Can We Turn Oranges into Apples on Fast Time Scales?
We think of the blink of an eye as being pretty quick. A blink takes about a third of a second or 300 to 400 milliseconds. The human eye can register visual information in as little as 13 milliseconds, according to neuroscientists at MIT. That’s much faster still, but we can’t get anywhere close to seeing the movements of atoms with our naked eye -- that takes place at one trillionth of a second time scale, called the picosecond. What could we learn if we could see electrons leaping across energy barriers? Perhaps something new about the many intermediate states of matter during the jump.
Recently, the Department of Energy put out a special call for new ideas on ultrafast phenomena. Because of recent advances in laser technology meant, the DOE believed there were untapped fields of science to be explored at time scales on the order of femtoseconds, or 1/1000 of a trillionth of a second or less.
“All these cool tools are out there now, these lasers,” explains Venkat Gopalan, Penn State professor of materials science and engineering. “DOE asks what can we do on these time scales? They are looking for exciting science. What are the most exciting things that you want to know about materials and phenomena?”
Gopalan and his Penn State colleague Roman Engel-Herbert put together a proposal with an interesting premise. It suggested that when viewed on ultrashort time scales, a certain class of materials called complex oxides would have hidden properties that could be revealed and even controlled by using ultrashort laser pulses. They suggested that by moving atoms around with a pulse of light, they could transform the properties of matter. It was not unlike the old alchemical idea of the philosopher’s stone that could turn base metals like lead into gold.
Gopalan illustrated the concept with a small orange left over from a hurried lunch. “Here is an orange. The reason you saw an orange 10 minutes ago when you came in, see it now, and you will see it tomorrow is because it is in what physicists call a ground state. In that state it is rather stable. It won’t last forever, of course, but we can call it metastable.”
A ground state is an object’s lowest energy level, and things tend to seek their lowest energy level. Most scientists like to look at things in the ground state because in that state they are stable and predictable, Gopalan said. However, things with high energy change quickly, and so they must be studied with very high speed equipment.
Going back to his example of the orange, Gopalan explained: “This orange, it looks pretty still. But if you keep zooming in and in, eventually you will come to atoms. Maybe if I wasn’t looking at this at its lowest energy state, but on a faster time scale where the atoms are moving and things are happening, I might see that halfway through the motion of atoms, there is a transient hidden state that is more like an apple instead of an orange. It sounds crazy, but how do you know it isn’t happening inside? You can only see the final stable state. What if there are hidden states of matter you can’t see on your macroscopic time scales? Those states may be just as exciting, but you don’t even know that they exist. Discovering such hidden states is what our proposal is about.”
Lasers were invented in the mid-20th century based on an idea proposed by Albert Einstein in a paper in 1917. He proposed that by stimulating atoms with a certain frequency of light, a cascade of atoms would emit photons of the same frequency and direction, lining up like soldiers marching in unison. The laser, which stands for light amplification by stimulated emission of radiation, was not invented for another 40 years. The key to the laser is that unlike normal light such as sunlight, which is disorganized, every photon of laser light is identical to every other photon in amplitude, in color, and in phase.“You have to have a pool of atoms. And you have to excite them somehow, apply a voltage or UV light. You need a lot of energy, and yet you only get a little back. It’s not like you get more light than you put in,” Gopalan explained. “You just get special kind of light.”
After the first laser was invented, based on Einstein’s idea of stimulated emission, lasers quickly became a part of everyday life – laser pointers, laser eye surgery, laser printers, and CD and DVD players. There have also been recent advances in ultrashort pulse lasers, which have given rise to the research areas of ultrafast laser physics and ultrafast optics. Keep in mind that ultrafast does not imply the light is moving faster in these lasers – the speed of light is unchanged. But the ultrashort pulses work like a strobe light to freeze action at faster and faster time scales.
“We can measure events at femtoseconds scale, no problem,” Gopalan said. “The new cutting edge is the attosecond scale, or one millionth of a trillionth of a second.”
The DOE call for proposals was an acknowledgement of the fact that although the U.S.is still in the lead as far as laser technology is concerned, the Europeans are pulling ahead in the science, especially materials science, of ultrafast phenomena. This program was meant to push U.S. scientist to regain the lead.
