Researchers use sound to slow down, speed up, and block light

This is an artist visualization of slow light, fast light, and one-way light blocking using BSIT in a series of silica microresonators (slow/red, fast/blue; central yellow shows blocking effect). Credit: Gaurav Bahl, University of Illinois

 http://phys.org/news/2015-01-block.html#jCp

by Rick Kubetz

How do you make an optical fiber transmit light only one way? Researchers from the University of Illinois at Urbana-Champaign have experimentally demonstrated, for the first time, the phenomenon of Brillouin Scattering Induced Transparency (BSIT), which can be used to slow down, speed up, and block light in an optical waveguide. The BSIT phenomenon permits light to travel in the forward direction while light traveling in the backward direction is strongly absorbed. This non-reciprocal behavior is essential for building isolators and circulators that are indispensible tools in an optical designer's toolkit.

In this study, the researchers demonstrated the BSIT phenomenon using nothing more complicated than a glass micro-fiber and a glass sphere adjacent to it.

"Light at certain wavelengths can be absorbed out of a thin optical waveguide by a microresonator—which is essentially a tiny glass sphere—when they are brought very close," explained Gaurav Bahl, an assistant professor of mechanical science and engineering at Illinois. "Through the BSIT phenomenon we can eliminate this opacity, i.e., we can make this system transparent again by adding another laser at a specially chosen wavelength nearby.

"The effect occurs due to the interaction of the light with sound waves present in the material, and is a new physical process that has never been seen before. The most significant aspect of our discovery is the observation that BSIT is a non-reciprocal phenomenon—the transparency is only generated one way. In the other direction, the system still absorbs light."

Time-reversal symmetry (i.e. reciprocity) is a fundamental tenet understood in most acoustic, electromagnetic, and thermodynamic contexts. Engineers are often forced to use tricks to break this time-reversal symmetry for specific device applications.

Current non-reciprocal optical devices—for example, isolators and circulators—are exclusively built using the Faraday magneto-optic effect. This method uses magnetic fields to break the time-reversal symmetry with certain specialized garnet and ferrite materials. However, these materials are challenging to obtain at the chip-scale through conventional foundry processes. Magnetic fields are also sources of interference in many applications such as cold atom microsystems. These constraints have deterred availability of Faraday effect isolators for on-chip optical systems till date.

"We have demonstrated a method of obtaining linear optical non-reciprocity that requires no magnets, can be implemented in any common optical material system without needing ferrites, and could be implemented today in any commercial optical foundry," Bahl added. "Brillouin isolators do already exist, but they are nonlinear devices requiring filtering of the scattered light. BSIT, on the other hand, is a linear non-reciprocal mechanism."

"Brillouin-Mandelstam scattering, originally discovered in the early 1920s, is the coupling of light waves and sound waves through electrostrictive optical forces and acousto-optic scattering. It is the fundamental physical process behind BSIT, and occurs in all solids, liquids, gases, and even plasmas," stated JunHwan Kim, a graduate student at Illinois and first author of the paper, "Non-Reciprocal Brillouin Scattering Induced Transparency," appearing in the journal, Nature Physics.

BSIT also enables the speeding up and slowing down of the group velocity of light. Physicists call this "fast" and "slow" light. "Slow" light techniques are extremely useful for quantum information storage and optical buffer applications. Some day, such buffers could be incorporated in quantum computers.

"While it is already known that the slow and fast light can be obtained using Brillouin scattering, our device is far smaller and uses far less power than any other previous demonstration, by several orders-of-magnitude. However, we must sacrifice bandwidth to obtain such performance," Kim added.

In their studies, Bahl's research group uses the extremely minute forces exerted by light to generate and control mechanical vibrations of microscale and nanoscale devices—a field called optomechanics. In resonant microcavities, these miniscule forces can be enhanced by many orders of magnitude. They are using these phenomena to unearth new physics behind how solids, liquids, and gases interact with light.

