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An interesting application of our 6fs laser system at MHz repetition rate could be ultrafast pump-probe spectroscopy of photoinduced electron transfer samples, respectively solar-cells. These kinds of samples require an energy density of only 1µJ/cm2 1_{. Current setups are} using commercial kHz laser systems with around 30fs pulses [54]. In order to resolve certain physical processes [55], it would be desirable to have 6fs pulses. Furthermore, to reduce the measurement time and to increase the signal to noise ratio one would like to have the laser system operating at MHz repetition rate. From our 500nJ chirped pulse oscillator we can generate a 6fs probe beam with a sufficient energy density as well as the probe by white light generation in a YAG crystal, directly from the 55fs long output laser beam. One point which is not yet clearly answered is the damage threshold of these samples concerning the average power density. Solar-cells are made for the intensity of the sun, respectively a power density of 1366 W/m2. Quite often the damage threshold of the solar-cell is at least a factor 10 higher. Working at 5MHz with a focus generating 1µJ/cm2 _{will result in a power density}

slightly above the damage threshold. However, setting down the repetition rate to 1MHz using a pulse picker and optimizing the focus diameter, respectively decreasing the energy density one should come below the sample’s damage threshold. Ultrafast spectroscopy with solar-cells is a very interesting and important research field. Therefore we drew your attention to the benefits which our laser system with pulse compression offers to this research field.

Other direct applications can be high harmonic generation (HHG) [40], HHG with reso- nant plasmonic field enhancement [41], material processing and waveguide writting [56], as high energy seed for succeeding OPCPA schemes, or ultrafast single electron diffraction exper- iments [57]. The current technique should also be able to compress the chirped pulse oscillator presented in Ref [58] which works at 70MHz with 62nJ and approximately 30fs pulse duration to (hopefully) 6fs. If we assume a final transmission of (only) 50% we still would have 30nJ of useable output energy for further experiments, which is around 6 times more as commercial oscillators currently generate at the same repetition rate. We believe that the combination of a commercial oscillator with such a simple pulse compression setup for 6fs makes it attractive for many research groups.

3.2

Evolution of spectral broadening

Spectral broadening with 55fs input pulses is on the border between pure self-phase-modulated spectra and beginning influences of other effects [1]. Furthermore the strong peak intensity will lead to saturation effects which was not yet reached in earlier experiments. Analyzing the spectral broadening expansion when increasing the input energy should allow us conclusions about such effects and therefore an optimization of our setup. We are not aware of a published systematical investigation addressing these points.

In the following experiments we recorded the spectral broadening of a LMA-25 when increasing the input energy. We used two different fiber lengths, 2.5cm and 4.0cm. For each length we recorded the spectral broadening for input pulse durations of 110fs respectively 190fs, measured using a GRENOUILLE (Model 8-9 USB; Swamp optics). We have chosen the chirped case to detect a possible influence of the average power more explicitly. For a

1

3.2 Evolution of spectral broadening 29

clear visualization of the spectral broadening we plot the analysis curves as shown in Fig.3.4. The difference between the FWHM points of the broadened spectrum one minus those of the original one plotted against the input power. In doing so, we differentiate between the visible part (VIS) and near-infrared (NIR) part. In Fig.3.5we show the spectral broadening

Figure 3.4: principle

expansion (VIS and NIR) of a 4.0cm long LMA-25 with 110fs long input pulses. Both develop approximately linear with the same slope but later the VIS part starts saturating. A weak saturation in the NIR is observed as well. Interestingly, the VIS always shows a bend whereas the NIR stays clearly linear for longer time. It seems that two different regimes of the spectral evolution exist. Furthermore, at a higher input power, oscillations appear which are present in both parts. In all measurements these oscillations never show up in the linear part. Therefore we do not attribute these oscillations to external reasons (laser) but to a combination of fiber, saturation and self-phase modulation behavior which is not yet fully understood. The average power (and with it thermal reasons) can be excluded as the oscillations do not appear at the same input power when using 190fs input pulses. The shown behavior in Fig.3.5is typical for the other experimental conditions as well. Comparing the development when using different fiber lengths we noted a surprising fact, shown in Fig.3.6. The shorter fiber results in a stronger spectral broadening concerning the VIS. This behavior is reproducible and was observed for the 190fs case too. For the NIR part (Fig.3.7, 110fs input pulse duration) we do not observe such a behavior. Concerning the VIS part this is a quite surprising observation as it is contrary to the outcome predicted by (simplified) theory. This indicates that one has to use already a more accurate theoretical description [1]. Searching the effect behind this saturation process in Fig.3.8, we compare the spectra generated by input pulses of 240nJ and 110fs, using a LMA-25 fiber length of 2.5cm respectively 4.0cm. The spectra are similar. Comparing the VIS and NIR part we notice that the NIR does not exhibit a spectral feature seen in the VIS part (A). The effect behind is called optical wave-breaking (OB) [9] and happens if frequency-shifted light in the leading and trailing edges of a pulse overtakes unshifted light

30 3. Characterization and few-cycle pulse compression

Figure 3.5: Spectral broadening expansion with saturation in the VIS for 110fs long input pulses in a 4.0cm long LMA-25

Figure 3.6: Comparing the VIS spectral expansion using different fiber lengths with (a) 110fs and (b) 190fs input pulse duration.

3.2 Evolution of spectral broadening 31

Figure 3.7: Linear spectral expansion for the NIR part with 110fs input pulse duration

Figure 3.8: Comparison of spectra using different fiber lengths but same input energy

in the pulse tails. Mixing of these overlapping frequency components generates sidelobes on the pulse spectrum. Normally it appears on both ends simultaneously. Optical wave- breaking can be described by the nonlinear Schr¨odinger equation. Comparing the OB of the different fiber lengths we notice that the OB of the longer fiber is much further advanced as the one of the shorter fiber. Therefore we assume that the OB process is responsible for the shorter VIS part. Consequently, the VIS should be larger before OB appears and later decreasing back. A simulation should confirm this. In Fig.3.9 we simulated the spectral evolution using a 4.0cm long LMA-25 with 240nJ of input energy and 77fs long input pulses. The simulation has been done with the program used in [7]. The shown spectra are taken at different lengths in the fiber, namely after 5, 10, 25 and 40mm. To stress the effect more clearly, we have chosen input pulse duration of 77fs instead of 110fs. One clearly observes how the self-phase modulation spectrum decreases as soon as the OB starts. With the simulation we have identified the process behind our observation. But why it starts to decrease has not been fully understood. We are not aware of a publication addressing this question even there are some articles published on OB [59]. We notice that optical wave-breaking stops respectively decreases the spectral bandwidth generated by the pure self-phase modulation process. Nevertheless, in total, the spectral broadening still grows due the contribution by OB which results then again in a shorter Fourier-limit of the pulse. The question is now if

32 3. Characterization and few-cycle pulse compression

Figure 3.9: Spectrum simulation using different fiber lengths

we still can compress such a pulse dominated by OB. To avoid long discussions we refer to section 3.1 where the 6fs spectral broadening is also strongly influenced by OB but we were able to compress the pulse close to its Fourier-limit.

A detailed analysis of the spectral broadening can help to identify and to understand many ongoing processes. We have shown only one example of optical wave breaking which dominates over the self-phase modulation. Another interesting aspect would be to clarify why we do not yet see OB in the NIR part. This would allow to draw conclusions about the input pulse shape which strongly influences the spectral broadening [59].