The coupling of longitudinal-optic (LO) phono ns and free-carrier collective excitations by "macroscopic" longitudinal electric fields in polar semiconductors has been treated theoretically by a number of investigators. 1 "" 9 In the absence of a magnetic field, the coupled modes involve plasmons (co =oop) as the freecarrier collective excitations, and LO phonons (W = CI>LO)' Tne cou Pl ec * modes are longitudinal in character and, as such, cannot interact with electromagnetic (EM) radiation in bulk samples or in thin films under normal incidence conditions. An interaction of the coupled modes with EM radiation can, however, be observed in thin-film, oblique-incidence transmission experiments 8 and also in Raman scattering experiments on semiconductors lacking a center of inversion. 5 ? 10 In the presence of a magnetic field H 0 there are two types of coupled modes, depending on the direction of the wave vector q of the modes relative to the direction of the magnetic field. One type involves jplasmons and LO phonons with q parallel to H 0 ; the other type involves collective cyclotron excitations {oo 2 = oo c 2 + oop 2 ) and LO phonons with q tilted away from H 0 . u Because of the requirement of momentum conservation, EM radia-tion will interact only with those modes having q parallel to the wave vector k of the radiation. Thus in thin-film transmission experiments carried out at normaMncidence in the Faraday configuration (kllH 0 ), one observes resonance absorption of EM radiation only by single-particle cyclotron excitations (co=a> c ) and by transverse optical (TO) phonons phonons (co = CO T Q). 12 The coupled plasmon-LO-phonon modes with qllH 0 , which are purely longitudinal in character, do not interact with the EM radiation for k!lH 0 . On the other hand, the coupled modes involving the collective cyclotron excitations and LO phonons are transverse as well as longitudinal in character. Thus they may be expected to interact with EM radiation when kllq is perpendicular to H 0 .We report here the observation of resonance absorption by the coupled modes involving collective cyclotron excitations and LO phonons in thin-film transmission experiments on ntype InSb carried out at normal incidence in the Voigt configuration, i.e., ki-H 0 . The frequencies of the normal modes with q ~ 0 and q±H 0 , which arise from the coupling of the collective cyclotron excitations with the LO phonons via the macroscopic longitudinal electric field, are given by 8 , 2 ±\(oo 2 + oo 2 + oo 2 ) 2 -4(co 2 co 2 + oo 2 co s LO LV p cwhere (j0p 2 = 4TrNe 2 /m*e QO , e and w* are the charge and the effective mass of the carriers, respectively, N is the carrier density, and e^ is the high-frequency dielectric constant. [When# o = 0, Eq. (1) reduces to the form which applies to the coupling of plasmons and LO phonons.] The normal-incidence transmission of a thin film for which 2Trd/\ 0 «l, where d is the thickness and X 0 is the vacuum wavelength of the radiation, is given by 8 ' 12" " 2-nd^.r, x T = l--Imx T (w),where XT r (w) = P j^(u))...
