Search

Terahertz near-field microspectroscopy is emerging as an essential tool for characterization of novel materials and biomolecules. It is important to ensure the near field geometry used does not introduce spectral artifacts. For example, many scanning techniques can be strongly influenced by the interaction between the scattering tip and the sample. Here we examine the spectroscopic effects of a $200\ \mu\mathrm{m}$ diameter aperture for THz near-field measurements. We use HFSS to model free-space transmission through samples with resonant absorbance as a function of sample thickness, lateral sample width, and aperture diameter. We examine the transmitted power and spectral fidelity for coupling of transmitted THz light onto a detector. These studies inform corrective post-measurement analysis algorithms and design of near-field detection systems.

We use massive averaging over the thermally populated energy landscape to calculate a realistic terahertz vibrational density of states (VDOS), isotropic absorption and anisotropic absorption for the protein chicken egg white lysozyme (CEWL). We find that the VDOS shows little variation with starting structure, however both the isotropic and anisotropic absorption vary considerably with starting structure. Averaging over the population leads to a smooth featureless average isotropic spectrum, in agreement with THz absorption measurements however resonant bands emerge for the anisotropic averaging. Further we find the vibrational ensemble in fact has frequency dependent features associated with large overlap with functional displacements.

We measure as a function of sample orientation, doping, temperature, and energy, the Faraday rotation of linearly polarized THz light by high temperature cuprate superconductors. We observe a linear dichroism signal which increases with decreasing frequency and may exhibit resonances in the THz (1–6 meV) range. Preliminary results also show strong temperature dependence to the Faraday rotation signal.

The dynamical transition (DT) corresponds to the rapid onset of atomic motions at ca. 200K seen for minimally hydrated biomolecules. The DT is present in solution phase terahertz protein absorption measurements suggesting a benchtop DT technique. To move towards this application, several outstanding questions must be addressed: 1) does the THz absorption of protein solution reflect the protein structural dynamics, or the biological water dynamics; and 2) are THz solution phase DT measurements consistent with direct root mean square displacement measurements of hydrated powders. We show that for T

Proteins are large complex biomolecules which have structural vibrations in the terahertz range. The vibrational spectrum is dependent on the protein structure, which fluctuates at biological temperatures. Recently it was shown that these fluctuations can be accounted for by massive sampling of the energy landscape to calculate the terahertz isotropic and anisotropic absorbance. The large number of vibrations contributing to the spectra however makes interpretation of resonant bands challenging. Here we examine how we can apply elastic network models to define fundamental vibrational displacements to analyze these massive data sets.

Terahertz time domain spectroscopy is used to measure the melting kinetics of fructose molecular crystals. Combining single crystal anisotropy measurements with density functional calculations we assign the phonon frequencies and interrogate how specific phonons behave with melting. While nearly all the low frequency phonons continuously red shift with heating and melting, the lowest energy phonon polarized along the c-axis blue shifts at the melting temperature, suggesting an initial structural change immediately before melting. We find that the kinetics follow a 3D growth model with large activation energies consistent with previous DSC measurements. The large activation energies indicate multiple H-bonds must break collectively for the transition. The results suggest THz TDS with sub picosecond resolution could be used to measure ultra-fast kinetics.

Orange carotenoid protein (OCP) controls efficiency of the phycobilisome (PBS), the light harvesting antenna in cyanobacteria, to prevent oxidative damage. The OCP switches from resting state to photo protective state with intense blue light illumination. Questions persist as to why OCPR interaction increases with the PBS over that with the OCPO. Here we examine the role of long-range intramolecular vibrations within OCP. Using Stationary Sample Anisotropic Terahertz Microscopy (SSATM) we measure changes in the intramolecular vibrations with photo switching. We report the first observation of switching in the intramolecular vibrations with photoexcitation. Results suggest that there is a stiffening of the molecule in the photo protective state. This increase in structural stability may enhance the interaction with the PBS change in OCP interaction with PBS. In low light, carotenoid bound OCP appears orange (OCPo) and is inactive. Illumination by strong light converts OCP to the active, red (OCPR) state, which interacts with the PBS. A comparison of anisotropic THz microscopy measurements of dark adapted (OCPo) and illuminated OCP crystals indicate differences in their vibrational modes that may be important for OCP-PBS interactions.

We have developed a stationary sample THz anisotropic dichroism microscope as a powerful tool for material characterization. The key innovation is the use of an iso-response curve defined by the terahertz and detection light polarizations. Here we show that following a continuous iso-response curve introduces angular distortion in the spectrum, and this distortion can be eliminated by using a discontinuous iso-response angle pairs avoiding large gradient regions. Employing this additional constraint on the relative polarizations provides a convenient technique for rapid anisotropic terahertz spectroscopy.

A stationary sample anisotropic terahertz microscopy technique is used to characterize the intramolecular vibrations for lysozyme. Tetragonal and triclinic crystals are compared. We find excellent reproducibility within a single crystal symmetry group. Several resonant bands are present for both symmetry groups, indicating they originate with the intramolecular vibrations and not crystal lattice phonons. Bands become more pronounced and higher frequency resonant bands begin to emerge with slight dehydration.

