We are immersed in light; light is our main sensory source of information about the outside world. Everything that we see is not only characterized by its form and shape but to an essential part by the colour that it appears in. For example, we know that a green strawberry will not taste as good as a red one.
However, the colour red is not very specific to strawberries. Their smell, on the other hand, is. The human nose can identify a vast number of smells, each corresponding to specific molecules. We can smell a fire from a long distance and we can smell traces of poisonous gases from rotten eggs or even chlorine in the swimming pool without seeing either. Interestingly, light can be seen as an alternative way to “sniff” for molecules. Light has the unique advantage of allowing one to sense a substance without the need to be in physical contact with it. Therefore, using photonics technology to detect molecules is of great interest.
Each molecule has a very specific fingerprint in the electromagnetic spectrum from infrared to the UV that uniquely identifies it. Hence, going back to the colour analogy, each molecule has a very characteristic colour to it. The challenge is to resolve that colour against a huge background of ambient light and at a distance. Given that many otherwise colourless organic molecules have unique absorption lines in the mid and far infrared spectrum, it is possible to make large concentrations of these molecules visible using infrared cameras in the broad daylight. However, at night, or when many different molecules are in the sample, infrared imaging fails or is unspecific.
Alternatively, using laser light, we can detect traces or dilute samples of these molecules. Lasers can easily be brighter than the sun for a single colour, and that colour can be tuned extremely accurately. This means that the specificity and the sensitivity of laser light can allow, for example, the trace detection of explosives on car doors or anthrax spores on mail. Another example of where this capability can be useful is in sensing radioactive sources over a distance. Air molecules that have been exposed to nuclear radiation can be made to fluoresce in the ultraviolet using a laser, and thereby help identify containers of radioactive materials at a distance of tens of meters. However, this detection is usually done when there is some kind of substrate to which the laser light can be directed.
The challenge for remote sensing is that over increased distances the signal level tends to decrease rapidly. If atmospheric aerosols are to be detected, the detection has to occur in the km-range.
One approach to overcome this limitation is to put a laser in the sky and have it shine back at you. This could be achieved with an airplane or a remotely controlled drone, however, it is not a very practical solution. Another possibility relies on the mechanism of laser filamentation.
Laser filamentation takes place when a very powerful laser pulse is focused in the atmosphere, causing the air molecules in the discharge, or filament, to glow as in a very thin fluorescent light tube. It turns out, that the filament can be seen itself as a laser. Since the filament is aligned with the generating laser beam, it emits its own laser light directly back to the source allowing for fingerprinting of molecules along the line of sight.
At the National Research Council of Canada a team in the Quantum Photonic Sensing and Security program is developing such a free-space airlaser. This advanced technology is expected to have major impact in the areas of security sensing and resource sensing for its ability to detect traces of molecules in the atmosphere in the broad daylight with increased accuracy, and at much greater distances than is currently possible.
André Staudte is a senior research officer in the Extreme Photonics group of the Quantum Photonic Sensing and Security Program (QPSS) at the National Research Council of Canada (NRC). NRC’s QPSS program is a catalyst for photonics in Canada, offering its clients and partners access to the critical research infrastructure and expertise required to advance the technology readiness of quantum applications in three high impact areas: cyber security, resource sensing and security sensing.