PHYS 252 Lasers1 Lasers What is stimulated emission? Well, there are two types of light emission that can occur with atoms! The kind that we have been talking about is the first type. spontaneous emission Einstein postulated the second type and gets credit for “discovering” it. (But it had always been occurring!) stimulated emission Light Amplification by Stimulated Emission of Radiation

PHYS 252 Lasers2 Lasers Spontaneous Emission atom emits photon on its own atom is stimulated to emit photon by an incoming photon Stimulated Emission emitted in random direction the two photons leave in the same direction incoming photon must have the same energy as the one that will be emitted

PHYS 252 Lasers3 Lasers Possible Idea! If we could get a bunch of excited atoms and have each one be stimulated to emit a photon, then we could get a whole bunch of photons with the same energy and wavelength travelling in the same direction. This would give us a bright output beam…..a laser beam! Problem! For most atomic energy levels, spontaneous emission occurs before stimulated emission. Spontaneous emission will not give us a nice beam and just one wavelength. Solution! Find atoms that have an energy level where stimulated emission is more likely to occur. This state is called the metastable state.

PHYS 252 Lasers4 Lasers Three Level Laser Operation Pump atom from 1 to 3 using excitation source. 5. Repeat Step Quick decay from 3 to 2. * When there are more atoms at level 2 than level 1, we have a population inversion. Lasing action will now occur. 3. Atom “waits” in metastable state for incoming photon*. 4. Incoming photon stimulates emission from 2 to 1.

PHYS 252 Lasers5 Lasers So what do you need to build a laser? 1. Gain Medium - a material with atoms that have a metastable state and that can achieve population inversion 2. Excitation Source - a way to supply energy to excite the atoms 3. Resonating Cavity (Mirrors) - allows the stimulated photons to travel back and forth through the gain medium so they can stimulate more photons (light amplification)

PHYS 252 Lasers6 Lasers gain medium solid liquid gas excitation source light, laser for solid lasers laser for liquid lasers high voltage for gas lasers full mirror partial mirror allows some photons to escape to produce beam

PHYS 252 Lasers7 Lasers Light Characteristics tight beam Operation Schemes continuous wave (cw) - beam is on all the time - scanning, optical ranging, light displays nearly one wavelength (monochromatic) relatively high output power, especially in pulsed wave operation pulsed wave – short pulses are emitted -high-power applications like surgery or cutting -low-power applications like communication

PHYS 252 Lasers8 Lasers

PHYS 252 Lasers9 Lasers Semiconductor Lasers (Laser Diodes) made from semiconductor solid crystals operation is similar to the previous lasers we discussed but there are significant differences use a flow of electrons (current) for the excitation source very small but with medium to high output powers laser light can be tuned sightly to different wavelengths used for: – optical reading & writing (dvd, cd, laser printer, bar code) – laser pointers – optical fiber communication – excitation sources for other lasers

PHYS 252 Lasers10 Lasers This is a AlGaInP/GaInP laser emitting light at 676 nm. It is 0.03 mm wide, 0.3 mm long, and about 0.1 mm thick. SEMICON- DUCTOR MaterialTunable Range AlGaAsnear IRcurrentfirst semiconductor lasers AlGaInPredcurrentreplaced He-Ne lasers, read & write technologies AlGaInAsPnear IRcurrentoptical communication InGaNgreen to bluecurrentread & write technologies Excitation

PHYS 252 Lasers11 Laser Applications

PHYS 252 Lasers12 Laser Applications

PHYS 252 Lasers13 Laser Applications