Gratings with fine gaps fabricated for magneto-plasmonic structures with VOYAGER

Working with a research group at TU Dortmund, scientists from Raith recently published an article in Nature explaining the fabrication of a plasmonic structure on a magnetic semiconductor Cd0.975Mn0.025Te/Cd0.73Mg0.27Te with the purpose of searching for new effects in the area of magneto–plasmonics.  

A few experimental challenges had to be solved in order to reach this goal. Parameters of gratings, 185 nm wide metal stripes separated by 65 nm gaps, presented no difficulty for VOYAGER’s beam size of the order 10 nm. However the technology selected for the grating – a lift-off process – relies on good adhesion of gold to the semiconductor. Usually adhesion is ensured by a thin layer of chemically active material, Ti or Cr.  

The decision was taken to abandon this traditional way for two reasons: first, plasmonic structures gain high efficiency if Nobel, a high purity metal with large electron scattering time, is used. Pure gold completely free from impurities is the best choice for this purpose.  

Second, the excitonic luminescence in the semiconductor, which was a key ingredient in the experiment, is fragile with respect to charges on the surface of a semiconductor. Titanium and chromium can easily interact with the ambient atmosphere and generate such charges.  

Therefore the components were kept free of Ti and Cr and adhesion was provided by a short, low–energy oxygen plasma clean (descumming) directly in the metal evaporation chamber. Figure 1 demonstrates an example of the grating. 

Figure 1. SEM image of fabricated grating with period of 250 nm, gold thickness was 45 nm, the gap between stripes was about 50 nm, and grating size was 200 x 200 µm2

Discovery of a new effect claimed in the paper, namely the transverse magnetic routing of light emission (TMRLE), obliged a careful investigation for its dependence on grating period. 13 gratings with periods between 200 and 320 nm with a step of 10 nm were fabricated and the directionality in all these structures was measured. Figure 2 shows the dependence. The result is a resonance peak which occurs due to crossing of the excitonic resonance by a dispersion branch for surface plasmon polariton (SPP).

Figure 2. Directionality factor as a function of the grating period

By reversing the magnetic field the TMRLE effect turns the light emission direction by an angle of 40 degrees. The phenomenon controls the emission directionality and is important both for practical applications and basic research in the nanoscale optics. 

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