New microscope freezes action in picosecond resolution

May 12, 1995

By Jeffery Kahn, [email protected]

Materials Sciences Division researchers have devised a breakthrough process that gives them the ability to image and stop the action of atomic scale events in picosecond freeze-frames.

Ultrafast scanning probe microscopy, as the new imaging process is called, promises to give researchers the ability to look at atoms in the time frames at which atomic scale phenomena unfold. Up to now, this world of ultrasmall, fleeting phenomena has been all but off-limits to exploration.

Scientists believe they will be able to image electrons as they move in their enigmatic paths across the atomic lattice of a semiconductor. Conceivably, a series of images of a plant transforming sunlight into chemical energy, or in essence, a movie of photosynthesis, could be recorded.

MSD's Shimon Weiss, UC Berkeley graduate student David Botkin, and MSD Director Daniel Chemla have led the effort to develop this new family of microscopes. Other contributors include MSD's Frank Ogletree and Miquel Salmeron. The inventors say the full potential of the technique is yet to be defined.

"This dynamic technique is very exciting and very broad," said Chemla, "but also very new. Certainly, this is an interdisciplinary tool. However, at this stage, we are still exploring just which approaches will be most effective."

First describing their efforts in the November 1, 1993 issue of Applied Physics Letters, the researchers report that they have obtained images with simultaneous two picosecond and 50 angstrom resolution.

Ultrafast scanning probe microscopy results from the wedding of two cutting edge tools. Scanning probe microscopes -- these include the scanning tunneling microscope and the atomic force microscope -- can image details as small as a single atom. Femtosecond lasers, developed by LBL Director Charles Shank, provide pulses of light that last a millionth of a billionth of a second. Coupling the microscope and the laser, the new ultrafast instruments use the laser pulse to optically activate a switch integrated in the tip of a scanning tunneling microscope. The technique captures images of ultrafast events, improving the time resolution attainable with the scanning tunneling microscope by nine orders of magnitude.

Weiss says that when a series of successive, time lapse images is combined, the result is a movie of surface dynamics.

Each image involves two pulses of light. The first pulse excites the surface that is being investigated. This excitation can be electronic or vibronic, generating either a pulse of electrons or an acoustic wave (phonons).

The second light pulse activates the switch, turning on the microscope for the duration of the pulse. This allows it to image the surface just for that instant. The Auston switch used in the instrument, which has subpicosecond speed, is the fastest known electrical switch. It was invented by David Auston, a former colleague of Chemla and Shank's at Bell Laboratories and a UC Berkeley graduate.

Chemla said the motivation underlying the development of ultrafast microscopy can be found in the growing interest in nanoscale features and phenomena by both chemists and physicists.

Chemla himself is particularly interested in semiconductors.

"Each time we create a new generation of chips," he said, "we reduce their dimensions. Up until now, making it smaller has meant that everything runs faster. But at a certain scale -- and we are now approaching that -- the standard design rules begin to break down. Electronic, magnetic, and optical properties are altered. They become dependent on size and shape."

To guide the design of future generations of semiconductors, researchers would like to be able to zoom in and stop the action, exploring just what happens in the nanoscale world. For example, launching a very fast packet of electrons and then recording their whereabouts as they cross a semiconductor device would reveal the processes governing their motions on such small space and time scales. Due to quantum effects, electrons "tunnel" across gaps but exactly when and where tunneling will take place is unknown. Chemla says the ultrafast microscope now makes it feasible for researchers to observe the dynamics of tunneling.

Before movies can be made of surface dynamics, the researchers first must understand the physics of the instrument. "The first thing we have to do," said Weiss, "is understand what we are observing when we measure a single point. Right now, we are investigating and characterizing the physics of tunneling -- in particular, the quantum capacitance effect -- at a single point. This is really the first science to be done with this new technique."

While this work is very fundamental, Weiss notes that it requires a tool that can count the number of electrons at a given point at any instant. Ultrafast microscopy, he notes, allows researchers to count fractions of electrons.

Ultrafast, stop-action "movies" long in coming

The ability to stop the action and image what happens in a given instant has a relatively short history.

Although the camera began to emerge in the 1500s, the first true photograph was not made until 1826. In 1877, the first successful photographs of motion -- of a running horse -- were captured. Working in California, British photographer Eadweard Muybridge set up a row of cameras with strings attached to their shutters. As a horse ran by, it broke the strings, tripped the shutters, and created a series of photos that amazed the world.

Since then, faster and faster cameras have been developed. Consumers can now buy models with shutter speeds as fast as 1/2000th of a second.

Science has developed a number of imaging techniques other than photography. Scanning probe microscopes, invented in the early 1980s, can zoom in and image individual atoms but essentially, they have been unable to record motion.

LBL's new ultrafast scanning probe microscopes change that. Events evolving at an atomic level often occur in lightning-like time frames. The new microscopes finally give science the ability to observe these ephemeral events. They will capture moments that are measured in picoseconds, or trillionths of a second.