We demonstrate anisotropic THz spectroscopy of sucrose using newly developed compact discretely tunable THz sources for turn-key spectroscopic systems.

We demonstrate THz anisotropy signature determination of a protein crystal model using newly developed compact tunable narrow band THz sources for turn-key spectroscopic systems for the bio molecular community.

The so called protein dynamical transition has long been associated with thermally activated excitations of the solvent adjacent to the polypeptide surface. Recently we reported the somewhat surprising result that the low temperature (

Modeling has predicted that intramolecular structural vibrations enables proteins to access functionally important structural change. We show that the vibrational density of states and the isotropic absorption in the terahertz range are only weakly dependent on the protein functional state for several bench marking proteins. At the same time the direction of motions changes dramatically with functional state and with a resulting impact on the anisotropic absorption. Our anisotropic THz microscopy (ATM) measurements confirm this sensitivity. Here we apply the technique to the question of whether the protein dynamical transition (DT) is important to protein function. We find a strong anisotropic resonance at 70 cm−1 rapidly increases in strength at temperatures above the DT. As these intramolecular vibrations enable protein structure to change conformation, the results suggest function will cease below DT for those proteins that require large scale conformational change.

The higher transmission through tissues of long wavelength light motivates the development of fluorescent proteins with excitation shifted to the red. However red fluorescent proteins (RFPs) are more susceptible to photobleaching than their shorter wavelength counterparts. In particular RFPs are more susceptible to photobleaching [1]. A possible reason for this is a decrease in the structural stability of the beta barrel. Measurements of structural stability include atomic root mean squared displacement measured by the X-ray B-factor and neutron quasi elastic scattering. To date, X-ray measurements of RFP's do not indicate a structural stability change and systematic scattering studies have not been performed. Using THz dielectric response we examine if the picosecond structural flexibility decreases with increasing FP stability.

We use a rapid data acquisition technique to test reproducibility and inhibitor binding sensitivity of the recently reported narrow band resonances for lysozyme protein crystals using THz microscopy. The spectra are reproducible, and change dramatically with inhibitor binding. The results indicate the resonances arise from intramolecular modes.

In this work we present mid-infrared (114 meV–224 meV) polarization-sensitive measurements on multilayer epitaxial graphene, which reveal graphene's rich and complex cyclotron resonant (CR) structure. By using Fourier transform Kerr spectroscopy we are able to separate and identify cyclotron resonances from graphene multilayers with various stacking orders and thicknesses. Moreover, from the identification of the layers present and their measured fundamental band parameters we are able to use these results to better understand previously unexplained CR features in far-infrared (0.1 meV – 87 meV) transmission measurements performed on the same sample.

A method is presented for measuring anisotropic THz response for small crystals, Crystal Anisotropy Terahertz Microscopy (CATM). Sucrose CATM measurements find the expected anisotropic phonon resonances. CATM measurements of protein crystals find the expected broadband water absorption is suppressed and strong orientation and hydration dependent resonant features.

We report a new broad band technique for rapidly measuring the complex Faraday and Kerr rotations in materials such as topological insulators and graphene, combining the distinct advantages of THz time domain spectroscopy and polarization modulation techniques. The performance of the system is demonstrated using GaAs two dimensional electron gas in a magnetic field.

The protein dynamical transition is investigated as a function of protein structure using terahertz time domain spectroscopy (THz-TDS). Measurements performed for native state and denatured hen egg white lysozyme (HEWL) show that protein structure is not necessary for the dynamical transition. We find the temperature dependence follows activated behavior and there is no evidence of a fragile to strong transition. Measurements of short chain poly alanine show a dynamical transition down to penta-alanine, however no transition is observed for di-alanine or tri-alanine. These measurements demonstrate that the temperature dependence arises strictly from the interaction of the side chains with the solvent. The lack of a transition for shorter chain polypeptides may indicate the temperature dependence arises from a net ordering of the adjacent water which scales with the length of the polypeptide chain.

Tertiary structural vibrational modes of proteins are at far infrared or terahertz frequencies. These modes involve collective motion of many atoms and are indicative of the protein structure, hydration and binding with ligands and other proteins[1-3]. Previously we have shown that the absorbance strongly resembles the vibrational mode density of states. Our measurements of conformational change have focused on the hydration, denaturing effects, and photoactive proteins bacteriorhodopsin. In addition we have shown a strong sensitivity of the terahertz response to heme protein oxidation state[4]. We will discuss the impact of these results on biomolecular switching mechanisms as well as applications to biomolecular sensing. In the case of bacteriorhodopsin the conformational state is controlled through photoexcitation and monitored by UV/Vis absorption spectroscopy[3]. In the "active" state, critical to the full photocycling and therefore biological activity, the terahertz absorbance and index are increased relative to the resting state. This increase could be associated with a red shifting of the density of states, consistent with increased structural flexibility in the active state. However in the case of photoactive yellow protein we see a decrease in the index and absorbance consistent with a decrease in tertiary structure suggesting partial unfolding in the active state. We have investigated the effect of unfolding directly by examining denatured samples of hen egg white lysozyme (HEWL). In Figure 1 we show the change in the absorbance and index as HEWL is thermally denatured.