The terahertz spectrum is tucked away between more commonly known electromagnetic wavelengths, like infrared and microwave. This spectrum, and the radiation wavelengths it emits, is still a decently new field of study, and researchers have learned that terahertz radiation (THz) is chock full of information that can add value to applications in imaging and communications; THz is often used today for material analysis in laboratories, as well as in airport security checkpoints. Its wavelength, in the millimeter range, is far larger than one of visible light, and its beams can only be manipulated with special techniques.
Switching from its more recent focus on 3D printing materials, researchers at the Technical University of Vienna (TU Wien) in Austria have been studying ways to shape the perfect terahertz beam, and have succeeded thanks to a 3D printed, precisely calculated plastic screen.
The TU Wien research team can use this simple 3D printed screen to precisely shape terahertz beams into desired shapes.
“Normal plastic is transparent for terahertz beams, in a similar way as glass is for visible light. However, terahertz waves slow down a little when they pass through plastic,” explained Professor Andrei Pimenov from TU Wien’s Institute of Solid State Physics. “This means that the crests and troughs of the beam become a little displaced – we call that phase shifting.”
Think of a glass optical lens, with a middle thicker than the edge. When a beam of light hits the middle of the lens, it will spend more time in the glass than a second beam that hits the edge at the same time. This causes the middle light beam to be more phase delayed than the one on the edge, which then changes the beam’s shape – for instance, a wider beam of light can focus on a single point. The TU Wien researchers used this same type of phase shifting to shape terahertz beams, though they’re far from done with this line of study.
“We didn’t just want to map a wide beam to a point,” said Jan Gosporadič, a PhD student on Professor Pimenov’s team. “Our goal was to be able to bring any beam into any shape.”
The researchers accomplished just that by inserting a precisely adapted, 3D printed plastic screen, with a diameter of only a few centimeters, into the beam. They have to adjust the thickness of the screen, from 0 to 4 mm, during the process in order to deflect different areas of the beam in a controlled manner. Then, they will end up with the desired image or shape at the end.
“The process is amazingly simple. You don’t even need a 3D printer with an especially high resolution,” Professor Pimenov said. “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 2mm wavelength.”
The team recently published a paper on their work, titled “3D-printed phase waveplates for THz beam shaping,” in Applied Physics Letters; co-authors include Gosporadič, A. Kuzmenko, Anna Pimenov, C. Huber and D. Suess from the University of Vienna, S. Rotter, and Professor Pimenov.
The abstract reads, “The advancement of 3D-printing opens up a new way of constructing affordable custom terahertz (THz) components due to suitable printing resolution and THz transparency of polymer materials. We present a way of calculating, designing, and fabricating a THz waveplate that phase-modulates an incident THz beam (λ0 = 2.14 mm) in order to create a predefined intensity profile of the optical wavefront on a distant image plane. Our calculations were performed for two distinct target intensities with the use of a modified Gerchberg-Saxton algorithm. The resulting phase-modulating profiles were used to model the polylactide elements, which were printed out with a commercially available 3D-printer. The results were tested in a THz experimental setup equipped with a scanning option, and they showed good agreement with theoretical predictions.”
Researchers developed a special calculation method to achieve the desired design for their 3D printed screens. To show how many possible designs have been opened up through their new THz beam shaping method, the TU Wien research team 3D printed several different screens, including an option that shapes a wide beam into the easily recognizable shape of the university’s logo.
Professor Pimenov said, “This shows that there are hardly any geometric limits to the technology. 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.”
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[Source/Images: TU Wien]