Naomi Halas at the CLEO 2019 Tuesday plenary session.
Following an opening talk on brain imaging by OSA Fellow Chris Xu, two other Tuesday morning CLEO plenary speakers—OSA Fellow Naomi Halas of Rice University, USA, and Mial Warren of TriLumina Corp., USA—looked at how plasmonic nanoparticles and lasers for lidar are driving a broad range of applications.
Hot electrons and hot holes
Like Xu, Halas focused a large part of her CLEO plenary talk on a particular application in biology—in her case, cancer treatment—but using a distinctly different technology, plasmonic nanoparticles. The specific nanoparticles her work has focused on are dielectric particles surrounded by a gold shell. These subwavelength metal-coated particles, she explained, can concentrate optical fields and create different kinds of electronic resonances, depending on particle shape and size.
“You can build nanoparticles of remarkably complex structures, where you can determine predictably where the plasmon resonances are,” and what their properties are, according to Halas. When such a plasmonically resonant nanoparticle is struck by the right wavelength of light, Halas said, concentration of energy heats up the particle on femtosecond timescales. And the particle also generates “hot electrons and hot holes, and you can do interesting applications with both.”
Battling cancers by concentrating light
One such application Halas’ group has pioneered is using plasmonic nanoparticles for photothermal cancer therapy—an opportunity opened up because the diminutive particles like to congregate in cancer cells. “Because of the nature of tumor vasculature,” Halas said, “any particle less than 200 nm will tend to accumulate in a tumor.” Thus, in principle, one can inject nanoparticles that are resonant at around 800 nm—the first “water window” for IR radiation—wait for them to accumulate in the tumor, and then use infrared radiation to heat the nanoparticles up and photothermally kill the tumor.
The technique, she noted, has been used in head and neck cancers and, more recently, is under clinical trials for prostate cancers using a specialized laser catheter. Those latter trials have, she said, now treated 39 patients successfully, without many of the quality-of-life-diminishing side effects of conventional prostate cancer treatments.
The hot electrons and hot holes generated by plasmonic nanoparticles also have some interesting implications for doing chemistry differently—and, in particular, for dramatically reducing its energy costs. Halas pointed out that “one of the biggest energy consumers” in industry is chemical manufacturing—just one of Dow Chemical’s reactors for creating ammonia for fertilizer “could power the city of Chicago,” according to Halas. Indeed, fertilizer manufacturing, while crucial for maintaining life, reportedly consumes 5 percent of the power used on Earth.
Plasmonic nanoparticles, Halas said, could get past this by allowing different approaches to photocatalysis—triggering otherwise energy-expensive chemical reactions, such as the dissociation of hydrogen, using cheap light energy. These techniques can help not only with production of energy-intensive chemicals such as fertilizers, but with the creation of hydrogen for use in non-polluting “hydrogen economy” applications such as fuel-cell-powered vehicles.
A start-up company, Syzygy Plasmonics, is now working on commercializing some of the technologies developed by Rice researchers in this sphere, Halas noted. The group even has its eye on a possible light-based reactor that could extract hydrogen from methane, a potent greenhouse gas, to create fuel for clean, hydrogen-powered vehicles—a potential one-two punch against climate change.
Looking at lidar
Mial Warren at CLEO 2019.
Tuesday’s final speaker, Mial Warren of TriLumina Corp., came to that company after a long career at Sandia National Laboratory, doing research on VCSELs, nanophotonics and other areas. He’s now working on the development of high-power VCSEL arrays for lidar in automobiles and 3-D imaging applications.
Those applications could become relevant sooner than many people think, according to Warren. He pointed out that, while the complete “self-driving” vision of truly autonomous vehicles is likely “very far out” in the future, so-called Level 3 systems—which involve detection of the environment, with the driver still expected to intervene—“could be coming quite quickly.” That’s one reason, according to Warren, that the consulting firm Yole Development recently pegged the lidar market as reaching some US$6 billion by 2024—with some 70 percent of that going to the automotive sector.
Lidar is, of course, only one of several sensors in next-gen vehicles, with other important ones being radar, for long-range sensing, and CMOS cameras, which require substantial signal processing after the sensing. Lidar, by contrast, seems to promise the best of all worlds: relatively high resolution, long range, low latency and the capability of operating day or night. But lidar still “rather expensive” compared with the other technologies, Warren said.
Tough price requirement
Warren reviewed some of the history of automotive lidar, starting with the DARPA grand challenges in the early to mid 2000s—during which some of the industry’s pioneers gained significant experience. In the past several years, as automakers have become increasingly interested in the technology, a “remarkable consensus” around the technical requirements for automotive lidar has emerged, according to Warren. And one particularly tough requirement to meet has been the industry’s price requirement: Less than US$200 per lidar unit, far below current levels.
Faced with that, he said, the race among the many companies seeking to put “lidar in every garage” has been “not for better lidar, but for cheaper lidar.” That race, Warren noted, could be aided by continued innovation in laser technology—which must meet a dizzying array of requirements for operating temperature range, reliability, eye safety and other factors.
Another area in which progress could help, he continued, is in the development of detection technologies such as silicon-based single-photon avalanche detector (SPAD) arrays for photon counting. SPAD arrays, he noted, have a particular advantage of high gain—which can allow much lower laser power levels, ameliorating some of the eye-safety issues that have been a consistent concern about wide deployment of lidar. Still other researchers are looking at techniques such as coherent detection, which can help with background noise and which are immune to crosstalk from other lidar systems—a big advantage in a situation where “everyone’s driving around with lidars and pinging each other.”
A lidar in every garage?
Taking these and other trends together, Warren said he thought it was “highly likely” that automotive lidar would come down in cost enough to be in every vehicle, driven largely by improvements in silicon detection technology and integration. Further, because of the nature of the automotive business and its preference for independent supply chains, Warren suggested that it’s “also likely that there will be more than one winner,” and that the market would be characterized by competing technologies for quite some time.