Lasers

1. Light Amplification by Stimulated Emission of Radiation Lasers Light Amplification by Stimulated Emission of Radiation 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 PHYS 252 Lasers

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

3. 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. Lasers PHYS 252 Lasers

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

5. 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 Lasers

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

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

8. Lasers PHYS 252 Lasers

9. 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 Lasers

10. SEMICON- Material Tunable Range Excitation DUCTOR AlGaAs near IR current first semiconductor lasers AlGaInP red current replaced He- Ne lasers, read & write technologies AlGaInAsP near IR current optical communication InGaN green to blue current read & write technologies 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. PHYS 252 Lasers

11. Laser Applications PHYS 252 Lasers

12. Laser Applications PHYS 252 Lasers

13. Laser Applications PHYS 252 Lasers