Electromagnetically Induced Transparency for X Rays

Abstract

Electromagnetically induced transparency (EIT) is predicted for x rays in laser-dressed neon gas. The x-ray photoabsorption cross section and polarizability near the Ne K edge are calculated using an ab initio theory suitable for optical strong-field problems. The laser wavelength is tuned close to the transition between 1s −1 3s and 1s −1 3p (∼ 800 nm). The minimum laser intensity required to observe EIT is of the order of 10 12 W/cm 2 . The ab initio results are discussed in terms of an exactly solvable three-level model. This work opens new opportunities for research with ultrafast x-ray sources.PACS numbers: 32.30. Rj, 32.80.Fb, 32.80.Rm, 42.50.Hz In a Λ-type medium characterized by atomic levels a, b, and c with energies E a > E b > E c , resonant absorption on the c → a transition can be strongly suppressed by simultaneously irradiating the medium with an intense laser that couples the levels a and b. This phenomenon is known as electromagnetically induced transparency (EIT) [1,2,3]. EIT enables one to control the absorption and dispersion of a gaseous medium. It has become a versatile tool for creating media with exceptional optical properties [4,5,6,7]. EIT forms the basis of a recent proposal for a high-accuracy optical clock [8].In this Letter, we study EIT for x rays. Specifically, we consider the near-K-edge structure of neon gas in the presence of a linearly polarized 800-nm laser with an intensity of 10 13 W/cm 2 . The decay widths of excited states involved in EIT in the optical regime typically do not exceed ∼ 10 −4 eV (see, e.g., Ref.[2]) and are often much smaller. In contrast, the decay width of core-excited neon, Γ 1s = 0.27 eV [9], is larger by a factor of ∼ 2000. Therefore, as we will show below, the intensity of the coupling laser must be extraordinarily high. This represents the first case of EIT where the laser causes strong-field ionization of the two upper levels.Because of their high binding energy, the core and valence electrons of Ne remain essentially unperturbed at 10 13 W/cm 2 . (Multiphoton ionization of Ne in its ground state is negligible.) However, laser dressing of the core-excited states introduces strong-field physics: For the laser parameters employed here, the ponderomotive potential [10] is U p = 0.60 eV, and the energy needed to ionize, for instance, the 3p electron in the 1s −1 3p state is I 1s −1 3p = 2.85 eV (1s −1 3p denotes the state produced by exciting a 1s electron to the 3p Rydberg orbital). Hence, the Keldysh parameter [11] is γ = I 1s −1 3p /(2 U p ) = 1.5. For γ ≪ 1, the atom-field interaction can be described in the adiabatic tunneling picture; the perturbative multiphoton regime is indicated by γ ≫ 1. In our case, where γ ≈ 1, the nonadiabatic strong-field regime persists; neither perturbation theory nor a tunneling description is adequate. It should also be noted that the transition energy between, e.g., the states 1s −1 3s and 1s −1 3p is 1.69 eV, which is, within the decay width of the core-excited states, in one-photon resonance with the 1.55-eV la...

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