  Integrating an absorbing thin film into a resonant cavity is the most
practical way to achieve perfect absorption of light at a selected wavelength
in the mid-to-far infrared, as required to target blackbody radiation or
molecular fingerprints. The cavity is designed to resonate and enable perfect
absorption in the film at the chosen wavelength \lambda. However, in current
state-of-the-art designs, a still large absorbing film thickness (\lambda/50)
is needed and tuning the perfect absorption wavelength over a broad range
requires changing the cavity materials. Here, we introduce a new resonant
cavity concept to achieve perfect absorption of infrared light in much thinner
and thus really nanoscale films, with a broad wavelength tunability using a
single set of cavity materials. It requires a nanofilm with giant refractive
index and small extinction coefficient (found in emerging semi-metals,
semi-conductors and topological insulators) backed by a transparent spacer and
a metal mirror. The nanofilm acts both as absorber and multiple reflector for
the internal cavity waves, which after escaping follow a fractal phasor
trajectory. This enables a totally destructive optical interference for a
nanofilm thickness more than 2 orders of magnitude smaller than \lambda. With
this remarkable effect, we demonstrate angle-insensitive perfect absorption in
sub - \lambda/100 bismuth nanofilms, at a wavelength tunable from 3 to 20 \mu
m.
