
Boltzmann distributions and thermal equilibrium
To understand the concept of a population inversion, it is necessary to understand some thermodynamics and the way that light interacts with matter. To do so, it is useful to consider a very simple assembly of atoms forming a laser medium[?].
Assume there are a group of N atoms, each of which is capable of being in one of two energy states, either:
The number of these atoms which are in the ground state is given by N_{1}, and the number in the excited state N_{2}. Since there are N atoms in total, N_{1} + N_{2} = N .
The energy difference between the two states, ΔE = E_{2}E_{1} determines the characteristic frequency ν_{21} of light which will interact with the atoms; This is given by the relation:
h being Planck's constant[?].
If the group of atoms are in thermal equilibrium[?], it can be shown from thermodynamics that the ratio of the number of atoms in each state is given by a Boltzmann distribution:
where T is the temperature of the group of atoms, and k is Boltzmann's constant.
We may calculate the ratio of the populations of the two states at room temperature (T≈300 K) for an energy difference ΔE that corresponds to light of a frequency corresponding to visible light (ν≈5*10^{14} Hz). Since E_{2}  E_{1} >> kT, if follows that the argument of the exponential in the equation above is a large negative number, and as such N_{2} / N_{1} is vanishingly small, i.e., that there are almost no atoms in the excited state.
When in thermal equilibrium, then, it is seen that the lower energy state is usually more populated than the upper energy state, and this is the normal state of the system. If the ratio could be inverted such that N_{2}/N_{1} > 1, then the system is said to have a population inversion. In fact, Boltzmann statistics predicts that, for all positive values of ΔE and temperature, the population of N_{1} will always exceed that of N_{2}, when the system is a thermal equilibrium. It is clear then, that to produce a population inversion, the system cannot be at thermal equilibrium.
The interaction of light with matter
There are three types of possible interactions between the system of atom and light that are of interest:
If light (i.e., photons) of frequency ν_{21} pass through the group of atoms, there is a possibility of the light being absorbed by atoms which are in the ground state, which will cause them to be excited to the higher energy state. The probability of absorption is proportional to the radiation density of the light, and also to the number of atoms currently in the ground state, N_{1}.
If an atom is in the excited state, it will spontaneously decay to the ground state at a rate proportional to N_{2}, the number of atoms in the excited state. The energy difference between the two states ΔE is emitted from the atom as a photon of frequency ν_{21} as given by the frequencyenergy relation above.
The photons are emitted stochastically[?], and there is no fixed phase relationship between photons emitted from a group of excited atoms; in other words, spontaneous emission is incoherent[?]. In the absence of other processes, the number of atoms in the excited state at time t, is given by:
where N_{2}(0) is the number of excited atoms at time t=0, and τ_{21} is the lifetime of the transition between the two states.
If an atom is already in the excited state, it may be perturbed by the passage of a photon which has a frequency ν_{21} corresponding to the energy gap ΔE of the excited state to ground state transition. In this case, the excited atom relaxes to the ground state, and is induced to produce a second photon of frequency ν_{21}. The original photon is not absorbed by the atom, and so the result is two photons of the same frequency. This process is known as stimulated emission. The rate at which stimulated emission occurs is proportional to the number of atoms N_{2} in the excited state, and the radiation density of the light.
The critical detail of stimulated emission is that the induced photon has the same frequency and phase as the inducing photon. In other words, the two photons are coherent. It is this property that allows optical amplification[?], and the production of a laser system.
During the operation of a laser, all three lightmatter interactions described above are happening. Initially, atoms are energised from the ground state to the excited state by a process called pumping, described below. Some of these atoms decay via spontaneous emission, releasing incoherent light as photons of frequency ν. These photons are fed back into laser medium, usually by an optical resonator[?]. Some of these photons are absorbed by the atoms in the ground state, and the photons are lost to the laser process. However, some photons cause stimulated emission in excitedstate atoms, releasing another coherent photon. In effect, this results in optical amplification.
If the number of photons being amplified per unit time is greater than the number of photons being absorbed, then the net result is a continuously increasing number of photons being produced; the laser medium is said to have a gain of greater than unity.
Recall from the descriptions of absorption and stimulated emission above that the rates of these two processes are both proportional to the number of atoms in the ground and excited states, N_{1} and N_{2}, respectively. If the ground state has a higher population than the excited state (N_{1} > N_{2}), the process of absorption dominates and there is a net attenuation of photons. If the populations of the two states are the same (i.e., N_{1} = N_{2}), the rate of absorption of light exactly balances the rate of emission; the medium is then said to be optically transparent. If the higher energy state has a greater population than the lower energy state (N_{1} < N_{2}), then the emission process dominates, and light in the system undergoes a net increase in intensity. It is thus clear that to produce a faster rate of stimulated emissions than absorptions, it is required that the ratio of the populations of the two states is such that N_{2} / N_{1} > 1; In other words, a population inversion is required for laser operation.
As described above, a population inversion is required for laser operation, but cannot be achieved in our theoretical group of atoms with two energylevels when they are in thermal equilibrium. In fact, any method by which the atoms are directly and continuously excited from the ground state to the excited state (such as optical absorption) will eventually reach equilibrium with the deexciting processes of spontaneous and stimulated emission. At best, an equal population of the two states, N_{1} = N_{2} = N/2, can be achieved, resulting in optical transparency but no net optical gain.
