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Here

ε

0

= 8

,

85

10

12

;

ε

is high-frequency dielectric permittivity;

ω

l

and

ω

0

are zero and pole of dielectric function

ε

(

ω

)

.

For longitudinal electromagnetic waves (

E

k

k

) from relationship (3) it

follows that

ε

(

ω

) =

ε

(

ω

l

) = 0

, i.e. frequency of longitudinal electromagnetic

waves for the material medium in question

ω

=

ω

l

=

const.

Usually it is assumed that in vacuum

ε

(

ω

) = 1

. According to formula

(3) it means that only the transverse waves are present. However, according

to the Maxwell equation div

~D

=

0

ε

(

~k ~E

) = 0

transverse waves are

present in vacuum when

ε

= 0

.

Polariton waves spectrum can be analyzed experimentally by the

spontaneous Raman scattering method. By selecting scattering geometry

and polarization installations transverse (TO) and longitudinal (LO) polar

modes can be detected. Spectra for TO and LO phonons at certain

polarization geometries in gallium phosphide monocrystal with planes

(100), (010) and (001) are presented in Fig. 1 [5]. Small-angle Raman

scattering for a gallium phosphide sample with a plane (111) perpendicular

to the the propagation direction of the exciting radiation ray allows

calculating frequencies of the lower branch polaritons and plotting a

dispersion curve piece in that crystal. So Raman scattering method in

Fig. 1. Spectra of Raman scattering by longitudinal and transverse polar modes

in a gallium phosphide crystal at scattering geometry

z

(

xz

)

y

when according to

the selection rules both transverse and longitudinal modes arise (

а

), at scattering

geometry

z

(

xz

)

x

when only transverse modes arise (

b

), at scattering geometry

x

(

zy

)

x

when only longitudinal modes arise (

c

)

38

ISSN 1812-3368. Herald of the BMSTU. Series “Natural Sciences”. 2015. No. 1