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Avalanche Photodiodes: A Status Report
John Swain, Northeasten
University, Boston, MA, USA
E-mail: John.Swain@cern.ch
Avalanche photodiodes (APD's) are essentially photodiodes designed in such a way as to have internal gain. A region with a large electric field allows electrons knocked loose by incoming photons to gain enough energy to release further electrons, giving rise to more than one electron in response to one photon. The quantum efficiencies of such devices are intrinsically very high, approaching 100% and far exceeding anything possible with a traditional photomultiplier. In addition the devices are robust, can be made small or pixellated, and require only modest voltages, well under 1kV. With a flat spectral response across the visible range, such devices are extremely attractive for biophotonics research but for one significant problem: the dark current (noise) is rather high at room temperature. Over the last year some progress has been made in cooling APD's with an attendant dramatic drop in dark current. I summarize the status of this work and indicate expectations for the near future, emphasizing that the main problems are only of a technical nature and should all be solvable.
Lasing in Random Media
Following pioneering theoretical work by Letokhov in 1968, recent experiments have demonstrated that lasing can take place in random media with the feedback provided by multiple scattering rather than the usual Fabry-Perot cavity using mirrors. Such lasing has several differences from that of conventional lasers, and suggests possible tests for coherence of biophotons. I review the deviations from Poisson statistics in such lasers, as well as the phenomena of spiking, gain narrowing, and coherent backscattering, all of which may be observable in living cells. In addition, I look at the issue of localization of photons in strongly scattering media with sufficiently high gain. Tests for spatial coherence are also discussed.
Life, Computability and Quantum Mechanics
In 1961, Eugene Wigner presented a clever argument that in a world which is adequately described by quantum mechanics, self-reproducing systems in general, and perhaps life in particular, would be incredibly improbable. The problem and some attempts at its solution are examined, and a new solution is presented based on computability theory. In particular, it is shown that computability theory provides limits on what can be known about a system in addition to those which arise from quantum mechanics and that these limits are significant when we come to try to define life. It is suggested that no single definition of life will ever be really adequate. Parallels are drawn to the way that geometry has been generalized from that of Euclid and the way in which Goedel's theorem has changed the way we think about mathematics.
