Imaging with Electrons, Aided by Light

Hank Hogan

Researchers at California Institute of Technology in Pasadena have developed an instrument with the spatial resolution of an electron microscope and the imaging speed of an optical microscope. They have dubbed it the ultrafast electron microscope.

The resolution of a microscope depends on the wavelength of the particles used for imaging. Electrons thus have an advantage over photons in that their wavelengths are orders of magnitude shorter. However, optical systems can capture events that happen much too quickly for a conventional transmission electron microscope to follow.

In the work, the team used a pair of pulse trains from a femtosecond laser to act as the electron-imaging source in a transmission electron microscope and to heat or excite the sample. Lead researcher Ahmed H. Zewail said that the use of two optical beams arose out of the team’s previous work in femtochemistry, in which two laser pulses chart the motion of atoms.

To build the ultrafast electron microscope, the researchers integrated a femtosecond laser and associated optical system with a transmission electron microscope. They added two ports, one near the sample and the other near the cathode, to a transmission electron microscope from FEI Co. of Hillsboro, Ore. In conventional operation, that microscope’s cathode is heated, and electrons boil off. These particles are focused by electron optics and are used for imaging.

Two beams

In the ultrafast electron microscope, a mode-locked Ti:sapphire laser generates two optical beams: a train of sub-100-fs pulses at 800 nm and another train of femtosecond pulses frequency-doubled to 400 nm. The scientists steer the second beam onto the cathode and focus it to a 50-μm spot to generate electrons. They dial back the intensity so that only one electron is produced per pulse, which keeps the mutual repulsion of the electrons from interfering with imaging. To detect their single electrons, they employ a phosphor scintillator and a four-megapixel CCD camera from Gatan Inc. of Pleasanton, Calif.

With the other beam, they heat the sample, controlling the delay between this pulse train and the other by adjusting the length of an optical path. In a demonstration of the instrument, they stepped the path distance in 1-μm increments to yield a time resolution of 3.3 fs. They used the arrival of the second pulse at the sample to provide the zero point of time for a measurement.

The group verified the performance of the ultrafast electron microscope by comparing the images it produced with those from a transmission electron microscope. The two were equivalent for calibration gratings, material diffraction patterns and biological samples.

Zewail sees the system being used to image dynamic events in various materials as well as in biological cells. It can operate as either a transmission electron microscope or an ultrafast electron microscope, enabling scientists to use a single instrument from the atomic to the much larger microscopic domain. Caltech has a patent pending on the technology, and commercialization of the instrument will need to account for this.

PNAS, May 17, 2005, pp. 7069-7073.