  Silicon photomultipliers (SiPMs) have a few advantages over conventional
photomultiplier tubes (PMTs) used in imaging atmospheric Cherenkov telescopes.
The first notable characteristic is their higher photon detection efficiency
(PDE) of up to about 60%, which is roughly 1.2-1.5 times better than that of
PMTs in the 300-450 nm range, enabling us to lower the energy threshold of
gamma-ray observations and increase the photon statistics. The second advantage
is that SiPMs are chemically stable after exposure to long and bright
illumination, while PMTs can cause gain and quantum efficiency degradation
after the same exposure. Therefore, the use of SiPMs under bright or full moon
conditions may extend the total observation time in the highest energy coverage
region of individual telescopes. However, the SiPM PDE is too high in
wavelengths longer than 500 nm; hence, the signal-to-noise ratio (S/N) of the
Cherenkov signal over the night-sky background (NSB) is not necessarily superb.
This is because the Cherenkov signal is dominant over the wavelength of 300-500
nm, while the NSB is brighter in the region of 550 nm or longer. To improve the
S/N with minimal and cost-effective additional hardware, we have developed
multilayer coating designs with only 8 layers and applied them to the specular
surfaces of light concentrators. The layers were designed to reflect more
photons in the 300-500 nm range but fewer in 550-800 nm. Using a prototype
light concentrator fabricated with the novel multilayer design, we demonstrated
that a SiPM array exhibits ~50% better photon collection efficiency at 403 nm
than that obtained with PMTs, agreeing with the result of a ray-tracing
simulation. The efficiency measured at 830 nm was also successfully reduced by
30-50%.