To achieve nonequilibrium conditions, and indirect method of populating the excited state must be used. To understand how this is done, we may use a slightly more realistic model, that of a threelevel laser. Again consider a group of N atoms, this time with each atom able to exist in any of three energy states, levels 1, 2 and 3, with energies E_{1},E_{2} and E_{3}, and populations N_{1}, N_{2} and N_{23}, respectively. An energy level diagram of these is shown below:
=================================== level 3, E3, N3 ^    R (fast, radiationless transition)  V  level 2, E2, N2      P   (pump  L (slow, laser transition)  transition)     V  level 1 (ground state), E1, N1
Note that E_{1} < E_{2} < E_{3}, that is, the energy of level 2 lies between that of the ground state and level 3.
Initially, the system of atoms is at thermal equilibrium, and the majority of the atoms will be in the ground state, i.e. N_{1} ≈ N, N_{2} ≈ N_{3} ≈ 0. If we now subject the atoms to light of a frequency ν_{31}, where E_{3}E_{1} = hν_{31} (h being Planck's constant), the process of optical absorption will excite the atoms from the grounds state to level 3. This process is called pumping, and in general does not always directly involve light absorption; other methods of exciting the laser medium, such as electrical discharge or chemical reactions may be used. The level 3 is sometimes referred to as the pump level or pump band, and the energy transition E_{1} > E_{3} as the pump transition, which is shown as the arrow marked P in the diagram above.
If we continue pumping the atoms, we will excite an appreciable number of them into level 3, such that N_{3} > 0. In a medium suitable for laser operation, we require these excited atoms to quickly decay to level 2. The energy released in this transition may be emitted as a photon (spontaneous emission), however in practice the 3>2 transition (labeled R in the diagram) is usually radiationless, with the energy being transferred to vibrational motion (heat) of the host material surrounding the atoms, without the generation of a photon.
An atom in level 2 may decay by spontaneous emission to the ground state, releasing a photon of frequency ν_{21} (given by E_{2}E_{1} = hν_{21}), which is shown as the transition L, called the laser transition in the diagram. If the lifetime of this transition, τ_{21} is much longer than the lifetime of the radiationless 3>2 transition τ_{32}, (i.e., if τ_{21} >> τ_{32}), the population of the E_{3} will be essentially zero (N_{3} ≈ 0) and a population of excited state atoms will accumulate in level 2 (N_{2} > 0). If over half the N atoms can be accumulated in this state, this will exceed the population of the ground state N_{1}. A population inversion (N_{2} > N_{2} ) has thus been achieved between level 1 and 2, and optical amplification at the frequency ν_{21} can be obtained.
Because at least half the population of atoms must be excited from the ground state to obtain a population inversion, the laser medium must be very strongly pumped. This makes threelevel lasers rather inefficient, despite being the first type of laser to be discovered (based on a ruby laser medium, by Theodore H. Maiman in 1960). In practice, most lasers are fourlevel lasers, as shown in the following energy diagram:
=================================== level 4, E4, N4 ^    Ra (fast, radiationless transition)  V  level 3, E3, N3      P   (pump  L (slow, laser transition)  transition)     V  level 2, E2, N2     Rb (fast, radiationless transition)  V  level 1 (ground state), E1, N1
Here, there are four energy levels, energies E_{1}, E_{2}, E_{3}, E_{4}, and populations N_{1}, N_{2}, N_{3}, N_{3}, respectively. The energies of each level are such that E_{1} < E_{2} < E_{3} < E_{4}.
In this system, the pumping transition P excites the atoms in the ground state (level 1) into the pump band (level 4). From level 4, the atoms again decay by a fast, nonradiative transition Ra into the level 3. Since the lifetime of the laser transition L is long compared to that of Ra (τ_{32} >> τ_{43}), a population accumulates in level 3 (the upper laser level), which may relax by spontaneous or stimulated emission into level 2 (the lower laser level). This level likewise has a fast, nonradiative decay Rb into the ground state.
As before, the presence of a fast, radiationless decay transitions result in population of the pump band being quickly depleted (N_{4} ≈ 0). In a fourlevel system, any atom in the lower laser level E_{2} is also quickly deexcited, leading to a negligible population in that state (N_{2} ≈ 0). This is important, since any appreciable population accumulating in level 3, the upper laser level, will form a population inversion with respect to level 2. That is, as long as N_{3} > 0, then N_{3} > N_{2} and a population inversion is achieved. Thus optical amplification, and laser operation, can take place at a frequency of ν_{32} (E_{3}E_{2} = hν_{32}).
Since only a small number of atoms must be excited into the upper laser level to form a population inversion, a fourlevel laser is much more efficient than a threelevel one, and most practical lasers are of this type. In reality, many more than four energy levels may be involved in the laser process, with complex excitation and relaxation processes involved between these levels. In particular, the pump band may consist of several distinct energy levels, or a continuum of levels, which allow optical pumping of the medium over a wide range of wavelengths.
Note that in both three and fourlevel lasers, the energy of the pumping transition is greater than that of the laser transition. This means that, if the laser is optically pumped, the frequency of the pumping light must be greater than that of the resulting laser light. In other words, the pump wavelength is shorter than the laser wavelength. It is possible in some media to use multiple photon absorptions between multiple lowerenergy transitions to reach the pump level; such lasers are called upconversion lasers.
While in many lasers the laser process involves the transition of atoms between different electronic energy states, as described in the model above, this is not the only mechanism that can result in laser action. For example, there are many common lasers (e.g. dye lasers, carbon dioxide lasers) where the laser medium consists of complete molecules, and energy states correspond to vibrational and rotational modes of oscillation of the molecules. This is the case with water masers, that occur in nature.
See also quantum electronics[?]
Search Encyclopedia

Featured Article
