U-M scientists’ ultrafast lasers put the squeeze on atoms
ANN ARBOR—Using ultrafast pulses of laser light, University of Michigan physicists have found a way to control the random oscillations of atoms in a crystal lattice. The U-M study, which was published in today’s issue of Science, describes the first experimental modification of one of the most fundamental quantum states of solid matter.
About 10 years ago, scientists discovered how to create this “squeezed” state for quantum particles of light energy called photons; but until the U-M experiment, no one has been able to do so with quantum particles called phonons which carry vibrational energy through a solid.
“Our goal was to learn how to control matter—to tell the atoms what to do, rather than just watch them do something,” said Roberto Merlin, U-M professor of physics and one of several authors of the Science article.
Appreciating the significance of the U-M study requires plunging into the murky world of quantum mechanics which governs how matter and energy interact within and between atoms. The traditional laws of classical physics are irrelevant in the quantum world where matter exists both as a particle and a wave and atoms can be located in two places simultaneously.
To visualize the atomic structure of a solid at the quantum level, it is important to forget the traditional ball-and-stick models used in introductory chemistry textbooks. A more helpful analogy is to think of the atom as a cloud whose area is defined by the atom?s random motion within the solid, according to Alberto Rojo, U-M assistant professor of physics and a co-author of the Science paper.
“We can calculate probabilities for an atom’s location within a specific area of uncertainty, but we can never know precisely where the atom will be,” Merlin said. “It’s like hide- and-seek. With short-pulse, high-power lasers, we can reduce the size of the volume where the atom can hide.”
“Points of higher density in the cloud represent points where the probability of finding the atom is higher,” Rojo said.
U-M graduate student Gregory Garrett conducted the experiment using a titanium-sapphire laser system at the U-M Center for Ultrafast Optical Science. Garrett fired 70- femtosecond laser pulses focused on a tiny spot on a potassium tantalate crystal. “A 70-femtosecond pulse is incredibly short in time,” Garrett said. “At the speed of light, for example, it takes 300 femtoseconds to travel a distance equal to the thickness of just one sheet of paper.”
Splitting the laser beam into two parts, Garrett diverted one beam along an alternate path, so the secondary pulse arrived at the target a few picoseconds after the initial pump pulse.
“The stronger pump pulse hits the atoms in the crystal lattice like a hammer producing a force which creates pairs of phonons and makes the atomic lattice oscillate,” Garrett said. “The weaker probe pulse is scattered by these phonons as it passes through the lattice later in time. Measuring the amount of probe pulse energy that makes it through the crystal gives us a picture of what’s happening inside.”
It helps to visualize the atoms in the lattice as bouncing balls tethered to a wall with a piece of elastic, Rojo explained. When the initial laser pulse strikes, it stretches the spring constant of the elastic expanding the quantum limit or the original range of the atom’s movement. After the pulse passes, the elastic suddenly snaps back reducing the atom’s range of movement to a fraction of its original size. For a very brief moment, equivalent to one-half its normal oscillation cycle time, the atom is squeezed below its quantum limit. This process repeats itself for many cycles until eventually the solid returns to equilibrium.
Merlin says it is too early to speculate on potential applications for the squeezed state in matter, but adds that the U-M team plans future experiments to replicate the squeezed state in other types of crystal.
Others collaborating on the experiment included A.K. Sood, of the Indian Institute of Science in Bangalore, India; and John F. Whitaker, associate research scientist at the U-M Center for Ultrafast Optical Science. The project was funded by the National Science Foundation, through its support for the Center for Ultrafast Optical Science, and the U.S. Army Research Office.