In this explainer, we will learn how to describe the functions of the different components of a laser.
Lasers allow us to use the energy from electron emissions to produce beams of light. The word “laser” is an acronym for “light amplification by stimulated emission of radiation.” This name tells us a lot about how lasers work: light radiation is produced by stimulated emission and then amplified.
Lasers are made of three main components:
- The active medium
- The energy source
- The resonant cavity
To understand how lasers work, we can look at each of these components in turn.
The active medium is a material made of atoms, which provide the electrons for radiation emission. Electrons in the active material can have different energies, depending on the material’s allowed energy levels. Most of the time, electrons will exist in the ground state, which is the lowest energy level. But an electron can be excited to a higher energy level by an incident photon if the energy of the photon is equal to the difference in energy between the two states.
To produce laser light, we need to have our electrons in an excited energy level. The energy needed to move the electrons from the ground level to the excited state is provided by the energy source. When an electron is in an excited state, an incident photon can cause the electron to undergo stimulated emission, where it releases a photon and moves down to a lower energy level. The emitted photon will have the same energy, direction, and phase as the photon that caused the emission. This means that both of these photons can contribute to a beam of laser light.
When the proportion of electrons in the atoms of an active medium that are in excited states is greater than the proportion of such electrons in the ground state, a population inversion is said to occur in the medium.
Example 1: Photon Production in Lasers
The diagrams shown represent three successive stages of the production of photons in a laser.
- In which diagram is energy being supplied to the active medium of the laser?
- The energy supplied to the active medium of the laser consists of photons with a range of wavelengths. Which of the following fractions of the photons shown in the diagram results in stimulated emission in the laser?
- How many photons are produced by the lasing process in the diagrams?
Answer
Part 1
Energy is being supplied in diagram I. Diagram I shows three photons, supplied by the energy source, interacting with electrons in the active medium. This is the first stage of the lasing process, and it is required to establish a population inversion.
Part 2
Diagram I shows three photons, provided by the energy source, interacting with the electrons in the active medium. The top and bottom atoms absorb the photons and become excited. However, the middle atom undergoes a stimulated emission; it produces another photon, identical to the photon that caused the emission.
We have three incoming photons, and only one causes a stimulated emission. Hence, the answer is .
Part 3
Diagram I shows three incoming photons. Two of these are absorbed, meaning there is one photon left. This photon causes the stimulated emission of a second, identical photon. So, the answer is 2 photons.
To produce laser light, we need to have most of our electrons in an excited state. We call this population inversion, because this is the opposite of how electrons are usually arranged in a material. Normally, as many electrons as possible will populate (fill) the ground state. But here we have the inverse situation, because we are using our energy source to raise the energy of the electrons.
However, if we only have two energy levels in our system, stimulated emission is very unlikely to occur. This is because an excited electron will naturally decay back down to the lower energy state, through a process called spontaneous emission. Spontaneous emission happens so quickly that an electron will only remain in the excited energy state for about 10 ns. Since this process is so fast, it is unlikely that an incident photon of the correct energy will reach the excited electron and cause a stimulated emission before the electron decays back to the ground state.
Like stimulated emission, spontaneous emission does produce photons. But we cannot use these photons for lasers because they all have different directions and phases. This means the photons do not “add up” to produce a strong laser beam.
To make sure that stimulated emission happens, we must choose an active material with three energy levels. We still have a ground state and an excited state, but we also need a third state in between them. We call this a metastable state. We can think of a metastable state as being “almost stable.” An electron in the metastable state will stay there for a long time before it eventually decays back down to the ground state. In fact, it will stay in the metastable state thousands of times longer than it will stay in the excited state.
We can use this three-state setup to produce laser light. Electrons will begin in the ground state, and then the energy source will excite them into the excited state. They will then quickly decay into the metastable state, where they will stay for a long time. Then, photons incident on the electrons in the metastable state will cause the stimulated emission of more photons.
