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Applied Physics Research

Symmetric Fano Profiles of Two-photon Electronic Transitions Observed in an Ultracold Rydberg Gas

Jianing Han, Brianna Provencher, Juliet Mitchell, Chandler Steade, Na Gong, Jinhui Wang

Published Nov 1, 2023• 0 citations
Publisher

Abstract

Fano profiles are caused by the interference between the discrete states and continuum. It is a very common phenomenon in multi-electron Rydberg states, such as the Rydberg states of group II elements in the periodic table. It often leads to autoionization due to the fact that a discrete state couples with an autoionizing state. Unlike the typical asymmetric line shape, here we report a symmetric unusual Fano line shape that has been predicted. In addition, this profile is caused by molecular states or polymer states instead of multi-electron atomic states. Moreover, it is shown that the fano profile caused by the multi-electron polymer states does not necessarily lead to autoionization. We propose that the Fano profile studied is caused by the interference between discrete levels and the multipole-multipole coupled continuum, or energy band.

DOI
10.5539/apr.v15n2p173

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Fano profiles are caused by the interference between the discrete states and continuum. It is a very common phenomenon in multi-electron Rydberg states, such as the Rydberg states of group II elements in the periodic table. It often leads to autoionization due to the fact that a discrete state couples with an autoionizing state. Unlike the typical asymmetric line shape, here we report a symmetric unusual Fano line shape that has been predicted. In addition, this profile is caused by molecular states or polymer states instead of multi-electron atomic states. Moreover, it is shown that the fano profile caused by the multi-electron polymer states does not necessarily lead to autoionization. We propose that the Fano profile studied is caused by the interference between discrete levels and the multipole-multipole coupled continuum, or energy band.
Photon absorption spectroscopy is a powerful tool for uncovering the structure of atoms, molecules, and solids. Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures related to the structural and dynamical properties. Recently, Ott et al. [Science 340, 716 (2013)] successfully transferred Fano profile into Lorentzian line shape using an intense infrared laser, after excitation of autoionizing states in helium by attosecond XUV pulse. This is a very important step forward in quantum phase control. However, here we show experimentally that an autoionizing state can have both Fano and Lorentzian behavior naturally, depending on the process involved. This study utilized the inverse process of photon absorption ionization, i.e., electron ion recombination with photon emission, making sure the resonant autoionizing state is not modified or decorated by the laser fields. Our result implies that excitation of the state through different paths---for example, one photon versus multiphoton excitation, or even one step versus multistep excitation---can lead to different Fano profiles for the same resonant state. We also report an experimental determination of the energy shifts in the recombination photon-intensity peaks due to the interference between the resonant and nonresonant processes.
The ordinary Fano effect occurs in many-electron atoms and requires an autoionizing state. With such a state, photo-ionization may proceed via pathways that interfere, and the characteristic asymmetric resonance structures appear in the continuum. Here we demonstrate 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. Using multi-photon processes gives complete, ultra-fast control over the interference. We show that a line-shape index q near unity (maximum asymmetry) may be produced in hydrogenic silicon donors with a relatively weak beam. 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. We discuss a variety of atomic and molecular spectroscopies, and in the case of silicon donors we provide a calculation for a qubit readout application.
We know that the Fano profile is obtained for single-photon absorption to the ionization continuum in the presence of an autoionizing state. We have shown that, in the case of (1+1)-photon absorption to an autoionizing state embedded in the ionization continuum, the absorption profile changes radically to a mirror image of the Fano profile for the value of q=\ifmmode\pm\else\textpm\fi{}1, q being the Fano q parameter. Moreover, for higher values of q, the (1+1)-photon absorption profile exhibits a broad minimum, while a distinct interference pattern is obtained in the profile of single-photon absorption. The absorption line shape has been found to oscillate between the Fano and a modified Fano profile for the autoionization through odd and even number of steps, respectively, via resonances.
Fano resonance is a well-celebrated weak-coupling phenomenon whereby a small interaction between a discrete and a continuum of excited states leads to an interference between the respective two transition amplitudes, resulting in an asymmetric line shape in the excitation spectra. We present the strong-coupling phase of the Fano's model where the interaction is localized in a region $\ensuremath{\xi}$ that is smaller than the collective coupling between the discrete state and the continuum. We explore the behavior of the excitation probability $\ensuremath{\sigma}$ as a function of $\ensuremath{\xi}$, the ratio of the bare transition amplitudes, and the energy of the discrete state. In the strong-coupling phase, the spectral function of the discrete state splits up and produces up to three complete destructive interferences or zeros of $\ensuremath{\sigma}$ along with ultrasharp features. We also find that the discrete state does not have to lie within the continuum to produce the well-known weak-coupling profiles of $\ensuremath{\sigma}$ as long as the collective coupling is strong enough. In addition, we consider a broadened state instead of a continuum and present a method that exactly treats such cases by explicitly including the source of the broadening---a weak coupling to the continuum of environment states---in the model.
Spectroscopic signatures associated with symmetric Lorentzian and asymmetric Fano line shapes are ubiquitous. Distinct features of Fano resonances in contrast with conventional symmetric resonances have found several applications in photonics such as optical switching, sensing, lasing, and nonlinear and slow-light devices. It is therefore important to effectively generate and control these resonances. In this work, we show through ab initio simulations of coupled light–matter systems that Fano interference phenomena can be realized in a multimode photonic environment by strong coupling to the electromagnetic continuum. Specifically, we show that by effectively enhancing the light–matter coupling strength to the photon continuum in an experimentally feasible way, we can achieve a transition from Lorentzian to Fano lines shapes for both electronic and polaritonic excitations. An important outcome of switching between these spectral signatures is the possibility to control the Purcell enhancement of spontaneous emission alongside electromagnetically induced transparency, which is a special case of Fano resonances. Switching from Fano back to a Lorentzian profile can be achieved by physically reducing the coupling strength to the continuum of modes. Our results hold potential for realizing tunable Fano resonances of molecules and materials interacting with the electromagnetic continuum within multimode photonic environments.
The Fano resonance, with a unique asymmetric line shape, is a well-known manifestation of quantum interference with numerous promising applications for light-harvesting devices. Here we report an experimental observation of the Fano effect in an ultracold atom-molecule coupled system realized by photoassociation near a magnetically tunable ddd-wave Feshbach resonance. Our result shows that, compared to ubiquitous single-peak Fano-type line shapes, the coupled Fano resonances feature a two-peak profile. We develop a theory of coupled Fano resonances which agrees very well with the experimental results. This work may open a new perspective for tuning ultracold collisional interaction using the Fano resonance and studying bound states in continuum.