Berkeley Lab scientists say marriage opens door to development of single device with exceptional range of optical capabilities
Bringing opposing forces together in one place is as challenging as you would imagine it to be, but researchers in the field of optical science have done just that.
Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have for the first time created a single device that acts as both a laser and an anti-laser, and they demonstrated these two opposite functions at a frequency within the telecommunications band.
Their findings, reported in a paper published today in the journal Nature Photonics, lay the groundwork for developing a new type of integrated device with the flexibility to operate as a laser, an amplifier, a modulator, and an absorber or detector.
“In a single optical cavity we achieved both coherent light amplification and absorption at the same frequency, a counterintuitive phenomenon because these two states fundamentally contradict each other,” said study principal investigator Xiang Zhang, senior faculty scientist at Berkeley Lab’s Materials Sciences Division. “This is important for high-speed modulation of light pulses in optical communication.”
Reversing the laser
The concept of anti-lasers, or coherent perfect absorber (CPA), emerged in recent years as something that reverses what a laser does. Instead of strongly amplifying a beam of light, an anti-laser can completely absorb incoming coherent light beams.
While lasers are already ubiquitous in modern life, applications for anti-lasers—first demonstrated five years ago by Yale University researchers—are still being explored. Because anti-lasers can pick up weak coherent signals in the midst of a “noisy” incoherent background, it could be used as an extremely sensitive chemical or biological detector.
A device that can incorporate both capabilities could become a valuable building block for the construction of photonic integrated circuits, the researchers said.
“On-demand control of light from coherent absorption to coherent amplification was never imagined before, and it remains highly sought after in the scientific community,” said study lead author Zi Jing Wong, a postdoctoral researcher in Zhang’s lab. “This device can potentially enable a very large contrast in modulation with no theoretical limits.”
The researchers utilized sophisticated nanofabrication technology to build 824 repeating pairs of gain and loss materials to form the device, which measured 200 micrometers long and 1.5 micrometers wide. A single strand of human hair, by comparison, is about 100 micrometers in diameter.
The gain medium was made out of indium gallium arsenide phosphide, a well-known material used as an amplifier in optical communications. Chromium paired with germanium formed the loss medium. Repeating the pattern created a resonant system in which light bounces back and forth throughout the device to build up the amplification or absorption magnitude.
If one is to send light through such a gain-loss repeating system, an educated guess is that light will experience equal amounts of amplification and absorption, and the light will not change in intensity. However, this is not the case if the system satisfies conditions of parity-time symmetry, which is the key requirement in the device design.
Balance and symmetry
Parity-time symmetry is a concept that evolves from quantum mechanics. In a parity operation, positions are flipped, such as the left hand becoming the right hand, or vice versa.
Now add in the time-reversal operation, which is akin to rewinding a video and observing the action backwards. The time-reversed action of a balloon inflating, for example, would be that same balloon deflating. In optics, the time-reversed counterpart of an amplifying gain medium is an absorbing loss medium.
A system that returns to its original configuration upon performing both parity and time-reversal operations is said to fulfill the condition for parity-time symmetry.
Soon after the discovery of the anti-laser, scientists had predicted that a system exhibiting parity-time symmetry could support both lasers and anti-lasers at the same frequency in the same space. In the device created by Zhang and his group, the magnitude of the gain and loss, the size of the building blocks, and the wavelength of the light moving through combine to create conditions of parity-time symmetry.
When the system is balanced and the gain and loss are equal, there is no net amplification or absorption of the light. But if conditions are perturbed such that the symmetry is broken, coherent amplification and absorption can be observed.
In the experiments, two light beams of equal intensity were directed into opposite ends of the device. The researchers found that by tweaking the phase of one light source, they were able to control whether the light waves spent more time in amplifying or absorbing materials.
Speeding up the phase of one light source results in an interference pattern favoring the gain medium and the emission of amplified coherent light, or a lasing mode. Slowing down the phase of one light source has the opposite effect, resulting in more time spent in the loss medium and the coherent absorption of the beams of light, or an anti-lasing mode.