For example, consider the following diagram.
An electron in the ground state, with energy , can absorb a photon of energy and move to the excited state. The electron will quickly decay through spontaneous emission, producing a photon of energy . The electron is now in the metastable state, where it will stay until it interacts with a photon of energy . This interaction will cause the electron to undergo a stimulated emission, releasing another photon of energy , with the same direction and phase as the incident photon. The electron is now back in the ground state. Although this process produces photons with two energies ( and ), most of the radiation emitted by the laser will be in the form of a photon with energy because these are the only photons produced by stimulated emission, as opposed to spontaneous emission.
Example 2: Excited Atoms in Different Media
The diagrams represent atoms in two media. Medium A has a ground state and an excited state and medium B has ground,
excited, and metastable states. Photons that excite atoms in the ground state of the medium to the excited state are
supplied to both media. In both media, excited atoms release photons when they transition to a lower energy state.
- The average time interval required for atoms to be excited from the ground state to a higher energy state is
. The average time interval required for atoms to relax to the ground state from a
higher energy state is . In which medium is ?
- Medium A
- Medium B
- Both media
- As energy is supplied to the media, in which of them will more atoms tend to be at higher energy levels than
at the ground state?
- Medium A
- Medium B
- Both media
- Under which of the following conditions does a medium undergo a population inversion?
- When there are more atoms in energy states with greater energy than the ground state than there are atoms in the ground state
- When all the atoms in the medium are ionized
- When none of the atoms in the medium are in the metastable state
- When the energy of the excited state is equal to the energy of the metastable state
- When there are fewer atoms in energy states with greater energy than the ground state than there are atoms in the ground state
- When a medium undergoes population inversion, in which energy state are most of the atoms that are at higher
energy levels than the ground state?
- The metastable state
- The excited state
- There is about the same number of atoms in the excited state as in the metastable state.
Answer
Part 1
For an atom to move from the ground state to a higher energy state, it must absorb a photon of energy equal to the difference between the two energy states. So, an atom has to wait until it is struck by a photon of the correct energy before it is excited. The time it takes for the right photon to come along is , and this is often a fairly long time.
For an atom to relax from a higher state to a lower state, it must emit a photon of energy equal to the difference between the two states.
In medium A, there is only a ground state and an excited state. Once an atom has reached the excited state, it will only remain there for about 10 ns, before it undergoes spontaneous photon emission and relaxes back into the ground state. The time it takes for this process to occur is . Because spontaneous emission happens so quickly, in medium A, .
In medium B, there is a ground state, an excited state, and a metastable state. An atom in the excited state will very quickly decay to the metastable state by spontaneous emission.
An atom in the metastable state can reach the ground state by undergoing spontaneous emission, but because this state is so stable, this process takes a very long time. The atom can also reach the ground state by stimulated emission, but it has to wait for a photon of the correct energy to come along. This can also take a very long time.
In medium B, is the time it takes for an atom to decay from the excited state to the metastable state and then from the metastable state to the ground state. Because the transition from the metastable state to the ground state takes so long, in medium B, .
So, the answer is medium B.
Part 2
When there is no external energy source, the atoms in a medium tend to exist at the lowest possible energy. So, most atoms will be in the ground state.
We can move atoms from the ground state to higher energy levels by supplying energy, which excites the atom.
In medium A, there is no metastable state. This means that an excited atom will decay back to the ground state very quickly.
In medium B, the metastable state means that excited atoms can stay at higher energy levels for much longer.
So, for a given energy input, it is much easier to keep atoms in medium B at higher energy levels than it is in medium A. This means that medium B will tend to have more atoms at higher levels than the ground state.
Part 3
When there is no external energy being supplied, atoms in a medium will, by default, exist at the lowest possible energy state. So, atoms will normally fill, or “populate,” the lowest energy level they possibly can. Atoms populate the ground state first and only begin to populate the higher levels when the ground state is full.
