Page 350 - Electrical Properties of Materials
P. 350

332                           Lasers

                                   be achieved by a second excitation in which they become ionized and can be
                                   collected by an electric field.
                                     A disadvantage of the process is that, once perfected, it will enable do-it-
                                   yourself enthusiasts (with possibly a sprinkling of terrorists among them) to
                                   make their own atomic bombs.


                                   12.13.18  Holography
                                   I would like to mention holography, a method of image reconstruction inven-
                                   ted by Dennis Gabor in 1948 (Nobel Prize, 1973). It is difficult to estimate
                                   at this stage how important it will eventually turn out to be. It may remain
                                   for ever a scientific curiosity with some limited applications in the testing of
                                   materials. On the other hand it might really take off and might have as much
                                   influence on life in the twenty-first century, as the nineteenth century invention
                                   of photography has upon our lives. The technique is by no means limited to
                                   the optical region; it could in principle be used at any frequency in the elec-
                                   tromagnetic spectrum, and indeed, holography can be produced by all kinds
                                   of waves including acoustic and electron waves. Nevertheless, holography
                                   and lasers became strongly related to each other, mainly because holographic
                                   image reconstruction can most easily be done with lasers at optical frequencies.
                                     The basic set-up is shown schematically in Fig. 12.27. The laser beam is
                                   split into two, and the object is illuminated by one of the beams. The so-called
                                   ‘hologram’ is obtained by letting the light scattered from the object interfere
                                   with the other beam. The pattern that appears depends both on the phase and




                                              Laser


                                                                                  Mirror
                                               Semi-transparent        Lens
                                               mirror


                                                                                  Hologram
                                                      Object















     Fig. 12.27
                                                  Lens
     Schematic representation of taking a
     hologram.                              Mirror
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