If the phase of the two wavelengths are equal and they enter the device at the same time, there is neither amplification nor absorption because the light spends equal time in each region.
The researchers targeted a wavelength of about 1,556 nanometers, which is within the band used for optical telecommunications.
“This work is the first demonstration of balanced gain and loss that strictly satisfies conditions of parity-time symmetry, leading to the realization of simultaneous lasing and anti-lasing,” said study co-author Liang Feng, former postdoctoral researcher in Zhang’s Lab, and now an assistant professor of electrical engineering at the University at Buffalo. “The successful attainment of both lasing and anti-lasing within a single integrated device is a significant step towards the ultimate light control limit.”
Laser applications may benefit from crystal research by scientists at the National Institute of Standards and Technology (NIST) and China’s Shandong University. They have discovered a potential way to sidestep longstanding difficulties with making the crystals that are a crucial part of laser technology. But the science behind their discovery has experts scratching their heads.
The findings, published today in Science Advances, suggest that the relatively large crystals used to change several properties of light in lasers – changes that are crucial for making lasers into practical tools – might be created by stacking up far smaller, rod-shaped microcrystals that can be grown easily and cheaply.
So far, the team’s microcrystals outperform conventional crystals in some ways, suggesting that harnessing them could signal the end of a long search for a fast, economical way to develop large crystals that would otherwise be prohibitively expensive and time-consuming to create. But the microcrystals also challenge conventional scientific theory as to why they perform as they do.
The color you see in a laser’s light is often different than the one it initially generates. Many lasers create infrared light, which then passes through a crystal converting its energy – and therefore its wavelength – to light of a visible color like green or blue.
Frequently, that crystal is made of potassium diphosphate (KDP), a common material that has properties that make it invaluable: Not only can a KDP crystal alter the light’s color, but it also can act as a switch that changes the light’s polarization (the direction in which its electric field vibrates) or prevent it from passing through the crystal until just the right moment. The data carried by laser light through fiber-optic cables depends on the light’s polarization, and many applications depend on a laser pulse’s timing.
Small KDP crystals are easy to make, and these find use in pocket laser pointers and telecommunications systems alike. But for higher-energy applications, scientists have searched for decades for a way to make large, high-quality crystals that can survive repeated exposure to intense laser pulses, but a solution has remained elusive.
The team has found useful results by growing KDP crystals in solution. These crystals take the form of hexagonal-shaped hollow tubes and long rods just a few micrometers wide. Individually, these KDP microcrystals have an energy-conversion efficiency surpassing even the best KDP crystals under the same conditions, raising the possibility of directly growing crystals for use in telecommunications.
The team also suggests the rods could be stacked up like firewood, building a larger piece out of billions of the tiny filaments. Before they are stacked together they could be coated by a thin layer of conductive material that carries heat away, rendering them capable of handling repeated pulses of high-intensity laser light – potentially broadening their application range if a way can be found to stack them.
The mystery is why the microcrystals perform as they do. Basic physics says they shouldn’t. Conventional physics models indicate that an optical medium like a crystal must not be symmetric about its center if it is to convert energy efficiently, yet these microcrystals appear to break this rule.
“We’ve spoken to a number of experts in different fields worldwide, and none of them can explain it,” says NIST physicist Lu Deng. “Currently no theory can explain the initial growth mechanism of this exotic crystal. It’s challenging our current understanding in fields from crystallography to condensed matter physics.”
While theory catches up with data, Deng said the team is concentrating on the engineering challenges of growing stackable microcrystal rods.
“We can grow more than 1,000 microstructures every 10 minutes or so on a single glass slide, so growing a large amount is not a problem,” he said. “What we need to figure out is how to grow a large fraction of them with nearly uniform cross-sections since that will be important in the final assembly stage.”
White light from lasers demonstrates data speeds of up to 2 GB/s
A nanocrystalline material that rapidly makes white light out of blue light has been developed by KAUST researchers.
While Wi-Fi and Bluetooth are now well established technologies, there are several advantages gained by shortening the wavelength of the electromagnetic waves used for transmitting information.
