Optics and Optomechanics

The design and construction of the optical layout were one of the main tasks in the design of the MCD-spectrometer. The material of the employed lenses should show a high transmission over the entire wavelength range and no birefringence affecting the light polarization should occur. Thus, UV-grade fused silica lenses (Thorlabs) suitable for wavelengths between 185 and 2100 nm were chosen. Uncoated lenses were chosen since anti-reflection coatings are available only for limited wavelength ranges.

Focusing the light first onto the sample within the magnet and afterwards onto the detector requires at least four lenses. Depending on the nature of these lenses, two possible basic layouts illustrated in Figure 26 and their combinations have been considered: Option 1 involves two pairs of biconvex lenses while option 2 employs two pairs of plano-convex lenses with the first lens of each pair collimating the light and the second one focusing it. In order to find out which option performs better, simulations using the ZEMAX 8.0 optical design software¹³¹ were carried out. The required data of the optical layout within the polarizer compartment were provided by Aviv and the corresponding light path is shown in Figure 27. Simulations based on these data showed that smaller spot sizes as well as lower wavelength dependencies can be achieved by implementation of option 2. Best results were obtained with four plano-convex lenses having diameters of 50.8 mm and focal lengths of 250 mm for lenses 1, 3 and 4 and a focal length of 200 mm for lens 2. The corresponding data and drawings are shown in the appendix, section 8.2.1.

Figure 26: Schematic illustration of the basic optical layouts considered for focusing the light onto the sample and the detector.

Figure 27: Optical layout within the polarizer compartment according to the data provided by Aviv biomedical, Inc. For reasons of clarity, only five rays belonging to one field point (center) and one wavelength (1000 nm) are shown.

This version of the optical layout sufficed for CD- and MCD-measurements in the visible range. However, due to the comparatively small active area of the InGaAs detector, satisfactory spectra in the NIR range required an improved focusing of the light beam onto the detector, i.e. a smaller spot size. According to equation (64), insufficient light intensity leads to high baseline offsets and unphysically large CD signals. Thus, simulations were revised by adding an additional small lens in front of the detector. Although the light beam is rectangularly shaped, which suggests that the integration of a cylindrical lens would be beneficial, better performance was simulated by adding another plano-convex lens with a diameter of 25.4 mm and a focal length of 75 mm. The simulated light path beginning from the polarizer compartment is shown in Figure 28, while Figure 29 shows the full field spot diagrams at the sample surface and at the detector position. The selected field points for the

Design and Setup of the MCD-Spectrometer 63 simulations correspond to the maximum monochromator slit width of about 3 mm and a rather small slit height of 4 mm. The full field spot diagrams show that a sample diameter of at least 6 mm and a detector surface diameter of at least 3 mm are necessary in order to catch the full light intensity of the given field points. Since the clear sample cell diameter is 12 mm, no intensity is cut off when the light passes the cell, even if the experimental slit height is larger than in the simulation. However, care has to be taken when preparing the sample, i.e. it has to be rather homogeneous. When single crystals are going to be studied, either the implementation of an additional lens or an aperture is required.

Figure 28: Simulated optical layout for the MCD-spectrometer using five plano-convex (PLX) lenses. Top: Light path from the spectrometer exit to the sample. Bottom: Light path from the sample to the detector. For reasons of clarity, only five rays corresponding to one field point (center) and one wavelength (1000 nm) are shown.

Figure 29: Simulated full field spot diagrams corresponding to the optical layout shown in figure 28. Left:

Sample surface. Right: Detector position. Different colors represent different wavelengths: 1000 nm (blue), 1500 nm (green) and 2000 nm (red).

With a diameter of only 3 mm, the illuminated area at the detector position is much smaller than at the sample surface because lens number 5 is positioned directly in front of the detector. Although the required area is still larger than the active area of the InGaAs detector, the spot size corresponds to a reduction of about 40 % compared to the first version of the MCD-spectrometer with only four lenses (compare Figure A 2 in the appendix). Figure 29 also shows a clear wavelength dependency of the focal spots, which is more pronounced at the detector site. The spot size is smallest for wavelengths around 1000 nm since the distances given in Figure 28 were optimized for 1000 nm. The utilization of achromatic lenses could reduce the wavelength dependency and has to be considered for future versions of the spectrometer.

The experimental realization of the simulated optical layout involved mounting the lenses onto aluminum rails, which were fixed to the tables where the CD-spectrometer, the magnet and the detector compartment are placed on (Figure 16). Appropriate rail carriers allow the fine positioning of the lenses parallel to the light path, which was performed by monitoring the detector signal while carefully moving the lenses. The experimentally determined optimum distances agree rather well with the simulated ones and only slight changes were necessary. Fine-positioning of the lenses perpendicularly to the light beam was performed with the help of linear translation stages placed on top of the rail carriers while vertical adjustment was possible by height adjustable optical post holders.

The light path had to be shielded from ambient light since ambient light reaching the detector leads to artificial CD signal lowering (compare equation 64). In the first version of the MCD-spectrometer, light shielding was achieved by a combination of rigid PVC pipes and flexible rubber hoses. However, although ambient light was efficiently shielded, this setup turned out to be rather cumbersome concerning maintenance work, e.g. readjusting the lenses or checking the magnet windows for impurities. Thus, for the second version of the MCD-setup a more user-friendly alternative was chosen by designing a wooden box that perfectly fits to the dimensions of the tables and the optomechanics. This wooden box was designed and assembled with the help of Michal Kern and Jan Vaverka as part of their Erasmus projects at the University of Stuttgart and with the help of Dr.-Ing. Petr Neugebauer (Institute of Physical Chemistry, University of Stuttgart). The top and the side covers of the box can be removed separately whenever it is necessary, e.g. for lens readjustment. For efficient light shielding, the fixed parts of the box are sealed with polyethylene sealant while black foam serves for sealing the flexible parts. Figure 30 shows the current state of the complete MCD-setup including the wooden box.

Design and Setup of the MCD-Spectrometer 65

Figure 30: Photographs showing the current state of the setup. Left: Front view onto the completed MCD-setup. Right: View into the opened box containing the lenses.

However, work is ongoing concerning the user-friendly change of the magnet between several applications, e.g. MCD-, torque-detected EPR- or FDMR-spectroscopy.