A few weeks ago, thanks to the smart thinking of Donna Cunningham, an indefatigable Bell Labs media relations manager, an intriguing book crossed my desk. Donna knew I would not be able to resist the title. The book is Stuff: The Materials the World is Made Of, by science writer Ivan Amato (Basic Books, New York, NY, 1997). The book is about just that--stuff--the materials that our world is made of, from rocks to iron and steel, from plastics to semiconductors. Amato describes the many ways humans have taken the stuff of raw materials and fashioned them into modern miracles ranging from the Brooklyn Bridge to fiberoptic networks. It`s a fascinating read: I recommend the book highly to scientists and nonscientists alike.
The chapter I found particularly fascinating is entitled "The Rite of Atomic Masons," which is largely about the work of Federico Capasso, the head of the Quantum Phenomena and Device Research Department at Bell Laboratories (Murray Hill, NJ), now a part of Lucent Technologies. Capasso and his research colleagues are the "atomic masons" who developed the quantum cascade laser, a device whose output wavelength can be tailored over a wide range. The quantum cascade laser will not only find particular uses in fiberoptic communications, as might be expected from a Bell Labs development, but it also promises a host of applications ranging from environmental sensing to collision-avoidance radar for cars.
Amato calls Capasso and his cohorts "atomic masons" because they literally design and build solid-state materials brick by brick and layer by layer from atomic scratch. Capasso`s term for this atomic masonry is more sober: he calls the work "band-gap engineering." The concept of band-gap engineering is simple in theory, but its execution is devilishly difficult in practice. When a sufficient number of electrons are prodded to jump from one electron energy level to another in, say, a semiconductor such as gallium arsenide or one of its compounds and then fall back, this recombination generates light. The electron levels, and hence the output wavelengths, are dictated by the energy band gaps that naturally occur in semiconductor materials. Capasso`s challenge was to find ways of expanding the range of band gaps beyond those in nature`s standard catalog. Thus, he coined the term "band-gap engineering." The stuff band-gap engineers use is comprised of compounds from the third and fifth columns of the periodic table (III-V compounds). It`s difficult stuff to engineer.
Fortunately, semiconductor process equipment makers have developed a method, molecular-beam epitaxy, that can spray-paint III-V crystals one atomic layer at a time. By using molecular-beam epitaxy to form wafers with carefully engineered layers of aluminum, gallium, indium, and arsenic, Capasso`s team eventually built a laser with a quantum mechanical staircase of 25 steps. As electrons cascade down this energy staircase, they emit photons of specific wavelengths. This may sound simple, but Capasso`s associates, Barbara Sivco and Jerome Faist, worked for more than three years to grow the intricately layered crystals without the defects that would inhibit the lasing action. Amato vividly describes the excruciating effort that went into building up a five-hundred-layer crystal that was only a few microns thick. All along, Capasso was convinced that his early intuition was correct. In early 1994, Faist succeeded in getting his experimental device to lase. The stuff worked!
Making optoelectronic miracles
The quantum cascade laser is a startling example of how a small team of researchers can take a few milligrams of stuff and turn it into another modern miracle. Bill Brinkman, vice president of research at Bell Labs, sees the quantum cascade laser as "a real tour de force. This is a step toward the ultimate situation in which you sit at a computer, design a material, and then let a machine make it for you." We have clearly come a long way from the days of primitive man forming arrowheads by chipping rocks.
On the wall at the entrance to Bell Labs` main facility in Murray Hill are the words of Alexander Graham Bell: "Leave the beaten track occasionally and dive into the woods. You will be certain to find something that you have never seen before." Capasso and his team of band-gap engineers left the beaten track to find something that promises to be an optoelectronic marvel.