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Transcript
Announcements: Today: LASERs! • Exam 2 is next week (Thu., 7:30-9:00pm) Topics: TZD, Chapters 3-5, 11.9, 11.10 • Bring your own formula sheet. • No written HW due for next week but reading assignments are due as usual. Now, let's make a gigantic LAZER! What’s so special about LASER light? Reading Quiz A) It doesn't diffract when it goes through two slits. B) All the photons in the laser beam oscillate inphase with each other. C) The photons in the laser beam travel a little bit faster because they all go the same direction. D) Laser light is pure quantum light, and therefore, cannot be described with classical EM theory. E) Laser light is purely classical light, and therefore, it is incompatible with the photon picture. What’s so special about LASER light? Reading Quiz A) It doesn't diffract when it goes through two slits. B) All the photons in the laser beam oscillate inphase with each other. C) The photons in the laser beam travel a little bit faster because they all go the same direction. D) Laser light is pure quantum light, and therefore, cannot be described with classical EM theory. E) Laser light is purely classical light, and therefore, it is incompatible with the photon picture. Spatial coherence What’s so special about LASER light? Remember the word 'coherence'? (We used it occasionally in context with interferometers, diffraction and with photons.) There are two types of coherence: - Spatial coherence - Temporal coherence LASERs produce light with excellent spatial and temporal coherence. Temporal coherence Collimating lens Flashlight (incoherent) Laserpointer (coherent) Flashlight (incoherent) 'Mach-Zehnder' interferometer c ·τ1 Laser (coherent) c ·τ2 For very stable lasers: (τ1- τ2)max ~ 1 second. (This corresponds to 300’000 km path difference!) Remember this one (from class 3): Let's make a coherent light pulp! Bad: Throw away most of the light and individual photons still have random phases φ E-field (for a single color): LASER E(x,t) = A0 sin(ωt+(x ·ω)/c +φ) ω = 2πf E λ A0 φ x Temporal coherence: ω and φ are very well defined constants (i.e. time independent). How can we make identical photons? Aperture Loose more than Color filter (passes only a trillion to one in power!(=bad idea) certain colors, ω) Remember: Coherent light requires that ω and φ are constants. How light interacts with atoms e out Clone them! 2 1 spontaneous emission of light: Excited atom emits one photon. in 2 e 1 absorption of light: Atom absorbs one photon e in 2 out 1 stimulated emission: clone the photon -- A. Einstein Surprising fact: Chance of stimulated emission of excited atom EXACTLY the same as chance of absorption by lower state atom. Critical when making a laser. Laser: Stimulated emission to clone photon many times (~1020/s) Light Amplification by Stimulated Emission of Radiation Spontaneous emission Stimulated emission Random phase Random direction Similar energy (as absorbed photon) Legend: Photon Identical phase Identical energy Identical momentum Legend: Atom in ground state Atom in excited state e Photon Atom in ground state e Atom in excited state e e Excited atom Chance of stimulated emission of excited atom is EXACTLY the same as chance of absorption by ground state atom. Atom in ground state Glass tube below contains 9 atoms. Some are excited some not excited (as shown). Light enters the tube on the left: b. less come out right 3 excited atoms can emit photons, 6 ground state atoms can absorb. Absorption wins. For the condition above: what do you expect? a. More photons will come out (on the right) than go in. b. Fewer photons will come out (on the right) than go in. c. Same number as go in, d. None will come out. Think about statistics / probabilities To increase the number of photons when going through the atoms, more atoms need to be in the upper energy level than in the lower. Need a “Population inversion” (This is the hard part of making laser, b/c atoms jump down so quickly.) How to get population inversion? e A) Use photons with hf < ΔE Nupper > Nlower, more cloned than eaten. ΔE B) Use photons with hf = ΔE excited e not excited C) Use photons with hf > ΔE D) Use very strong lamp with hf ≈ ΔE. E) Will never get population inversion in this system. Nupper < Nlower, more eaten than cloned. No population inversion in 2 level atom! e out 2 1 spontaneous emission of light: Excited atom emits one photon. in 2 e 1 absorption of light: Atom absorbs one photon e in 2 Need at least 3 energy levels! Use a second color of light to create population inversion out 1 stimulated emission: clone the photon 2 2 t2 t1 1 Equal probability Population inversion means: More atoms are in the excited state than in the ground state. As soon as we have the same number of atoms in the excited state as in the ground state, the probability of creating an excited atom is same (or smaller, when considering spontaneous emission) as the probability of having stimulated emission! Can never reach population inversion in 2-level atom! 1 t3t1<>t2t2 3 t1 < t2 2 G G t2 1 “Pumping” process produces population inversion t1< t2 G Optical resonator Ok, if we have 'population inversion' we get 'gain'. But where are these photons coming from? Mirror Partially-silvered mirror out } out From here! Some fraction of the photons are 'recycled' through the amplifier (feedback!). The rest is used as the laser's output. This is done with an 'optical resonator/cavity' Continuously supply energy to the atoms to maintain population inversion. Let's play! Summary Mirror Half-silvered mirror out LASERs need: • Population inversion Gain • Optical feedback (optical resonator) Coherent light http://phet.colorado.edu/simulations/lasers/lasers.jnlp Semiconductor lasers are tiny! But others can be fairly ‘involved’ …or very involved… Various 'flavors' of Lasers Gas lasers All the energy of this laser is focused into this! (Trigger Nuclear fusion.) Dye lasers (liquid) Chemical lasers Solid state lasers Fiber lasers Diode lasers Gas dynamic lasers … Practically unlimited numbers of applications for lasers Just to name a few: Medicine: Surgery (no bleeding, noncontact eye) Diagnostics (Two Photon-Microscopy, tomography) Machining: Tight focus allows very high intensity. (100 W cut through hardened steel like butter) Science: Huge variety of applications (Ultra precise spectroscopy / light matter interaction...) Commercial: DVD/CD, range finding, leveling, telecommunication… Sharks with frickin' laser beams attached to their heads…