We present supercontinua generated in LiF and CaF2 revealing that LiF is advantageous especially in the near UV region since it pushes the cut-off wavelength about 17 nm towards lower wavelengths and the occurrence of color centers, which has been considered as a drawback up to now, is not a limitation for its applicability in femtosecond transient absorption spectroscopy. Even though the color centers occur within a short time of illumination, they do not influence the supercontinuum generation significantly and they can furthermore, if desired, be eliminated from the substrate simply by heating. Thus LiF is a promising substrate for broad band measurements in the UV/vis range.
© 2013 OSA
For femtosecond transient absorption spectroscopy in the visible spectral range a white light source is needed to probe the dynamics over a preferably wide spectral range. Especially the accessibility of the near UV range is of high interest since most organic compounds have their main absorbance in that region, therefore each method to generate a more blueish white light is valuable. Usually the white light is generated by focusing femtosecond laser pulses into a transparent material where a so called supercontinuum (SC) is produced. SCs were first described by Alfano and Shapiro in 1970 in glasses and crystals  but a generation in liquids and even gases is possible [2, 3]. Several processes contribute to the process of SC generation whereas the exact mechanisms are not entirely understood up to now . Nevertheless, several effects, which are believed to play an essential role, can be identified. One is the optical Kerr-effect that induces a self focusing of the pulse due to its spatial intensity distribution. Another process, having the opposite effect is plasma defocusing, which occurs above a threshold value and prevents the pulse to collapse onto itself at high intensities. Since mainly these two processes stabilize the transversal width of the pulse over huge distances, longer than the diffraction length, the propagation process is called filamentation [5, 6]. The spectral broadening itself is mostly due to self phase modulation as a result of the temporal variation of the index of refraction caused by the creation of free electrons depending on the temporal variation of the pulse intensity and thus depends on the nonlinear refractive index n2[7, 8]. One limitation for the spectral broadening of the pulse is of course the band gap of a material, since at high intensities multi photon absorption becomes likely leading to losses on the high energy side of the SC spectrum. The correlation between the band gap size and the spectral width of a SC has been demonstrated .
Sapphire offers some advantages in comparison to other bulk materials since it has a relatively wide band gap of 6.9 eV  and is stable against material degradation if used at moderate intensities, due to its high thermal conductivity and hardness. However, the efficiency of SC generation in the near UV is low. A material that overcomes this limitation is CaF2 since it provides a higher band gap than sapphire but is prone to material degradation. This negative effect on the long term stability of a SC can be reduced by moving the substrate. If one only takes the band gap as the limiting factor for the generation of a SC in the UV LiF should provide even better results than CaF2 since it has a gap of 13.6 eV in comparison to only 12.0 eV for CaF2. However, in literature the formation of color centers (F-centers) is mentioned and rated as a drawback for SC generation in that material . F-centers are a result of point defects in ionic crystals caused by high temperatures where anionic vacancies are produced which are occupied by one or more electrons . These electrons can be excited by radiation in the UV and visible spectral range. In the case of LiF the F-centers absorb in the range of 250 nm whereas the M-centers, that are a result of aggregating F-centers, absorb around 450 nm. The absorption coefficients of these color centers are 121 and 192 cm−1, respectively .
We show that the generation of a SC in LiF is more efficient in the near UV region than CaF2 and that the color centers do not influence the generated spectrum noticeably since they are formed mainly at the focal position whereas the gross of the SC generation is accomplished out of that region. They can furthermore be eliminated by simple heating of the substrate.
2. Results and discussion
To study the applicability of LiF in comparison with CaF2 we used a setup as shown in Fig. 1. A small fraction (6 μJ) of the output beam with a central wavelength of 775 nm and a pulse length of 170 fs, produced in a Clark CPA2110, is transmitted through an aperture and focused into the 5 mm thick substrate (purchased from Korth Kristalle GmbH). where the beam diameter in the focal region is 46 μm in air. This results in a peak power of 35 MW, which is more than the tree-fold threshold intensity for SC generation in LiF and CaF2. The generated SC is subsequently collimated and coupled via a Teflon cosine corrector (Avantes BV, transparent in the range from 200–800 nm) into a fiber which is attached to a spectrometer (Ocean Optics, MC2000 Production). During the measurement the substrate is translated orthogonally to the propagation direction to avoid material degradation.
