International Journal of High Speed Electronics and Systems

Indium nitride (InN) is identified as a promising terahertz (THz) emitter based on the optical and electronic properties of high quality In- and N-face samples. Time domain THz spectroscopy has been employed to measure the pump wavelength and background carrier concentration dependence of THz emission from InN. There is no discernable difference between the In- and N-face InN samples, as expected for the improved crystalline quality and concomitant low background electron density and high mobility for both polarities. While there is only a weak dependence of THz signal on pump wavelength from 800 nm to 1500 nm, there is a strong dependence on background electron density. Modeling shows that the dominant mechanism for THz generation in bulk InN is the current associated with the diffusion of the photo-generated electrons at elevated electron temperature (photo-Dember effect) and the redistribution of the background electrons under drift, with larger screening from the higher mobility electrons as compared to holes. Compensation or p-type doping in conjunction with manipulation of the large internal electric fields in InN/InGaN nanostructures should lead to significant improvements in THz emitters.

Both selectivity and sensitivity of chemical sensors can be significantly improved by exploiting the information contained in microfluctuations present in the sensor system. We call our collection of methods to extract information from these microfluctuations Fluctuation-Enhanced Sensing (FES). In this review paper we discuss general FES principles and two types of applications; gas sensing with commercial solid state sensors and the Sensing of Phage-Triggered Ion Cascde (SEPTIC) technique to detect and identify bacteria.

Conventional munitions emit intense radiation upon detonation which spans much of the electromagnetic spectrum. The phenomenology of time-resolved visible, near- and mid-IR spectra from these fast transient events is poorly understood. The observed spectrum is driven by many factors including the type, size and age of the chemical explosive, method of detonation, interaction with the environment, and the casing used to enclose the explosive. Midwave infrared emissions (1800–6000 cm^-1, 1.67–5.56 μm) from a variety of conventional military munitions were collected with a Fourier transform spectrometer (16 cm^-1, 21 Hz) to assess the possibility of event classification via remotely sensed spectra. Conventional munitions fireballs appear to be graybodies in the midwave. Modeling the spectra as a single-temperature Planckian (appropriately modified by atmospheric transmittance) provided key features for classification and substantially reduced the dimensionality of the data. The temperature cools from ~1800 K to ambient conditions in 3–5 s, often following an exponential decay with a rate near 1 s^-1 second. A systematic, large residual spanning 2050–2250 cm^-1 was consistently observed shortly after detonation and may be attributable to hot CO₂ emission at the periphery of the fireball. For two different explosive types detonated under similar conditions, features based on the temperature, area and fit residuals could be used to distinguish between them. This paper will present the phenomenology of detonation fireballs and explore the utility of physics-based features for explosive classification.

We have developed a new broadband Chirped-Pulse Fourier Transform Microwave (CP-FTMW) spectrometer that allows the microwave spectrum in the 7.5-18.5 GHz range to be measured in a single data event. This technique produces a pure rotational spectrum that can be used for unambiguous identification of any species having a permanent electric dipole moment. CP-FTMW is a gas phase technique that is ideally suited for the detection of airborne chemical warfare agents (CWA) which must be detected in trace amounts (<10 ppm in air). The high resolution of the technique allows the identification of complex mixtures without the need for a preliminary separation step, such as gas chromatography, which significantly reduces analysis time. The technique is “blind” to major atmospheric components (N₂, O₂, CO₂, H₂O) as they either do not posses a permanent dipole moment or do not absorb in the range of the spectrometer, thereby eliminating large background signals. In this paper we will present preliminary results that are focused on early detection of airborne CWA, including acquisition time, sensitivity limits, and sample handling requirements for several of these species.

The interactions of charges in DNA with the vibrational modes in DNA depend on the spectra of these vibrational modes. Using (a) the Su-Schrieffer-Heeger (SSH) Hamiltonian approach, (b) integrated structures of DNA and manmade nanostructures, and (c) gel electrophoresis techniques,¹ the interaction between charges in DNA and the vibrational modes of DNA are investigated. As is well-known, DNA has a rich spectrum of modes in the THz spectral regime. The use of manmade nanostructures integrated with DNA facilitates the engineering of nanoscale systems useful in studying the role of environmental effects on the vibrational modes of DNA as well as the interaction of these modes with charge carriers in DNA. Among the DNA-based structures considered in this account are: B-DNA and Z-DNA strands related by a conformational change; and DNA molecules bound on one terminal to indirect bandgap semiconductor quantum dots. Gel electrophoresis is used as a tool for the analysis of carrier interactions in novel integrated DNA-manmade-nanostructure complexes, and models based on the SSH Hamiltonian² are employed as a means of analyzing the interactions between the vibrational modes of DNA and charge carriers in DNA.^3-4

Detection and identification of chemical warfare simulants based on multidimensional phase shaped femtosecond laser pulses coupled to mass spectrometry (MS) is demonstrated. The presented approach is based on binary phase shaping (BPS) and aims to improve the accuracy and precision required for security applications. It is based on multiphoton intrapulse interference of femtosecond laser pulses. Spectra retrieved by applying n-differently shaped pulses represent n-dimensions of the analysis. We present a multidimensional technique for detection and identification of analogues to chemical agents and mixtures in real-time. Experimental results for dimethyl phosphate, pyridine, and three isomers of nitrotoluene are presented.

