In a phenomenon known as the acousto-optic effect, ultrasonic waves can alter the behavior of light as it passes through liquids. When a fluid is subjected to high-frequency vibrations (above 20 kHz), its molecules compress and expand rhythmically, creating variations in the refractive index. These variations act as a dynamic diffraction grating that deflects and scatters light beams.
Research at the Max Planck Institute has demonstrated that applying ultrasound to a vial of water generates visible interference patterns as concentric rings of light. The sound's frequency determines the spacing between these rings: the higher the frequency, the greater the number of fringes. This principle is used in acousto-optic devices to modulate laser intensity in telecommunications and spectroscopy.
- Typical frequency: 1-100 MHz for practical applications
- Most effective material: Molybdenum telluride crystals (MoTe₂)
- Modulation speed: Microseconds, enabling real-time control
Most surprisingly, this effect occurs at room temperature and requires no external electric fields, only the mechanical energy of sound. In 2022, a team at Stanford University managed to project moving images using only ultrasound and a low-power laser, opening possibilities for holographic displays without physical screens.
Source: Max-Planck-Institut für Quantenoptik (2021), Stanford University - Department of Engineering (2022)
