As IEEE Spectrum’s Alexander Hellemans points out, terahertz waves are difficult to create and detect. Indeed, this frequency band is squeezed in between the far infrared and very short wave radio frequency region of the electromagnetic spectrum.
Nevertheless, a SwissFEL laser team led by Christoph Hauri at the Paul Scherrer Institute near Zurich used a common megapixel CCD device to capture images produced by terahertz waves. More specifically, the team leveraged a silicon CCD of 1360 x 1024 pixels to obtain images of THz beams with a resolution 25 times higher than currently available bolometer-based terahertz imaging systems – and at a fraction of the cost.
Image Credit: Markus Fischer/Paul Scherrer Institute (via IEEE Spectrum)
Essentially, microbolometer imagers comprise two-dimensional arrays of metal-and-insulator pixels that change their resistances when exposed to terahertz radiation. However, the arrays are slower than CCDs, with lower resolutions due to pixels that are several times larger (24 micrometers instead of 4.65 µm).
“In common CCDs, individual photons of visible light liberate individual electrons, a phenomenon known as the internal photoelectric effect,” Hellemans explained. “These electrons, have sufficient energy to cross silicon’s band gap, and end up stored in a potential well, from which they can be read out. Terahertz photons, with their longer wavelengths, carry much less energy and the dislodged electrons simply don’t make it across the band gap.”
As such, the SwissFEL group chose to engage in an experimental trick of sorts, with a mode of CCD operation that differed from those used at optical frequencies. Simply put, the researchers obtained images by exposing the CCD directly to an intense terahertz beam. Interestingly, optimizing the CCD’s sensitivity doesn’t seem to require more powerful terahertz lasers. In fact, visible images can still be obtained using just 2 to 5 percent of the laser power employed in the above-mentioned experiment.
Mostafa Shalaby, lead author of the Nature Communications paper describing the THz imaging, told IEEE Spectrum there are a wide range of additional improvements that could ultimately be implemented.
“If you use CMOS instead of CCD you have a much higher sensitivity,” he confirmed. “Also, with structured metal or metamaterials on top of the substrate, one can enhance the sensitivity of the imager.”
Commenting on the IEEE Spectrum article, Jay Endsley, the Sr. Director of Imaging Engineering at Rambus, told us it was quite impressive that SwissFEL team had leveraged a silicon CCD of 1360 x 1024 pixels to obtain images of THz beams with a resolution 25 times higher than currently available bolometer-based terahertz imaging systems.
“Using commercial, off the shelf (COTS) equipment to see terahertz waves significantly cuts costs and opens the door to future innovation. It is also particularly interesting to see how the researchers exploited a previously unused mode to accomplish this,” Endsley told Rambus Press. “In general, terahertz waves are fairly elusive because scientists lack standardized systems to measure them. Plus, they aren’t exactly simple to generate, as the waves are subject to interference from other forms of energy. This is a good first step towards designing a more practical system suitable for commercial purposes.”