University of Liverpool
How a Laser Works:
The Basics of an Atom
Everything we see within the universe is made up of an infinitesimally large number
of combinations of the 100 different kinds of atoms. The arrangement and bonding of
these atoms determines what material/object they constitute.
Atoms are constantly in motion. They continuously vibrate and move. Although all
atoms are vibrating to a degree, atoms can be in a different state of excitation (i.e.
they can have different levels of energy). If a large degree of energy is applied to an
atom then it can leave what is referred to as ground-state energy level and go to an
excited level. The level of excitation is proportional to the amount of energy applied.
A simple atom as shown in Figure 1 consists of a nucleus, which consists of protons
and neutrons and what is often referred to as an electron cloud. For a simplistic
interpretation of the atom model it is easy to think of the electrons within the electron
cloud following discrete paths or orbits within the cloud. This analogy suits our
purpose as we can then consider these orbits to be the different energy levels that
make up the atom. If we add some form of energy to the atom we can assume that
electrons from the lower-energy orbitals will transfer to the higher-energy orbitals at a
greater distance from the nucleus, resulting in a higher level of excitation.
Figure 1 - Simple Atom Model
When atoms reach a higher-energy orbital the eventually seek to return to the groundstate energy level. Upon returning to ground-state energy level the excess energy is
released in the form of a photon - a particle of light.
The Connection Between Atoms and Lasers
Laser is an abbreviation for Light Amplification by Stimulated Emission of Radiation.
A laser is a device that controls the way in which energised atoms release protons.
There are many different types of laser available; all the different types of laser rely
on the same basic elements. In all types of laser there is a lasing medium, which is
pumped to get the electrons within the atoms to a higher-energy orbital i.e. to get the
atoms excited. Typically, very intense flashes of light or an electrical discharge pump
the lasing medium and create a large number of excited-state atoms. This creates a
high degree of population inversion (the number of excited state atoms versus the
number of atoms at ground-state energy level). At any stage the excited state atoms
can release some of the energy and return to a lower-energy orbital. The energy
released, which comes in the form of photons, has a very specific wavelength that is
dependant on the level of energy or excitation of the electron when the photon is
released. Two identical atoms with electrons in identical states will release photons
with identical wavelengths. This forms the basis for laser light.
Laser light has the following properties:
· Laser light is monochromatic. It contains one specific wavelength of light, which as
described earlier is determined by the amount of energy released when the electron
drops to a lower-energy orbital.
· Laser light is coherent. Each proton moves in step with the other (i.e. all protons
have wave fronts that move in unison).
· Laser light is highly directional (i.e. a laser beam is very tight and concentrated
To ensure the aforementioned properties are apparent within the laser light the process
briefly mentioned earlier, 'stimulated emission' must occur.
Any photon that has been released by an atom, (which therefore has a wavelength,
phase and energy level dependant on the difference between the excited atom state
and the ground-state energy level) should encounter another atom that has another
electron in the same excited state, stimulated emission can occur. The first photon can
stimulate or induce atomic emission so that the emitted photon vibrates with the same
frequency and direction.
To produce laser light it is necessary to have a pair of mirrors at either end of the
lasing medium. These mirrors are often known as an optical oscillator due to the
process of oscillating photons between the two mirrored surfaces. The mirror
positioned at one end of the optical oscillator is half-silvered, therefore it reflects
some light and lets some light through. The light that is allowed to pass through is the
light that is emitted from the laser. During this process photons are constantly
stimulating other electrons to make the downward energy jump, hence causing the
emission of more and more photons and an avalanche effect, leading to a large
number of photons being emitted of the same wavelength and phase.
Below is a graphical illustration of what has been detailed above. The graphics
illustrate how laser light is created using a ruby laser, the first fully functioning laser.
Theodore Maiman invented the ruby laser on May 16th 1960 at the Hughes Research
Laboratories (for more information on dates relating to laser invention see a Brief
History of Lasers).
Figure 2 - Schematic of Laser in Non-Lasing State
Figure 3 - Schematic Illustrating the Excitation of Atoms Using Light Source
Figure 4 - Schematic Showing Photon Emission
Figure 5 - Schematic Showing the Stimulated Emission of Further Photons
Figure 6 - Schematic Showing Column of Laser Light Leaving Optical Oscillator