Applying ultrafast phenomena to complex oxides
Gopalan and Engel-Herbert’s program is called “Dynamic Visualization and Control of Emergent Phases in Complex Oxide Heterostructures.” “That sounds complex, but what it means is -- dynamic visualization is like watching things in action. And control – not only do I want to watch stuff, but can I actually make something happen? An electron is jumping up. Can I make it go the other way on that fast time scale?” Gopalan asked.
He proposes to apply the tools of ultrafast lasers on a large family of materials called complex oxides. (Most rocks are oxides.) Complex oxides are widely studied for their many unusual properties -- some are insulators, some are dielectrics that can be used as capacitors, some are ferroelectrics, others ferromagnetic, and some are even superconductors. Gopalan has recruited a group of top U.S. scientists to be part of his team.
The experts include three leaders in the thin film growth of complex oxides – Roman Engel-Herbert at Penn State, Lane Martin at UC Berkeley, and Jak Chakhalian at the University of Arkansas. Once the films are made, they will be studied using a variety of tools. With each spectrum of light, from x-rays to infrared, a new piece of the puzzle will be revealed.
“With hard X-rays you are looking at electrons, electron states, crystal structure, and how the electrons are moving,” Gopalan said. “When you go to softer X-rays, you can study things like magnetism.”
Infrared is the frequency that makes atoms vibrate, while the visible spectrum is where many electronic processes take place. The femtosecond timescale is the correct one for looking at atoms in the visible frequency, he continues. Two colleagues at Argonne National Lab near Chicago, John Freeland and Hayden Wen, will do the X-ray work at the X-ray synchrotron source in conjunction with a post-doc from Penn State. Other X-ray work will be done at the Stanford Linear Accelerator Laboratory in Menlo Park, CA.
At the University of California, San Diego, two scientists, Dimitry Basov and Rick Averitt, will look at atoms with a unique kind of microscope invented by Basov that uses different color frequencies to look at the materials with both high resolution and at fast time scales, a combination which can be challenging to accomplish.
Two theorists, James Rondinelli at Northwestern University and Andrew Millis at Columbia University, are experts in techniques for predicting states of matter both dynamically (Millis) and at ground state (Rondenelli). “The combination of these two people is perfect for us,” Gopalan remarked. His own group has expertise in ferroelectrics and also in X-ray synchrotron techniques. He is also upgrading his laser lab with much more powerful lasers. His group will look at complex oxides that have built-in polarization as well as study materials that can transition from metal to insulator, something of a magic trick with possible applications for very fast switching in transistors. By this summer, he will also have the first terahertz spectroscopy setup on campus. Terahertz is a very long wavelength that is useful for vibrating atoms. None of the new instruments are funded through the DOE grant.
Finding hidden phases
One technique they will use for finding hidden phases in matter is called a pump-probe. The material is first hit with a laser pulse that excites the electrons – this is the pump -- which is followed with a weaker pulse -- the probe -- that will give information about the transition as the electrons fall back into the ground state. Rick Averitt at UCSD used this technique on a complex oxide called lanthanum calcium manganese oxide that has an insulator-to-metal transition under a high magnetic field at low temperatures. What Averitt found was that the material could be made to do the same thing at room temperature using a laser pulse.
“If you wanted to make a very fast switch that goes from conducting to non-conducting at room temperature and without applying a magnetic field, what our phase diagrams say is you can’t do it,” Gopalan said. “At room temperature it is always going to be an insulator. But when Rick hits it with a laser light with 1.5 electron volt photons, it turns into a metal and it’s magnetic. When he hits it again, it’s an insulator and it’s no longer magnetic. It’s changing two properties at the same time. It’s like a magic wand, like me changing this orange into an apple and back.”
Intel, for one, is looking for metal-insulator transitions to make their transistors go even faster than silicon switches can. But with so many materials to look at, which ones should they try? That is what Gopalan and colleagues will explore using both theory and experiment.
They will be probing a range of oxides – titanates, manganates, vanadates, ferrites, cobalites, and nickelates – with a rich range of phenomena such as ferroelectricity, magnetism, metal-insulator transitions, and charge ordering, which is a uniform arrangement of electrons related to superconductivity. “How can we dramatically change these materials just by exciting them?” he asked. That is the question his team will be looking to answer.
The DOE grant is for ~ $3 million over three years with Penn State as the lead university.
The goal for this program is ambitious. If they can find a way to manipulate the arrangements of atoms in a material, maybe then they can take that orange and move its atoms in a slightly different way until they have an apple.
Contact Venkat Gopalan at firstname.lastname@example.org.