Power/Performance Bits: March 29

Photonic-phononic circuit

Researchers at the National Institute of Standards and Technology (NIST) developed a piezo-optomechanical circuit that converts signals among optical, acoustic and radio waves.

At the heart of the piezoelectric optomechanical circuit is an optomechanical cavity, which consists of a suspended nanoscale beam. Within the beam are a series of holes that act like a hall of mirrors: photons of a very specific color or frequency bounce back and forth between these mirrors thousands of times before leaking out. At the same time, the nanoscale beam confines mechanical vibrations, phonons, at a frequency of billions of cycles per second. The photons and phonons exchange energy so that vibrations of the beam influence the buildup of photons inside the cavity, while the buildup of photons inside the cavity influences the size of the mechanical vibrations. The strength of this mutual interaction is one of the largest reported for an optomechanical system.

Acoustic waveguide channels phonons into the optomechanical cavity, enabling the group to manipulate the motion of the suspended nanoscale beam directly. (Source: K. Balram/K. Srinivasan/NIST)

One of the researchers’ main innovations came from joining these cavities with acoustic waveguides. By channeling phonons into the optomechanical device, the team was able to manipulate the motion of the nanoscale beam directly. Because of the energy exchange, the phonons could change the properties of the light trapped in the device. To generate the sound waves, which were at GHz frequencies, they used piezoelectric materials, which deform when an electric field is applied to them. By using an interdigitated transducer, which enhances this piezoelectric effect, the group was able to establish a link between radio frequency electromagnetic waves and the acoustic waves. The strong optomechanical links enabled them to optically detect this confined coherent acoustic energy down to the level of a fraction of a phonon.

Solar recycling

Scientists at the University of Cambridge discovered that hybrid lead halide perovskites can recycle light, a finding that they believe could lead to large gains in the efficiency of solar cells.

Solar cells absorb photons from the sun to create electrical charges, but the process also works in reverse: when the electrical charges recombine, they can create a photon. The researchers showed that perovskite cells have the extra ability to re-absorb these regenerated photons in a process known as photon recycling. This creates a concentration effect inside the cell, as if a lens has been used to focus lots of light in a single spot.

The study involved shining a laser on to one part of a 500nm thick sample of lead-iodide perovskite. Perovskites emit light when they come into contact with it, so the team was able to measure photon activity inside the sample based on the light it emitted.

Close to where the laser light had shone on to the film, the researchers detected a near-infrared light emission. Crucially, however, this emission was also detected further away from the point where the laser hit the sample, together with a second emission composed of lower-energy photons.

“The low-energy component enables charges to be transported over a long distance, but the high-energy component could not exist unless photons were being recycled,” Luis Miguel Pazos Outón, lead author on the study, said. “Recycling is a quality that materials like silicon simply don’t have. This effect concentrates a lot of charges within a very small volume. These are produced by a combination of incoming photons and those being made within the material itself, and that’s what enhances its energy efficiency.”

“It’s a massive demonstration of the quality of this material and opens the door to maximizing the efficiency of solar cells,” said Felix Deschler, a corresponding author of the study. “The fabrication methods that would be required to exploit this phenomenon are not complicated, and that should boost the efficiency of this technology significantly beyond what we have been able to achieve until now.”