William Baughman

Localized excitons mediate defect emission in ZnO powders

A series of continuous-wave spectroscopic measurements elucidates the mechanism responsible for the technologically important green emission from deep-level traps in ZnO:Zn powders. Analysis of low-temperature photoluminescence (PL) and PL excitation spectra for bound excitons compared to the temperature-dependent behavior of the green emission reveals a deep correlation between green PL and specific donor-bound excitons. Direct excitation of these bound excitons produces highly efficient green emission from near-surface defects. When normalized by the measured external quantum efficiency, the integrated PL for both excitonic and green emission features grows identically with excitation intensity, confirming the strong connection between green emission and excitons. The implications of these findings are used to circumscribe operational characteristics of doped ZnO-based white light phosphors whose quantum efficiency is almost twice as large when the bound excitons are directly excited.

Observation of Hydrofluoric Acid Burns on Osseous Tissues by Means of Terahertz Spectroscopic Imaging

Terahertz technologies have gained great amount of attention for biomedical imaging and tissue analysis. In this study, we utilize terahertz imaging to study the effects of hydrofluoric acid on both compact bone tissue and cartilage. We compare the differences observed in the exposure for formalin fixed and raw, dried, tissue as well as those resulting from a change in hydrofluoric (HF) concentration. Measurements are performed with THz-TDS, and a variety of spectroscopic-based image reconstruction techniques are utilized to develop contrast in the features of interest.

Comparative reconstructions of THz spectroscopic imaging for non-destructive testing and biomedical imaging

Imaging with electromagnetic radiation in the THz frequency regime, between 0.2 THz and 10 THz, has made considerable progress in recent years due to the unique properties of THz radiation, such as being non-ionizing and transparent through many materials. This makes THz imaging and sensing promising for a plethora of applications; most notably for contraband detection and biomedical diagnostics. Though many methods of generation and detection terahertz radiation exist, in this study we utilize Terahertz Time Domain Spectroscopy (THz TDS) and THz digital holography using a coherent, tunable CW THz source. These methods enable access to both the amplitude and phase information of the traveling THz waves. As a result of the direct time-resolved detection method of the THz electric field, unique spectroscopic information about the objects traversed can be extracted from the measurements in addition to being able to yield intensity imaging contrast. Utilizing such capabilities for THz based imaging can be useful for both screening and diagnostic applications. In this work, we present the principles and applications of several reconstruction algorithms applied to THz imaging and sensing. We demonstrate its ability to achieve multi-dimensional imaging contrast of both soft tissues and concealed objects.

Nanoscale characteristics of single crystal zinc oxide nanowires

In this work, we report the growth and nanoscale characterization of single crystal zinc oxide nanowires synthesized by thermal chemical vapor deposition. Scanning electron microscopy, high-resolution transmission electron microscopy, x-ray diffraction, photoluminescence and Raman spectroscopy confirmed the high quality nature of the materials. To analyze their electrical properties, terahertz time domain spectroscopy was used. Atom probe tomography experiments and analysis were successfully developed and carried out, for the first time, on individual ZnO nanowires. This analysis revealed the incorporation of small concentration levels of atomic nitrogen homogeneously in nanowires grown when nitrogen gas was present during synthesis. Atom probe tomography can yield valuable information on the distribution of dopants and other impurities in wide bandgap semiconductor nanostructures and thus help understand better the material characteristics at the nanoscale.

