Overcoming the far-field diffraction limit using Stimulated Emission Depletion (STED) microscopy enabled the effective focusing of light to deep subwavelength dimensions, resulting in unprecedented resolution in all 3 axes for optical microscopy. In this paper, the authors utilized STED-inspired direct-write lithography (DWL) to significantly increase the axial resolution of optical patterning. To demonstrate the efficacy of this technique, they fabricated and characterized a novel 3-helix metamaterial structure that demonstrates circular dichroism without polarization conversion. The importance of this achievement is evident from the fact that helical metamaterials have, for a long time, borne the promise of large bandwidth and high extinction ratios, making them applicable in sensor technology, circular dichroism spectroscopy, circular polarization converters, etc.

Single-helix metamaterials are known to generate unwanted polarization conversions due to lack of rotational symmetry. This can be overcome using N-helices where N = 3 or 4 since these structures possess discrete rotational symmetry. Theoretical analysis shows that such structures exhibit zero polarization conversion and significant circular dichroism. The authors provide sound basis of this theory using simulations. Following this, they proceed to fabricate the triple-helix structures with helical pitch of 2.66μm, helix radius of 750nm and wire radius of 270nm. The high axial resolution of STED-DLW enabled the fabrication these structures. The patterns are created first in polymer templates, whose voids are filled with electrochemically deposited gold. Subsequently, the polymer is removed by oxygen plasma, leaving behind gold helix structures. The measured transmittance spectra of the triple-helix structures compare well with those of the single-helix structures. The measurements show excellent agreement with the simulations and effectively demonstrate circular dichroism, while any cross-polarization conversion is less than 2%. The demonstrated spectra is from 46 THz to 80 THz. Measurement data for the case of reflectance is also provided and shown to agree well with theory. One key challenge with the demonstrated structure is its low extinction ratio. However, by increasing the number of helix turns, this can be mitigated at the expense of a more complex fabrication process.

This work demonstrates the power of overcoming the diffraction barrier in lithography by enabling optics to achieve nanometric resolution. Compared to alternatives, the STED-DLW can not only achieve the desired 3D resolution, it can also be fast and potentially lead to a very robust fabrication process. Metamaterials in 3D are just one of the many possible applications, of which we will certainly more in the near future

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