Speaking of the earliest holography (Holography), everyone may have invariably think of the rescue clip of Princess Leia in the science fiction classic "Star Wars". The film was released in 1977, and under the limited technical conditions of the time, it used imagination and ancient special effects to create a mythical world for humanity. Holographic display is one of the most important concepts, and even today, more than forty years later, human beings are still pursuing to explore and realize the technology.
Princess Leia sends a hologram of Luke Skywalker and Obi-Wan, Source: Star Wars
You may wonder, didn't Front-projected holographic display exist for a long time? For holographic in the strict sense, air is the medium for imaging objects. And now most of the "holograms" that we can come into contact with every day, such as the return of the late singer to the stage at concerts, are actually the application of the Pepper's Ghost introduced in our past public account article "'Holography' Takes You to the Entrance to the Wizarding World" - this is a "quasi-holographic projection technology", which requires the help of holographic film, tulle screen or fog and other media, and people can observe the image at a certain angle. So it's just a flat and not a true 360-degree stereoscopic image.
Teresa Teng's holographic projection on stage, Source: Google
Recently, however, the journals Nature and Science also reported on the news headlines a Multimodal acoustic trap display (MATD) introduced by researchers from the University of Sussex in the United Kingdom, which is expected to make holograms in science fiction films a reality.
"Holographic" images in MATD, Source: mittrchina.com
MATD display devices are not complicated, just a tap on the keyboard, can make a 2mm wide suspension particle - polystyrene beads (foam balls) come to life. The researchers can manipulate the white dots to make them jump, hover and circle in the air; with a click, the small dots will swirl around in the black box to form a glowing colored butterfly, and it will even flap its wings.
MATD device structure, Source: youtube
Behind the butterflies deforming and flying in mid-air is a relatively simple device: above and below the suspended particles there are two slender arrays of 256 miniature speakers, which control the movement of the particles by sound levitation. The red, green, and blue light emitted by the LEDs hits the particles, mapping out the trajectories left by their high-speed movements, forming the evolving 3D images we see with the naked eye. This is the "visual persistence" phenomenon mentioned in the previous public account article ("long dance" is not slow, your eyes have not been able to keep up) - because the display system controls the particle speed in the vertical and horizontal directions as high as 8.75m/s and 3.75m/s, respectively, and its actual movement distance is only a few millimeters to a few centimeters, so its movement time is much smaller than the time required for our optic nerve response (about 1/16 second, and the light reaction time for different frequencies will vary), This reaction duration causes visual images to lag in the brain.
High-speed moving particles draw out a countdown with a smiley face image, Source: Youtube
Butterflies and smiling faces or other patterns are thus drawn directly in the air, like sketches with a high-speed etching machine.
How does the speaker array rely on sound to control tiny particle entities to form a suspended image? Acoustic levitation is a method of using acoustic radiation pressure from high-intensity sound waves to suspend a substance in the air under the action of gravity.
We all know that sound is a wave that travels through the air, similar to the wave of water that travels in water. However, water waves are transverse waves, and their vibration direction is perpendicular to the direction of propagation; while sound waves are longitudinal waves, and their vibration direction is in the direction of propagation. The continuous vibration of sound waves causes the air molecules to continuously compress and relax alternately, resulting in rapid fluctuations in atmospheric pressure, and objects in the sound field are affected by the forces generated by the pressure difference.
Standing waves, source: wikipedia
In the image above, red and blue represent two identical waves that propagate in opposite directions, and black waves are the result of their synthesis. Adding the amplitudes of the two columns of red and blue waves produces a wave that seems to stand still in place, which is also the origin of the name "Standing wave".
Careful observation can be found that the amplitude of the sound wave at the red dot position on the black standing wave is always zero, so the theoretical pressure is also zero, and these places are called "wave nodes"; and those places with the largest amplitude are called "wave belly", and the distance between the two nodes is exactly half of the original sound wave wavelength. The use of standing waves to suspend objects is when the high pressure generated at the belly of the wave pushes the object into the low pressure area (wave node) of the force field, and when the gravity of the suspended particle/water droplet itself reaches equilibrium, it can be stopped in the air.
The particle trajectories that move faster and faster form the "holographic" image in MATD, Source: Youtube
Because the particles moving fast in MATD are light but physically present objects, they can produce an actual tactile sensation when you try to reach out and touch it —"the touch feels like gently spraying your hand with pressurized air." The researcher said in an interview. After demonstrating the full display, turn off the monitor and the flapping butterfly disappears, leaving the beads that created it bouncing on the monitor base.
Touch the butterflies flying in MATD, Source: Youtube
These seemingly mysterious scientific principles are always exquisite and simple, and there are more "magician hats" waiting for people to explore in the world, so let's look forward to the future to develop more exciting scientific and technological magic like sound levitation.
Reference:
https://en.wikipedia.org/wiki/Acoustic_levitation
https://www.youtube.com/watch?v=tzWP-NL3Lck
https://www.youtube.com/watch?v=jOnxr9Ez_Kc
https://futurism.com/scientists-create-holograms-see-hear-feel
https://www.mittrchina.com/news/detail/3988
The reproduced content represents the views of the author only
Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
Source: Center for the Study of Science and Arts
EDIT: just_iu