Spectroscopy is the measurement of the response of a material as a function of frequency. In its most basic form, it is how we see the world around us. Light from the sun, or a lightbulb, reflects from objects into our eyes. Different wavelengths of light are reflected differently by different objects, and our eyes process that reflected light to tell us the color and texture of the objects we look at. The measurement of this frequency dependence of the reflectivity, performed by our eyes and our brain, is an example of spectroscopy.
Visible light is a small fraction of the electromagnetic spectrum, and its photons have energies in the range of 2 eV. This energy range works fine to give us some information about the world, but it is not perfect. For example, polished aluminum and polished silicon both have a mirror-like finish when examined with visible light. However, aluminum is a metal and silicon is an insulator! The electronic properties of the two materials are completely different, but visible light doesn’t allow us to distinguish between the two. By using "light" (electromagnetic radiation) of other wavelengths, we can learn more about these and other materials.
In terahertz spectroscopy, we use photons with energies of about 1 meV (one one-thousandth of the energy of a visible light photon). This energy scale is important for learning about many important aspects of condensed matter physics. In particular, terahertz spectroscopy is useful for studying collective effects in materials (superconductors, charge density waves, heavy fermion materials, etc.).
Collective effects are at the forefront of condensed matter physics, because they are so difficult to understand. In the past 50 years, physicists have become very good at understanding electrons in solids in a mean field model. This means that each electron is assumed to be interacting with an average of the positions of all the other electrons. When the mean field model breaks down, it becomes necessary to consider pairs of interactions between each electron with all the other electrons (~1 mole of electrons). This is clearly an intractable problem, so it is necessary to make certain simplifying assumptions and then to compare these results to experimental measurements.
This is where terahertz spectroscopy becomes important. The energy scale for most collective effects is on the order of 1 meV, which historically has been very difficult to access. However, since the development of terahertz spectroscopy, this energy scale has become accessible, albeit by a rather complex and difficult laboratory technique. This technique has been implemented at Colgate University.
Below is the time-dependence of a THz pulse, along with its frequency spectrum, found from the Fourier transform. The frequency ranges from about 100 GHz to 1 THz, a frequency range that is above that of microwave techniques and below that of far infrared techniques.
The experiment starts with a femtosecond laser, a source that produces pulses of light that are about 100 femtoseconds (fs) long. This is 10-13 seconds! It's hard to comprehend just how short this time is. Here's one comparison that may help. A light pulse can travel to the moon in just over 1 second. In 100fs, light travels 0.03mm (less than the diameter of a human hair!).
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The femtosecond laser pulses then hit a "terahertz antenna," which generated the terahertz pulse. |
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This is the THz spectrometer. The antenna is in the brass holder located in the lower left of the picture. The large parabolic mirrors shown are used to focus the terahertz radiation to the sample and back to a receiver antenna. |
Many students have worked on this project: Ian Hoffman '98 constructed an autocorrelator to measure the length of the pulses, Carol Finn '99 and Lara Northrop '99 worked on the optical setup. Dennis Bauer '00 and Joe Loomis '01 worked last summer on taking the first terahertz signals along with setting up a cryostat for measurements at low temperatures. Nick Gould '98 and Eliza Michiels '02 have worked on the data acquisition system. Joe Loomis '01 measured transition energies and linewidths in Mn12 acetate for his independent research project in the Spring of 2001. Lea Vacca '02 checked that linewidth in loose nanocrystals pressed into a pellet, and Varun Sondhi '05 found that it was possible to orient crystals in a magnetic field.
Recent publications:
“Effect of mechanical stress on the linewidth of single photon absorptions in Mn12-acetate” Beth Parks, Lea Vacca*, Evan Rumberger, David N. Hendrickson, George Christou (in preparation for submission to Physica B).
“Inhomogeneous broadening of single photon transitions in molecular magnets,” Beth Parks, Joseph Loomis*, Evan Rumberger, En-Che Yang, David N. Hendrickson, George Christou, Journal of Applied Physics, 91, 7170 (2002).
“Linewidth of single-photon transitions in Mn12-acetate,” Beth Parks, Joseph Loomis*, Evan Rumberger, David N. Hendrickson, George Christou, Physical Review B, 64, 184426 (2001).
Recent presentations:
2000 Meeting of the American Physical Society
2001 Meeting of the American Physical Society
2002 Meeting of the American Physical Society
2003 Meeting of the American Physical Society
Low Temperature Physics Conference in Hiroshima, Japan, August 2002.
Conference on Magnetism and Magnetic Materials, Seattle, WA, Nov. 2001.
The laboratory was constructed using funds from Colgate University and the National Science Foundation.
This work is funded by an award from Research Corporation (2000-present).
[The background picture is a picture of new catalyst pads used for measuring the conducting properties of nanotubes. Photo taken by Chris Hall '03]