Work by an international group of researchers is fine-tuning the way our eyes detect and ultimately see light via frequency upconversion. They say it could advance several technologies, including spectroscopy, imaging, and sensing.
According to the team, we are able to see well in the visible spectrum: between 400 and 750 trillion Hertz. At lower frequencies, “energy transported by light isn’t enough to trigger photoreceptors in our eyes and in many other sensors.” This presents a problem, given the “rich information” within frequencies below 100 THz—mid- and far-infrared (mid- and far-IR) spectrum.
In a study published in Science, the researchers have exhibited a way around that problem, thanks to continuous-wave frequency upconversion. With the development of a new device, they have discovered it’s possible to detect IR light—by changing its frequency to that of visible light, subsequently extending the sight of common, very sensitive light detectors.
“So far, however, the device’s light-conversion efficiency is still very low,” says Dr. Wen Chen, a scientist at École Polytechnique Fédérale de Lausanne (EPFL) and first author of the study, noting that the team is now working on ways to improve it. Increasing the efficiency of the new device’s light conversion will be key in developing any commercial applications.
As noted in the study, “molecules have rich signatures in their spectra at infrared wavelengths and are typically accessed with dedicated spectroscopic instrumentation.” The team “demonstrated optomechanical frequency upconversion from the mid-IR to the visible domain using molecular vibrations, coupled to a plasmonic nanocavity at ambient conditions.”
Their work involved using a mediator—tiny vibrating molecules—to add energy to IR light. That light is then directed to the molecules; there, it is transformed into vibrational energy. The researchers found that a higher-frequency laser beam interrupts those same molecules for extra energy, allowing the vibration to be converted from IR to visible light (see figure). The molecules are “sandwiched between metallic nanostructures,” which perform as optical antennas, to boost the frequency conversion process—in all of this, the IR light as well as the laser energy are concentrated at the molecules.
“The new device has a number of appealing features,” says lead researcher Christophe Galland, a professor in EPFL’s School of Basic Sciences. “First, the conversion process is coherent, meaning that all information present in the original IR light is faithfully mapped onto the newly created visible light.” He notes that this allows high-resolution IR spectroscopy to be performed with standard detectors, such as those found in cell phone cameras. “Each device is about a few micrometers in length and width, which means it can be incorporated into large pixel arrays.”
Galland adds that the method is “highly versatile and can be adapted to different frequencies by simply choosing molecules with different vibrational modes.”
The research team was comprised of scientists from EPFL (Switzerland), Wuhan Institute of Technology (China), Valencia Polytechnic University (Spain), and AMOLF, a research institute in the Netherlands.
