Scientists use light to accelerate supercurrents, access forbidden light, quantum properties

This illustration shows light wave acceleration of supercurrents, which gives researchers access to a new class of quantum phenomena. That access could chart a path forward for practical quantum computing, sensing and communicating applications. Larger image. Image courtesy of Jigang Wang.

https://www.news.iastate.edu/news/2020/05/19/forbidden-light

AMES, Iowa – Scientists are using light waves to accelerate supercurrents and access the unique properties of the quantum world, including forbidden light emissions that one day could be applied to high-speed, quantum computers, communications and other technologies.

The scientists have seen unexpected things in supercurrents – electricity that moves through materials without resistance, usually at super cold temperatures – that break symmetry and are supposed to be forbidden by the conventional laws of physics, said Jigang Wang, a professor of physics and astronomy at Iowa State University, a senior scientist at the U.S. Department of Energy’s Ames Laboratory and the leader of the project.

Jigang Wang. Photo by Christopher Gannon.

Wang’s lab has pioneered use of light pulses at terahertz frequencies– trillions of pulses per second – to accelerate electron pairs, known as Cooper pairs, within supercurrents. In this case, the researchers tracked light emitted by the accelerated electrons pairs. What they found were “second harmonic light emissions,” or light at twice the frequency of the incoming light used to accelerate electrons.

That, Wang said, is analogous to color shifting from the red spectrum to the deep blue.

“These second harmonic terahertz emissions are supposed to be forbidden in superconductors,” he said. “This is against the conventional wisdom.”

Wang and his collaborators – including Ilias Perakis, professor and chair of physics at the University of Alabama at Birmingham and Chang-beom Eom, the Raymond R. Holton Chair for Engineering and Theodore H. Geballe Professor at the University of Wisconsin-Madison – report their discovery in a research paper just published online by the scientific journal Physical Review Letters. (See sidebar for a list of the other co-authors.)

“The forbidden light gives us access to an exotic class of quantum phenomena – that’s the energy and particles at the small scale of atoms – called forbidden Anderson pseudo-spin precessions,” Perakis said.

(The phenomena are named after the late Philip W. Anderson, co-winner of the 1977 Nobel Prize in Physics who conducted theoretical studies of electron movements within disordered materials such as glass that lack a regular structure.)

Wang’s recent studies have been made possible by a tool called quantum terahertz spectroscopy that can visualize and steer electrons. It uses terahertz laser flashes as a control knob to accelerate supercurrents and access new and potentially useful quantum states of matter. The National Science Foundation has supported development of the instrument as well as the current study of forbidden light.  

The scientists say access to this and other quantum phenomena could help drive major innovations: 

• “Just like today’s gigahertz transistors and 5G wireless routers replaced megahertz vacuum tubes or thermionic valves over half a century ago, scientists are searching for a leap forward in design principles and novel devices in order to achieve quantum computing and communication capabilities,” said Perakis, with Alabama at Birmingham. “Finding ways to control, access and manipulate the special characteristics of the quantum world and connect them to real-world problems is a major scientific push these days. The National Science Foundation has included quantum studies in its ‘10 Big Ideas’ for future research and development critical to our nation.”

• Wang said, “The determination and understanding of symmetry breaking in superconducting states is a new frontier in both fundamental quantum matter discovery and practical quantum information science. Second harmonic generation is a fundamental symmetry probe. This will be useful in the development of future quantum computing strategies and electronics with high speeds and low energy consumption.”

Before they can get there, though, researchers need to do more exploring of the quantum world. And this forbidden second harmonic light emission in superconductors, Wang said, represents “a fundamental discovery of quantum matter.”

Physicists use terahertz flashes to uncover new state of matter hidden by superconductivity

Jigang Wang of Iowa State and the Ames Laboratory led experiments that switched on a hidden state of matter in a superconductive alloy. Larger photo. Photo by Christopher Gannon.

https://www.news.iastate.edu/news/2018/06/04/terahertzflashes

AMES, Iowa – Using the physics equivalent of the strobe photography that captures every twitch of a cheetah in full sprint, researchers have used ultrafast spectroscopy to visualize electrons interacting as a hidden state of matter in a superconductive alloy.

It takes intense, single-cycle pulses of photons – flashes – hitting the cooled alloy at terahertz speed – trillions of cycles per second – to switch on this hidden state of matter by modifying quantum interactions down at the atomic and subatomic levels.

