  We experimentally demonstrate ultrathin flat lenses with a thickness of 7
{\AA}, which corresponds to the fundamental physical limit of the thickness of
the material, is fabricated in a large area, monolayer, CVD-prepared tungsten
chalcogenides single crystals using the low-cost flexible laser writing method.
The lenses apply the ultra-high refractive index to introduce abrupt amplitude
modulation of the incident light to achieve three-dimensional (3D) focusing
diffraction-limited resolution (0.5{\lambda}) and a focusing efficiency as high
as 31%. An analytical physical model based diffraction theory is derived to
simulate the focusing process, which shows excellent agreement with the
experimental results.
