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The multi-photon induced Fano effect

Date of news: 22 January 2021

An international team of researchers demonstrated that Fano structure may also be induced without need of auto-ionization, by dressing the continuum with an ordinary bound state in any atom by a coupling laser, in this case the Free Electron Laser FELIX, which emits tunable picosecond pulses of high intensity. Using multi-photon processes gives complete, ultra-fast control over the interference. Since the Fano lineshape has both constructive and destructive interference, the laser control opens the possibility of state-selective detection with enhancement on one side of resonance and invisibility on the other. The results are published in Nature Communications.

Nature commm Fano effect

a Here, |g|g⟩ is the ground state, |c|c⟩ (the orange band) is the one-electron continuum, and |a|a⟩ is an auto-ionizing state, i.e., it is discrete and localized, but it has enough total energy that one electron can be kicked out of the atom into |c|c⟩|kω|kω⟩ (dashed blue line) is the state within the continuum that is resonant with excitation from |g|g⟩ by dipole allowed transitions with perturbation Hamiltonian D at frequency ω, indicated here such that |kω|kω⟩ is above the level of |a|a⟩, i.e., positive detuning, Δ. V is an energy-conserving perturbation (such as a configuration interaction) that couples |a|a⟩ with |c|c⟩, and creates a second pathway from |g|kω|g⟩→|kω⟩ via |a|a⟩ and the pathways interfere. As the detuning Δ is tuned across the resonance with |a|a⟩, the interference between the pathways changes sign. Resonance-Enhanced Multi-Photon Ionization (RE-MPI) is similar to the Fano process, involving an intermediate state. In Laser-Induced Continuum Structure (LICS), V is a dipole transition from a bound state |b|b⟩ and now |g|kω|g⟩→|kω⟩ interferes with a third-order process |g|k|b|kω|g⟩→|k⟩→|b⟩→|kω⟩, where |k|k⟩ is a virtual intermediate state and no autoionizing state is involved. In Multi-Photon Induced Continuum Structure (MPICS), interference occurs between two second-order processes |g|b|k2ω|g⟩→|b⟩→|k2ω⟩ and |g|k|k2ω|g⟩→|k⟩→|k2ω⟩, where the latter is a two-photon transition via a virtual intermediate state with Hamiltonian D(2). In degenerate MPICS (used in this work), V = D. b The pump-probe experiment geometry in time and space. The pulse duration tp and delay time t are indicated. c The primary processes in a standard pump-probe experiment (black) and the additional ionization-related processes (blue) of interest here.


The multi-photon induced Fano effect, K. L. Litvinenko, Nguyen H. Le, B. Redlich, C. R. Pidgeon, N. V. Abrosimov, Y. Andreev, Zhiming Huang & B. N. Murdin, Nature Communications 12, 454 (2021)

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Britta Redlich