And then it takes a second terahertz light to trigger an ultrafast camera to take images of the state of matter that, when fully understood and tuned, could one day have implications for faster, heat-free, quantum computing, information storage and communication.

The discovery of this new switching scheme and hidden quantum phase was full of conceptual and technical challenges.

To find new, emergent electron states of matter beyond solids, liquids and gases, today’s condensed matter physicists can no longer fully rely on traditional, slow, thermodynamic tuning methods such as changing temperatures, pressures, chemical compositions or magnetic fields, said Jigang Wang, an Iowa State University professor of physics and astronomy and a faculty scientist at the U.S. Department of Energy’s Ames Laboratory.

“The grand, open question of what state is hidden underneath superconductivity is universal, but poorly understood,” Wang said. “Some hidden states appear to be inaccessible with any thermodynamic tuning methods.”

The new quantum switching scheme developed by the researchers (they call it terahertz light-quantum-tuning) uses short pulses of trillionths of a second at terahertz frequency to selectively bombard, without heating, superconducting niobium-tin, which at ultracold temperatures can conduct electricity without resistance. The flashes suddenly switch the model compound to a hidden state of matter.

The scientific journal Nature Materials has just published a paper describing the discovery. Wang is corresponding author. Leading authors are Xu Yang and Chirag Vaswani, Iowa State graduate students in physics and astronomy. (See sidebar for other co-authors.)

In most cases, exotic states of matter such as the one described in this research paper are unstable and short-lived. In this case, the state of matter is metastable, meaning it doesn’t decay to a stable state for an order of magnitude longer than other, more typical transient states of matter.

The fast speed of the switch to a hidden quantum state likely has something to do with that.

“Here, the quantum quench (change) is so fast, the system is trapped in a strange ‘plateau’ and doesn’t know how to go back,” Wang said. “With this fast-quench, yet non-thermal system, there’s no normal place to go.”

A remaining challenge for the researchers is to figure out how to control and further stabilize the hidden state and determine if it is suitable for quantum logic operations, Wang said. That could allow researchers to harness the hidden state for practical functions such as quantum computing and for fundamental tests of bizarre quantum mechanics.

It all starts with the researchers’ discovery of a new quantum switching scheme that gives them access to new and hidden states of matter.

Said Wang: “We are creating and controlling a new quantum matter that can’t be achieved by any other means.”

Physicists use terahertz flashes to uncover new state of matter hidden by superconductivity

https://www.news.iastate.edu/news/2018/06/04/terahertzflashes

Jigang Wang of Iowa State and the Ames Laboratory led experiments that switched on a hidden state of matter in a superconductive alloy. Larger photo. Photo by Christopher Gannon.

AMES, Iowa – Using the physics equivalent of the strobe photography that captures every twitch of a cheetah in full sprint, researchers have used ultrafast spectroscopy to visualize electrons interacting as a hidden state of matter in a superconductive alloy.

It takes intense, single-cycle pulses of photons – flashes – hitting the cooled alloy at terahertz speed – trillions of cycles per second – to switch on this hidden state of matter by modifying quantum interactions down at the atomic and subatomic levels.

And then it takes a second terahertz light to trigger an ultrafast camera to take images of the state of matter that, when fully understood and tuned, could one day have implications for faster, heat-free, quantum computing, information storage and communication.

The discovery of this new switching scheme and hidden quantum phase was full of conceptual and technical challenges.

To find new, emergent electron states of matter beyond solids, liquids and gases, today’s condensed matter physicists can no longer fully rely on traditional, slow, thermodynamic tuning methods such as changing temperatures, pressures, chemical compositions or magnetic fields, said Jigang Wang, an Iowa State University professor of physics and astronomy and a faculty scientist at the U.S. Department of Energy’s Ames Laboratory.

“The grand, open question of what state is hidden underneath superconductivity is universal, but poorly understood,” Wang said. “Some hidden states appear to be inaccessible with any thermodynamic tuning methods.”

The new quantum switching scheme developed by the researchers (they call it terahertz light-quantum-tuning) uses short pulses of trillionths of a second at terahertz frequency to selectively bombard, without heating, superconducting niobium-tin, which at ultracold temperatures can conduct electricity without resistance. The flashes suddenly switch the model compound to a hidden state of matter.

The scientific journal Nature Materials has just published a paper describing the discovery. Wang is corresponding author. Leading authors are Xu Yang and Chirag Vaswani, Iowa State graduate students in physics and astronomy. (See sidebar for other co-authors.)

