Using light to carry messages, just like carrier pigeons
The essential human need to stay in touch has driven some of our most innovative technology, from carrier pigeons to the telegraph, satellites, cellular phones and, someday soon, a laser with light-emitting regions too small to see with the naked eye.
That is the sweeping history of human communication Distinguished University Professor Pallab Bhattacharya will lay out in his April 4 lecture, "From Pigeons to Spin-Polarized Lasers: Carriers of Information Through the Ages." His talk will be at 4 p.m. in the Rackham Amphitheatre.
"I'm not going to spend much time on the pigeons, obviously," adds the Charles M. Vest Distinguished University Professor of Electrical Engineering and Computer Science.
Yet, for all the advances since Guglielmo Marconi first bounced radio waves across the North Atlantic, our very extended conversation with the Voyager spacecraft, as well as the latest episode of "the Sopranos," still rely on radio waves, Bhattacharya says.
"It amazes me that a lot of it is still microwave communication."
All of this is going to change, however, and the future is light, he says. Fiber optics for high-bandwidth telephone and data that put information into light waves already are quite commonplace. But conventional lasers used for fiber-optic communication have some drawbacks, he adds.
The last two decades of Bhattacharya's career have been devoted to making and understanding lasers that would be more suitable for communication, including lasers that could be integrated with a silicon chip.
Like many great scientific breakthroughs, Bhattacharya's story begins with a happy accident. While trying to fabricate nano-scale transistors, Bhattacharya's students kept finding little rogue islands of semiconductor material in the transistor channel region. These islands, now known widely as self-assembled quantum dots initially were an annoyancea defect, Bhattacharya explains.
Bhattacharya and Jasprit Singh, a collaborator and colleague in the department, were troubled with this unwanted roughness in the devices. Soon after, Professor Brad Orr in physics performed scanning/tunneling electron microscopy of the nano-sized islands as they were forming. What he found were discrete, well-organized islands of semiconductor material, just 100-200 angstroms across and 50-100 angstroms high.
"As soon as I saw Brad's images, I knew we could make lasers out of them," Bhattacharya says. "That's what we now call a nanophotonic device."
A few early attempts to make the quantum dots produce lasers met with failure, however, and Bhattacharya abandoned the ideaor thought that he had.
Another group picked it up and found that, indeed, lasers could be made from quantum dots if the device was chilled sufficiently. Bhattacharya says his dots would have worked, too, if he'd thought to chill them.
"I learned my lesson: that you should never throw anything out, or at least look through the trash once in a while," he says.
Upon his return to the field about 10 years ago, Bhattacharya and his students used improved materials and laser design and became one of the first labs to make a quantum dot laser work at room temperature, a key factor if they ever are to be practical drivers of fiber optic networks.
Working with Singh, Rachel Goldman in materials science and engineering, Ted Norris in the Center for Ultrafast Optical Science and dozens of graduate students during the years, Bhattacharya has perfected the quantum dot lasers and a related technology that photo-detects in the infrared spectrum.
"These are just graduate students, beating out all the big corporations," he says, displaying an image of a semiconductor photoreceiver array that is in widespread use today.