Researchers at the University of Michigan have created a device that enables them to control the flow of electrons through a semiconductor using only laser light—no electrical power source required.

This device was built ¹ to explore fundamental physics and realize a previously unobserved behavior, but it could also open doors for new applications in areas that bridge optics and electronics, including sensing, imaging and telecommunications. The phenomenon could help improve how signals are sent through and between devices, as well as create new opportunities to store more information in those signals.

“This electrical device that we manufactured at the Lurie Nanofabrication Facility has the potential to turn into something that measures different aspects of light,” said Yiming Gong, who helped lead the project as a doctoral student in the U-M Department of Physics. “But this originates from a very fundamental level of physics, which is the interference between different optical absorption processes.”

The researchers showed they could induce the orderly flow of electrons through a semiconductor using two different colors of light. By rotating the polarization of the two optical fields—the direction that the light waves wiggle—the researchers could also control the direction of the electronic current.

“This isn’t the way things normally work. When you think about electrons moving through a material, they’re moving because you’ve applied an electrical field and they actually bounce around and drift across the materials. Here, using light, you can actually sort of squirt the electrons in a specific direction without applying an electric field,” said U-M physicist Steven Cundiff, senior author of the team’s report.

Although previous work had shown the ability of light to get electrons flowing with light alone, this work goes a step further and shows the light gets electrons flowing in a narrow beam in a specific direction.

“The light no longer merely switches the current on; it also aims it,” Cundiff said.

Cundiff likens this to a lighthouse, which sweeps a beam of light by rotating its lamp. Here, the beam is made of electrons and that beam can be rotated by rotating the polarization of the two phase-coherent optical fields.

The phenomenon relies on quantum interference, which arises when two colors of light drive different absorption pathways to the same final state. The light transfers energy to the semiconductor material in discrete, or quantized, packets called photons, which mobilize the material’s electric charge carriers.

In the set-up, the incident light is absorbed by two routes at once, and the routes can be thought of as overlapping ripples, Cundiff said. Those ripples line up and enhance each other for electrons traveling a certain direction, but cancel each other out for electrons moving in other directions.

A collaborator on an earlier project, J.E. Sipe of the University of Toronto, had predicted it would be possible to create such an “electron lighthouse.” Now, working with the Lurie Nanofabrication Facility, Gong was able to realize such a device.

“The LNF is an amazing facility,” Gong said, adding it was a painstaking process to meld the device’s materials together in a way that didn’t introduce any extraneous electric fields.

“That was the biggest puzzle to solve for me, because there isn’t a standard way to do that. So I worked with the LNF staff to play around with different recipes and temperatures to come up with a manufacturing process.”

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Sugars are key biomolecules in living organisms, as they form the backbone of DNA and RNA and play a fundamental role in metabolic processes. In theories of the origin of life, sugars are also essential for the synthesis of the first nucleic acids. Despite their importance, one of the major questions in origin-of-life research is how the first sugars formed on Earth, since laboratory experiments show that they do not form in enough quantities under prebiotic conditions. Sugars such as ribose and glucose have previously been detected in meteorite and asteroid samples, suggesting that some of these molecules may have originated in the primordial molecular cloud from which our Solar System formed. However, until now, no sugar had ever been directly detected in the interstellar medium.

An international team led by Centro de Astrobiología (CAB, CSIC-INTA) researcher Izaskun Jiménez-Serra has now identified ¹ the first sugar in interstellar space: erythrulose. This molecule is the only possible four-carbon ketose, and on Earth it is commonly found in raspberries and sunless tanning products. Erythrulose was detected toward the molecular cloud G+0.693−0.027, located near the centre of our Galaxy, the Milky Way. The discovery was made possible by ultra-sensitive, broadband spectroscopic surveys carried out with the 40-m Yebes radio telescope and the 30-m telescope of the Institute for Radio Astronomy in the Millimeter Range (IRAM).

The team identified 12 spectral lines matching the laboratory spectrum of erythrulose measured at the University of the Basque Country. The study also shows that this sugar is at least eight times more abundant than similar three-carbon sugars, none of which were detected in the same region. “This finding was unexpected, as the prevailing view in astrochemistry is that interstellar molecules grow in size through the sequential addition of carbon atoms”, says Izaskun Jimenez-Serra.

