  The bubble-assisted Liquid Hole Multiplier (LHM) is a recently-proposed
concept for the combined detection of ionization electrons and primary
scintillation photons in noble-liquid time projection chambers. The LHM
comprises a perforated micro-pattern electrode (e.g. Thick Gas Electron
Multiplier - THGEM, or Gas Electron Multiplier - GEM) immersed in the liquid,
with a bubble of the noble gas supported underneath. Ionization electrons and
scintillation-induced photoelectrons extracted from a cesium iodide
photocathode drift through the electrode's holes and induce electroluminescence
(EL) signals in the bubble; these are recorded by photon detectors located
closely below the electrode. We present recent results in the development of
LHMs, comparing the response of different electrodes to ionization and
photon-induced electrons.
