Design

Inspired by the advantages and disadvantages of the reviewed literature and the Vis-TA setup of Ernstinget al.⁸, we tried to balance user friendliness and perfor-mance of the setup. In the following, the different components of the instrument are discussed.

White light generation and intensity balancing

Based on the work of Riedle et al. the WL is generated by focusing the 800 nm output of the Ti:Sa laser in a 4 mm thick, undoped YAG crystal (Laser Compo-nents).⁵After WLG, the spectral region around the fundamental laser wavelength still contains 95% of the energy.⁶ The intensity in this spectral region must be re-duced to avoid a significant photoexcitation of the sample by the probe beam as well as a saturation of the detector. Inspired by the TA instrument of Ernstinget al.⁸, we use a central beam stop to absorb a significant part of the residual laser light close to the optical axis (Figure 2.3). The intensity close to the fundamental is further reduced by passing through a solution of a IR-140 in methanol with an absorption band close to 800 nm. In addition, we utilize the spatial separation of different spectral regions after dispersion at the prism to balance the spectrum.

Since the region of the fundamental around 800 nm is in the center of the 600-1600

2.3 NIR Detection 31 nm observation window after dispersion, a normal, variable neutral density filter cannot be used. Therefore we turned to apodizing neutral density filters (AP, NDY20B, Thorlabs) which are normally used to reshape Gaussian beam profiles into near-flat-top ones. They are coated with a reflective metallic layer that has a near-Gaussian density distribution with an optical density of 2 in the center which decreases to 0.04 at the edges. The Gaussian change of the optical density allows us to decrease the intensity in the center without considerably decreasing the region at the edges where the WL is already weak.

The WL can be further fine tuned by changing the energy of the 800 fundamental beam with a variable neutral density filter (VND) and the aperture by an iris before focusing on the YAG crystal.

All reflective optics

In line with the UV-Vis detection, we decided to only use spherical mirrors instead of lenses after WLG to avoid chromatic aberration. To minimize the astigmatism, the reflection angles are kept small by placing them in a folded geometry. Further-more, instead of using 1 inch optics we chose spherical mirrors with a diameter of 75 mm and a focal length of 200 mm (M1-M3, CM750-200-P01, Thorlabs). This is necessary to (i) decrease the reflection angles (ii) facilitate the absorption of the fundamental close to the optical axis by a central beam stop and (iii) enable a small angle between pump and probe beams. Metallic mirrors with a silver coating (-P01,Thorlabs) are used due to their high reflectivity between 600 and 1600 nm.

reference beam

sample beam

AF YAG

Probe Pulse

L₁ M₂

M₁ IR 140

Sample cell

Pump Pulse

WG

STD STD

λ/2

VND

1500 mm

700 mm

BS

WG

M₃

M₄

M₅ WG L₂

InGaAs

InGaAs

Figure 2.3: Layout of the NIR transient absorption spectrometer. STD: Standard containing rare earth elements (NIST 2065)

Spectrograph

In contrast to the UV-Vis detection, where a grating based polychrometer is used, we built a prism based spectrograph to disperse the Vis-NIR light. The advan-tages of a prism in the NIR outweight the disadvanadvan-tages and are summarized in the following:

Pros:

(i) High transmission throughout the whole spectral window.

(ii) Circumventing the second order diffraction problem.

(iii) Resolution which is nearly linear in energy.

(iv) Flexibility to change the width of the observation window by changing the prism angle without changing the optical elements.

Cons:

(i) More complicated pixel to lambda conversion (see section 2.5.1).

(ii) If the direction of the input into the spectrograph changes, not only the horizontal position on the detector is different but also the spectral region and the resolution which is mapped on the detector. The day-to-day alignment is therefore more time consuming and prone to errors.

For recollimation of the beam a 1 inch mirror (M4, f=100, CM254-100-P01, Thorlabs) is used, to obtain a collimated beam with a size slightly smaller than the 20x20 Brewster prism (SF10, apex angle = 60.84, Halle) which is cut for 1100 nm. The spherical mirror (M4, f= 200) together with the prism dispersion and length of the detector active area (12.8 mm) gives a spectral region from 550 to 1650 nm.^c The maximal resolution is obtained by placing the camera at the focus of the sagittal plane where a vertical line can be observed. However, at this position the beam exceeds the height of the active area and therefore light is lost (Figure 2.4b and c). The position of the camera is therefore optimized for the highest resolution under the condition that the intensity is not decreased. Since this position is not very well defined and has to be identical for the sample and reference channel, it is more time consuming to align the NIR-detection compared to the UV-Vis detection.

Detector

The two detectors consist of an InGaAs photodiode array with 512 pixels (G11608-512, Hamamatsu) and a CMOS chip, which does not have to be cooled. Its spectral response increases from the visible towards 1600 nm and then decreases steeply at higher wavelengths (Figure 2.4a). The size of photoactive area of each pixel is 10x500µm, whereas the pitch is 25 leading to a pixel size of 25x500 µm (Figure2.4c). For the read out of the detector, the Low Speed Camera System

csee equation 2.4 and 2.5

2.3 NIR Detection 33

10 μm

500 μm saggital focal plane

tangential focal plane circle of least confusion

a) b) c)

InGaAs

pitch = 25 μm

Figure 2.4: a) Spectral response curve of the InGaAs detector. b) A sketch to illustrate the images at several important points along the optical axis for an astigmatic beam. c) At the saggital focal plane the resolution is best but the beam surpasses the height of the pixel. This is avoided by imaging a position between the saggital focal plane and the circle of least confusion on the detector at the cost of resolution.

FLCC3001 (Entwicklungsb¨uro Stresing) is used.

The height of the active area (500µm) is more than a factor of 2 smaller than the height of the active area in the UV-Vis detection (1.392 mm). This is the reason the beam exceeds the detector in the focus of the saggital plane and renders the alignment of the NIR detection more difficult. (see section 2.3.2)