The Wonders of Waves
Future Technologies Built on Basic Research
In the future, a regular physical examination may not be necessary to identify any potential health crises. Instead, a smart device in a mirror will report to us any anomalies in our body and provide health recommendations when we wake up every morning. This sort of assessment will be made possible by airborne ultrafast ultrasonic imaging technology, in which a time-reversal device records a video of skin vibrations at 1,000 frames per second. Details unnoticed by human eyes are compared to population data and can serve as first screening tools of medical conditions. Such technology is not science fiction, but an actual study in progress by IAS Visiting Professor Mathias Fink.
Fink is a physicist trained in condensed matter physics. His best-known innovation is the ultrafast ultrasound imaging scanner—the Aixplorer—sold by the company Supersonic Imagine and widely used around the world for cancer diagnosis. Fink enjoys the luxury of serving in various capacities in scientific research and development, from conducting basic research and developing theoretical frameworks to designing product applications for implementation in commercial operations. He is the originator, researcher, inventor of and entrepreneur behind the concepts of time reversal signal processing and its related technology.
Time Reversal Technique: The Idea
After he received his PhD in Solid-State Physics, Fink decided to move to acoustic holography in the hope of developing an apparatus that would allow archaeologists to reveal images of objects deep under soil or water. Because the development of this kind of tool was largely military-driven, funding opportunities were limited for academia. Therefore, he changed his research direction to the application of holography in medicine.
“What is the core in making an image of an object?” This was the question Fink asked in the late 1970s. From there, he developed the general concept of a time-reversal mirror and time reversal processing. “When we see an object, our eyes receive the light waves reflected from it. A mathematical property common in all waves is time reversal symmetry. If all the details of a wave radiated by any object could be recorded quickly enough on a two-dimensional array of antennae (surrounding the object) and then reversed to go backwards in time, we would be able to construct the object exactly. The same applies for acoustic waves, ultrasound waves, microwaves and light, and has a lot of applications in different areas.”
The Rise of Computers
Building a time-reversal mirror was very challenging, and Fink decided to build the first one for ultrasonic waves. The idea was to use an array of piezoelectric transducers (reversible antennae with both microphones and loudspeakers) to record in electronic memories the ultrasonic signals radiated by an object, reverse them in time, and play them back using the same transducers as transmitters. In 1978, however, it was not technically feasible to put this idea into practice because no electronic memory was fast enough to store such a large amount of information. The theory remained a theory and was forgotten for 10 years. The computer technology matured in 1988, when the technological advancements in computer performance offered the possibility for an apparatus to record, store, time-reverse, and send back the ultrasonic wave signals.
The first time-reversal mirror was built for ultrasonic waves, but Fink and his group later developed another one for acoustic waves and for water waves. In one spectacular experiment conducted in a swimming pool, they set up a number of controllable paddles around a pool of water to record and reverse waves on the water surface. A stone was thrown into the water, and the splash became waves flowing across the pool and bouncing back. These waves were recorded. “When the recorders played the time-reversed sound waves backwards in time,” Fink said, “the waves traveled from one end of the pool to the other, bounced back and they ultimately brought back the splash to its original spot.”
\\ We forgot this idea for 10 years, until the emergence of computers and fast memory enabled us to make it happen.\\
Attempts to Commercialize
The first application of the time-reversal mirror technology was shockwave therapy. “Using time reversal process, we built the first apparatus to locate and record the echoes of a kidney stone in a human body, and then time-reversed them to send back exactly strong shock waves to the stone. The energy fitted its size and shape, such that the kidney stone could be perfectly destroyed.”
Another technical service was offered to aircraft companies when an apparatus was developed to scan complex engines. Ultrasonic waves were sent into aircraft engines. Using the time reversal technique, the apparatus could exactly locate very small defects and factory errors.
\\ When the business model did not work, we always came back to basic research and found a way. \\
Both of these technologies achieved successful technical results with time-reversal mirrors made of hundreds of transducers, but they were hardly marketable in the early 1990s because of the prohibitively high costs of the fast electronic memories associated with each of these transducers.
Revising the Theory
Fink recalled that “we were very disappointed in 1992, when we returned to basic research and tried to understand how a time-reversal mirror works in complex media in which reverberation and multiple scattering become prominent. We found a very surprising result, that the time reversal technique does not require many antennae surrounding the object if the propagation medium is sufficiently complex. Contrary to our earlier thoughts, only one antenna should suffice in the time reversal process. Another finding was that, quite unlike our initial thoughts, it was actually easier to reconstruct an object if waves have gone through complex media, which supposedly had distracted the spread of waves.”
