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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities 4 NEAR-INFRARED, VISIBLE, AND ULTRAVIOLET REGION: 10 µm to 200 nm PRESENT CAPABILITIES Laboratory lasers are the dominant sources of tunable light for wavelengths between about 10 µm and 200 nm. Noncommercial instrumentation can extend the limits of commercial laboratory lasers with some difficulty to the range from 20 µm to 100 nm. Essentially all of these systems, except for some of the most ambitious wavelength extension schemes, are commercial devices or have been assembled from commercial components. The transformation of lasers developed initially as research tools to commercial devices has continuously advanced capabilities over the last two decades. Conventional laboratory lasers have produced an enormous body of innovative and useful science because of their utility in the laboratories of individual investigators and their availability in many important wavelength regions. These lasers are typically small enough and inexpensive enough ($10K to $200K) to operate in an individual investigator's laboratory with the users and the manufacturer's service staff providing support. Many experiments in this wavelength region involve using multiple lasers in preparation and interrogation steps and require independent timing of pulses from the different lasers. Although continuous-wave lasers are very important in fields such as high-resolution spectroscopy, laboratory lasers with pulses between a few nanoseconds and tens of femtoseconds are the most suitable devices for comparison to free electron lasers and other coherent sources. Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figures 6 in Chapter 1 compare the peak power, pulse energies, and average power of laboratory laser sources with those of synchrotrons and FELs for different wavelengths. (The plots also show data for terawatt lasers and the high-order harmonics that one can generate using them.) The large impact of conventional laser technology comes not only from its affordability but also from the fundamental importance of the wavelength regions that it reaches. In particular, photons in the infrared and ultraviolet portions of the spectrum interact with numerous different vibrational and electronic transitions in gases, liquids, and solids. The fingerprint region containing fundamental molecular vibrations is in the infrared portion of the spectrum, and excitation of valence electrons in chemical bonds requires light of ultraviolet wavelengths. Excitation of semiconductors using photons with energies larger than or comparable to the band gap requires light with wavelengths ranging from the visible to the infrared. In part because of their convenience but primarily because they are well matched to many atomic, molecular, and solid-state processes, laboratory lasers are certain to play a central role even when sources at longer and shorter wavelengths become available. It
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities is often possible to obtain pulse energies, durations, bandwidths, and repetition rates that are optimum for probing particular processes. Currently, the most common laser system for producing tunable radiation in the range of a few microns to 100 nm is a visible dye or tunable solid-state laser using associated nonlinear techniques to reach shorter and longer wavelengths. The pump source for dye lasers is often a frequency doubled or tripled Nd:YAG laser that is either Q-switched or mode locked. Typically, nonlinear techniques—such as frequency doubling and mixing, stimulated Raman scattering, and four-wave mixing—provide wavelengths beyond the fundamental range (400 nm to 800 nm) of a pulsed dye laser. This is a well-developed, proven commercial technology that is currently undergoing potentially revolutionary advances. Nonlinear crystals are likely to be central to developments in this area, but the limited resources of the small businesses that usually produce them slow their development into optimized optical elements. The development of titanium-doped sapphire lasers into practical devices has changed the production of ultrafast light pulses with durations from a few picoseconds to less than 10 femtoseconds. These solid-state devices, which produce tunable fundamental radiation in the range of 700 nm to 1100 nm, are excellent for mode-locking to create the short pulses that are efficient for producing other wavelengths by nonlinear processes. Other tunable solid-state lasers such as those based on forsterite or alexandrite, although less developed commercially, also provide tunable fundamental radiation, and, in all cases, nonlinear doubling and mixing techniques extend the wavelength range far beyond the fundamental region. The other seminal development that is just beginning to change the landscape for commercial pulsed lasers is the introduction of optical parametric oscillators (OPOs) for nanosecond-duration pulses and optical parametric amplifiers (OPAs) for shorter