Quantum Properties of Light

Quantum processes dominate the fields of atomic and molecular physics. The treatment here is limited to a review of the characteristics of absorption, emission, and stimulated emission which are essential to an understanding of lasers and their applications.

Atomic transitions which emit or absorb visible light are generally electronic transitions, which can be pictured in terms of electron jumps between quantized atomic energy levels.

Note that the frequency that is emitted when an electron makes the downward transition is the same as the frequency absorbed by this two-level system. This can be generalized to the multiple energy levels of atoms. The emission spectra of atoms are the series of frequencies emitted by those atoms in gaseous form. If these same gases were cool, the same series of frequencies would be selectively absorbed.

Lasers

The stimulated emission of light is the crucial quantum process necessary for the operation of a laser.

Population Inversion

The achievement of a significant population inversion in atomic or molecular energy states is a precondition for laser action. Electrons will normally reside in the lowest available energy state. They can be elevated to excited states by absorption, but no significant collection of electrons can be accumulated by absorption alone since both spontaneous emission and stimulated emission will bring them back down.

A population inversion cannot be achieved with just two levels because the probabability for absorption and for spontaneous emission is exactly the same, as shown by Einstein and expressed in the Einstein A and B coefficients. The lifetime of a typical excited state is about 10^-8 seconds, so in practical terms, the electrons drop back down by photon emission about as fast as you can pump them up to the upper level. The case of the helium-neon laser illustrates one of the ways of achieving the necessary population inversion.

Coherent Light

Ordinary light is not coherent because it comes from independent atoms which emit on time scales of about 10^-8 seconds. There is a degree of coherence in sources like the mercury green line and some other useful spectral sources, but their coherence does not approach that of a laser.

Parallel Light from a Laser

The light from a typical laser emerges in an extremely thin beam with very little divergence. Another way of saying this is that the beam is highly "collimated". An ordinary laboratory helium-neon laser can be swept around the room and the red spot on the back wall seems about the same size at that on a nearby wall.

The high degree of collimation arises from the fact that the cavity of the laser has very nearly parallel front and back mirrors which constrain the final laser beam to a path which is perpendicular to those mirrors. The back mirror is made almost perfectly reflecting while the front mirror is about 99% reflecting, letting out about 1% of the beam. This 1% is the output beam which you see. But the light has passed back and forth between the mirrors many times in order to gain intensity by the stimulated emission of more photons at the same wavelength. If the light is the slightest bit off axis, it will be lost from the beam.

The highly collimated nature of the laser beam contributes both to its danger and to its usefulness. You should never look directly into a laser beam, because the highly parallel beams can focus to an almost microscopic dot on the retina of your eye, causing almost instant damage to the retina. On the other hand, this capacity for sharp focusing contributes to the both the medical applications and the industrial applications of the laser. In medicine it is used as a sharp scalpel and in industry as a fast, powerful and computer-controllable cutting tool.

Monochromatic Laser Light

The light from a laser typically comes from one atomic transition with a single precise wavelength. So the laser light has a single spectral color and is almost the purest monochromatic light available.

That being said, however, the laser light is not exactly monochromatic. The spectral emission line from which it originates does have a finite width, if only from the Doppler effect of the moving atoms or molecules from which it comes. Since the wavelength of the light is extremely small compared to the size of the laser cavities used, then within that tiny spectral bandwidth of the emission lines are many resonant modes of the laser cavity.

Characteristics of Laser Light

1. Coherent. Different parts of the laser beam are related to each other in phase. These phase relationships are maintained over long enough time so that interference effects may be seen or recorded photographically. This coherence property is what makes holograms possible.

2. Monochromatic. Laser light consists of essentially one wavelength, having its origin in stimulated emission from one set of atomic energy levels.

3. Collimated. Because of bouncing back between mirrored ends of a laser cavity, those paths which sustain amplification must pass between the mirrors many times and be very nearly perpendicular to the mirrors. As a result, laser beams are very narrow and do not spread very much.