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Dr. Markus Walther
Molecular and Optical Physics
Physikalisches Institut
Albert-Ludwigs-Universität
Hermann-Herder-Str. 3
79104 Freiburg
Germany
Phone: (+49) 203 7606
Fax: (+49) 203 5955
email: walther@physik.uni-freiburg.de
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Research
interest:
Terahertz time-domain
spectroscopy (THz-TDS) is a useful extension of traditional
far-infrared spectroscopic methods. It allows the direct determination
of the optical properties, i.e. the complex dielectric function, of a
variety of
samples in the submillimeter wavelength range. The technique
relies on the coherent generation and detection of ultra-short
electromagnetic transients using femtosecond laser pulses. Due to their
duration of less than a picosecond THz pulses exhibit a large frequency
bandwidth extending from a few GHz up to some THz (1 THz = 33 cm-1
= 4.1 meV), thereby allowing broadband and time-resolved spectroscopy
in the far-IR.
We study the characteristic THz spectra of individual biomolecular
systems in the crystalline and amorphous phase. In particular the
correlation between the observed low-frequency modes and molecular
structure and association is investigated. Our results show that the
modes in the THz region represent a unique fingerprints for the
molecular structure and conformation, allowing to distinguish related
compounds [Biospectroscopy, 67 (4-5), 310-313 (2002)]. It has
been demonstrated, that the method can be used to even discriminate
between different
Isomeric conformations of a molecule, as in the case of retinal [Chem.
Phys. Lett., 332, 389-395 (2000)]. Potential applications in
chemical
recognition and drug screening are feasible.
THz-TDS has also been applied to study non-covalent intermolecular
forces. In molecular crystals temperature dependent frequency shifts of
associated intermolecular vibrational modes reflect the delicate
interplay between the different weak non-covalent forces, that
stabilize the crystal lattice [Chem. Phys., 288, 261-268 (2003)].
The technology:
We use femtosecond laser
pulses to discharge a biased metall-semiconductor-metall structure. The
stored capacitative energy is initially converted into pulsed
far-infrared radiation propagating into free space.
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The figure shows our homebuild FreiTEC
("Freiburg THz Emitter Chip"), that we use for THz pulse
generation.
It consists of five
emitter structures, that can be used individually.
(Click image for a detailed view)
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The THz pulse is
focussed onto a similar detector chip, that measures the temporal
shape of the electric field of the incident THz pulse.
Such an emitter together with a corresponding detector and the
appropriate optics, makes a nice THz time-domain spectrometer as we
have it in our lab.
How spectroscopy works is demonstrated in the next figures:
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The
left figure shows the measured THz pulse shape as measured online on the scope.
When a sample is introduced into the THz beam path, the transmitted
signal gets attenuated due to absorption and phase-shifted due to
dispersion.
In the right figure we hold a piece of plastics into the THz-beam
(click image for movie sequence). Click left image to see how the
THz pulse shifts and changes shape due to absorption and dispersion
of the plastic piece.
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Finally you can
take a tour around our laser table (...but don't touch the
optics!) 
Selected
results:
Large organic molecules,
such as e.g. chromophores, sugars and biomolecules, show low-frequency
vibrational modes with frequencies in the THz range. The central
frequencies and intensities are characteristic for the molecular
structure, conformation and association (e.g. in a crystal lattice).
THz time-domain spectroscopy allows the direct determination of the
characteristic mode structure of individual molecular systems.
As an example the two figures below show the molecular structure of the
chromophores retinal and isoalloxazine. An animation of the lowest
modes can be viewed by clicking the movie icons.
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The
normal modes have been calculated with GAUSSIAN 98, and animated by
using gOpenMol-2.30.
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Low-frequency modes of all-trans
retinal:
ring
deformation at 52 cm-1
and chain torsion at 82 cm-1
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"Butterfly"-mode of isoalloxazine
at around 50 cm-1
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Due to the sensitivity to
molecular structure and conformation, the measurement of the THz
spectra allows an unambigous identification of different chemicals or a
determination of the molecular configuration, as for example in the
case of different isomers.
Potential applictaions are chemical sensing or drug recognition.
