The physics of laser light revolves around the creation of a coherent and amplified beam of light through a process called Light Amplification by Stimulated Emission of Radiation (LASER).
Here's a breakdown of the key physical principles:
1. Stimulated Emission: The Core Phenomenon
- Absorption: An atom absorbs a photon of light and transitions to a higher energy state.
- Spontaneous Emission: An atom in an excited state spontaneously decays back to its lower energy state, emitting a photon in a random direction. This is how normal light sources work.
- Stimulated Emission: Crucially, if an atom in an excited state is struck by a photon with energy equal to the energy difference between the excited and ground states, it will emit an identical photon. This emitted photon travels in the same direction, with the same phase, and same polarization as the incident photon. This is stimulated emission.
2. Population Inversion: Setting the Stage
For stimulated emission to dominate and create a net amplification, you need more atoms in the excited state than in the ground state. This is called a population inversion. Achieving population inversion is non-equilibrium and usually requires a 'pumping' mechanism. Common pumping methods include:
- Optical Pumping: Using intense light to excite atoms.
- Electrical Discharge: Passing an electric current through a gas to excite atoms.
- Chemical Reactions: Using chemical reactions to produce excited atoms.
3. Optical Cavity: Amplification and Coherence
Lasers typically use an optical cavity, formed by two mirrors, to amplify the light produced by stimulated emission.
- Reflection and Amplification: Photons emitted along the axis of the cavity bounce back and forth between the mirrors, repeatedly stimulating more emission from excited atoms. This leads to exponential amplification of the light.
- Coherence: Because each stimulated emission event produces a photon identical to the stimulating photon, the light within the cavity becomes highly coherent (all photons have the same phase and direction).
- Output Coupling: One of the mirrors is partially transmissive, allowing a portion of the amplified, coherent light to escape as the laser beam.
4. Key Properties of Laser Light
These processes result in laser light having unique properties:
| Property | Description |
|---|---|
| Coherence | All photons have the same phase and direction, resulting in a highly ordered wave. |
| Monochromaticity | The light consists of a very narrow range of wavelengths (or frequencies). |
| Directionality | The light is emitted as a narrow, focused beam with minimal divergence. |
| High Intensity | The light is concentrated into a small area, resulting in a high power density. |
5. Examples of Lasers
Different types of lasers utilize different materials and pumping mechanisms, each affecting the achievable wavelength and power output. Some common examples include:
- Solid-state lasers (e.g., Nd:YAG): Use a solid gain medium doped with rare-earth ions.
- Gas lasers (e.g., HeNe, Argon): Use a gas as the gain medium.
- Semiconductor lasers (e.g., laser diodes): Utilize a semiconductor junction as the gain medium.
- Fiber lasers: Employ optical fibers doped with rare-earth ions.
In summary, the physics of laser light involves creating a population inversion, using stimulated emission to amplify light, and shaping the light using an optical cavity to produce a coherent, monochromatic, and directional beam.
