Near-forward Raman selection rules for the phonon-polariton in (Zn, Be)Se alloys

Abstract

The Raman selection rules of the (Zn-Se,Be-Se)-mixed phonon-polariton (PP) created by "alloying" in the three-mode [1x(-Zn-Se),2x(Be-Se)] Zn 1-x Be x Se system, whose dramatic S-like dispersion (∼200 cm -1 ) covers the large frequency gap between the Zn-Se and Be-Se spectral ranges, is studied in its wave vector ‫)ݍ(‬ dependence by near-forward scattering. Both the collapse regime away from Γ and the reinforcement regime near Γ are addressed, using appropriate laser lines and Be contents. We find that in both regimes the considered PP, in fact a transverse mode with mixed mechanical-electrical character, obeys the same nominal Raman selection rules as its purelymechanical (PM) variant commonly observed in the backscattering geometry. Besides, marked differences in the PP Raman lineshapes in the two regimes give a hint about how the PP-like electrical field ‫ܧ‬ ሬԦ develops while descending the S-like dispersion towards Γ. In the reinforcement regime ‫ܧ‬ ሬԦ is large, leading to intra-mode on top of inter-mode ‫ܧ‬ ሬԦ −mediated transfers of oscillator strength between the two Be-Se modes, that both exhibit a fine structure on account of the alloy disorder. In contrast, in the collapse regime ‫ܧ‬ ሬԦ remains weak, as testified by the absence of intramode transfer. The discussion is supported by contour modeling of the multi-PP Raman lineshapes in their ‫-ݍ‬dependence within the linear dielectric approach.

Multi-phonon (percolation) behavior and local clustering of CdxZn1−xSe-cubic mixed crystals (x ≤ 0.3): A Raman–ab initio study

“…Such patterns are fairly well reproduced with Cd 0.075 Zn 0.925 Se-c but not with Cd 0.170 Zn 0.830 Se-c. The significant deviation at x ¼ 0:170 is not due to the increased alloy disorder as compared to x ¼ 0:075, since the TO modes of the alternative ZnSe-based Zn 1−x Be x Se-c mixed crystal ideally obey the nominal Raman selection rules even at x 0:50 (corresponding to the maximum alloy disorder), 47 though the contrast in bond physical properties (bond length and covalency) is larger than with Cd x Zn 1−x Se. 11,70 The deviation is rather attributed to the persisting Fano-like interference of TO Zn Zn-Se with the 2A-band nearby, which cannot be fully suppressed.…”

“…The TO and PP Raman selection rules are studied at the same sample spot, following the procedure described in Ref. 47. The polarized backward/forward Raman spectra are labeled by using Porto's notationk i (ẽ i ,ẽ s )k s .…”

“…and parallel (k) polarizations depending on the azimuth angle α ¼ (ẽ i ,X) at the sample surface are compared to the corresponding theoretical (X,Ỹ)-centered Greek crosslike and butterflylike patterns, respectively. 47 In the reported data, care was taken that the Raman signals at the start (α ¼ 0 ) and at the end of a complete revolution (α ¼ 360 ) are (nearly) the same, meaning that the laser is stable throughout the experiment. When needed, the scattering angle θ ¼ (k i ,k s ) inside the crystal can be varied for a given (ẽ i ,ẽ s ) polarization setup in the near-forward scattering geometry by slightly departing from the normal incidence (θ ¼ 0 ) of the laser beam at the rear crystal face.…”

“…A refined contour modeling of the TO, PP, and LO Cd x Zn 1−x Se Raman lineshapes is achieved by using the generic expression of the RCS given in Ref. 47. Crude approximations for the TO (and LO) modes used by Kozyrev 13,18,29 (see Sec.…”

“…Each Lorentzian is characterized by several parameters: its resonance at the p-related TO frequency, noted ω TO,p (x); its damping γ p (x) (sample-dependent), which governs the full width at half maximum of all p-related Raman lines; and its oscillator strength S p (x), which fixes the magnitude of the p-related TO-LO splitting and also the Raman intensities of all p-type optic modes. The p-related Faust-Henry coefficient C FH,p (x) (ratio of the lattice to the electronic contributions to the polar vibrational modes), which comes as a weighting factor of the main ε r (ω, x)-related term in the RCS, 47 also contributes to the p-related Raman intensities.…”