In most cases, exotic states of matter such as the one described in this research paper are unstable and short-lived. In this case, the state of matter is metastable, meaning it doesn’t decay to a stable state for an order of magnitude longer than other, more typical transient states of matter.

The fast speed of the switch to a hidden quantum state likely has something to do with that.

“Here, the quantum quench (change) is so fast, the system is trapped in a strange ‘plateau’ and doesn’t know how to go back,” Wang said. “With this fast-quench, yet non-thermal system, there’s no normal place to go.”

A remaining challenge for the researchers is to figure out how to control and further stabilize the hidden state and determine if it is suitable for quantum logic operations, Wang said. That could allow researchers to harness the hidden state for practical functions such as quantum computing and for fundamental tests of bizarre quantum mechanics.

It all starts with the researchers’ discovery of a new quantum switching scheme that gives them access to new and hidden states of matter.

Said Wang: “We are creating and controlling a new quantum matter that can’t be achieved by any other means.”

Scientists directly observe light-to-energy transfer in new solar cell materials

https://phys.org/news/2017-06-scientists-light-to-energy-solar-cell-materials.html

Scientists at the U.S. Department of Energy's Ames Laboratory are now able to capture the moment less than one trillionth of a second a particle of light hits a solar cell and becomes energy, and describe the physics of the charge carrier and atom movement for the first time.

The generation and dissociation of bound electron and hole pairs, namely excitons, are key processes in solar cell and photovoltaic technologies, yet it is challenging to follow their initial dynamics and electronic coherence.

Using time-resolved low frequency spectroscopy in the terahertz spectral region, the researchers explored the photo-excitations of a new class of photovoltaic materials known as organometal halide perovskites. Organometallics are wonder materials for light-harvesting and electronic transport devices, and they combine best of both worlds— the high energy conversion performance of traditional inorganic photovoltaic devices, with the economic material costs and fabrication methods of organic versions.

"These devices are so new and so unique that the mechanism by which a particle of light, or photon, converts to charge carriers and how they move in a concerted way for energy conversion is not well understood, and yet that is the most fundamental processes in in solar cell and photovoltaic technologies," said Jigang Wang, an Ames Laboratory scientist and associate professor of physics at Iowa State University. "Why is this material so distinct? That has been the big question in the scientific community, and it has led to a fever of research and publication."

Ames Laboratory researchers wanted to know not only how the generation and dissociation of bound electron and hole pairs, namely excitons, happened in the material, they wanted to find out the quantum pathways and time interval of that event.

"If you look at the natural process, in photosynthesis, it's an extremely efficient process in some biological molecules, so it's also very coherent. We see a similar thing in a man-made system of a laser; a laser oscillates in a fixed wave pattern," said Wang. "If we can measure such a memory in the charge transport and energy migration in these materials, we can understand and control it, and have the potential to improve them by learning from Mother Nature."

Conventional multimeters for measuring electrical states in materials do not work for measuring excitons, which are electrically neutral quasiparticles with no zero current. Ultrafast terahertz spectroscopy techniques provided a contactless probe that was able to follow their internal structures, and quantify the photon-to-exciton event with time resolution better than one trillionth of a second.

Wang credited the contributions of researchers from multiple areas of expertise across the Ames Laboratory with the significance of the discovery. "This was only possible with the collaboration of experts in material design and fabrication, computational theory, and spectroscopy," he said. "Having those capabilities in one place is what makes Ames Laboratory one of the most forward looking places in this kind of photonic materials research."

The research is further discussed in a paper, "Ultrafast terahertz snapshots of excitonic Rydberg states and electronic coherence in an organometal halide perovskite", authored by Liang Luo, Long Men, Zhaoyu Liu, Yaroslav Mudryk, Xin Zhao, Yongxin Yao, Joong M. Park, Ruth Shinar, Joseph Shinar, Kai-Ming Ho, Ilias E. Perakis, Javier Vela, and Jigang Wang; and published in Nature Communications.

 Explore further: Perovskite edges can be tuned for optoelectronic performance

More information: Liang Luo et al. Ultrafast terahertz snapshots of excitonic Rydberg states and electronic coherence in an organometal halide perovskite, Nature Communications (2017). DOI: 10.1038/ncomms15565

Iowa State Physicists Win W.M. Keck Foundation Grant To Develop Terahertz Nanoscope

http://www.publicnow.com/view/7C85525E85A1BFCC59F34B952BF85C496329DB61

AMES, Iowa - Researchers from Iowa State University's department ofphysics and astronomy are working to build a powerful instrument capable of exploring and tuning materials in ways that could help solve the world's energy, information processing and data storage needs.

