The detection of light, the basis of many modern technologies, is relatively simple in the visible spectrum (region of the electromagnetic spectrum perceived by the human eye) and in the near infrared. However, detection becomes more complicated as the wavelength gets larger and smaller (and the frequency smaller) and we move towards the mid-infrared and beyond in the range of terahertz .
The reason is that in these spectral bands light carries very little energy compared to ambient heat at room temperature. This ambient ‘noise’ obscures infrared light unless specialized detectors are used that operate at very low temperatures, which is expensive and energy intensive.
These new nanoantennas and nanotechnology convert the invisible infrared light into another visible one that is more accessible for conventional detectors, which opens the door to new systems for environmental surveillance and observation of the universe
The Universitat Politècnica de València (UPV) , with its Center for Nanophotonics Technology (NTC), has participated together with researchers from the United Kingdom, Switzerland and other countries in the development and validation of new technology with which they have managed to convert infrared light into visible, a range in which it can be detected with conventional systems.
The experiments were carried out within the framework of the European project THOR and the results are published this week in the journal Science .
“The basic idea is that matter vibrates at very high frequencies, on the order of tens of terahertz. Thus we can use molecules as mixers and manage to convert the frequency of incident infrared radiation into visible light ”, explains Alejandro Martínez , researcher at the NTC and professor at the UPV.
At the moment, these results open the door to new detection systems for application in thermal imaging, observation of the universe, detection of pollutants and greenhouse gases, as well as in chemical and biological analysis. Furthermore, being able to detect light at frequencies where it is not easy to do so can lead to unforeseen applications.
Ambient temperature operation
“This technology will allow us to inspect a frequency regime in which we now detect practically nothing, because current detectors are inefficient, slow, bulky and need to operate at cryogenic temperatures ”, emphasizes Martínez. Infrared detectors now introduced, however, operate at room temperature.
Its experimental validation has been arduous: dual nano-antennas were needed that work in very different spectral regimes and that were capable of both efficiently collecting incident infrared light and locating visible light in the nanometric regions where the molecules are located.
The fundamental thing is to use gold nanostructures, which are those that allow us to capture and locate light in regions the size of the molecule
Alejandro Martínez (UPV)
“The fundamental thing is to use gold nanostructures , which are what allow us to capture and locate light in regions the size of the molecule,” explains Martínez, who has participated in both independent groups of scientists now publishing their results in Science .
The difference between both experiments is the nanoantenna used: in one carried out at the Federal Polytechnic School of Lausanne (Switzerland) they placed the gold nanoparticle inside a nanometric slot on a film also made of gold, while in another from the University of Cambridge (United Kingdom ) they placed it on a disk of the precious metal. In both cases biphenyl-4-thiol molecules were deposited in the medium.
“Our next goal is to reach lower frequencies, in the terahertz band , where there are no efficient detectors that work at room temperature, and for this, what we will do is change the molecule ”, explains Martínez, who concludes:“ We also want to implement it on a silicon chip, so the technology would be very cheap and compatible with microelectronics ”.
W. Chen et al .: “Continuous-Wave Frequency Upconversion with a Molecular Optomechanical Nanocavity”. A. Xomalis et al .: “Detecting mid-infrared light by molecular frequency upconversion with dual-wavelength hybrid nanoantennas”. Science , 2021
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