Context
Light has a special relationship with information. In the form of visible light, it can create images of the world in the human brain. But visible light is just a small portion of a larger electromagnetic spectrum, and that spectrum makes astonishing applications possible.
These include radio transmissions, microwave-based wi-fi telecommunications, and the medical applications associated with X-rays.
Technology aimed at informational, analytical and sensing applications are especially advanced by the study of electromagnetism.
At the University of Melbourne, there is a special focus on the infrared range that is associated with heat radiation and that makes night vision goggles possible.
Currently, certain types of imaging can be dramatically improved by a shift into the mid and long-wave range infrared (MWIR and LWIR), such as the ability to see through fog, smoke and other atmospheric obstructions, and to bring even distant objects into fine focus.
The problem with existing LWIR image sensors is that in order to work they need to be cooled, for example, with liquid nitrogen. This severely limits their operational applications.
Now, Professor of Physics and Electronic Engineering at the University of Melbourne, Kenneth Crozier, is using nanotechnology to come up with new approaches to high-performance infrared imaging technology. In the process, the new design has created novel opportunities for innovation of additional communication and sensing technology.
Professor Crozier is funded by the US Defense Advanced Research Projects Agency (DARPA) and works in collaboration with the University of California, Berkeley. The advances are targeted for military, civilian and industrial sectors.
The project
Professor Crozier wanted the project to capitalise on the improvement to infrared image quality associated with a shift to longer wavelengths. The problem he faced was that LWIR photons do not carry much energy, making them harder to detect.
Currently, semiconducting materials (made of mercury-cadmium-telluride) are used as LWIR light sensors. These materials have a ‘small bandgap’, meaning that at room temperature the sensor picks up a lot of noise, even in the absence of any light. To reduce that noise, the sensor has to be cooled to extreme temperatures.
To avoid the need for cooling, Professor Crozier focused on a different kind of semiconductor, using materials that are extremely thin and known as two-dimensional (2D) materials.
At the University of Melbourne, the focus for development work fell on graphene (at less than a billionth of a metre thick), while the University of California worked with tellurium and black phosphorus combined with molybdenum disulphide.
Making the light-sensitive region as small as possible can reduce noise, but it introduced its own challenge: it reduced the signal.
In response, Professor Crozier designed, built and tested nanostructures that function as antennae to concentrate the light signal at the 2-D photodetector. These were manufactured using gold at the Melbourne Centre for Nanofabrication.
Outcomes
When the tiny gold antennae were included in the new 2D photodetector, they were found to dramatically increase the signal, and the resulting device creates a new, more flexible basis for infrared technology development.
Professor Crozier and his team also used the nanofabrication methods to add another layer to the tiny 2D photodetector and antenna. They constructed an array of 116 nanofilters, with each one restricting the light that it transmits based on its wavelength. This is designed to sit on top of the 2D photodetector.
The team showed that by measuring the amount of light transmitted through each filter, it is possible to deconstruct the light’s makeup. Software has been developed that uses this information to produce a forensic ‘spectrum’ of the incoming light. In other words, this device can function as a miniature spectrometer, to read the chemical identity of unknown materials using infrared light.
This device has applications as a handheld spectrometer for medical, food safety, military and industrial applications, including the rapid detection of toxic gas.
In the process of optimising the photodetector’s performance, Professor Crozier’s team made an additional, serendipitous discovery.
“We were applying a voltage to the photodetector and found we could change the device’s ‘reflection spectrum’, converting it into a modulator,” Professor Crozier says. “That means you can shine a light onto the photodetector to generate a reflected light beam but use voltage to control the power of the light to introduce a signal.”
That creates communication applications, including using lasers to relay information through the ambient atmosphere. There are also uses as ‘scene projectors’ that mimic the visible light modulators used to screen movies.
“Today electromagnetic technology is a tiny fraction of what is possible,” Professor Crozier says. “So it’s exciting when a blend of physics, mathematical modelling and nanofabrication can push forward the boundaries.”