Application and Entrepreneurship
With this new concept in mind, Fink decided to apply time reversal techniques in many other areas in which the medium complexity allows the use of a small number of antennae. Wave-related technologies can be used in four main areas: telecommunications, imaging, therapy, and tactile objects.
“Through basic research, we used the time reversal technique to manipulate electromagnetic waves and focus them on a certain point. This has great implications for communications. Focusing waves to a certain point means that we can encrypt our message to the desired target without spreading the waves elsewhere. It can bring more secure communication and better signal quality.”
“4G and WiFi use electromagnetic waves to communicate, and the time reversal technique serves as a way to bring the technology to a new level. We have developed various electromagnetic smart mirrors that reflect the waves to concentrate WiFi signals to certain spots in an indoor space. This technique can save up to 90% of the electromagnetic waves required for telecommunication and massively increase the amount of information that can be transmitted over the air. We have created the company Greenerwave to develop and commercialize this technology.”
“Acoustic waves can be translated into images using the time reversal technique in computer software. Traditional ultrasound images typically produce 50 frames per second when we scan a growing baby in the womb. In our body, ultrasonic waves are compressional waves that propagate 1,500 meters per second and deliver information on tissue density. However, another kind of mechanical waves, known as shear waves, travels at 1-10 meters per second. Shear waves reflect the stiffness of body tissue.” With the use of a time reversal processor, Fink built the first ultrafast imaging scanner (10,000 frames per second), an apparatus known as the Aixplorer, that can follow the tissue motions induced by low-speed shear waves and measure the stiffness of tissues and detect potential tumors at an early stage in areas such as the breast, thyroid, and prostate. The company Supersonic Imagine developed the Aixplorer and has already sold thousands of such devices to medical and research institutions worldwide.”
“Following the shockwave therapy, we are using ultrasound to tackle calcified heart valves. Operative treatment is very difficult in this part of the body. The new ultrasound application tracks movement and delivers a shockwave treatment to remove the blockade. A startup company Cardiawave in France is testing the new technology in animals, and a clinical study will be carried out in 2018.”
“The time reversal technique can turn every surface into an interactive platform, like a touch screen. Using the elastic waves radiated by your fingers moving over a solid surface, a smart device can reflect the exact finger location and present with computer programs the appropriate image at the desired location. These can be helpful for communication and learning.” A company named Sensitive Object developed this technique and it was then sold to Tyco Electonics.
Metamaterials are man-made materials designed to have wave functional properties not found in nature. They are typically composite materials comprising an array of subwavelength resonators (whereby the lattice constant " α " is much smaller than the wavelength " λ "), as illustrated in the schematic diagram.
Greater Precision, More Information
One problem that faced Fink and his fellow scientists was diffraction limits. Waves cannot naturally be focused on a spot smaller than half of the wavelength. Many scientists have attempted to overcome this limitation.
Fink and his colleagues first developed a new approach that combined time reversal techniques and metamaterials, but instead of using negative-index metamaterials such as that developed by another IAS Visiting Professor, John Pendry, they showed that some metamaterials with positive indices allow diffraction limits to be overcome when using time reversal focusing. “We developed the concept of resonant metalens and designed these materials for microwave communications. For example, the wavelength of wireless communication is 12 cm, but the waves can be focused on an area of 4 mm with this technique. Likewise, acoustic waves with a wavelength of 80 cm can be controlled to knock down an empty soft-drink can,” Fink explained.
Fink is now collaborating with physicists Ping Sheng at HKUST and Guancong Ma, former IAS Postdoctoral Fellow (now at Hong Kong Baptist University) at HKUST to design an “acoustic sink” made of metamaterial to focus sound waves on a subwavelength perfect absorber.
Further Research with Time Reversal Approach
“Time reversal is a very general principle in physics. At first, we did not think that it would have so many applications,” Fink said. Research derived from this theoretical ground still has a long way to go, and Hong Kong has some favorable conditions to offer to researchers. “Hong Kong is very open to innovation,” he continued, “and “excellence in research and development is well supported with resources. Extending our current collaboration at HKUST, we are now exploring whether time reversal may bring some insight to quantum physics and cosmology. We are developing a new approach to time reversal that we call the ‘instantaneous time mirror.’ Rather than using an antenna array, ultrafast changes of the refractive index are made in a large volume. We believe that this new approach can be applied in graphene to time-reversed electrons and may help to explain some fundamental problems in the effect of exponential expansion of the universe just after the Big Bang—the inflation period when the refractive index changed very quickly.”