... like I'm 5 years old
A laser is a device that emits light. But it's not like any regular source of light such as a bulb or a candle. The light from a laser is special because it's made up of waves that are all in sync with each other; they have the same wavelength and move together in a single direction. This synchronization of light waves is what makes a laser beam so focused and intense, capable of performing a variety of tasks, from cutting through metal to performing delicate eye surgeries.
A laser works by energizing atoms and causing them to emit light. This is done by placing a material (like a gas, crystal, or semiconductor) inside a tube and pumping energy into it. The energy can be in the form of light, electricity, or even another laser. As the atoms in the material absorb this energy, they get excited and emit particles of light (also known as photons) which are then reflected back and forth within the tube. The reflected photons trigger other atoms to emit more photons, creating a chain reaction. This process is known as "stimulated emission".
Imagine you're in a room full of sleeping people and you suddenly shout, causing a few people to wake up and shout as well. Their shouting wakes up more people who then also shout. Soon, the entire room is full of people shouting in unison. This is similar to how a laser works. The initial shout is like the energy pumped into the laser, the people are like the atoms, and their shouts are like the photons being emitted.
... like I'm in College
In a laser, the material or "gain medium" used (which can be a gas, crystal or a semiconductor) plays a crucial role. The type of gain medium determines the wavelength of the laser light produced.
The physical structure of a laser includes the gain medium placed inside a resonant cavity, which is essentially a pair of mirrors. One mirror is fully reflective, bouncing all the light back into the cavity, while the other mirror is partially transparent, allowing some light to escape. This escaping light forms the laser beam.
The process begins with an external source of energy, also known as a "pump", which excites the atoms in the gain medium. This is called "population inversion". As these excited atoms return to their normal state, they emit photons. These photons bounce between the mirrors, triggering more atoms to emit photons in a process known as "stimulated emission". This amplifies the light, causing it to grow in intensity and creating a beam of coherent light - the laser beam.
Imagine a box of Lego bricks as our laser. The different Lego pieces represent atoms in various energy states. The ground state atoms are the standard 2x4 bricks while the excited state atoms are the 2x2 bricks.
Start by having most of your Lego bricks as 2x4s (ground state atoms) and a few 2x2s (excited state atoms). Now, imagine a magical Lego separator tool (the pump) that can split the 2x4 bricks into two 2x2 bricks, representing the energy that excites atoms to a higher energy state.
As you use your Lego separator tool, more and more 2x4 bricks become 2x2 bricks. This is population inversion. Now, imagine that whenever two 2x2 bricks touch each other, they turn back into a 2x4 brick, and a special shiny Lego piece (photon) is released.
Now, imagine a mirror at each end of your box. Every shiny Lego piece that bounces off these mirrors turns another 2x2 brick into a 2x4 brick, producing another shiny Lego piece. This is stimulated emission. The mirrors keep the shiny Lego pieces bouncing back and forth, producing more and more shiny pieces until some of them escape through a small hole in one of the mirrors. This stream of shiny Lego pieces represents the laser beam.
... like I'm an expert
The foundation of laser operation lies in the principles of quantum mechanics. The process of creating a laser beam involves manipulating the quantum state of atoms within the laser's gain medium to achieve population inversion, a state where more atoms are in an excited state than in the ground state.
The energy source or pump provides the energy required to excite the electrons in the atoms of the gain medium to higher energy levels, called excited states. However, these higher energy states are unstable and the electrons quickly drop back down to their ground state. The energy difference between the excited state and the ground state is released as a photon with a specific wavelength.
As this photon travels through the gain medium, it can interact with other excited atoms, stimulating them to emit their own photons that are in phase and have the same direction and wavelength, in a process called stimulated emission. This results in a coherent beam of light.
The mirrors at either end of the gain medium act as an optical resonator, creating an optical feedback that amplifies the light by reflecting it back through the gain medium multiple times. This amplification process continues until the beam is emitted through the partially transparent mirror, forming the laser.