Inside every cell phone is a tiny mechanical heart, beating billions of times per second. These micromechanical resonators play an essential role in cellular communication. Shaken by the cacophony of radio frequencies on the airwaves, these resonators select just the right frequencies to transmit and receive signals between mobile devices.
With the growing importance of these resonators, scientists need a reliable and efficient way to ensure the devices are working properly. This is best achieved by carefully studying the acoustic waves generated by the resonators.
Now, researchers at the National Institute of Standards and Technology (NIST) and their colleagues have developed an instrument to image these acoustic waves over a wide range of frequencies and produce “movies” of them in unprecedented detail.
Researchers have measured acoustic vibrations as fast as 12 gigahertz (GHz, or billions of cycles per second) and may be able to extend these measurements to 25 GHz, providing the frequency coverage needed for 5G communications as well as for potentially powerful future applications in the quantum field. information.
The challenge of measuring these acoustic vibrations is likely to increase as 5G networks dominate wireless communications, generating even smaller acoustic waves.
NIST’s new instrument captures these waves in action using a device called an optical interferometer. The illumination source of this interferometer, usually a stable beam of laser light, is in this case a laser that pulses 50 million times per second, which is significantly slower than the measured vibrations.
The laser interferometer compares two pulses of laser light that travel along different paths. A pulse passes through a microscope which focuses laser light onto a vibrating micromechanical resonator and is then reflected. The other pulse serves as a reference, moving along a path that is continually adjusted so that its length is within one micrometer (one millionth of a meter) of the distance traveled by the first pulse.
When the two pulses meet, the light waves from each pulse overlap, creating an interference pattern – a set of dark and light fringes where the waves cancel or reinforce each other. As subsequent laser pulses enter the interferometer, the interference pattern changes as the microresonator vibrates up and down. From the changing fringe pattern, researchers can measure the height (amplitude) and phase of the vibrations at the location of the laser spot on the micromechanical resonator.
NIST researcher Jason Gorman and his colleagues deliberately chose a reference laser that pulses between 20 and 250 times slower than the frequency at which the micromechanical resonator vibrates. This strategy allowed the laser pulses illuminating the resonator to slow down the acoustic vibrations, in the same way that a strobe light seems to slow down dancers in a nightclub.
The slowdown, which converts acoustic vibrations that oscillate at GHz frequencies to megahertz (MHz, million cycles per second), is important because the light detectors used by the NIST team operate much more accurately, with less noise, at these lower frequencies.
“Switching to lower frequencies removes interference from communication signals typically found at microwave frequencies and allows us to use photodetectors with lower electrical noise,” Gorman said.
Each pulse lasts only 120 femtoseconds (quadrillionths of a second), providing highly accurate instantaneous vibration information. The laser scans the micromechanical resonator so that the amplitude and phase of the vibrations can be sampled across the entire surface of the vibrating device, producing high resolution images over a wide range of microwave frequencies.
By combining these measurements, averaged over many samples, researchers can create three-dimensional films of the vibrational modes of a microresonator. Two types of microresonators were used in the study; one had dimensions of 12 micrometers (millionths of a meter) by 65 micrometers; the other was 75 micrometers on a side, about the width of a human hair.
Not only can images and movies reveal whether a micromechanical resonator is working as expected, but they can also indicate problem areas, such as where acoustic energy is leaking from the resonator. Leakage makes resonators less efficient and leads to loss of information in quantum acoustic systems. By identifying problem areas, the technique gives scientists the information they need to improve resonator designs.
In the February 4, 2022 edition of Nature Communication, the researchers reported that they could image acoustic vibrations with an amplitude (pitch) as small as 55 femtometers (quadrillionths of a meter), or about one five-hundredth the diameter of a hydrogen atom.
Over the past decade, physicists have suggested that micromechanical resonators in this frequency range could also be used to store fragile quantum information and transfer data from one part of a quantum computer to another.
Establishing an imaging system capable of routinely measuring micromechanical resonators for these applications will require further research. But the current study is already an important step in assessing the ability of micromechanical resonators to operate accurately at the high frequencies that will be needed for efficient communication and for quantum computing in the near future, Gorman said.