A. Suarez-Garcia

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.

Nanostructuring the Er–Yb distribution to improve the photoluminescence response of thin films

Thin films of amorphous aluminum oxide (a-Al2O3) codoped with Er3+ and Yb3+ ions have been in-depth nanostructured by distributing the rate earth (RE) ions in layers separated in the 0–3 nm range. The Yb to Er concentration ratio is varied from 0 to 3.6. The photoluminescence (PL) response at 1.53 μm exhibits an increase of up to two orders of magnitude with respect to that of films doped only with Er. The PL intensity is improved when Yb3+ and Er3+ ions are in separate layers and the results show that efficient Yb3+ to Er3+ energy transfer can be achieved for separations up to 3 nm. Furthermore, it is shown that designing an adequate RE distribution, for the same total RE content and Yb to Er concentration ratio, can enhance the PL intensity by a further factor of two. It is shown that the Er3+ PL response is improved because of a reduction of the RE clustering and an improvement of the energy transfer from Yb3+ to Er3+ ions.

Photoluminescence performance of pulsed-laser deposited Al2O3 thin films with large erbium concentrations

Erbium doped Al2O3 films with concentrations up to 6×1020 Er cm−3 have been prepared in a single step process by pulsed-laser deposition. Alternate ablation of Al2O3 and Er targets has been used to control the in-depth distribution and in-plane concentration of Er3+ ions independently. The characteristic Er3+ photoluminescence response at 1.53 μm has been studied as a function of the Er3+ distribution. It is found that lifetime values can be greatly increased by increasing the Er3+–Er3+ in-depth separation above 3 nm. This result can be related to a reduced Er3+–Er3+ energy migration process. The in-plane Er3+ concentration was increased by either increasing the number of pulses on the Er target or the laser energy density for ablation. By the latter method in-plane concentrations as high as 1.1×1014 Er cm−2 per layer (corresponding to 2×1020 Er cm−3) were achieved, while keeping lifetime values as high as 6 ms. This result is explained in terms of shallow Er3+ implantation during deposition.

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.

Resputtering and morphological changes of Au nanoparticles in nanocomposites as a function of the deposition conditions of the oxide capping layer

A multilayer nanocomposite consisting of Au nanoparticles separated by layers of amorphous Al2O3 was prepared by pulsed laser deposition. The laser energy density used to deposit the Al2O3 layers was varied to investigate the effects on the resulting microstructure. When Al2O3 is deposited on the Au it results in resputtering and interfacial mixing of the Au nanoparticles into the Al2O3 matrix being deposited onto them. This leads to a reshaping of the nanoparticles, elongating them in the growth direction and in some cases forming cavities in the nanoparticles. These effects become more pronounced with increasing laser energy density. The high kinetic energies of the species involved also result in implantation of Au into the Al2O3 where it forms nanoparticles of about 1 nm mean diameter.

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.

Enhanced luminescence performance by controlling the er and yb distribution in the nanometer scale

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

The shallow implantation of bismuth during the growth of bismuth nanocrystals in Al2O3 by pulsed laser deposition

The effect of the laser energy density used to deposit Bi onto amorphous aluminum oxide (a-Al 2 O 3 ) on the growth of Bi nanocrystals has been investigated using transmission electron microscopy of cross section samples. The laser energy density on the Bi target was varied by one order of magnitude (0.4 to 5 J cm -2 ). Across the range of energy densities, in addition to the Bi nanocrystals nucleated on the a-Al 2 O 3 surface, a dark and apparently continuous layer appears below the nanocrystals. Energy dispersive X-ray analysis on the layer have shown it is Bi rich. The separation from the Bi layer to the bottom of the nanocrystals on top is consistent with the implantation range of Bi species in a-Al 2 O 3 . As the laser energy density increases, the implantation range has been measured to increase. The early stages of the Bi growth have been analyzed in order to determine how the Bi implanted layer develops.

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.