Okt. 92
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Studies
in Physics und Mathematics at the Albert-Ludwigs Universität,
Freiburg (Germany)
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1994-95
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ERASMUS-scholarship
at the University of Glasgow (GB)
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1999
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Staatsexamen
in physics and mathematics (equivalent to a M.S., for teaching at high
schools) at Freiburg University
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Sept.
1999
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Researchproject
in the group of
Prof. S. R. Keiding, Dep.
of Physics and Chemistry, University of Arhus (Denmark)
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1999
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Diploma
degree in Physics, University Freiburg
Thesis title: “Resonant excitation of biomolecules and solids in the
Terahertz Frequency range“
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2000-2003
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start
of PhD project at the University of Freiburg (Germany)
with Dr. P. Uhd Jepsen in the group of Prof. Dr. H. Helm
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Jul.-Dez.
2002
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Researchproject
at the University of Toronto (Canada)
in the group of Prof. R. J. Dwayne Miller, Dep. of Chemistry and Physics
on ”Transient phase-grating spectroscopy on myoglobin”
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Dez.
2003
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PhD
thesis:
“Modern Spectroscopy on Biological Molecules – Structure and
Bonding investigated by THz time-domain and transient phase-grating
spectroscopy“
(full text (PDF))
(15 Mb)
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Jan.
2004
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PhD in
physics, Albert-Ludwig University Freiburg / Germany
Awarded the Pfizer Research Award 2004 |
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2004-2006
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Postdoctoral Fellow
at the University of Alberta, Edmonton, Canada
in the Ultrafast Spectroscopy Group of Prof. Hegmann
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from
June 2006 |
Position as "Akademischer Rat" at the University of Freiburg / Germany
leading the THz-Group |
Pfizer
Research Award 2004 (Pfizer Forschungspreis)
at the Albert-Ludwigs Universitat Freiburg, Faculty for Mathematics
and Physics
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PhD Thesis
"Modern Spectroscopy on
Biological Molecules – Structure and Bonding investigated by THz
time-domain and transient phase-grating spectroscopy“
(November
2003) (PDF-version) (15 Mb)
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Feature Article (in German)
"Chemische Analytik und Bildgebung mit gepulster Terahertz-Strahlung“
on www.analytik-news.de
(June
2009) (PDF-version)
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Journal papers:
- V. A. Fedotov, N. Papasimakis, E. PLum, A. Bitzer, M. Walther, P. Kuo, D. P. Tsai, and N. I. Zheludev
"Spectral collapse in ensembles of metamolecules"
Phys. Rev. Lett. 104, 223901 (2010)
(abstract, pdf)
- D. A. Turton, J. Hunger, A. Stoppa, A. Thoman M. Candelaresi, G. Hefter, M. Walther, R. Buchner, and K. Wynne
"Rattling the cage: Micro-to mesoscopic structure in liquids as simple as argon and as complicated as water"
J. Mol. Liq., in press - (2010)
(abstract, pdf)
- L. Öhrström, A. Bitzer, M. Walther, and F. J. Rühli
"Terahertz imaging of ancient mummies and bone"
American Journal of Physical Anthropology 142 (3), 497-500 (2010)
(abstract, pdf)
- A. Bitzer, A. Ortner, and M. Walther
"Terahertz near-field microscopy with subwavelength spatial resolution based on photoconductive antennas"