So-called visible-light communication (VLC) makes use of parts of the electromagnetic spectrum that are unregulated and is potentially more energy-efficient. VCL also offers a way to combine information transmission with illumination and display technologies–for example, using ceiling lights to provide internet connections to laptops.
Many such VLC applications require light-emitting diodes (LEDs) that produce white light. These are usually fabricated by combining a diode that emits blue light with phosphorous that turns some of this radiation into red and green light. However, this conversion process is not fast enough to match the speed at which the LED can be switched on and off.
“VLC using white light generated in this way is limited to about one hundred million bits per second,” said KAUST Professor of Electrical Engineering Boon Ooi.
Instead, Ooi, , Associate Professor Osman Bakr and their colleagues use a nanocrystal-based converter that enables much higher data rates.
The team created nanocrystals of cesium lead bromide that were roughly eight nanometers in size using a simple and cost-effective solution-based method that incorporated a conventional nitride phosphor. When illuminated by a blue laser light, the nanocrystals emitted green light while the nitride emitted red light. Together, these combined to create a warm white light.
The researchers characterized the optical properties of their material using a technique known as femtosecond transient spectroscopy. They were able to show that the optical processes in cesium lead bromide nanocrystals occur on a time-scale of roughly seven nanoseconds. This meant they could modulate the optical emission at a frequency of 491 Megahertz, 40 times faster than is possible using phosphorus, and transmit data at a rate of two billion bits per second.
“The rapid response is partly due to the size of the crystals,” said Bakr. “Spatial confinement makes it more likely that the electron will recombine with a hole and emit a photon.”
Importantly, the white light generated using their perovskite nanostructures was of a quality comparable to present LED technology.
“We believe that white light generated using semiconductor lasers will one day replace the LED white-light bulbs for energy-efficient lighting,” said Ooi.
Learn more: Wi-fi from lasers
Cancer treatments based on laser irridation of tiny nanoparticles that are injected directly into the cancer tumor are working and can destroy the cancer from within.
Researchers from the Niels Bohr Institute and the Faculty of Health Sciences at the University of Copenhagen have developed a method that kills cancer cells using nanoparticles and lasers. The treatment has been tested on mice and it has been demonstrated that the cancer tumors are considerably damaged. The results are published in the scientific journal, Scientific Reports.
Traditional cancer treatments like radiation and chemotherapy have major side affects, because they not only affect the cancer tumors, but also the healthy parts of the body. A large interdisciplinary research project between physicists at the Niels Bohr Institute and doctors and human biologists at the Panum Institute and Rigshospitalet has developed a new treatment that only affects cancer tumors locally and therefore is much more gentle on the body. The project is called Laser Activated Nanoparticles for Tumor Elimination (LANTERN). The head of the project is Professor Lene Oddershede, a biophysicist and head of the research group Optical Tweezers at the Niels Bohr Institute at the University of Copenhagen in collaboration with Professor Andreas Kjær, head of the Cluster for Molecular Imaging, Panum Institute.
After experimenting with biological membranes, the researchers have now tested the method on living mice. In the experiments, the mice are given cancer tumors of laboratory cultured human cancer cells.
“The treatment involves injecting tiny nanoparticles directly into the cancer. Then you heat up the nanoparticles from outside using lasers. It is a strong interaction between the nanoparticles and the laser light, which causes the particles to heat up. What then happens is that the heated particles damage or kill the cancer cells,” explains Lene Oddershede.
Design and effect
The small nanoparticles are between 80 and 150 nanometers in diameter (a nanometer is a millionth of a millimeter). The tested particles consist of either solid gold or a shell structure consisting of a glass core with a thin shell of gold around it. Some of the experiments aimed to find out which particles are most effective in reducing tumors.
“As physicists we have great expertise in the interaction between light and nanoparticles and we can very accurately measure the temperature of the heated nanoparticles. The effectiveness depends on the right combination between the structure and material of the particles, their physical size and the wavelength of the light,” explains Lene Oddershede.
