Optical response size dependence

Although these acoustical probes can be made very small they will always slightly disturb the ultrasonic field. Just as in the case of coated thermal probes, the response signal depends on the nature and size of the probe, thus it is important that the microphones are carefully calibrated. They are however widely used, especially to calibrate medical ultrasonic equipment. Recently, very small and sensitive devices using PVDF membranes [68,69] or fiber optics [70] have been described. PVDF has piezoelectric properties and miniature membrane hydrophones (about 0.5 mm in diameter) are available. Fiber optic probes can even be smaller and a spatial resolution of 0.1 mm has been claimed [70],... 

Ballesteros, J.M., Solis, J., Serna, R., Afonso, C.N. Nanocrystal size dependence of the third-order nonlinear optical response of Cu Al2O3 thin films. Appl. Phys. Lett. 74, 2791-2793 (1999)... 

Ma, G., Sun, W., Tang, S.-H., Zhang, H., Shen, Z., Qian, S. Size and dielectric dependence of the third-order nonlinear optical response of Au nanocrystals embedded in matrices. Opt. Lett. 27, 1043-1045 (2002)... 

In the second part of this work we review our theoretical and experimental works to obtain enhanced two-photon cross-sections by using the super-linear response of centrosymmetric monomers that are coherently coupled. In this alternative approach, the nonlinear material consists of an assembly of nonsubstituted /r-electron systems that are coupled by dipole-dipole interactions. The monomer two-photon term is a pure transition dipole term ( UQ,jU,2). Typical materials can be molecular aggregates, nanocrystals, oligomers, and dendrimers. The dipole-dipole interactions determine the size dependency of optical properties, and in particular of two-photon cross-sections. 

It should be noted that optical humidity sensors as a rule use similar effects, which were discussed above (Russell and Flecher 1985 Ballantine and Wohltjen 1986 Boltinghouse and Abel 1989 Wang et al. 1991 Kharaz and Jones 1995 Ando et al. 1996 Zhou et al. 1998 Skrdla et al. 1999 Alvarez-Herrero et al. 2004). The water adsorption in a porous matrix produces a variation in the optical response of the device, because the refractive index of the layer changes when the hydration of sensing material takes place and the pores are filled or emptied. The water adsorption isotherms and, therefore, the sensor response depend on the size and shape of the pores. One can find in Posch and Wolfbeis (1988), Otsuki and Adachi (1993), Papkovsky et al. (1994), Costa-Femandez et al. (1997), Costa-Femandez and Sanz-Medel (2000), Choi and Tse (1999), Choi and Shuang (2000), and Bedoya et al. (2001, 2006) a description of optical humidity sensors used and other principles. 

Size and shape dependency is very important property in SPR [33, 34] of metal NPs observed in the range between 10 and 100 nm. The optical response of the SP absorption in these metal NPs can be demonstrated by the electron dynamics (electron-electron and electron-phonon scattering). It is found that the electron-phonon relaxation processes in NPs, which are smaller than the electron mean free path (MFP), are independent of their size or shape (Fig. 13.8). 

Moreover, in considering the effects of the size in the optical response of a metallic nanoparticle, we must put in evidence that in the case of particles with dimensions comparable or smaller than the mean free path of its oscillating electrons (i.e. for gold and silver particles of radius o < 10 nm) the dielectric function of the particles becomes strongly size-dependent and the additional surface damping must be considered for a correct treatment of the resonances intensity. 

The permittivity discussed so far depends only on frequencies and, through the relaxation times, on the particle size. It is well-known that the dielectric response of materials, in particular metal, is non-local, i.e. the polarization vector induced at a certain point depends on the values of the electric field in all other points. In the reciprocal space language, we can say that e(plane-waves in which the probing electric field can be decomposed. The permittivity of metals such as Ag, Au and Cu at optical frequencies mainly depends on the behavior of both the valence electrons, which is close to that of a free-electron gas, and the core of the metal. As we did before, the total dielectric constant of the metal ([Pg.239]

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