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FEL Research Highlights

Pulsed Laser Deposition – Magnetic thin films

Well-behaved magnetic thin films of stoichiometric alloys, such as an alloy of nickel and iron (NiFe), are not easily formed. Anne Reilly and colleagues at Jefferson Lab and The College of William & Mary excited bulk NiFe with the Jefferson Lab FEL and found a strikingly different response than that found with a conventional titanium-sapphire laser.

The Jefferson Lab FEL caused a plume to be emitted when it struck the bulk material, similar to what happened with the conventional laser. However, the spectrum of this plume indicated that the atoms were in a thermally, but not electronically, excited state, as judged by the broadband thermal emission spectrum.

The thin films formed when these plumes condensed onto substrates also showed striking differences, as shown in the figure. Furthermore, the measured hysteresis of the Jefferson Lab FEL produced smoother films, matching that of the native material, whereas the films produced conventionally were dramatically different and very lossy. These experiments demonstrate the high potential for a laser with a high repetition rate to produce materials in novel forms for technological developments in energy, data storage, and in testing dynamical theories of material behavior far from equilibrium and under extreme conditions.

A. Reilly et al. J. Appl. Phys. 95 3098 (2003)

Pulsed Laser Deposition

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Photodynamic therapy

Graph 1

Fig. 1. Fat and water have nice colors in the infrared.

The Jefferson Lab Free-Electron Laser was used by Rox Anderson’s group from the Wellman Center at Massachussetts General Hospital, part of Harvard Medical School, for a series of experiments aimed at curing acne, a debilitating disease of over-active sebaceous glands. Currently the drug Accutane® (generically called isotretinion) is used to treat acne, but its side effects can be worse than the disease.

The principle behind the experiments is to tune the Jefferson Lab FEL light to a "color" or wavelength, which the sebaceous glands will absorb much more strongly than the surrounding tissue. As the fat cells heat up, they die, while the rest of the tissue is unharmed.

Graph 2

Fig. 2. FEL-induced photothermal excitation of porcine fat and porcine skin, normalized to FEL exposure energy (see text). Preferential heating of fat is maximum at the lipid absorption band maxima near 1,210 nm (A) and 1,720 nm (B). The data are consistent with water as the dominant absorber in skin and lipid as the dominant absorber in fat.

The initial experiments were successful but required carefully controlled conditions, with the skin held under a cold window, and with careful selection of the wavelength.

R. Rox Anderson, William Farinelli, Hans Laubach, Dieter Manstein, Anna N. Yaroslavsky, Joseph Gubeli III, Kevin Jordan, George R. Neil, Michelle Shinn, Walter Chandler, Gwyn P. Williams, Steven V. Benson, David R. Douglas, H.F. Dylla, "Selective photothermolysis of lipid-rich tissues: A free electron laser study", Lasers in Surgery and Medicine 38 913 (2006).

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Dynamics of Impurities in Semiconductors

Silicon is perhaps one of the most important materials in our technological world, but its performance is always ultimately limited by impurities. Mitigation by impurity elimination is not possible, and theoretical understanding is very limited. Thus, these experiments, in which real-time dynamical evolution of excited impurity dynamics is measured, are of high fundamental as well as technological importance.

The principle behind the experiments is to use two photons, both tuned to a "color" or wavelength, at which the impurities absorb energy. The first photon is absorbed by the impurity, and the second one interrogates the impurity to study how the energy dissipates in the time following the first photon– like hitting a piano key and listening to the consequences in time.

The experiments were led by Gunter Luepke of The College of William & Mary. The group measured many impurity modes of hydrogen in silicon and were also able to compare their behavior with that of deuterium (an isotope of hydrogen), which behaves quite differently even though it is electronically similar.

Luepke et al.
Phys. Rev. Letts 85, 1452 2000
Phys. Rev. Letts 88, 135501, 2002
Phys. Rev. Letts 87, 145501, 2001
Phys. Rev. B63 195203 2001
J. Appl. Phys. 93 2316, 2003

Dynamics of Impurities in Semiconductors

Red dots indicate hydrogen atoms in the silicon lattice, shown as blue dots. The time response of the excited HBC case is depicted in the graph.

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