High-speed magnetic bearing chopper for infrared nanospectroscopy

08. November 2021

Infrared (IR) nanospectroscopy is a non-destructive method for the analysis of molecular structures below the wavelength scale. It is used in a broad range of research applications spanning from biochemistry to geology. A magnetic bearing chopper is a core component to bring this IR method to the next level of spatial resolution. It provides the beam chopping functionality in vacuum, at high frequency and with very low jitter.

Field of application
Infrared nanospectroscopy is used in applications such as medical biochemistry and histology, physical chemistry of materials, studies of cultural heritage and archaeology, biomineralogy and geology [1].

Working principle of IR microspectroscopy

A sample is exposed to the broadband IR microbeam of a synchrotron e.g. at Diamond Light Source. The IR beam absorbed by the molecules in the sample results in a tiny temperature change (less than 10 millidegrees) and a thermal expansion. This mechanical change is measured by an atomic force microscope (AFM). The AFM consists of a very fine probe, mounted on a sensitive cantilever arm. The topography is measured by following  the surface via the cantilever’s arm while moving the sample. As this thermal expansion is very small it has not been possible to directly resolve and measure an IR photothermal spectrum at the sub micrometre scale so far.

Improved infrared microspectroscopy

The IR beamline B22 scientists at Diamond Light Source improved this method by modulating the IR beam with an optical chopper at the mechanical resonance of the AFM cantilever. Larger mechanical signals are now produced and a spatial resolution to the 100 nm scale is obtained in the IR map. This method is known as resonance enhanced atomic force microscope infrared spectroscopy (RE-AFM IR).
The first two mechanical resonances of the AFM cantilever, at 63 kHz and 188 kHz (Figure 1), are used. The sensitivity and resolution allowed by the second resonance mode is higher than the first. However, the thermal expansion drops at higher modulation frequencies ~1/f, but the spatial resolution is better at higher chopper frequencies because of the smaller thermal diffusion of the IR excitation and reduction in noise. Therefore, it is preferred to operate the chopper at the second resonance mode of 188 kHz. This frequency was not achievable with the standard chopper technology (limited to <100 kHz) at Diamond Light source  [2, 3].
Besides requiring a high chopper frequency, it is very important to have a very low jitter on the chopped IR beam, as any jitter reduces the contact mode resonance frequency and with that the resolution. Furthermore, any vibration of the chopper will be propagated, even when damped, to the cantilever of the AFM and will generate vibrational noise on the measurement, which must be reduced to minimum.

                                              

Figure 1: Calculated power spectral density (PSD) of the cantilever deflection response to a 1 pm expansion of the sample. The first two resonance modes at 63 kHz and 188 kHz are clearly visible, reprinted from [3].

The chopper motor for RE-AFMIR

For this project, Celeroton developed a customized chopper rotor to fit the existing standard magnetic bearing CM-AMB-400 motor. With that, the development time and cost could be reduced compared to a new chopper motor design. The rotor has 57 holes in axial direction for chopping the beam and can be operated up to 211 krpm, yielding a chopper frequency of up to 200 kHz. The low jitter (<1%) of the chopped beam is achieved by precision manufacturing of the rotor (specifically the holes), fine balancing of the rotor and an accurate rotational speed control. The low vibration values are possible due to advanced control algorithms that use a notch filter to rotate the rotor around the centre of inertia and thus preventing the dominant rotational speed synchronous vibrations. This is only possible due to the active magnetic bearings.

                                                  

Figure 2: Celeroton’s magnetic bearing motor CM-AMB-400 (left) with customized chopper rotor (right) with 57 holes developed for the IR beamline B22 at Diamond Light Source.

The high-speed beam chopper has been integrated into Diamond’s RE-AFMIR spectroscope as shown in Figure 2. First measurements from the new chopper have shown promising results, which will be part of a scientific publication under preparation.

                                                            

Figure 3: Optical setup of RE-AFMIR with Celeroton’s active magnetic high-speed beam chopper within a vacuum box at the IR beamline B22 at Diamond Light Source.

The chopper has been developed, in close collaboration between Celeroton and Diamond Light Source, as a dedicated project upgrade for the IR beamline B22 with a team lead by Dr Gianfelice Cinque (Principal Beamline Scientist).

At the end of the project, Dr. Mark Frogley (Senior Beamline Scientist at Diamond Light Source) stated: “We have ended up with a system that we can be proud of and that will genuinely make a big difference to the Diamond IR nanospectroscopy as well as other scientists in the future.”

With the customized magnetic bearing motor CM-AMB-400, Celeroton provides an ultra-fast chopper, with low jitter and vibrations, which will enable researchers a closer insight to organic and inorganic matter and other experimental results.


References

[1]    G. Cinque et al., “World First for Diamond in Synchrotron-Based IR Photothermal Nanospectroscopy,” Synchrotron Radiation News, vol. 29, no. 4, pp. 37–39, 2016, doi: 10.1080/08940886.2016.1198675.
[2]    K. L. A. Chan et al., “Synchrotron Photothermal Infrared Nanospectroscopy of Drug-Induced Phospholipidosis in Macrophages,” Analytical chemistry, vol. 92, no. 12, pp. 8097–8107, 2020, doi: 10.1021/acs.analchem.9b05759.
[3]    M. D. Frogley, I. Lekkas, C. S. Kelley, and G. Cinque, “Performances for broadband synchrotron photothermal infrared nano-spectroscopy at Diamond Light Source,” Infrared Physics & Technology, vol. 105, p. 103238, 2020, doi: 10.1016/j.infrared.2020.103238.

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