The researchers call their proposed instrument an extreme quantum terahertz nanoscope. Its primary function will be to discover materials and material functions at unprecedented scales of space, time and energy, enabling studies at scales that are ultrafast, ultrasmall and at very low frequency. The instrument will also allow researchers to control the materials they study.

'We want to develop a microscope that doesn't exist yet,' said Jigang Wang, the project leader, an Iowa State associate professor of physics and astronomy and an associate of the U.S. Department of Energy's Ames Laboratory. 'We're trying to see new things by looking across these three dimensions simultaneously. We want to understand how the electrons move and communicate in order to produce fascinating properties of materials.

'All of this will create a new paradigm to understand materials, control their properties and have far-reaching consequences to promote science and future technology.'

The W.M. Keck Foundation of Los Angeles - one of the country's largest philanthropic organizations - recently awarded a three-year, $1.3 million grant to support construction, commissioning and initial use of the nanoscope. The project will be known as the W.M. Keck Initiative in Ultrafast Quantum Microscopy of Emergent Orders.

Iowa State is also supporting the project with funds from the Office of the Vice President for Research, the College of Liberal Arts and Sciences and the department of physics and astronomy. And the Ames Laboratory is supporting the project with specialized laboratory space in the laboratory's new Sensitive Instrument Facility.

The grant was made through the Iowa State University Foundation, a private, nonprofit corporation dedicated to securing and managing gifts and grants that benefit Iowa State.

A diverse team and extreme scales

Wang said the quantum nanoscope project was made possible by the expertise, technical resources and close collaboration of Iowa State and Ames Laboratory scientists. The project team includes:

• Wang who has expertise in ultra-fast optics and terahertz spectroscopy of complex materials.
• Zhe Fei, an Iowa State assistant professor of physics and astronomy and an Ames Laboratory associate, who has expertise in scanning near-field optical microscopy.
• Paul C. Canfield, an Iowa State Distinguished Professor of physics and astronomy and an Ames Laboratory senior physicist, who has expertise in new materials design and discovery.
• Costas Soukoulis, an Iowa State Distinguished Professor of physics and astronomy and an Ames Laboratory senior physicist, and Thomas Koschny, an Ames Laboratory associate scientist, who have expertise in plasmonics, the study of light waves and metal surfaces, and metamaterials, materials with properties not found in nature.

Wang said the researchers will work together to develop a nanoscope that collects data at three extreme scales: billionths of a meter in space (nanometers), quadrillionths of a second in time (femtoseconds) and thousandths of electron volts in energy (milli-electron volts or terahertz).

He said each of those three dimensions - space, time and energy - is like the side of a triangle.

'On each side of that triangle humanity has achieved tremendous progress and understanding,' he said. 'But what about the region on the inside of the triangle, this inaccessible region that combines the best of all three fields? For many outstanding scientific and technological problems, our answers are largely limited by our inability to see inside this region. Through this award from the Keck Foundation, this instrument we're building will break down the barriers and allow us to see what's there.'

When the instrument's superconducting magnet, scanning near field microscopy probes, ultrafast lasers and other components are assembled, Wang said the nanoscope will be commissioned by studying graphene, a thin layer of carbon atoms packed in a honeycomb. The instrument's initial experiments will study a high-temperature, iron-based superconductor, a material that can conduct electricity with no resistance and support exotic magnetic properties when cooling in a fashion different from conventional metals.

The researchers will also experiment with using the nanoscope to manipulate electrons so they can tune materials with minimal heating of the samples. Study goals include discovering new states of matter and establishing the shortest times and smallest lengths for these states to switch.

The researchers wrote that they expect the nanoscope and its measurements to 'reveal the secrets of emergent-order phenomena and manipulate them at will, a monumental challenge in this age of novel materials.'

----

Based in Los Angeles, the W. M. Keck Foundation was established in 1954 by the late W. M. Keck, founder of the Superior Oil Company. The Foundation's grant making is focused primarily on pioneering efforts in the areas of medical, science and engineering research. The Foundation also maintains an undergraduate education program that promotes distinctive learning and research experiences for students in the sciences and in the liberal arts, and a Southern California Grant Program that provides support for the Los Angeles community, with a special emphasis on children and youth from low-income families, special needs populations and safety-net services. For more information, please visitwww.wmkeck.org.

