Laser cooling of positronium (Ps) is an important technique to advance the frontier of science by, for instance, realizing the first Bose-Einstein condensation of antimatters. It was proposed that performing laser cooling on Ps trapped in a cryogenic porous material would be more efficient, but a recent study (Cooper, et al., Phys. Rev. B 97, 205302 (2018)) reported that Ps in the excited (2P) state inside nano pores of silica had a large decay rate into γ-rays, and the resonant spectrum of the transition was significantly broadened, both of which are critically problematic for the proposed cooling scheme. A possible cause of these unexplained results was proposed to be residual impurities which trapped Ps in the vicinity of the pore surface and then enhanced interactions between Ps and the material. This work reports fabrication and inspection of a new porous silica cavity without such impurities, and construction of an experimental system to induce the Lyman-α transition of Ps trapped in pores of the cavity.

Airborne ultrasound is a promising technology in food and agriculture industries. However, its practical application is limited by its high energy attenuation and short propagating distance. In this work, we introduced a new system called Air-DPLUS (airborne double-parabolic-reflector wave-guided ultrasonic transducer), consisting of a focusing mechanism combined with a cylindrical airborne transducer. We evaluated the output pressure and its axial distribution in simulations and experiments. Under transient and continuous excitation, our prototype Air-DPLUS amplified the maximum pressure 6.9 and 24.4 fold, respectively, in comparison with the use of an airborne transducer alone.

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