4. Experimental Setup
4.4. In-Situ Reference Spectrometer for High Sensitivity Transient Absorption
Sensitivity Transient Absorption Measurements
As stated above, the non-linearity of HHG and slight instability of the laser parameters (the fiber compressor is also driven by non-linear processes) cause the spectrum of the
input XUV pulses to be different for each recorded absorption spectrum. This repre- sents a significant difficulty for the measurement of the optical density, as it can only be determined if both the absorption and reference spectrum are known. Considering the fluctuations of the XUV spectrum over time, they have to be recorded ideally at the same time. However, up to this point, the reference spectrum could only be recorded separate to the absorption signal. In previous measurements the reference was recorded by mov- ing the target in and out of the beam, or by removing the target gas from the target cell. Thus spectra and references were always recorded sequentially which caused differences between input and reference spectrum.
TEM grid Modified target cell
a
c
b
Installed grid + filter array Gas cell opening 200 µm
CCD Chip Image Absorption Spectrum Reference Spectrum grid separation: d = 12.5 µm, 16.5 µm
I (w)
0I (w)
sigI(w)
XUV-VLS spectrometer grating diffraction pattern th 0 st +1 st -1 th 0 st +1 st +1 th 0Figure 4.12.: Illustration of the in-situ reference measurement method: After passing the concentric filter assembly for spatial beam separation the XUV beam hits a TEM transmission grating which acts as a beam splitter. These gratings are mounted on Kapton wire shown in inset a) and placed behind the metal filters on the filter holder. The installed overall assembly is shown in inset b). The diffraction pattern of such a TEM grid is shown on the lower left. The 0th diffraction order is used for the absorption measurement while the +1st order does not pass the sample and gives the reference spectrum. For this a target cell with a very small separation between the gas reservoir holes and a slit that allows the diffracted part to pass above without going through the sample is necessary. The design is shown in inset c). On the right, a typical full chip image of the CCD camera is shown which contains all information about the absorption (area between orange dashed lines) and reference spectrum (area between red dashed lines).
One of the main technical achievements of this work is the implementation of the simul- taneous measurement of signal and reference. To achieve this, it is first of all necessary to
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create a copy of the input signal I0(ω). In the visible spectral range this is easily possible
using a beamsplitter (most simply a thin fused silica plate). However, in the XUV region this is not possible. The developed setup illustrated in figure 4.12 uses a standard copper micro grid for transmission electron microscopy (TEM) as a dispersive element for the XUV-radiation. This element is a square grid of 5µm wires on a 3 mm ring with a wire
separation of severalµm depending on the spectral range of the measurement. The idea is
to use the 0th order transmission as the input for the absorption measurement Isig(ω) and
the +1st order diffraction as the reference signal Iin(ω). The filters are mounted on the
other side of the filter holder described in section 4.2.2 using the self-built Kapton wire mount in figure 4.12 a). To keep the spatial pulse separation they are installed together with the concentric Kapton-metal filters by LEBOW shown in figure 4.12 b). With the setup at hand, the two beams can only be separated by a maximum of ∼5 mm after the full travel distance a = 1219 mm from the TEM grid to the CCD chip. Otherwise, the two beams cannot pass all filters unhindered, fit on the spectrometer grating and be measured simultaneously. Furthermore, a minimum separation of ∼1.5 mm of the beams is given by the requirement that they have to be distinguishable along the chip height (see data presented in figure 4.12). Using the grating equation
mλ = d · (sin(arctan(dx/a))), (4.5)
with m the diffraction order, d the grating groove distance, dx the position of the first max- imum with respect to the zeroth order and a the distance to the grating, the TEM grids with d = 16.5µm for the spectral bandwidth between 25 nm (50 eV) to 60 nm (20 eV,
lower Al edge) and d = 12.5µm for the spectral bandwidth between 17 nm (70 eV, upper
Al edge) to 35 nm (35 eV) were selected. Like this, a reference can be recorded over the whole range accessible with Al-filters.
A further restriction of the setup is the beam separation at the target focus and how only one of the foci can be exposed to the gaseous spectroscopy sample. In the current geom- etry the separation at the target position will roughly be a third of the final separation on the CCD chip. Therefore a new design for the target cell illustrated in figure 4.12 inset c) was worked out and very precisely manufactured in the mechanical workshop at MPIK. With this new setup reference spectra can be recorded simultaneously with the absorption spectra like it is shown on the full chip image in figure 4.12. The lower region shows the 0th order with the absorption signal imprinted. The upper region gives +1st order of the diffraction pattern as the reference spectrum. It is tilted upwards toward lower energies, because they represent higher wavelengths which are diffracted more. This diffraction is overlaid with the spectrometer diffraction along the horizontal axis and gives the tilted image of the reference spectrum. In order to get the correct reference, the rotation of the TEM grid has to be adjusted before installing it. The reason for this is that both beams have to enter the spectrometer exactly in the same horizontal position. In other words, they have to hit the spectrometer grating in the same position vertically. Otherwise the different path lengths in the spectrometer lead to different dependencies on the wave- length in the lower and upper sections of the CCD image. This would further complicate the correct analysis and make separate calibration of the regions of interest necessary. Careful pre-alignment of the TEM-gratings is thus done outside the vacuum with a HeNe
laser. The aim is to adjust the rotation of the diffraction pattern parallel to the optical table as precisely as possible at a great distance from the TEM grid.
Figure 4.13.: Comparison of the sequential and in-situ reference method: a) 300 op- tical density (OD) spectra recorded at different time delays for zero NIR intensities. The references have been recorded after the absorption spectra and afterward matched to achieve the best fit to the shape of the harmon- ics. The lower panels show the mean value and the standard deviation us- ing all spectra in the time delay scan. b) Analogous measurement showing ODs measured with the in-situ reference method. The lower panels again show the mean value and standard deviation of these spectra. Comparing the standard deviations of the different methods taking a difference of 3σ
for detectable changes in the signal it can be seen, that the in-situ reference increases the sensitivity for changes in the OD down to ∼ 10 mOD. This represents an improvement of one order of magnitude.
The capabilities of this reference measurement technique are illustrated in figure 4.13, which compares the in-situ reference with the method of recording the reference sequen- tially (target in/out, gas on/off). Figure 4.13 a) shows the optical densities (OD) of a measurement of a time-delay scan over 300 values for zero NIR intensity. The ODs have been evaluated using a reference scan measured after the absorption spectra were recorded without the medium in the target cell. The references were matched to the absorption spectra by comparing the structure of the harmonics and choosing the best fit. The lower panels show the statistics from these 300 spectra. The mean value still exhibits slight modulations in the OD resulting from slight mismatches between sequentially recorded absorption signals and references. The standard deviation showing the magnitude of fluc- tuations in the measurements which ultimately limits the sensitivity to changes in the OD caused by physical effects. Here, the fluctuations are rather high across the whole spec- trum. Compared to this method, figure 4.13 b) shows the same measurement performed with the in-situ reference method. Already the scan data is much more smooth without easily visible fluctuations between single OD spectra. Looking at the statistics of this data set, the mean value of the OD spectrum shows no slow modulations anymore. Further- more, the overall standard deviation is much lower, with high values only at the positions
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of the resonances which naturally fluctuate more. If a difference in OD of 3σ is consid-
ered as necessary for a detectable change in OD the in-situ method is sensitive to changes down to ∼ 10 mOD, while the sequentially recorded reference only manages to detect changes of ∼ 100 mOD on average. This is an improvement of one order of magnitude in sensitivity, which enables detailed studies of time-delay dependent absorption changes even in the continuum and precise determination of spectral line shapes as well as their modifications due to interactions.