Factores de riesgo del maltrato infantil

The first XUV generation experiments carried out with the system presented in this work are in good agreement with the theoretical predictions, see Section 4.5. The current limita- tion for experimentally investigating the HHG process in the new power regimes available in femtosecond enhancement cavities is given by the output coupling mechanism. In order to access the XUV radiation generated in these power regimes, novel XUV output cou- pling techniques need to be implemented first. In the course of this work, several new output couplers were developed and proposed, however, not yet tested. In the following, the techniques coming into consideration for the systems developed in our group will be discussed.

5.2 Power Scalable XUV Output Coupling 73

5.2.1

Thin Plates and WOMOC

A conceivable way to increase the damage threshold of thin plates, operated in transmission for the fundamental radiation and reflecting the XUV (such as Brewster plates or the GIP [108]), is improving their mounting with respect to cooling. In contrast to free-standing plates (see Fig. 4.11), the WOMOC is expected to allow a better cooling due to the fact that one surface of the wedged top layer, acting as the thin plate, is entirely attached to the underlying multilayer structure. To optimize cooling, the quantitative investigation of the heating effects is planned in our group. On the one hand, the in-situ measurement of the heating of these elements (and of the cavity mirrors) using a high-resolution thermal camera is planned. On the other hand, as for the cavity mirrors, we plan to develop a theoretical model for this process by numerically solving the heat conduction equation with appropriate boundary conditions.

5.2.2

Mirrors with Apertures, Direct Output Coupling

As mentioned in Section 4.4.3, mirrors with a circular hole with a suitable diameter have only very recently become available. Thus, to this day we have not carried out experiments with these novel mirrors. We expect that for a successful implementation of this method, additional precautions concerning the excited mode need to be taken. For instance, a position in the stability range will need to be chosen at which higher-order modes are not simultaneously resonant with the fundamental mode, thus avoiding a coupling among transverse modes. Additional spatial filtering with appropriate apertures might be neces- sary. Furthermore, the effect of the high intensity at the fundamental mode center on the hole edges is subject to future investigations. However, the major advantage of using the fundamental cavity mode is the already existing understanding of the HHG process for this transverse field distribution.

In parallel, quasi-imaging (QI) as presented in [37] is being investigated. To this end, curved mirrors with horizontal slits have been produced at the ILT in Aachen [107]. The manufacturing constraints of such slits are somewhat more relaxed than those of small holes, since the QI mode avoids the slit. In our cavity, the QI mode consisting of the degenerate Gauss-Hermite eigen-modesGH00andGH40 experiences low losses even at slits

with_∼300µm width. Initial power scaling results with such a mirror have revealed thermal limitations at around 5 kW of circulating power. The origin of this limitation is subject to further investigations. The production of new slotted mirrors with improved slit edges is planned in collaboration with the ILT. With those, further power scaling experiments and the investigation of thermal effects are planned. Moreover, in contrast to the fundamental mode, QI modes change their shape upon propagation along the optical axis due to the different Gouy phases of the involved resonator eigen-modes, cf. [37]. The relative rapid shape change in the region with an on-axis intensity maximum might affect the phase- matching of the HHG process. Furthermore, the spatial distribution of the generated high harmonics still needs to be investigated and the question whether the generated XUV light will pass through the slit needs to be answered.

74 5. Outlook

Two further variations of the above methods are conceivable. Firstly, realizing QI in both transverse directions simultaneously could be achieved by canceling out the difference between the Gouy parameters for the two transverse directions, i.e. setting ψx =ψy. This

could be done by horizontally tilting the beam incident on one of the curved mirrors in the current setup or by inserting an additional convex mirror [125]. In this case, a circular hole could be used instead of the slit. The rotational symmetric mode, which is a linear combination of Gauss-Laguerre eigen-modes in this case, might be beneficial for the HHG process. A second approach concerns the ACR design. To obtain relatively small foci, this resonator needs to be operated close to a stability range border. Therefore, QI in the center of the stability range cannot be achieved in this LMA cavity. However, as the stability range border is approached, the Gouy phase differences among higher-order modes decrease and the coupling among them increases. A quantitative description of this coupling is subject to further study3_{. We plan to investigate whether this coupling could}

be used to excite a QI-like mode in the ACR resonator with losses small enough so that they allow the desired power enhancement.

Finally, we mention that the currently implemented XUV detection setup (see Fig. 4.15) can easily be modified to suit the implementation of collinear, direct output coupling through a hole. The geometrically output coupled XUV beam can be steered to the XUV spectrometer by two parallel Au mirrors (or plates, anti-reflection coated for the IR [108]) under (close-to-) grazing incidence while the XUV photodiode mounted on the translation stage can remain at the same position.