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Ultrafast and Continuous-Wave Terahertz Spectroscopy and Imaging

An effort to develop Terahertz (THz) technologies, first supported by the NIST Director’s Competence Program between 1998 to 2003, continues to increase the Division’s expertise in long-wavelength, 30 µm to 300 µm (0.1 THz to 15 THz), pulsed and continuous-wave (CW) coherent laser spectroscopy and imaging. The technology is being applied to model biomolecules to understand the complex dynamics involved in such processes as molecular recognition and protein folding. This program is divided into Pulsed Terahertz Spectroscopy and Imaging and Continuous Wave Terahertz Spectroscopy of Biomolecules.

Solid-state photomixers, based on electrically biased antenna-structures lithographically printed on low-temperature-grown GaAs substrates, are used as sources of pulsed and continuous wave THz radiation from 0.1 THz to 2 THz. For the pulsed THz system, a Nd+3:YVO4-pumped, mode-locked, Ti+3:Sapphire high-repetition-rate femtosecond laser is used to illuminate the photomixer to produce broad-band THz radiation, which is detected with a similarly designed optically gated antenna structure. Alternatively, the photomixers can be replaced with nonlinear crystals (e.g., ZnTe, GaAs, or GaP<110>) to generate and detect broadband THz pulses. Fourier-transform methods are employed to extract spectra. The spectrometers have successfully recorded absorption spectra of pressed DNA pellets, peptides and small proteins, and is presently being used to study polypeptides and sugars.

A second type of pulsed THz instrument utilizes high-power Ti+3:Sapphire pulses at 1 kHz repetition rate with GaP crystals for THz generation and electro-optic (EO) detection. This spectrometer schematic pictured below covers a broader frequency range (0.2 THz to >8 THz) than the photomixer-based systems and is being applied to the study of hydrogen-bonding dynamics, low frequency biomolecule motions and THz imaging of biological and organic conductor thin films and semiconductor substrates. A similar instrument is available which utilizes ZnTe and large aperture GaAs generators for pulsed imaging applications. High power THz pulses are propagated through or reflected from samples and the resultant image is detected using the EO effect by a large area ZnTe detection crystal. Weak 800 nm probe pulses are passed through the EO detector crystal and imaged onto a 512×512 CCD array to read out the THz image. Acquisition of entire pulsed waveforms throughout the image enables one to spectrally identify materials within the image (hyper-spectral imaging). This approach using corrective collection lenses is being developed for pharmaceutical tablet identification, homeland security and related imaging applications.

Diagram showing pump-probe terahertz apparatus

Diagram showing the pump-probe terahertz apparatus.
(Diagram courtesy of Klaas Wynne)


One high resolution CW THz system combines two near infrared laser beams, one from a fixed-frequency 855 nm CW diode laser and the other from a tunable CW Ti+3:Sapphire laser in a photomixer antenna to produce THz radiation at the difference frequency of the two pump beams. Tuning the frequency of the Ti+3:Sapphire laser varies the THz output frequency. The THz output power is approximately 1 µW, which is 6 orders of magnitude above the noise equivalent power (NEP) of the 1.6 K liquid-He-cooled bolometer detector for a 1 Hz bandwidth. A second CW system uses backward-wave oscillator (BWO) (see picture) stabilized to better than 1 kHz to produce tunable radiation from 0.1 THz to 0.8 THz. The BWO's produce 1 mW to 10 mW of output power, sufficient in some cases for room-temperature detectors. The spectrometers are being applied to the investigation of the torsional force fields of model biomolecules in vacuum and in the condensed phase. Torsional modes are responsible for the conformation flexibility of biomolecules and have fundamental frequencies in the THz spectral region. In addition to the study of biomolecules, the CW systems are being developed for the chemical and temperature characterization of etching plasmas using in semiconductor processing. Photograph of a backward-wave oscillator electronic tube.

Photograph of a backward-wave oscillator electronic tube.

