Chapter 8: Lasers and Masers

Unique Properties of Lasers• Monochromaticity - light with a single

frequency (one colour).

• Coherence – all rays of the beam are in phase.

• Directionality

Three fundamental phenomena

• Absorption

• Spontaneous emission • Stimulated emission

Absorption

• An atom initially at level one (ground level) receives an external stimulus (energy).

• Energy raises it to level two (an excited state)

Spontaneous emission• Atom in an excited state (high energy level)

falls back to a less excited state, emitting a photon of light.

• This is called spontaneous or radiative emission.

• Spontaneous emission is random

• Atoms involved in spontaneous emission may produce monochromatic light…

• … but the phases of the waves will be different (non-coherent) and …

• … they will travelling in different directions.

Stimulated emission

• Emission stimulated by another photon.

• The emitted photon is in the same direction and in phase with the incident photon.

Population inversion

• Stimulated emission is the basis of the laser, but you need many more atoms in the excited state than the ground state. This is called population inversion. It is achieved by pumping.

Most atoms in ground state

Most atoms in excited state (population inversion)

Metastable states

To achieve population inversion we must have metastable states. These are excited states where electrons stay for unusually long times.

Metastable states

Normally an electron in an excited state will make the transition to a lower state in a time of 10-7s. In contrast an electron may stay in a metastable state for 10-3s.

• Laser equipment has three basic components:

– a resonant optical cavity

– a gain medium (active medium) and

– a pump source to excite the particles in the gain medium.

• Active medium placed between two mirrors.

• Light bounces up and down between the mirrors resonantly.

• Laser cavity made up of two mirrors, one of which is semi-transparent to release the laser that is generated in the cavity.

• Stimulated emission process takes place in the gain medium.

• Gain medium amplifies light of any direction.

• However, only the light that bounces up and down between the resonator mirrors is amplified many times and therefore reaches a high intensity.

• Monochromatic because the photon energy has to match a particular energy transition.

• Since the amplification process maintains the phase and direction of the light, the laser output is directional and coherent.

• The active particles in the laser gain medium need to be in a state of inversion for the laser to operate.

• This is done through a process known as pumping.

• The beam gains an extra photon whenever an atom returns to the ground state

• The beam (laser) is reflected back and forth in the cavity until it has enough intensity to be sent out of the cavity.

• After that, the flash bulb can again excite enough atoms for the creation of a new pulse.

• Laser is released continuously in a Continuous Wave (CW) laser.

• An obstacle is placed in the laser cavity to stop the laser discharge.

• The obstacle is removed when the flash bulb has excited a large number of atoms it releasing one gigantic pulse.

Helium-Neon laser

Helium-Neon laser

• The helium is used to create population inversion in the neon. An electric field is used to excite the Helium to a metastable state 20.61 eV above its ground level. When this metastable helium atom collides with a neon atom it excites the neon atom to a 20.66 eV metastable state. The extra 0.05 eV comes from the helium’s kinetic energy and the helium atom is now back in its ground state.

Helium-Neon laser

The neon atoms transition to a 18.7 eV state, giving out photons of λ = 632.8 nm. These photons are reflected by mirrors at the ends of the laser tube causing stimulated emission in other metastable neon atoms.

Helium-Neon laser

One of the mirrors is semi transparent, allowing the monochromatic, coherent, directional and very intense laser beam to emerge.

Uses of lasers

Destroying tumours

Reattaching damaged retinas

The laser damages part of the tissue so that the resulting scar “welds” the retina back into place.

Unblocking arteries and heart valves

Cutting and sealing nerves in neurosurgery

Correcting myopia in cornea operations

Welding

Cutting and drilling metals

Compact discs

Measuring distances

Reading barcodes

Optical fibre communication

Laser guided “smart” weapons