LASERS AGAIN? Phys 1020, Day 17: Questions? LASERS: Next Up: Cameras and optics Eyes to web: Final Project Info

Transcription

1 LASERS AGAIN? Phys 1020, Day 17: Questions? LASERS: 14.3 Next Up: Cameras and optics Eyes to web: Final Project Info 1

2 Group Exercise Your pennies will simulate a two state atom; tails = ground state, heads = excited state Start with all of the pennies tails up. Each round you will turn 30% of the tails to heads and 30% of the heads to tails to simulate the stimulated processes Go through 10 rounds what do you find? Now make a change to allow for spontaneous emission by turning an additional 10% of the heads to tails what do you find?

3 To amplify number of photons going through the atoms I need a. more atoms in lower energy level, b. half in lower, half upper, c. more in upper energy level, d. a sufficient number in upper level and it does not matter how many are in the lower. 3

4 so get amplification if more in upper level than lower this condition very unnatural known as population inversion how to get population inversion? We already know we can t do it with two levels 4

5 Another exercise Place all of your pennies on state 1 the paper Every round take 10% of the pennies in state 1 and place them on state 3, take 50% of the pennies originally on state 3 and place them on state 2 and take 20% of the pennies on state 2 and place them on state 1. Go through 10 rounds - what do you find? Now do it again but have 20% go from 1 to 3 and 10% go from 2 to 1. What do you find?

6 Therefore, To get population inversion, need at least one more energy level involved. 2 also can kick up by bashing with electron 3 to have most of the atoms piled up in state 3 (pop. inversion with lower) you would need a. rate out of 3 slower than rate up to 2. b. rate from 2 to 3 faster than 3 to 1, c. rate out of 3 faster than 2 to 3 but slower than 1 to 2, d. a and b 1 pumping process to produce population inversion 6

7 Can you get a sustained population inversion in a two level system? 7

8 What color light could come out on 3 to 1 transition if green light is absorbed for the 1 to 2 transition? a. green, b. blue, c. red, d. a and b., e. a and c 2 also can kick up by bashing with electron c. red. as to be lower energy than the green needed for 1 to 2. Smaller the separation between 3 and 1, slower the rate from 3 to 1. So these two factors why easier to get lasers at longer wavelengths 3 than at shorter wavelengths. 1 pumping process to produce population inversion 8

9 so now see how to get population inversion, will give amplification of red light. If enough atoms in upper, will lase. PhET Simulation But much easier if not let light all escape. Reuse. mirror to reflect the light. 9

10 what will come out on the right? Think before you pick a. 1 photon, b. 2 photons, c. 3 photons, d. 4 photons, e. 8 10

11 gas laser like elium Neon. Just like neon sign with with helium and neon mixture in it and mirrors on end. Diode laser- same basic idea, but light produced like in light emitting diode at P-N diode junction. 11

12 What have we learned in this section: 1) Lasers (pump up to population inversion, put mirrors around it, stimulated emission will take care of the rest) 2) For operation, lasers need at least 3 energy levels (ground state and 2 excited states). It helps if the middle level has a long lifetime ( metastable) 3) ow glow in the dark toys work 4) Lots of cool demonstrations. Looked at emission spectra. Disassembled a working laser. 12

13 Many applications of lasers igh energy small area: Cutting: surgery, laser welding communication (and weapons) Focus light into extremely small spot: (diffraction limit, because in phase!) CDs, DVDs, Collimated beam Tracking, leveling, Pure color LIDAR. 13

14 Is a laser really an amplifier or is it an oscillator? ow do you make an oscillator? Notice the transistor amplification required

15 Why do we care about photons? Interaction of Electromagnetic Radiation and Matter: Photon Energy is what counts! Energy = h x f = Planck s Constant x Frequency Application: Interaction of EM radiation with molecules in our skin cells. Many energy levels in molecules. Radio waves (0) really long wavelength, really low energy photon, no levels that close in energy, no effect. E 2 Microwave, infrared waves (1) low energy photon, electron hops up small level, turns into atom motion as goes back down, heat. 0 1 Visible light (2) Medium energy photon, electron jumps to medium level, energy sometimes turns into heat or chemistry or sometimes back out as light. (Example: sunlight on plant, red and blue into chemical changes and heat, light absorbed, green spit back out as light in all directions scattered.) 15

16 What happens if energy is greater than visible light UV light? Electrons in molecules in skin cells. Many energy levels. Send in EM radiation of different wavelengths. UV light (3) igher energy- jumps up to such high level, electron has enough energy to fly out of molecule, break it up. Result is damage to these molecules!! These molecules are DNA. Cause of SUNBURN 16

17 typical biomolecule in skin cell c c UV! c c before UV hits it 17

18 typical biomolecule in skin cell after exposure to UV c c e c c c c before UV hits it red and heat of sunburn is body working on cleaning out and replacing dead cells. 18

19 UV light-- lots of energy, break apart molecules in skin and damage it. Skin cancer. Same thing but damage DNA molecule without enough damage to kill cell. Maimed DNA- turns cancerous- skin cancer. Which would give the worst sunburn? a photons of green light (Total E =3.2 x 10^-24 J) b. 2 photons of UV light (Total E = 8 x 10^-27 J) c. 100,000 photons of radio waves 19

20 QUESTION: Your image conscience friend is worried about getting a tan in time for bathing suit season. It is still too cold for him to tan outside naturally, and he can not afford the steep prices charged at your local tanning salon, as he is a deadbeat. e decides that he is going to steal a few of the infrared heat lamps from a nearby restaurant and set up a homemade tanning booth inside his apartment. Explain why this is a bad idea. 20

21 Light A photon particle or a EM wave? When do you need to consider light as POTONS: Electronic transition in atom or molecule matches the energy of a photon of the light you are interested in: Interaction of light with dye molecules, Causes for sunburn, Color of light emitted by LEDs Absorption of photon Also need to consider photons if you have so few atom light interactions that you cannot consider average result Isolated atoms or small clusters of atoms, materials thinner than the wavelength of the light. Emission of photon Moving along 21

22 When can you consider light as a LIGT RAY (EM wave) good for raindrop, lenses, particles in paint: 3 Criteria: 1. Atoms in material have no electronic transitions near photon energy of the light. Same as saying material is transparent at that wavelength. Glass transparent in visible, not transparent in UV Water is transparent in visible, not in IR s Atoms Per l e.g. GLASS Physical dimensions of material much bigger wavelength of light, so many, many oscillations between entrance and exit. Spacing of atoms much, much smaller than wavelength of light, so can just look at average response of atom light interaction 22