摘要：

An inventive new method of peering inside the body with high resolution illustrates the role that liquid crystalline components will play in the next generation of optical technologies [ 1 1. Myers A. A high-resolution endoscope as thin as a human hair. Phys Org [Internet]. 2013 Mar 13; [cited 2013 Apr]. Available from: http://phys.org/news/2013-03-high-resolution-endoscope-thin-human-hair.htmlView all references, 2 2. Mahalati RN, Gu RY, Kahn JM. Resolution limits for imaging through multi-mode fiber. Opt Exp. 2013;21:1656.[CrossRef], [PubMed], [Web of Science ®]View all references]. An endoscope, a device which can be inserted into the body and which transmits an image of its surroundings, is generally a bundle of thousands of single-mode optical fibres, each one of which produces a single image pixel. There have been efforts recently to make an endoscope which instead uses a single multi-mode fibre tens of microns in diameter. The difficulty is that the waveguide modes depend very sensitively on the precise characteristics of the fibre and cannot be predicted, and since any received light will excite multiple modes with different dispersion relations, interpreting the signal is difficult. Commonly, a laser shined through a liquid crystalline spatial light modulator (SLM) is used to excite various combinations of modes, with the image reconstructed from the different optical signals received back. The usual method is to use the feedback from some initial test signals to work out how to change the pattern on the SLM to produce a spot which can be scanned around the object to be imaged [ 3 3. Bianchi S, Di Leonardo R. A multi-mode fiber probe for holographic micromanipulation and microscopy. Lab Chip. 2012;12:635.[CrossRef], [PubMed], [Web of Science ®]View all references]. The maximum number of information-bearing pixels is then the number of waveguide modes. A team at Stanford University, however, has devised a different method which, surprisingly, can produce four pixels for every waveguide mode. Rather than scanning across the object and looking at signals from each point separately, its method solves a linear optimisation problem to find the image that best fits all the output signals given all the input signals, which do not now have to produce a localised spot (they simply produce a series of random patterns on their SLM). The additional resolution stems from making better use of the phase information from the sample. The team has managed to produce images with a resolution of 3 μm, similar to the best existing endoscopes, with the fibre a fraction of the diameter. The future goal is to make a flexible version, a difficult undertaking for a multi-mode fibre. The moving hologram, which has long been a mainstay of science fiction, is an ideal which the 3D displays currently being developed fall short of. They are essentially stereoscopic rather than holographic, using clever optical tricks to send two different images to the eyes rather than actually producing a three-dimensional image. However, researchers at the Tokyo University of Science have demonstrated a device using a doped ferroelectric liquid crystal that could hope to change this [ 4 4. Sasaki T, Ikegami M, Abe T, Miyazaki D, Kajikawa S, Naka Y. Real-time dynamic hologram in photorefractive ferroelectric liquid crystal with two-beam coupling gain coefficient of over 800 cm−1 and response time of 8 ms. Appl Phys Lett. 2013;102:063306.[CrossRef], [Web of Science ®]View all references]. A hologram can be formed by recording the interference pattern between a laser exposed to the object to be imaged and a reference laser. When another laser is shone onto the pattern, it acts as a diffraction grating and reproduces the light field around the object. In this case, the pattern was written onto the liquid crystal layer using the photorefractive effect, a change in refractive index induced by incident light. The liquid crystalline mixture consists of three compounds which form smectic phases, a photoconductive molecule, which loses an electron when expo

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