Applied Optics 49 (19), E1-E6 (2010)
(abstract, pdf)
- M. Walther, B. M. Fischer, A. Ortner, A. Bitzer, A. Thoman, and H. Helm
"Chemical sensing and imaging with pulsed terahertz radiation"
Anal. Bioanal. Chem. 397 (3), 1009-1017 (2010)
(abstract, pdf)
- M. Walther, A. Ortner, H. Meier, U. Löffelmann, P. J. Smith, and J. G. Korvink
"Terahertz metamaterials fabricated by inkjet printing"
Appl. Phys. Lett. 95, 251107 (2009)
(abstract, pdf)
- A. Bitzer, J. Wallauer, H. Helm, H. Merbold, T. Feurer, and M. Walther
"Lattice modes mediate radiative coupling in metamaterial arrays"
Opt. Express, 17 (24), 22108-22113 (2009)
(cover movie, abstract, pdf)
- D. A. Turton, J. Hunger, A. Stoppa, G. Hefter, A. Thoman, M. Walther, R. Buchner, and K. Wynne
"Dynamics of Imidazolium Ionic Liquids from a Combined Dielectric Relaxation and Optical Kerr Effect Study: Evidence for Mesoscopic Aggregation"
J. Am. Chem. Soc. 131, 11140-11146 (2009)
(abstract, pdf)
- A. Knübel, R. Aidam, V. Cimalla, L. Kirste, M. Baeumler, C.-C. Leancu, V. Lebedev, J. Wallauer, M. Walther, and J. Wagner
"Transport characteristics of Indium Nitride (InN) films grown by plasma assisted molecular beam epitaxy (PAMBE)"
phys. stat. sol. (c), 6 (6), 1480-1483 (2009)
(cover, abstract, pdf)
- A. Bitzer, H. Merbold, A. Thoman, T. Feurer, H. Helm, and M. Walther
"Terahertz near-field imaging of electric and magnetic resonances of a planar metamaterial"
Opt. Express, 17 (5), 3826-3834 (2009)
(abstract, pdf)
- J. Hunger, A. Stoppa, A. Thoman, M. Walther, and R. Buchner
"Broadband Dielectric Response of Dichloromethane"
Chem. Phys. Lett. 471, 85-91 (2009)
(abstract, pdf)
- A. Bitzer and M. Walther
"Terahertz near-field imaging of metallic subwavelength holes and hole arrays"
Appl. Phys. Lett. 92, 021107 (2008)
(abstract, pdf)
- A. Thoman, A. Kern, H. Helm, and M. Walther
"Nanostructured gold films as broadband terahertz antireflection coatings"
Phys. Rev. B 77 (19), 195405 (2008)
(abstract, pdf)
- A. Kern and M. Walther
"Subgridding in the FDTD method for simulating the interaction of terahertz radiation with metal"
J. Opt. Soc. Am. B 25 (3), 279-285 (2008)
(abstract, pdf)
- A. Bitzer, H. Helm and M. Walther
"Beam-Profiling and Wavefront-Sensing of THz Pulses at the Focus of a Substrate-Lens"
IEEE J. Select. Top. Quant. Electron. 14 (2), 476-481 (2008)
(abstract, pdf)
- M. Franz, B. M. Fischer, and M. Walther
"The Christiansen effect in terahertz time-domain spectra of coarse-grained powders"
Appl. Phys. Lett. 92 (2), 021107 (2008)
(abstract,
pdf)
- M. Walther, D. G. Cooke, C. Sherstan, M. Hajar, M. R. Freeman, and F. A. Hegmann
"Terahertz conductivity of thin gold films at the metal-insulator percolation transition"
Phys. Rev. B 76 (12) 125408 (2007)
(abstract,
pdf)
- A. Bitzer, M. Walther, A. Kern, S. Gorenflo, and H. Helm
"Examination of the spatial and temporal field distributions of single-cycle terahertz pulses at a beam focus"
Appl. Phys. Lett. 90 (7), 071112 (2007)
(abstract,
pdf)
- M. Walther, M. R. Freeman, and F. A. Hegmann
"Metal-wire terahertz
time-domain spectroscopy"
Appl. Phys. Lett. 87 (26), 261107 (2005)
(abstract,
pdf)
- M. Walther, G. S.
Chambers,
Z. Liu, M. R. Freeman, and F. A. Hegmann
"Emission and detection
of terahertz pulses from a metal tip antenna"
J. Opt. Soc. Am. B 22 (11), 2357-2365 (2005)
(abstract, pdf)
- M. Walther, V. Raicu,
J. P. Ogilvie, R. Phillips, R. Kluger, and R. J. D. Miller
"Determination
of Fe-CO bond energy in myoglobin using heterodyne detected transient
thermal phase grating spectroscopy"
J. Phys.
Chem. B 109, 20605-20611 (2005)
(abstract, pdf)
- M. Walther, B. M.
Fischer,
and P. Uhd Jepsen
"Noncovalent intermolecular forces in
polycrystalline
and amorphous saccharides in the far infrared"
Chemical Physics
288 261-268 (2003)
(abstract, pdf)
- B. M. Fischer, M.
Walther,
and P. Uhd Jepsen
"Far-infrared vibrational modes of DNA
components
studied by terahertz time-domain spectroscopy"
Phys. Med. Biol.
