A significant portion of the electromagnetic spectrum is currently used in many fields and technologies. However, the gap between radio waves and infrared radiation (known as the “terahertz gap“) has not yet been successfully harnessed for practical use.
Nobel laureate Felix Bloch theorized that a unique material could conduct terahertz gap signals as long as it permitted electrons to oscillate in a particular way. The challenge since has been to create the ideal material with these desired properties.
Finding the Right Material
Almost half a century after Bloch’s terahertz theory, Stanford researchers have created a two-dimensional superlattice material by making a sort of chemical sandwich: thin graphene just a few atoms wide was placed between two sheets of insulating boron nitride.
Stacking boron nitride and graphene on top of each other created an interference pattern known as the moiré pattern.
From inside the boron nitride bread layers, researchers found that electrons in the graphene flowed along smooth paths without deflection. Once the electrons passed through the graphene, researchers inferred the activity of the electrons still moving through the graphene.
In a moiré superlattice, it is possible to confine electrons to narrower bands of energy. When any voltage is applied, electrons are deflected. Because this deflection takes time and hence oscillates in place, the result is radiation in the terahertz gap range being emitted. This achievement is one step toward developing a way to efficiently and practically sense and control terahertz frequencies emissions.
“new junctions using the boron nitride could emit several electrons per photon.”
Using the Terahertz Gap
Researchers also found that the structure of the electrons in the superlattice material changed in an unexpected way.
When more electrons were added to the superlattice material, it became more positively charged. On the other hand, removing electrons resulted in an increase in negative charge.
It is important to note that standard electron behavior using silicon-based semiconductors is just the opposite.
This means that this discovery could be leveraged specifically to create more efficient solar panels. One photon absorbed from light shined on one p-n junction might normally send one electron, but new junctions using the boron nitride could emit several electrons per photon.