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The nonlinear optical response of Ag and Au nanoparticles encapsulated in hybrid block copolymer micelles was
investigated by means of the Optical Kerr Effect (OKE) [5] and the Z-scan techniques [6]. The OKE technique provided the
Reχ (3) values of the samples, as well as its temporal evolution, whereas the Z-scan technique allowed the simultaneous
investigation of both the magnitude and the sign of the real and imaginary parts of the third-order susceptibility χ (3) (i.e.
Reχ (3) , Imχ (3) respectively). For the needs of these studies a mode locked Nd:YAG laser was employed, delivering 35 ps laser
pulses at 532 nm. A variety of systems with different polymers, encapsulating the metal nanoparticles, as well as with
different polymer/metal ratios were systematically investigated. Moreover, in the case of solutions, the dependence of the
nanoparticles concentration in the corresponding solvent (e.g. THF, toluene) on the nonlinear optical properties was also
examined.
All studied samples were found to exhibit very large nonlinear optical response (order of χ (3) , 10 -13 esu). This fact
can be attributed not only to the large inherent third-order nonlinear optical response of the metal nanoparticles, but also to
the surface plasmon resonance (SPR) feature, which is characteristic of such systems, where the metal nanoparticles are not
allowed to aggregate and form “macroscopic-scale” clusters. In the case of solutions this is feasible by using a polymer,
whereas in the case of films, thin layer construction, followed by post-treatment (e.g. annealing) ensures that the percolation
threshold is not exceeded and as a result the surface plasmon resonance will be present. The presence of the SPR in principle
can increase the nonlinear optical response of the sample by enhancing the local electric field at the vicinity of the
nanoparticles with the surrounding dielectric. During the present work it was found that increasing the polymer/metal ratio
resulted in decrease of the nonlinear optical response (scheme 2). Moreover, very good linear dependence of the Reχ (3) values
on the metal nanoparticles concentration in the solvent was found, as theoretically predicted [7].
The large third-order nonlinear optical response of these nanoparticle systems, combined with the fact that
changing preparation parameters, as well as applying post-treatment can affect the nonlinear optical response makes them
very promising candidates for a variety of photonic applications in optical telecommunications, optical computing, optical
data storage, information processing, etc.
2,5 PS:Ag 2:1
PS:Ag 4:1
2,0
χ (3) (10 -13 esu)
1,5
1,0
0,5
0,0
0 2 4 6 8 10
Concentration (mM)
Scheme2. χ (3) as a function of the nanoparticles concentration. It is obvious that increase of the
polymer/metal ratio decreases the nonlinear optical response
References
[1] R.Shenhar, T.B.Norsten, V.M.Rotello, Adv.Mater.2005, 17, No 6
[2] J.Li, L. Shi, Y.An, Y. Li, X.Chen, H.Dong, Polymer 2006, 47, 8480-8487
[3] S.Mössmer, J.Spatz, M.Möller, T.Aderle, J.Scmidt, W.Burchard, Macromolecules 2000, 33, 4791-4798
[4] M.Vamvakaki, D.Palioura, A.Spyros, P.Armes, S.H.Anastasiadis, Macromolecules 2006, 39, 5106-5112
[5] P. Ho, R.R. Alfano, Phys. Rev. A, 1979, 20, 2170-2187
[6] M. Sheik-Bahae, A. Said, T. Wei, D. Hagan, E.V. Stryland, IEEE J. Quantum Electron. 1990, 26, 760-769
[7] E. Koudoumas, M. Konstantaki, A. Mavromanolakis, X. Michaut, S. Couris, S. Leach, J. Phys. B: At. Mol. Opt.
Phys., 2001, 34, 1-14
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