Interaction with External Fields
191
resonance cavity where the F = 1 atoms undergo stimulated transitions to the
F = 0 state. This maser is remarkable for the stability of the frequency of its
radiation, v H = 1.420 405 7518 × 10
9
s
–1
and can be used as a time standard with
nearly the same accuracy as the caesium clock.
Dye lasers: A major constraint in the lasers discussed so far is that the
frequency of the outcoming radiation is essentially fixed, though a small variation
can be achieved by varying temperature, etc. This constraint was removed by
the development of dye lasers which use solutions of organic dyes as the active
medium for stimulated emission, e.g., rhodamine 6 G in methanol solution.
The output wavelength of these lasers is continuously tunable over a large range
of frequencies, which makes them very versatile. The main features of a dye
laser are described here in terms of a schematic representation of the energy
levels in Fig. 6.3(c).
The large degrees of freedom in an organic dye molecule give rise to
relatively broad energy bands with closely spaced vibrational and rotational
levels. At ordinary temperatures, most of the molecules occupy energy levels
close to the ground state level S 0 in the lowest energy band S characterized by
the property that the molecule is in a singlet state, i.e., total spin S = 0. If a
solution of the dye is exposed to an intense radiation from a laser, usually a
nitrogen laser, or a flash lamp, the molecules undergo transition to one of the
excited singlet states S*. Nonradiative transitions quickly bring them down (in
about 10
–11
to 10
–12
s) to the bottom of the S* band, i.e., S 0
*
. Since there are very
few molecules in the upper part of S, population inversion is obtained between
the lower part of the energy band S* and the upper part of the S band. This gives
rise to stimulated transitions to almost any part of the S band. These transitions
can be tuned to any frequency within this range by using a suitable diffraction
grating and a partially reflecting mirror, placed on the opposite sides of the
active medium.
Alternatively, in some cases the molecules may undergo nonradiative
transitions from S* to the metastable triplet states T* with total spin S = 1. Then
stimulated transition can occur between the lowest triplet state T 0 *, and S states.
Since T 0 * is metastable, usually the transitions from T 0 * states to the S states
take place after a time delay. For nitrogen-laser-pumped dye lasers, it is the
transitions from the S 0 * to the S band that produce the dominant laser action.
Over the years, a large number of materials that can produce laser action
have been developed. A special mention should be made of semiconductor lasers
which are sturdy, compact and inexpensive, and therefore suitable for practical
applications. It is reasonable to expect that many more laser materials will be
developed in the coming years.
Resonance cavity: The laser material is usually kept between mirrors (plane
or concave) so that the photons are reflected back and forth many times to build
191
resonance cavity where the F = 1 atoms undergo stimulated transitions to the
F = 0 state. This maser is remarkable for the stability of the frequency of its
radiation, v H = 1.420 405 7518 × 10
9
s
–1
and can be used as a time standard with
nearly the same accuracy as the caesium clock.
Dye lasers: A major constraint in the lasers discussed so far is that the
frequency of the outcoming radiation is essentially fixed, though a small variation
can be achieved by varying temperature, etc. This constraint was removed by
the development of dye lasers which use solutions of organic dyes as the active
medium for stimulated emission, e.g., rhodamine 6 G in methanol solution.
The output wavelength of these lasers is continuously tunable over a large range
of frequencies, which makes them very versatile. The main features of a dye
laser are described here in terms of a schematic representation of the energy
levels in Fig. 6.3(c).
The large degrees of freedom in an organic dye molecule give rise to
relatively broad energy bands with closely spaced vibrational and rotational
levels. At ordinary temperatures, most of the molecules occupy energy levels
close to the ground state level S 0 in the lowest energy band S characterized by
the property that the molecule is in a singlet state, i.e., total spin S = 0. If a
solution of the dye is exposed to an intense radiation from a laser, usually a
nitrogen laser, or a flash lamp, the molecules undergo transition to one of the
excited singlet states S*. Nonradiative transitions quickly bring them down (in
about 10
–11
to 10
–12
s) to the bottom of the S* band, i.e., S 0
*
. Since there are very
few molecules in the upper part of S, population inversion is obtained between
the lower part of the energy band S* and the upper part of the S band. This gives
rise to stimulated transitions to almost any part of the S band. These transitions
can be tuned to any frequency within this range by using a suitable diffraction
grating and a partially reflecting mirror, placed on the opposite sides of the
active medium.
Alternatively, in some cases the molecules may undergo nonradiative
transitions from S* to the metastable triplet states T* with total spin S = 1. Then
stimulated transition can occur between the lowest triplet state T 0 *, and S states.
Since T 0 * is metastable, usually the transitions from T 0 * states to the S states
take place after a time delay. For nitrogen-laser-pumped dye lasers, it is the
transitions from the S 0 * to the S band that produce the dominant laser action.
Over the years, a large number of materials that can produce laser action
have been developed. A special mention should be made of semiconductor lasers
which are sturdy, compact and inexpensive, and therefore suitable for practical
applications. It is reasonable to expect that many more laser materials will be
developed in the coming years.
Resonance cavity: The laser material is usually kept between mirrors (plane
or concave) so that the photons are reflected back and forth many times to build