Ionization of Rydberg atoms in Intense, Single-cycle THz field

Transcription

1 Ionization of Rydberg atoms in Intense, Single-cycle THz field 4 th year seminar of Sha Li Advisor: Bob Jones Dept. of Physics, Univ. of Virginia, Charlottesville, VA, April. 15 th, 2013

2 Outline Outline Background Introduction of Rydberg atoms Brief review of several typical types of field ionization My work Intense THz generation via optical rectification THz streak THz ionization of low-lying Rydberg atoms THz ionization of Rydberg stark states Future Plan Electron Scattering

3 Rydberg atoms Energy (a.u.) Hydrogen-like atoms Rydberg atoms Valance electron at highly excited orbit, with large principle quantum number n Experience effectively +1 net charge, can be considered as one electron system, but due to finite core size, QUANTUM DEFECT Sketch of a Kepler orbit with low angular momentum 1 En 2( n nl, ) 1 E n 1 E n 3 n 2 r T 3 2 n 2 n 1 3 n 3 2 n=6 n= eV 0.06eV 0.13eV 0.008eV 54a 0 338a ps 0.5ps 30THz 2THz n=6 n= Z (a 0 ) Hydrogen coulomb potential and energy diagram

4 Examples of Field ionization Examples of Field ionization ω Field ~ω atom

5 Examples of Field ionization Field ω F >ω atom Half Cycle Pulse: impulsive regime Time F 1 n 2 ~ 1 n R. R. Jones, D. You, and P. H. Bucksbaum, Phys. Rev. Lett. 70, (1993)

6 Examples of Field ionization Field ω F < ω atom, static or quasi-static field. Classical field ionization. Time F = 1 Cn 4 T. F. Gallagher, Rydberg Atoms, Cambridge University Press (1994)

7 Field Examples of Field ionization ω F < ω atom, Multi-cycle Microwave ionization. n=29 n=28 n=27 n=26 n=25 n=24 n=23 Energy Classical Ionization Limit n=22 Time n=21 n= Field (V/cm) F = 1 3n 5 P. Pillet, T. F. Gallagher, et.al Phys. Rev. A 30, (1984)

8 THz ionization of low-lying Rydberg atoms Field Our Experiment Low-lying n levels (n=6-15), which require very high field for ionization Oscillating field, but single cycle Very fast pulsed field T~4ps compared to ns or μs scale pulsed field usually used. And very strong peak field strength ~500 kv/cm n=6 n= Z (a 0 ) But still relatively slow (ω F < ω atom ) compared to the Kepler period of the Rydberg atoms studied: T (n=6-15) range from 0.03ps~0.5ps Time

9 THz ionization of low-lying Rydberg atoms Field Relatively slow: ω F < ω atom Absolutely fast: ps scale F 1 n? Time

10 THz generation Characteristics of THz radiation Frequency: ν = 1 THz = Hz Period: τ = 1/ν = 1 ps = S Wavelength: λ = c/ν = 0.3 mm = 300 μm Wavenumber: 1/λ = 33.3 cm 1 Photon energy: ħω = 4.14 mev Temperature: T = hν/kb = 48 K Xi-Cheng Zhang, Jingzhou Xu, Introduction to THz Wave Photonics, Springer (2009) Yun-Shik Lee, Principles of Terahertz Science and Technology, Springer (2008)

11 THz generation THz generation via Optical Rectification χ 2 Phase matching LiNbO3 Second order nonlinear effect Difference-frequency mixing among the spectral components contained within the ultrashort pulse bandwidth v g pump v phase THz Large nonlinear coefficient Less THz absorption

12 THz generation Tilted-Pulse-Front Pumping J. Hebling, Keith A. Nelson, et.al JOSA B, Vol. 25, Issue 7, pp. B6-B19 (2008)

13 THz generation Field strength (arb. unit) Electro-Optic Sampling n2 1/ 2{1, 1, 2} n1 1/ 2{ 1,1, 2} n n n n 1 0 n 2 0 n n0 41E 2 3 n0 41E 2 THz THz d ( n n ) E c 1 2 THz n3 1/ 2{ 2, 2, 0} time(ps)

14 THz Streak THz Streak e- p t F() t dt e- e-

15 THz Streak: Experimental setup THz Streak: Experimental setup nm Na beam Na+ & e- THz 390nm 150fs blue Pump LiNbO3 3p 3s 589.8nm Polarizers THz 390nm 150fs blue Delay nm 390nm 150fs blue Time

16 THz Streak: Result THz Streak: Result

17 THz ionization of low-lying Rydberg atoms THz ionization: Experimental Setup

18 THz ionization of low-lying Rydberg atoms Ionization Probability Ion yield curve THz field (kv/cm) 10% n=6 n=7 n=8 n=9 n=10 n=11 n=12 n=13 n=14 n=15

