Optical forces are sometimes called “tractor beams” and are actually the pressure of light. Optical tweezers are thus far the only practical application that uses optical forces. They are mainly used in biological applications and can be used to move or hold minuscule objects.
In new research conducted by a multidisciplinary team led by Ali K. Yetisen at Harvard University and MIT, and Yunuen Montelongo at Imperial College London, a new type of rewritable, dynamic 3D holographic material has been created. The new material can be written and erased many times rapidly, unlike other 3D holographic materials. Information can also be stored on it without using any external energy. Potential applications for this material includes metamaterials, biosensors, large-scale volumetric data storage devices, 3D holographic displays, optical lenses and tunable lasers.
The team manufactured memory devices, 3D holograms and lenses by using nanostructured materials that have been reversible optical manipulated. The fundamental principal to creating 3D holographic material with these advantages is to modify the material’s properties reversibly by using optical forces. Two or more laser beams are used to interfere with each other. This produces the optical forces that creates an optical pressure that is able to move nanoscale structures.
The potential applications of optical forces have been expanded in the new study. The team showed that big numbers of silver nanoparticles could be arranged reversibly within well-defined 3D patterns inside a solid material. Rearranging these patterns and the information stored in them is dependent on the angle, energy and quantity of the laser pulses.
Montelongo noted that the study has shown that nanoparticles can be assembled inside a solid by using optical pressure. Optical forces have been used for quite some time, but this is the first time that nanostructured materials have been configured using them. This new method can be scaled making it possible to assemble big arrays of nanoparticles in a reversible fashion.
This video shows the holographic nanoassembly of a photonic crystal. In this process, metallic nanoparticles absorb light, which increases the temperature of the surrounding medium and causes the nanoparticles to move to lower-energy configurations. After the thermal energy dissipates, the nanoparticles remain in stable positions. Credit: Montelongo et al. Nature Communications
Although nanoparticles can be manipulated in 3D patterns via other existing methods, most of these change the material permanently so that it cannot be rewritten. For the first time scientists have 3D arrangements that are reversibly controlled. There is also no need for external energy to maintain the recorded information. The nanoparticle configurations remain stable without needing external energy because the laser produces a high temperature while the writing process occurs.
The nanostructured material assumes a rubber state at this temperature in which it is easily configured. When the heat effect is removed, the temperature decreases and transforms the material into a glass state, effectively “freezing in” the configuration.
The researchers compare the process to a “knife in the butter.” The medium changes its toughness depending on the temperature of the metal. Just as a cold knife has more difficulty to slice through butter than a hot one, the high temperature allows the optical forces to manipulate the nanomaterials in the rubber state easily.
The researchers hope that the new method’s scalability, reconfigurability and fast recording will make optical forces an encouraging approach for creating dynamic 3D holograms for numerous applications.
Montelongo intends to develop a new family of photoactive materials that can be used to produce rewritable 3D holograms. One of the examples he cites is to replace conventional 2D visualization methods in different areas of medicine with 3D holograms. The advantage of a hologram is that it can provide a full reconstruction with a sense of volume in the space. As the new materials are active, information can be recorded and erased rapidly. Montelongo believes this mechanism will be used for real-time holographic 3D displays in the long term.