NanoESCA MARIS

Photo-excited electrons leave the sample (1) and are accelerated towards the extractor lens (2). In the back focal plane (3) the electrons are separated by their angle. A contrast aperture is used to restrict the angular acceptance. In the Real Space Mode, this reduces spherical aberrations and improves the lateral resolution. A set of deflectors/ stigmators (4) and a transfer lens form an optimized real space projection in the first image plane (5). Retardation lenses (6) project an electron image with a correct pass energy into the second image plane (7). Fourier lenses (7, 14) transcode the lateral image coordinates into angular coordinates and back for the passing of the two hemispheres (9, 12) of the Imaging Energy Filter.

In the Aberration Corrected Mode, the energy separation is done in the first hemisphere (9) using the energy dispersion and the entrance slit (8) and first exit slit (10) to cut out a small energy band. The second hemisphere (12) corrects the energy dis Photo-excited electrons leave the sample (1) and are accelerated towards the extractor lens (2). In the back focal plane (3) the electrons are separated by their angle. A contrast aperture is used to restrict the angular acceptance.

In the Real Space Mode, this reduces spherical aberrations and improves the lateral resolution. A set of deflectors/ stigmators (4) and a transfer lens form an optimized real space projection in the first image plane (5). Retardation lenses (6) project an electron image with a correct pass energy into the second image plane (7). Fourier lenses (7, 14) transcode the lateral image coordinates into angular coordinates and back for the passing of the two hemispheres (9, 12) of the Imaging Energy Filter. In the Aberration Corrected Mode, the energy separation is done in the first hemisphere (9) using the energy dispersion and the entrance slit (8) and first exit slit (10) to cut out a small energy band. The second hemisphere (12) corrects the energy dispersion and therefore higher order aberrations.

After passing the second Fourier lens, a monochromatic image sits in the first image plane behind the Imaging Energy Filter (14). This can be magnified by the downstream projection lenses (15), which map the electron image on a detector unit (16). The detector unit consists of an electron multiplier (Double MCP), a scintillation screen, camera optics and a high frame rate CMOS camera (17). In the Momentum Space Mode, the only essential difference is, that the Retardation lenses (6) are set to project the angular/momentum distribution into the second image plane (7). The electron trajectories before are the same, so that the apertures (3, 5) can be used complementary in both modes. 

In the Momentum Space Mode, the contrast aperture (3) will be opened, while a field aperture (5) in the first image plane can be closed to limit the accepted lateral region from the sample (micro ARPES). In the Double Dispersive Imaging Energy Filter Mode, the first exit slit (10) is open. The Transfer Lens (11) performs a 1:1 transfer of the electron trajectories and the second hemisphere (12) is used as an additional dispersive element. The second exit slit (13) is used to filter out an energy band. In this mode, one reaches the same energy resolution with twice as high pass energies. Thus one achieves the double transmission at the cost of a slightly lower image resolution.

*The presented modes are examples. Each microscopy mode can be combined with each Imaging Energy Filter mode.
** For clarity only the electron trajectories traveling along the optical axis are shown in the Imaging Energy Filter.

The biggest specification improvement comes from the completely revised microscope lens. It was simulated and build to achieve a better angular / momentum resolution while keeping the same good real space resolution from its predecessor. This new microscope doesn’t make compromises. In real space mode < 35 nm lateral resolution are specified. In momentum space mode, the resolution improved by a factor of 3, from 0.015 Å^-1 to 0.005 Å^-1. The performance is nicely shown on the Rashba split surface state of a Au (111) single crystal.

Left: Momentum Space Image of a Au (111) single crystal at the Fermi level, showing more than one Brillouin zone (room temperature, 50 meV instrumental energy resolution).

Right: Zoom onto the Rashba split surface state of Au (111). This image was taken at a sample stage temperature of 45 K. The instrumental energy resolution was evaluated to be < 24 meV by fiting the Fermi edge and subtracting the thermal broadening.

The line profiles of the bands were fitted with Lorentzian peak functions. The natural broadening of the bands are significantly bigger than the Gaussian broadening of the instrumental resolution.

The Brillouin zone is of scientific interest for ARPES measurements. The NanoESCA can zoom into any interesting feature within a typical Brillouin zone. The deflectors behind the entrance slit (8) and before the exit slit (13) can be used to navigate through the momentum map, while the projection lenses (15) zoom into the features with adjustable magnifications. Here the sp-bands of Au (111) at the Fermi edge are shown.

Real space images of (the same) Ag (111) surface terraces were acquired to compare the new Double Dispersive Energy Filter mode with the standard mode. The imaging works nicely in the new mode, with a slight compromise on the ultimate image quality.

Left: The image was acquired in the standard Aberration corrected mode. Right: The same sample was acquired with the same settings, but in the Double Dispersive Mode of the NanoESCA.

Sometimes a direct Energy Dispersion Image is of interest in some time critical experiments. A new mode offers this by projecting a momentum image into the entrance slit (8) of the Imaging Energy Analyzer and then using the Double Dispersive mode, which transfers an Energy vs. Momentum image into the second exit slit (13). 

In contrast to the Double Dispersive Imaging Mode, the exit slit is fully open in this case. This energy dispersion image does not undergo the usual Fourier transformation behind the Imaging Energy Filter, but is directly projected onto the imaging unit (16). The measurement on the right shows the surface state of a Au (111) single crystal acquired by using the Energy Dispersion Mode of the NanoESCA · MARIS.