47 3807 (2002)
(abstract, pdf)
- M. Walther, P.
Plochocka,
B. Fischer, H. Helm, and P. Uhd Jepsen
"Collective Vibrational
Modes
in Biological Molecules investigated by THz Time-Domain Spectroscopy"
Biopolymers (Biospectroscopy), 67 (4-5), 310-313 (2002)
(abstract, pdf)
- M. Schall, M. Walther,
and P. Uhd Jepsen
"Fundamental and second-order phonon processes
in CdTe and ZnTe"
Phys. Rev. B 64
(9) 094301 (2001)
(abstract,
pdf)
- M. Walther, B.
Fischer,
M. Schall, H. Helm, and P. Uhd Jepsen
"Far-infrared vibrational
spectra of all-trans, 9-cis and 13-cis retinal measured by THz
time-domain
spectroscopy"
Chem. Phys. Lett., 332, 389-395 (2000)
(abstract, pdf)
- M. Walther, K. Jensby,
S. R. Keiding, H. Takahashi, and H. Ito
"Far-infrared properties
of DAST"
Opt. Lett., 25 (12),
911 (2000)
(abstract, pdf)
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C. Winnewisser, F.
Lewen,
M. Schall, M. Walther, and H. Helm
"Characterization and
application
of dichroic filters in the 0.1 to 3 THz region"
IEEE Trans.
Microwave
Theo. Tech., 48 (4), 744 (2000)
(abstract,
pdf)
- Markus Walther, "Terahertz near-field imaging of metamaterials", CIMTEC - 12th Int. Ceramics Congress & 5th Forum on new Materials, 13.-18. June 2010, Montecatini Terme (Italy)
- A. Bitzer, J. Wallauer, H. Helm, H. Merbold, T. Feurer, and M. Walther, "Lattice modes mediate radiative coupling in metamaterial arrays", SPIE Photonics Europe, 12.-16. April 2010, Brussels (Belgium)
- M. Walther, A. Bitzer, A. Ortner, S. Waselikowski, "Terahertz metamaterials under a near-field microscope", SPIE Photonics Europe, 12.-16. April 2010, Brussels (Belgium)
- Markus Walther, "Terahertz metamaterials investigated in the far- and near-field", Annual Meeting of the German THz-Center (DTZ), 6./7. May. 2010, Konstanz (Germany)
- Markus Walther, "Terahertz near-field imaging", THz-Symposium by the Danish Academy of the Technical Sciences, 5. Oct. 2009, DTU Kopenhagen (Denmark)
- Markus Walther and Andreas Bitzer, "Terahertz near-field imaging of electric and magnetic resonances in metamaterials", SPIE Conference on Optics + Photonics, 4.-6. August 2009, San Diego(USA), (abstract, pdf)
- Markus Walther, "Sensing and imaging with pulsed terahertz radiation", 6. Deutsches BioSensor Symposium, University of Freiburg, 29. March - 01. April 2009, Freiburg (Germany)
- Markus Walther and Andreas Bitzer, "Imaging the electromagnetic near-fields in terahertz metamaterials", ORC Seminar Series, Optoelectronic Research Centre, University of Southampton, 23. January 2009, Southampton (UK)
- Andreas Bitzer and Markus Walther, "Imaging the electric and magnetic near-fields in terahertz metamaterials", 2nd European Topical Meeting on Nanophotonics and Metamarials (NanoMeta), 5.-8. January 2009, Seefeld (Austria)
- Markus Walther, “Terahertz near-field microscopy of metallic microstructures”, Physikalisch Technische Bundesanstalt (PTB) Braunschweig, 02. December 2008
- Markus Walther, “Terahertz near-field imaging of metallic microstructures”, GDR-E 2008 THz Workshop: Semiconductor sources and detectors of THz radiation, 25.-26. September 2008, University of Paris7, Paris (France)
- F. A. Hegmann, D. G. Cooke, and M. Walther, "The Nature of Terahertz Conductivity in Nanomaterials", Optical Terahertz Science and Technology (OTST), March 18, 2007, Orlando, USA, (abstract, pdf)
- Markus Walther, "Collective motion
