Where vision meets know-howXiang Zhang’s XLab
Xiang Zhang greets the visitor to his airy, ground-floor office in Etcheverry Hall wearing cargo shorts, a colorful sport shirt and a broad smile. He’s relaxed enough to be on vacation, a deceptive look for someone who holds the Ernest S. Kuh chair as professor in the mechanical engineering department, directs the National Science Foundation’s Nanoscale Science and Engineering Center, was recently appointed director of the materials sciences division at Lawrence Berkeley National Laboratory and who in less than two decades has published more than 240 papers in such journals as Science, Nature and Physical Review.
Zhang is a world leader in the burgeoning field of photonic metamaterials. He makes light perform astonishing tricks by means of artificial structures that don’t occur in nature: “superlenses” that capture light that ordinary lenses can’t see; tiny motors called “light mills,” powered by beams of light; “invisibility cloaks” to make objects vanish.
He calls his group of more than 30 Ph.D. students, postdocs and visiting scientists the XLab, “not because we’re mutants, but because we’re really looking for nontraditional research topics. X stands for explore, for experiment, for excellence.”
As much as depth of scientific thinking, Zhang seeks creativity in recruiting XLab members, even if their test scores are not the absolute highest. “I might ask,” he speculates, “‘What would you like to do if you had unlimited resources?’ If the answer is ‘I want to go to Mars,’ I’d ask, ‘What would you need? How would you do it?’”
Getting an idea off the ground can mean not worrying about feasibility too quickly, a point Zhang makes by describing a kooky scheme from his early years on the UCLA faculty. “The neighborhood was expensive, and a lot of the faculty couldn’t afford housing. I came up with the idea of an inflatable apartment we could pump up at night, float up in the air — great views! — and come down in the morning. We did a cost estimate; the price was actually reasonable, compared to the market.” He laughs. “Housing for the future, maybe.”
Into the XLab
Visionary from the start, metamaterials are a young field of research, wide open for exploration. Joining disparate components — metal versus insulator, say — metamaterials owe their surprising new properties not to chemistry or crystallography but to their architecture.
“Everything we do begins in nature,” says Zhang. “But we think of the natural materials as ‘parents,’ lending their DNA to designs with properties the parents don’t have.”
To make these artificial structures possible, the labyrinthine shops and workstations of the XLab wind through three campus buildings, housing an armory of tools including lasers, atomic force microscopes, ion mills, electron beam evaporators and other precision instruments.
Metamaterials embrace such macrodevices as the XLab’s acoustic hyperlens, bringing high resolution to sonar and ultrasound images ranging from a school of fish to a fetus in the womb; the fan-shaped array of 20-centimeter-long brass strips can focus meter-scale sound waves to a fraction of their wavelength. The nanoscale, however, opens new vistas for metamaterials.
In the realm of acoustics, XLab Ph.D. candidate Kevin O’Brien uses lasers in Sutardja Dai Hall to create trillionth-of-a-second pulses that excite plasmons (quantized waves of electrons) in gold nanostructures shaped like Swiss crosses, having 90-nanometer short arms and 120-nanometer long arms.
“The cross-shaped nanostructures heat up and expand rapidly to generate coherent acoustic phonons,” O’Brien explains. These quantized sound waves travel through solids at 10 gigaherz, a frequency many hundreds of times higher than conventional ultrasound. Detecting the in-phase or out-of-phase oscillation of the nano-arms, says O’Brien, “is a step toward potential applications, in principle including higher-resolution imaging.”
Scale is a secondary concern for theorist Hamidreza Ramezani, an XLab postdoc. His whitewashed-concrete office in Hesse Hall houses only a computer; with it he is designing a unidirectional laser by applying quantum mechanical principles and symmetries of time and parity (mirror symmetry) on the scale of long-established classical physics.
Instead of a traditional laser cavity with mirrors, “we’re looking for an open cavity that amplifies light in only one direction,” Ramezani says. “It’s an optical structure that allows photons to take many different paths, but eventually they find one way out, as a laser beam.” The test version of this resonant cavity combines three distinct materials.
