Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory, Berkeley, CA, say that they have demonstrated a technique for producing, detecting, and controlling ultrahigh frequency sound waves at the nanometer scale. Through a combination of subpicosecond laser pulses and unique nanostructures, the team produced acoustic phonons (quasi-particles of vibrational energy that move through an atomic lattice as sound waves) at a frequency of 10 gigahertz. In comparison, medical ultrasounds typically reach a frequency of only about 20 megahertz.

The 10GHz phonons not only promise unprecedented resolution for acoustic imaging, they also can be used to “see” subsurface structures in nanoscale systems that optical and electron microscopes cannot.

The ability of sound waves to safely pass through biological tissue has made sonograms a popular medical diagnostic tool. Phonons at GHz frequencies can pass through materials that are opaque to photons, the particles that carry light. Ultrahigh frequency phonons also travel at the small wavelengths that yield a sharper resolution in ultrasound imaging.

Their biggest challenge was to find effective ways of generating, detecting, and controlling ultrahigh frequency sound waves. They met this challenge by designing nanostructures that support multiple modes of both phonons and plasmons. The nanostructures are made of gold and shaped like a Swiss-cross. Each is 35 nanometers thick with horizontal and vertical arm lengths of 120 and 90 nanometers, respectively. When the two arms oscillate in phase, the crosses generate symmetric phonons. When the arms oscillate out of phase, anti-symmetric phonons are generated.