Research Bits: April 18

Simplified microwave photonic filter for 6G

Researchers from Peking University developed a new chip-sized microwave photonic filter to separate communication signals from noise and suppress unwanted interference across the full radio frequency spectrum.

“This new microwave filter chip has the potential to improve wireless communication, such as 6G, leading to faster internet connections, better overall communication experiences and lower costs and energy consumption for wireless communication systems,” said Xingjun Wang from Peking University.

The team created a simplified photonic architecture for the filter. First, a phase modulator serves as the input of the radio frequency signal, which modulates the electrical signal onto the optical domain. Next, a double-ring acts as a switch to shape the modulation format. An adjustable microring is the core unit for processing the signal. Finally, a photodetector serves as the output of the radio frequency signal and recovers the radio frequency signal from the optical signal.

“The greatest innovation here is breaking the barriers between devices and achieving mutual collaboration between them,” said Wang. “The collaborative operation of the double-ring and microring enables the realization of the intensity-consistent single-stage-adjustable cascaded-microring (ICSSA-CM) architecture. Owing to the high reconfigurability of the proposed ICSSA-CM, no extra radio frequency device is needed for the construction of various filtering functions, which simplifies the whole system composition.”

In tests, the simplified photonic architecture showed comparable performance with lower loss and system complexity compared with previous programmable integrated microwave photonic filters composed of hundreds of repeating units.

The researchers plan to further optimize the modulator and improve the overall filter architecture to achieve a high dynamic range and low noise while ensuring high integration at both the device and system levels.

Manipulating multiple lasers on chip

Researchers at the National Institute of Standards and Technology (NIST) developed chip-scale devices for simultaneously manipulating the color, focus, direction of travel, and polarization of multiple beams of laser light. It could be used to create portable sensors that could measure such fundamental quantities as rotation, acceleration, time, and magnetic fields with high accuracy outside a laboratory, and enable miniature optical atomic clocks.

The single-chip design combines integrated photonic circuits and an optical metasurface, which consists of glass wafers imprinted with millions of tiny structures that manipulate the properties of light without the need for bulky optics. The one chip was able to perform the work of 36 optical components. It was also able to direct two beams of different colors to travel alongside each other, a requirement for some types of advanced atomic clocks.

NIST researchers designed and fabricated this on-chip system to shape multiple laser beams (blue arrows) and control their polarization before the light is sent into space to interact with a device or material. Three components all contribute to manipulating the laser beams: An evanescent coupler (EVC), which couples light from one device to another; a metagrating (MG), a tiny surface imprinted with millions of tiny holes that scatter light just as a large-scale diffraction grating would; and a metasurface (MS), a small glass surface studded with millions of pillars that acts as lenses. (Credit: NIST)

“Replacing an optical table full of bulky optical components with a simple semiconductor wafer that can be fabricated in the clean room is truly game-changing,” said Amit Agrawal, a member of the NIST team. “These kinds of technologies are needed since they are robust and compact and can be easily reconfigured for different experiments under real-world conditions.”

The chip-based optical system is a work in progress, said NIST scientist Vladimir Aksyuk, noting that the laser light is not yet powerful enough to cool atoms to the ultra-low temperatures required for a miniaturized advanced atomic clock, but believes the work to be a key stepping stone.

Attosecond observation

Researchers from Harvard University and Graz University of Technology developed and tested a meta-optics lens for microscopes that could enable attosecond observations.

The lens makes it possible to use extreme ultraviolet radiation in microscopy. The extremely short wavelength of this light enables it to follow ultra-fast physical processes in the attosecond range, such as taking real-time images of the inside of transistors.

However, the extreme ultraviolet wavelength also presents challenges for typical optics materials, which are opaque to this light. “I asked myself whether the classical principle of optics could not be reversed. Can you use the absence of material in small areas as the basis of an optical element?” said Marcus Ossiander of the Institute of Experimental Physics at TU Graz.

Using this concept, the team created a precisely calculated arrangement of tiny holes in an extremely thin silicon foil. These holes conduct and focus the incident attosecond light. The researchers observed that these vacuum tunnels transmit more light energy than should be possible due to the hole-covered surface, suggesting that the meta-optics literally sucks the ultraviolet light into the focal point.

The meta-optics consists of an approximately 200-nanometer-thin film into which tiny hole structures have been etched. The entire lens consists of many hundreds of millions of holes; there are about ten of these structures per micrometer on the membrane. A single hole measures between 20 and 80 nanometers in diameter. The diameters of the holes vary and decrease from the center of the membrane outwards. Depending on the size of the hole, the incident light radiation there is delayed and thus collapses into a tiny focal point.

Next, the researchers plan to develop a microscope that works with this lens. The attosecond microscope would enable researchers to closely observe transistors and track the ultrafast movement of charge carriers in space and time.