electron lasers in a specific wavelength region may be important to the improvement of free electron lasers in all wavelength regions.

FAR-INFRARED REGION (1000 to 10 µm)

The most compelling case for a free electron laser facility is in the far infrared, the region between 1000 and 10 µm. Because of the lack of suitable nonlinear materials, techniques for generating tunable light using commercial laboratory lasers are not effective at wavelengths longer than 10 µm. It is possible to extend this limit to longer wavelengths with some difficulty using noncommercial instrumentation, but it is unlikely that conventional laboratory lasers will ever be effective at wavelengths longer than 20 µm.

  1. The scientific case for a tunable, short-pulse (picosecond) source in the far infrared is compelling, but at present there are no picosecond far-infrared FEL user facilities in the United States. This is the spectral region where molecule-surface vibrations, intermolecular cluster vibrations, and transitions in semiconductor quantum wells can be excited. It is also the region for probing transitions between adjacent high-lying Rydberg states of atoms and low-frequency motions in large biomolecules. There is sufficient scientific interest to efficiently use a far-infrared user facility capable of producing picosecond pulses, and the committee believes that the scientific opportunities justify the establishment of such a facility. Operation of such a facility would provide information on whether additional user facilities might be needed subsequently. Two relevant issues that should then be considered are the quality of the proposed science that cannot be accommodated by a single facility and the existence of alternative infrared sources such as laboratory-sized far-infrared FELs.

  2. Free electron lasers already exist in the far-infrared region, even as user facilities, and therefore uncertainty about the technical requirements and cost of establishing a far-infrared free electron laser facility is relatively low. The relatively modest accelerator and undulator requirements mean that a user facility might be built for several million dollars using existing technology.

  3. There is some promise that further development will lead to a significantly less expensive free electron laser that could be used by a single academic department or perhaps an individual investigator.

  4. A photocathode electron source may increase the utility of a user facility both because of the free electron laser's improved performance and because the laser used to excite the photocathode could also serve as a second, synchronous photon source for pump-probe experiments.

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