Mapping quantum structures with light to unlock their capabilities

Quantum combs illuminated: Upon light excitation (red and yellow beams), electrons are discovered to form comb-like wave patterns. The narrow width of the comb lines enables detecting (illuminated peaks) super-resolution images of quantum-material properties - much sharper than earlier efforts. Credit: Markus Borsch, Quantum Science Theory Lab

Researchers at the University of Michigan, University of Regensburg and University of Marburg propose a new tool that uses light to map out the electronic structures of crystals. This could reveal the capabilities of emerging quantum materials and pave the way for advanced energy technologies and quantum computers.

Silicon-based solar cells are already becoming the cheapest form of electricity, although their sunlight-to-electricity conversion efficiency is rather low, about 30%. Emerging “2-D” semiconductors, which consist of a single layer of crystal, could do that much better—potentially using up to 100% of the sunlight. They could also elevate quantum computing to room temperature from the near-absolute-zero machines demonstrated so far.

The ability to map these properties down to the atoms could help streamline the process of designing materials with the right quantum structures. But these ultrathin materials are much smaller and messier than earlier crystals, and the old analysis methods don’t work. Now, 2-D materials can be measured with the new laser-based method at room temperature and pressure.

The quantum mapping method uses a 100 femtosecond (100 quadrillionths of a second) pulse of red laser light to pop electrons out of the ground state and into the conduction band. Next the electrons are hit with a second pulse of infrared light. This pushes them so that they oscillate up and down an energy “valley” in the conduction band, a little like skateboarders in a halfpipe.

The team uses the dual wave/particle nature of electrons to create a standing wave pattern that looks like a comb. They discovered that when the peak of this electron comb overlaps with the material’s band structure electrons emit light intensely. That powerful light emission along, with the narrow width of the comb lines, helped create a picture so sharp that researchers call it super-resolution.

By combining that precise location information with the frequency of the light, the team was able to map out the band structure of the 2-D semiconductor tungsten diselenide.

Applications include LED lights, solar cells and artificial photosynthesis. (

The paper has been published in Science.

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