  Solar sails with the capability of generating a tangential radiation pressure
at the sun-pointing attitude, such as refractive sails can provide more
efficient methods for attitude and orbital control of sailcraft. This paper
presents the concept of gradient-index sail as an advanced class of refractive
sail, which operates by guiding the solar radiation through a structure made of
graded refractive index material. The design of the sail's refractive index
distribution is performed by transformation optics, and the resultant index
realized by the effective refractive index of non-resonant bulk metamaterials
made of silica. The performance of the sail was evaluated by using ray tracing
for a broad spectrum of solar radiation under the normal incidence angle, which
showed an efficiency of 90.5% for generation of a tangential radiation
pressure. We also studied the orbital control of the
tangential-radiation-pressure-generating sails, and showed that the full
orbital control, including the modification of orbital axes, eccentricity, and
inclination can be applied by changing the attitude of the sail merely around
the sun-sail axis, while the sail keeps the sun-pointing attitude at every
point of the orbit.
