Deformable mirrors were developed to enable arbitrary deformation of their shapes. This is achieved by attaching a large number of piezoelectric elements symmetrically on both sides of the mirror substrate.
A deformable mirror with 18 piezoelectric elements can be deformed into the shape of a sine curve. Up to 9 cycles and surface shape deformation tens of micrometers deep is possible.
In a conventional optical system with mirrors of fixed shape, it is impossible to change the focusing size without changing the arrangement of the mirror and sample position (focal position). Using a deformable mirror, the focal area can be changed freely under the diffraction limit without changing the experimental setup of the optical system.
Deformable mirrors are also used for wavefront compensation. In the focusing experiment by SPring-8, the world's smallest focus size (7 nm) was achieved using a wavefront compensation optical system with a deformable mirror for conventional mirror focusing of the optical system.