Over the past twenty years, surface plasmon resonance has been developed as an effective technique for use in real-time biotechnological measurements of the kinetics of label-free biomolecular interactions with high sensitivity.^1-16 On a fundamental level, it is the dielectric-imaging involvement of the adsorbed biomolecular layer (DNA for example) in shifting the surface plasmon resonance (SPR) frequency by means of electrostatic coupling at the interface with the metal film substrate that facilitates SPR-based optical sensing. Of course, there are various factors that can influence surface plasmon resonance, including plasma nonlocality, phonons, multiplicity of layers, all of which should be carefully examined. Moreover, tunable SPR phenomenology based on the role of a magnetic field (both classically and quantum mechanically) merits consideration in regard to the field's effects on both the substrate¹⁷ and the adsorbed layer(s).¹⁸ This paper is focused on the establishment of the basic equations governing surface plasmon resonance, incorporating all the features cited above. In it, we present the formulation and closed-form analytical solution for the dynamic, nonlocal screening function of a thick substrate material with a thin external adsorbed layer, which can be extended to multiple layers. The result involves solution of the random phase approximation (RPA) integral equation for the spatially inhomogeneous system of the substrate and adsorbed layer,^19-25 given the individual polarizabilities of the thick substrate and the layer. (This is tantamount to the space-time matrix inversion of the inhomogeneous joint dielectric function of the system.) The frequency poles of the resulting screening function determine the shifted surface (and bulk) plasmon resonances and the associated residues at the resonance frequencies provide their relative excitation amplitudes. The latter represent the response strengths of the surface plasmon resonances (oscillator strengths), and will be of interest in optimizing the materials to be employed.

ALTAIR Center develops long-wave infrared (LWIR) and terahertz-frequency (THz) lasers operating at room temperature employing intraband luminescence in colloidal semiconductor nanocrystals, in which the optical transition frequencies can be easily tuned to the desired values by an appropriate choice of the semiconductor material and radius of the nanocrystals.

A novel concept of a compact mm/submm integrated spectrometers for environmental monitoring for hazardous materials of chemical and biological origin as well as for remote monitoring of the Earth atmosphere is discussed. The agents will be exactly identified by their unique spectral signatures. The assembled on a multi-chip module, cryocooler-mounted Superconducting Integrated SPectromer (SISP) exploits the superior performance of superconducting Josephson junction technology and unique on-chip integration of analog components, analog-to-digital converter, and digital components. Analog components include a superconductor-insulator-superconductor (SIS) mixer with integrated quasioptical antenna, mm-wave local oscillator, and SQUID amplifier for the down-converted (IF) signals. Upon amplification, the IF signal is digitized using a bandpass delta-sigma modulator, followed by real time processing with rapid single flux quantum (RSFQ) circuitry. Experimental results showing both operation of spectrometer components and the way to their successful integration are presented.

An optical characterization unit based on fluorescence spectroscopy-on-a-chip is described. It comprises a compact fluidic platform that is integrated onto a chip-size spectrometer. The analyte is continuously excited within a novel waveguide. Fluorescence spectra are recorded as the analyte traverse the detection area.

In order to achieve a strong interaction between excitation light and analyte we use an anti-resonant waveguide, in which the light is guided within the target-containing medium, thereby enabling a continuous excitation of a large volume. The excitation light is guided in the lower-refractive-index fluid when the light is coupled into the waveguide at an appropriate angle.

Compact spectrometers can be integrated along the fluidic channel. The spectrometers are composed of a detector array which is coated with a linear variable band-pass filter. The filter converts the spectral fluorescence information into a spatially dependent signal that is analyzed by the detector array. These chip-size spectrometers are especially applicable for characterization of moving analytes.

The experimental and computational study of bacterial thioredoxin, an E. coli protein, at THz frequencies is presented. The absorption spectrum of the entire protein in water was studied numerically in the terahertz range (0.1 – 2 THz). In our work, the initial X-ray molecular structure of thioredoxin was optimized using the molecular dynamical (MD) simulations at room temperature and atmospheric pressure. The effect of a liquid content of a bacterial cell was taken into account explicitly via the simulation of water molecules using the TIP3P water model. Using atomic trajectories from the room-temperature MD simulations, thioredoxin's THz vibrational spectrum and the absorption coefficient were calculated in a quasi harmonic approximation.

