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October 26, 2006
Light Moving Through Condensed Matter: the First Molecular Movie

While it won't compete at the box office with Titanic, a movie made by an international collaboration of scientists working at Berkeley Lab's Advanced Light Source (ALS) stands alone for originality: it's the first ever based on recorded observations of light moving through matter at the molecular level. Unlike a box office blockbuster, this movie carries ramifications well beyond summer entertainment, as it illuminates a possibility for using light to alter material properties.

Elemental mercury
A computer graphic movie shows a wave of terahertz radiation propagating through a crystal of lithium tantalate, against an x ray diffraction pattern of the material. (Go here for the mpg movie file.)

"If you want to understand the propagation of light through condensed matter at a microscopic level, especially in some of the complex materials that are of interest for modern optoelectronic applications, you need to make a molecular movie of how the atoms and electrons wiggle in the light field," says Andrea Cavalleri, a physicist now at Oxford University who was a member of Berkeley Lab's Materials Sciences Division at the time of the experiments. "We were able to do this by combining ultrafast pulses of laser light with the electron beam at the ALS to create a camera with an extremely quick shutter speed."

The shutter speed of this "camera" was measured in femtoseconds, billionths of a billionth of a second. The femtosecond timescale is the speed at which changes in matter at the atomic and molecular levels occur, such as the making or breaking of chemical bonds or phase transitions from solid to liquid to gas. With their technique, Cavalleri and his colleagues were able to measure the movement of T-rays — terahertz waves of light, at frequencies of trillions of cycles per second — through a crystal of ferromagnetic material.

"Being able to observe the displacement of atoms in condensed matter on a femtosecond timescale provides a new window to understanding the properties of materials and the underlying physics behind these properties," says Schoenlein, a physicist in the Materials Sciences Division who is a leading authority on femtosecond spectroscopy. "This is particularly challenging for ferroelectric and other materials, which feature strong microscopic coupling mechanisms."

Schoenlein says, "In our experiment, we measured the one-dimensional Bragg diffraction and its periodic modulation on the femtosecond time scale, from which we quantified the milli-angstrom displacements of atoms along certain coordinates. Our measurements were compared with a model calculation which became the basis for the movie."

The researchers reported their results in the journal Nature in a paper titled "Tracking the motion of charges in a terahertz light field by femtosecond X-ray diffraction." Co-authoring the paper with Cavalleri and Schoenlein were Berkeley Lab's Matteo Rini; Keith Nelson, Eric Statz, and David Ward of MIT; and Simon Wall and Chris Simpson of Oxford University.

The motion of light

Early in their science education school kids learn that light, depending on how you measure it, moves either as waves or as particles of energy called photons. When light moves through the condensed matter of a crystalline solid, it interacts with the vibrations of the crystal's atomic lattice, called phonons, which can be thought of as atomic sound waves. The interaction of photons with phonons creates an exotic new particle-cum-wave called a "polariton."

To create and study polaritons, the Berkeley-Oxford-MIT team sent waves of terahertz-frequency light through a crystal of lithium tantalate. Terahertz light falls between microwaves and infrared radiation on the electromagnetic spectrum; shining it through the crystal heated the atoms in the lattice, causing them to vibrate.

Lithium tantalate is ferroelectric, which means that exposure to an electric field can induce a change in the positions of the atoms that make up the crystal. This results in the coupling of oscillating electromagnetic fields of terahertz light waves to vibrations in the lattice.
MIT's Nelson explains that "the terahertz frequency waves we generated in the lithium tantalate crystals were not purely light — that is, not just electromagnetic waves — but also lattice vibrational waves. As they move through a region of the crystal, there is a coupling of the electromagnetic field oscillations and the vibrational oscillations of the atoms in that region."

  A typical U.
Spacer image Robert Schoenlein (inset), a physicist with Berkeley Lab's Materials Sciences Division, working with Matteo Rini (center) and Andrea Cavalleri, used femtosecond x-rays produced at the Advanced Light Source to measure the movement of T-rays through a crystal of ferromagnetic material.

While femtosecond pulses of laser light can be used to generate and measure terahertz polariton waves, recording images of the waves interacting with atoms as they pass through a crystal requires x-rays. Until a year ago, there was no reliable means of obtaining femtosecond scale pulses of x-rays. That changed when another study at the ALS, also led by Cavalleri and Schoenlein, demonstrated a technique in which laser light can be used to "slice off" femtosecond x-ray pulses from the ALS's primary beam.

Berkeley Lab's ALS is a synchrotron designed to accelerate electrons to energies of nearly 2 billion electron volts (2 GeV), focus them into a hair-thin beam, and send this beam around the curved path of a storage ring for several hours. Beams of photons, primarily x-rays, can be extracted from the electron beam in the storage ring when it passes through a bend, wiggler, or undulator magnetic device. Because the ALS electron beam is pulsed on a picosecond time scale (trillionths of a second), photon beams directly extracted from the electron beam are also bunched in picosecond pulses.

However, the group led by Cavalleri and Schoenlein demonstrated that if femtosecond pulses of laser light are sent through a wiggler magnet at the same time as the ALS electron beam, femtosecond bunches of electrons can be spatially separated or sliced from the main beam. When these displaced electron bunches are then sent through a bend magnet, they generate femtosecond pulses of x-ray light. In the latest study, Cavalleri, Schoenlein, Nelson and their colleagues used the laser-slicing technique to record femtoscale "snapshots" of terahertz waves interacting with the vibrations of the lithium tantalate's atomic lattice.

Elemental mercury
By sending femtosecond pulses of laser light through a wiggler magnet as the ALS electron beam passes through the wiggler, the electron beam can be sliced into femtosecond bunches. These are then sent through a bend magnet to generate femtosecond pulses of x ray light.

"We were able to see which lattice vibrations were involved — the microscopic paths along which the atoms were moving, and the amplitude of the vibrational motion," says Nelson, "that is, how far the atoms were moving."

The researchers are now exploring the possibility of generating larger vibrational amplitudes with terahertz waves to reposition atoms in a crystal lattice and thereby alter the material's macroscopic properties. This manipulation of polaritons in materials is similar to photonics — the manipulation of photons — and has been dubbed "polaritonics."

Says Cavalleri, "With our technique we can selectively control the direction of motion, which atoms are being excited and which remain still, and precisely at what frequency they are being agitated. Through this vibrational control, we should be able to control the conductive and magnetic properties of a solid, and its optical and possibly other properties as well."

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