Graphene light detector could put heat vision tech in a contact lens

Thermal imaging has already found its way onto smartphones, but a team of researchers from the University of Michigan (U-M) have gone even further with the creation of an ultrathin graphene-based light detector. Being only slightly thicker than two sheets of graphene, the approach has the potential to put infrared heat detecting technology into a contact lens.

Under ordinary conditions, it is impossible to detect certain light frequencies, such as infrared light, with the naked eye. However, technology has long been in use that grants us the ability to extend the spectrum of visible light. One such example of this technology being the use of infrared light detectors as a method of night vision.

The infrared spectrum, that begins just beyond the wavelengths of visible light, is made up of near, mid, and far infrared radiation. Traditionally, devices that allow for the detection of the full range of infrared frequency light require bulky cooling systems in order to function, as the device must generally be kept at very cold temperatures to allow the sensors to function properly. However, the new graphene-based technique pioneered by the U-M research team works at room temperature without requiring a cooling system, and thus can be easily miniaturized.

“We can make the entire design super-thin,” states Zhaohui Zhong, assistant professor of electrical engineering and computer science at U-M, "It can be stacked on a contact lens or integrated with a cell phone."

How does it work?

The team created the compact light detector by utilizing graphene, a wonder material with the ability to detect the entire infrared spectrum. Whilst researchers have attempted to use graphene for infrared imaging applications in the past, until the breakthrough by the U-M research team, the material had not been suitable for the task.

"The challenge for the current generation of graphene-based detectors is that their sensitivity is typically very poor," stated Zhong. In basic terms, the one-atom-thick graphene is unable to absorb enough light to produce an electric signal, rendering the material useless as a sensor.

The team solved this problem by finding a different way in which to create the electrical signal. Instead of directly measuring the electrons freed from the graphene when the light hits its surface, (the method of observation used in the past), the researchers magnified the signal by observing how the electrical charges on the graphene affected a nearby current.

The device itself was created by placing an insulating layer between two sheets of graphene, with an electrical current running through the bottom sheet. As infrared light impacted on the upper layer, electrons were freed from the graphene, creating holes that acted as a positive charge between the electrons. The electrons were then able to slip through the insulating barrier and on to the bottom layer of graphene where the team was then able to observe changes in the flow of the current running through the bottom layer of graphene. From this, the team was able to deduce the brightness of the light impacting on the upper layer and thus create a viable method for detecting infrared light that is only slightly thicker than two sheets of graphene.

The future uses of this light detecting technology could span from military applications – replacing the clunky infrared gear sported by special forces around the world – to medical innovation – aiding doctors in monitoring the blood flow of patients. There's also a possibility the technology could find more general commercial applications.

"If we integrate it with a contact lens or other wearable electronics, it expands your vision," Zhong said. "It provides you another way of interacting with your environment."

The team's research appears in the journal Nature Nanotechnology.

Source: University of Michigan