“A splendid light has dawned on me… -Albert Einstein
A Short History of Lasers
As the 20th century began, physicists were analyzing the fundamental nature constituting the substance and interaction of matter and energy. Foremost in these studies was the scientific evolution in describing the essential properties underlying the physics of light energy.
Around 1900, German theoretical physicist, Max Planck – investigating blackbody radiation (a theoretical object that absorbs 100% of incident radiation, reflecting nothing and therefore appears black) – proposed that atoms absorb and emit radiation in discrete quantities he termed “quanta”. This emerging field study of quantum physics was led by the preeminent theoretical physicist Albert Einstein.
Using a derivation on Planck’s quantization of energy principle, Einstein’s 1917 paper Zur Quantentheorie der Strahlung (On the Quantum Theory of Radiation) described the theory of ‘stimulated emission’. This concept established the underlying principles for the physical properties of the laser. It formulated that in addition to absorbing and emitting light spontaneously, electrons could be stimulated to emit light of a particular wavelength. Thus Light Amplification by Stimulated Emission of Radiation took theoretical birth.
The laser itself took another 40 years for its creation.
Interestingly, the Nobel Committee did not confer upon Einstein their 1921 Physics prize for either of his more famous theories relating to General or Special Relativity – rather “…for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect”.
How then did Einstein’s theoretical concept of LASER become today’s laser?
Laser light emission owes its properties in measure to the ways in which light energy interacts with electrons. Electrons exist at discrete energy levels characteristic of a particular atom or molecule. These levels could be conceived as orbits around a central nucleus. Outer orbit ring electrons exhibit higher energy levels than inner rings. Also, electrons can be elevated to higher energy levels by injecting them with energy – for instance, a flash of light.
As electrons pass from an outer to an inner level (excited state to ground state), excess energy is produced as light. This emitted wavelength of light is directly related to the amount of energy released. Depending upon the specific lasing material (Ruby, Nd:YAG, GaAlAs, CO2, etc.), specific wavelengths of light are absorbed (thus re-energizing/exciting those electrons) and subsequently are emitted (as those electrons return to their initial state) as laser light.
Putting these theoretical concepts to practical application, here’s a simplified explanation of how on May 16th, 1960 at Hughes Research Laboratories (Los Angeles) Theodore Maiman’s first ruby laser functioned:
A ruby crystal is formed into a cylinder. A fully reflecting mirror is created at one end along with a partially reflecting mirror at the other end of the cylinder. A high-intensity lamp forms a spiral around the ruby rod and its light excites the stimulated emission of lasing activity. The green and blue wavelengths in the light flash excite electrons in the ruby rod’s chromium to a higher energy valence. Upon returning to their initial state, the electrons emit their characteristic ruby red (694.3nm) light. The mirrors reflect some of this stimulated (laser) light back and forth within the ruby crystal. Accordingly, other excited chromium atoms produce more red light – until achieving sufficient intensity. This ‘gain’ emits the energy stored in the crystal as ruby laser light out of the partially reflecting mirror end.
Recently celebrating its 50 year anniversary, the ruby laser now coexists with a vast and varied assortment of other lasers. Developmental laser technology continues to create new laser systems and to further expand their usage into a wide diversity of disciplines and applications.