Understanding the nature of light and its interactions with matter has
always been a challenge for Physics. New concepts have emerged from
these investigations, such as the quantum nature of the microscopic
world and the wave-particle duality. New mechanisms for the generation
of light have also been discovered, leading to the realization of new
light sources, called "lasers", with remarkable properties. The light
emitted or absorbed by atoms is not only a valuable source of
information on the structure of the world which surrounds us; it is also
a powerful tool for acting on atoms, for manipulating them, for
controlling their various degrees of freedom.
We will show how it is possible to use the basic conservation laws in
atom-photon interactions for polarizing atoms, for cooling them to very
low temperatures, in the microkelvin, and even in the nanokelvin range.
A review will be given of recent developments in this field, including
atomic clocks with atomic fountains, interference of atomic de Broglie
waves, the realization of new states of matter such as Bose-Einstein
condensates, matter waves and atom lasers. New perspectives opened by
these results will be briefly discussed.
The first excited state of helium atoms is the metastable 23S1 state
lying 20 eV above the ground state. So, laser excitation of helium atoms
from the ground state would be extremely difficult. It is however
possible to populate the long lived metastable state 23S1 in a discharge
and to manipulate atoms in this state by exciting them with a laser beam
driving the transition 23S1 --> 23PJ (J=3D0,1,2) at 1080 nm.
A few recent experiments using this excitation scheme will be described:
Bose-Einstein condensation of metastable 4He atoms, which is the only
case where atoms are condensed, not in the electronic ground state, but
in a highly excited electronic state; photo-association of metastable
helium atoms in a pure long range potential and formation of giant
helium dimers with atom-atom separations as large as 60 nm.