Robert S. Donnan

Sub-terahertz spectroscopy reveals that proteins influence the properties of water at greater distances than previously detected.

The initial purpose of the study is to systematically investigate the solvation properties of different proteins in water solution by terahertz (THz) radiation absorption. Transmission measurements of protein water solutions have been performed using a vector network analyser-driven quasi-optical bench covering the WR-3 waveguide band (0.220-0.325 THz). The following proteins, ranging from low to high molecular weight, were chosen for this study: lysozyme, myoglobin, and bovine serum albumin (BSA). Absorption properties of solutions were studied at different concentrations of proteins ranging from 2 to 100 mg/ml. The concentration-dependent absorption of protein molecules was determined by treating the solution as a two-component model first; then, based on protein absorptivity, the extent of the hydration shell is estimated. Protein molecules are shown to possess a concentration-dependent absorptivity in water solutions. Absorption curves of all three proteins sharply peak towards a dilution-limit that is attributed to the enhanced flexibility of protein and amino acid side chains. An alternative approach to the determination of hydration shell thickness is thereby suggested, based on protein absorptivity. The proposed approach is independent of the absorption of the hydration shell. The derived estimate of hydration shell thickness for each protein supports previous findings that protein-water interaction dynamics extends beyond 2-3 water solvation-layers as predicted by molecular dynamics simulations and other techniques such as NMR, X-ray scattering, and neutron scattering. According to our estimations, the radius of the dynamic hydration shell is 16, 19, and 25 Å, respectively, for lysozyme, myoglobin, and BSA proteins and correlates with the dipole moment of the protein. It is also seen that THz radiation can serve as an initial estimate of the protein hydrophobicity.

Micromachined Thick Mesh Filters for Millimeter-Wave and Terahertz Applications

This paper presents several freestanding bandpass mesh filters fabricated using an SU-8-based micromachining technique. The important geometric feature of the filters, which SU8 is able to increase, is the thickness of the cross-shaped micromachined slots. This is five times its width. This thickness offers an extra degree of control over the resonance characteristics. The large thickness not only strengthens the structures, but also enhances the resonance quality factor ( Q-factor). A 0.3-mm-thick, single-layer, mesh filter resonant at 300 GHz has been designed and fabricated and its performance verified. The measured Q-factor is 16.3 and the insertion loss is 0.98 dB. Two multi-layer filter structures have also been demonstrated. The first one is a stacked structure of two single mesh filters producing a double thickness, which achieved a further increased Q-factor of 27. This is over six times higher than a thin mesh filter. The second multilayer filter is an electromagnetically coupled structure forming a two-pole filter. The coupling characteristics are discussed based on experimental and simulation results. These thick mesh filters can potentially be used for sensing and material characterization at millimeter-wave and terahertz frequencies.

Determination of the Gyrotropic Characteristics of Hexaferrite Ceramics From 75 to 600 GHz

The work reported in this paper is in support of applications of ferrites in measurement systems operating at frequencies in the range 100-600 GHz. Measurements of the magneto-optical characteristics of selected grain-oriented hexaferrite ceramics from 75 to 600 GHz are described; a quasi-optical transmissometer driven by a multiband vector-network-analyser is used to provide the required high dynamic range, spectral-resolution, and scan-speeds. The implications of the results of the measurements are examined.

Terahertz spectral domain computational analysis of hydration shell of proteins with increasingly complex tertiary structure.

Solvation dynamics of biomolecules and water in a hydration shell have been studied by different methods; however, a clear picture of this process is not yet established. Terahertz (THz) studies of molecular behavior in binary solutions present unique information on the picosecond motions of molecules. A complete mechanical interpretation of THz absorption spectra associated with solvated biomolecules remains a challenging task. The Gromacs molecular dynamics (MD) simulation package is used here to examine the spectral characteristics of water molecules in close proximity to biomolecules using vibrational density of states (VDOS). Systematic simulation of solvation dynamics of proteins of different size and tertiary structure are presented. The following have been selected for analysis. They range from less to more complex tertiary structure (corresponding to an increased number of secondary structure elements): TRP-cage13-20 peptide, TRP-cage, BPTI, and lysozyme. The initial study predicts that the depth of the hydration shell, determined by VDOS of water, extends to approximately 10 Å and does not depend on protein size. Furthermore the integral perturbation coefficient of the whole solvation layer is found to be increased for larger proteins due to a higher retardation rate of water molecules in their shells. Differences in solvation dynamics among the proteins considered originate primarily from the water molecules buried in the interior of the protein and not from the on-surface molecules.

