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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70<<strong>br</strong> />
A fase da onda eletromagnética<<strong>br</strong> />
superior a ω, como veremos quando tratarmos a interação entre a luz e a<<strong>br</strong> />
matéria no Cap. 9. Vamos expandir k em torno de ω0, de acordo com:<<strong>br</strong> />
2 [ ( ω − ) ]<<strong>br</strong> />
dk<<strong>br</strong> />
k( ω ) = k 0 + ( ω − ω0<<strong>br</strong> />
) + ϕ ω<<strong>br</strong> />
( 4.7)<<strong>br</strong> />
0<<strong>br</strong> />
dω<<strong>br</strong> />
0<<strong>br</strong> />
O termo quadrático pode ocorrer no caso em que houver<<strong>br</strong> />
dispersão<<strong>br</strong> />
no índice de refração, isto é, quando n = n(ω). Desprezando termos de<<strong>br</strong> />
ordens superiores à linear em ω (caso sem dispersão) temos:<<strong>br</strong> />
ω<<strong>br</strong> />
Δω<<strong>br</strong> />
+<<strong>br</strong> />
0<<strong>br</strong> />
2<<strong>br</strong> />
⎪⎧ ⎡⎛<<strong>br</strong> />
dk ⎞ ⎤⎪⎫<<strong>br</strong> />
E ( z,<<strong>br</strong> />
t)<<strong>br</strong> />
= ∫ E ⎨ ⎢⎜<<strong>br</strong> />
+ ω − ω ⎟<<strong>br</strong> />
0exp<<strong>br</strong> />
i ⎜k<<strong>br</strong> />
0 ( 0)<<strong>br</strong> />
⎟ z−<<strong>br</strong> />
iωt⎥⎬<<strong>br</strong> />
dω<<strong>br</strong> />
(4.8)<<strong>br</strong> />
Δω<<strong>br</strong> />
⎪⎩ ⎣⎝<<strong>br</strong> />
dω<<strong>br</strong> />
0 ⎠ ⎦⎪⎭<<strong>br</strong> />
ω0<<strong>br</strong> />
−<<strong>br</strong> />
2<<strong>br</strong> />
Fazendo a substituição Ω = ω − ω0 obtemos:<<strong>br</strong> />
E(z, t)<<strong>br</strong> />
Δω<<strong>br</strong> />
+<<strong>br</strong> />
2<<strong>br</strong> />
⎧ ⎡ dk<<strong>br</strong> />
E 0 exp{<<strong>br</strong> />
i(<<strong>br</strong> />
k 0z<<strong>br</strong> />
ω0t<<strong>br</strong> />
) } . ∫ exp iΩ z t dΩ<<strong>br</strong> />
Δω dω 0 ⎭ ⎬⎫<<strong>br</strong> />
⎤<<strong>br</strong> />
= −<<strong>br</strong> />
⎨ ⎢ − ⎥ (4.9)<<strong>br</strong> />
⎩ ⎣ ⎦<<strong>br</strong> />
−<<strong>br</strong> />
2<<strong>br</strong> />
O primeiro termo desta expressão representa a onda portadora e o<<strong>br</strong> />
segundo é a função forma ou modulação que passaremos a chamar g(z,t).<<strong>br</strong> />
Assim,<<strong>br</strong> />
Δω<<strong>br</strong> />
+<<strong>br</strong> />
2<<strong>br</strong> />
g 0<<strong>br</strong> />
( z,<<strong>br</strong> />
t)<<strong>br</strong> />
= E 0<<strong>br</strong> />
x ∫<<strong>br</strong> />
Δω<<strong>br</strong> />
−<<strong>br</strong> />
2<<strong>br</strong> />
⎧ ⎡ dk<<strong>br</strong> />
exp⎨<<strong>br</strong> />
iΩ<<strong>br</strong> />
⎢<<strong>br</strong> />
⎩ ⎣dω<<strong>br</strong> />
0<<strong>br</strong> />
⎤⎫<<strong>br</strong> />
z − t⎥⎬<<strong>br</strong> />
dΩ<<strong>br</strong> />
= 2E<<strong>br</strong> />
⎦⎭<<strong>br</strong> />
sen φ ⎛ Δω<<strong>br</strong> />
⎞<<strong>br</strong> />
⎜ ⎟<<strong>br</strong> />
φ ⎝ 2 ⎠<<strong>br</strong> />
(4.10)<<strong>br</strong> />
onde ⎛ Δω<<strong>br</strong> />
⎞⎡<<strong>br</strong> />
dk ⎤<<strong>br</strong> />
φ = ⎜ ⎟⎢<<strong>br</strong> />
z − t . A Fig. 4.3 mostra o pacote de ondas obtido<<strong>br</strong> />
⎥<<strong>br</strong> />
⎝ 2 ⎠⎣dω<<strong>br</strong> />
0 ⎦<<strong>br</strong> />
através das equações (4.9) e (4.10). Seu valor máximo ocorre quando φ =<<strong>br</strong> />
0, ou seja, quando dk z = t. A velocidade com que o pacote se propaga,<<strong>br</strong> />
dω 0<<strong>br</strong> />
que é a já conhecida velocidade de grupo, é:<<strong>br</strong> />
dz<<strong>br</strong> />
v g = =<<strong>br</strong> />
dt<<strong>br</strong> />
dω<<strong>br</strong> />
dk<<strong>br</strong> />
0<<strong>br</strong> />
(4.11)<<strong>br</strong> />
S. C. Zilio <strong>Óptica</strong> <strong>Moderna</strong> – <strong>Fundamentos</strong> e Aplicações