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  • richardmitnick 3:22 pm on August 22, 2014 Permalink | Reply
    Tags: , Lightsource Technology,   

    From M.I.T.: “Teaching light new tricks” 

    MIT News

    August 22, 2014
    Denis Paiste | Materials Processing Center

    Light is a slippery fellow. Stand in a darkened hallway and close a door to a lighted room: Light will sneak through any cracks — it doesn’t want to be confined. “Typically, in free space, light will go everywhere,” graduate student Chia Wei (Wade) Hsu says. “If you want to confine light, you usually need some special mechanism.”

    Chia Wei (Wade) Hsu

    Last summer, Hsu demonstrated a new way to confine light on the surface of a photonic crystal slab. “We were the first ones to experimentally demonstrate this new way to confine light,” says Hsu, a graduate student in physics who is conducting research under Marin Soljacic, a professor of physics as MIT.

    The photonic crystal is a thin slab whose structure has a periodicity, or repeating pattern, that is comparable in size to the wavelength of light — extremely short distances measured in nanometers (billionths of a meter). “Light can interact with the structure in a non-trivial way. Typically one observes modes called ‘guided resonance,’ where light is semi-confined in the slab but it can radiate outside. It’s not perfectly confined; it still leaks out,” Hsu explains.

    However, at a certain angle (35 degrees in the study), light stays bound to the surface, oscillating indefinitely. Hsu, Soljacic, co-author and MIT graduate student Bo Zhen, and others reported these findings recently in Nature. This phenomenon is called an embedded eigenstate, also known as a “Bound State in the Continuum.” The bound state affects just one wavelength of light that reaches the slab. The particular wavelength that is bound is related to the structure of the photonic crystal slab. So for a different structure, the bound state will appear at a different wavelength and wavevector, or angle of propagation. By manipulating the structure, researchers can manipulate the wavelength and the angle of this special state. Separately, Hsu and colleagues detailed their physical and mathematical analysis of the bound state in a theoretical paper in Light: Science and Application.

    One way to visualize the bound state effect, Hsu says, is to think of the difference between dropping a stone into a lake — where the waves ripple out without being confined — and using a drum stick to hit a drum membrane — which vibrates back and forth, but does not spread because it’s blocked by the boundary of the drum. “Eigenstate or eigenvalue refers to a sustained oscillation,” Hsu explains.

    At a particular angle, or wavevector, as light tries to escape, outgoing waves of the same amplitude, but opposite phase, cancel each other — which is known as destructive interference. “All of the outgoing waves are cancelled, so light becomes confined,” Hsu says. “There are no outgoing waves anymore and then it becomes perfectly confined in the slab.”

    Unexpected finding

    In 1929, scientists John von Neumann and Eugene Wigner theoretically predicted such a state, known as an embedded eigenvalue. The trapped state is in contrast to what typically happens when light resonates on the surface for a time, but then escapes or decays.

    “This bound state was certainly an unexpected discovery. We happened upon it when we were looking for something else,” Soljacic explains.

    The researchers are looking for a practical use of this finding. “The same mechanism we described about this interference cancellation mechanism can also be applied to a structure that’s similar to a fiber, so it may have potential use in optical communication too,” Hsu says. Although light does not escape the typical optical fiber because of its total internal reflection, the fiber confines all angles of light above a critical angle. “All the light above some cut off will be confined. In our mechanism, cancellation only happens at one particular angle. Only light at that particular angle is confined, so it has some more selectivity,” Hsu explains.

    Breakthrough from simplicity

    Prior examples of theoretically predicted embedded eigenstates were too complicated to realize. “Here we found a structure that is very simple to realize,” Hsu says. Fellow graduate student in Soljacic’s group, Jeongwon Lee, fabricated the photonic crystal structure, using a structure which the group had already studied.

