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We report the complex dielectric function of the quasi-one-dimensionalchalcogenide Ta$_2$NiSe$_5$, which exhibits a structural phase transition thathas been attributed to exciton condensation below $T_c = 326$ K, and of theisostructural Ta$_2$NiS$_5$ which does not exhibit such a transition. Usingspectroscopic ellipsometry, we have detected exciton doublets with pronouncedFano lineshapes in both the compounds. The exciton Fano resonances inTa$_2$NiSe$_5$ display an order of magnitude higher intensity than those inTa$_2$NiS$_5$. In conjunction with prior theoretical work by E. Rashba, weattribute this observation to the giant oscillator strength of spatiallyextended exciton-phonon bound states in Ta$_2$NiSe$_5$. The formation ofexciton-phonon complexes in Ta$_2$NiS$_5$ and Ta$_2$NiSe$_5$ is confirmed bythe pronounced temperature dependence of sharp interband transitions in theoptical spectra, whose peak energies and widths scale with the thermalpopulation of optical phonon modes. The description of the optically excitedstates in terms of strongly overlapping exciton complexes is in good agreementwith the hypothesis of an EI ground state.

Giant exciton Fano resonance in quasi-one-dimensional Ta$_2$NiSe$_5$