A new horizontal Infrared microscope for large volume samples developed at SOLEIL

SMIS beamline at SOLEIL is dedicated to infrared microspectroscopy. It is equipped with commercial instruments allowing to perform microanalysis with diffraction-limited spatial resolution. It has two operational branches, each of them equipped with one of these instruments, which serve a wide range of applications in various disciplines: Biology and Biomedicine, Earth Science, Chemistry, Soft Matter, Archaeology…

The microscopes available on the beamline operate with a pair of commercial mirror objectives, commonly called “Schwarzschild objectives”, used as condenser and imaging objective in a confocal configuration. Two pairs of objectives are available with respectively a x15 magnification (M) and 0.5 numerical aperture (N.A.) for the first pair, and M=x32 and N.A.= 0.62, for the second one. However, for sake of compacity, these commercial objectives have a quite short working distance: 15 mm for the X15 objective, and 9 mm for the x32 one.

A growing demand for infrared microspectroscopy (and imaging) on larger volume sample has emerged (> 40 mm working distance). More particularly, high pressure measurements in Diamond Anvil Cell request less space constraints due to the need to accommodate heating devices, cells of larger dimension (and compatible with those used for X-rays diffraction and absorption). In addition, reaching very high pressure inside DAC cells ( > 100 GPa) requires gaskets with small holes, often of the order of 20 microns or less. For such experiments, the synchrotron radiation brilliance is a crucial advantage, therefore coupling this microscope to the synchrotron infrared beam should be made easy.

Facing such a challenge, a group of several scientists at SOLEIL decided to design and build a new horizontal microscope with large working distance Schwarzschilds, while keeping a large numerical aperture. In addition, the constraints were placed to keep the same capability that exists in the commercial instrument: transmission and reflection mode, mid-infrared and far-infrared detection, while adding in situ fluorescence and Raman spectroscopy capabilities.

The two Schwarzschild objectives were calculated by the Optics Group of SOLEIL (Bruno Lagarde and Francois Polack), to exhibit the following parameters: x15 magnification, 0.5 N.A. and a working distance of 47 mm, with minimum spherical aberration. The mirrors have been manufactured by diamond turning, and assembled at SOLEIL. The mechanics holding the two spherical mirrors and allowing all necessary motions for alignment were designed at SOLEIL by Stephane Lefrançois (SMIS) in close collaboration with the Optics group. 

 
Figure 1: shows the size of the secondary spherical mirror (right) and all the mechanical parts of the objective before assembly (left).
 

Each Schwarzschild has been assembled and aligned individually, in the Optical Laboratory of SOLEIL, by Blandine Capitanio and Gilles Cauchon. Then, the pair has been aligned for optimum confocal operation. Figure 2 shows the two Schwarzschilds precisely aligned in their confocal configuration in the Optics Laboratory of SOLEIL.

Figure 2: Alignment of the two Schwarzschilds with an autocollimator telescope and a CCD camera.

 

The horizontal microscope table and accessories have been fully designed at SOLEIL by Stephane Lefrançois (SMIS). The complete set up includes possibility for transmission, reflection geometry, mid- and far-IR detection (with a 50 microns MCT detector, and two bolometers). Figure 3 shows the microscope on the SMIS beamline, coupled to the synchrotron beam.

 
Figure 3: Picture of the microscope during an experiment on the SMIS beamline. One can note the presence of a DAC cell between the two Schwarzschild, and the relaxed space constraints provided by these two large working distance objectives. Blue tubing ensures a clean and dried purged atmosphere inside the focusing optical elements and in mirrors boxes.

The projected beam size at the sample location using the new horizontal microscope has been recorded by raster scanning a 10 microns pinhole as the focal plane position between the two Schwarzschilds. The beam profile, shown on Figure 4 indicates that the small and brilliant synchrotron source is properly demagnified to produce a narrow beam profile at the sample location.

Figure 4: Beam profile at 10 microns wavelength, recorded at the sample location. This profile gives a Full Width at Half Maximum (FWHM) of 22 ± 3 μm.

The microscope has been recently used, for high pressure experiments, with small sized holes in the gasket of the DAC cells (Paul Loubeyre, Florent Occelli and Agnes Dewaele). Infrared spectra of good quality have been obtained with gaskets having a 24 micron hole. (Fig. 5)

Figure 5: Spectrum recorded through the 24 micron hole in the gasket of a Type IIA diamond anvil cell. The spectrum has been acquired in 23 seconds at a spectral resolution of 4 cm^-1.

 
The newly designed microscope is now operational on the SMIS beamline. In a next future, a Raman spectrometer will be added to allow simultaneous measurement of Infrared, Raman, and Fluorescence spectra on the sample.
The new equipment developed at SOLEIL has benefited from a close collaboration between the members of the Optics Group of SOLEIL (François Polack, Bruno Lagarde, Blandine Capitanio and Gilles Cauchon), of the SMIS beamline (Stephane Lefrançois and Paul Dumas), and of the members of CEA/DIF, Bruyères-le-Chatel (Florent Occelli and Paul Loubeyre).