    Lee fabricated the photonic crystal on silicon nitride slab, using interference photolithography to etch the periodic structure or repeating pattern. Hsu and Zhen measured the sample in the lab and analyzed the data to confirm the phenomenon. “In this simple structure, we found this phenomenon of this new type of light confinement. Since the structure is simple, we were able to demonstrate it, which other people were not able to because their systems are more complicated,” Hsu explains.

    Hsu is working toward a deeper understanding of why this phenomenon occurs where light gets confined, as well as exploring potential applications in photonic crystal lasers. “We are investigating where this new type of light confinement can give rise to different behaviors of lasers,” he adds.

    Watch how MIT and Harvard University researchers confine light to a crystal slab surface and design a transparent display.

    Video by the Materials Processing Center

    Creating transparent displays

    Besides his light confinement work, Hsu led the demonstration of a blue transparent display composed of a clear polymer coating with embedded resonant nanoparticles made of silver.

    Such displays work because the wavelength of blue light is strongly scattered by interaction with silver. “In this case, we only want to scatter the particular wavelengths of our projector light. We don’t want to scatter other wavelengths because we will need it to be transparent,” he says.

    “We can take a piece of glass which is originally transparent and put in nanoparticles that only scatter a particular, narrow bandwidth of light. Light in the visible spectrum is made of many different wavelengths from 300 nanometers to 750 nanometers. If we have such a structure, then most of the light can pass through, so it is still transparent, but if we project light of that particular narrow bandwidth, light can be scattered strongly as if it were hitting a regular screen,” Hsu explains. The results were published in a Nature article, “Transparent displays enabled by resonant nano particle scattering,” in January.

    Hsu’s theoretical design consisted of a nanoparticle with a silica (silicon dioxide) core and a silver shell, but the experiment was done using purely silver particles. “Silver-only is good enough if we want to scatter only blue light,” he says. A very tiny amount of silver, just six-thousandths of a milligram, produced the effect in the demonstration, making it a potential economical approach.

    Silver has conducting electrons, and when the particular blue wavelength interacts with them, those conducting electrons will oscillate back and forth strongly. “It’s a resonance phenomenon. At that point, you’ll get very strong light scattering,” Hsu explains. The phenomenon is called a localized surface plasmon resonance.

    One advantage of this approach is that the projected image has a broad viewing angle. “Nanoparticle scattering will send light in all different directions, so you will be able to see the image no matter which angle you look at. So it will be useful for applications where you would want people to see it from all different directions,” Hsu says.

    Hsu received his bachelor’s in physics and mathematics at Wesleyan University in 2010. His doctoral thesis will be split between the nanoparticle display work and the confinement of light work.

    See the full article here.

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  • richardmitnick 6:55 pm on August 12, 2014 Permalink | Reply
    Tags: , , , , Lightsource Technology,   

    From SPACE.com: “What?! The Universe Appears to Be Missing Some Light” 

    space-dot-com logo


    August 12, 2014
    Charles Q. Choi

    An extraordinary amount of ultraviolet light appears to be missing from the universe, scientists have found.

    One potential source of this missing light might be the mysterious dark matter that makes up most of the mass in the cosmos. But a simpler explanation could be that ultra violet light escapes from galaxies more easily than is currently thought, according to the new research.

    New data from the Hubble Space Telescope and computer simulations have revealed that the universe has much less ultraviolet light than previously thought. Credit: Ben Oppenheimer and Juna Kollmeier

    This puzzle begins with hydrogen, the most common element in the universe, which makes up about 75 percent of known matter. High-energy ultraviolet light can convert electrically neutral hydrogen atoms into electrically charged ions. The two known sources for such ionizing rays are hot young stars and quasars, which are supermassive black holes more than a million times the mass of the sun that release extraordinarily large amounts of light as they rip apart stars and gobble matter.

    Astronomers previously found that ionizing rays from hot young stars are nearly always absorbed by gas in their home galaxies. As such, they virtually never escape to affect intergalactic hydrogen.