 Explore further: Subwavelength optical fibers to diffuse light

More information: Nature Physics, dx.doi.org/10.1038/nphys3236

Journal reference: Nature Physics  

Provided by University of Illinois at Urbana-Champaign  

Researchers demonstrate novel, tunable nanoantennas

This is an illustration of the pillar-based Au bowtie nanoantenna arrays undergoing selective actuation due to an electromagnetic-induced force. Credit: University of Illinois

Read more at: http://phys.org/news/2014-07-tunable-nanoantennas.html#jCp

A research team from the University of Illinois at Urbana-Champaign has developed a novel, tunable nanoantenna that paves the way for new kinds of plasmonic-based optomechanical systems, whereby plasmonic field enhancement can actuate mechanical motion.

"Recently, there has been a lot of interest in fabricating metal-based nanotextured surfaces that are pre-programmed to alter the properties of light in a specific way after incoming light interacts with it," explained Kimani Toussaint, an associate professor of mechanical science and engineering who led the research. "For our approach, one can take a nanoarray structure that was already fabricated and further reconfigure the plasmonic, and hence, optical properties of select antennas. Therefore, one can decide after fabrication, rather than before, how they want their nanostructure to modify light.

The researchers developed a novel, metal, pillar-bowtie nanoantenna (p-BNA) array template on 500-nanometer tall glass pillars (or posts). In doing so, they demonstrated that the gap size for either individual or multiple p-BNAs can be tuned down to approx. 5 nm (approx. 4x smaller than what is currently achievable using conventional electron-beam lithography techniques).

"On a fundamental level, our work demonstrates electron-beam based manipulation of nanoparticles an order of magnitude larger than previously possible, using a simple SEM operating at only a fraction of the electron energies of previous work," said Brian Roxworthy, who earned his PhD in electrical and computer engineering (ECE) at Illinois and was first author of the paper published in Nature Communications.

"The dramatic deformation of the nanoantennas we observe is facilitated by strong in-gap plasmonic modes excited by the passing electrons, which give rise to nanoNewton-magnitude gradient forces on the constituent metal particles."

The interdisiciplinary research team—that included Abdul Bhuiya (MS student in ECE student), Xin Yu (ECE post-grad), and K.C. Chow (a research engineer at the Micro and Nanotechnology Laboratory)—also demonstrated that the gap size for either individual or multiple p-BNAs can be tuned down to approximately 5 nm (roughly 4x smaller than what is currently achievable using conventional electron-beam lithography techniques).

The team demonstrated that an electron beam from a standard scanning electron microscope (SEM) can be used to deform either individual p-BNA structures or groups of p-BNAs within a sub-array with velocities as large as 60 nanometers per second. A photonic-crystal fiber was used to generate (quasi-white light) supercontinuum to probe the spectral response of select regions within the array.

The researchers said the importance of this work is three-fold: It enables tuning of the optical (plasmonic) response of the nanoantennas, down to the level of a single nanoantenna (approximately 250 nanometers across); it could lead to unique, spatially addressable nanophotonic devices for sensing and particle manipulation, for example; and, it provides a fertile platform for studying mechanical, electromagnetic, and thermal phenomena in a nanoscale system.

The team believes that the relatively high aspect ratio (pillar height-to-thickness) of 4.2 for the p-BNAs, along with a significant thermal contribution, permit sufficient compliance of the pillars to be actuated by electron-beam-induced gradient forces. Based on the observed experiments, the gradient force is estimated to be on the order of nanoNewtons.

"Our fabrication process shows for the first time an innovative way of fabricating plasmonic nanoantenna structures under the SEM, which avoids complications such as proximity effects from conventional lithography techniques," Bhuiya said. "This process also reduces the gap of the nanoantennas down to ~5 nm under SEM with a controlled reduction rate. With this new fabrication technique, it opens an avenue to study different phenomena which leads to new exciting research fields."

