Three RIKEN scientists have proven the ability of an ultrafast version of transmission electron microscopy to measure sound waves in nanostructures. This could aid in the development of a high-resolution imaging approach that uses ultrahigh-frequency sound waves to image structures as small as nanometers.
In clinics and hospitals, ultrasound is commonly used to examine internal organs and babies in the womb. Because the sound waves utilized have a wavelength of a few millimeters, they can picture structures down to that level.
While such a resolution is enough for medical imaging, physicists want to employ sound waves to image structures in materials as small as a few nanometers.
“We can use sound waves with wavelengths of about 100 nanometers or so to inspect materials, such as finding defects,” says Asuka Nakamura of the RIKEN Center for Emergent Matter Science (CEMS). “However, the sensitivity to small defects is truly wavelength dependent.”
This necessitates the generation and detection of sound waves with substantially shorter wavelengths (and thus higher frequencies). It is quite simple to generate such high-frequency sound waves—ultrashort laser pulses have been employed to generate them in metals and semiconductors for decades. However, detecting them is significantly more difficult because it needs constructing detectors with resolutions of nanometers in space and picoseconds in time.
Nakamura and CEM colleagues Takahiro Shimojima and Kyoko Ishizaka have now proved the capability of a specific type of electron microscope for imaging such ultrahigh-frequency sound waves. The findings were published in the journal Nano Letters.
They used an ultrafast transmission electron microscopy (UTEM) to detect sound waves produced by a 200-nanometer hole in the center of a thin silicon plate. A UTEM employs two laser beams separated by a little delay (see diagram above). One beam illuminates the material, while the other produces an ultrashort electron pulse in the microscope. This configuration allows for the resolution of very short timescales.
When the trio theoretically calculated the waves and compared the simulations to experimentally observed photos, they discovered good agreement.
The photos’ quality exceeded the team’s expectations, allowing them to run a Fourier-transform analysis (a common mathematical analytic approach) on them. “We didn’t intend to characterize the sound waves before performing these experiments,” Nakamura explains. “However, once we had the data, we noticed it was very beautiful, and we could apply the Fourier transformation.” That took me by surprise.”
The researchers propose to use UTEM to examine ultrafast structural and magnetic dynamics in solids caused by such nanometric sound waves.
