From lidar to food quality control, IR imaging technology is in increasingly high demand. Traditional imaging methods have made advances difficult, as they can ultimately block the transmission of visible light and disrupt “normal” vision. Now, researchers say clearer, more efficient IR imaging is possible.
A team from The Australian National University (Canberra, Australia), in conjunction with researchers from Nottingham Trent University (Nottingham, UK) and others, have demonstrated IR imaging via nonlinear metasurfaces (ultrathin subwavelength-scale layers), comprised of small, ultrathin, transparent semiconductor nanocrystals. In their study, the team designed “a multiresonant metasurface to enhance the field at all of the frequencies participating in the [sum-frequency generation] SFG process.”1
The metasurface was fabricated and transferred to a transparent glass, forming a layer of nanocrystals on the glass surface.
In the study, an IR image of a Siemens-star target illuminated the metasurfaces. A second beam was combined with the IR image of the target; through the SFG process, this up-converted to a visible wavelength at 550 nm. These visible green images were captured with a conventional camera and corresponded to different transverse positions of the target, “including the case when the target was fully removed from the path of the IR beam and the SFG emission from the metasurface was observed.” The team found that the images were well reproduced into the visible spectrum, regardless of different parts of the IR signal beam being upconverted by independent nanocrystals in the metasurface.
According to the researchers, the use of nonlinear metasurfaces is “a promising way to upconvert IR photons to visible and thereby image IR objects through coherent conversion using ultrathin and ultralight devices.” Also, transparent metasurfaces may be able to perform IR imaging in a transmission configuration while simultaneously transmitting visible light and allowing for “normal” vision.
The use of nonlinear optical processes to convert IR light into visible light is an all-optical alternative to traditional cameras. In this study, “electrical signals are no longer involved in the IR detection process, and the image, converted to the visible, can be captured by eye or phone-type camera.”
This new approach could bring with it opportunities not currently possible in conventional upconversion systems. The researchers cite the use of counter-propagating excitation beams, incidence at different angles, and multicolor IR imaging by an appropriately designed metasurface. This shows potential for advanced compact night vision instruments, sensor devices, and standard glasses (see figure), as well as multicolor IR imaging at room temperature.
REFERENCE
1. R. Camacho-Morales et al., Adv. Photonics, (2021); doi:10.1117/1.ap.3.3.036002.
