Ultrafast Lasers in Materials Research
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Ultrafast Lasers in Materials Research David G. Cahill and Steve M. Yalisove, Guest Editors Abstract With the availability of off-the-shelf commercial ultrafast lasers, a small revolution in materials research is underway, as it is now possible to use these tools without being an expert in the development of the tools themselves. Lasers with short-duration optical pulses—in the sub-picosecond (less than one-trillionth of a second) range—are finding a variety of applications, from basic research on fast processes in materials to new methods for microfabrication by direct writing. A huge range of pulse energies are being used in these applications, from less than 1 nJ (a billionth of a joule) to many joules. Keywords: laser, ablation.
Introduction Ultrafast lasers—that is, lasers that produce optical pulses with a duration of less than a picosecond—are playing an increasingly important role in many science and technology disciplines. Ultrafast, time-resolved measurements are well established in physical chemistry, where fundamental time scales of chemical reactions become accessible,1 and in solid-state physics and electrical engineering, where carrier dynamics and transport are probed on picosecond time scales directly relevant to the operation of modern highspeed devices. Applications in materials research have been slower to emerge, because only within the past decade have commercial instruments reached a level of reliability and sophistication that make them practical tools for scientists and engineers who may not be experts in laser technology and the manipulation of short-duration, high-intensity optical pulses. The rapid development of ultrafast laser technology is a principal motivation for this issue of MRS Bulletin: the time is ripe for ultrafast lasers to take on a rapidly expanding role in studies of the science of materials and in materials characterization, materials modification, and microfabrication. Ten years ago, very little materials research was conducted using high-intensity, ultrafast lasers. With the commercial availability of off-the-shelf ultrafast tools, a small revolution in materials research is underway.
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Ultrafast optical pulses of ⬍1 ps duration are generated by mode-locked laser oscillators. The phases of the longitudinal optical modes of the laser cavity are locked together by either an active element (e.g., an acousto-optic modulator) or by passive effects such as Kerr-lensing in the gain medium or the use of a saturable absorber. Mode-locking produces shortduration optical pulses with a high repetition rate determined by the length of the optical cavity. The wide bandwidth of optical gain in sapphire doped by Ti enables extremely short-duration pulses. Ti:sapphire lasers dominate the market, but Ti:sapphire laser oscillators are, of course, actually a set of three lasers. Continuous-wave (cw) diode lasers pump a cw solid-state laser, which is frequency-doubled and used to pump the Ti:sapphire oscillator. Ultrafast laser oscillators and amplifiers that can be directly pumped by
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