An ideal light-absorbing surface is able to collect light energy from wide ranges of wavelengths and angles of incidence. It has been reported that photonic structures can manipulate and couple light on the nanoscale surface. These photonic nanostructures can enhance light absorption and improve solar energy conversion also have been expected. In this article, we will describe how to calculate the optical properties of photonic nanostructures, especially for the biomimetic antireflecting structures on semiconductor substrates. The optical properties of biomimetic nanostructures were been analyzed using finite difference time domain (FDTD) calculations. Our FDTD simulation results show that the antireflecting structures utilize design parameters of spacing/wavelength and length/spacing, which could be expected exhibiting ∼99% optical absorption over wavelength from UV-vis region and angle of incidence up to 60° in high-index semiconductor materials.
A simple way of detecting melamine in raw milk is demonstrated via surface-enhanced Raman spectroscopy (SERS) using fractals of bare and nonfunctionalized ∼30 nm gold nanoparticles (AuNP) distributed on a solid support. The technique demonstrates the formation of AuNP fractals, from a random distribution, upon exposure to melamine, that enhance the Raman scattering cross-section to enable detection by SERS. The agglomeration, which is pronounced at higher melamine concentrations, is demonstrated directly through imaging, and the red-shift of the plasmon absorption peak of the AuNP fractal away from 530 nm by finite difference time domain (FDTD) calculations. The agglomeration results in a strong plasmon field, shown by FDTD, over the interparticle sites that enhances the Raman scattering cross-section of melamine and ensures unambiguous detection. Limit of detection of 100 ppb could be achieved reproducibly.
Three different antireflecting structures (ARS), namely, single-diameter nanorods, dual-diameter nanorods, and biomimetic nanotips (resembling moth-eye’s submicrostructures) were compared to each other analytically for their reflectivities, using finite difference time domain calculations. Simulation results establish the biomimetic nanotips as better ARS than the others, in the visible and near-infrared wavelength zone and over a wider angle of incidence. The reflectance values in the nanotips are significantly lower compared to both types of nanorods and also the planar silicon below the Brewster angle (∼75 deg ). The low antireflection translated to enhanced optical absorption in these subwavelength structures. A general antireflection design rule emerged from the simulation results.
Biomimetic structures provided important clues for nano-synthesis in pursuit of enhanced performances. Here, we
report a wide angle and broadband antireflection is observed on a
6-inch silicon nanotip array (SiNTs) substrate
fabricated using a single step electron cyclotron resonance plasma etching technique. This subwavelength structure
consists of the SiNTs with apex and bottom diameter of ~5 nm and ~200 nm, respectively, length of ~1600 nm and
density of 10<sup>9</sup>/cm<sup>2</sup>. This aperiodic array of SiNTs with geometry designed in the sub-wavelength level to demonstrate a low hemispherical reflectance of < 1% in the ultraviolet to infrared region. The antireflection property holds good for a wide angle of incidence and both, s and p, forms of polarizations of light. The effective refractive index distribution
related to the structure of SiNTs is built. The equivalent three-layered thin films with gradient refractive index can be
applied in interpretation of the low reflection phenomenon. The equivalent admittance of the system is shown to be near
that of air even the wavelength is varied from 400 nm to 800 nm (or angle of incidence is varied from 25 to 70 degree).
The configuration to have broadband and wide-angle antireflection is different from the previous design because the
equivalent rare film adjacent to air in our case is much thinner than the requirement proposed by J. A. Dobrowolski. This
near ideal antireflection property suggests enhanced performances in renewable energy, and electro-optical devices in defense applications.