LASER Light Amplification by Stimulated Emission of Radiation
Laser Fundamentals The light emitted from a laser is monochromatic, that is, it is of one color/wavelength. In contrast, ordinary white light is a combination of many colors (or wavelengths) of light. Lasers emit light that is highly directional, that is, laser light is emitted as a relatively narrow beam in a specific direction. Ordinary light, such as from a light bulb, is emitted in many directions away from the source. The light from a laser is said to be coherent, which means that the wavelengths of the laser light are in phase in space and time. Ordinary light can be a mixture of many wavelengths. These three properties of laser light are what can make it more hazardous than ordinary light. Laser light can deposit a lot of energy within a small area. 4
Incandescent vs. Laser Light 1. Many wavelengths 2. Multidirectional 3. Incoherent 1. Monochromatic 2. Directional 3. Coherent 5
Energy Level, Definitions The valence band is the highest filled band The conduction band is the next higher empty band The energy gap has an energy, E g, equal to the difference in energy between the top of the valence band and the bottom of the conduction band
Conductors When a voltage is applied to a conductor, the electrons accelerate and gain energy In quantum terms, electron energies increase if there are a high number of unoccupied energy levels for the electron to jump to It takes very little energy for electrons to jump from the partially filled to one of the nearby empty states
Insulators The valence band is completely full of electrons A large band gap separates the valence and conduction bands A large amount of energy is needed for an electron to be able to jump from the valence to the conduction band The minimum required energy is E g
Semiconductors Electrons A semiconductor has a small energy gap Thermally excited electrons have enough energy to cross the band gap The resistivity of semiconductors decreases with increases in temperature The white area in the valence band represents holes Holes
Atomic Transitions Stimulated The blue dots represent electrons When a photon with energy ΔE is absorbed, one electron jumps to a higher energy level These higher levels are called excited states ΔE = hƒ = E 2 E 1 In general, ΔE can be the difference between any two energy levels Absorption
Atomic Transitions Spontaneous Emission Once an atom is in an excited state, there is a constant probability that it will jump back to a lower state by emitting a photon This process is called spontaneous emission
Atomic Transitions Stimulated Emission An atom is in an excited state and a photon is incident on it The incoming photon increases the probability that the excited atom will return to the ground state There are two identical emitted photons, the incident one and the emitted one The emitted photon is exactly in phase with the incident photon
Before After Absorption Unexcited molecule Excited molecule Spontaneous Emission Stimulated Emission
Stimulated emission leads to a chain reaction and laser emission. If a medium has many excited molecules, one photon can become many. Excited medium This is the essence of the laser. The factor by which an input beam is amplified by a medium is called the gain and is represented by G.
Absorption and Spontaneous Emission Consider a two-level system E 2, N 2 E 2, N 2 absorption emission E 1, N 1 E 1, N 1 Light from bulbs are due to spontaneous emission
Absorption and Stimulated Emission E 2, N 2 E 2, N 2 absorption stimulated emission E 1, N 1 E 1, N 1 Laser light results from stimulated emission
Two-Level System E 2, N 2 E 2, N 2 E 1, N 1 E 1, N 1 Even with very a intense pump source, the best one can achieve with a two-level system is excited state population = ground state population
Population Inversion We must have a mechanism where N 2 > N 1 This is called POPULATION INVERSION Population inversion can be created by introducing a so call metastable centre where electrons can piled up to achieve a situation where more N 2 than N 1 The process of attaining a population inversion is called pumping and the objective is to obtain a non-thermal equilibrium. It is not possible to achieve population inversion with a 2-state system. If the radiation flux is made very large the probability of stimulated emission and absorption can be made far exceed the rate of spontaneous emission. But in 2-state system, the best we can get is N 1 = N 2. To create population inversion, a 3-state system is required. The system is pumped with radiation of energy E 31 then atoms in state 3 relax to state 2 non radiatively. The electrons from E 2 will now jump to E 1 to give out radiation.
Energy Introduction Lasing Action Diagram Excited State Spontaneous Energy Emission Metastable State Stimulated Emission of Radiation Ground State 20
Three-Level System The first laser, the ruby laser, was a three-level system upper lasing state lower lasing state Laser light due to transition from 2 E state to 4 A 2 state
He-Ne laser Four-Level System
Four-Level System Nd:YAG laser upper laser state lower laser state Laser light due to transition from 4 F to 4 I
Dye Lasers: Four-level systems
efficient pumping slow relaxation Requirements for Laser Action fast Metastable state slow Population inversion Fast relaxation