Pre-excitation of Rb atoms by frequency chirped laser pulses for efficient plasma generation

G.P. DjotyanWigner Centre for Physics of the Hungarian Academy of Sciences

Contributors: D. Dzsotjan Zs. Sörlei J. Bakos G. Demeter N. Sandor

Outline

Frequency chirped laser pulses and their application in manipulation of inner and translational states of multilevel atoms: some results achieved in the field in the Wigner Centre

Robust pre-excitation of Rb atoms by frequency chirped ultra-short laser pulses for efficient OBI or impact ionization

Over-barrier and impact ionization of excited Rb atoms

Action of a laser pulse with area A = ∫ Ω(t) dt = 5 π

Action of a frequency-chirped laser pulse on a two-level atom

Coherent Population Transfer to a Target State Without Excitation of the Atom:

Interaction of a single frequency chirped laser pulse with L –atom

Dynamics of the populations of the L-atom in the field of single “narrowband” frequency-chirped laser pulse

n2

n3

n1

-3 -2 -1 0 1 2 3TIME

0

0.2

0.4

0.6

0.8

1

SNOITALUPOP

n1

n3n2

G.P. Djotyan, J.S. Bakos, Zs. Sörlei, J.Szigeti, Phys.Rev.A, 70, 063406 (2004)

Creation of superposition between a ground and excited state

G.P. Djotyan, N. Sandor, J.S. Bakos, Zs. Sörlei, Opt. Express, 19, n. 18, 17493(2011)

Electromagnetically self-induced transparency of a single narrowband frequency-chirped laser pulse

The propagation distance X is measured in the units of the absorption lengthG. Demeter, D. Dzsotjan, and G. P. Djotyan.

Phys.Rev.A, 76, 023827 (2007)

Using frequency-chirped pulses in manipulation of translational states of atoms

Experiment: Momentum transfer to Rb atoms by partially overlapping laser pulses

J.S. Bakos, G.P. Djotyan, et al., Eur. Phys. J. D, 39, 59 (2006);

Eur. Phys. J. D 44, 149 (2007)

Population transitions in Rb85 atom (D2 line) induced by frequency-chirped laser pulses

GP. Djotyan, JS. Bakos, G. Demeter, PN. Ignacz, MA. Kedves, Zs. Sorlei, J. Szigeti, Z.Toth, Phys.Rev A, 68, 053409 (2003)

Parameters of the MOT and of the trapped and cooled atomic cloud

• Volume ~ 1 mm3

• Number of atoms ~ 10 7

• Lifetime ~ 500 msec• Temperature ~ 10 -100 μK

Rb transition lines and the ionization scheme

Dynamics of the populations for positive and negative frequency chirp

I0>

I1>

I2>

I3>I4>

I5>

Final populations of the ground state 5s and first excited state

5s

5p

Final populations of the excited states 5d and 7s

5d 7s

ConclusionsFrequency chirped laser pulses may be successfully applied for pre-excitation of Rb atoms before further one-photon or over the barrier ionization by a strong laser pulse, or by the impact ionization. The pre-excitation of a gas of Rb atoms will drastically diminish the threshold of the ionization. Advantages of using frequency chirped laser pulses:i. Robustness of the atomic population transfer against variation of parameters of the laser radiationii. Applicability in both homogeneously and in-homogeneously broadened media, e.g. hot vapors (Doppler-broadening) or solid-state environmentii. Ability to select a target state even in the case of broadband (e.g. ultra-short) laser pulses in the case of spectrally non-resolved manifold of atomic (molecular) energy levels.

Our plans in the field of the theory of laser plasma generation:

-Atomic physics: numerical simulations of interaction of multilevel atoms with ultra-short and intense laser pulses, including ones with phase modulation. Analysis and simulation of different regimes of ionization, including resonance-enhanced multi-photon, tunneling, OBI and impact ionization.

-Simulation of propagation and nonlinear interaction of intense laser pulses in highly nonlinear resonant multi-level atomic media, including pulses with frequency modulation, electromagnetically induced transparency regime, etc.

- At special interest: Propagation and nonlinear self-action effects of intense ultra-short pulses in gaseous media under conditions of plasma generation.