The experiments showed that the researchers got the best results with nanoparticles that were 150 nanometers in size and consisted of a core of glass coated with gold. The nanoparticles were illuminated with near-infrared laser light, which is the best at penetrating through the tissue. In contrast to conventional radiation therapy, the near-infrared laser light causes no burn damage to the tissue that it passes through. Just an hour after the treatment, they could already directly see with PET scans that the cancer cells had been killed and the effect continued for at least two days after the treatment.
“Now we have proven that the method works. In the longer term, we would like the method to work by injecting the nanoparticles into the bloodstream, where they end up in the tumors that may have metastasized. With the PET scans we can see where the tumors are and irridate them with lasers, while also effectively assessing how well the treatment has worked shortly after the irradiation. In addition, we will coat the particles with chemotherapy, which is released by the heat and which will also help kill the cancer cells,” explains Lene Oddershede.
TACC Stampede, Lonestar supercomputers help discover gamma ray creation from lasers
Ever play with a magnifying lens as a kid? Imagine a lens as big as the Earth. Now focus sunlight down to a pencil tip. That still wouldn’t be good enough for what some Texas scientists have in mind. They want to make light even 500 times more intense. And they say it could open the door to the most powerful radiation in the universe: gamma rays.
Comic book readers might know about gamma rays. The Incredible Hulk was transformed from mild scientist into wild superhero by gamma rays from a nuclear explosion. The real gamma rays form in nature from radioactive decay of the atomic nucleus. Besides hazardous materials, you’d have to look in exotic places like near a black hole or closer to home at lightning in the upper atmosphere to find natural forces capable of making gamma rays.
Scientists have found that gamma rays, like the Hulk, can do heroic things too — if they can be controlled. Hospitals now eradicate cancer tumors using a ‘gamma ray knife’ with surgical precision. The rays can also image brain activity. And gamma rays are used to quickly scan cargo containers for terrorist materials. But it’s near impossible to make gamma rays with non-radioactive materials. To do that today one needs a colossal atom smasher like at CERN or SLAC. No one has been able to make a gamma ray beam from lasers. But it can be done, say scientists at The University of Texas (UT) at Austin.
Supercomputers might have helped unlock a new way to make controlled beams of gamma rays from a laser that fits on a table-top, according to research physicist Alex Arefiev, who has a dual appointment at the Institute for Fusion Studies and at the Center for High Energy Density Science at UT Austin. Arefiev co-authored the study, “Enhanced multi-MeV photon emission by a laser-driven electron beam in a self-generated magnetic field,” published May 2016 in the journal Physical Review Letters.
“One of the key results that we found is that a laser pulse can be efficiently converted into a beam of very energetic photons,” Arefiev said. “They are more than one million times more energetic than the photons in the laser pulse. Until recently, there hasn’t been a method for producing a beam of such energetic photons. So the proposed regime can be groundbreaking for a number of applications and also for fundamental science studies.”
A new technique, developed at EPFL, combines microfluidics and lasers to guide cells in 3D space, overcoming major limitations to tissue engineering.
Future medicine is bound to include extensive tissue-engineering technologies such as organs-on-chips and organoids – miniature organs grown from stem cells. But all this is predicated on a simple yet challenging task: controlling cellular behavior in three dimensions. So far, most cell culture approaches are limited to two-dimensional environments (e.g. a Petri dish or a chip), but that neither matches real biology nor helps us sculpt tissues and organs. Two EPFL scientists have now developed a new method that uses lasers to carve out paths inside biocompatible gels to locally influence cell function and promote tissue formation. The work is published in Advanced Materials.
In the body, cells grow in 3D microspaces that are specific to each type of tissue – liver, kidney, lung, heart, brain etc. These microenvironments are important because they control the behavior of the cells, e.g. how they interact with other parts of the tissue to help it develop, function, and repair. In addition, the microenvironments themselves are very dynamic and adaptable, sending the cells various biochemical signals to adapt their behavior to physiological changes.
This means that any successful merging of biology and engineering must first be able to grow cells in custom-built yet biologically active 3D spaces. Working at EPFL’s Institute of Bioengineering, Matthias Lütolf and his PhD student Nathalie Brandenberg have developed a method that uses a laser to cut three-dimensional pathways and networks for cells inside a hydrogel scaffold that matches their natural environment.
