The brilliant metallic blue wings of Morpho butterflies dazzle the eye. Branched nanostructures on the wings scatter light in complex ways to produce this characteristic iridescent hue. By harnessing the optical properties of these nanostructures, researchers have made a device that detects sound waves and human voices (Nano Lett. 2019, DOI: 10.1021/acs.nanolett.9b00468). The technology could lead to acoustic detectors that are smaller, faster, and more sensitive than current ones, its developers say.
Acoustic sensors usually consist of a vibrating diaphragm or piezoelectric material that converts sound waves into an electrical signal. Such sensors are used for testing pipelines for leaks and to monitor machines and medical implants to catch faint sounds or unusual vibrations long before components fail. Newer types of acoustic sensors detect sound waves using light; they are based on thin metallic films or optical fibers that, when deformed by sound waves, reflect or scatter light waves. Because light waves interact with nanoscale structures and are immune to electromagnetic interference, optical acoustic sensors can be smaller and more sensitive and have a faster response time.
Materials scientists and engineers at Shanghai Jiao Tong University decided to take advantage of the unique photonic nanostructures on Morpho butterfly wings to sense the sound waves. Others have already used the wings or tried to mimic their structures to make gas and heat sensors and photocatalysts. Morpho wings are covered with rows of tile-like scales. Each scale has long parallel ridges that, in cross section, look like pine trees. Light waves bounce off these tree-like nanostructures and interfere to create shimmering colors; the colors and brightness change when the ridges deform under mechanical or chemical changes.
To make their acoustic sensor, Tao Deng, Wen Shang, and their colleagues sandwiched a piece of a butterfly wing between two steel films and cut a 5 mm wide hole in the middle. They direct sound from a speaker onto the wing, shine bright white light on it, and use a photodetector to look at the reflected light.
The wing vibrates due to the acoustic wave, causing the nanostructures to deform. “This leads to an oscillation in the intensity of the reflected light from the wing surface, which could be detected by a photodetector,” Deng says. The detected frequency spectrum matched that of the input sound waves. As a simple demonstration of the sensor’s ability to recognize human voice, the team broadcast long vowel sounds in a female voice and a male voice. The sensor created an accurate frequency spectrum for each vowel, with the frequency peaks slightly different for the male and female voice speaking the same vowel.
The wing-based sensor can detect sounds as low as 20 decibels, which is an order of magnitude less sensitive than today’s best acoustic sensors. “But this sensitivity can be further improved by applying a better photodetector and light source,” Shang says. The device’s response speed of 50 MHz matches the scanning speed of the photodetector, so theoretically the butterfly wing sensor should be able to detect high-frequency ultrasonic waves. The researchers are now planning to make microsized acoustic sensors based on a single scale of the butterfly wing, she says. Practical sensors would use artificial butterfly nanostructures made with lithographic processes rather than real butterfly wings, she adds.
“This is a brilliant concept,” says Radislav A. Potyrailo, a micro- and optoelectronics researcher at GE Research who has made gas sensors based on Morpho butterfly wings. High-performance acoustic detection is an “unexpected application” for the natural photonic nanostructures on wings. Such 3-D nanostructures can be made in a cost-effective way, he says. And since iridescent butterflies are visible from kilometers away, he imagines that the wing-based sensor could allow remote noise detection, which could be especially useful for security and surveillance applications.