The Era of Modern Spectroscopy
The era of modem spectroscopy began with the invention of the laser,
which provides intense, collimated monochromatic radiation throughout
optical spectral range. Historically, the laser was an extension
of the maser, a microwave oscillator developed by N.G. Basov and
A.M. Prokhorov in the USSR and C.H. Townes in the US (1954). In
a brilliant intellectual breakthrough, in 1958 Townes and A.L. Schawlow
proposed ex- tending the maser principle into the optical regime.
They pointed out that an interferometer of the type developed by
Fabry and Perot in 1900 would also function as an optical resonator.
When atoms or molecules in a state of inversion (more atoms in the
upper level of the atomic transition than in the lower one) are
placed within the resonator, the emission should dramatically increase
in intensity and decrease in line width. They called their device
an optical maser, but the term laser (for light amplification using
stimulated emission of radiation) soon caught on instead. The first
working prototypes were built soon after. The ruby laser, a pulsed
solid state laser emitting red light at a wavelength of 0.694 µm,
was developed by T.H. Maiman in 1960, and the He-Ne laser, a continuous-wave
gas laser emitting infrared light at 1.15 µm, was developed shortly
afterwards by A. Javan. There rapidly followed a host of new laser
sources: CO2 ( λ=9.4-10.6 µm), Nd:YAG (λ=1.064 µm),
argon and krypton ion lasers, which produce multiple visible wavelengths,
and many others. The dye laser, developed by P.P. Sorokin, F.P.
Schafer, B.B. Snavely and others in 1966, is of particular significance
because it provides monochromatic radiation that can be broadly
tuned over the visible spectral range.
Laser light, with its high intensity, narrow spectral linewidth
and phase coherence, immediately stimulated new interest in atomic
and molecular spectroscopy. Because the first lasers were fixed
in frequency, the earliest work was done on the laser medium itself,
and on spectral lines that happened to coincide with the frequency
of a laser source. Transitions under study were sometimes Stark-
or Zeeman-tuned into resonance. Tunable lasers, the dye laser in
particular, greatly extended the scope of possible measurements.
Laser light opened the field of ultra-high resolution spectroscopy.
Saturation spectroscopy, developed by Javan, Schawlow, W.E. Lamb,
Jr. and others, provided sub-Doppler resolution of spectral lines
of atomic and molecular vapors. Ordinarily, such lines are limited
in resolution by thermal atomic motion ("Doppler broadening").
Intense, monochromatic laser light can selectively saturate an optical
transition, producing extremely narrow Doppler- free resonances.
Techniques for producing two-photon Doppler-free resonances
were devised (V.P. Chebotayev). Atomic beams were also used to eliminate
Doppler broadening and produce narrow spectral lines. Figure 1 illustrates
the advances made over conventional spectroscopy measurements.
By studying the shapes of these narrow lines, collisional
processes could be studied. Further, by locking the laser frequency
to a narrow spectral line, its wavelength (or frequency) could be
accurately defined, making possible laser wave- length and frequency
standards, and eventually laser atomic clocks (J. Hall, K. Evenson).
Laser light also opened the possibility of conducting spectroscopy
in the time domain. Coherent transient processes such as free induction
decay and photon echoes, which had been observed earlier at radio
frequencies, now began to be studied in the optical domain (S.G.
Hartmann). In addition, the interaction of laser light with optically
thick samples, in which coherent re-radiation can significantly
modify the response of the sample, was studied for the first time.
Effects observed included self-induced transparency (E. Hahn), in
which a normally opaque sample becomes transparent to an intense
light pulse, and Dicke superradiance (M.S. Feld), in which atoms
are made to undergo emission proportional to the square of the number
Intense laser light has opened the field of nonlinear optics, the
formalism of which was developed by N. Bloembergen. Frequency mixing
processes such as second harmonic generation (P. Franken) were discovered
and exploited to generate coherent light at new wavelengths deep
in the ultra-violet and far infrared. Other nonlinear optical effects
studied include the stimulated Raman effect (R. Hellworth), four-wave
mixing and other stimulated processes induced by the high intensities
of laser light (large number of photons per mode). Multiphoton absorption
in atoms was used to ionize and detect trace quantities, and multiphoton
absorption in molecules was used to produce large quantities of
highly excited state species (V.S. Letokhov).
New results in laser spectroscopy continue to advance the field.
The development of atom traps by H. Dehmelt and W. Paul, combined
with laser slowing of atomic beams due to photon recoil, called
laser cooling, has led to techniques for highly localized confinement
of isolated atoms, as well as dense collections of atoms (S.Chu)
and, Bose-Einstein condensation (C. Weimann, E. Cornell
and W. Ketterle). These species, arrested in space, can then be
studied free of the usual external perturbations, providing spectroscopic
data of extremely high resolution for fundamental studies and applications.