The Art of the Jedi Knight: the Chemistry of Laser

The laser action is notably used in science fiction movie such as the lightsabers in Star Wars movies. However, laser itself also has been used in field of science since for a long time such as spectroscopy. This time, we will see the basic principles of the chemistry behind laser and also some real examples of the application of laser. 


Laser is an acronym of light amplification by stimulated emission of radiation and from this acronym there are two main keywords, light amplification and stimulated emission.

When a molecule absorbs photons, it will excite to higher energy level and there will be 2 possible ways of the molecule returning to its ground state. The first possibility is the molecule directly or spontaneously returns to its ground state by emitting a photon, so this process is called by spontaneous emission. The other possibility is the molecule returns to its ground state with the aid of external electromagnetic wave; in the other words the transition is stimulated so this transition is called stimulated transitions. The photon required for this transitions has to have the same frequency (or energy) with the emission frequency. This implies that the more photons that are present, the greater possibility of the emission and this essential for laser action. The emitted photons then trigger another stimulated emission within the confined laser tube which also has several mirrors, one of the mirrors is partially reflective. This mirror will keep the photons within the tube and also amplified the emission (light).

One of the requirement for laser action is the existence of metastable excited state, an excited state with a long enough lifetime for it to participate in stimulated emission. This metastable state can be achieved  by exciting a molecule into the excited state where the molecule cannot return to the ground state due to forbidden transition. For example, the electron of He in ¹S₀ level can be excited to ³S₁ level. However, the electron cannot return to the ground state level because the transition is forbidden due to change of spin state. The colour of laser corresponds to the energy gap of the transition with smaller energy gap will lead to red-shift emission.

Besides that, another requirement for laser action is the existence of greater population in the excited state than the ground state so there will be a net of emission radiation. However, in the thermal equilibrium, the opposite is true, so it is necessary to achieve population inversion for laser action.

Three-level laser (left) and four-level laser (right)
(courtesy: Atkin's Physical Chemistry)

There are 2 possible way to achieve this population inversion. The first step is the molecule to intermediate state I, then releasing energy so the molecule will fall to lower level, let say A. Then, the laser action is the transition between A to the ground state. Overall, there are 3 different energy levels involve, so this is called three-level laser. Another possibility is the laser action happens when the transition between state A to unpopulated state A' other which is slightly higher than ground state. This method uses four different energy levels, so this is called four-level laser. Because of the unpopulated state of A', it is easier to achieve population inversion than three-level laser. Furthermore, this transition can be achieved when the transitions from A' to X are rapid.

Anatomy of the HeNe laser

One of the example of four-level laser is in HeNe laser where the active medium is the mixture of He and Ne gas in a mole ratio of 5:1. The initial step is the excitation of He atom to the metastable state by electrical discharge. The excitation energy happens to match an excitation energy of Ne and during He-Ne collision efficient transfer energy may occur, leading to the production of highly excited metastable Ne atoms with unpopulated intermediate states. The laser action generates radiation at 633 nm.

HeNe laser action

Another gas lasers such as Ar ion and Kr ion lasers work under similar principle of four-level laser.

This laser action can also be used to for the alternative method calculating the concentration of a sample using UV-Visible spectroscopy which employs Beer-Lambert Law. However, this law has a limitation when the sample has very small concentration, so an alternative method is required. Hence, to overcome this problem, indirect technique is required. There are two indirect technique that can be used, which are Laser-Induced Fluorescence (LIF) and Resonance-Enhanced Multiple-Photon Ionisation (REMPI).

Laser-Induced Fluorescence

Both methods use laser to irradiate the molecule to cause excitation from S₀ to S₁, but rather than measuring the absorbance, they measure another parameter. When, the molecule is excited, it will return to ground state emitting fluorescence radiation. Therefore, in LIF method, the detector is placed not at the laser pathway to detect the weak response from the fluorescence radiation. Meanwhile in REMPI, the laser causes ionisation of the molecule and the ions formed are measured, ion measurement can be done by coupling the machine with mass spectrometry. Both methods does not depend on the Beer-Lambert Law.

Another form of laser which is also important for science is called maser. Maser produces a radiation at microwave region rather than visible light such as in laser. The maser was originally observed in 1965 from the space and it was thought to be a new element found in space. However, it turns out to be the emission of metastable OH which formed from the photodissociation of water molecules by star light in interstellar molecular clouds in space. The transition of this OH molecule corresponds to microwave radiation (around 1700 MHz). Another known form of astrophysical maser is formaldehyde.

Summary of laser action
(courtesy: HowStuffWorks)

To sum up "the art of the jedi knights", laser action happens by stimulated emission which requires the presence of metastable state and population inversion. In the thermal equilibrium, the opposite of population inversion is true, so it is necessary to achieve this inversion and it can be done by three-level or four-level laser (such as HeNe laser). This laser can be used for the alternative of Beer-Lambert Law using LIF or REMPI method. When the stimulated emission happens in the microwave region, it is called maser which is known for the emission of OH particle in interstellar cloud in space. 

Hopefully, when you think or see laser, you wouldn't think as a mere pointer in a presentation or props from Star Wars movies but you can see it as related to you and your world.