Phys 570V: Advanced Topics in Optics and Photonics Professor Tongcang Li
Lecture 20: Laser cooling of atoms
Course website: http://www.physics.purdue.edu/academic- programs/courses/course_detail.php?SEM=fall2015&c=phys570V Syllabus, Lecture notes, etc.
Purdue University Fall 2015 Physics 570V Room: Phys 331 Time: MW 2:30-3:45 PM 1 Take-home exam: due on Wed., 11/11/2015
Final presentations: 11/30-12/9 • 12-minute talk + 3 minute Q&A. • The content of the presentation can be about any recent (after 2000) development in optics and photonics. It can be about one particular paper, an overview of a research topic, or your own research if it is related to this course. • After each presentation, every student (including the speaker) will give a grade (0- 10) to it. The instructor will also grade it. The final grade of the presentation will be the average of the grades given by students (50%) and the instructor (50%).
• Presentation sequence: to draw lots on Nov. 11
2 Last lecture: Radiation Pressure
ℎ 퐸 Photon momentum: 푝 = = 휆 푐 푑푝 푑 퐸 푃 퐹 = = = for absorbed light 푑푡 푑푡 푐 푐 2푃 퐹 = for reflected light 푐
If we reflect 100W of light, 퐹~7 × 10−7 N
3 Last lecture: optical tweezers
Rayleigh approximation
4 Our vacuum optical tweezer at Purdue University
Nanodiamond (100 nm)
5 This lecture: laser cooling of atoms
6 Laser cooling
2005 1981 2012 1989
D.J. Wineland, H. Dehmelt: Bull. Am. Phys. SOC. 20, 637 (1975) Neutral atoms Ions
Laser cooling/trapping
1997 Steven Chu, Claude Cohen-Tannoudji, William D. Phillips
Atomic Bose-Einstein condensation (BEC) Eric A. Cornell, Wolfgang Ketterle, Carl E. Wieman 2001 7 Laser cooling: basic idea
Absorption-emission
8 Doppler cooling
force
(Doppler limit)
9 10 11 Zeeman slower
Wikipedia 12 Optical Molasses
(Doppler temperature) (240 µK for Na atoms) 13 Sub-Doppler cooling
14 k 2 Recoil Velocity: vR m mL
1 (k)2 Recoil Energy: E mv2 R 2 R 2m
= 2.95cm/s For sodium atoms in 589nm laser: vR 1 E = 2 25.0 kHz = k 2.4 µK R 2 B
15 Magneto Optical Trap (MOT)
Nature Nanotechnology, 8, 317–318 (2013) 16 Magnetic trap H B
(BmF gF )Bz 1 For sodium atoms in the state: Fm1, 1, g F F 2 V
Atoms can be trapped in a local minimum of magnetic field. x,y z
17 Evaporative cooling
Waferboard / Flickr
18 Bose-Einstein Condensate
19 Vacuum Chamber 1.1∙10-11 Torr 3∙10-9 Torr 800 m/s 30 m/s
6∙10-11 Torr
20 Creation of a Bose-Einstein condensate
800 m/s 30 m/s
24 cm/s
Na BEC 21 Signatures of BEC
• Bimodal distribution • Anisotropic expansion • Matter interference • superfluidity
Cornell/Weiman
ketterle 22 Atom Optics & Atom Laser
23 Atoms in an optical lattice
24 Creating a periodic potential
Magnetic BEC Waveguide
Mirror
532nm Laser Waist: 120 µm Max Power: 1 W Optical Lattice
Well Depth: 0 – 22 ER
YAG Tweezer (1064nm) Waist ≈ 180 µm Trap Depth ≈ 4 E R 25 Single particle in a periodic potential
2 V(z) V0 sin (kL z) V0
1. Classical: Trapped in a well if V0 Eatom 2. Quantum: Forms a band structure.
ikRn (r Rn ) e (r)
26 Expansion in a periodic potential Dilute Thermal Atoms
V0 2.24ER 6.5K
11.8 ms 22.8 ms 33.7 ms
The expansion of thermal atoms (T=0.52 µK) is in excellent agreement with the single-particle model. 27 Quantum phase transition from a superfluid to a Mott insulator in a gas of ultracold atoms
Nature 415, 39-44 (3 January 2002) 28 Next lecture: Optical refrigeration of solids
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