Tag Archives: Semiconductors/Optoelectronics

Photonics Meets Plasmonics in New Switch that Could Steer Lidar Laser Beams

Post Syndicated from Jeff Hecht original https://spectrum.ieee.org/tech-talk/semiconductors/optoelectronics/new-electro-mechanical-switch-integrated-photonics

The synergy of electronic processing and optical communications has powered the decades-long boom in information technology. But the need to convert signals back and forth between electrical and optical forms is becoming a bottleneck for the emerging field of integrated photonics.

A new type of switch that combines electrical and mechanical effects to redirect light could open the door to large-scale reconfigurable photonic networks for several applications including beam steering for lidars and optical neural networks for computing. 

Currently, integrated photonics are used in high-performance fiber-optic systems , and a joint government-industry program called AIM Photonics is pushing their manufacture. However, current optical switches are too big and require too much power to blend well into integrated photonics. The new hybrid nano-opto-electro-mechanical switch has a footprint of 10 square micrometers and runs on only one volt—making it compatible with the CMOS (complementary metal-oxide-semiconductor) silicon electronics used in integrated photonics, says Christian Haffner from the Swiss Federal Institute of Technology in Zurich now working at the National Institute for Standards and Technology (NIST) in Gaithersburg, Maryland and the researcher who led the team that developed it.

The root of the problem Haffner set out to solve is that photons and electrons behave very differently.

Photons are great for communications because they travel at the speed of light and interact weakly with each other and matter, but they are much larger than chip features and require high voltages to redirect them because of their weak interactions.

Electrons are much smaller and interact much more strongly than photons, making them better for switching and for processing signals. However, electrons move slower than light, and more energy is needed to move them.

Long-distance communication systems process signals electronically and convert the signal into light for transmission, but converting between signal formats is cumbersome for local transmission. The new switch makes it possible to redirect optical signals on the integrated photonic circuit without having to convert them to electrical format and then back to optical format for further transmission. 

In Science, Haffner and colleagues describe a hybrid nano-opto-electro-mechanical switch that would occupy only about 10 square micrometers on an integrated photonic circuit. Their switch is a small multilayered disk sitting at a T-junction between two optical waveguides—stripes of transparent silica that guide light—that meet at a right angle. The top layer of the disk is a four-micrometer circle of 40-nanometer gold membrane resting on a small piece of alumina on layer of silicon deposited on silica. That structure acts as a curved waveguide resonant with both the input and output waveguides, so it can transfer resonant light between the two. 

Light within the silica waveguides remains as photons, but within the switch the light excites oscillations of surface electrons in the gold, producing plasmons that vibrate at the frequency of the light wave but over a light much smaller than the optical wavelength. That tight confinement of the plasmonic part of the energy in the air gap between the gold and silicon creates a strong opto-electro-mechanical effect concentrated in the small volume of the switch. 

With no voltage applied to the switch, the plasmonic waveguide remains resonant with the silica waveguides, so it couples light from the input waveguide to the output waveguide with minimal loss, as shown in the animation.

Applying one volt to the switch produces a static charge that pulls the gold membrane toward the silicon layer, changing the shape of the waveguide in the switch so it shifts the phase of the light by 180 degrees. This causes destructive interference in the switch, breaking the resonance and the coupling of light into the side waveguide, so the light instead continues through the input waveguide to another switch.

“What we have in the end,” says Haffner, “is a hybrid [switch], partly photonic and partly plasmonic, that manipulates light very efficiently.” The plasmonic part concentrates the switching in a small area; the photonic part experiences low loss. Applying a one-volt bias compatible with CMOS electronics across such a short distance can produce a very strong force. That gives the switch a small footprint, low loss, and lower power consumption, which conventional electro-optic switches cannot achieve simultaneously.

Mass of the gold film is so low that the switch can operate millions of times a second. That’s adequate for most switching, says Haffner, but it does have limits. The mechanical part of the switch cannot reach the picosecond speeds needed to modulate light in an optical transmitter. 

The first applications are likely to be in laser beam steering for lidar, particularly for autonomous vehicles where continual information on the local environment is vital for safety. Another potential application is optical routing of signals on integrated photonic chips to create optical neural networks for deep-learning applications. The switch can redirect signals millions of times a second, a time scale needed by such applications.

“I don’t see any issues in fabricating [the switches] with high yield,” says Haffner. 

A Simple Filter Turns Blue OLED Light Into White

Post Syndicated from XiaoZhi Lim original https://spectrum.ieee.org/tech-talk/semiconductors/optoelectronics/a-simple-filter-makes-blueemitting-oleds-give-off-white-light

Organic light-emitting diodes (OLEDs) have come a long way since the first working device was reported three decades ago. Prized for their dark blacks, crisp image reproduction, and power efficiency, today’s OLEDs dominate the screens of Android phones and LG televisions. They may take over iPhones as early as next year.

