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 ...
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Ondas eletromagnéticas 63<<strong>br</strong> />
r r r<<strong>br</strong> />
S = E xH<<strong>br</strong> />
(3.44)<<strong>br</strong> />
Usando a relação entre H r e E r dada logo após as eq. (3.17) temos:<<strong>br</strong> />
r r<<strong>br</strong> />
r r ( k x E)<<strong>br</strong> />
1 r r r r r r<<strong>br</strong> />
S = Ex<<strong>br</strong> />
= [ − E(<<strong>br</strong> />
k.<<strong>br</strong> />
E)<<strong>br</strong> />
+ k(<<strong>br</strong> />
E.<<strong>br</strong> />
E)<<strong>br</strong> />
] =<<strong>br</strong> />
μω μω<<strong>br</strong> />
2 2<<strong>br</strong> />
E r E r r<<strong>br</strong> />
0 2 r<<strong>br</strong> />
= k = cos [ k.<<strong>br</strong> />
r − ωt]k<<strong>br</strong> />
μω μω<<strong>br</strong> />
(3.45)<<strong>br</strong> />
Os detetores existentes não possuem velocidade suficiente para<<strong>br</strong> />
acompanhar a variação rápida do campo elétrico e fazem<<strong>br</strong> />
uma média<<strong>br</strong> />
temporal do sinal. Portanto, devemos calcular a média temporal do vetor<<strong>br</strong> />
de Poynting, isto é:<<strong>br</strong> />
t0<<strong>br</strong> />
+ T<<strong>br</strong> />
2 t0<<strong>br</strong> />
+ T<<strong>br</strong> />
r 1 r r E r<<strong>br</strong> />
0<<strong>br</strong> />
2 r r<<strong>br</strong> />
< S > = S(<<strong>br</strong> />
r,<<strong>br</strong> />
t)<<strong>br</strong> />
dt = cos ( k.<<strong>br</strong> />
r − ωt)<<strong>br</strong> />
dt k<<strong>br</strong> />
T ∫ μωT<<strong>br</strong> />
∫<<strong>br</strong> />
t<<strong>br</strong> />
0<<strong>br</strong> />
t<<strong>br</strong> />
0<<strong>br</strong> />
[ 1 + cos2y]<<strong>br</strong> />
2 1<<strong>br</strong> />
Usando a identidade cos y = obtemos:<<strong>br</strong> />
2<<strong>br</strong> />
r<<strong>br</strong> />
S<<strong>br</strong> />
2<<strong>br</strong> />
Eo<<strong>br</strong> />
= 2<<strong>br</strong> />
2μω<<strong>br</strong> />
T<<strong>br</strong> />
−<<strong>br</strong> />
1<<strong>br</strong> />
sen<<strong>br</strong> />
2<<strong>br</strong> />
r<<strong>br</strong> />
{ 1 r<<strong>br</strong> />
ωT<<strong>br</strong> />
+ sen[<<strong>br</strong> />
2(<<strong>br</strong> />
k.<<strong>br</strong> />
r − ωt<<strong>br</strong> />
0 − ωT)<<strong>br</strong> />
]<<strong>br</strong> />
2<<strong>br</strong> />
r r [ 2 ( k.<<strong>br</strong> />
r − ωt<<strong>br</strong> />
0 ) ]}<<strong>br</strong> />
Integrando em um período, que é dado por T = 2π/ ω , obtemos:<<strong>br</strong> />
(3.46)<<strong>br</strong> />
(3.47)<<strong>br</strong> />
r 2<<strong>br</strong> />
E r 1 r r<<strong>br</strong> />
0<<strong>br</strong> />
*<<strong>br</strong> />
< S > = k = Re{<<strong>br</strong> />
E x H}<<strong>br</strong> />
(3.48)<<strong>br</strong> />
2μω<<strong>br</strong> />
2<<strong>br</strong> />
Definimos densidade de fluxo radiante ou irradiância como:<<strong>br</strong> />
r E<<strong>br</strong> />
I = < S > =<<strong>br</strong> />
2μω<<strong>br</strong> />
2 2<<strong>br</strong> />
0k E0<<strong>br</strong> />
1<<strong>br</strong> />
= = cn<<strong>br</strong> />
2μv<<strong>br</strong> />
2<<strong>br</strong> />
ε<<strong>br</strong> />
0<<strong>br</strong> />
2<<strong>br</strong> />
0<<strong>br</strong> />
E (3.49)<<strong>br</strong> />
que possui unidades de W/m il na<<strong>br</strong> />
prática, pois permite relacionar a intensidade da luz com o campo elétrico.<<strong>br</strong> />
2 . Esta é uma expressão bastante út<<strong>br</strong> />
S. C. Zilio <strong>Óptica</strong> <strong>Moderna</strong> – <strong>Fundamentos</strong> e Aplicações