Prof. Ruihua He's team at Westlake University discovered the first photocathode quantum material, whose performance shatters known records by at least an order of magnitude. The observed emergence of coherence points to the development of an underlying novel process on top of those encompassed in the current theoretical photoemission framework, opening new prospects for research and development, applications and theory progress in the field.

The paper titled “Anomalous intense coherent secondary photoemission from a perovskite oxide” was published in Nature on March 8. Ph.D. students Caiyung Hong, Wenjun Zou and Pengxu Ran are co-first authors, while Ruihua He, associate professor at the School of Science, is a corresponding author. All experiments and theoretical work were done at Westlake University.

Link to the paper in Nature：

https://www.nature.com/articles/s41586-023-05900-4

Strontium titanate, the first photocathode quantum material.

01

Photocathode 

One of the cornerstones of modern science

German physicist Heinrich Hertz observed sparks when ultraviolet rays hit metal surface electrodes in 1887. In 1905, Einstein came up with a theoretical explanation based on the conjecture of the quantization of light, for which he was awarded a Nobel Prize in Physics in 1921. That ushered the world into the era of quantum mechanics, where concepts such as the photoelectric effect and photocathode material caught the attention of scientists.

As scientists learned more about the photoelectric effect, they developed better theories to explain the basic properties of all photocathode materials, and successfully predicted the existence of photocathode materials. Existing photocathodes such as conventional metals and semiconductors were mostly discovered six decades ago, and they now serve as the core components of cutting-edge technological devices such as particle accelerators, free electron lasers, ultrafast electron microscopes, and high-resolution electron spectrometers. They are seen often in the labsbut  and also daily life. For example, particle accelerators are now used to treat cancer, kill bacteria, develop packaging materials, and improve fuel injection in cars. The efficacy of photocathode directly affects the performance of such devices.

However, traditional photocathode materials suffer from poor coherence: The emission angle of the electron beams is too big, and the velocity of the electron movement can be unstable. It takes a series of processes and electrical engineering techniques to enhance the coherence. This increases the complexity of the electron gun system and in turn drives up the cost and production threshold.

02

Strontium titanate

A potential reboot for the photocathode

Though the photocathode-based electron gun system has made significant progress over the past decades, it no longer meets the needs of material science. Cutting-edge science demands an upgrade of an order of magnitude, requiring innovations at the material and theoretical levels.

He’s team, which has long been invested in the research of physical properties of materials,  made a breakthrough on a common lab substance: strontium titanate.

In recent years, the rich features and diverse properties of quantum materials have theirdriven rising interest in them, including the perovskite-based strontium titanate (SrTiO₃) . The subtle and complex phase transition of strontium titanate inspired Nobel laureate Karl Alexander Müller to dub the material the “Drosophila of solid-state physics”, after the ubiquitous fruit fly whose many changes of structure formed the basis for much of modern genetics.

In the past, research on strontium titanate and its derivative oxide quantum material only explored it as a potential substitute for silicon-based semiconductors and focused on unique electronic-related properties. But He and his team discovered it was also capable of triggering a novel photoelectric effect, which distinguished it from all existing photocathode materials.

In comparison to other materials, the initial electron beam ejected by strontium titanate is significantly superior. The electron emission kinetic energy divergence is less than 0.01 eV (b), while the divergence angle is less than 2° (a).

A reviewer for Nature wrote, “I think the most important aspect of the photocathode performance of SrTiO₃ is perhaps an orders-of-magnitude improvement on the electron beam coherence as compared with other existing photocathodes under similar experimental conditions. Such a giant leap in performance allows the inherently coherent electron beam to be obtained in full without the need of sacrificing intensity for coherence. This finding will likely lead to a paradigm shift in the photocathode technology that has long been suffering from the intensity-coherence trade-off as imposed by the incoherent nature of the initial electron beam.”

Changxi Zheng, co-author of the paper and expert on the ultrafast electron microscope, said that the importance of their work went beyond adding one more magical feature to strontium titanate. “Very likely, this discovery would change the rules of the game. It might have rebooted photocathode technology, which is generally considered a mature field of research.”

03

Angle-resolved photoelectron spectroscopy 

Defeat the magician with his own tricks

He owed his discovery to angle-resolved photoelectron spectroscopy.

Over the past 135 years, research on the photocathode was mainly conducted with photocurrent detection to work with the polycrystalline or amorphous structures on the surface of traditional photocathode materials. This excluded newer equipment such as angle-resolved photoelectron spectroscopy.

Angle-resolved photoelectron spectroscopy is used to detect the electron structure of the material and understand how electrons move in them. Over the past decades, it has been mostly used to study the electronic structure related to the optical, electrical, and thermal properties of materials. As a result, many experiments maximized electron structure measurements in their design.

He’s team used the quantum material research tool derived from the photoelectric effect, and unexpectedly captured the unique photoemission characteristics of single crystal quantum materials.

By applying “irregular” settings to the angle-resolved photoelectron spectroscopy to realize the electronic structure measurement related to the photoelectric effect in the unconventional energy region, He’s team found that the superior photocathode performance of strontium titanate came from its unique photoemissive properties. “The observed emergence of coherence in secondary photoemission points to the development of an underlying novel process on top of those encompassed in the current theoretical photoemission framework,” said the press release.

The difference between the initial electron beams ejected by a) conventional photocathode materials and b) strontium titanate.

Prof. Arun Bansil of Northeastern University, a  co-author of the paper, said, “It shows that something fundamental is missing from our complete understanding of the physical processes involved in the photoelectric effect. This missing element could be the key to unlocking an entire family of photocathode quantum materials with unique photocathode properties not found in existing materials.”

04

Looking ahead 

From theory to application

Discovery is only the first step into the vast world of knowledge. After publishing their findings, He and his team quickly went back to work.

Caiyun Hong, Ph.D. student in the 2019 cohort and a co-first author of the paper, said they would move their focus to the theory and application of strontium titanate materials.

Since existing theories have failed to explain the observed performance of strontium titanate photocathodes, new ones are needed. He proposed to Bansil to establish a new photoemission mechanism to verify a series of new photocathode quantum materials that could meet the needs of modern-day research.

They are also in contact with other research teams to expand the application value of strontium titanate.

On Ruihua He’s personal page, he wrote, “I hope Westlake could be the wild west that encourages cross-disciplinarity research and bold innovation.” In fact, their discovery may represent the first bucket of gold found in this adventure.

At the beginning, they were only researching the work function of quantum materials. Thanks to the ultra-high vacuum interconnect system, He was about to batch measure the work function of various materials at high throughput, which created the chance for them to see strontium titanate under a different light.

This moment of serendipity echoed the discovery of the photoelectric effect. Back in 1887, Hertz conducted experiments consisting of a coil with a spark gap while trying to verify Maxwell’s equations. The rest is history.

These adventure-loving scientists at Westlake continue to go where no one has gone before, hoping to discover more secrets of the photocathode material.

Ruihua He’s personal page：

https://www.westlake.edu.cn/faculty/ruihua-he.html

Job vacancies at He’s lab：

https://www.westlake.edu.cn/Careers/OpenPositions/research/202203/t20220318_19251.shtml