For our terahertz transmission measurements, we used solutions of thioredoxin in distilled water obtained from Sigma. The experimental and simulated signatures are correlated and dominant peaks are close in frequencies. The results of this study demonstrate that terahertz spectroscopy is a promising tool in generating spectral data for cellular components of bio agents such as bacterial cells and spores.

We study the transition pathways of a Lennard-Jones cluster of seven particles in three dimensions. Low lying saddle points of the LJ cluster, which can be reached directly from a minimum without passing through another minimum, are identified without any presumption of their characteristics, nor of the product states they lead to. The probabilities are computed for paths going from a given minimum to the surrounding saddle points. These probabilities are directly related to prefactors in the rate formula. This determination of the rate prefactors includes all anharmonicities, near or far from saddle points, which are pertinent in the very sophisticated energy landscape of LJ clusters and in many other complex systems.

We present a theoretical formulation of van der Waals (VdW)^1-19 molecule-substrate attraction in which the second order vdW energy is explicitly exhibited as a correlation/self-energy of the molecular/atomic electrons generated by a dynamic nonlocal image potential due to polarization of the electrons of the bounded metal/semiconductor substrate in the electrostatic limit.^20-23 This formulation can also be applied to the vdW interaction between an adsorbate layer and the substrate, as well as the interaction between layers. We have already applied it in the case of atom-surface vdW attraction in the presence of a normal magnetic field which induces both classical and Landau quantization magnetic effects,²⁴ incorporating the role of dynamic and nonlocal plasma effects. The extension to multiple adsorbate layers and their mutual interactions as well as their attraction to the substrate is straightforward.²⁵ The dependence of the atom/molecule-surface vdW energy on magnetic field strength provides an adjustable parametrization of the underlying zero-point photon energy (represented in terms of the nonretarded longitudinal plasmon-photons of the Coulomb interaction), opening the possibility of analyzing the concomitant fundamental quantum phenomenology in detail with material parameters that can be examined experimentally.

The spin Hall effect provides a new possible way to effectively inject spins into paramagnetic semiconductors. Here, we investigate the spin-Hall effect in a p-type Luttinger semiconductor employing a two-band kinetic equation analysis. The long-range disorder effect on spin-Hall current (SHC) is considered within the self-consistent Born approximation. We find that in addition to the intrinsic SHC proposed previously, there is a nonvanishing SHC that originates from long-range electron-impurity scattering, but which is independent of impurity density in the diffusive regime. This SHC has an opposite sign from the intrinsic one, leading to a significant reduction of the total SHC. We also carry out a numerical analysis of the hole density dependencies of SHC and spin mobility, finding that with increasing hole density, the SHC first increases and then falls, while the spin mobility monotonically decreases.

Several aspects of the theory of plasmon resonant DC photoconduction are discussed here, in connection with recent observations involving a THz-irradiated grid-gated double-quantum-well FET.¹ In this, we construct a classical model of nonlinear polarizability to second order in the THz field using a “hydrodynamic” type formulation including the roles of a stress-tensor and friction/viscosity. The resulting second order polarizability exhibits resonant behavior when the THz frequency matches plasmon frequencies of the system, sharply reducing the effectiveness of screened impurity scattering potentials which can admit resonant DC photoconduction. Furthermore, we also show that an asymmetric double-quantum-well system with lateral periodicity can mix optical and acoustic plasmons, giving rise to an interlayer THz field which becomes very strong when tuned by gate voltage into the “mode-mode-repulsion” regime wherein the optical and acoustic modes equally share amplitude. This can enhance interlayer electron tunneling and may contribute to photoconductivity.

The objective of the present study was to better understand the photophysics of explosives and chemical warfare simulants in order to develop better performing analytical tools. Photoionization mass spectra were taken using three optical schemes. The first was resonance-enhanced multiphoton ionization (REMPI) using few-ns duration 248 or 266 nm laser pulses. The second scheme was non-resonant multiphoton ionization (MPI) using 100 fs duration laser pulses at wavelengths between 325 and 795. The third approach was single photon ionization (SPI) using few-ns duration 118 nm laser pulses. For all the molecules investigated, mass spectra resulting exposure to ns-duration 248 or 266 nm laser pulses consisted of only low molecular weight fragments. Using fs-duration laser pulses produced more complicated, potentially analyzable, fragmentation patterns, usually with some parent peak. Single photon ionization gave the best results, with mass spectra consisting of almost only parent peak, except for the case of TATP.

