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 of radiators.

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.