in biomolecules studied by THz time-domain spectroscopy" , Workshop
on Infrared and THz Radiation 2001, Forschungszentrum Rossendorf,
Dresden
- M. Schall, M. Walther, C. Winnewisser, H. Helm, and P. Uhd Jepsen,"Subpicosecond time-resolved terahertz time-domain spectroscopy of transient carrier dynamics in semiconductors", SPIE/EOD Int. Symposium on Terahertz Radiation, June 15-18, 1999, Munich, Germany, (abstract, pdf)
Selected conference contributions
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- H. Merbold, A. Bitzer, J. Wallauer, H. Helm, M. Walther, and T. Feurer, "Adjusting the Functionality of THz Metamaterials", 2nd Int. Workshop on Terahertz Technology, Osaka, Japan, 2009, Poster 1P-29
- A. Bitzer and M. Walther, "Imaging electric and magnetic near-fields in terahertz metamaterials", CLEO/Europe, Munich, Germany, 14-19 Jun 2009
- V. A. Fedotov, N. Papasimakis, A. Bitzer, M. Walther, and N. I. Zheludev, "Metamaterial analogue of the Mössbauer effect", CLEO/Europe-EQEC, Munich, Germany, 14-19 Jun 2009
- A. Thoman, A. Stoppa, J. Hunger, R. Buchner, H. Helm, and M. Walther, "THz time-domain spectroscopy of room-temperature ionic liquids", Optical Terahertz Science and Technology Topical Meeting (OTST) of the OSA 2009, Santa Barbara, USA
- A. Bitzer and M. Walther, "Imaging the electric and magnetic near-fields in terahertz metamaterials", Optical Terahertz Science and Technology Topical Meeting (OTST) of the OSA 2009, Santa Barbara, USA
- M. Walther and A. Bitzer, "Time- and frequency-resolved terahertz nearfield imaging of electric fields in metamaterials", European Optical Society (EOS) Annual Meeting 2008, Paris, France
- M. Walther, D. G. Cooke, C. Sherstan, A. Thoman, and F. A. Hegmann, "Terahertz-conductivity of nano-structured gold films", Int. Bunsen Discussion Meeting - Exploring THz Spectroscopy 2007, Bad Honnef, Germany
- M. Walther, A. Thoman, C. Sherstan, D. G. Cooke, and F. A. Hegmann, "Terahertz-conductivity of nano-structured gold films", Optical Terahertz Science and Technology Topical Meeting (OTST) of the OSA 2007, Orlando, USA (abstract, pdf)
- M. Walther, G. Chambers, Z. Liu,
M. R. Freeman, R. R. Tykwinski, J.-B. Green, R. A. Wolkow, and F.A. Hegmann
"Towards ultra-fast Nano-Spectroscopy with THz pulses", NanoForum CANADA
2005, Montréal, Québec, Canada
- D. Turchinovich, M. Walther, P. Uhd Jepsen, M. Koch, S. Lahmann, U. Rossow, and A. Hangleiter, "Terahertz pulse emission from strained GaN/GaInN quantum wells", THz 2002, Sept. 9-10, 2002, Cambridge, UK, (abstract, pdf)
- M. Walther, B.
Fischer, H. Helm, and P. Uhd Jepsen, "THz time-domain spectroscopy
as a diagnostic tool: Fundamental aspects and applications", BISAT 2001,
Leeds, UK
- M. Walther, P.
Plochocka, B.
Fischer, H. Helm, and P. Uhd Jepsen, "Investigation of
Collective Vibrational Modes in Biological Molecules by THz Time-Domain
Spectroscopy" , 9th European Conference on the Spectroscopy
of Biological Molecules 2001, Praha, Czech Rep.
- M. Walther, B.
Fischer, M.
Schall, F. Rutz, H. Helm, and P. Uhd Jepsen, "Collective motion
in biomolecules studied by THz time-domain spectroscopy" , Workshop
on Infrared and THz Radiation 2001, Forschungszentrum Rossendorf,
Dresden
- M. Walther, B.
Fischer, M.
Schall, H. Helm, P. Uhd Jepsen, "THz time-domain spectroscopy of
biomolecules", Int. THz Workshop 2000, Sandbjerg Castle, Denmark
THz Group Freiburg
THz Group UofA
Last edited: 19.07.2010
