David S. Wilbert

Design and analysis of perfect terahertz metamaterial absorber by a novel dynamic circuit model

Metamaterial terahertz absorbers composed of a frequency selective layer followed by a spacer and a metallic backplane have recently attracted great attention as a device to detect terahertz radiation. In this work, we present a quasistatic dynamic circuit model that can decently describe operational principle of metamaterial terahertz absorbers based on interference theory of reflected waves. The model comprises two series LC resonance components, one for resonance in frequency selective surface (FSS) and another for resonance inside the spacer. Absorption frequency is dominantly determined by the LC of FSS while the spacer LC changes slightly the magnitude and frequency of absorption. This model fits perfectly for both simulated and experimental data. By using this model, we study our designed absorber and we analyze the effect of changing in spacer thickness and metal conductivity on absorption spectrum.

Equivalent-Circuit Interpretation of the Polarization Insensitive Performance of THz Metamaterial Absorbers

Polarization insensitive metamaterial perfect absorbers were investigated through finite element numerical method and a new equivalent-circuit electric model was proposed to interpret this polarization insensitivity. The devices were fabricated to validate the model and experimental measurements were shown to be in good agreement with the simulated results. This absorber device is suitable for future use in THz sensing and detection applications.

Surface optical phonons in GaAs nanowires grown by Ga-assisted chemical beam epitaxy

Surface optical (SO) phonons were studied by Raman spectroscopy in GaAs nanowires (NWs) grown by Ga-assisted chemical beam epitaxy on oxidized Si(111) substrates. NW diameters and lengths ranging between 40 and 65 nm and between 0.3 and 1.3 μm, respectively, were observed under different growth conditions. The analysis of the Raman peak shape associated to either longitudinal or surface optical modes gave important information about the crystal quality of grown NWs. Phonon confinement model was used to calculate the density of defects as a function of the NW diameter resulting in values between 0.02 and 0.03 defects/nm, indicating the high uniformity obtained on NWs cross section size during growth. SO mode shows frequency downshifting as NW diameter decreases, this shift being sensitive to NW sidewall oxidation. The wavevector necessary to activate SO phonon was used to estimate the NW facet roughness responsible for SO shift.

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.

Terahertz metamaterials perfect absorbers for sensing and imaging

Devices operating at THz frequencies have been continuously expanded in many areas of application and major research field, which requires materials with suitable electromagnetic responses at THz frequency ranges. Unlike most naturally occurring materials, novel THz metamaterials have proven to be well suited for use in various devices due to narrow and tunable operating ranges. In this work, we present the results of two THz metamaterial absorber structures aiming two important device aspects; polarization sensitivity and broad band absorption. The absorbers were simulated by finite element method and fabricated through the combination of standard lift-off photolithography and electron beam metal deposition. The fabricated devices were characterized by reflection mode THz time domain spectroscopy. The narrow band absorber structures exhibit up to 95% absorption with a bandwidth of 0.1 THz to 0.15 THz.

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.

Theoretical and experimental investigation of hybrid broadband terahertz metamaterial absorber

Among electromagnetic spectrum, terahertz region has been utilized less due to the lack of appropriate devices that works well in these frequencies But recently growing interest has been focused to design devices with functionality in terahertz region because of potential terahertz applications. We present a novel structure that broadens bandwidth of terahertz metamaterial absorber. Our structure takes a benefit of multiband absorber by making the bands close enough to each other but in a multilayer pattern. The absorber has composed of two concentric copper rings in two different layers followed by polyimide and a metal back layer. Simulation shows 100 GHz bandwidth which is double of that of a single layer single ring absorber.

Highly Efficient, Polarization Insensitive Terahertz Metamaterial Perfect Absorber and Imaging

We demonstrate performance and characteristics of a metamaterial absorber designed for operation in the THz regime. This absorber exhibits upto 90% absorbance and shows potential for use in sensor and detector devices.

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].