15.3 Lasers

At some time a laser photon will run into another excited electron. Then stimulated emission occurs and the electron drops down to the ground state and emits an additional identical photon as shown in [link] . Since the laser material typically has a large number of atoms, one laser photon passing through this material will rapidly cause a large number of photons just like it to be emitted.

The optical cavity keeps the laser photons inside the laser cavity so that they can build up the laser beam. At each end is a concave mirror; one is a full mirror and one is a partial mirror. The full mirror is totally reflective. The partial mirror transmits a small amount of the light that hits it (less than 1%). The mirrors are carefully aligned so that photons that reflect off one mirror become “trapped”, and bounce back and forth between the mirrors many times causing more and more stimulated emission. The photons that eventually escape through the partially-silvered mirror become the laser beam that we see.

As the photons bounce between mirrors, they continually pass through the laser material, stimulating those atoms to emit more photons. This creates an ever increasing beam of photons, all with the same characteristics, all traveling in the same direction. In this way, the optical cavity helps to amplify the original laser photons into a concentrated, intense beam of photons.

The laser cavity also helps to narrow the frequency range of laser light emitted. The distance between the two mirrors defines the cavity mode which only allows light of a narrow range of frequencies to continue being reflected back and forth. Light of other frequencies damped out. (This is just like in the chapter on the physics of music where a pipe of a certain length corresponds to a particular wavelength of sound.) Therefore only a narrow frequency of light can be emitted.

Laser applications and safety

Although the first working laser was only produced in 1958, lasers are now found in many household items. For example, lasers are well-known through their use as cheap laser pointers. However, lasers can be very dangerous to the human eye since a large amount of energy is focused into a very narrow beam. NEVER POINT A LASER POINTER INTO SOMEBODY'S EYES - IT CAN BLIND THEM FOREVER.

Other uses include:

• Semiconductor lasers which are small, efficient and cheap to make are used in CD players.
• He-Ne Lasers are used in most grocery shops to read in the price of items using their barcodes. This makes the cashiers' job much quicker and easier.
• High energy lasers are used in medicine as a cutting and welding tool. Eye surgery in particular make use of the precision of lasers to reattach the retinas of patients' eyes. The heat from cutting lasers also helps to stop the bleeding of a wound by burning the edges (called cauterising).

Case study : uses of lasers

Do research in a library or on the Internet on one application of laser technology. Explain how the technology works by using a laser.

You will need to present your findings to the class in the form of a poster. You can think of any useful application, but to give you some ideas of where to start, some applications are listed below:

• laser printers
• laser communication and fibre optics
• optical storage
• using lasers as precision measurement tools
• your own ideas...

Lasers

1. Explain what is meant by spontaneous emission of radiation .
2. Explain what is meant by stimulated emission of radiation .
3. List the similarities and differences between spontaneous emission of radiation and stimulated emission of radiation.
4. How is the light emitted by a laser different from the light emitted by a light bulb?
5. Describe using a simple diagram, how a laser works. Your description should include the following concepts: metastable state and population inversion.
6. Give examples of some materials that have been used for lasers. What do all these materials have in common?
7. Describe how the laser cavity affects:
   • increasing amplification
   • concentrating beam intensity
   • narrowing the frequency of the beam
8. List some applications of lasers.

Summary

1. Light of the correct frequency can eject electrons from a metal. This is called the photoelectric effect.
2. A metal has a work function which is the minimum energy needed to emit an electron from the metal.
3. Emission spectra are formed by glowing gases. The pattern of the spectra is characteristic of the specific gas.
4. Absorption spectra are formed when certain frequencies of light are absorbed by a material.
5. Lasers are devices that produce a special type of light that has many uses.
6. Lasers have many uses,for example, in CD and DVD players, to cut material, in surgery, in printing, in telecommunications and as laser pointers.

End of chapter exercise

1. What is the photoelectric effect?
2. Calculate the energy of a photon of red light with a wavelength of 400 nm.
3. Will ultraviolet light with a wavelenth of 990 nm be able to emit electrons from a sheet of calcium with a work function of 2,9 eV?
4. What does the acronym LASER stand for?
5. Name three types of lasers and their uses.
6. Write a short essay on the benefits lasers have had on modern society.