R. del Coso

Limits to the determination of the nonlinear refractive index by the Z-scan method

We analyze the limitations imposed by sample absorption on the determination of the nonlinear refractive index by the Z-scan technique. By using a nanostructured thin film consisting of Cu nanocrystals embedded in a dielectric Al2O3 matrix as an example, we show that thermo-optical effects appearing when linear absorption is significant can be strongly misleading in the interpretation of the results of a Z scan. Even though this effect is not new, the widespread use of the Z-scan technique during the past several years makes it necessary to analyze explicitly the conditions under which the technique can be reliably applied and when more sophisticated techniques should be used instead. We discuss the contributions to the signal under different experimental conditions, several diagnostic techniques to discriminate true nonlinear effects from thermally induced phenomena, and different methods to reduce the thermal contribution.

Third order nonlinear optical susceptibility of Cu:Al2O3 nanocomposites: From spherical nanoparticles to the percolation threshold

The third order optical susceptibility of metal-dielectric nanocomposite films (Cu:Al2O3) has been determined by degenerate four wave mixing. The films have been synthesized by alternate pulsed laser deposition and consisted of Cu nanoparticles in an amorphous Al2O3 matrix. They have metal volume fractions, p, ranging from 0.07 to 0.45, and morphologies that range from spherical particles (diameter, φ∼2 nm) to a random network when close to the percolation threshold. In nanocomposites containing isolated oblate spheroids (p⩽0.17), the optical response at wavelengths close to that of the surface plasmon resonance (SPR) can be described in the frame of the Maxwell-Garnett effective medium theory. Above the particle coalescence threshold, in nanocomposites with higher Cu content (p⩾0.2), both the linear absorption in the near-infrared and the third order nonlinear optical susceptibility at the SPR are greatly enhanced, the latter achieving values as high as 1.8×10−7 esu. These results are discussed in terms ...

Large enhancement of the third-order optical susceptibility in Cu-silica composites produced by low-energy high-current ion implantation

Low-energy high-current ion implantation in silica at a well-controlled substrate temperature has been used to produce composites containing a large concentration of spherical Cu clusters with an average diameter of 4 nm and a very narrow size distribution. A very large value for the third-order optical susceptibility, χ(3)=10−7 esu, has been measured in the vicinity of the surface plasmon resonance by degenerate four-wave mixing at 585 nm. This value is among the largest values ever reported for Cu nanocomposites. Additionally, the response time of the nonlinearity has been found to be shorter than 2 ps. The superior nonlinear optical response of these implants is discussed in terms of the implantation conditions.

Controlling the transmission at the surface plasmon resonance of nanocomposite films using photonic structures

The transmission of nanocomposite thin films formed by Cu nanocrystals embedded in an amorphous aluminum oxide (Al2O3) matrix has been enhanced in the vicinity of the surface plasmon resonance wavelength, by the design of one-dimensional photonic structures. The nanocrystals are distributed in layers, whose separation and periodicity are used to control the optical response of the films. It is found that at the SPR a transmission enhancement up to 36% can be achieved with respect to a film with an homogeneous distribution of the nanocrystals. These photonic structures have been successfully produced by pulsed laser deposition.

Role of surface-to-volume ratio of metal nanoparticles in optical properties of Cu:Al2O3 nanocomposite films

We report on the role of the surface-to-volume ratio of Cu nanoparticles (NPs) both in the linear and nonlinear optical properties of Cu:Al2O3 nanocomposite films. The results show that when the shape of the NPs deviates sufficiently from that of a sphere, the increase of the fraction of metal atoms present at the surface (NS) with respect to the total amount of atoms (NT) in the NP leads to a substantial reduction of the enhancement of the local field. As a consequence, for NS∕NT values above a certain threshold (≈0.4–0.5), the surface–plasma resonance is smeared out and the nonlinear optical response of the nanocomposite film becomes very weak and independent of the dimensions of the NPs or their volume fraction in the matrix.

Thin film photonic structures based on metal nanocrystals periodically distributed in a dielectric host

In this work, we have produced nanostructured thin films with an absorption that has been varied as a function of the distribution of the nanocrystals (NCs). The NCs have been distributed in layers whose periodicity has been modeled in order to design the required response. To achieve consistent results an experimental-modeling feed-back approach has been followed. The results show that the optical transmission of such a system can be varied from that of a nanocomposite material in which the metal NCs are uniformly distributed in the dielectric volume and thus exhibiting a standard absorption resonance to that of a photonic structure. Some of these configurations have been experimentally produced and the agreement between modeled and experimental results is excellent.

Enhancement of the nonlinear optical response of Cu: Al/sub 2/O/sub 3/ nanocomposite films by multiple particle interaction effects

CLEO/EUROPE ; EQEC European Quantum Electronics Conference, Munich ICM, Germany, 22-27 June, 2003

Role of nanocrystal morphology on the third order non-linear response of Cu:Al/sub 2/O/sub 3/ nanocomposite films

Summary form only. Nanocomposite Al/sub 2/O/sub 3/:Cu films have been prepared by alternate pulsed laser ablation of the matrix (Al/sub 2/O/sub 3/) and the metallic (Cu) targets using an ArF excimer laser. The non-linear third order susceptibility of the composite film has been determined by the complementary use of Z-scan spectroscopy and degenerate four wave mixing (DFWM) using a cavity-dumped, mode locked tunable dye laser.