LASER (semiconducting Lasers) Dr Zainovia Lockman LASER 1 EBB 424E

Lecture Contents Definition of lasers Emission and absorption of radiation Population Inversion Semiconducting lasers Materials used for semiconducting laser Laser for fibre optics communication Quantum Well devices

Diode Laser

Typical Application of Laser The detection of the binary data stored in the form of pits on the compact disc is done with the use of a semiconductor laser. The laser is focused to a diameter of about 0.8 mm at the bottom of the disc, but is further focused to about 1.7 micrometers as it passes through the clear plastic substrate to strike the reflective layer. The reflected laser will be detected by a photodiode. Moral of the story: without optoelectronics there will no CD player!

1. Definition of laser A laser is a device that generates light by a process called STIMULATED EMISSION. The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation Semiconducting lasers are multilayer semiconductor devices that generates a coherent beam of monochromatic light by laser action. A coherent beam resulted which all of the photons are in phase.

Another Typical Application of Laser – Fibre Optics An example of application is for the light source for fibre optics communication. Light travels down a fibre optics glass at a speed, = c/n, where n = refractive index. Light carries with it information Different wavelength travels at different speed. This induce dispersion and at the receiving end the light is observed to be spread. This is associated with data or information lost. The greater the spread of information, the more loss However, if we start with a more coherent beam then loss can be greatly reduced.

Fibre Optics Communication

3 Mechanisms of Light Emission For atomic systems in thermal equilibrium with their surrounding, the emission of light is the result of: Absorption And subsequently, spontaneous emission of energy There is another process whereby the atom in an upper energy level can be triggered or stimulated in phase with the an incoming photon. This process is: Stimulated emission It is an important process for laser action Absorption Spontaneous Emission Stimulated Emission Therefore 3 process of light emission:

Absorption E1 E2

Spontaneous Emission

Stimulated Emission

Background Physics In 1917 Einstein predicted that: under certain circumstances a photon incident upon a material can generate a second photon of Exactly the same energy (frequency) Phase Polarisation Direction of propagation In other word, a coherent beam resulted.

Background Physics Consider the ‘stimulated emission’ as shown previously. Stimulated emission is the basis of the laser action. The two photons that have been produced can then generate more photons, and the 4 generated can generate 16 etc… etc… which could result in a cascade of intense monochromatic radiation.

Stimulated Emission

Background Physics In a system, all three mechanisms occur. However the stimulated emission is very very sluggish compared to the spontaneous emission We need to have a much stimulated emission as possible for lasing action How? Refer to the board for the derivation of the Einstein’s

Einstein;s

Absorption of Light Through a Medium Light or photon must be absorbed in order for us to have a lasing action I(x) = I(o) exp (-x) I(o) I(x)

Absorption Light that falls on a piece of material will decrease exponentially.  = (N1-N2)B21(hf) n/c N1 is often more than N2 (N1 < N2) Example for tungsten  is typically 106m-1 (+ve) If we want implication,  must be –ve i.e. N2 > N1

Population Inversion Therefore we must have a mechanism where N2 > N1 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 N2 than N1 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 N1 = N2. To create population inversion, a 3-state system is required. The system is pumped with radiation of energy E31 then atoms in state 3 relax to state 2 non radiatively. The electrons from E2 will now jump to E1 to give out radiation.

3 states system

Population Inversion When a sizable population of electrons resides in upper levels, this condition is called a "population inversion", and it sets the stage for stimulated emission of multiple photons. This is the precondition for the light amplification which occurs in a LASER and since the emitted photons have a definite time and phase relation to each other, the light has a high degree of coherence.

Typical Exam Question… Define the term population inversion for a semiconducting laser (diode) explain what is the condition of population inversion. Why is population inversion required for a lasing action? (40 marks)

Optical Feedback The probability of photon producing a stimulated emission event can be increased by reflecting back through the medium several times. A device is normally fashioned in such a way that the 2 ends are made higly reflective This is term an oscillator cavity or Fabry Perot cavity

Therefore in a laser…. Three key elements in a laser •Pumping process prepares amplifying medium in suitable state •Optical power increases on each pass through amplifying medium •If gain exceeds loss, device will oscillate, generating a coherentoutput