  The energy absorption on nanometer scale is vital for many bio and chemical
systems. We report here that a two times amplification in absorption efficiency
can be achieved by water molecules confined in carbon nanotubes with small
radius, compared with situations in normal bulk water, under irradiations of
short mid-infrared pulses. The effect of confinement due to a (6,6) carbon
nanotube is found to be very robust, equivalent to that of a 5 $V/nm$ static
electric field. These findings are instructive not only for designing
high-efficiency nano devices but also for understanding behaviours in
biological channels.
