Researchers have developed a technique for imaging terahertz (THz) photocurrents with nanoscale resolution, applicable to visualising compressed THz waves (plasmons) in a graphene photodetector.
The extremely short wavelengths and highly concentrated fields of these plasmons open new venues for the development of miniaturised optoelectronic THz devices.
Radiation in the THz frequency range is attracting large interest because of its manifold application potential for non-destructive imaging, next-generation wireless communication or sensing.
However, the generating, detecting and controlling of THz radiation faces numerous technological challenges. Particularly, the relatively long wavelengths of THz radiation require solutions for nanoscale integration of THz devices or for nanoscale sensing and imaging applications.
Now, researchers at CIC nanoGUNE, Spain, in collaboration with The Institute of Photonic Sciences (ICFO), Spain, Istituto Italiano di Tecnologia (IIT), Italy – members of the EU Graphene Flagship – Columbia University, US, Radboud University, the Netherlands, the National Institute for Materials Science (NIM), Japan, and Neaspec, Germany, could visualise strongly compressed and confined THz plasmons in a room temperature THz detector based on graphene.
To see the plasmons the scientists recorded a nanoscale map of the photocurrent that the detector produced while a sharp metal tip was scanned across it. The tip focused the THz illumination to a spot size of about 50 nanometres. This new imaging technique, named THz photocurrent nanoscopy, provides unprecedented possibilities for characterising optoelectronic properties at THz frequencies.
Former nanoGUNE researcher Pablo Alonso, now at the University of Oviedo, Spain, and first author of the work, said: “In the beginning we were quite surprised about the extremely short plasmon wavelength, as THz graphene plasmons are typically much less compressed.
“We managed to solve the puzzle by theoretical studies, which showed that the plasmons couple with the metal gate below the graphene.
“This coupling leads to an additional compression of the plasmons and an extreme field confinement, which could open the door towards various detector and sensor applications.”
The technique for THz photocurrent nanoimaging could also assist the study of other 2D materials, classical 2D electron gases or semiconductor nanostructures.