A collaboration between École Polytechnique fédérale de Lausanne (EPFL) in Switzerland and the University of California, Santa Barbara (UCSB) has developed a long-anticipated breakthrough and demonstrated CMOS technology that is used for building microprocessors and memory chips and that allows wafer-scale manufacturing of chip-scale optical frequency combs.

Optical frequency combs consist of light frequencies made of equidistant laser lines. They have already revolutionized the fields of frequency metrology, timing, and spectroscopy. The discovery of ‘’soliton microcombs’’ by Professor Tobias Kippenberg’s lab at EPFL in the past decade has enabled frequency combs to be generated on-chip. In this scheme, a single-frequency laser is converted into ultra-short pulses called dissipative Kerr solitons.

Soliton microcombs are chip-scale frequency combs that are compact, consume low power, and exhibit broad bandwidth. Combined with a large spacing of comb “teeth”, microcombs are uniquely suited for a wide variety of applications, such as terabit-per-second coherent communication in data centers, astronomical spectrometer calibration for exoplanet searches and neuromorphic supercomputing, optical atomic clocks, absolute frequency synthesis, and parallel coherent LiDAR.  

Yet, one outstanding challenge is the integration of laser sources. While microcombs are generated on-chip via parametric frequency conversion (two photons of one frequency are annihilated, and a pair of two new photons are generated at a higher and lower frequency), the pump lasers are typically off-chip and bulky. Integrating microcombs and lasers on the same chip can enable high-volume production of soliton microcombs using well-established CMOS techniques developed for silicon photonics, however, this has been an outstanding challenge for the past decade.

For the nonlinear optical microresonators, where soliton microcombs are formed, silicon nitride (Si₃N₄₎) has emerged as the leading platform due to its ultralow loss, wide transparency window from visible to mid-infrared, absence of two-photon absorption, and high power-handling capability. But achieving ultralow-loss Si₃N₄ microresonators is still insufficient for high-volume production of chip-scale soliton microcombs, as co-integration of chip-scale driving lasers is required.

Fifteen years ago, Professor John Bowers’s lab at UCSB pioneered a method for integrating semiconductor lasers onto a silicon wafer. Since silicon has an indirect bandgap and cannot emit light, scientists bond indium phosphide semiconductors on silicon wafers to form laser gain sections. This heterogeneous integration laser technology has now been widely deployed for optical interconnects to replace the copper-wire ones that linked servers at data centers. This transformative laser technology has been already commercialized, and Intel ships millions of transceiver products per year.

In an article published in Science, the two labs at EPFL and UCSB now demonstrate the first heterogeneous integration of ultra low-loss Si₃N₄ photonic integrated circuits (fabricated at EPFL) and semiconductor lasers (fabricated at UCSB) through wafer-scale CMOS techniques. 

The method is mainly based on multiple wafer bonding of silicon and indium phosphide onto the Si₃N₄ substrate. Distributed feedback (DFB) lasers are fabricated on the silicon and indium phosphide layers. The single-frequency output from one DFB laser is delivered to a Si₃N₄ microresonator underneath, where the DFB laser seeds soliton microcomb formation and creates tens of new frequency lines.

This wafer-scale heterogeneous process can produce more than a thousand chip-scale soliton microcomb devices from a single 100-mm-diameter wafer, lending itself to commercial-level manufacturing. Each device is entirely electrically controlled. Importantly, the production level can be further scaled up to the industry standard 200- or 300-mm-diameter substrates.

“Our heterogenous fabrication technology combines the three mainstream integrated photonics platforms, namely silicon, indium phosphate, and Si₃N₄, and can pave the way for large-volume, low-cost manufacturing of chip-based frequency combs for next-generation high-capacity transceivers, data centers, sensing and metrology,” says Dr. Junqiu Liu who leads the Si₃N₄ fabrication at EPFL’s Center of MicroNanoTechnology (CMi).

The results indicate masks and proper ventilation may be key to allowing more capacity in schools, businesses, and other indoor areas

A new study from the University of Central Florida suggests that masks and a good ventilation system are more important than social distancing for reducing the airborne spread of COVID-19 in classrooms.

The research, published recently in the journal Physics of Fluids, comes at a critical time when schools and universities are considering returning to more in-person classes in the fall.

"The research is important as it provides guidance on how we are understanding safety in indoor environments," says Michael Kinzel, an assistant professor in UCF's Department of Mechanical and Aerospace Engineering and study co-author.

"The study finds that aerosol transmission routes do not display a need for six feet social distancing when masks are mandated," he says. "These results highlight that with masks, transmission probability does not decrease with increased physical distancing, which emphasizes how mask mandates may be key to increasing capacity in schools and other places."

In the study, the researchers created a supercomputer model of a classroom with students and a teacher, then modeled airflow and disease transmission, and calculated airborne-driven transmission risk. 

The classroom model was 709 square feet with 9-foot-tall ceilings, similar to a smaller-size, university classroom, Kinzel says. The model had masked students -- any one of whom could be infected-- and a masked teacher at the front of the classroom.

The researchers examined the classroom using two scenarios -- a ventilated classroom and an unventilated one -- and using two models, Wells-Riley and Computational Fluid Dynamics. Wells-Riley is commonly used to assess indoor transmission probability and Computational Fluid Dynamics is often used to understand the aerodynamics of cars, aircraft, and the underwater movement of submarines.

Masks were shown to be beneficial by preventing direct exposure of aerosols, as the masks provide a weak puff of warm air that causes aerosols to move vertically, thus preventing them from reaching adjacent students, Kinzel says.

Additionally, a ventilation system in combination with a good air filter reduced the infection risk by 40 to 50% compared to a classroom with no ventilation. This is because the ventilation system creates a steady current of airflow that circulates many of the aerosols into a filter that removes a portion of the aerosols compared to the no-ventilation scenario where the aerosols congregate above the people in the room.

These results corroborate recent guidelines from the U.S. Centers for Disease Control and Prevention that recommend reducing social distancing in elementary schools from six to three feet when mask use is universal, Kinzel says.

"If we compare infection probabilities when wearing masks, three feet of social distancing did not indicate an increase in infection probability with respect to six feet, which may provide evidence for schools and other businesses to safely operate through the rest of the pandemic," Kinzel says.

"The results suggest exactly what the CDC is doing, that ventilation systems and mask usage are most important for preventing transmission and that social distancing would be the first thing to relax," the researcher says.

When comparing the two models, the researchers found that Wells-Riley and Computational Fluid Dynamics generated similar results, especially in the non-ventilated scenario, but that Wells-Riley underpredicted infection probability by about 29 percent in the ventilated scenario.

As a result, they recommend some of the additional complex effects captured in Computational Fluid Dynamics be applied to Wells-Riley to develop a more complete understanding of the risk of infection in space, says Aaron Foster, a doctoral student in UCF's Department of Mechanical and Aerospace Engineering and the study's lead author.

"While the detailed Computational Fluid Dynamics results provided new insights into the risk variation and distance relationships, they also validated the more commonly used Wells-Riley models as capturing the majority of the benefit of ventilation with reasonable accuracy," Foster says. "This is important since these are publicly available tools that anyone can use to reduce risk."