Alex Heltzel

Nanostructuring Borosilicate Glass With Near-Field Enhanced Energy Using a Femtosecond Laser Pulse

A model based on the evolution of electron density derived from the Fokker-Planck equation has been built to describe ablation of dielectrics during femtosecond laser pulses. The model is verified against an experimental investigation of borosilicate glass with a 200 fs laser pulse centered at 780 nm wavelength in a range of laser energies. The ablation mechanisms in dielectrics include multi-photon ionization (MPI) and avalanche ionization. MPI dominates the ionization process during the first stages of the laser pulse, contributing seed electrons which supply avalanche ionization. The avalanche process initiates and becomes responsible for the majority of free-electron generation. The overall material removal is shown to be highly dependent upon the optical response of the dielectric as plasma is formed. The ablation model is employed to predict the response of borosilicate glass to an enhanced electromagnetic field due to the presence of microspheres on the substrate surface. It is shown that the diffraction limit can be broken, creating nanoscale surface modification. An experimental study accompanies the model, with AFM and SEM characterizations that are consistent with the predicted surface modifications. DOI: 10.1115/1.2360595

Analytical and Experimental Investigation of Laser-Microsphere Interaction for Nanoscale Surface Modification

An investigation on the features created on a silicon substrate by the irradiation of nanospheres on the substrate surface with a pulsed laser is presented. Silica nanospheres of diameter on the order of laser wavelength are deposited on silicon substrate and irradiated with a pulsed Nd: YAG laser. As a result, nanofeatures are created on the surface by the melting and resolidification of silicon. The experiment is repeated for different laser wavelengths (532 nm, and 355 nm), sphere diameters (640 nm, and 1.76 μm) and laser energies, and the effect of each of these parameters on the features created are studied. An analytical model based on Mie Theory complements the results. The model includes all evanescent terms and does not rely on either far field or size-parameter approximations. The predicted intensity distributions on the substrate indicate a strong near field enhancement confined to a very small area (nanometer scale). The results correlate well with the feature geometries obtained in the experiment.Copyright

Simulation of a plasmonic tip-terminated scanning nanowire waveguide for molecular imaging

Finite difference time domain simulation reveals plasmon coupling and local field enhancement at the gap between the gold nanoparticle (NP) tip of a ZnO nanowire (NW) waveguide and a gold-coated substrate or a gold NP probe. The region of field enhancement is about three times smaller than the 100 nm diameter of the gold NP tip, making the NW waveguide grown on a transparent microcantilever well-suited for near field imaging of single molecules immobilized on a gold substrate or gold NP-labeled cell membranes with superior spatial resolution and signal to noise ratio.

Optoelectronic property modeling of carbon nanotubes grafted with gold nanoparticles

A three-dimensional (3D) electrodynamic model is built using the finite-difference time-domain (FDTD) method to investigate the optical response of carbon nanotubes grafted with gold nanoparticles. Theoretical characterizations suggest an anisotropic response, in line with previously observed absorption peaks of such systems in the optical range. An investigation of geometric and wavelength dependences is conducted, predicting the ability to tune the sub-wavelength intensity enhancement for efficient localization and propagation. The support of electric field enhancement along the nanotube walls raises the possibility of utilizing such systems as plasmon generators and waveguides for optical signal propagation.

A microsphere coupler for a nanowire waveguide plasmonic probe for molecular imaging

Highly forward scattering of light by a silica microsphere is predicted by Mie theory. Finite-difference time-domain simulations are used to examine the use of a silica microsphere for coupling light into a ZnO nanowire terminated with a gold nanoparticle (NP) tip, intended for near-field imaging of single molecules immobilized on a gold substrate or gold nanoparticle-labeled cell membranes. The results show that plasmonic coupling at the Au tip is dependent on the incident angle of the excitation. Pre-conditioning the signal with the microsphere amplifies the coupling while reducing background energy levels, significantly boosting the signal-to-noise ratio.

Broadband Absorption Enhancement in Thin Film Solar Cells Using Inverse Optimization of Light Trapping Mechanisms

In this paper, global optimization techniques are used to design broadband solar absorption enhancement in thin film amorphous silicon (a-Si) solar cells, using periodic nanostructures on the top and bottom surfaces of the cell. Considering a combination of silver rectangular gratings and indium tin oxide (ITO) coatings on both surfaces of the a-Si, numerical optimization techniques such as Simulated Annealing and a local constrained Quasi-Newton algorithm are used to optimize the surface texture patterns. Numerical results indicate that, unlike the case of metallic gratings on the front surface, a periodic silver grating structure on the back surface results in a modification of the absorption spectrum largely independent of the effect of anti-reflection ITO coatings on the front of the cell. Furthermore, additional improvement can be obtained by using a thin rear surface ITO layers. Therefore, using a combination of metallic gratings and ITO coatings on both sides, a wideband absorption spectrum enhancement is achievable. Simulations predict integrated enhancement factors as high as 2.0 (100% improvement) for the case of metallic grating on the back surface and ITO layers on the front, and as high as 2.2 (120% improvement) when a combination of grating and ITO coatings on both sides is used. Such noteworthy improvements are made possible by efficient multi-parameter optimization supplanting an intractable exhaustive search.Copyright

