A research work published in Nano Letters explains the advance of a graphene-enabled detector for terahertz light that is more sensitive and rapid when compared to current room-temperature technologies.
(Left) Schematic representation of the central part of the graphene-based THz photodetector device, containing the hBN-encapsulated graphene channel, on top of the narrow-gap antenna structure. By applying distinct voltages to the left and right antenna branches, a pn-junction is created in the graphene channel with unequal Seebeck coefficients on the left and right of the junction. Incident light is focused by the antenna above the gap, which is where the photoresponse is generated. (Right) Measurement of a THz focus, obtained by scanning the THz detector in the plane of the focus. The observation of several rings of the Airy pattern indicate the high sensitivity of the detector. (Credit: ICFO)
It is very helpful to detect terahertz (THz) light due to two major reasons. First, THz technology is turning out to be a vital component in applications concerning wireless data communication, security (such as airport scanners), and quality control, to name a few. However, existing THz detectors have exhibited strong restrictions when it comes to simultaneously fulfilling the demands for speed, sensitivity, being able to operate at room temperature, spectral range, and so on.
Second, the low-energy photons render it a very safe type of radiation, with over a hundred times less energy when compared to those photons in the visible light range.
Several graphene-based applications are likely to become known from its use as material for detecting light. In comparison with the standard materials used for photodetection, such as silicon, graphene has the specialty of not possessing a bandgap. Due to the bandgap in silicon, incident light with wavelengths greater than one micron is not absorbed and hence not detected.
On the contrary, graphene can absorb and detect even terahertz light with a wavelength of hundreds of microns. Graphene-based THz detectors have offered promising results until now; however, none of the detectors to date could defeat commercially available detectors when it comes to sensitivity and speed.
In a recent research, ICFO scientists Sebastián Castilla and Dr Bernat Terrés, headed by ICREA Prof. at ICFO Frank Koppens and former ICFO researcher Dr Klaas-Jan Tielrooij (now Junior Group Leader at ICN2), together with researchers at CIC NanoGUNE, NEST (CNR), Nanjing University, Donostia International Physics Center, University of Ioannina and the National Institute for Material Sciences, have managed to get over these issues. They have created a new graphene-enabled photodetector that functions at room temperature, is very quick and highly sensitive, has a broad dynamic range, and includes a wide range of THz frequencies.
In their experiment, the researchers could optimize the photoresponse mechanism of a THz photodetector using the below-mentioned strategy. They incorporated a dipole antenna into the detector to focus the incident THz light around the antenna gap region. By making an extremely small (100 nm, around one thousand times smaller than the thickness of a hair) antenna gap, they were able to acquire a great intensity concentration of THz incident light in the photoactive region of the graphene channel.
They noticed that the light absorbed by the graphene forms hot carriers at a pn-junction in graphene; then, the uneven Seebeck coefficients in the p and n regions create a local voltage and current through the device producing a very large photoresponse, thereby resulting in a very high sensitivity, high-speed response detector, with a broad dynamic range and a wide spectral coverage.
The outcomes of this research pave the way for the development of a completely digital low-cost camera system. This could be as economical as the camera inside the smartphone because such a detector has demonstrated to have an extremely low power consumption and is wholly compatible with CMOS technology.
This study was funded by the Cellex foundation, the Graphene Flagship, and also a Mineco Young Investigator grant.
