Optical properties of MWCNT and thin graphitic grating arrays using FDTD and EMA methods

The fascinating optical properties of nanostructured materials find important applications in a number of solar energy utilization devices. Nanotechnology provides methods for fabrication and use of structures and systems with size corresponding to the wavelength of visible light. This opens a wealth of possibilities to explore the new, often of resonance character, phenomena observed when the object size and the electromagnetic field periodicity (light wavelength λ) match. Multiwalled carbon nanotube (MWCNT) arrays are modeled using the finite difference time domain (FDTD) method to study the effects of various structural randomness including randomness in position, diameter, length and orientation, on their optical properties. In low density random diameter arrays we see an enhancement in the absorptance as compared to arrays with random position or random length; this can be attributed to the strong optical scattering in random structures. Lower reflectance was observed for arrays with random CNT length as the variation in length results in a low local dielectric constant near the surface of the array. For oblique arrays enhanced absorption was observed for higher inclination angles when TM polarized light is incident. The reason for this could be that the component of dielectric constant along the direction of the electric field of the incident light is much higher in value. For the TE polarized light a slight decrease in the absorption with the angle is observed. Thermal radiative properties of vertical graphene petal arrays are theoretically and experimentally investigated to show that they are superior absorbers of radiation. Finite difference time domain (FDTD) simulations are first performed to calculate optical properties of vertical graphitic arrays of different configurations, namely, graphitic gratings, periodic graphitic cavities, and random graphitic cavities. The effect of polarization of incident radiation on optical properties of such structures is systematically evaluated. When the incident electric field is parallel to the graphitic plane (S polarization) in graphitic gratings, the absorptance is very high, but the reflectance is also significant when compared to reflectance from a MWCNT array. On the other hand, when the electric field is polarized perpendicular to the graphitic plane (P polarization), the absorptance is significantly lower, as well as the reflectance. This contrast is due to the significantly stronger optical response for the S polarization. Ordered graphitic petal cavity arrays show optical properties falling between the above two cases because of the presence of both polarizations. The random graphitic petal cavity arrays with various angles of orientation show similar properties to ordered petal arrays, and the simulated reflectance results agrees very well with experimental data measured on a fabricated thin graphite petal sample