Ultrafast Terahertz Spectroscopy and Imaging

We are developing novel measurement and theoretical techniques to explore the low frequency intramolecular dynamics of model biological molecules and species including amino acids, peptides and DNAs. Our current efforts focus on obtaining broadband Terahertz (THz) spectra of protein constituents (e.g., serine, cysteine, D/L and D,L Tryptophans), synthetic peptides (e.g., dipeptides, tripeptides and poly(Alanine) and sugars. We also employ UV/Vis pump-THz probe methods to induce molecular isomerization or chemical reaction in order to identify molecular structures that occur along the reaction path. Examples include trans-cis isomerization of Azobenzene and related conjugated systems. Related studies are conducted on electronic materials. Solid-state periodic boundary calculations using CPMD, VASP and CHARMM are employed to determine the origin of internal and external vibrational modes in solid-state samples. These types of measurements in conjunction with theoretical modeling are envisioned as a means to extract important protein-folding rates and determine mechanisms responsible for DNA base pair hydrogen-bonding, surface interactions and helix dynamics.

In parallel to our THz spectroscopy efforts, we are also developing novel methods to perform hyperspectral imaging of inhomogeneous materials, threat agents and other species relevant to Homeland Security needs. This wavelength range is particularly attractive because THz radiation readily transmits through most plastics, cloth, papers and other non-metallic low density matierals. Detailed analysis of source and THz propagation is required as well as specialized collection optics to optimize image resolution. Spectral characterization of materials in an image scene at several meter reflection distances is being explored.

These investigations use state-of-the-art, KHz rep-rate amplified ~45 femtosecond pulsed lasers for broadband 0.2-10 THz generation and detection including GaAs antennas (for generation and detection), as well as GaP and ZnTe nonlinear crystals for broadband spectroscopic determinations and imaging.

The above projects are conducted in conjunction with collaborators from SPARTA, Inc., Brimrose of America, FDA, and University of Maryland Baltimore Campus.

Resources:

Time-resolved THz spectrometer for spectral determinations of samples (0.2 THz to ~10 THz) and transient spectroscopy of biomolecular and material characterization.

Representative THz spectra of solid tripeptides demonstrating that molecular structure, amino acid sequence and crystal environmental effects are uniquely “fingerprinted” in this spectral range. Solid-state theory is employed to understand detailed atomic motions responsible for these spectral features.

Terahertz imaging apparatus employing transmission and reflection geometries for hyperspectral studies relevant to Homeland Security applications.

References:

"Terahertz Spectroscopy of Solid Serine and Cysteine,"
T.M. Korter, R. Balu, M.B. Campbell, M.C. Beard, S.K. Gregurick, and E.J. Heilweil,
Chem. Phys. Lett. 418, 65-70 (2005).

"Comparative OHD-RIKES and THz-TDS Probes of Ultrafast Structural Dynamics in Molecular Liquids,"
M.C. Beard, W.T. Lotshaw, T. Korter, E.J. Heilweil, and D. McMorrow,
J. Phys. Chem. A 108(43), 9348-9360 (2004).

"Non-invasive detection of weapons of mass destruction using THz radiation,"
M.B. Campbell and E.J. Heilweil,
in Proc. SPIE 5070 Terahertz for Military and Security Applications, ed. by R. Jennifer Hwu, Dwight L. Woolard, (SPIE, Bellingham, WA, July, 2003), p. 38.

"Terahertz Spectroscopy of Short-Chain Polypeptides,"
M. Kutteruf, C. Brown, L. Iwaki, M. Campbell, T.A. Korter and E.J. Heilweil,
Chem. Phys. Lett. 375, 337-343 (2003).

"Pulsed Terahertz Spectroscopy of DNA, Bovine Serum Albumin and Collagen between 0.1 and 2.0 THz,"
A.G. Markelz, A. Roitberg, and E.J. Heilweil,
Chem. Phys. Lett. 320 42-48 (2000). (Preprint)

"Temperature Dependent Terahertz Output from Semi-Insulating GaAs Photoconductive Switches,"
A.G. Markelz and E.J. Heilweil,
Appl. Phys. Lett. 72(18), 2229 (1998).