Utilizing Light in the Nanoworld

RIKEN Advanced Science Institute
Nanophotonics Laboratory
Chief Scientist, Satoshi Kawata, Dr. Eng.

A nanomachine works its way through a bloodstream to a lesion and emits a laser to pinpoint cancer cells. An optical microscope is used to distinguish different kinds of molecules and atoms according to the colors of living cells. An optical memory device as small as a cube of sugar is used to store all the information in Japan's National Diet Library. Giving these examples, Satoshi Kawata, a chief researcher of RIKEN, specifically explains what will be feasible with expected advances from the institution's research activities. "I want to develop an idea and technology that would totally refute the 'common knowledge' that the nanoworld cannot be imaged by light. It's my dream," says Kawata. He is working to foster nanophotonics, a discipline that focuses on the exploration and operation of the nanoworld using light, and to pioneer plasmonics, another new field of science.

Using light to see the nanoworld
The nanoworld cannot be imaged by light-that has been the "common knowledge." One nanometer (nm) is equal to one-billionth of a meter, or the size of several atoms. On the other hand, the light that can be perceived (visible light) is a kind of electromagnetic wave with a wavelength of approximately 400 to 700 nm. It has been the common belief that things measuring less than half the wavelength of visible light cannot, in principle, be seen with light.
Electromagnetic waves are divided by frequency (number of oscillations per second) into radio waves, infrared rays, visible light, ultraviolet rays, x-rays, and gamma rays, in ascending order. As the frequency increases, the wavelength shortens. This is the reason why shorter waves such as X-rays are used to visualize the nanoworld.
It has long been considered impossible to shorten the wavelength of electromagnetic waves sufficiently to visualize smaller worlds unless the frequency of such waves is increased. Kawata says, however, that this common knowledge can be refuted. "One possible approach is to decrease the velocity of light. Decreasing the velocity of light without changing the number of wave oscillations per second would result in a shorter wavelength."
However, can light be slowed down? "Light travels approximately 300,000 kilometers per second. This, however, is the velocity obtained in a vacuum. The velocity of light decreases approximately 23% in water and about 34% in glass." Other situations can be created in which the velocity of light decreases. For example, if light is shone on a piece of metal with an aperture smaller than the light's wavelength, a very small amount of light will come out from the other side of the aperture. The light's velocity will have decreased and its wavelength shortened. This type of light occurs only in close vicinity to the metal's surface and is therefore known as "near-field light."
In 1972, British researchers successfully applied this principle to observe a 0.5-mm structure using a radio wave with a wavelength of 30 mm. In 1985, a method of observing images by means of near-field light coming from a metal-covered aperture approximately 100 nm in diameter at the tip of an optical fiber was proposed. However, near-field light coming out from such a small aperture is rather dim. Near-field light of a shorter wavelength cannot be obtained unless the aperture size is further reduced. If so reduced, the near-field light becomes increasingly dim.
"In 1992, I had spent months trying to come up with a way to obtain brighter near-field light and discussed this everyday at my university. Then one morning, I was struck with an idea while driving to the university." The idea was to aim an intense laser beam to a very small particle of metal or the sharply pointed tip of a metal needle. Intense near-field light is thus produced under the needle. About 70% of the near-field optical microscopes currently available in the world are of the metal needle type. This method invented by Kawata is now the world's standard in the relevant field.

Seeing the nanoworld in color
If we have electron microscopes and other tools to explore the nanoworld, why do we need to use light to see the nanoworld? "This is because under an electron microscope, the world is monochrome, but under light, the world is in color," Kawata answers.
The principle of electron microscopy is that an image is formed as gradations in density according to the intensity of electron rays passing through the sample. Color images obtained using an electron microscope are products of artificial coloring. Optical microscopes enable us to see the world in color. However, cells look transparent when seen under an optical microscope. Accordingly, the technology to investigate cell structures and the functions of proteins and other ingredients therein has been developed using a variety of dyes and fluorescent substances. This technology serves as a powerful tool to support advances in medicine and life sciences.
Kawata and members of his laboratory developed a microscope capable of examining the structures and functions of living cells in their natural colors without staining them, using a specially designed picosecond (one-trillionth of a second) laser. This new microscope has already been commercialized in a venture business started by Kawata and his members. How can the colors of cells, which are essentially transparent, be seen without using dyes?
"The molecules that constitute the cellular organelles and proteins oscillate constantly. Upon exposure to light, these molecules emit a kind of light known as Raman scattered light, which has a wavelength (color) that differs slightly from that of the light applied. This is because the wavelength shifted due to molecular oscillation. Although these color differences are imperceptible to the human eye, the most advanced optical technology enables us to perceive them. The color differences result from a variation in the mode of oscillation, depending on the molecular species. Hence, we can distinguish different kinds of molecules according to their colors."
Recently, Kawata's laboratory succeeded in distinguishing the four bases of DNA according to color differences in Raman scattered light. Kawata's method is a powerful tool in evaluating and analyzing nanomaterials as well. For example, different thicknesses (diameters) of carbon nanotubes (nano-sized tubes formed by carbon atoms) can be distinguished by color differences (Figure 1). This is because tubes of different diameters produce different frequencies of carbon molecular structure.