The method combines lasers with microfluidics – the science of controlling fluids in micrometer-sized spaces. Here, the scientists used focalized short-pulsed lasers, which can generate enough power to create tiny tunnels in different biocompatible gels already used in cell biology and tissue engineering. The laser can be applied before or even during 3D cell culture, meaning that the cells can be controlled in real time to match their natural growth.
A new class of lasers developed by a team that included physics researchers at Kansas State University could help scientists measure distances to faraway targets, identify the presence of certain gases in the atmosphere and send images of the earth from space.
These energy-efficient lasers also are portable, produce light at difficult-to-reach wavelengths and have the potential to scale to high-powered versions.
The new lasers were invented by Brian Washburn and Kristan Corwin, both associate professors of physics at Kansas State University’s College of Arts & Sciences, along with Andrew Jones, a May 2012 doctoral graduate in physics, and Rajesh Kadel, a May 2014 doctoral graduate in physics. Other contributors include three University of New Mexico physics and astronomy researchers: Wolfgang Rudolf, a Regents professor and department chair, Vasudevan Nampoothiri, a research assistant professor, and Amarin Ratanavis, a doctoral student; and John Zavada, a Virginia-based optic and photonic physicist who brought them all together.
The new lasers are fiber-based and use various molecular gases to produce light. They differ from traditional glass-tube lasers, which are large and bulky, and have mirrors to reflect the light. But the novel lasers use a hollow fiber with a honeycomb structure to hold gas and to guide light. This optical fiber is filled with a molecular gas, such as hydrogen cyanide or acetylene. Another laser excites the gas and causes a molecule of the excited gas to spontaneously emit light. Other molecules in the gas quickly follow suit, which results in laser light.
China is well-known for investing in technological advancements – from space explorations to military warfare. In its latest groundbreaking invention, soldiers are now in possession of Star Wars-like laser guns.
This laser gun could soon have the ability to attack heat-seeking sensors on missiles, satellites and other warfare using breakthrough portable-laser technology. When the laser comes in contact with infrared missiles, their sensors become disabled, rendering them useless.
A team of Chinese researchers, led by Professor Zhi-Yuan Li of the Chinese Academy of Sciences’ Institute of Physics, decreased the size and mechanism that produces high-frequency laser down using a portable device the size of a suitcase. These can be easily mounted on tanks, aircrafts and can be used by the soldiers themselves.
“This is a groundbreaking achievement,” said a professor at Tsinghua University, Beijing who requested anonymity in the issue.
“Nobody has generated a laser at such a high frequency on a single piece of crystal before. Their technology will significantly simplify the process of ultrafast laser production and reduce the size of relevant devices,” added the anonymous professor.
Laser Technology Is Banned To Use In Warfare
The Protocol on Blinding Laser Weapons which was updated on October 1995, an international convention and treaty, bans the use of lasers and blinding weapons used against enemies. It states that it is prohibited to employ laser weapons specifically designed to cause permanent blindness to unenhanced vision like the naked eye.
Though China aims to use its newest laser guns on optical and thermal sensors on vehicles, drones, robots and aircraft, the possession of this kind of technology is still fair game. Aside from damaging enemy targets, it can also pick up encrypted communications and detect stealth aircraft.
New drug delivery method targets cancer cells – not the entire body – and limits chemotherapy side effects
Now, researchers are developing a better delivery method by encapsulating the drugs in nanoballoons – which are tiny modified liposomes that, upon being struck by a red laser, pop open and deliver concentrated doses of medicine.
Because the nanoballoons encapsulate the anti-cancer drugs, they diminish the drugs’ interaction with healthy bodily systems.
“The nanoballoon is a submarine. The drug is the cargo. We use a laser to open the submarine door which releases the drug. We close the door by turning the laser off. We then retrieve the submarine as it circulates through the bloodstream.” Lovell will continue fundamental studies to better understand why the treatment works so well in destroying tumors in mice, and to optimize the process.