Óptica Moderna Fundamentos e aplicações - Fotonica.ifsc.usp.br ...
Óptica Moderna Fundamentos e aplicações - Fotonica.ifsc.usp.br ...
Óptica Moderna Fundamentos e aplicações - Fotonica.ifsc.usp.br ...
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56<<strong>br</strong> />
iQ + kP'=<<strong>br</strong> />
0<<strong>br</strong> />
Ondas eletromagnéticas<<strong>br</strong> />
(3.26b)<<strong>br</strong> />
Desta forma, obtemos equações diferenciais, que embora não lineares, são<<strong>br</strong> />
de primeira ordem, e consequentemente fáceis de serem resolvidas. A<<strong>br</strong> />
solução da eq. (3.26a) resulta em:<<strong>br</strong> />
Q(<<strong>br</strong> />
z)<<strong>br</strong> />
k<<strong>br</strong> />
=<<strong>br</strong> />
z + q<<strong>br</strong> />
0<<strong>br</strong> />
(3.27)<<strong>br</strong> />
onde q0 é uma constante de integração, que será analisada posteriormente.<<strong>br</strong> />
Utilizando este resultado na eq. (3.26b) é fácil mostrar que:<<strong>br</strong> />
⎛ z ⎞<<strong>br</strong> />
P(<<strong>br</strong> />
z)<<strong>br</strong> />
= −iln<<strong>br</strong> />
⎜<<strong>br</strong> />
⎜1<<strong>br</strong> />
+<<strong>br</strong> />
⎟<<strong>br</strong> />
⎝ q0<<strong>br</strong> />
⎠<<strong>br</strong> />
(3.28)<<strong>br</strong> />
Podemos agora substituir os valores de P(z) e Q(z) na eq. (3.24)<<strong>br</strong> />
para encontrarmos a função ψ(r,z). Antes porém, vamos re-escrever a<<strong>br</strong> />
constante de integração como q 0 = iz0, com z0<<strong>br</strong> />
real. A razão de se<<strong>br</strong> />
considerar q0 imaginário é que esta é a única maneira de se obter uma<<strong>br</strong> />
solução que está confinada em torno do eixo z; caso contrário, o campo<<strong>br</strong> />
elétrico se estenderia exponencialmente até o infinito e esta é uma solução<<strong>br</strong> />
que não nos interessa. Desta forma temos:<<strong>br</strong> />
e<<strong>br</strong> />
{ − iP(<<strong>br</strong> />
z)<<strong>br</strong> />
} = exp{<<strong>br</strong> />
− ln[<<strong>br</strong> />
1−<<strong>br</strong> />
i(<<strong>br</strong> />
z / z ) ] }<<strong>br</strong> />
exp<<strong>br</strong> />
0<<strong>br</strong> />
1<<strong>br</strong> />
= =<<strong>br</strong> />
1−<<strong>br</strong> />
i(<<strong>br</strong> />
z / z0<<strong>br</strong> />
)<<strong>br</strong> />
1<<strong>br</strong> />
1+<<strong>br</strong> />
( z / z<<strong>br</strong> />
−1<<strong>br</strong> />
exp{<<strong>br</strong> />
i tg ( z / z0<<strong>br</strong> />
) }<<strong>br</strong> />
2<<strong>br</strong> />
)<<strong>br</strong> />
⎧ Q(<<strong>br</strong> />
z)<<strong>br</strong> />
r<<strong>br</strong> />
exp⎨−<<strong>br</strong> />
i<<strong>br</strong> />
⎩ 2<<strong>br</strong> />
⎫<<strong>br</strong> />
⎬ =<<strong>br</strong> />
⎭<<strong>br</strong> />
⎪⎧<<strong>br</strong> />
2<<strong>br</strong> />
kr ⎛ z − iz<<strong>br</strong> />
= exp⎨−<<strong>br</strong> />
i ⎜<<strong>br</strong> />
⎪⎩<<strong>br</strong> />
2 ⎜ 2<<strong>br</strong> />
⎝ z + z<<strong>br</strong> />
2<<strong>br</strong> />
⎪⎧<<strong>br</strong> />
2<<strong>br</strong> />
k ⎛ r<<strong>br</strong> />
exp⎨−<<strong>br</strong> />
i ⎜<<strong>br</strong> />
⎪⎩ 2 ⎜<<strong>br</strong> />
⎝ z + iz<<strong>br</strong> />
0<<strong>br</strong> />
2<<strong>br</strong> />
0<<strong>br</strong> />
0<<strong>br</strong> />
0<<strong>br</strong> />
⎞⎪⎫<<strong>br</strong> />
⎟<<strong>br</strong> />
⎬<<strong>br</strong> />
⎠⎪⎭<<strong>br</strong> />
⎪⎫<<strong>br</strong> />
2<<strong>br</strong> />
2<<strong>br</strong> />
⎞ ⎧ r ikr ⎫<<strong>br</strong> />
⎟<<strong>br</strong> />
⎬ = exp⎨−<<strong>br</strong> />
− 2 ⎬<<strong>br</strong> />
⎠⎪⎭<<strong>br</strong> />
⎩ w ( z)<<strong>br</strong> />
2R(<<strong>br</strong> />
z)<<strong>br</strong> />
⎭<<strong>br</strong> />
onde as grandezas w(z) e R(z) foram introduzidas como:<<strong>br</strong> />
{ } { } 2<<strong>br</strong> />
2 2<<strong>br</strong> />
1+<<strong>br</strong> />
(z/<<strong>br</strong> />
z ) = w 1 (z/<<strong>br</strong> />
z )<<strong>br</strong> />
2<<strong>br</strong> />
0<<strong>br</strong> />
w (z)<<strong>br</strong> />
0<<strong>br</strong> />
0 +<<strong>br</strong> />
(3.29)<<strong>br</strong> />
(3.30)<<strong>br</strong> />
2z<<strong>br</strong> />
= 0 (3.31a)<<strong>br</strong> />
k<<strong>br</strong> />
S. C. Zilio <strong>Óptica</strong> <strong>Moderna</strong> – <strong>Fundamentos</strong> e Aplicações