Power/Performance Bits: Dec. 21

Compact optical amplifier; efficient optical switch; cooling with kirigami.


Compact optical amplifier
Researchers at Chalmers University of Technology propose a new optical amplifier design that is compact, high-performance, and doesn’t generate excess noise.

“We have developed the world’s first optical amplifier that significantly enhances the range, sensitivity and performance of optical communication, that does not generate any excess noise – and is also compact enough to be of practical use,” says Ping Zhao, Postdoc at the Photonics Laboratory at Chalmers.

The new amplifier fits in a small chip just a few millimeters in size, compared to previous amplifiers that have been several thousand times larger. The light amplification in the project is based on the Kerr effect, which can amplify light without causing significant excess noise.

They researchers noted that the level of performance the amplifiers offer allows them to be placed more sparingly. They also work in a continuous wave (CW) operation rather than a pulsed operation only.

“What we demonstrate here represents the first CW operation with an extremely low noise in a compact integrated chip. This provides a realistic opportunity for practical use in a variety of applications. Since it’s possible to integrate the amplifier into very small modules, you can get cheaper solutions with much better performance, making this very interesting for commercial players in the long run,” said Professor Peter Andrekson, Head of the Photonics Laboratory at the Department of Microtechnology and Nanoscience at Chalmers.

“This amplifier shows unprecedented performance. We consider this to be an important step towards practical use, not only in communication, but in areas including quantum computers, various sensor systems and in metrology when making atmospheric measurements from satellites for Earth monitoring.”

Efficient optical switch
Researchers from Skolkovo Institute of Science and Technology (Skoltech), IBM Research, and Bergische Universität Wuppertal developed an optical switch that can perform 1 trillion operations per second while being energy efficient and requiring no cooling.

In addition to its primary transistor-like function, the switch could act as a component that links devices by shuttling data between them in the form of optical signals. It can also serve as an amplifier, boosting the intensity of an incoming laser beam by a factor of up to 23,000.

The device relies on two lasers to set its state to “0” or “1” and to switch between them. A very weak control laser beam requiring only a few photons is used to turn another, brighter laser beam on or off.

“What makes the new device so energy-efficient is that it only takes a few photons to switch,” said Anton Zasedatelev of Skoltech.

The switching occurs inside a microcavity created from a 35-nanometer thin organic semiconducting polymer sandwiched between highly reflective inorganic structures. The microcavity keeps incoming light trapped inside for as long as possible to favor its coupling with the cavity’s material.

The researchers said that this light-matter coupling forms the basis of the new device. “When photons couple strongly to bound electron-hole pairs — aka excitons — in the cavity’s material, this gives rise to short-lived entities called exciton-polaritons, which are a kind of quasiparticles at the heart of the switch’s operation.

“When the pump laser — the brighter one of the two — shines on the switch, this creates thousands of identical quasiparticles in the same location, forming so-called Bose-Einstein condensate, which encodes the “0” and “1” logic states of the device.”

The team used a control laser pulse seeding the condensate shortly before the arrival of the pump laser pulse to switch between the two levels of the device. This stimulates energy conversion from the pump laser, boosting the amount of quasiparticles at the condensate. The high amount of particles in there corresponds to the “1” state of the device.

“In our Skoltech labs we achieved switching with just one photon at room temperature. That said, there is a long way to go before such proof-of-principle demonstration is utilized in an all-optical co-processor,” added Professor Pavlos Lagoudakis, who heads the Hybrid Photonics Labs at Skoltech.

The team plans work to continue lowering the power consumption further and integrate it with a toolkit of all-optical components.

Cooling with kirigami
Researchers from Osaka University, Oita National College of Technology, and Tokyo Polytechnic University used kirigami techniques to improve passive convective cooling using cellulose nanofiber films. Kirigami is a paper art that uses cutting in addition to folding to create structures.

By stretching a simple kirigami “net decoration” pattern called amikazari, the cuts in the cellulose nanofiber film can open into holes that allow air to flow through them. They have better cooling properties than uncut films by default, but applying air flow to them dramatically improves the cooling ability.

Using laser-cut films based on traditional designs, the team performed a heat-dissipation test with irradiated light on a graphite-blackened area and observed a dramatic difference in the maximum temperature between the kirigami film and the uncut film under air flow. “We also developed ‘cool’ light sources by creating powder electroluminescent devices, which are normally prone to overheating, on top of the cellulose nanofiber films” said Kojiro Uetani of Osaka University.

Thermal resistance was reduced to about one-fifth of what it was before applying the kirigami system. “The kirigami heat-dissipation concept enables new thermal designs using various film materials as heat-dissipation components and is expected to inspire a wide range of new cooling devices and methods for use in next generation electronics,” said Masaya Nogi of Osaka University.

The researchers believe that the flexibility of the cooling material could benefit wearable devices and non-rigid electronics.

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