  In this work we present how to fabricate large-area (15 cm2), ultra-low
threshold germanium bolometric photo-detectors and how to operate them to
detect few (optical) photons. These detectors work at temperatures as low as
few tens of mK and exploit the Neganov-Trofimov-Luke (NTL) effect. They are
operated as charge-to-heat transducers: the heat signal is linearly increased
by simply changing a voltage bias applied to special metal electrodes,
fabricated onto the germanium absorber, and read by a (NTD-Ge) thermal sensor.
We fabricated a batch of five prototypes and ran them in different facilities
with dilution refrigerators. We carefully studied how impinging spurious
infrared radiation impacts the detector performances, by shining infrared
photons via optical-fiber-guided LED signals, in a controlled manner, into the
bolometers. We hence demonstrated how the radiation-tightness of the test
environment tremendously enhances the detector performances, allowing to set
electrode voltage bias up to 90 volts without any leakage current and
signal-to-noise gain as large as a factor 12 (for visible photons). As
consequence, for the first time we could operate large-area
NTD-Ge-sensor-equipped NTL bolometric photo-detectors capable to reach sub
10-eV baseline noise (RMS). Such detectors open new frontiers for rare-event
search experiments based on low light yield Ge-NTD equipped scintillating
bolometers, such the CUPID neutrinoless double-beta decay experiment.