New metamaterial efficiently captures sunlight and if scaled could capture or emit electromagnetic radiation at microwave and terahertz frequencies

Tapered ridges, made from alternating layers of metal and insulating material deposited on a surface, can produce a metamaterial that is tuned to a range of specific frequencies of light. Light of different wavelengths is absorbed by the material at different levels, where the light's wavelength matches the width of the ridges. Image: Yanxia Cui

http://www.physorg.com/news/2012-03-metamaterial-efficient-capturing-sunlight-solar.html
March 9, 2012 by David L. Chandler
Metamaterials are a new class of artificial substances with properties unlike anything found in the natural world. Some have been designed to act as invisibility cloaks; others as superlenses, antenna systems or highly sensitive detectors. Now, researchers at MIT and elsewhere have found a way to use metamaterials to absorb a wide range of light with extremely high efficiency, which they say could lead to a new generation of solar cells or optical sensors.

Nicholas X. Fang, the Brit (1961) and Alex (1949) d’Arbeloff Career Development Associate Professor in Engineering Design in MIT's Department of Mechanical Engineering, says that most thin materials used to fully capture light are limited to a very narrow range of wavelengths and angles of incidence. The new design uses a pattern of wedge-shaped ridges whose widths are precisely tuned to slow and capture light of a wide range of wavelengths and angles of incidence.
These metamaterials can be extremely thin, saving weight and cost. Fang compares the tapered structures to the cochlea of the inner ear, which responds to different frequencies of sound at different points along its narrowing structure. “Our ears separate different frequencies and gather them at different depths,” he says; similarly, the metamaterial wedges harvest photons at different depths.
The actual structure of the material is etched from alternating layers of metal and an insulating material called a dielectric, whose response to polarized light can be varied by changing an electric field applied to the material. The creation of this new material is described in a paper to be published in a forthcoming issue of the journal Nano Letters. A preliminary version of Fang’s paper — co-authored with researchers at Zhejiang University and Taiyuan University in China, and the University of Illinois at Urbana-Champaign — is available online now.
Kin Hung Fung, an MIT postdoc and co-author of the Nano Letters paper, says, “What we have done is to design a multilayer sawtooth structure that can absorb a wide range of frequencies” with an efficiency of more than 95 percent. Previously, such efficiency could only be achieved with materials tuned to a very narrow band of wavelengths. “High-efficiency absorption has been achieved before, but this design has an extremely wide window” for colors of light, Fung says

Metamaterials have been “a very hot topic this decade,” he says, “because they can help us to design functional materials that interact with light in unconventional ways.” By using the tuned metamaterial, he says, his team was able to slow light to less than one-hundredth of its normal speed in a vacuum, making it much easier to trap inside the material. “When something is going very fast, it’s difficult to catch it,” he says, “so we slow it down so it’s easier to absorb.”
The material can easily be fabricated using equipment that is already standard in conventional photovoltaic-cell manufacturing. Although the initial work was based on computer simulations, the team is now working on lab experiments to confirm their findings.
Besides solar cells, the design could be used to make efficient infrared detectors for a selected range of wavelengths. “We can selectively enhance the material’s interaction with infrared light at the wavelengths we want,” Fung says.
Fang says that by its nature, the material would be both a very efficient emitter and absorber of photons — so in addition to potential uses in new kinds of solar cells or infrared detectors, the material could be used for infrared-light emitting applications, such as devices for generating electricity from heat. In addition, the researchers say the principle can be scaled so that it could be used to capture or emit electromagnetic radiation at other wavelengths, such as microwave and terahertz frequencies. It could even be used to produce visible light with extremely low energy loss, creating a new kind of high-efficiency light bulb.
Richard Averitt, a professor of physics at Boston University who was not involved in this research, calls the sawtooth-shaped structure developed by this team “a unique and impressive approach toward realizing functional broadband absorbers” that could have applications in thermal detection and in light harvesting for energy applications. He cautions that further work is needed to ease fabrication and integration of the materials, but adds, “This is an intriguing slow-wave structure that should inspire new developments in this field.”
Provided by Massachusetts Institute of Technology (news : web)
This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.