Shaping of Terahertz Beams

Terahertz radia­tion can be used for a wide variety of appli­cations and is used today for airport security checks just as much as it is for material analysis in the lab. It also requires specialised tech­niques to mani­pulate the beams and get them into the right shape. At the Tech­nical Univer­sity Wien, shaping terahertz beams is now something of a resounding success: with the help of a precisely calcu­lated plastic screen produced on the 3D printer, terahertz beams can be shaped as desired.

When the screens are inserted into the beam, the desired patterns emerge: a cross or the logo of TU Vienna. (Source: TUW)

“Normal plastic is trans­parent for tera­hertz beams, in a similar way as glass is for visible light,” explains Andrei Pimenov from the Insti­tute of Solid State Physics at TU Wien. “However, tera­hertz waves slow down a little when they pass through plastic. This means that the crests and troughs of the beam become a little displaced.” This phase shifting can be used to shape a beam. Exactly the same thing happens in a much simpler form with an optical lens made of glass.

When the lens is thicker in the middle than on the edge, a light beam in the middle spends more time in the glass than another beam that simul­taneously hits the edge of the lens. The light beams in the middle are there­fore more phase delayed than the light beams on the edge. This is exactly what causes the shape of the beam to change; a wider beam of light can be focussed on a single point. And yet the possi­bilities are still far from being exhausted. “We didn’t just want to map a wide beam to a point. Our goal was to be able to bring any beam into any shape,” says Jan Gos­poradič, a PhD student in Andrei Pimenov’s team.

This is achieved by inserting a precisely adapted plastic screen into the beam. The screen has a diameter of just a few centi­meters, its thickness varying from 0 to 4 mm. The thickness of the screen must be adjusted step by step so that the different areas of the beam are deflec­ted in a controlled way, resulting in the desired image at the end. A special calcu­lation method has been deve­loped in order to obtain the desired screen design. From this we can then produce the matching screen from an ordinary 3D printer.

“The process is amazingly simple,” says Andrei Pimenov. “You don’t even need a 3D printer with an espe­cially high reso­lution. If the precision of the structure is signi­ficantly better than the wave­length of the radiation used, then it’s enough – this is no problem for tera­hertz radiation with a 2 mm wave­length.” In order to highlight the possi­bilities offered by the technique, the team have produced different screens, including one which brings a wide beam into the shape of the TU Wien logo. “This shows that there are hardly any geometric limits to the tech­nology,” says Pimenov. “Our method is rela­tively easy to apply, which leads us to believe that the tech­nology will be rapidly intro­duced for use in many areas and that the tera­hertz tech­nology that is currently emerging will make it a bit more precise and versatile.” (Source TUW)

Reference: J. Gospodaric et al.: 3D-printed phase waveplates for THz beam shaping, Appl. Phys. Lett. 112, 221104 (2018); DOI: 10.1063/1.5027179

Link: Solid State Spectroscopy, Institute of Solid State Physics, Vienna University of Technology, Vienna, Austria

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