Lasers & Holography. Ulrich Heintz Brown University. 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 1

Transcription

1 Lasers & Holography Ulrich Heintz Brown University 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 1

2 Lecture schedule Date Topic Thu, Jan 28 Introductory meeting Tue, Feb 2 Safety training Thu, Feb 4 Lab 1: Johnson noise and shot noise Tue, Feb 9 Lab 2: Tunneling in thin film superconductors Thu, Feb 11 Measurement and Error Tue, Feb 16 Parameter Estimation Thu, Feb 25 Lab 3: Electron paramagnetic resonance Tue, Mar 1 Lab 4: Pulsed nuclear magnetic resonance Tue, Mar 8 Setting limits (1) Tue, Mar 22 Lab 5: Doppler free spectroscopy Tue, Apr 5 Lab 6: Lasers and holography Tue, Apr 12 Setting limits (2) 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 2

3 Lab schedule Dates Teams A & C Teams B & D Lab reports due Feb 2-15 Feb 16-Mar 1 Mar 2-15 Mar 16-Apr 5 Mar 27-Apr 4 Apr 6-19 Apr 20-May 3 Johnson noise and shot noise Tunneling in thin film superconductors Pulsed nuclear magnetic resonance Electron paramagnetic resonance Spring break Lasers and holography Doppler free spectroscopy Tunneling in thin film superconductors Johnson noise and shot noise Electron paramagnetic resonance Pulsed nuclear magnetic resonance Doppler free spectroscopy Feb 24 Mar 11 Mar 21 Apr 11 Apr 25 Lasers and holography May 9 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 3

4 Lasers and holography Goal of experiment Understand lasers Create your own holograms Hologram Photograph which records intensity and phase of light Requires two temporally and spatially coherent light beams Procedure Build a laser (may not be available) Observe transverse modes of He-Ne laser Use high speed photodetector to measure beat frequency of longitudinal modes Use Michelson interferometer to measure coherence length Use a camera to make some holograms 9/9/2015 Ulrich Heintz - PHYS

5 Laser Light Amplification by Stimulated Emission of Radiation 1964 Nobel Prize in Physics Charles Hard Nicolay Aleksandr Townes Gennadiyevich Mikhailovich Basov Prokhorov "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle". 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 5

6 Interaction of radiation and matter Spontaneous emission Excited state ground state + photon Emmission probability = B (Einstein coefficients) Typical lifetime of excited states 10 8 s E = ħω Absorption Photon + ground state excited state E = ħω Stimulated emission Photon + excited state ground state + 2 photons Photon triggers transition to lower state Both photons in same quantum state (same direction, energy, phase, ) coherent radiation E = ħω 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 6

7 Principle of a laser Photons are all coherent and form intense beam of light E = ħω I N 2 N = number of atoms in excited state 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 7

8 Ordinary light source Photons are all emitted independently of each other Each photon has its own phase and direction I N N = number of atoms in excited state 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 8

9 Preconditions for sustained laser operation Population inversion In thermal equilibrium at room temperature n 2 E = e kt n 1 Therefore n 2 n 1 and probability of emission << probability of absorption For light amplification need n 2 n 1 Metastable state Excited atoms decay in 10 8 s Need atoms with excitations that have half life 10 3 s Multistate system Atoms that fall back to ground state absorb photons Reduced light amplification 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 9

10 Example: ruby laser first working laser made by Theodore H. Maiman at Hughes Research Laboratories in 1960 Gain medium: solid ruby Al 2 O 3 with Cr impurities Red color from absorption bands of red s complementary color (green) 3-level system short lived (10 8 s) spontaneous emission metastable (10 3 s) fully silvered mirror T.H. Maiman, Nature, (1960) partially silvered mirror lasing transition Pulsed laser Optical flash creates population inversion Lasing restores population in ground state flashlamp laser beam 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 10

11 Continuous wave laser 4-level system short lived spontaneous emission metastable lasing transition pumping short lived spontaneous emission ground state Few atoms in this state little absorption of lasing radiation If pumping is continuous there will always be more atoms in metastable state than in shortlived states high efficiency Most powerful laser: National Ignition Facility at Lawrence Livermore Lab 500 TW in very short pulse 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 11

12 Helium-Neon laser Gain medium: 10:1 mixture of He and Ne Pumping He atoms are excited by HV discharge to ev level He atoms transfer energy to Ne by collision Ne atoms are excited to 5s 1 state More atoms with be in 5s 1 state than in 3p 1 state (population inversion) ev 1s 1 2s 1 (metastable) pumping through HV discharge collisions ev 2p 5 5s 1 lasing transition λ = nm ev 2p 5 3p 1 spontaneous emission Helium 0 1s 2 spontaneous emission Neon 2p 5 3s 1 2p 6 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 12

13 Why laser? A laser is not a very efficient device Typical efficiency of lab laser: Input power = W Output power = few mw Efficiency ε However the output is a coherent beam of light with high intensity A beam of power 10 mw and a diameter of 1 mm has an intensity I = 10 2 W π 10 6 m 2 3 kw m 2 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 13

14 Optical oscillator Setup of He-Ne laser in lab fully silvered mirror partially silvered mirror Brewster angle: angle of incidence at which light polarized in the plane of incidence is transmitted without reflection No loss as light traverses glass of He-Ne tube Light is reflected by mirrors In the optical cavity between the mirrors the laser photons form a standing wave 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 14

15 Optical profile of laser light Homogeneous line broadening Same conditions everywhere in the material Result in Lorentzian line shape Examples are half life τ from spontaneous emission and collisions between atoms (pressure broadening) Line width E = ħ (Heisenberg s uncertainty principle) τ Uncertainty in photon frequency Δν = 1 Inhomogeneous line broadening Conditions vary across sample Result in Gaussian line shape Examples are Doppler broadening and Strain variations in a solid Typically population inversion is achieved over a wide frequency band (GHz) However, the actual frequencies of the laser are determined by the frequencies for which standing waves can exist in the optical resonator 2πτ 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 15

16 Longitudinal modes Standing waves can only exist for wave lengths λ L = n λ 2, n = 1,2,3, L = distance between mirrors These are called longitudinal modes 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 16

17 Coherence length Distance l over which phase changes by π 2 l Δλ = 2l Observe beats in laser intensity with wavelength Δλ = 2l 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 17

18 Transverse modes Waves that travel at a small angle to the mirror axis such that they bounce off the cavity lead to transverse modes Laser with cylindrical symmetry Laser with rectangular symmetry 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 18

19 Holography Ordinary photographs record only the intensity (and color) of the light Holograms also record the relative phase of the light Invented in late 1940s by Dennis Gabor 1971 Nobel Prize in Physics Dennis Gabor "for his invention and development of the holographic method" 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 19

20 reference beam laser Setup to create hologram object beam mirror lens photographic film records interference pattern of object and reference beams 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 20

21 Setup to view hologram lens photographic film recreates object beam 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 21

22 Hologram Life Magazine Hologram by Upatnieks, Juris; Leith, Emmett N.; Goro, Fritz 1966 MIT museum 4/5/2016 Ulrich Heintz - PHYS 1560 Lecture 10 22