  Light plays the central role in many applications. The key to unlocking its
versatility lies in shaping it into the most appropriate form for the task at
hand. Specifically tailored refractive index modifications, directly
manufactured inside glass using a short pulsed laser, enable an almost
arbitrary control of the light flow. However, the stringent requirements for
quantitative knowledge of these modifications, as well as for fabrication
precision, have so far prevented the fabrication of light-efficient aperiodic
photonic volume elements (APVEs).
  Here we present a powerful approach to the design and manufacturing of
light-efficient APVEs. We optimize application-specific 3D arrangements of
hundred thousands of microscopic voxels and manufacture them using femtosecond
direct laser writing inside millimeter-sized glass volumes. We experimentally
achieve unprecedented diffraction efficiencies up to 80%, which is enabled by
precise voxel characterization and adaptive optics during fabrication.
  We demonstrate APVEs with various functionalities, including a spatial mode
converter and combined intensity shaping and wavelength-multiplexing. Our
elements can be freely designed and are efficient, compact and robust. Our
approach is not limited to borosilicate glass, but is potentially extendable to
other substrates, including birefringent and nonlinear materials, giving a
preview of even broader functionalities including polarization modulation and
dynamic elements.