    However, when scientists performed supercomputer simulations of the amount of intergalactic hydrogen that should exist and compared their results with observations from the Hubble Space Telescope’s Cosmic Origins Spectrograph, they found the amount of light from known quasars is five times lower than what is needed to explain the amount of electrically neutral intergalactic hydrogen observed.

    “It’s as if you’re in a big, brightly-lit room, but you look around and see only a few 40-watt lightbulbs,” lead study author Juna Kollmeier, a theoretical astrophysicist at the Observatories of the Carnegie Institution of Washington in Pasadena, Calif., said in a statement. “Where is all that light coming from? It’s missing.”

    The researchers are calling this giant deficit of ultraviolet light “the photon underproduction crisis.”

    “In modern astrophysics, you very rarely find large mismatches like the one we are talking about here,” Kollmeier told Space.com. “When you see one, you know that there is an opportunity to learn something new about the universe, and that’s amazing.”

    “The great thing about a 400 percent discrepancy is that you know something is really wrong,” study co-author David Weinberg at Ohio State University said in a statement. “We still don’t know for sure what it is, but at least one thing we thought we knew about the present day universe isn’t true.”

    Strangely, this missing light only appears in the nearby, relatively well-studied cosmos. When telescopes focus on light from galaxies billions of light years away — and therefore from billions of years in the past — no problem is seen. In other words, the amount of ultraviolet light in the early universe makes sense, but the amount of ultraviolet light in the nearby universe does not.

    “The authors have performed a careful and thorough analysis of the problem,” said theoretical astrophysicist Abraham Loeb, chairman of the astronomy department at Harvard University, who did not take part in this research.

    The most exciting possibility these findings raise is that the missing photons are coming from some exotic new source, not galaxies or quasars at all, Kollmeier said. For example, dark matter, the invisible and intangible substance thought to make up five-sixths of all matter in the universe, might be capable of decay and generating this extra light.

    “You know it’s a crisis when you start seriously talking about decaying dark matter,” study co-author Neal Katz at the University of Massachusetts at Amherst said in a statement.

    There still may be a simpler explanation for this missing light, however. Astronomers could be underestimating the fraction of ultraviolet light that escapes from galaxies in the nearby universe. “All that one needs is an average escape probability on the order of 15 percent to relieve the discrepancy,” Loeb told Space.com.

    Nearby, recent “low-redshift” galaxies have less gas to absorb ultraviolet rays that more distant, early “high-redshift” galaxies, Loeb noted.

    “The more I think about it, the more plausible it appears that the escape fraction of ultraviolet photons is higher in local galaxies than in high-redshift galaxies,” Loeb said.

    On the other hand, “the biggest problem with this possible solution is that there are measurements of local galaxies that indicate the average escape fraction is significantly lower than 15 percent — more like 5 percent,” Kollmeier said.”In principle, it is possible that these galaxies are not representative and therefore we need to do more such measurements, but we cannot just dismiss the data.”

    Another potential explanation is ionization of intergalactic hydrogen by x-rays and cosmic rays, Loeb said. Although he noted this radiation does not play a major role in ionizing intergalactic hydrogen in the most distant corners and earliest times in the universe, astronomers may want to see how much of a role x-rays and cosmic rays play in the nearby universe, “where they are produced more vigorously,” he said.

    The scientists detailed their findings in the July 10 issue of the Astrophysical Journal Letters.

    See the full article here.

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  • richardmitnick 4:06 am on June 7, 2014 Permalink | Reply
    Tags: , Lightsource Technology,   

    From SLAC Lab: “A New Way to Create Compact Light Sources” 

    SLAC Lab

    June 6, 2014
    Glenn Roberts Jr.

    SLAC scientists have found a new way to produce bright pulses of light from accelerated electrons that could shrink “light source” technology used around the world since the 1970s to examine details of atoms and chemical reactions.

