University of Minnesota
School of Physics & Astronomy


The interaction between light and sound

Semere Tadesse
Semere Tadesse
Richard Anderson

Semere Tadesse is a physics graduate student working on optics with Electrical Engineering and Physics graduate faculty member Professor Mo Li. His research integrates optical and surface acoustic wave devices on piezoelectric aluminum nitride film, which enables interaction of light and sound on the same chip.

The study of light-sound interaction dates back to as early as 1922 when Léon Brillouin predicted the scattering of light from thermally excited acoustic waves. Until now studies have focused on regimes where the acoustic wave has a wavelength much longer than light wave. Tadesse’s research focuses on studying the interaction in a regime where the phonon (sound) wavelength is shorter than the photon (light) wavelength.

To generate the acoustic waves, he deposits periodically arranged interlocking metal electrodes on the piezoelectric substrate. Application of time varying voltage to these electrodes excites acoustic waves of wavelength matching the electrodes periodicity, and frequency determined by the electrodes period and speed of sound in the piezoelectric material.

The optical devices he studies are optical cavities that store photons for a finite amount of time. These cavities are deliberately introduced defect states in artificially created perfect photonic crystals. The photonic crystals are composed of periodically arranged dielectric materials which affect propagating photons in the same way as semiconductor crystals affect electron waves. The crystal creates a bandgap where photons of a certain frequency range cannot propagate. By introducing defect states in this otherwise perfect crystal, Tadesse can manage to store photons in the cavity for a finite amount of time.

Using the nanofabrication facility at Minnesota Nanofabrication Center (MNC), Tadesse has realized acoustic transducers that generate surface acoustic waves with wavelengths as low as 400nm, much shorter than wavelength of near infrared photons. The photonic crystal cavities he fabricated can store photons with lifetime up to half nanosecond. This test device was able to modulate the optical cavities at a speed of 10 GHz, an order of magnitude better than what had already been demonstrated. Tadesse is working on improving his device to make it even more efficient by eliminating the leak of the acoustic wave into the substrate beneath the aluminum nitride.
From a commercial point of view, Tadesse says, optical modulators like his device can be used in signal processing, linking wireless communication with fiber optic communications. Understanding the physics behind the interaction of sound and light can also lead to the development of more efficient communication devices.