21 Nonlinear Optical Properties of Polymer Nanocomposites. . .
339
The observed asymmetry of Z-scan transmittance curves may be the result of a
large nonlinear phase shift induced by a pulsed laser [42] that requires additional
analysis and verification.
Under femtosecond excitation at a wavelength of 800 nm, the nonlinear absorption is absent in the random structure and decreases in the ordered one (Fig. 21.6a).
Unlike the nanosecond experiment, the sign of Im χ (3) changes from negative
to positive under femtosecond excitation of ordered structures. This transformation reveals that another nonlinear effect such as reverse saturated absorption or
two-photon absorption determines the nonlinear response of the nanocomposite.
Transmittance curves reflecting a purely nonlinear refractive effect indicate a negative value of the nonlinear refractive index in both chaotic and ordered structures
(Fig. 21.6b).
By using the mentioned fitting [40], we calculated the values of Im χ (3) , Re
χ (3) , nonlinear absorption coefficient β, and nonlinear refractive index n 2 for
random and ordered structure. They are given in Tables 21.1 and 21.2. In both
cases of nanosecond and femtosecond excitation, we found a significant increase in
transmission, absorption, and nonlinear refraction for ordered structures compared
to disordered ones.
We have compared the values of nonlinear parameters of the proposed nanocomposite with the results obtained by other authors under similar experimental
(a)
(b)
Fig. 21.6 Nonlinear response of Ag nanocomposite at 800 nm excitation (τ = 180 fs, f = 75 kHz,
I 0 = 0.142 GW/cm 2 ): open-aperture (a) and closed-aperture (CA/OA) (b) Z-scan results for random
(open circles) and ordered (black squares) structures. The lines are the theoretical fits
Table 21.1 Nonlinear parameters calculated for random and ordered structure (λ ex = 532 nm, τ =
20 ns, f = 0.5 Hz)
Structure α,
I 0 ,
n 2 ,
β,
Re χ (3) ,
Im χ (3) ,
|χ (3) |,
type
cm −1 MW/cm 2 cm 2 /W
cm/W
esu
esu
esu
Random 1381 5.11
3×10 −10
−1.13×10 −4 1.36×10 −8 −2.88×10 −8 3.19×10 −8
Ordered 1381 5.11
1.14×10 −9 −1.77×10 −4 5.17×10 −8 −4.52×10 −8 6.87×10 −8
339
The observed asymmetry of Z-scan transmittance curves may be the result of a
large nonlinear phase shift induced by a pulsed laser [42] that requires additional
analysis and verification.
Under femtosecond excitation at a wavelength of 800 nm, the nonlinear absorption is absent in the random structure and decreases in the ordered one (Fig. 21.6a).
Unlike the nanosecond experiment, the sign of Im χ (3) changes from negative
to positive under femtosecond excitation of ordered structures. This transformation reveals that another nonlinear effect such as reverse saturated absorption or
two-photon absorption determines the nonlinear response of the nanocomposite.
Transmittance curves reflecting a purely nonlinear refractive effect indicate a negative value of the nonlinear refractive index in both chaotic and ordered structures
(Fig. 21.6b).
By using the mentioned fitting [40], we calculated the values of Im χ (3) , Re
χ (3) , nonlinear absorption coefficient β, and nonlinear refractive index n 2 for
random and ordered structure. They are given in Tables 21.1 and 21.2. In both
cases of nanosecond and femtosecond excitation, we found a significant increase in
transmission, absorption, and nonlinear refraction for ordered structures compared
to disordered ones.
We have compared the values of nonlinear parameters of the proposed nanocomposite with the results obtained by other authors under similar experimental
(a)
(b)
Fig. 21.6 Nonlinear response of Ag nanocomposite at 800 nm excitation (τ = 180 fs, f = 75 kHz,
I 0 = 0.142 GW/cm 2 ): open-aperture (a) and closed-aperture (CA/OA) (b) Z-scan results for random
(open circles) and ordered (black squares) structures. The lines are the theoretical fits
Table 21.1 Nonlinear parameters calculated for random and ordered structure (λ ex = 532 nm, τ =
20 ns, f = 0.5 Hz)
Structure α,
I 0 ,
n 2 ,
β,
Re χ (3) ,
Im χ (3) ,
|χ (3) |,
type
cm −1 MW/cm 2 cm 2 /W
cm/W
esu
esu
esu
Random 1381 5.11
3×10 −10
−1.13×10 −4 1.36×10 −8 −2.88×10 −8 3.19×10 −8
Ordered 1381 5.11
1.14×10 −9 −1.77×10 −4 5.17×10 −8 −4.52×10 −8 6.87×10 −8