    Traditionally this light is created by wiggling accelerated electrons with heavy magnets in a device called an undulator. Undulators currently produce bright X-ray pulses for experiments at dozens of sites, including SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL) and Linac Coherent Light Source (LCLS) free-electron X-ray laser. The new approach replaces the magnets with microwaves, and designers say it could be scaled to produce X-ray light.

    “This is a radical new design that can be used to create light sources that are more compact than can be achieved using a conventional undulator,” said Sami Tantawi, a SLAC particle physics and astrophysics professor who led the development effort.

    The microwave undulator can produce shorter wavelengths of light – using less driving energy from electrons than conventional undulators. This means the accelerator that feeds it electrons can shrink considerably, Tantawi said.

    A Step Toward Tabletop X-ray Lasers

    “This brings us much closer to the dream of a tabletop X-ray free-electron laser,” Tantawi said. “We have proved, in principle, that this is possible.”

    Electrons beamed into the undulator encounter rapidly alternating high-power microwave fields that cause the electrons to wiggle, emitting light at specific wavelengths. The wavelength and other properties of this light can be precisely tuned to suit many different types of experiments by adjusting the energy of the electrons and the microwave power.

    Tantawi said, “Because it doesn’t have any moving parts, the microwave undulator is amenable to scaling down to very short wavelengths.” Increasing its length and supplying more electron energy could allow it to produce X-ray pulses like those generated by the LCLS X-ray laser. Its scalability and rapid tuning sets it apart from other next-generation undulators, he noted.

    A Next-generation Source of Highly Tunable Light

    The new device, featured on the cover of the April 25 edition of Physical Review Letters, could be used to provide more highly tailored light pulses that open new realms of experimental possibilities, particularly for studies of materials with exotic properties. It could be incorporated into next-generation lasers, synchrotrons and particle colliders and installed in existing facilities, such as SLAC’s LCLS and SSRL.

    Also, Tantawi said he would like to pursue a superconducting version of the undulator that would require supercooling in order to produce a more continuous stream of light for experiments.

    “This technology has vast potential to dynamically control the properties of the light for use in many scientific experiments,” Muhammad Shumail, a PhD student who is writing his thesis about the undulator, said.

    Muhammad Shumail, a PhD student, inspects the microwave undulator that he worked to design and build. (Fabricio Sousa/SLAC)

    With further miniaturization, it is conceivable that the microwave undulator could serve as a portable medical imaging source, Tantawi said. “The next step of the research is to really make this technology more available.”

    SLAC technologist Gordon Bowden said building the prototype device was challenging, requiring specialized tools to cut a series of tiny ribs from the inside of the copper pipe the undulator is made of. The ends of the device had to be precisely tapered to better trap the microwave energy in the device.

    “No conventional undulator has the agility that this device can offer,” Bowden said. “The unique properties of the light that emerges from the undulator are due to the precise comb-like pattern of the microwave fields trapped inside,” he said.

    Unique Expertise

    Tantawi said SLAC’s historic and unique expertise in accelerators and microwave technology was well-suited to the development of the new device. “This is one of the few places on the planet we could have built this,” he said. The first magnetic undulator was built by Stanford researchers in the 1950s.

    This microwave undulator, designed and built at SLAC, uses microwave power and electrons to produce shorter-wavelength pulses, and is highly tunable compared to conventional undulators that use magnets. (Glenn Roberts Jr./SLAC)

    The pursuit of the microwave undulator was inspired by Tantawi’s conversations with X-ray free-electron laser pioneer Claudio Pellegrini and by Joachim Stöhr, SLAC photon science professor, and the design stemmed from his work on a similar design for an antenna that could be used to explore the so-called cosmic microwave background radiation, which is believed to hold clues to the earliest origins of the universe.

    Also participating in the microwave undulator project: Jeffrey Neilson, Chao Chang, Erik Hemsing and Michael Dunning. The project was funded by the U.S. Department of Energy and the Defense Advanced Research Projects Agency AXiS program.

    See the full article here.

    SLAC Campus
    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.

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