2.2.1 Overview
The layout of the UV-Vis TA setup is illustrated in Figure 2.2a. Probing is achieved using white light pulses generated by focusing (L1, f=200)∼1 µJ of the output of the 800 nm pulses in a 3 mm CaF2 plate (Korth Kristalle GmbH). After recolli-mation (M1, f=100), the white-light beam passes a custom-made mirror (M800)b to remove the residual fundamental at 800 nm and is split into the sample and reference channels by a reflective neutral density filter (BS, NDUV05B,Thorlabs).
The beams in both channels are focused (M2, f=250) to an approximately 60µm diameter spot. In the sample path, the beam passes the sample cell at the focal point and overlaps with the pump beam. To spectrally balance the spectrum, the high intensity region close to the fundamental was reduced by passing through the same CuSO4solution in both channels. Both beams are then focused (M3, f=100) in a pair of grating based spectrographs (SR163, Andor Technology) equipped with a 512x58 pixel back-thinned CCD (S7030-0906, image size = 12.288x1.392 mm, Hamamatsu). The spectrographs and readout electronics were assembled by Entwicklungsb¨uro Stresing, Berlin.
Figure 2.2: a) Layout of the UV-Vis transient absorption spectrometer. b) Schematic description of major alignment steps by moving the spectrgraphs along a rail, place two irises (Ir4and Ir5) and align with the last two mirrors.
2.2.2 Changes implemented during this thesis
Reflective Optics
During the thesis all lenses were exchanged with reflective optics in order to avoid chromatic aberration. This precludes spectrally distorted spectra that originate
bHigh reflectivity at 800 nm and transmittance>90% from 300 to 700 nm
2.2 UV-Vis Detection 27 from a λdependence of the focal position. If the pump spot is not significantly larger than the probe close to the focal point, the pump-probe overlap is dependent onλ. Therefore, the pump power experienced by the probed volume can differ for different λleading to distorted spectra. Furthermore, the observation window in the blue could be extended from 360 nm to 330 nm by avoiding lenses in the probe path. The probing range is now limited by the grating in the spectrographs and the beam splitter. Due to the high reflectivity in the UV, we use planar and spherical UV enhanced aluminium mirrors (-F01, Thorlabs). To limit the astigmatism, we keep the reflection angles of all spherical mirrors minimal by placing them in a folded geometry as illustrated in Figure 2.2a. Since the height of the CCD chip is significantly larger than the width of one pixel, a slight astigmatism is actually beneficial to illuminate the whole chip.
Polarization of pump and probe beams
The reflectivity and transmission curves of the neutral density filter used as beam splitter are dependent on the polarization of the white light. Therefore the polar-ization, of the fundamental is turned with a half-wave plate (λ/2) before WLG to fine tune the intensity in both beam paths. It has to be noted that recent studies have highlighted that the WLG is a function of the CaF2 plate orientation.2,3 Therefore, the CaF2 plate has to be rotated to obtain the highest intensity for a given polarization of the fundamental. After the BS, a broadband wire grid polar-izer (WG1, WP25M-UB,Thorlabs) is used to clean the polarization followed by WG2 after the sample, which filters the pump and is used to measure the optical Kerr effect. If not stated otherwise, the polarization of the pump is set to magic angle with WG3 and the intensity can be modulated by aλ/2.
2.2.3 Standard operating procedure
On a daily basis, the fundamental beam is aligned using a pair of mirrors and a pair of irises. For better reproducibility a CMOS camera (DMK 72AUC02, The Imaging Source) is used to center the beam on two irises. Now a Holmium oxide filter (HO) is placed in the collimated beam and the narrow absorption lines are observable with both the sample and reference cameras. The grating of the two spectrographs are rotated until the spectral features in both cameras overlap. The intensity of the fundamental beam can then be optimized using a variable neutral density filter (VND). To decrease the scattering light of the pump at the detector, two irises (Ir2, Ir3) are placed after the sample cuvette. Since it is crucial that IS = IR (see equation 2.2), an additional iris (Ir1) is placed before the beam splitter. After reducing the size of Ir2and Ir3, Ir1should be always closed as much to be the limiting iris in order to guarantee IS =IR.
Major alignment
If the alignment is degraded due to accidentally touching an optical element, major alignment steps are necessary. It is important to guarantee that the height is
constant in both channels, the beam it is not clipped anywhere and the angle of reflection of the spherical mirrors is minimal. Furthermore, it is crucial to enter in both spectrographs at the same position and angle. This can be achieved by the following steps:
(i) The spectrograph is unscrewed from the table and can now be moved forward and backwards along a rail. The direction of the movement is parallel to the optical entrance axis in the spectrograph.
(ii) A metal cone is inserted in the entrance slit of the spectrograph which marks the position of the entrance at position 1 (Figure 2.2b). Now an iris (Ir4) is placed with the center at the entrance position marked by the center of the metal cone.
(iii) The spectrograph is moved back along the rail as far away as possible and the entrance position is marked with a second iris (Ir4).
(iv) The white light is now aligned using the two mirrors before the spectrograph and Ir4 and Ir5 to guarantee an optimal input angle and position into the spectrograph.
(v) The irises are removed and the HO is placed in the collimated beam. The spectrometer is now moved along the rail until the spectral features of the HO are the narrowest. At this position the sagittal focal point is imaged at the CCD camera and the resolution is highest.