Metamaterials shine bright as new terahertz source

A metamaterial that consists of a two-dimensional array of U-shaped gold structures (square background in the picture) efficiently emits terahertz frequency electromagnetic waves (red axis) when illuminated by a wavelength tunable near-infrared pump laser (blue axis). Credit: Ames Laboratory
 http://phys.org/news/2015-04-metamaterials-bright-terahertz-source.html#jCp

Metamaterials allow design and use of light-matter interactions at a fundamental level. An efficient terahertz emission from two-dimensional arrays of gold split-ring resonator metamaterials was discovered as a result of excitation by a near-infrared pulsed laser.

Terahertz waves are used in non-invasive imaging and sensing technology, in addition to information, communication, processing, and data storage technologies. Despite their widely recognized importance, however, there are few terahertz sources presently available due to the limitations of natural materials. This discovery opens new ways to use metamaterials for these important applications.

Broadband terahertz sources offer exciting possibilities to study fundamental physics principles, to develop non-invasive material imaging and sensing, and make possible terahertz information, communication, processing, and storage. The terahertz spectral range sits between infrared and typical radar frequencies, and the challenges of efficiently generating and detecting terahertz radiation has limited its use.

To solve these challenges, consider metamaterials; materials that allow control of the properties of light-matter interactions at the fundamental level. The building blocks of metamaterials, known as split-ring resonators, can be designed to exhibit strong electric and magnetic response to electromagnetic fields over a wide frequency range, from terahertz to infrared. Scientists at Ames Laboratory, Iowa State University, and Karlsruhe Institute of Technology in Germany have discovered that when a two-dimensional array of nanometer-sized gold metamaterial resonators is illuminated by a tunable near-infrared femtosecond laser, with wavelengths matching the magnetic resonance of the metamaterial, a strong broadband of terahertz electromagnetic waves is emitted.

The efficiency of this conversion to terahertz waves was significantly better than conventional materials that are presently used for these applications. Detailed analysis of the directionality and polarization of the emitted radiation reveals the fundamental nature of this efficient wavelength conversion. Further, these new metamaterials could allow integration of terahertz optoelectronics with high-speed telecommunications.

 Explore further: Improving terahertz optics with efficient broadband antireflection coatings

More information: "Broadband terahertz generation from metamaterials." Nature Communications 5, 3055 (2014). DOI: 10.1038/ncomms4055

OT-Quantum tricks drive magnetic switching into the fast lane-promising THz speeds

https://www.ameslab.gov/news/news-releases/quantum-tricks-drive-magnetic-switching-fast-lane

All-optical switching promises terahertz-speed hard drive and MRAM memory

Researchers at the U.S. Department of Energy’s Ames Laboratory, Iowa State University, and the University of Crete in Greece have found a new way to switch magnetism that is at least 1000 times faster than currently used in magnetic memory technologies. Magnetic switching is used to encode information in hard drives, magnetic random access memory and other computing devices. The discovery, reported in the April 4 issue of Nature, potentially opens the door to terahertz (10¹² hertz) and faster memory speeds.

Ames Laboratory physicist Jigang Wang and his team used short laser pulses to create ultra-fast changes in the magnetic structure, within quadrillionths of a second (femtosecond), from anti-ferromagnetic to ferromagnetic ordering in colossal magnetoresistive materials, which are promising for use in next-generation memory and logic devices. Scientists, led by Ilias E. Perakis, at the University of Crete developed the theory to explain the observation.

“The challenge facing magnetic writing, reading, storing and computing is speed, and we showed that we can meet the challenge to make the magnetic switches think ultra-fast in the femtosecond range – one quadrillionth of a second – by using quantum ‘tricks’ with ultrashort laser pulses ” said Wang, who is also an assistant professor of physics and astronomy at Iowa State University. In current magnetic storage and magneto-optical recording technology, magnetic field or continuous laser light is used. For example, photo-excitation causes atoms in ferromagnetic materials to heat up and vibrate, and the vibration, with the help of a magnetic field, causes magnetic flips. The flips are part of the process used to encode information.  “But the speed of such thermal magnetic switching is limited by how long it takes to vibrate the atoms, and by how fast a magnetic field can reverse magnetic regions” said Wang.  “And it is very difficult to exceed the gigahertz (109 hertz) switching speed limit of today’s magnetic writing/reading technology.”

