Holograms are a very special image recording format that records not only the intensity of light from an object, as an ordinary photographic film does, but also records the phase of light. We are all familiar with static holograms on the cover of National Geographic or on our credit cards. One of the seemingly magical qualities of holograms is that they appear to move as we change our angle of view. In cases of excellent holographic recording, we can actually see behind an object as we move our head.
But these holograms are fixed and unmoving. The object is permanently recorded in the holgraphic film, and although the hologram contains some 3-D information that is impossible to record on ordinary film, the holograpic record itself is static. Static holograms are ubiquitous, but have rather limited usefulness.
What would happen if a hologram recorded on holographic film could change in time, tracking and adjusting to changes in the image intensity and phase? What could we do with this?
An amazing number of things!
We can, for instance, "see" through turbid media, such as skin or other tissue, looking for lesions and carcinomas that cannot be seen by eye alone. We can create a sensitive holographic ear that can listen for ultrasound signals using lasers. We can receive femtosecond laser pulses that have been distorted by propagation down a fiber optic, and remove the distortion automatically as the system even adapts in real-time to changing distorations. In each of these applications, we rely on the moving hologram to adapt to a changing environment. The holograms "shimmer", so to speak, as they track chaning image information.
All these applications, and others, have already been demonstrated in holographic thin films that are constructed from nanometer thin layers of sandwiched semiconductor structures. The thin layers are only 100 Å wide, producing strong quantum-confinement effects. These holographic thin films are called photorefractive quantum wells (PRQW), invented by Prof. Nolte at AT&T Bell Labs, and developed at Purdue in the Quantum Optoelectronics Group in the Department of Physics. They are the most sensitive dynamic holographic media currently known.