“The excitons we observed can be tuned using an electrical field, have a high quality factor, strongly absorb light and lie in the technologically important mid-infrared to terahertz wavelength range,” explains team member and lead author of the study Long Ju, who is at Cornell University. “No other conventional semiconductor contains such excitons.”

Graphene is a sheet of carbon atoms just one atom thick arranged in a honeycomb lattice. It is a semi-metal and does not contain a bandgap in its pristine state. Bilayer graphene is different, however, in that a large and tunable bandgap can be induced in it using an applied electric field – something that cannot be done for single-layer graphene.

Researchers theorize that bilayer graphene also supports tunable excitons (electron-hole pairs) but these had never been actually observed in an experiment until now.

Electron-hole pairs produce a significant photocurrent

A team led by Paul McEuen at Cornell, Feng Wang at UC Berkeley, Jiwoong Park of the University of Chicago and James Hone of Columbia University has now observed excitons in high-quality hexagonal boron nitride-encapsulated bilayer graphene (hBN-BLG) devices. The researchers made their device by placing BN-BLG-BN stacks on a piece of graphite local back gate and depositing a 14-nm layer of nickel/chrome on top as a semitransparent top gate. They then used source and drain electrodes to apply voltage to the device and measured the photocurrent generated.

When illuminated with infrared light, electron-hole pairs are generated in the BLG and they produce a significant photocurrent, which is proportional to the amount of light absorbed by the bilayer material. McEuen and colleagues then obtained optical absorption spectra using a modified Fourier transform infrared (FTIR) spectroscopy technique and found two prominent exciton resonances. They found that they could tune the frequencies of these resonances across a large wavelength range (from the mid-infrared to terahertz) by applying various magnitudes of electric fields.

The researchers then looked at how the excitons in BLG behave under an applied magnetic field and observed a very large magnetic moment that originates from pseudospins in the material. At the microscopic level, the magnetic moment of this pseudospin is a manifestation of the so-called Berry curvature effect, which determines how electron states evolve in external fields. “Bilayer graphene provides a model system to understand this effect in BLG and indeed other materials,” says Ju.

BLG excitons obey quite different optical selection rules

And that is not all: the Cornell team also found that the excitons in BLG obey optical selection rules that are quite different to those in conventional semiconductors. “These rules determine whether a specific optical transition is allowed to occur in a material and they can be understood as the conservation of angular momentum before and after the material has absorbed a photon,” explains Ju. “The pseudospin in graphene is just like real electron spin and carries angular momentum, which thus affects the rules. Indeed, we found that that BLG has a ‘winding number’ of two, different from that in monolayer graphene or other materials.”

Long says that key to his and his colleagues’ discovery, which they report in Science DOI: 10.1126/science.aam9175, was their photocurrent spectroscopy measurement technique. “Conventional optical spectroscopy is not suitable for studying our BLG device since its size is much smaller than the diffraction limit of the infrared beam,” he says. “We overcame this problem by collecting the photocurrent generated by optical absorption in the device. This approach is not limited by the size of the sample or indeed light wavelengths, thus providing a much better signal-to-noise ratio than other optical absorption methods.

“Another important element is the high quality of the BLG device, made by encapsulating it in hBN. This reduces disorder form oxides and allows us to observe the material’s intrinsic properties,” he adds.

A platform for studying exciton physics

Observing tunable excitons in the mid-infrared to terahertz range of the electromagnetic spectrum will be important for a number of technological applications as well as for fundamental physics, he tells nanotechweb.org. “BLG provides a platform in which we can study exciton physics in a tunable semiconductor system and could help us better understand phenomena like many-body interactions and their interplay with a material’s electronic band structure and magnetic pseudospin.

“Technologically speaking, the high-quality factor and the fact that excitons can be tuned with an electric field open up optical and optoelectronic applications, such as photodetectors and light-emitting diodes. The mid-infrared and terahertz range is crucial for molecular spectroscopy, thermal imaging and astronomical applications, so these could benefit too since technology in this wavelength range is underdeveloped compared to the visible range. Only a handful of semiconductors have bandgaps in the mid-IR to terahertz and BLG is a new and unique one – especially since its bandgap is electrically tunable.”