Making Waves with Groundbreaking Research

An invisibility cloak might sound like something from Star Trek or Harry Potter but scientists say they may soon be able to turn fiction into fact.

Dr Yun Lai, a physics professor at Nanjing University in Nanjing, China, has been conducting frontier research in cloaking and metamaterials since his PhD studies at HKUST on blocking sound waves.

Research into the unique materials, which demonstrates how light could be bent around objects to render the objects invisible, has become a hot topic since its initial discovery in 2006. 

A brand-new way of cloaking

Hong Kong has been playing an important role in some of the advances in the field, and Lai is among one of the physicists leading the way. After completing his PhD at HKUST in 2005, Lai worked under Prof Che Ting Chan to study metamaterials and cloaking. They were inspired by the research of British theoretical physicist Prof Sir John Pendry, a pioneer in cloaking using metamaterials which are artificially designed structures with unique optical properties non-existent in nature.

The initial work relied on an effect similar to that of mirage, where light is bent to create optical illusions. Then in 2009, Lai and his colleagues made an important discovery, opening a new area of research.

“I accidentally found that there is another way of cloaking,” he said. “With the original cloaking method, you have to cover the object with something resembling Harry Potter’s cloak. We, on the other hand, found that sometimes you do not need to cover the object. Instead, you can put a cloaking device right next to the object to be cloaked and obtain the same effect.”

This brand new way of cloaking is based on creating wave interference between the cloaking device and the object. “This was actually quite an astonishing result at that time,” Lai continued.

Amazing as it appears, one should not expect to be walking around Hong Kong under an invisibility cloak anytime soon. At the moment, invisibility is limited to long wavelengths such as microwaves. Creating invisibility in the optical spectrum remains work-in-progress. “For optical cloaking, it may take some time to fabricate a material and device that can realize a nice effect,” he mentioned.

Illusion optics surpassing current VR technology

Lai entered the field after researching artificial acoustic structures such as phononic crystals during his PhD studies. The structures are materials that can be designed to prevent selected ranges of frequencies from being transmitted through the material. While this might seem to be a far cry from invisibility cloaks, the large-scale calculations and extensive programming, required to engineer the sound-blocking properties of such crystals, provided Lai with a solid background for understanding how waves interact with microstructures.

In fact, Lai has not completely left this work behind. Instead, he continues to study acoustics and has conducted research to design a solid material that only allows acoustic waves to propagate. The research may have applications on earthquake-proofing buildings. In future, he hopes to create a new type of mesh structure that will allow air to flow across it while blocking sound at the same time.

Metamaterials and cloaking continue to be Lai’s core field of study for now, thanks to the exciting applications in relation to the realization of such technologies. His work on cloaking led to the discovery of a new technique for creating optical illusions that might one day be used to make 3D movies and games indistinguishable from reality.

“If you have an apple and you cloak it by using a free space, we are effectively replacing the apple with the free space,” said Lai. “What if we use something else instead of the free space? Based on this question, we proposed the concept of replacing an apple with a banana, which is now called illusion optics. This may have some important applications in future because we can create ultra illusions using this technique that are perfect.”

The word ‘perfect’ was not used casually. Lai explained that Maxwell’s equations – forming the foundation of classical electromagnetism, optics and electric circuits – guarantee that the illusion is perfect. This is a very different concept to holography – the science and practice of making holograms – where it is always possible to see the illusion as an illusion. It also differs from conventional 3D effects used in movies, which rely on the optical difference between your left and right eyes to create an artificial sense of three dimensions.

With illusion optics, it should be possible to create a 3D display device that can easily surpass current Virtual Reality (VR) technology. “I am still working on it,” said Lai. “In principle, it is possible to solve the problems of virtual reality devices in relation to users’ feeling of dizziness and discomfort after using the devices for a long time.”

It is believed that today’s VR headsets bring about discomfort because they do not create a true three-dimensional optical field, which can be achieved with illusion optics. “With illusion optics, if you have an illusion banana, it is exactly the same as a real banana,” Lai noted.

A vision for thinner invisibility cloaks

Despite challenges to be met, Lai predicts that the technology could be turned into practical applications very soon in the form of television screens or glasses.

Other areas for future research include ultra-transparent materials that may one day enable the development of much thinner invisibility cloaks. “We have done experiments to verify the effect, and the results are reasonably good,” Lai remarked. “With these meta-surfaces, we can shrink the cloaking shell into something much thinner and much more similar to Harry Potter’s cloak.”

Lai has been selected to be one of China’s high-profile Young Thousand Talents. In future, he aspires to continue making waves with more ground-breaking research.