  We show that the powerful framework of transformation optics may be exploited
for engineering the nonlocal response of artificial electromagnetic materials.
Relying on the form-invariant properties of coordinate-transformed Maxwell's
equations in the spectral domain, we derive the general constitutive
"blueprints" of transformation media yielding prescribed nonlocal
field-manipulation effects, and provide a physically-incisive and powerful
geometrical interpretation in terms of deformation of the equi-frequency
contours. In order to illustrate the potentials of our approach, we present an
example of application to a wave-splitting refraction scenario, which may be
implemented via a simple class of artificial materials. Our results provide a
systematic and versatile framework which may open intriguing venues in
dispersion engineering of artificial materials.