Metamaterial Window Glass for Adaptable Energy Efficiency

A computational analysis of a metamaterial (MTM) window design is presented for the purpose of increasing the energy efficiency of buildings in seasonal or cold climates. Commercial low-emissivity windows use nanometer-scale Ag films to reflect infrared energy, while retaining most transmission of optical wavelengths for functionality. An opportunity exists to further increase efficiency through a variable emissivity implementation of Ag thin-film structures. 3-D finite-difference time-domain simulations predict nonlinear absorption of near-infrared energy, providing the means to capture a substantial portion of solar energy during cold periods. The effect of various configuration parameters is quantified, with prediction of the net sustainability advantage. MTM window glass technology can be realized as a modification to current, commercial low-emissivity windows through the application of nanomanufactured films, creating the opportunity for both new and after-market sustainable construction.Copyright

Simulation of electromagnetic field distribution at a nanowire probe for near field scanning optical microscopy

This work studies a new design of a near field scanning optical microscopy (NSOM) probe based on a ZnO nanowire sub-wavelength waveguide terminated with a plasmonic gold nanoparticle. Three-dimensional finite difference time domain (FDTD) simulation is used to visualize light guiding in the nanowire and near field coupling between the plasmonic nanoparticle and the substrate. The simulation results reveal local field enhancement at the gap between the nanoparticle and a gold substrate when the nanowire axis is tilted from the substrate normal by a small angle. The enhancement occurs only along the cross section plane that is parallel to the polarization of the excitation laser beam. The regime of field enhancement is much smaller than the diameter of the 100 nm plasmonic particle, making the nanowire probe well suited for NSOM with superior spatial resolution and signal to noise ratio compared to the state of the art.Copyright

Design and Optimization of Thermal Selective Emitters for High-Efficiency Thermophotovoltaic (TPV) Power Generation

Thermophotovoltaic (TPV) devices are popular energy converters due to providing low noise, low thermal-mechanical stresses and portability. The conversion efficiency of TPVs is still low due to mistuned spectral properties between thermal selective emitters and the TPV cell. Using thermal selective emitters that are well-matched to the TPV cell spectrum enhances the conversion efficiency of TPVs. Several thermal selective emitters, composed of 1-D complex multilayer structures with rectangular gratings, have been proposed. Cost, fabrication and stability factors have been major problems for their application on TPV modules. In this paper, a 1-D tungsten thermal emitter is optimized which exhibits close to blackbody emittance near the band-gap of a GaInAsSb TPV cell and sharp cutoff for longer wavelengths. The emitter is at 1200K, and is designed and optimized by modeling triangular grooves to excite localized groove modes which are well-matched to the GaInAsSb TPV cell external quantum efficiency (EQE) for high efficiency energy conversion. We suggest that a quasi-monochromatic, narrow-band and coherent emitter at a frequency near the energy band-gap of the converter is an ideal source to achieve high conversion efficiency.© 2015 ASME

Simulation of Charge Density and Field Distribution of a Gold Nanoparticle Tip-Terminated Scanning Nanowire Waveguide for Molecular Imaging

A Finite Difference Time Domain (FDTD) simulation is employed to calculate electromagnetic field and charge density distributions at the junction between a gold nanoparticle (NP) tip of a scanning ZnO nanowire and gold NP bio-markers. This three-dimensional simulation calculates the magnetic and electric field components in a large matrix of Yee cells by solving Maxwell’s curl equations. An absorbing boundary condition is included to eliminate reflection back into the simulation chamber. In the specific simulations considered here, a laser pulse of single wavelength is incident on the backside of a transparent silicon dioxide micro-cantilever, and coupled into a ZnO nanowire grown from an opening on a metal coating of the front side of the cantilever. The simulation results reveal local field enhancement between the gold NP tip of the nanowire and only one of three 20 nm gold NPs with a 28 nm empty spacing between two adjacent NPs. The charge density distributions in the gold tip and the gold NP are calculated and correlated with the local field enhancement, which makes the gold tip of the scanning nanowire waveguide attractive for use in imaging gold NP bio-labels on cell membranes.© 2009 ASME