Magnetic structure in a colossal magneto-resistive manganite is switched from antiferromagnetic to ferromagnetic ordering during about 100 femtosecond (10-15 s) laser pulse photo-excitation. With time so short and the laser pulses still interacting with magnetic moments, the magnetic switching is driven quantum mechanically – not thermally. This potentially opens the door to terahertz (1012 hertz) and faster memory writing/reading speeds.

So, some scientists have turned their attention to colossal magnetoresistive (CMR) materials because they are highly responsive to the external magnetic fields used to write data into memory, but do not require heat to trigger magnetic switching.

“Colossal magnetoresistive materials are very appealing for use in technologies, but we still need to understand more about how they work,” said Wang. “And, in particular, we must understand what happens during the very short periods of time when heating is not significant and the laser pulses are still interacting with magnetic moments in CMR materials. That means we must describe the process and control magnetism using quantum mechanics. We called this ‘quantum femto-magnetism.’”  

Wang’s team specializes in using ultra-fast spectroscopy, which Wang likens to high-speed strobe photography, because both use an external pump of energy to trigger a quick snapshot that can be then re-played afterwards. In ultra-fast laser spectroscopy, a short pulse of laser light is used to excite a material and trigger a measurement all on the order of femtoseconds.

“In one CMR manganite material, the magnetic order is switched during the 100-femtosecond-long laser pulse. This means that switching occurs by manipulating spin and charge quantum mechanically,” said Wang. “In the experiments, the second laser pulse ‘saw’ a huge photo-induced magnetization with an excitation threshold behavior developing immediately after the first pump pulse.”

Jigang Wang (center) and his team, Tianqi Li (left) and Aaron Patz (right), specialize in ultra-fast spectroscopy, which helps scientists understand changes in materials in very short time scales.

The fast switching speed and huge magnetization that Wang observed meet both requirements for applying CMR materials in ultra-fast, terahertz magnetic memory and logic devices. “Our strategy is to use all-optical quantum methods to achieve magnetic switching and control magnetism. This lays the groundwork for seeking the ultimate switching speed and capabilities of CMR materials, a question that underlies the entire field of spin-electronics,” said Wang. “And our hope is that this means someday we will be able to create devices that can read and write information faster than ever before, yet with less power consumed.”

Tianqi Li, Aaron Patz, Jiaqiang Yan and Thomas Lograsso collaborated on the experimental work at Ames Laboratory and Iowa State University. Leonidas Mouchliadis at the University of Crete and the Institute of Electronic Structure and Laser at the Foundation for Research and Technology - Hellas in Greece helped develop the theory used to interpret the experiments.

The research is supported by the Department of Energy’s Office of Science (sample synthesis and characterization) and the National Science Foundation (ultrafast experimental work). DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit the Office of Science website at science.energy.gov/.

The Ames Laboratory is a U.S. Department of Energy Office of Science national laboratory operated by Iowa State University. The Ames Laboratory creates innovative materials, technologies and energy solutions. We use our expertise, unique capabilities and interdisciplinary collaborations to solve global problems.

Quantum Tricks Drive Magnetic Switching Into the Fast Lane

Magnetic structure in a colossal magneto-resistive manganite is switched from antiferromagnetic to ferromagnetic ordering during about 100 femtosecond (10^-15 s) laser pulse photo-excitation. With time so short and the laser pulses still interacting with magnetic moments, the magnetic switching is driven quantum mechanically -- not thermally. This potentially opens the door to terahertz (10¹² hertz) and faster memory writing/reading speeds. (Credit: Image courtesy of DOE/Ames Laboratory)

http://www.sciencedaily.com/releases/2013/04/130403200312.htm

Apr. 3, 2013 — Researchers at the U.S. Department of Energy's Ames Laboratory, Iowa State University, and the University of Crete in Greece have found a new way to switch magnetism that is at least 1000 times faster than currently used in magnetic memory technologies. Magnetic switching is used to encode information in hard drives, magnetic random access memory and other computing devices.

The discovery, reported in the April 4 issue of Nature, potentially opens the door to terahertz (10¹² hertz) and faster memory speeds.