Surface plasmon and evanescent wave sensors are attractive for chemical and biological sensing applications. They can work in aqueous media and when used in conjunction with the appropriate surface chemistry they can have high specificity and high sensitivity. However, most surface plasmon sensors are relatively complex as they are based on the use of attenuated total internal reflection to excite surface plasmon resonance in a thin gold film and require light to be incident at the appropriate angle and polarization. Other surface plasmon and optical affinity sensors have used the evanescent waves in planar waveguides to interact with the environment. These devices are sensitive but, have strict optical coupling requirements and are difficult to fabricate. In optical fiber evanescent wave sensors the interaction with the surrounding environment is usually obtained by tapering an optical fiber, which significantly weakens the structure, or by just utilizing the end of the optical fiber. In this paper, in-line optical fiber structures are presented that are mechanically robust, and provide a large interaction length for high sensitivity. They are compatible with standard chemistries for optical affinity sensing of biological compounds.

We present material and device characterization of 280 nm semiconductor ultraviolet light emitting diodes. These devices exhibit low series resistance, wavelength stability with increasing current, and have a half-life in excess of 570hrs, depending upon the injection current. Time-resolved photoluminescence studies of these materials prior to fabrication have been correlated with the device performance. We also discuss the potential for use in water purification.

There is considerable interest in electrical sensing of biomolecular binding since it has the potential to be label free, to work easily in aqueous environments native to the biomolecules, and to be integrated with small, fast, and inexpensive microelectronoics as detection instrumentation. Although electrochemical methods have been used successfully in detections of DNA molecules with Ag labels at very high sensitivity (~ p ml), detection of DNA molecules in terms of label free techniques has a lower sensitivity (~ μ ml). Here, the surface attachment chemistry is critical towards the detection of ultra-low concentration of biomolecules. In this article, based on density functional theory, we have calculated and analyzed the electrical characteristics of the contact between aromatic molecules and silicon (100) − 2×1 surfaces. Design principles for silicon based electrodes of electrochemically biomolecular sensing instruments for label-free sensing of single or a few biomolecular molecules have also been discussed.

The sol-gel method has been employed in the fabrication of mesoporous composite films consisting of a nonionic surfactant, Pluronic P123, as the organic component, and silica as the inorganic component. The hybrid nature of these films resulted in their having an internal structure consisting of nanometer size self-assembled organic mesostructures surrounded by a silica framework. These films served as the host matrix for the laser dye coumarin 481 (C481) and an energy transfer complex formed between C481 and J-aggregated meso-tetra(4-sulfonatophenyl)porphyrin (TSPP). Upon exposure to methanol vapor, a rapid and reversible decrease in fluorescence intensity occurs for films containing C481 alone as well as containing both C481 and TSPP. Steady-state and time-resolved spectroscopic studies suggest that the decrease in fluorescence intensity is primarily due to an excited state interaction between methanol and C481; while, additionally, morphological changes within the film appear to play a role for films containing both C481 and TSPP.

A novel nanoscale-engineering methodology is presented that has potential for the first-time development of a microscope-system capable of collecting terahertz (THz) frequency spectroscopic signatures from microscopic biological (bio) structures. This unique THz transmission microscopy approach is motivated by prior studies on bio-materials and bio-agents (e.g., DNA, RNA and bacterial spores) that have produced spectral features within the THz frequency regime (i.e., ~ 300 GHz to 1000 GHz) that appear to be representative of the internal structure and characteristics of the constituent bio-molecules. The suggested THz transmission microscopy is a fundamentally new technological approach that seeks to avoid the limitations that exist in traditional experiments (i.e., that must average over large numbers of microscopic molecules) by prescribing a viable technique whereby the THz frequency signatures may be collected from individual bio-molecules and/or microscopic biological constructs. Specifically, it is possible to envision the development of a “nanoscale imaging array” that possesses the characteristics necessary (e.g., sub-wavelength resolution) for successfully performing “THz-frequency microscopy.”

The so-called “mountain-pass” theorem allows for the finding of a critical point on the path between two points on a multidimensional contour where the maximal elevation is minimal. By implementing the “elastic string algorithm”, it is possible to not only find the critical point, but to compute the mountain-pass itself on a finite-dimensional contour. For a given molecule that sits between two probes making up a nanostructure device, we propose that the mountain-pass will be a likely path of electron transport through the molecule, the contour being the electronic potential of the molecule. This potential along this path will be used as the input potential for SETraNS, a 1D Wigner-Poisson electron transport solver in order to explore the current-bias characteristics of such the molecule in such a device. In order to calculate the mountain pass, the elastic string algorithm is used to set up a constrained non-linear optimization problem which is, in turn, solved via a Monte Carlo method. We will compute the mountain-pass for a well-known test contour in order to show the validity of this approach. The procedure developed here is to be combined with conformational analysis via the molecular modeling program AMBER and the quantum transport program SETraNS in order to predict molecular function. When achieved, this combined procedure will allow for the better design and implementation of nanoscale molecular devices for application such as sensing, switching and data processing.