== Phonons in Perfect 4H-SiC

不同方向入射光的声子位置不同。

声子的性质与它们在倒空间中的位置（即波矢）有关，而参与拉曼散射的声子波矢则由激光波长和拉曼入射配置（正入射、掠入射、边入射）决定。
在我们的实验（绿光）中，参与拉曼散射的声子都位于 Gamma 点附近（距离仅约 0.005 埃-1），而与 Gamma 点的相对方向则根据入射配置而定。
对于正入射，声子大致位于 Gamma-A 线上；对于边入射，声子大致位于 Gamma-K 线上。对于掠入射，声子不位于任何高对称线上。

The properties of phonons depend on their positions in reciprocal space (i.e., their wavevectors),
  and the wavevectors of phonons participating in a Raman scattering process
  are determined by the Raman experiment configuration.
In our experiments (back-scattering with 532 nm laser light),
  the phonons involved are located very close to the #sym.Gamma point (approximately $5 times 10^(-3) angstrom$),
  and direction relative to the #sym.Gamma point is determined by the incidence configurations.
For normal and edge incidence,
  the relevant phonons lie approximately along the #sym.Gamma–A and #sym.Gamma–K lines, respectively,
  as illustrated in @figure-discont.
For grazing incidence,
  the relevant phonons do not reside on any high-symmetry lines and are therefore not depicted in the figure.

#include "figure-discont.typ"

每个点都有 21 个声子模式。我们把它分成 18 个极性很小的和 3 个极性很强的两类。

对于每一个 Gamma 附近的点，都有 21 个声子模式（简并模式按简并数计）。
我们将这 21 个声子模式分为两类：18 个极性可以忽略的声子模式（即极性为零或非常弱的声子），在拉曼散射过程中它们的电极性造成的效应可以忽略；
  和 3 个强极性声子模式，在拉曼光谱中电极性效应是可观测的。

At each position near the #sym.Gamma point,
  there are 21 phonon modes (counting degeneracies).
These modes were categorized into two groups according to their electrical polarities:
  18 negligible-polar phonons (i.e., zero or very weak electrical polarity),
    where atoms of the same species vibrate in opposite directions
    resulting in effective cancellation of electrical polarization during the Raman process
    (gray lines in @figure-discont);
  and three strong-polar phonons,
    where atoms of the same species vibrate in phase,
    resulting in strong electrical polarization and observable effects in Raman spectra,
    (colored lines in @figure-discont).

#include "non-polar/default.typ"
#include "polar/default.typ"
