  We report on recent advances in the operation of bubble-assisted Liquid Hole
Multipliers (LHM). By confining a vapor bubble under or adjacent to a
perforated electrode immersed in liquid xenon, we could record both
radiation-induced ionization electrons and primary scintillation photons in the
noble liquid. Four types of LHM electrodes were investigated: a THGEM, standard
double-conical GEM, 50 $\mu$m-thick single-conical GEM (SC-GEM) and 125
$\mu$m-thick SC-GEM - all coated with CsI photocathodes. The 125 $\mu$m-thick
SC-GEM provided the highest electroluminescence (EL) yields, up to ~400 photons
per electron over 4$\pi$ with an RMS pulse-height resolution reaching 5.5% for
events comprising ~7000 primary electrons. Applying a high transfer field
across the bubble, the EL yield was further increased by a factor of ~5. The
feasibility of a vertical-mode LHM, with the bubble confined between two
vertical electrodes, and the operation of a two-stage LHM configuration were
demonstrated for the first time. We combine electrostatic simulations with
observed signals to draw conclusions regarding the location of the liquid-gas
interface and suggest an explanation for the observed differences in EL yield
between the investigated electrodes.