pulses. These systems use b-barium borate (BBO), potassium trihydrogen phosphate (KTP), lithium niobate (LiNbO3), and new nonlinear crystals such as AgGaSe2. For example, several commercial manufacturers now offer OPOs that span the range from 440 nm to 2 µm with 10-ns, 20- to 100-mJ pulses of light and produce 1- to 10-mJ pulses in the range of 2 to 3.5 µm. Others promise an OPA that can generate 1-ps, tunable infrared pulses at a kilohertz repetition rate at wavelengths between 1 and 5 µm. Nonlinear frequency difference and mixing schemes should routinely provide radiation with wavelengths as short as 200 nm and as long as 5 µm from these devices. Many laboratory devices operate within a factor of a few of the Fourier transform limit of bandwidth determined by the pulse duration. A 100-fs ultrafast laser pulse has a transform-limited bandwidth of 150 cm−1, corresponding to a fractional bandwidth of Δλ/λ = 0.01 at 15,000 cm−1. Sophisticated seeding technologies can produce longer-duration (nanosecond) pulses that are near the transform-limited bandwidth as well, but, even without these measures, bandwidths are routinely less than 0.05 cm−1, corresponding to a fractional bandwidth of Δλ/λ ≤ 3 × 10−6 .The good spatial properties, large pulse energies, and small bandwidths of conventional laboratory lasers make them very bright sources in the regions where they operate.
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities There are currently several free electron lasers operating in the 1- to 9-µm region of the spectrum, such as those at Duke University, Stanford University, Vanderbilt University, and Los Alamos National Laboratory. (Chapter 2 lists the FELs in the United States and abroad.) Typically, FELs in the infrared wavelength region generate trains of 1-ps micropulses in 0.5- to 6-µs macropulses and produce 100 mJ of energy in a macropulse. Scientists at these facilities and visitors have carried out experiments in tissue ablation and laser surgery and have studied vibrational photon echoes, and they plan studies of the biophysics of large molecules such as proteins. The surgical studies, for example, have explored basic and applied aspects of the interaction of light with tissue to discover the optimum wavelength and pulse width for clinical applications, which are likely to be implemented eventually with conventional lasers. The research advantage of the FEL in this context is its wide tunability and its flexible pulse structures, with the possibility of using chirped pulses being a capability that is unavailable with conventional lasers in this wavelength region. Cost is one important factor in assessing free electron lasers and other coherent light sources, particularly if one is considering establishing a center for shared use of a large device. Because conventional lasers are so ubiquitous and productive, the spending on research lasers in the United States is one point of comparison. An annual survey of worldwide laser sales (Laser Focus) found that in 1993 (the last year for which figures are available) the worldwide market for lasers used in research and development was $110M. The committee learned from two large scientific laser companies (Spectra Physics and Coherent) that the average fraction of their sales of lasers for scientific research in the United States is 34%, and, thus, it estimates that the annual expenditure on laboratory research lasers in this country is about $37M. The distribution of this amount among types of laboratories is roughly 44% for universities, 35% for industrial laboratories, and 21% for government laboratories. These laboratory lasers cost between $5K and $250K, with $100K being the cost of a typical system. FINDINGS Laboratory lasers have been and will continue to be an important photon source for research in the region from 10 µm to 200 nm. Much of the science in this wavelength region involves simultaneous use of several sophisticated laboratory lasers. When cost and convenience are considered, it is unlikely that free electron lasers will be competitive with laboratory lasers in this spectral region in the foreseeable future. RECOMMENDATIONS In the near-infrared, visible, and ultraviolet regions, continued support of lasers in individual investigators' laboratories is the highest priority.
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities The development of nonlinear materials for extending the wavelength capabilities of laboratory lasers deserves support in both universities and industry (e.g., through the Small Business Innovation Research program). Support for this activity should receive a high priority, as even a modest investment is likely to have a considerable impact on scientific research. In the near-infrared, visible, and ultraviolet regions, support for additional free electron lasers should receive relatively low priority with respect to support of conventional lasers and with respect to support for free electron lasers in other wavelength regions.
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