Ames Laboratory physicist Jigang Wang and his team used short laser pulses to create ultra-fast changes in the magnetic structure, within quadrillionths of a second (femtosecond), from anti-ferromagnetic to ferromagnetic ordering in colossal magnetoresistive materials, which are promising for use in next-generation memory and logic devices. Scientists, led by Ilias E. Perakis, at the University of Crete developed the theory to explain the observation
.
So, some scientists have turned their attention to colossal magnetoresistive (CMR) materials because they are highly responsive to the external magnetic fields used to write data into memory, but do not require heat to trigger magnetic switching.

"Colossal magnetoresistive materials are very appealing for use in technologies, but we still need to understand more about how they work," said Wang. "And, in particular, we must understand what happens during the very short periods of time when heating is not significant and the laser pulses are still interacting with magnetic moments in CMR materials. That means we must describe the process and control magnetism using quantum mechanics. We called this 'quantum femto-magnetism.'"

Wang's team specializes in using ultra-fast spectroscopy, which Wang likens to high-speed strobe photography, because both use an external pump of energy to trigger a quick snapshot that can be then re-played afterwards. In ultra-fast laser spectroscopy, a short pulse of laser light is used to excite a material and trigger a measurement all on the order of femtoseconds.

"In one CMR manganite material, the magnetic order is switched during the 100-femtosecond-long laser pulse. This means that switching occurs by manipulating spin and charge quantum mechanically," said Wang. "In the experiments, the second laser pulse 'saw' a huge photo-induced magnetization with an excitation threshold behavior developing immediately after the first pump pulse."

The fast switching speed and huge magnetization that Wang observed meet both requirements for applying CMR materials in ultra-fast, terahertz magnetic memory and logic devices.
"Our strategy is to use all-optical quantum methods to achieve magnetic switching and control magnetism. This lays the groundwork for seeking the ultimate switching speed and capabilities of CMR materials, a question that underlies the entire field of spin-electronics," said Wang. "And our hope is that this means someday we will be able to create devices that can read and write information faster than ever before, yet with less power consumed."

Tianqi Li, Aaron Patz, Jiaqiang Yan and Thomas Lograsso collaborated on the experimental work at Ames Laboratory and Iowa State University. Leonidas Mouchliadis at the University of Crete and the Institute of Electronic Structure and Laser at the Foundation for Research and Technology -- Hellas in Greece helped develop the theory used to interpret the experiments.

Viewpoint: Stimulated Near-Infrared Light Emission in Graphene

Ilias E. Perakis, Department of Physics, University of Crete and Institute of Electronic Structure & Laser, Foundation for Research and Technology-Hellas, P. O. Box 2208, Heraklion, Crete, 71110, 

http://physics.aps.org/articles/v5/43

Published April 16, 2012  |  Physics 5, 43 (2012)  |  DOI: 10.1103/Physics.5.43

The electronic properties of graphene allow a population inversion to be established within the duration of a 35-femtosecond light pulse.

Graphene—a carbon sheet only one atom thick—provides physicists with a playground to explore exotic quantum phenomena, and engineers with a material with which they may be able to miniaturize electronic devices and catch up with Moore’s law [1, 2]. New results reported in Physical Review Letters by Tianqi Li and colleagues at Ames Laboratory and Iowa State University suggest that graphene has also earned its place in the arsenal of promising photonic materials [3]. Li et al. established two necessary conditions for possible lasing applications—a population inversion of charge carriers and optical gain—by exciting graphene with very short pulses of light.

Graphene’s unique mechanical, transport, chemical, and linear optical properties are fairly well established. Up to now, however, the effects of exciting graphene with femtosecond (1fs=10−15s) pulses of light that create nonequilibrium charge states—a highly nonlinear process—haven’t been well studied. Yet, nonlinear properties are often the basis of functional optical devices, as they enable functions such as ultrafast modulation and control, and gain. For example, lasing can occur through a process where first an external light source creates an excess of excited states (population inversion) and then a spontaneously emitted photon stimulates these states to emit radiation via a coherent cascade. The coherent light emitted in this way is many times more powerful than the original photon that triggers it. This is why this nonlinear phenomenon is called optical gain.

