PHONON AMPLIFICATION USING EVAPORATION AND ADSORPTION OF HELIUM

We report the results of,experiments designed to investigate the feasi bility of amplifying a phonon signal using the evaporation of helium f rom a superfluid film and its subsequent readsorption onto a helium-fr ee surface. We envision a multistage amplifier in which helium is evap orated from a wafer with a helium film only on one side add then adsor bed onto the film-free surface of a similar wafer. The phonons created by the adsorption reach the film on the opposite side of the wafer an d potentially desorb more helium than was evaporated by the first wafe r. The amplification would come from the high ratio of the binding ene rgy of a helium atom to a film-free surface relative to the binding en ergy to the liquid. A number of experiments are reported that investig ate the efficiencies of the individual steps of the process. The gain per stage is found to be about 3 for high-energy densities in which mu ltiphonon processes are possible. At low-energy densities, the energy deposited into a film-free wafer is found to be less than the original input energy, with the ratio of output to input energy 0.2. Since in applications requiring amplification the phonon density produced by th e adsorption of helium on a wafer will be low, the configuration we ha ve,studied-phonons produced in silicon coated with a saturated He-4 fi lm-will nor result in amplification. However, other configurations mig ht improve the efficiency enough to make an amplifier possible.