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Transcript
AC Stark Effect Travis Beals Physics 208A UC Berkeley Physics (picture has nothing whatsoever to do with talk) What is the AC Stark Effect? Caused by time-varying (AC) electric field, typically a laser. Shift of atomic levels Mixing of atomic levels Splitting of atomic levels (another pretty but irrelevant picture) DC Stark Shift |2, 0, 0! |2, 1, −1" |2, 1, 0! |2, 1, +1! Constant “DC” electric field Usually first-order (degenerate) pert. theory is sufficient DC Stark Effect can lift degeneracies, mix states ! Hstark =p·E = −eẑE = −eEr cos θ |2, 1, 0! − |2, 0, 0! √ 2 |2, 1, −1" |2, 1, +1! |2, 1, 0! + |2, 0, 0! √ 2 Hydrogen n=2 levels AC/DC: What’s the difference? (highly relevant picture) AC →time-varying fields Attainable DC fields typically much smaller (105 V / cm, versus 1010 V / cm for AC) AC Stark Effect can be much harder to calculate. One-level Atom Monochromatic variable field Atom has dipole moment d, polarizability α. Thus, interaction has the following form: 1 2 2 Hint = −dF cos ωt − αF cos ωt 2 Now, we solve the following using the Floquet theorem: dΨ i = Hint Ψ dt One-level Atom (2) Get solution: ➊ Ψ(r, t) = exp(−iEa t) k=∞ ! Ck (r) exp(−ikωt) k=−∞ ➋ 1 2 with Ea (F ) = − αF , 4 " # " # ∞ 2 ! αF dF k Ck = (−1) JS Jk+2S 8ω ω S=−∞ AC Stark energy shift is Ea, kω’s correspond to quasi-energy harmonics One-level Atom (3) Weak, high frequency field: dF << ω, αF 2 << ω Arguments of Bessel functions in ➋ are small, so only the k=S=0 term in ➊is significant. Quasi-harmonics not populated, basically just get AC Stark shift Ea One-level Atom (4) Strong, low-frequency field: 2 dF >> ω, αF << ω Bessel functions in ➋ kill all terms except S=0, and k=±dF/ω Only quasi-harmonics with energies ±dF are populated, so we get a splitting of the level into two equal populations One-level Atom (5) Very strong, very low-frequency field: 2 dF >> ω, αF >> ω Only populated quasi-energy harmonics are those with 2 dF αF k!± ± ω 4ω Thus, have splitting of levels, get energies αF 2 αF 2 E(F ) = ±dF ± − 4 4 Multilevel AC Stark Effect width of excited state transition co-efficient: μij = cij ||μ|| 2 ! c 3πc Γ ij I ∆Ei = 3 2ω0 δij 2 electronic ground state |gi> shift excited state energy: ħω0 intensity detuning: δij = ω - ωij Assumptions & Remarks Used rotating wave approximation (e.g. reasonably close to resonance) Assumed field not too strong, since a perturbative approach was used Can use non-degen. pert. theory as long as there are no couplings between degen. ground states In a two-level atom, excited state shift is equal magnitude but opposite sign of ground state shift AC Stark in Alkalis (a) 2 P3 F’=3 , ! HFS 2 0 , ! FS 2 P1 F’=2 F’=1 2 " I = 3/2 F=2 2 S1 ! HFS 2 F=1 (Figure from R Grimm et al, 2000) (b) 2 πc Γ Udip (r) = 2ω03 ! (c) 2 +L’=1 PgF mF 1 − PgF m J’= F + J’= ∆2,F ∆1,F 3 1 L=0 J= 1 2 2 2 " I(r) AC Stark in Alkalis (2) laser polarization 0: linear, ±1: σ± 2 πc Γ Udip (r) = 2ω03 Landé factor ! 2 + PgF mF 1 − PgF mF + ∆2,F ∆1,F " I(r) detuning between 2S1/2,F=2 and 2P3/2 detuning between 2S1/2,F=1 and 2P1/2 F, mF are relevant ground state quantum numbers What good is it? Optical traps Quantum computing in addressable optical lattices — use the shift so we can address a single atom with a microwave pulse References N B Delone, V P Kraĭnov. Physics-Uspekhi 42, (7) 669-687 (1999) R Grimm, M Weidemüller. Adv. At., Mol., Opt. Phys. 42, 95 (2000) or arXiv:physics/9902072 A Kaplan, M F Andersen, N Davidson. Phys. Rev. A 66, 045401 (2002)