Gradient Chemical Micropatterning Enables Fabrication of Reference Substrates for Calibration of Scanning Surface Nanome
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RESEARCH/RESEARCHERS
Photorefractive Polymer Device Records High-Resolution 3D Holographic Images Holograms hold a special place in the popular imagination, from the cover of National Geographic to episodes of Star Trek. But three-dimensional holography is fast becoming a reality for high-value applications, including biomedical imaging of living tissue. Devices for such 3D holographic imaging have so far been limited by either slow response times or low sensitivity. Now, in an article in the August 1 issue of Optics Letters (p. 1914), a group from the University of Manchester in the United Kingdom has introduced holographic imaging with a photorefractive polymer device that is both fast and has high 3D resolution, even through a scattering medium similar to organic tissue. A hologram is formed by creating (“writing”) a pattern of refractive-index contrast inside a material. When the material is later illuminated with a “read” laser, this pattern diffracts the coherent laser light and creates the hologram. A common technique for recording refractiveindex patterns is to use a photorefractive material in which incident light from the object being holographed liberates charge carriers in the material, which get trapped at points of low- or high-incident intensity. The electric field of these trapped charge carriers then modifies the local index of refraction through the electro-optic effect. The Manchester group—P. Dean, M.R. Dickinson, and D.P. West—used as their holographic medium a photorefractive polymer host poly(N-vinylcarbazole) and doped it with 50 wt% of the electrooptic chromophore 1-(2’-ethylhexyloxy)2,5-dimethyl-4–(4”-nitrophenylazo) benzene and 2 wt% of the sensitizer 2,4, 7-trinitro-9-fluorenone dimalenitrile to enhance the photorefractive effect. The holographic reference beam intensity was 161 mW/cm2, while the object beam intensity was 0.7 mW/cm2, and the reading beam intensity was 0.5 mW/cm2. With an exposure time of only a few seconds, they were able to record a high-resolution hologram of an object with transverse details as small as 42 μm and a depth resolution of 15 μm over an object area of 0.42 mm2. Then, to demonstrate the usefulness of this technique for biomedical imaging, the researchers placed a suspension of polystyrene microspheres between the holographed object and the recording laser, which simulated the optical scattering effects of an intervening layer of biological tissue. The optical depth of the microspheres corresponded to seven scattering mean free paths (round-trip). With this layer present, the required recording time 686
increased to 20 s, but the hologram resolution remained very high. What makes the group’s technique particularly noteworthy is that it operates at a near-infrared wavelength of 794 nm, which is only weakly absorbed by biological tissue. This, combined with the technique’s relatively fast time scale and high spatial resolution at low optical intensity, make it an especially promising advance toward the clinical goal of holographic biomedical imaging. COLIN M
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