Power/Performance Bits: Sept. 9

Smaller, cheaper integrated photonics
Researchers from the University of California Santa Barbara, California Institute of Technology (Caltech), and Ecole Polytechnique Fédérale de Lausanne (EPFL) developed a way to integrate an optical frequency comb on a silicon photonic chip.

Optical frequency combs are collections of equally spaced frequencies of laser light (so called because when plotted, the frequencies resemble a hair comb). They are used with integrated lasers, which can only produce one frequency at a time.

Generating combs used to require bulky and expensive equipment, but this can be now managed using the recently emerged microresonator-based soliton frequency combs, which are miniaturized frequency comb sources built on CMOS photonic chips. Using this “integrated photonics” approach, the team has developed the smallest comb generator in the world.

The system consists of a commercially available feedback laser and a silicon nitride photonic chip. “What we have is a source that generates all these colors out of one laser and one chip,” said John Bowers, Chair in Nanotechnology at UC Santa Barbara and director of the Institute for Energy Efficiency. “That’s what’s significant about this.”

The entire setup fits in a package smaller than a match box, with lower overall price and power consumption than previous systems. It is also easier to operate: previously, producing a coherent soliton comb required just the right frequency and power, and still was not guaranteed. “The new approach makes the process as easy as switching on a room light,” said Kerry Vahala, Professor of Applied Physics and Information Science and Technology at Caltech.

In the team’s design, when the pump laser and resonator are integrated, their interaction forms a highly coupled system that is self-injection-locking and simultaneously generates “solitons” – pulses that circulate indefinitely inside the resonator and give rise to optical frequency combs.

“What is remarkable about the result is the full photonic integration and reproducibility with which frequency combs can be generated on demand,” added Tobias J. Kippenberg, Professor of Physics at EPFL who leads the Laboratory and Photonics and Quantum Measurement (LPQM).

Beyond multicolor light sources for communications, the researchers see optical clocks as one application for the technology.

“Optical clocks used to be large, heavy, and expensive,” said Bowers. “There are only a few in the world. With integrated photonics, we can make something that could fit in a wristwatch, and you could afford it.”

“Low-noise integrated optical microcombs will enable a new generation of optical clocks, communications and sensors,” said Gordon Keeler, the project’s manager at DARPA. “We should see more compact, more sensitive GPS receivers coming out of this approach.”

Cobalt-free battery
Researchers at the University of Texas at Austin developed a lithium-ion battery cathode without using cobalt, an expensive component of current batteries.

Most cathodes for lithium-ion batteries use combinations of metal ions, such as nickel-manganese-cobalt (NMC) or nickel-cobalt-aluminum (NCA). Cathodes can make up roughly half of the materials costs for the entire battery, with cobalt being the priciest element. At a price of approximately $28,500 per ton, it is more expensive than nickel, manganese and aluminum combined, and it makes up 10% to 30% of most lithium-ion battery cathodes.

“Cobalt is the least abundant and most expensive component in battery cathodes,” said Arumugam Manthiram, a professor in the Walker Department of Mechanical Engineering at UT Austin and director of the Texas Materials Institute. “And we are completely eliminating it.”

Instead of cobalt, the cathode is 89% nickel. Manganese and aluminum make up the other key elements. More nickel means an increased energy density for the battery, but in the past that has come with a shorter cycle life, leading the battery to lose capacity more quickly.

Eliminating cobalt usually slows down the kinetic response of a battery and leads to lower rate capability, or how quickly the cathode can be charged or discharged. However, the researchers said they’ve overcome the short cycle life and poor rate capability problems through finding an optimal combination of metals and ensuring an even distribution of their ions.

During synthesis, the researchers were able to ensure the ions of the various metals remained evenly distributed across the crystal structure in the cathode. When these ions bunch up, performance degrades, and that problem has plagued previous cobalt-free, high-energy batteries, Manthiram said. By keeping the ions evenly distributed, the researchers were able to avoid performance loss.

“Our goal is to use only abundant and affordable metals to replace cobalt while maintaining the performance and safety,” said Wangda Li, a Ph.D. graduate at UT Austin, “and to leverage industrial synthesis processes that are immediately scalable.”

The researchers have received grants from the U.S. Department of Energy, and they have formed a startup called TexPower to bring the technology to market.