InP/ZnS core-shell quantum dots sensitized ZnO nanowires for photovoltaic devices

With the increasing worldwide need for energy, fossil fuels will not be able to keep up with demand in the coming decades. Solar energy is one of the best solutions to this problem with the advantages of being clean and sustainable. Among the various designs of solar cells, dye-sensitized solar cells provide relatively high efficiency with large scale for a low cost [1]. Conventional dye-sensitized solar cells operate with light harvesting organic dye molecules adsorbed at the interface between TiO2 nanoparticles and a hole-conducting liquid electrolyte [2]. However, new combinations of materials potentially present an opportunity to further improve the performance and lower the cost of solar cells. One such promising approach is to use quantum dots (QDs) instead of the organic materials as the main photosensitive constituent. Several types of semiconductor QDs, including CdSe [3], CdS [4] and InP [5], have been investigated to realize quantum dot sensitized solar cells by taking advantage of the tunable absorption spectrum of the QDs through changes in their size. Complementarily to this approach, it is desirable to use a semiconductor material with a high structural uniformity and surface area as the framework on which to attach these QDs and achieve efficient electron transport, such as well-aligned ZnO nanowires [6].

Acquisition and analysis of Terahertz Time Domain imaging and sensing

We present results of an acquisition of Terahertz imaging and its analysis in terms of frequency based on a Time Domain Terahertz Spectroscopy System. THz radiation was generated using a photoconductive antenna based on LT-GaAs and was detected by the electro-optical sampling in a ZnTe crystal. Broadband THz pulses were obtained in the range from 0.1 THz to 4 THz, with a peak intensity at 1 THz. The full transmission spectrum for different materials was measured, and frequency-dependent imaging characteristics were analyzed.

Identification of tissue interaction of terahertz radiation toward functional tissue imaging

In this study, we utilize Terahertz imaging to study the effects of hydrofluoric acid on both compact bone tissue and cartilage. We compare the differences observed in the exposure for formalin fixed and raw, dried, tissue as well as those resulting from a change in Hydrofluoric (HF) concentration. Measurements are performed with THz-TDS, and a variety of spectroscopic based image reconstruction techniques are utilized to develop contrast in the features of interest.

Design, simulation, and characterization of THz metamaterial absorber

In recent years a great amount of research has been focused on metamaterials, initially for fabrication of left-handed materials (LHM) for use in devices such as superlenses, or electromagnetic cloaking device. [1, 2]. Such devices have been developed and demonstrated in regimes from radio frequency all the way up to infrared and near optical frequencies [3–5]. Metamaterials can be characterized by electric permittivity, e(ω), and magnetic permeability, μ(ω). By manipulating these properties, metamaterials can be engineered to exhibit un-natural phenomena such as negative index of refraction (n eff = Z 0 ), the reflection at some resonance frequency, ω 0 , can be minimized.

Synthesis and optical properties of ZnO nanowires for nanophotonics

High quality ZnO nanowires were synthesized by chemical vapor deposition using both a catalyst-assisted vapor-liquid-solid and a catalyst-free vapor-solid deposition approach. and their optical properties studied using photoluminescence and Raman spectroscopy combined with confocal laser scanning microscopy. Strong UV near band edge along with defect related visible luminescence emissions were observed and their relative intensity compared. We report here the growth of ZnO nanowires by chemical vapor deposition using both a catalyst-assisted vapor-liquid-solid and a catalyst-free vapor-solid deposition approach. The nanowires were characterized through scanning electron microscopy, x-ray diffraction, optical absorption, micro-photoluminescence, confocal Raman spectroscopy, and Terahertz time domain spectroscopy.

Aperture-less terahertz near-field imaging

Terahertz based spectroscopy and imaging has become an active field of research in the past decade for a plethora of applications including security screening, biomedical imaging, chemical analysis, and investigation of carrier dynamics. Several advantages exist for the use of THz techniques since investigation of a sample can be performed without contact or ionization; however, fine detail is difficult to determine due to the diffraction limit of the radiation. The resolution limit of THz imaging and sensing can be overcome by the incorporation of near-field optical techniques; which can allow image resolution as fine as tens of nanometers at THz frequencies. With this expanded resolution capability, THz imaging can decipher micro- and nano-structural information which, when coupled with the non-contact features of these techniques, makes THz spectroscopy ideal for the analysis micro and nano-optical devices. In this study, we demonstrate the development and performance of an aperture-less near-field system which has been integrated to perform highly-spatially resolved Terahertz Time-Domain Spectroscopic (THz-TDS) imaging.