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Acoustic microscopy and nonlinear effects in pressurized superfluid helium
A A Moulthrop1, M S Muha, B Hadimioglu
1Aerospace Corp., Los Angeles, CA.
Summary
A new acoustic microscope uses superfluid helium as a coupling medium for high-resolution imaging. This innovative technique reveals details invisible to electron microscopes and explores nonlinear sound propagation in helium.
Area of Science:
- Acoustic microscopy
- Superfluid helium physics
- Nonlinear acoustics
Background:
- Scanning electron microscopy (SEM) has limitations in imaging certain materials.
- High-frequency acoustic microscopy requires specialized coupling media.
- Superfluid helium (⁴He) exhibits unique properties at low temperatures and high pressures.
Purpose of the Study:
- To demonstrate the operation of a novel acoustic microscope with 15 μm resolution.
- To image objects with low contrast in SEM using acoustic microscopy.
- To investigate nonlinear sound propagation in superfluid helium at high acoustic intensities.
Main Methods:
- Utilized a superfluid (⁴)He coupling medium below 0.9 K and above 20 bar.
- Operated the acoustic microscope at a frequency of 15.3 GHz.
- Analyzed received signal power versus input power to study nonlinear effects.
Main Results:
- Achieved a resolution of 15 μm, enabling imaging of previously unresolvable features.
- Observed significant nonlinear acoustic behavior in superfluid helium near the focal point.
- Demonstrated source depletion and frequency reconversion, indicative of harmonic generation.
Conclusions:
- The acoustic microscope successfully imaged samples with poor SEM contrast.
- Superfluid helium exhibits extreme nonlinear acoustic properties at high intensities.
- Harmonic generation is proposed as the underlying mechanism for the observed nonlinear phenomena.
