Small (and Big) Talk at FiO

The superbig and the supersmall were the subjects of two plenary sessions Monday at Frontiers in Optics 2009.

In the first session, Andrea M. Ghez, professor of physics and astronomy at UCLA, a leading expert in observational astrophysics, explained to attendees the process by which she has concluded that a dormant supermassive black hole lies not only at the center of our Milky Way galaxy, but every galaxy.

A black hole is a region of space in which the gravitational field is so powerful that nothing, not even light, can escape, which makes them essentially invisible when you try to find them.

Dr. Andrea Ghez, professor of physics and astronomy at UCLA, discusses black holes during her plenary talk at Frontiers in Optics 2009. (Photonics Media photos by Melinda Rose)

Black holes come in two "flavors," as Ghez said, ordinary and supermassive. A supermassive one has a mass 1 million to 1 billion times that of the sun. But even supermassive black holes are invisible.

"So how do you find something you can't see? You watch how stars at the center of a nearby galaxy move," Ghez said. The way those stars move can reveal whether or not a black hole is nearby.

Ghez uses the ground-based Keck telescopes, the largest in the world, to make her observations, but their spatial resolution is limited because of atmospheric turbulence. Adaptive optics techniques have dramatically improved astronomers' diffraction-limited capabilities, Ghez said, and allow the taking not only of pictures, but spectra.

"Adaptive optics is an order of magnitude better," Ghez said, which means astronomers can view stars 10 times fainter than before.

To make her observations, she creates an artificial star by shooting a laser beam into the sky, then uses that "guide star" to track the movements of a real star. In the future, astrophysicists will use multiple laser adaptive optic systems, she said.

Through her observations of star movements at the center of the Milky Way galaxy, she discovered that the space contains "mass 4 million times that of our sun, but shoved into something the size of our galaxy," she said. In other words, a supermassive black hole. (For more on black holes, both supermassive and superlame, read Gary Boas' FiO blog.)

The next speaker, Janos Kirz, scientific advisor at the Advanced Light Source (ALS) at Lawrence Berkeley Laboratory, turned to the nanoworld with his update on the field of x-ray microscopy. ALS was designed to produce soft x-ray and ultraviolet synchrotron radiation with the highest possible brightness and is at least 100 times brighter than previous sources.

Janos Kirz, scientific advisor at the Advanced Light Source at Lawrence Berkeley National Laboratory, brings FiO attendees up-to-date on x-ray microscopy techniques.

The advantage of using x-rays is that they offer short wavelengths that can penetrate samples more deeply, he said. Electron microscopy has to be used with samples that are quite thin and provides limited contrast in unstained biological samples.

Kirz discussed various methods for imaging samples, including with tabletop laser sources and liquid-jet laser systems. The lower limits appears to be about a 1-nm resolution, he said.

A completely different approach, Kirz said, is to get rid of the optics through diffractive microscopy. That method is lenseless, and involves using a computer to phase the scattered light rather than a lens.

Other lenseless imaging schemes include Fourier transform holography and ptychography, he said, with the latter being "a very exciting development because it may make it easier to do numerical reconstruction and doesn't require the sample to be isolated," Kirz said.

For more information, visit: www.frontiersinoptics.com

Melinda Rose
Senior Editor