Atomic and vibrational origins of mechanical toughness in bioactive cement during setting

Bioactive glass ionomer cements (GICs) have been in widespread use for ∼40 years in dentistry and medicine. However, these composites fall short of the toughness needed for permanent implants. Significant impediment to improvement has been the requisite use of conventional destructive mechanical testing, which is necessarily retrospective. Here we show quantitatively, through the novel use of calorimetry, terahertz (THz) spectroscopy and neutron scattering, how GICs developing fracture toughness during setting is related to interfacial THz dynamics, changing atomic cohesion and fluctuating interfacial configurations. Contrary to convention, we find setting is non-monotonic, characterized by abrupt features not previously detected, including a glass–polymer coupling point, an early setting point, where decreasing toughness unexpectedly recovers, followed by stress-induced weakening of interfaces. Subsequently, toughness declines asymptotically to long-term fracture test values. We expect the insight afforded by these in situ non-destructive techniques will assist in raising understanding of the setting mechanisms and associated dynamics of cementitious materials.

Accurate determination of terahertz optical constants by vector network analyzer of Fabry–Perot response

We present a method based on a Fabry-Perot model to efficiently and accurately estimate optical constants of wafer samples in transmission-only measurements performed by a vector network analyzer (VNA). The method is demonstrated on two separate wafer samples: one of silicon and the other of polymethylmethacrylate. Results show that the method can not only acquire optical constants accurately and simply over a broad frequency domain but also overcome the limitations of calculation for dispersive and lossy materials to which existing methods are susceptible, such as those based on VNA-driven quasi-optical transmissometers and terahertz time-domain spectrometry.

Revised metrology for enhanced accuracy in complex optical constant determination by THz-time-domain spectrometry

THz time-domain spectrometry (TDS) probes the complex polarization response of materials. Various analytical procedures are applied by many to extract the associated material optical constants. This has commonly been done by iteratively varying material parameters in order to achieve a match between experiment and a theoretical transfer function (TF). The poly root behavior of a TF is emphasized for measurements with reflections in the time domain. This study provides a comprehensive analysis of the influence of the initial guesses on the accuracy of extracted material parameters. In addition, various ways of representing multiple reflections inside the sample (a Fabry-Perot-like effect) are compared and their contribution to the uncertainty of material parameters is analyzed. Experimental evidence is provided where appropriate to support theoretical statements. Furthermore, this paper offers a basis for data comparison between different THz-TDS systems in transmission mode. Finally, a clear distinction is made between a commonly used basic analysis and an enhanced one, in terms of associated uncertainties in determination of the real and imaginary parts of the complex refractive index.

Experimental characterization of hexaferrite ceramics from 100 GHz to 1 THz using vector network analysis and terahertz-time domain spectroscopy

Gyrotropic, magnetically-hard hexaferrite ceramics, are a very promising material for deployment as quasioptical nonreciprocal devices at high frequencies (⪢90 GHz). In this paper, a quasioptical measurement bench driven by a vector-network analyzer and a terahertz (THz) time domain spectroscopy measurement system are used in conjunction to characterize a thin (2.02 mm thickness) hexaferrite plate for two orthogonal states of beam polarization. From these data, the intrinsic circular-polarization transmittance for each state (or case), of polarization, left and right, is computed. Inherent magneto-optical constants of the plate over the range of 100 GHz to 1 THz are then determined. Analysis reveals low attenuation of the hexaferrite ceramics over millimeter and submillimeter wave bands.

An optically controlled phase shifter employing the organic semiconductor poly(3-hexylthiophene)

The transmission characteristics of optically controlled phase shifters employing the organic semiconductor poly(3-hexylthiophene) (P3HT) are reported. Two microstrip structures were fabricated onto a P3HT coated indium tin oxide substrate supported by glass. Experimental results on these unoptimized prototype structures yield reversible differential phase shifts >10° at 2.0GHz under tens of milliwatt optical power illumination. These devices demonstrate great potential as linear analog phase shifters.

Complex Permittivity of Pure Water Measured by Vector Network Analysis at W-Band

Preliminary pure-water transmittance measurements over the W-band (75 – 110 GHz) are performed using a well-designed quasi-optical bench and vector network analyser (VNA/QO). The measurements are an initial trial to explore vector network analysis in studies of micro-biological systems.