Fischer makes the tiny cloak—less than half the cross-section of a human-hair—by direct laser writing (i.e. lithography) into a polymer material to create an intricate structure that resembles a miniature woodpile. The precisely varying thickness of the "logs" enables the cloak to bend light in new ways. The key to this achievement was incorporating several aspects of a diffraction-unlimited microscopy technique into the team’s 3-D direct writing process for building the cloak. The dramatically increased resolution of the improved process enabled the team to create log spacings narrow enough to work in red light.
"If, in the future, we can halve again the log spacing of this red cloak, we could make one that would cover the entire visible spectrum," Fischer added.
Practical applications of combining transformation optics with advanced 3-D lithography (a customized version of the fabrication steps used to make microcircuits) include flat, aberration-free lenses that can be easily miniaturized for use in integrated optical chips, and optical "black holes" for concentrating and absorbing light. If the latter can also be made to work for visible light, they will be useful in solar cells, since 90 percent of the Sun’s energy reaches Earth as visible and near-infrared light.
Presentation QTuG5 "Three-dimensional invisibility carpet cloak at 700 nm wavelength," by Joachim Fischer et al. is at 11 a.m. Tuesday, May 3. Fischer et al. will also present CML1, "Three-Dimensional Laser Lithography with Conceptually Diffraction-Unlimited Lateral and Axial Resolution," at 10:15 a.m. Monday, May 2.
2. 3-D Terahertz Cloaking
Close-up micrograph of the bump region of the Terahertz cloaking structure. The size variations in the holes, which extend through the entire structure, is a key feature that guides the light around the bump, beneath which objects become invisible. The view is about 2 millimeters square.
A research team from Northwestern and Oklahoma State universities claims to be first to cloak a three-dimensional object from view in a broad range of Terahertz frequency light, which lies between infrared and microwaves. In the team’s paper at CLEO: 2011, Cheng Sun of Northwestern describes how a rigid sponge-like cloaking structure less than 10 millimeters long on a side was built up in 220 layers, each precisely defined to vary the index of refraction and bend light to render invisible anything located beneath a shallow concave bump on the cloak’s bottom surface. The group showed that both the physical geometry and the spectrographic signature of a chemical strip about the width of 10 human hairs disappeared when cloaked.
Despite its Harry Potter-like allure, concealing tiny objects from view is not the team’s ultimate goal, Sun said. Rather, this latest demonstration shows that the new "transformation optics" principles and 3-D lithography techniques they used to make the cloak can also enable optical components for guiding, collimating, and focusing terahertz light in a variety of ways—in new medical and scientific diagnostic tools, airport security scanners, and data communication devices.
3. Nuclear Photonics: Gamma Rays Search For Concealed Nuclear Threats
Gamma rays are the most energetic type of light wave and can penetrate through lead and other thick containers. A powerful new source of gamma rays will allow officials to search for hidden reactor fuel/nuclear bomb material.
These gamma rays, called MEGa-rays (for mono-energetic gamma rays), are made by using a beam of fast-moving electrons to convert laser photons (light at a lesser energy) into the gamma ray part of the spectrum. The incoherent gamma rays can be tuned to a specific energy so that they predominantly interact with only one kind of material. A beam of MEGa-rays, for example, might be absorbed by the nuclear fuel uranium-235 while passing through other substances including the more common (but less dangerous) isotope uranium-238. That sort of precision opens the door to "nuclear photonics," the study of nuclei with light. "It is kind of like tunable laser absorption spectroscopy but with gamma-rays," says Chris Barty of Lawrence Livermore National Laboratory, who will present on MEGa-rays at CLEO: 2011.
In the last couple of years, MEGa-ray prototypes have identified elements like lithium and lead hidden behind metal barriers. The next-generation of MEGa-ray machines, which should come on-line in a couple of years, will be a million times brighter, allowing them to see through thick materials to locate specific targets in less than a second.
Barty will present several MEGa-ray applications in use today and will describe the attributes of next-generation devices. Work is under way on a MEGa-ray technology that could be placed on a truck trailer and carried out into the field to check containers suspected of having bomb material in them. At nuclear reactors, MEGa-rays could be used to quickly identify how enriched a spent fuel rod is in uranium-235. They could also examine nuclear waste containers to assess their contents without ever opening them up. MEGa-ray technology might also be employed in medicine to track drugs that carry specific isotope markers.
4. Green UV Sterilization: Switching on LEDs to Save Energy and the Environment
Ultraviolet light can safely sterilize food, water and medical equipment by disrupting the DNA and other reproductive molecules in harmful bacteria. Traditionally, mercury lamps have supplied this UV light, however mercury release from power generation and lamp disposal have generated discussion of harmful environmental impact. A potentially energy efficient and non-toxic alternative is the light-emitting diode, or LED, which can be made to emit at almost any desired wavelength. LEDs are also more rugged and operate at lower voltages than glass containing mercury bulbs. Thus, LEDs are more compatible with portable water disinfection units, which could also be solar-powered and used in situations where centralized facilities are not available, such as disaster relief. LEDs currently require a lot of electricity to produce UV light, but researchers from around the world are focused on improving this efficiency.
LEDs are semiconductor devices that operate in much the same way as the tiny elements on a computer chip. The difference is that some of the electrons flowing into an LED are captured and release their energy as light. Because these are solid materials rather than gas-filled bulbs, LEDs are more compact and durable than alternative light sources. The first commercial LEDs were small red indicator lights, but engineers have developed new materials that emit in a rainbow of colors. Nitride-based LEDs are the most promising for pushing beyond the visible into the ultraviolet. Some of these UV LEDs are already being used in the curing of ink and the testing for counterfeit money, but for sterilization, shorter wavelength light is required. These short wavelength, or "Deep UV" LEDs, present a number of technical challenges and are predominantly implemented in highly-specialized disinfection systems in industrial and medical applications, as well as other non-disinfection markets.
The Joint Symposium on Semiconductor Ultraviolet LEDs and Lasers at CLEO: 2011 will feature several talks addressing these challenges, while highlighting current efforts to improve the efficiency of nitride-based LEDs. Max Shatalov of Sensor Electronic Technology in Columbia, S.C., will report an improved design for making high-power UV LEDs that would be especially good for knocking out bacteria. From the birthplace of nitride (blue and white) LEDs, Motoaki Iwaya from Meijo University in Japan will describe a joint effort with Nagoya University to extend the range and improve the efficiency of UV LEDs.
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