X-ray refractive index of laser-dressed atoms

Abstract

We investigated the complex index of refraction in the x-ray regime of atoms in laser light. The laser (intensity up to 10^13 W/cm^2, wavelength 800nm) modifies the atomic states but, by assumption, does not excite or ionize the atoms in their electronic ground state. Using quantum electrodynamics, we devise an ab initio theory to calculate the dynamic dipole polarizability and the photoabsorption cross section, which are subsequently used to determine the real and imaginary part, respectively, of the refractive index. The interaction with the laser is treated nonperturbatively; the x-ray interaction is described in terms of a one-photon process. We numerically solve the resolvents involved using a single-vector Lanczos algorithm. Finally, we formulate rate equations to copropagate a laser and an x-ray pulse through a gas cell. Our theory is applied to argon. We study the x-ray polarizability and absorption near the argon K edge over a large range of dressing-laser intensities. We find electromagnetically induced transparency (EIT) for x rays on the Ar 1s --> 4p pre-edge resonance. We demonstrate that EIT in Ar allows one to imprint the shape of an ultrafast laser pulse on a broader x-ray pulse (duration 100ps, photon energy 3.2keV). Our work thus opens new opportunities for research with hard x-ray sources.Comment: 12 pages, 10 figures, 1 table, RevTeX4, corrected typo

Transient absorption and reshaping of ultrafast XUV light by laser-dressed helium

“…[18,19,27]. Here we provide a brief account to highlight the essential steps in the derivation and to facilitate a discussion of the other two formalisms.…”

“…The interaction with light is expressed in terms of nonrelativistic quantum electrodynamics [18,19,27]; the free IR laser and XUV fields areĤ EM,L andĤ EM,X , respectively, and the interaction of the atomic electrons with the laser field isĤ I,L . The weak interaction with the XUV light is represented byĤ 1 =Ĥ I,X [19].…”

“…Finally, we diagonalize the involved Floquet-type matrices to obtain the cross section. Without the laser field this is done exactly; when the laser is present we use 4000 Lanczos iterations [18].…”

“…The fundamental problem of a two-color field such as this has been studied before in the context of x-ray absorption by neon [2,7,[15][16][17], argon [18], and krypton [19] atoms. Specifically, the examination of laser-dressed atoms led to the discovery of electromagnetically induced transparency (EIT) for x rays [7], better characterized as Autler-Townes splitting [12] because the transparency is not predominantly caused by destructive interference.…”

Dressing effects in the attosecond transient absorption spectra of doubly excited states in helium

“…. If the spectrum of the XUV pulse is narrower than 2ω VIS , as in [20,46,47], transient absorption only probes the response close to the diagonal ω r = ω e . This provides valuable yet incomplete information on the ultrafast optical response of the dressed system.…”