2.1 Ultrafast solid-state lasers - ETH - the Keller Group
2.1 Ultrafast solid-state lasers - ETH - the Keller Group
2.1 Ultrafast solid-state lasers - ETH - the Keller Group
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130 <strong>2.1</strong>.9 Conclusion and outlook [Ref. p. 134<br />
regime where high-field laser physics such as high-harmonic generation [88Fer, 94Lew, 98Sal] and<br />
laser-plasma-generated X-rays [91Mur] are possible at more than 10 MHz pulse repetition rate.<br />
This improves signal-to-noise ratios in measurements by 4 orders of magnitude compared to <strong>the</strong><br />
standard sources at kHz repetition rates. This would be important for low-power applications<br />
such as X-ray imaging and microscopy [02Sch1], femtosecond EUV and soft-X-ray photoelectron<br />
spectroscopy [01Bau] and ultrafast X-ray diffraction [99Sid, 01Rou].<br />
– Very high pulse repetition rates with 100 GHz should be possible from passively mode-locked<br />
diode-pumped <strong>solid</strong>-<strong>state</strong> <strong>lasers</strong> at different wavelengths, even in <strong>the</strong> wavelength range around<br />
1.5 μm and 1.3 μm for telecom applications.<br />
– As an alternative to ion-doped gain media at high pulse-repetition rates (i.e. > 1GHz)optically<br />
pumped VECSELs and even hopefully electrically pumped VECSELs will become very<br />
interesting alternatives [06Kel]. The integration of <strong>the</strong> SESAM into <strong>the</strong> VECSEL structure will<br />
provide even more compact ultrafast <strong>lasers</strong> [07Maa].<br />
– So far octave-spanning frequency combs have been mostly generated with KLM Ti:sapphire<br />
<strong>lasers</strong> and supercontinuum generation in a microstructured fiber. However, many applications<br />
need more compact sources. The progress in femtosecond diode-pumped <strong>solid</strong>-<strong>state</strong> <strong>lasers</strong> and<br />
VECSELs reported here will make this possible in <strong>the</strong> near future.<br />
Thus, all <strong>the</strong>se examples show that <strong>the</strong> development of ultrafast diode-pumped sources has not<br />
come to its end but will continue to deliver superior performances for many established and new<br />
applications.<br />
In addition, research to produce pulses of even shorter duration is underway. Currently, <strong>the</strong><br />
most promising path to attosecond pulse generation and attosecond spectroscopy is high-harmonic<br />
generation (recent reviews are given with [98Sal, 95DiM, 04Ago]). High-order-Harmonic Generation<br />
(HHG), being an up-conversion process, provides a laser-like source of temporally and spatially<br />
coherent radiation consisting of odd multiples of <strong>the</strong> laser driving frequency down to <strong>the</strong> XUV region<br />
of <strong>the</strong> spectrum. Since its discovery in 1987 in Chicago and Saclay [88Fer, 87McP], it has been<br />
speculated early on that pulses from existing sources of high-harmonic generation exhibit attosecond<br />
time signature [95Cor, 96Ant]. Meanwhile much progress has been made and by controlling its<br />
properties, Attosecond Pulse Trains (APT) [01Pau2] and isolated attosecond pulses [02Dre] have<br />
been successfully produced and applied in first proof-of-principle experiments. The higher orders<br />
are produced simultaneously and fully phase-coherent with <strong>the</strong> driving IR field, which makes this<br />
source ideally suited for two-color or even multi-color pump probe experiments. Fur<strong>the</strong>rmore, it is<br />
important to note, that much of this progress is directly related to advancements in laser science.<br />
Isolated attosecond pulse generation depends on <strong>the</strong> maximum amplitude of <strong>the</strong> driving pulse’s<br />
electric field, i.e. <strong>the</strong> exact position of <strong>the</strong> electric field with regard to <strong>the</strong> pulse envelope (Carrier<br />
Envelope Offset: CEO) [99Tel]. Generally in mode-locked <strong>lasers</strong>, <strong>the</strong> CEO phase exhibits large<br />
fluctuations, which have to be measured and be corrected for. Until recently <strong>the</strong> only successful<br />
demonstration of CEO-phase-locked intense pulses was based on a CEO-stabilized Ti:sapphire<br />
laser, chirped-pulse amplification and pulse compression in a hollow fiber (Sect. <strong>2.1</strong>.6.9). Meanwhile,<br />
<strong>the</strong>re are some new promising ways to achieve this goal. One is based on Chirped-Pulse<br />
Optical Parametric Amplification (CPOPA) [92Dub] and <strong>the</strong> o<strong>the</strong>r on pulse compression through<br />
filamentation [04Hau1]. We have demonstrated CEO-phase-stabilized CPOPA for <strong>the</strong> first time<br />
with near-transform-limited 17.3-fs pulses [04Hau2]. But even more amazing was that even though<br />
filamentation is a highly nonlinear process involving plasma generation, <strong>the</strong> CEO stabilization was<br />
maintained and intense CEO-stabilized pulses as short as 5.1 fs with 180 μJ pulse energy have been<br />
generated [05Hau]. It is expected that both pulse duration and energies will be fur<strong>the</strong>r improved in<br />
<strong>the</strong> near future. For example just recently CPOPA resulted in 9.8 fs pulses with 10.5 mJ at 30 Hz<br />
pulse repetition rate [05Wit].<br />
We would expect that with attosecond time resolution we open up a new world of physics with<br />
as much impact as has been demonstrated in <strong>the</strong> 70’s and 80’s with <strong>the</strong> transition from picosecond<br />
to femtosecond time resolution. First time-resolved measurements have been done [02Dre]. At this<br />
Landolt-Börnstein<br />
New Series VIII/1B1