Human eye is a perfect example of an optoelectronic device. As light is focused into the retina, it is converted into electrical signals that are then processed in our brain. Such a device is known as a photodetector.
What is interesting is that “light” is not just what we see, because what is visible is only an extremely small part of the entire electromagnetic spectrum. This portion is called “visible” spectrum, just because it is “visible” to us, humans.
What is even more interesting is that other animals, such as some fishes or birds, are able to convert, for instance, ultraviolet (UV) light (that is invisible to our eyes), meaning they see different colors for different UV radiations.
On the other side of the spectrum lies the Infrared Radiation (IR), which is, again, invisible to our eyes. This is the reason why we don’t see, for example, the IR rays emitted from our television remote control.
Well, some animals, like snakes, are able to see IR light. The amazing feature of IR is that any warm body naturally emits IR.
This means that we just “shine” (IR) light from our body, but we don’t notice it because our eyes are not designed to see such light. Snakes exploit exactly this mechanism and they are able to spot a warm-blooded prey (e.g. a mammal) in the total darkness; just because such a prey is warmer than the environment and, therefore, emits more IR light.
MIMICKING SNAKE EYES
The technology to realize devices that “mimic” the snake eyes already exists and it is used to make “thermal cameras”. The main problem is that they are very expensive (often more than £100k) and their performance are not really high (i.e. the image contrast is poor).
One parameter to assess how good are these particular devices, called “bolometers,” is the temperature coefficient of resistance (TCR), which is usually between 2 and 4 %/K.
A recent research, in collaboration with Nokia technologies, ICFO, Spain and the University of Ioannina in Greece; researchers have designed, fabricated and demonstrated a graphene-based bolometer.
This device works at room temperature (i.e. it does not require low temperatures to operate) and it shows ultra high performance: a TCR as high as 900%/K. This is because of the extraordinary properties of graphene and the implementation of a novel device concept.
APPLICATIONS OF THIS TECHNOLOGY
This technology can be used in astronomy (to see “special” stars or other invisible features), security (for the human detection, such as alarms, human tracking, etc), medicine (imaging diseases) and automotive (to see in the dark). Another example would be using heat sensing to enable a firefighter to spot the exact location of a fire.
This report is written in collaboration with Ugo Sassi and his original research is published in Nature Communications. Ugo is working at the Nokia Bell Labs and currently finishing his PhD degree at the Engineering Department of the University of Cambridge. His main field of expertise is the fabrication and characterization of opto-electronic devices based on graphene and two-dimensional (2-D) materials.
Edited by Ken Adams.