In their experiment, Li et al. excited an epitaxial graphene monolayer sample with 1.55−eV pump laser pulses of 35-fs duration. With a “probe” pulse of light tuned to 1.55 eV or just below that, they then recorded the difference in reflectivity with or without the pump—a quantity called the ultrafast differential reflectivity. In metals, the relationship between how much light is reflected and absorbed is complex, but because graphene is exceptionally thin and the optical conductivity doesn’t depend on frequency, the reflectivity provides a measure of the absorbed light, which in turn depends on the optical conductivity. The experimental criterion for identifying optical gain in any pumped state is a negative optical conductivity, or negative absorption. This means that there are more photons coming out than coming in. So, when Li et al. observed a negative optical conductivity above a certain pump intensity threshold, they interpreted this as evidence of optical gain.

The fact that Li et al. are able to observe this behavior in graphene relates to this material’s unique properties. When the electrons in a material are excited by light on time scales of the order of femtoseconds, they occupy highly nonequilibrium states. In most materials, the excited electron states are coherent during or immediately after the pulse duration τp, but as the charges collide with themselves and with phonons, this coherence is lost. After a characteristic time interval τth, the charges can be described by a quasithermal transient distribution. Studies of semiconductors and two-dimensional semiconductor quantum wells have shown that when these materials are excited by 10-fs pulses, τth is much longer than τp[4, 5]. Such a relaxation parameter regime gives rise to excited states that are most likely to lie within a narrow range of energies around the photon energy (top left, Fig.1). However, since the Pauli exclusion principle prevents more than one charge from occupying each state, this situation results in a small population inversion and limits the light that could be emitted to a small range of frequencies.

Li et al. show that, in graphene, the excited states thermalize on a time scale that is less than the duration of the optical pulse, i.e., τth is less than τp even for a pulse duration that is only ∼10 fs. Their measurements provide evidence that femtosecond laser excitations almost instantaneously produce a broadband population inversion in graphene. The population inversion, where the electronic states are described by an hourglass-type distribution, is broadband and in quasiequilibrium after a mere 10 fs (top right, Fig.1).

The group found that broadband optical gain emerges over a wide energy window, up to hundreds of meV’s below the pump photon energy. They observed for the first time femtosecond stimulated emission and gain above a threshold photoexcited carrier density of ∼1013–1014 per cm2 for 1.55-eV pump photons, which is close to a complete nonlinear absorption saturation of the pump field. Though this is not the first ultrafast spectroscopy study of graphene, earlier experiments used relatively weak laser intensities to excite the charge carriers, so they did not see these nonlinear effects [6, 7, 8, 9, 10, 11].

Li et al.’s current study opens exciting opportunities to explore optical gain from terahertz to ultraviolet frequencies, a much wider range of frequencies than what is seen in conventional lasing materials. This broad optical gain is a special feature of graphene because, due to its characteristic linear energy dispersion, the photoexcited carriers scatter extremely fast among themselves and relax into a wide band of excited states. Another distinct feature in Li et al.’s work is that it takes less than 35 fs for them to build up the broadband gain. This result is quite impressive, as what normally happens in materials when electrons are excited by short pulses is that the states fill up and the Pauli exclusion principle blocks more carriers from being excited. In graphene, however, an ultrabroadband gain ranging from the conical point to the excitation energy can be established within∼10 fs. In the context of nonlinear materials and ultrafast materials science, this is a unique result.

Li et al.’s result that a pronounced population inversion can be created in graphene and can lead to optical gain in the infrared spectrum makes it possible to explore optical applications of graphene in laser technology and telecommunications that might surpass the present performance of semiconductor quantum wells. Such applications include extremely fast modulators, better absorbers that saturate at high intensities, broadband gain media, and lasing from the visible to the terahertz. Although the unique nonlinear optical properties in graphene, revealed by Li et al., set the stage for using this material as an emerging gain medium, there is still a long way to go before we can fully use it. For example, the short lifetime and high pumping rate for the gain states in graphene will require some clever engineering for novel optical functionalities, and it will be necessary to develop reliable ways to fabricate high-quality monolayer and few-layer graphene with a large surface area.

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About the Author: Ilias E. Perakis

Ilias Perakis is a Physics Professor and former chair of the Materials Science Department at the University of Crete. He is an Adjoint Professor at Vanderbilt University and Affiliated Faculty with the Foundation of Research & Technology–Hellas. He is an expert on the theory of ultrafast many-body processes in condensed matter systems, currently focusing on photoinduced phase transitions and attosecond dynamics in correlated and magnetic systems. He is an OSA Fellow and recipient of the NSF CAREER award. He received his Ph.D. in Physics from the University of Illinois and joined Vanderbilt University after four years at Bell Laboratories and Rutgers.