19 THz ionization of low-lying Rydberg atoms THz Field (kv/cm) New Scale Law: F 1!!! n Experimental * CMC Simulation 50 F = 1 16n n

20 THz ionization of low-lying Rydberg atoms THz Field (kv/cm) Experimental * CMC Simulation 50 F = 1 16n n n=15 n=6

21 Analyze electron energy distributions Electron energy distributions at Max THz field (470 kv/cm)

22 Probability Analyze electron energy distributions Ionization Probability ΔP Qualitative analysis d d THz Field (kv/cm) E-3 1E d 6d Energy (ev) p Time t F() t dt

23 Field Analyze electron energy distributions Time

24 Analyze electron energy distributions Ponderomotive Energy Up: The cycle averaged quiver energy of a free electron in an oscillating electric field: Up = e2 F 2 4mω 2 = F2 4ω 2 (a.u.) 2Up: The max energy a free electron can get from a oscillating electric field that slowly decreases in aptitude: Max energy=2up What if the field have only a few cycles, or even one single cycle?

25 Analyze electron energy distributions ev ~0.25THz and ~322 kv/cm F(t)=F 0 sin(ωt) ΔP = F t dt t0 E = (2P 0 P + ( P) 2 )/2 Up = F2 4ω ev Max energy~ 5. 4 Up!!!

26 Energy (a.u.) THz ionization of Rydberg Stark states What if the Rydberg atoms are initially prepared in Stark states? 11p 10d F = 1 Cn 4 11s Field (kv/cm) Na n=10, m=0 Stark manifold

27 Field (arb. unit) THz ionization of Rydberg Stark states Na+/e- elec. ion +/ elec. ion nth stark level

28 THz ionization of Rydberg Stark states Landau-Zener Transition Simplest case: 2-level H(F)= E 1(F) 1 2 E 1 2 E E 2(F) E 2 E + E E + E 1 E E: coupling/quantum defect P dia =e 2πΓ E 1 E E E 2 Γ= (1 2 E)2 = ( 2 E)2 de1 dt de2 dt de1 F 1 df de2 df F

29 THz ionization of Rydberg Stark states THz field E + E + E E F 0 E F E + E + t E E F 0 E F

30 THz ionization of Rydberg Stark states What may influence the result? Aptitude of static field Direction of static field (Relative to THz) Initial stark level Asymmetry of THz and THz slew rate

31 THz ionization of Rydberg Stark states Multi-level Landau-Zener Transitions () t i H( t) ( t) t ien ( t) Cn( t) n( t) e n Spherical basis n, l, m > : Ψ n, E n ( t ) t i E fn i C ( t) C ( t) H ( t) e f n fn n Parabolic/Diabatic basis n, k, m > : Ψ n, E n (t) i C ( t) C ( t) H ( t) e f n fn n Adiabatic/Local basis : (no good quantum #) t 0 i E Ψ n (t), E n (t) fn t ( t ) dt

32 THz ionization of Rydberg Stark states Still working on numerical simulation codes!

33 Future plan Future plan Electron Scattering Electron been dragged back by THz to the nucleus and knocks off more electrons.

34 Conclusion Conclusion Intense THz generation via optical rectification Can generate THz with peak field strength as high as 500 kv/cm THz streak & Electron energy distributions Can measure THz waveform inside the chamber. Electrons from ionization of lower n states have higher energies Electron energy distribution shows the asymmetry of the field In THz streak, have electrons with energy exceed 2Up. THz ionization of low-lying Rydberg atoms and Rydberg stark states Ionization of low-lying Rydberg atoms: Scales as 1/n^3 Ionization of Rydberg stark states: Shows the asymmetry and fast property of THz field.

35 Thank you!

36 Field Analyze electron energy distributions Time

37 THz ionization of Rydberg Stark states Numerical Simulation: 2 -Level Γ= (1 2 E)2 = ( 2 E)2 de1 dt de2 dt de1 F 1 df de2 df Initially in state, with a linear ramp F = kt from very large negative time to very large positive time. work in 1>, 2> basis P1 P1 500k 50k 10k 5k k E 10 E E 2 E + E E + E 1 25 E E 50 E E 1 E E E E F t t +

38 THz ionization of low-lying Rydberg atoms signal/pi (arb. u.) 8.0E E E E E E E E E Energy/n^3 arb. u. 3s-nd excitation prob. curves 6d/ d/ d/ d/ d/1 sig i i. Pe PN i 3s E Pe 3 n sig P N P sig P E 3 n E n N e 3s 3 3s

39 Analyze electron energy distributions Time of Flight (ToF) spectrometer MCP md 2 2d1eU d2 t t0 ( v0 v0 ) eu md 2 2d1eU v0 md d2 T (ns) v0 d d1 U -U (V)