Technology: Holographic movies come in from the cold

 作者:山氯铤     |      日期:2019-02-28 07:05:02
By CHARLES ARTHUR The science-fiction dream of films in three dimensions has been brought a step closer to reality by NTT, the giant Japanese telecommunications company. NTT says that it has developed a recording material which can capture holographic pictures of still or moving objects, and has used it to record and play back a 20-second sequence. However, the era of the ‘holofilm’ in your living room is still some time away. The system devised in NTT’s laboratories only works at 7 K (-266 degree C), and requires special materials and a precisely tunable laser. NTT started work on the project three years ago, with the aim of investigating the basic science of the optical properties of ions in solids. It was clear, says the team, that existing optical recording systems, in which a laser beam is used to burn data into the recording medium one bit at a time, would be too slow to capture films in real-time, and would not pack data densely enough. Instead, the researchers opted for a photochemical method that records each frame as a complete holograph, just as conventional movies are recorded as a sequence of still images. But whereas movies use a separate piece of film for each image, the NTT system records all the images on the same piece of material, using laser light of different frequencies to record successive frames. For the storage medium, the NTT team developed a material consisting of yttrium silicate crystals doped with ions of the metal europium, which has the chemical formula Eu3+:Y2SiO5. To record the data they built a laser with a basic frequency of 5 times 1014 hertz (a wavelength of 580 nanometres) tunable in 1-kilohertz steps over a range of 500 megahertz. Like any material, the crystals absorb light in varying amounts across a wide spectrum of frequencies. But when the material is cooled almost to absolute zero, each of the ions in the solid has a very precisely defined absorption frequency. When laser light of a particular frequency strikes the material, it excites any ions with a matching absorption frequency. Excited ions cannot absorb any more energy, so the material absorbs less at points where light of that frequency has already struck it – in other words, it acquires an image of the pattern of light that was shining on it. The advantage of europium-doped yttrium silicate is that the ions’ absorption frequencies are more narrowly defined than those of other materials. To record the holograms the NTT team used the reflection method, which is used to generate the holographic images found on credit cards, for example. In this process, a single laser beam is split into two: a reference beam that is shone directly onto the recording material and an object beam that is aimed at the scene to be recorded, before being recombined with the reference beam. Because the different parts of the object beam have travelled different distances, their light waves are out of phase with each other. The pattern of the interference fringes formed when the object and reference beams recombine therefore encodes the information about every point illuminated by the object beam. This allows an exact three-dimensional representation of the scene to be captured at the output point. A hologram of an object can be recovered by shining light through the recording medium or reflecting light off it. Because the images could make use of the whole of the crystal’s absorption spectrum, which ranges across several gigahertz, NTT’s material could, in theory, hold about 10 million still pictures, equivalent to 100 hours of TV broadcasts. NTT says it should also be feasible to develop an ultrafast recording system capable of taking a separate picture every nanosecond – the time that light takes to travel 30 centimetres. But, the team adds, ‘as long as the operating temperature is 7 K, applications will be limited, though we expect some in the recording of fast-moving objects.’ The team still has to overcome a number of problems, however. At the moment the holographic images on the recording material survive only for a matter of hours. Also,