The metamaterial that Ph.D. student David Barth is making in a cramped corner of Etcheverry Hall combines two materials that could hardly be more ordinary: silicon and air. Using a technique called photoelectrochemical etching, “we’re drilling little holes in the silicon, most about 30 nanometers big,” Barth says. “We control the density and the size of the holes to change and control the refractive index of the material,” manipulating the speed of light through the material to steer its path.
A medium’s refractive index indicates how fast light travels through it; at index 1, the vacuum is fastest. Air’s refractive index is only a few ten-thousandths greater than 1, while silicon, through which light travels slowly, has an index well over 3. A light ray traveling through silicon riddled with air holes does not respond to each hole, which are smaller than the light’s wavelength. But the light bends or accelerates according to how the density of holes changes the refractive index.
To etch the hole patterns, Barth begins with nothing fancier than a PowerPoint image projected onto the silicon wafer; the test case was a photo of Xiang Zhang himself. Depending on the size and pattern of the nanoscale holes, how light moves through the material and emerges from it can be controlled at will. Barth says, “Now that we can bend light in various interesting ways, we’re making devices that concentrate light where we want it.”
A visionary bolt of lightning
The XLab’s evolving design methodologies, Zhang says, make possible “new materials based on the imagination.” Most are destined to produce practical commercial advances in fields including telecommunications, biomedicine and national security. Yet some of the most startling ideas are pure speculative science.
In 2012 two independent groups, Zhang’s at Berkeley and that of Nobel Prize-winning theorist Frank Wilczek at MIT, were musing on special relativity and the possibility of broken-time symmetry — which suggested the possibility of a “space-time crystal,” a clock to keep perfect time for eternity.
“If a student tells me they want to go to Mars, I’d say, ‘What would you need? How would you do it?’ ”
– Xiang Zhang
Zhang and postdoc Tongcang Li proposed a unique system of trapped ions, rotating perpetually at their lowest quantum energy level in a static magnetic field. “By leading to a breaking of time-translational symmetry, this makes time — for the first time! — discrete, not continuous as we experience it,” Zhang says. “This exotic four-dimensional crystal is not only periodic in space, but also in time.”
Explorations of space-time only a little less far out are made possible by “continuous-index photon traps,” described in 2009 by Zhang, visitor Dentcho Genov and then-postdoc Shuang Zhang. In these metamaterial devices, light behaves like massive bodies, allowing investigations of celestial mechanics, including black holes and other manifestations of general relativity, right on the lab benchtop.
Zhang’s fascination with physics began as an undergraduate at Nanjing University. He then spent a year’s research at Fermilab, hoping to pursue particle physics when the Superconducting Super Collider started operations; its defunding in 1993 caused an abrupt career switch. Zhang was soon pursuing a master’s degree in environmental engineering at the University of Minnesota.
“I designed a few monitoring devices for air pollution, and I learned hard lessons about instrumentation,” he says. The most fundamental lesson: “Creativity must be combined with practical skills.”
Working toward his 1996 Ph.D. at Berkeley, Zhang learned from mechanical engineering professor Costas Grigoropoulos “how to use laser light to pattern devices.” It unleashed his imagination. A few short years later, Zhang was a full professor at UCLA, where he founded the first large metamaterials program in the U.S.
He returned to join Berkeley’s ME faculty in 2004 and today views himself as an applied scientist, but one who “sees things the other way around: I’m interested in how technology enables new science.”
That’s only part of the picture. Ask him where his all-embracing vision, from the mundane to the cosmic, could have originated, and his answer is quick: “I went to school during the Cultural Revolution. Crazy people were running the schools. Education was done with.”
There was an upside, he says: no homework and total freedom for the mind to grow. To guide that growth he depended on his father, a middle-school history teacher. Zhang remembers when he was six or seven, and the two of them were bicycling into a thunderstorm; they took shelter and watched the lightning streak across the sky. “My father said, ‘Someday someone will harvest all that energy.’”
“He wasn’t a scientist,” Zhang says, “but he taught me you can think about anything.”
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