Working in collaboration with chemists from the University of Extremadura and Radboud University (the Netherlands), the CAB team discovered that erythrulose can form within interstellar ices from simpler two-carbon alcohols and aldehydes.

Based on the abundance of erythrulose measured in the G+0.693−0.027 molecular cloud, the researchers estimate that between 0.5 and 50 million tonnes of this sugar could have reached Earth’s surface during the Late Heavy Bombardment, which occurred approximately 4.1 to 3.8 billion years ago. The presence of erythrulose in interstellar space therefore provides an alternative source of sugars that may have contributed to the emergence of the first metabolic and replication processes on the early Earth.

“The detection of erythrulose is very exciting because it opens up the possibility of discovering in space other sugars such as ribose, which is part of RNA, and other important molecules for the origin of life,” says Carlos Briones, co-author of the study.

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For millennia, farming in Switzerland did not reduce plant diversity but helped increase it, researchers have shown in a detailed reconstruction covering the past 7000 years. Only recent decades paint a different picture.

The fall of the Roman Empire and major plague outbreaks: These events not only affected people but also reduced plant diversity on the Swiss Plateau, as human land use temporarily declined. Researchers led by Dr. Fabian Rey and Professor Oliver Heiri at the University of Basel report ¹.

Their analyses are based on sediment deposits in three Swiss lakes: Moossee near Bern, Burgäschisee near Herzogenbuchsee, and Hüttwilersee in Thurgau. The researchers extracted sediment cores from these lakes and then analyzed the material deposited in the sediments over the course of millennia. This allowed them to infer and date variations in both plant diversity and agricultural land use in the areas surrounding the lakes.

“This is an exceptionally comprehensive and precisely dated dataset,” says Oliver Heiri. “It allows us to reconstruct changes in plant diversity around the lakes over the past 7000 years at a time resolution comparable to long-term ecosystem studies in modern ecology – and for a period long before modern ecology existed.”

Agriculture made the landscape more diverse

Since the Neolithic period, plant diversity increased with increasing agricultural activity. “You might think that human impact must be bad for plant diversity, because that’s what we see today,” says Fabian Rey. “But agriculture back then made the landscape more diverse.” Before early agricultural activities, the Swiss Plateau was largely covered by forest and was therefore a relatively uniform ecosystem.

“As agriculture expanded, a mosaic of habitats emerged over time,” says Rey. Fields, pastures, hedgerows, and later orchards of tall fruit trees alternated across relatively small areas. This provided varied conditions for plants adapted to those specific environments.

However, there were also recurring periods when plant diversity declined sharply: For example, during the Migration Period following the fall of the Roman Empire, or when plague outbreaks claimed many lives in the Middle Ages. “In times when people were less able to continue farming, the forest grew back and plant diversity declined at the landscape level,” explains Heiri.

Times of crisis caused diversity to decline

More agriculture, more biodiversity: However, this parallel trend lasted only until around World War II. Over the past 80 years, plant diversity has declined sharply. The research team attributes this to the intensification of agriculture since then.

Instead of a fragmented mosaic of habitats, large, uniform areas have emerged that are easier to farm using heavy machinery. The increasing use of fertilizers and pesticides also caused many specialized plant species to retreat.

“However, our data also show that plant diversity has recovered from earlier declines when people returned to farming practices that included varied landscapes,” says Rey. This suggests that the trend of the past 80 years could also be reversed if farming practices change again.

Pollen from seven millennia

The data for the study are based on more than a decade of analyses of sediment cores, conducted in close collaboration with researchers from the University of Bern and the Department of Archaeology of Canton Thurgau. To collect the data, Fabian Rey extracted pollen samples from every centimeter of the cores, which he chemically processed, prepared, and analyzed under a microscope. For each sample, he identified 500 pollen grains, enabling him to determine the diversity of plants around the lake.

The intensity of agricultural use during each period was determined based on the presence of specific pollen in the samples – including both cultivated plants and species that thrive on farmland – as well as a comparison with archaeological and historical data.

The scientists dated the layers using the 14C method, a technique for determining the age of organic material. The sediment cores date back approximately 7000 years, to the Neolithic period.

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