The demand for detecting infrared light, invisible to human eyes, is constantly growing, due to a wide variety of applications ranging from food quality control and remote sensing to night vision devices and lidar. Commercial IR cameras require the conver­sion of infrared light to electrons and the projection of the resultant image on a display. This display blocks the transmission of visible light, thereby disrupting normal vision. Moreover, such IR detectors require low tempera­ture and even cryogenic cooling due to the low energies of the IR photons, making IR detectors bulky and heavy.

An all-optical alternative to traditional cameras is the use of a nonlinear optical process to convert IR light into visible. In this case, 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. The optical process employed in this technique is nonlinear sum-frequency gene­ration (SFG). In the SFG process, two incident photons, one of them in the IR spectrum, interact within a nonlinear material to generate emission at higher and visible fre­quencies. However, in the usual approaches this conversion relies on bulky and expensive nonlinear crystals.

A very attractive platform to overcome these limitations is the use of ultrathin nanocrystal layers known as meta­surfaces. Meta­surfaces are planar arrays of densely packed nanoantennas, designed to manipulate various pro­perties of the incident light including its frequency. Among various examples, dielectric and semi­conductor meta­surfaces have shown great promise to enhance nonlinear optical processes at the nanoscale. Such metasurfaces can exhibit enhanced frequency conver­sion due to the excitation of optical resonances and good coupling to free space. Thus, the use of nonlinear meta­surfaces is a promising way to up-convert IR photons to visible and thereby image IR objects through coherent conversion using ultrathin and ultralight devices. Importantly, transparent meta­surfaces could perform IR imaging in a trans­mission configuration and simul­taneously transmit visible light to allow for normal vision.

With this idea in mind, researchers from the Australian National University, Nottingham Trent University, and colla­borators worldwide managed to demons­trate IR imaging via nonlinear meta­surfaces composed of small semi­conductor nanocrystals. The researchers designed a multiresonant metasurface to enhance the field at all the frequencies parti­cipating in the SFG process. The designed metasurface was fabricated and transferred to a transparent glass, forming a layer of nanocrystals on the glass surface. In the experiment, an IR image of a Siemens-star target illu­minated the meta­surfaces. The IR image of the target was mixed with a second beam and, through the SFG process, up-converted to a visible wavelength at 550 nanometers. The visible green images, captured with a conven­tional camera, correspond 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. Despite different parts of the IR signal beam being up-converted by independent nanocrystals composing the meta­surface, the images were well reproduced into the visible.

The proposed metasurface-based IR imaging approach offers novel oppor­tunities not possible in conven­tional up-conversion systems. For example, the use of counter-propagating excitation beams, as well as incidence at different angles and, most impor­tantly, multicolor IR imaging by an appro­priately designed meta­surface. Therefore, the results obtained by the researchers can benefit the future development of compact night vision instruments and sensor devices, offering an ultrathin and ultra­compact platform and new func­tionalities such as multicolor IR imaging at room temperature. (Source: SPIE)

Reference: R. Camacho-Morales et al.: Infrared upconversion imaging in nonlinear metasurfaces, Adv. Phot. 3, 036002 (2021); DOI: 10.1117/1.AP.3.3.036002

Link: Dept. of Electronic Materials Engineering, Australian National University, Canberra, Australia