"We have already achieved a resolution of 10 nm, equivalent to one-fiftieth the wavelength of light. However, this is only halfway toward fulfilling my dream. I want to spend the rest of my research life improving the resolution to one-fiftieth the current level, or approximately 0.3 nm."
Each hydrogen atom measures approximately 0.1 nm in diameter. With a resolution of approximately 0.3 nm, different atoms and molecules can be distinguished by their colors in the nanoworld.

Smallest laser sculptured bull featured in Guinness World Records
Kawata succeeded in making a microscopic sculpture of a bull measuring 8 μm (0.008 mm) long using laser beams (Figure 2). Light enables us to not only see things but also process them. "My family was indifferent to my work even when my achievements were published in Nature and other well-known journals, both in Japan and abroad. However, when my microscopic bull appeared as the world's smallest laser sculpture in the 2004 edition of Guinness World Records, they finally thought highly of me (laughter)."

The bull was sculpted from an acrylic resin that sets upon exposure to ultraviolet rays. In accordance with data stored in a computer on the contours of a real bull, a near-infrared laser was irradiated for an extremely short period of 100 femtoseconds (one-ten trillionth of a second) to define the contours of the microscopic sculpture. Finally, the surrounding resin, which remained unset, was washed off to reveal a three-dimensional bull.
The bull had details measuring only 50 nm long. How could we create a sculpture with a precision of 50 nm using near infrared rays-which have a wavelength of 800 nm, longer than that of visible light-and how could we set the resin, which normally sets upon exposure to ultraviolet rays, using-near infrared rays?
Light behaves as a particle does. This particle is known as a photon. Under normal conditions, any substance exposed to light absorbs one photon. However, when extremely intense light, which has a large number of photons, is used, two photons are absorbed at one time. In this state, the energy of two photons of infrared rays reaches a level equivalent to that of one photon of ultraviolet rays, thus enabling us to set the resin.
"This is another method of shortening the wavelength of light without changing its frequency. Do the multiplication for light. Because a wave can be expressed by a cosine function, its wavelength shortens as two cosines are multiplied."

Three-dimensional optical memory device
Light can also be used to write and retrieve information three-dimensionally. "In 2000, U.S. President Bill Clinton proposed the creation of a memory device the size of a sugar cube to store all the information in the nation's Library of Congress. However, there was no one who carried out specific work to realize this proposal. We have actually made an optical memory about the size of a cube of sugar that can store information written three-dimensionally," says Kawata (Figure 3)

"Optical memory devices that are currently available, including compact discs (CDs) and digital versatile discs (DVDs), suffer the limitation of information being written only on their surfaces," Kawata points out. "What is called blue-ray discs and HD-DVDs, both generically referred to as new-generation DVDs, can write and store information at very high densities on their surfaces using short-wavelength light, such as blue lasers. However, the method will not be able to improve much further. As the density of the information written on the disc increases, noise from dust likewise increases to the extent where the information will be irretrievable because the dust particles will be larger than the particles of disc material. Information should be written three-dimensionally in multiple layers rather than two-dimensionally at higher densities."
Kawata says that further advances in the technology of three-dimensional information writing would lead to an optical memory device capable of storing one terabyte (1,000 gigabytes) of data in its sugar-cube-sized body. This is equivalent to more than 200 units of the currently available 4.7-Gb DVD and several tens of units of the new-generation DVD under development.
"I believe this three-dimensional optical memory device will be out on the market within five years, provided that companies devote themselves to commercializing it," says Kawata confidently.