Taking a look at the respective generated SCs shows that the color of the SC generated in LiF is blueish whereas the color of the SC generated in CaF2 appears to be more yellowish. This observation is supported by the SC spectra shown in Fig. 2. Both show a maximum around 500 nm which is more pronounced in CaF2. Below that wavelength the LiF spectrum shows another maximum around 350 nm whereas the CaF2 spectrum remains at almost the same intensity level down to 350 nm. Below 350 nm the intensities of both spectra decrease with about the same slope down towards the respective cut-off wavelength which is at 287 nm for CaF2 and at 270 nm for LiF. These results show that LiF not only produces a higher SC intensity around 350 nm but also pushes the cut-off wavelength for SC generation about 17 nm towards lower wavelengths compared to CaF2, as predicted from the difference in the size of the respective band gaps.
Besides these valuable properties of LiF there is also the effect of creating color centers during the SC generation whose aggregates, especially the M-centers, can be observed with bare eyes as a yellowish color on the substrate (inset in Fig. 3). The black curve in Fig. 3 shows the absorption spectrum of the substrate after four hours of continuous usage where a major absorption is created around 250 nm due to the formation of the F-centers and a minor one around 450 nm due to F-center aggregation. Interestingly, those color centers do not, as one would expect, influence the SC spectrum since after four hours of continuous usage the SC spectrum remained unchanged (dotted line in Fig. 2) even though the M-centers are clearly visible and reach an optical density of 0.3. Since the M-center absorption rose from 0 to 0.3 during the time interval of the measurement one would at least expect a reduction of the SC intensity of about 50 %. Taking a closer look at the substrate explains this seeming contradiction: the defects are produced and are spatially restricted close to the focal region. Since during the measurement of our spectrum the focal point was close to the entrance surface, the centers were only created in that area and could not influence the SC that is mainly created after the region of color centers and thus does not influence the generated spectrum. Obviously, the threshold power for SC generation is lower than for F-center creation. To demonstrate the spatial restriction of the color centers to the focal region we performed a measurement where the focal point was moved to different depths within the material (Fig. 4). Here the substrate was moved only in y-direction during SC generation for about 10 min before it was moved to the next x-position and a few mm in z-direction. Figures 4(a) and 4(b) show the same substrate where in Fig. 4(b) the respective color centers were highlighted for a better representation. One can observe that the centers move according to the change in the spatial position of the focal point (I–III). Even at much higher intensities (IV) they remain restricted to the focal region.
The production of the color centers is reversible since we could eliminate them by heating the substrate for 2 hours at 400 °C. The spectrum (red curve in Fig. 3) shows that the M-centers are eliminated completely, whereas the F-centers still produce a small absorbance around 250 nm, but the main contribution to the absorption spectrum is obviously due to scattering as a result of material degradation, which also occurs in CaF2. Bringing the substrate to higher temperatures or heating it longer should eliminate the residual absorbance of the F-centers completely .
3. Summary and conclusion
With our observations we demonstrate that the process of SC generation is not necessarily disturbed by the creation of color centers. If the effect of re-absorption by these centers shall be circumvented, the focal point should be close to the entrance surface so that the subsequently created SC remains undisturbed. By using LiF instead of CaF2 as a substrate for SC generation we were able to push the cut-off wavelength about 17 nm further into the UV region, which is beneficial for numerous measurements in the UV/vis region since it allows to access a broader spectral range than any other material currently in use. Table 1 sums up the central parameters influencing SC generation. An alternative to generate a SC at even lower wavelengths is to use UV pulses but this does not provide a spectral coverage of the visible spectral range.
Experiments using LiF SC as a spectrally broad probe in femtosecond transient absorption spectroscopy are currently underway.
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