Kevin Santiago

Electrical conductivity and photoresistance of atomic layer deposited Al-doped ZnO films

Al-doped ZnO films were deposited by the atomic layer deposition (ALD) on both glass and sapphire (0001) substrates. The Al composition of the films was varied by controlling the Zn:Al pulse cycle ratios. The films were characterized by the atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and optical measurements. The Film resistivity was measured as a function of Zn:Al cycle ratios as well as temperature for films grown at various substrate temperature used for ALD deposition. The resistivity of the ALD grown films decreases significantly, and so as the increase in the carrier concentration as the cycle ratio increases. The systematic measurements of temperature dependence of resistivity of films at various cycle ratios clearly demonstrate the crossover of the metal–semiconductor–insulator phase with the function of temperature as well as the cycle ratios. The average transmission of all films is greater than 85% and the optical absorption increases significantly in the visib...

Topographically Engineered Large Scale Nanostructures for Plasmonic Biosensing.

We demonstrate that a nanostructured metal thin film can achieve enhanced transmission efficiency and sharp resonances and use a large-scale and high-throughput nanofabrication technique for the plasmonic structures. The fabrication technique combines the features of nanoimprint and soft lithography to topographically construct metal thin films with nanoscale patterns. Metal nanogratings developed using this method show significantly enhanced optical transmission (up to a one-order-of-magnitude enhancement) and sharp resonances with full width at half maximum (FWHM) of ~15nm in the zero-order transmission using an incoherent white light source. These nanostructures are sensitive to the surrounding environment, and the resonance can shift as the refractive index changes. We derive an analytical method using a spatial Fourier transformation to understand the enhancement phenomenon and the sensing mechanism. The use of real-time monitoring of protein-protein interactions in microfluidic cells integrated with these nanostructures is demonstrated to be effective for biosensing. The perpendicular transmission configuration and large-scale structures provide a feasible platform without sophisticated optical instrumentation to realize label-free surface plasmon resonance (SPR) sensing.

Nanopatterning of atomic layer deposited Al:ZnO films using electron beam lithography for waveguide applications in the NIR region

We have demonstrated the nanopatterning of atomic layer deposited (ALD) Al:ZnO (AZO) films using electron beam lithography (EBL) for plasmonic waveguide applications. The influence of grains on repeatable planar nanostructures by nanolithography process was studied for annealed films in order to avoid effects of granularity. Our results demonstrate that the nanopatterning of AZO by the EBL technique is limited due to granularity of ALD grown AZO films. This finding suggests the limitations of ALD grown samples for optical applications where nanopatterns are fabricated by the EBL technique. Furthermore, the ALD grown films lose conductivity orders of magnitude on annealing.

Enhanced optical transmission and Fano resonance through a nanostructured metal thin film

Artificial and engineered nanostructures expand the degrees of freedom with which one can manipulate the intricate interplay of light and matter. Certain nanostructural arrangements in the excited state enable the efficient electromagnetic coupling of propagating light with localized fields. Here, we demonstrate that light transmitted through a nanostructured metal thin film without any apertures can be significantly enhanced. Distinct asymmetric Fano resonances are observed in the zero-order transmission spectra using an incoherent light source. The transmission efficiency surpasses that of a metal thin film with the same area and thickness at the resonance maxima. The transmission minima and the sharp resonance maxima bear a strong resemblance to the extraordinary optical transmission observed in sub-wavelength nanohole array structures The resonance wavelength closely matches the nanostructural periodicity. The sensitivity of the resonances to the surrounding medium and the transmission efficiency demonstrate the potential for use in energy harvesting, imaging, optical processing and sensing applications.

Effects of dielectric thickness on optical behavior and tunability of one-dimensional Ag/SiO 2 multilayered metamaterials

We fabricated one-dimensional periodic multilayered metamaterial structures consisting of Ag and SiO₂alternating layers. Optical responses, such as transmission and absorption, are consistent well within finite difference time domain (FDTD) simulations. Angle dependent real and imaginary dielectric permittivity reflection spectra demonstrate their operational capability in the visible wavelength region. This multilayer metamaterial can be converted into a photonic crystal by manipulating the thickness of SiO₂ and we demonstrate that proper filling of SiO₂/Ag layers the operating wavelength can be tuned to higher wavelength region. However, absolute value of transmission reduces with increasing number of multilayer pairs due to metal absorption.