Nanomachines that combat cancer cells
"It took three hours to fabricate the microscopic bull statue featured in Guinness World Records. Now, it would take about 13 minutes to do the same thing. Recently, our laboratory succeeded in making 1,000 microscopic bull statues at one time using 1,000 light spots produced simultaneously with a specially designed lens."
At the Nanophotonics Laboratory, Kawata and his colleagues applied the technology used to simultaneously make the 1,000 microscopic bulls and created a nanostructure with an array of metal antennae.
Working the technology to fabricate a nanomachine the size of a red blood cell would make scenes from the science fiction movie Fantastic Voyage a reality. The microscopic bull, by the way, is the size of a single red blood cell. "Although it is impossible to reduce the size of physicians and deliver them into blood vessels as depicted in the movie, a nanomachine the size of a single blood cell would enable us to explore everywhere in the body, from the heart to the lungs and even the brain. The smallest blood vessel is as thin as a red blood cell in diameter. Injected into the blood vessel of a fingertip, the nanomachine would make its way in the bloodstream to a lesion. Receiving light transmitted from outside the body, the nanomachine would emit a laser to fight the cancer cells."
The next question is how to transmit light to the nanomachine from outside the body? "We use near infrared rays. Visible light is quickly absorbed in superficial layers, whereas near infrared rays go into the body." Through trial and error, Kawata and his colleagues have created a pacemaker that is rechargeable using extracorporeally applied near infrared rays, thus obviating the need to replace batteries. Current pacemakers require wearers to undergo surgery to replace batteries every several years.
"Light is gentle to the body and is useful as a source of energy to drive nanomachines in our bodies. In the nanoworld, the amount of energy does not necessarily have to be large; nanomachines can operate fully as long as a very small amount of thermal or optical energy is available."

Pioneering plasmonics
Kawata served as a professor in the Graduate School of Engineering at Osaka University and was the head of the University's Frontier Research Center, founded in 2001 (until March 2004). The center aims at developing new disciplines and new industries that go beyond the borders separating different academic fields as well that between industry and university. Kawata, as a research leader in nanotechnology both in Japan and abroad, established the Nanophotonics Laboratory at the Discovery Research Institute of RIKEN in 2002.
"During the first two years of my service at RIKEN, I prohibited those under me from publishing their findings, telling them not to write papers and refrain from giving presentations. I said that we should take our time doing our work. Even if they make a groundbreaking discovery, they should keep the fact a secret for a while. Being busy writing papers and filing patent applications are of no pleasure to researchers. The point of evaluating papers should be in their content rather than their number. I instructed them to store energy before embodying their work. Then, after two years, they all began getting desirable results."
Kawata says that he wants to do two things at RIKEN. "One of my own goals is to advance research into nanoscale science by means of the photon; this is my lifework. The other is to pioneer a new discipline we call plasmonics."
When light is applied to metal, the electrons in the metal oscillate as a whole. Scaling down a metal structure to the order of nanometers limits the space available for oscillation, causing the electrons' behavior to change from that found on larger scales. Plasmonics is the field of research into this behavior of electrons as a whole. For example, near-field light is emitted by a sharply pointed metal needle upon exposure to light, and a laser is emitted by the nanoscale structure of metal. Both of these are phenomena found in plasmonics.
"There have been discussions on the possible interaction of light applied to the nanoscale structure of metal and the electrons in the metal causing various unusual phenomena, including light curving in the direction opposite to its ordinary direction. However, there are no methods that demonstrate such phenomena. Owing to the compilation of technical knowledge on optics and nanoscience, it has finally become possible to conduct experimental investigations of the potential of plasmonics. We presented a paper in November 2004 demonstrating that the structure emits a laser upon exposure to light and the other science in cyclic structure fabrication (Figure 4).This is what I want to do at RIKEN. Plasmonics promises to provide our society with new materials that would facilitate the use of light in the nanoworld."

According to Kawata, "the 20th century was the age of Edison." In other words, technologies that relied on the use of electrons supported science, technology, and society in the last century. "It is certain, however, that the times are seeing a shift from the electron to the photon," says Kawata definitely. For example, conventional telephones, which depend on copper wires, are being replaced by mobile phones, which employ optical fibers or electronic waves, and television displays are changing from cathode-ray tubes, which employ electron rays, to liquid crystals.
"This is because light is friendly to the body and, in addition, can easily be utilized without the need of a vacuum like those of cathode-ray tubes. However, it has been impossible to utilize long-wavelength light in the nanoworld. If we can learn how to freely manipulate photons in the nanoworld, the transition from the electron to the photon will accelerate. At RIKEN, many of my colleagues are engaged in research into light. I would like to go along with them to pioneer the new field of optics and nanoscience at RIKEN, a world center of scientific research."