But in lasers, we need atoms to populate higher energy levels and leave the ground state as empty as possible. This is because lasers rely on atoms in excited states decaying to lower levels and producing photons in the process.
We call this “population inversion,” because this is the opposite of how atoms would normally populate the energy levels of a medium. So, the answer is A, when there are more atoms in energy states with greater energy than the ground state than there are atoms in the ground state.
The ionization of the atom is irrelevant, as this simply changes the number of electrons in each atom.
If none of the atoms were in the metastable state, all of the atoms would be in the ground state or the excited state. If most of the atoms were in the excited state, this would technically be a population inversion, but because spontaneous emission happens so quickly, this could not be sustained for very long.
If the energy of the excited state were equal to the energy of the metastable state, we would only have two distinct energy levels, meaning population inversion could not occur.
Part 4
Most higher energy atoms will be in the metastable state.
An atom in the highest excited state will very quickly decay to the metastable state by spontaneous emission, so atoms do not stay in the excited state very long. But once an atom reaches the metastable state, it will stay there for a very long time. This means that most higher energy atoms will collect in the metastable state.
The photons released by a stimulated emission process can interact with other excited electrons, causing more stimulated emissions to occur. This produces a large number of photons with identical energy, phase, and direction, which form a laser beam.
However, it is also possible for a photon to pass through the active medium without interacting with any electrons. In this case, stimulated emission does not occur, and we cannot produce a laser beam. To fix this problem, we use a resonant cavity. To build the resonant cavity, we simply place mirrors on every side of the active medium. Most of these should be full mirrors, meaning they reflect every photon that hits them, but one of them needs to be a partial mirror, meaning it allows some photons to pass through.
If a photon passes through the active medium without causing any stimulated emissions, it will reach the mirror at the end and be reflected back. This means the photon will pass through the active medium again and have another chance at interacting with an electron. When the photon finally interacts with an electron, it will stimulate the emission of another photon, which can also be reflected back and forth until it causes another photon emission. By reflecting the photons back and forth through the active medium, we produce more and more photons in total, creating a stronger laser beam. This process is known as amplification. The photons that are allowed to pass through the partial mirror are what we see as laser light.
Example 3: Resonant Cavities
One of the reflecting faces of the resonant cavity of a laser must be less than completely reflective for the laser to be effective. Which of the following explains why this is the case?
- If all faces of the resonant cavity are equally reflective, light waves traveling in opposite directions through the cavity destructively interfere.
- Energy cannot be emitted from the resonant cavity if all its faces are perfectly reflective.
- External energy cannot be supplied to the resonant cavity if all its faces are perfectly reflective.
- Coherent light cannot be emitted from the resonant cavity if all its faces are perfectly reflective.
- All the reasons given are true.
Answer
The answer is D, coherent light cannot be emitted from the resonant cavity if all its faces are perfectly reflective.
The resonant cavity uses mirrors to reflect photons back and forth through the active medium in order to increase the chances of a photon causing a stimulated emission. But if all of the faces of the resonant cavity were perfectly reflective and allowed no photons to pass through, we would not be able to see any of the laser beam that we had produced. So, one of the faces must be partially reflective so that our laser beam can pass through the resonant cavity.
Note that destructive interference does not occur. The resonant cavity is designed to have a certain length, such that standing waves are produced in the cavity, preventing destructive interference.
The energy source does not provide energy by directly inputting high-energy photons. Instead, it usually takes the form of an electrical power source, which could be applied even if all the mirrors were perfectly reflective.
Key Points
- Lasers use the stimulated emission of photons to produce beams of light.
- The active material of a laser needs three energy levels: a ground state, a metastable state, and an excited state.
- An energy source is needed to create a population inversion in the active material, where more electrons are in excited states than in the ground state.
- A resonant cavity, consisting of mirrors placed on either side of the active material, amplifies the beam of laser light.
