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-We investigate by near-forward Raman scattering a presumed reinforcement of the (A-C,B-C)-mixed phonon-polariton of a A 1-x B x C zincblende alloy when entering its longitudinal optical (LO-)regime near the Brillouin zone centre Γ, as predicted within the formalism of the linear dielectric response. A choice system to address such issue is ZnSe 0.68 S 0.32 due to the moderate dispersion of its refractive index in the visible range, a sine qua non condition to bring the phononpolariton insight near Γ . The LO-regime is actually accessed by using the 633.0 nm laser excitation, testified by the strong emergence of the (Zn-Se,Zn-S)-mixed phonon-polariton at ultimately small scattering angles.Due to the polarity of the chemical bonding in such a ionic crystal as a zincblende AB semiconductor compound, the long-wavelength (Γ-like, q~0) transverse optical (TO) phonon, corresponding to anti-phase displacement of the intercalated A-like and B-like fcc sublattices (mechanical character), is likely to be accompanied by a macroscopic electric field [1]. The latter is transversal to the direction of propagation, thus identical in nature to that carried by a pure electromagnetic wave, namely a photon. Now, due to the quasi vertical dispersion of a photon at the scale of the Brillouin zone, the electromagnetic character of a TO mode can only emerge very close to Γ. The concerned q values are of the order of one per ten thousands of the Brillouin zone size [2]. At this limit the electromagnetic and mechanical characters combine, conferring on a TO mode the status of a so-called phonon-polariton (PP). For certain q values the PP might acquire a dominant electromagnetic character, thus propagating at lightlike speeds. This stimulates interest in view of ultrafast (photon-like) signal processing at THz (phonon-like) frequencies [3].The ω vs. q dispersion of PP's propagating in the bulk of various AB zincblende compounds have been abundantly studied, both experimentally and theoretically [2,[4][5][6][7][8][9]. In a nutshell, it can be grasped within four asymptotic behaviors, i.e. two photon-like ones (ω-related) and two phonon-like ones (q-related). For large q values, i.e. falling within few percent of the Brillouin zone size, as routinely accessible in a conventional backscattering Raman experiment (schematically operating in a "reflection mode", see below), a transverse electric field cannot propagate at THz (phonon-like) frequencies, because the considered (ω,q)-domain falls far away from the natural dispersion of a photon (quasi vertical). In such so-called q ∞ -regime, a TO mode thus reduces to a purely mechanical oscillator (abbreviated P T hereafter, deprived of electric field), whose frequency, noted , constitutes the first phonon-like asymptote, i.e. away from Γ. The frequency of the nondispersive longitudinal optical (LO) mode, noted ω LO , larger than ω TO [10], defines the second phonon-like

Raman scattering and ab initio Raman/phonon calculations, supported by X-ray diffraction, are combined to study the vibrational properties of Zn1−xBexTe under pressure. The dependence of the Be–Te (distinct) and Zn–Te (compact) Raman doublets that distinguish between Be- and Zn-like environments is examined within the percolation model with special attention to x ~ (0,1). The Be-like environment hardens faster than the Zn-like one under pressure, resulting in the two sub-modes per doublet getting closer and mechanically coupled. When a bond is so dominant that it forms a matrix-like continuum, its two submodes freely couple on crossing at the resonance, with an effective transfer of oscillator strength. Post resonance the two submodes stabilize into an inverted doublet shifted in block under pressure. When a bond achieves lower content and merely self-connects via (finite/infinite) treelike chains, the coupling is undermined by overdamping of the in-chain stretching until a «phonon exceptional point» is reached at the resonance. Only the out-of-chain vibrations «survive» the resonance, the in-chain ones are «killed». This picture is not bond-related, and hence presumably generic to mixed crystals of the closing-type under pressure (dominant over the opening-type), indicating a key role of the mesostructure in the pressure dependence of phonons in mixed crystals.

International audienceWe study the bulk and surface phonon-polaritons of the Zn0.67Be0.33Se zincblende alloy by near-forward Raman scattering. The short (Be-Se) bond exhibits a distinct percolation doublet in the conventional backscattering Raman spectra, corresponding to a three-mode behavior in total [1××(Zn-Se), 2××(Be-Se)] for Zn0.67Be0.33Se. This offers an opportunity to achieve a refined understanding of the phonon-polariton modes of a zincblende alloy beyond the current two-mode approximation, corresponding to a [1××(Zn-Se), 1××(Be-Se)] description in the present case. The discussion is supported by contour modeling of the Raman signals of the multi-mode bulk and surface phonon-polaritons within the formalism of the linear dielectric response

Near-forward Raman scattering combined with ab initio phonon and bond length calculations is used to study the 'phonon-polariton' transverse optical modes (with mixed electrical-mechanical character) of the II-VI ZnSe1-x S x mixed crystal under pressure. The goal of the study is to determine the pressure dependence of the poorly-resolved percolation-type Zn-S Raman doublet of the three oscillator [1  ×  (Zn-Se), 2  ×  (Zn-S)] ZnSe0.68S0.32 mixed crystal, which exhibits a phase transition at approximately the same pressure as its two end compounds (~14 GPa, zincblende  →  rocksalt), as determined by high-pressure x-ray diffraction. We find that the intensity of the lower Zn-S sub-mode of ZnSe0.68S0.32, due to Zn-S bonds vibrating in their own (S-like) environment, decreases under pressure (Raman scattering), whereas its frequency progressively converges onto that of the upper Zn-S sub-mode, due to Zn-S vibrations in the foreign (Se-like) environment (ab initio calculations). Ultimately, only the latter sub-mode survives. A similar 'phonon freezing' was earlier evidenced with the well-resolved percolation-type Be-Se doublet of Zn1-x Be x Se (Pradhan et al 2010 Phys. Rev. B 81 115207), that exhibits a large contrast in the pressure-induced structural transitions of its end compounds. We deduce that the above collapse/convergence process is intrinsic to the percolation doublet of a short bond under pressure, at least in a ZnSe-based mixed crystal, and not due to any pressure-induced structural transition.