Growth of CdS Nanostep Structured Arrays to Improve Photoelectrochemical Performance.

CdS nanostep-structured arrays were grown on F-doped tin oxide-coated glasses using a two-step hydrothermal method. The CdS arrays consisted of a straight rod acting as backbone and a nanostep-structured morphology on the surface. The morphology of the samples can be tuned by varying the reaction parameters. The phase purity, morphology, and structure of the CdS nanostep-structured arrays were characterized by X-ray diffraction and field emission scanning electron microscopy. The light and photoelectrochemical properties of the samples were estimated by a UV-Vis absorption spectrum and photoelectrochemical cells. The experimental results confirmed that the special nanostep structure is crucial for the remarkable enhancement of the photoelectrochemical performance. Compared with CdS rod arrays, the CdS nanostep-structured arrays showed increased absorption ability and dramatically improved photocurrent and energy conversion efficiency. This work may provide a new approach for improving the properties of photoelectrodes in the future.

Diffraction in a stratified region of a high numerical aperture Fresnel zone plate: a simple and rigorous integral representation.

An algorithm for calculating the field distribution of a high numerical aperture Fresnel zone plate (FZP) in stratified media is presented, which is based on the vector angular spectrum method. The diffraction problem of FZP is solved for the case of a multilayer film with planar interfaces perpendicular to the optical axis. The solution is obtained in a rigorous mathematical manner and it satisfies the homogeneous wave equations. The electric strength vector of the transmitted and reflected field in the multilayer media is obtained for any polarized beam normally incident onto a binary phase circular FZP. For radially-, azimuthally- and linearly-polarized beam, the electric field in the focal region can be simplified as double or single integral, which can be readily used for numerical computation.

Diffraction theory of high numerical aperture subwavelength circular binary phase Fresnel zone plate.

An analytical model of vector formalism is proposed to investigate the diffraction of high numerical aperture subwavelength circular binary phase Fresnel zone plate (FZP). In the proposed model, the scattering on the FZP's surface, reflection and refraction within groove zones are considered and diffraction fields are calculated using the vector Rayleigh-Sommerfeld integral. The numerical results obtained by the proposed phase thick FZP (TFZP) model show a good agreement with those obtained by the finite-difference time-domain (FDTD) method within the effective extent of etch depth. The optimal etch depths predicted by both methods are approximately equal. The analytical TFZP model is very useful for designing a phase and hybrid amplitude-phase FZP with high-NA and short focal length.