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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Interação luz-matéria: tratamento semi-clássico<<strong>br</strong> />
3<<strong>br</strong> />
⎛ 8πn<<strong>br</strong> />
ν<<strong>br</strong> />
ρ(<<strong>br</strong> />
ν)<<strong>br</strong> />
= ⎜ 3<<strong>br</strong> />
⎝ c<<strong>br</strong> />
2<<strong>br</strong> />
⎞⎛<<strong>br</strong> />
⎟ ⎜<<strong>br</strong> />
⎜e<<strong>br</strong> />
⎠⎝<<strong>br</strong> />
⎞<<strong>br</strong> />
−1<<strong>br</strong> />
⎟<<strong>br</strong> />
⎠<<strong>br</strong> />
S. C. Zilio <strong>Óptica</strong> <strong>Moderna</strong> – <strong>Fundamentos</strong> e Aplicações<<strong>br</strong> />
hν<<strong>br</strong> />
KT<<strong>br</strong> />
−1<<strong>br</strong> />
hν<<strong>br</strong> />
219<<strong>br</strong> />
(10.9)<<strong>br</strong> />
onde a primeira fração representa a densidade de modos para a radiação<<strong>br</strong> />
isotrópica de freqüência ν, a segunda fração o número de ocupação destes<<strong>br</strong> />
modos e o termo hν é a energia por modo (fóton). A consideração deste<<strong>br</strong> />
tipo de radiação específica não implica em que<strong>br</strong>a de generalidade uma<<strong>br</strong> />
vez que é de se esperar que os coeficientes A e Bij dependam apenas do<<strong>br</strong> />
átomo e não da radiação a que está exposto. Substituindo (10. 9) em (10.<<strong>br</strong> />
8) obtemos:<<strong>br</strong> />
N B<<strong>br</strong> />
1<<strong>br</strong> />
12<<strong>br</strong> />
3<<strong>br</strong> />
8πn<<strong>br</strong> />
hν<<strong>br</strong> />
3<<strong>br</strong> />
c<<strong>br</strong> />
3<<strong>br</strong> />
⎛<<strong>br</strong> />
⎜<<strong>br</strong> />
⎜e<<strong>br</strong> />
⎝<<strong>br</strong> />
hν<<strong>br</strong> />
KT<<strong>br</strong> />
⎞<<strong>br</strong> />
−1<<strong>br</strong> />
⎟<<strong>br</strong> />
⎠<<strong>br</strong> />
−1<<strong>br</strong> />
= N<<strong>br</strong> />
2<<strong>br</strong> />
⎡<<strong>br</strong> />
⎢B<<strong>br</strong> />
⎢⎣<<strong>br</strong> />
21<<strong>br</strong> />
3<<strong>br</strong> />
8πn<<strong>br</strong> />
hν<<strong>br</strong> />
3<<strong>br</strong> />
c<<strong>br</strong> />
3<<strong>br</strong> />
⎛<<strong>br</strong> />
⎜<<strong>br</strong> />
⎜e<<strong>br</strong> />
⎝<<strong>br</strong> />
hν<<strong>br</strong> />
KT<<strong>br</strong> />
⎞<<strong>br</strong> />
−1<<strong>br</strong> />
⎟<<strong>br</strong> />
⎠<<strong>br</strong> />
−1<<strong>br</strong> />
⎤<<strong>br</strong> />
+ A⎥<<strong>br</strong> />
⎥⎦<<strong>br</strong> />
(10.10)<<strong>br</strong> />
Como os átomos estão em equilí<strong>br</strong>io térmico, a razão entre as populações<<strong>br</strong> />
dos níveis 1 e 2 é dada pelo fator de Boltzmann:<<strong>br</strong> />
N<<strong>br</strong> />
N<<strong>br</strong> />
hν<<strong>br</strong> />
2 g −<<strong>br</strong> />
2 KT e<<strong>br</strong> />
1<<strong>br</strong> />
= (10.11)<<strong>br</strong> />
g<<strong>br</strong> />
onde gi é a degenerescência do i-ésimo nível. Substituindo esta razão na<<strong>br</strong> />
eq. (10.10) e re-arranjando os termos obtemos:<<strong>br</strong> />
g<<strong>br</strong> />
g<<strong>br</strong> />
3 3<<strong>br</strong> />
3 3<<strong>br</strong> />
hν<<strong>br</strong> />
1 8πn<<strong>br</strong> />
hν<<strong>br</strong> />
⎛ 8πn<<strong>br</strong> />
hν<<strong>br</strong> />
⎞ −<<strong>br</strong> />
KT<<strong>br</strong> />
B 3 12 − A = ⎜ B 3 21 − A⎟<<strong>br</strong> />
e<<strong>br</strong> />
2 c<<strong>br</strong> />
⎝ c<<strong>br</strong> />
⎠<<strong>br</strong> />
que será válida para qualquer temperatura somente se:<<strong>br</strong> />
3 3<<strong>br</strong> />
A 8πn<<strong>br</strong> />
hν<<strong>br</strong> />
B<<strong>br</strong> />
21<<strong>br</strong> />
1<<strong>br</strong> />
(10.12)<<strong>br</strong> />
= (10.13a)<<strong>br</strong> />
3<<strong>br</strong> />
c<<strong>br</strong> />
B 12 g 2<<strong>br</strong> />
= (10.13b)<<strong>br</strong> />
B<<strong>br</strong> />
21<<strong>br</strong> />
Como num sistema atômico de dois níveis isolado a taxa de<<strong>br</strong> />
decaimento A é o inverso do tempo de vida espontâneo, A = 1/τesp, usando<<strong>br</strong> />
ν = c/λ obtemos:<<strong>br</strong> />
g<<strong>br</strong> />
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