And because OLEDs are cheap and easy to make, we ought to also use them to make white light for general illumination, says Konstantinos Daskalakis, a post-doctoral researcher at Aalto University in Finland.

Except white is an OLED’s Achilles’ heel. Typically, to get white light, individual red, green, and blue emitters shine at the same time. This makes white the most power-hungry color, reportedly requiring six times as much power as it takes to produce the color black on a Google Pixel. Other strategies to generate white light include carefully doping emitting layers with chemicals, but this approach makes it harder to fabricate devices.

In a proof-of-concept experiment, Daskalakis and his supervisor Paivi Torma converted conventional blue-emitting OLEDs to white-emitting ones simply by depositing a distributed Bragg reflector (DBR)—a stack of two alternating materials with high and low refractive indexes—on top of the OLEDs.

Microsize Lens Pushes Photonics Closer to an On-Chip Future

Post Syndicated from Mark Anderson original https://spectrum.ieee.org/nanoclast/semiconductors/optoelectronics/microsize-lens-pushes-photonics-closer-to-an-onchip-future

Optical microcomputing, next-generation compact LiDAR units, and on-chip spectrometers all took a step closer to reality with the recent announcement of a new kind of optical lens.

The lens is not made of glass or plastic, however. Rather, this low-loss, on-chip lens is made from thin layers of specialized materials on top of a silicon wafer. These “metasurfaces” have shown much promise in recent years as a kind of new, microscale medium for containing, transmitting, and manipulating light.

Photonics at the macro-scale is more than 50 years old and has applications today in fields including telecommunications, medicine, aviation, and agriculture. However, shrinking all the elements of traditional photonics down to microscale—to match the density of signals and processing operations inside a traditional microchip—requires entirely new optical methods and materials.

A team of researchers at the University of Delaware, including Tingyi Gu, an assistant professor of electrical and computer engineering, recently published a paper in the journal Nature Communications that describes their effort to build a lens from a thin metasurface material on top of a silicon wafer.

Gu says that metasurfaces have typically been made from thin metal films with nanosized structures in it. These “plasmonic” metasurfaces offered the promise of, as a Nature Photonics paper from 2017 put it, “Ultrathin, versatile, integrated optical devices and high-speed optical information processing.”

The problem, Gu says, is that these “plasmonic” materials are not exactly transparent like windowpanes. Traveling just fractions of a micrometer can introduce signal loss of a few decibels to tens of dB.

“This makes it less practical for optical communications and signal processing,” she says.

Her group uses an alternate kind of metasurface made from etched dielectric materials atop silicon wafers. Making optical components out of dielectric metasurfaces, she says, could sidestep the signal loss problem. Her group’s paper notes that their lens introduces a signal loss of less than one dB.

Even a small improvement (and going from handfuls of dB down to fractions of a dB is more than small) would make a big difference, because a real-world photonics chip might one day have many such components in it. And the more lossy the photonics chip, the greater the amount of laser power needed to be pumped through the chip. More power means more heat and noise, which might ultimately limit the extent to which the chip could be miniaturized. But with her team’s dielectric metasurface lens, “We can make a device much smaller and more compact,” she says.

Her group’s lens is made from a configuration of gratings etched in the metasurface — following a wavy pattern of vertical lines that looks a bit like the Cisco company logo. Gu’s group was able to achieve some of the familiar properties of lenses, including converging beams with a measurable focal length (8 micrometers) and object and image distance (44 and 10.1 µm).

The group further used the device’s lensing properties to achieve a kind of optical signal Fourier Transform—which is also a property of classical, macroscopic lenses.

Gu says that next steps for their device include exploring new materials and to work toward a platform for on-chip signal processing.

“We’re trying to see if we can come up with good designs to do tasks as complicated as what traditional electronic circuits can do,” she says. “These devices have the advantage that they can process signals at the speed of light. It doesn’t need logic signals going back and forth between transistors. … It’s going to be fast.”

This MicroLED Display Is Smaller Than a Bug

Post Syndicated from Samuel K. Moore original https://spectrum.ieee.org/tech-talk/semiconductors/optoelectronics/this-microled-display-is-smaller-than-a-bug

Mojo Vision’s microLED display has record-breaking pixel density and a somewhat mysterious purpose

A Silicon Valley-based startup has recently emerged from stealth mode to reveal what it claims is the smallest, most pixel-dense dynamic display ever built. Mojo Vision’s display is just 0.48 millimeters across, but it has about 300 times as many pixels per square inch as a typical smartphone display.