The Optical Technology Division's Terahertz project includes joint research with the Center for Neutron Research (Materials Science and Engineering Laboratory) to utilize complementary tools to study dynamical processes of proteins and DNAs. We are comparing state-of-the-art pulsed and CW THz optical measurements to high-resolution neutron scattering data to explore the microscopic, concerted, nuclear motions associated with molecular conformational changes. Determining the time-dependent variation in torsional motions and biomolecular interactions is crucial for understanding the biological function of enzymes, protein-drug interactions, and DNA helix transitions at a molecular level. We also collaborate with UMBC and others to perform ab initio, Density Functional and molecular mechanics periodic boundary solid-state spectral calculations to aid in the interpretation and understanding of the observed THz spectral measurements.

Resources:

Femtosecond laser systems (YVO4 solid-state pumped 20 fs Ti+3:sapphire oscillator and 45 fs, 2.5 mJ/pulse kilohertz Ti+3:sapphire amplifier with mid-IR and two synchronized OPAs) generating ultrafast pulses in the far-IR through UV; infrared and visible multichannel detector arrays and instrumentation for capturing transient spectra and upconverted images of samples; Linear-scanning CW Ti+3:sapphire laser; 0.3 THz to 0.8 THz BWO's; 0.1 THz to 2 THz CW-laser-pumped GaAs photomixer.

References:

"High Resolution THz Spectroscopy of Crystalline Trialanine: Extreme Sensitivity to β-sheet Structure and Co-crystallized Water,"
K. Siegrist, C.R. Bucher, I. Mandelbaum, A.R. Hight Walker, R. Balu, S.L. Gregurick, and D.F. Plusquellic,
J. Am. Chem. Soc. (submitted).


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


"Continuous-Wave Terahertz Spectroscopy of Biotin. Vibrational Anharmonicity in the Far-Infrared,"
T.M. Korter and D.F. Plusquellic,
Chem. Phys. Lett. 385 45-51 (2004).


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


"Continuous-Wave Terahertz Spectroscopy of Biomolecules and Plasmas,"
D.F. Plusquellic, T.M. Korter, G.T. Fraser, R.J. Lavrich, E.C. Benck, C.R. Bucher, J. Domench, and A.R. Hight Walker,
in Terahertz Sensing Technology. Volume 2: Emerging Scientific Applications and Novel Device Concepts, ed. by D.L. Wollard, W.R. Loerop, and M.S. Shur, 13(4) (World Scientific, 2003), 385-404.


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

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

"Pulsed Terahertz Spectroscopy of DNA, Bovine Serum Albumin and Collagen between 0.1 and 2.0 THz,"
Markelz, A.G., Roitberg, A., and Heilweil, E.J.,
Chem. Phys. Lett. 320, 42 (2000). (Preprint 81 kB Get a PDF viewer)


"Rotation-Tunneling Spectrm of Deuterated Ammonia Dimer States Correlating to the A-Symmetry States of the Monomer,"
Karyakin, E.N., Fraser, G.T., and Saykally, R.J.,
J. Chem. Phys. 110, 2856 (1999).


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


"A Terahertz Photomixing Spectrometer: Applicaiton to SO2 Self Broadening,"
Pine, A.S., Suenram, R.D., Brown, E.R., and McIntosh, K.A.,
J. Mol. Spectrosc. 175, 37 (1996).


"Photomixing up to 3.8 THz in Low-Temperature-Grown GaAs,"
Brown, E.R., McIntosh, K.A., Nichols, K.B., and Dennis, C.L.,
Appl. Phys. Lett. 66, 286 (1994).

For technical information or questions, call:
Edwin J. Heilweil
Phone: (301) 975-2370
Fax: (301) 869-5700
Email: edwin.heilweil@nist.gov 		   Gerald T. Fraser
Phone: (301) 975-3797
Fax: (301) 975-2950
Email: gerald.fraser@nist.gov 		   David F. Plusquellic
Phone: (301) 975-3896
Fax: (301) 975-2950
Email: david.plusquellic@nist.gov


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Online: December 2000   -   Last updated: March 2006