From H.G. Wells’ “The Invisible Man” to the “Harry Potter” novels, the notion that a person could become invisible has fascinated science fiction writers and readers for generations.
Researchers from the Nano-Scale Science and Engineering Center at the University of California’s Berkeley campus have taken a major step in converting this dream into reality.
Metamaterials are composite substances that exhibit behaviors resulting from their unique structural designs. In separate research articles, the investigators from UC Berkeley describe two new types of composites that can bend electromagnetic waves, which should lead to significant advances in optical imaging and other applications.
Eyeglasses, microscopes and telescopes all depend on the principle of refraction, which essentially states that a light wave will bend as it passes from one transparent medium to another because of a change in its average speed. The speed of light is 186,000 miles per second in a vacuum but is reduced by 25 percent in water, by 33 percent in glass and by 59 percent in diamond. These differences in speed are from varying rates of absorption and re-emission of photons by electrons in the different materials through which the light is traveling, creating time delays and resultant changes in the average speeds of the light wave in these media.
When sunlight travels from air to water, it slows and bends toward the direction that is perpendicular to the boundary layer between the media. One example of the practical consequences of this change in direction or refraction of the light wave is that a fish will appear to be at only 75 percent of its actual depth location below the surface of the water. Other examples include the appearance of mirages in hot and less dense air through which light travels more quickly, and the use of convex lenses in magnifying glasses to produce enlarged virtual images of objects.
All materials found in nature have positive indices of refraction, each of which describes the average velocity of light in a specific medium relative to the speed in a vacuum. These indices can be used to calculate the locations of virtual images generated by the bending of light waves traveling through the different media.
The UC Berkeley metamaterial profiled in a recent online edition of the journal Nature consists of 21 alternating layers of silver and magnesium fluoride that were cut into a fishnet pattern. Upon application of infrared light, the material became the first 3-D solid to exhibit a negative index of refraction with little absorption of the light as it leapt from one portion of the pattern to another.
The second metamaterial described in the journal Science became the first 3-D substance to bend visible light backward. It consists of vertically oriented silver wires with diameters of only 60 nanometers placed inside aluminum oxide. Only the electrical portion and not the magnetic field of an applied light wave will interact with the material, ensuring that little energy is lost in the transmitted wave.
If such a material suitably bends light with wavelengths in the visible range of 400 to 700 nanometers, that substance could become invisible to the human eye. Although much further research is needed to create practical invisibility shields from durable forms of metamaterials, the likelihood that such shields eventually will become a reality now is much greater due to the designs developed by the UC Berkeley research teams
And although invisibility shields might not be available in the near future, it would be wise for society to begin discussing possible safeguards to ensure that these materials will not be misused, rather than waiting passively for potential problems to develop.