Second and third-order nonlinear optical properties of Er³⁺/Yb³⁺ doped PbO-GeO₂-Ga₂O₃ glasses with Au nanoparticles

Highlights

• Examined glasses were prepared by melting and quenching method.

• The non-linear optics (NLO) parameters were obtained by classic Z-scan technique.

• Low efficiency of the second harmonic generation were detected.

• Annealing process changes the NLO parameters of examined glasses.

• Role of gold nanoparticles on the non-linear optics parameters has been discussed.

Abstract

This paper reports the second- and third- order nonlinear properties of Er³⁺/Yb³⁺ -doped PbO-GeO₂-Ga₂O₃ glasses with Au nanoparticles. The glasses were prepared by the melt-quenching technique, followed by annealing for the nucleation of gold nanoparticles. A 200 kV high-resolution transmission electron microscope (HR-TEM) was used to verify the presence of Au nanoparticles and estimate their sizes. The role of annealing on the possible variation of the third-order nonlinear electric susceptibility is studied in details following the possible changes in the Au nanoparticle concentration. Two-photon absorption (TPA) coefficient (β) and nonlinear refractive index (n₂) was measured by the conventional Z-scan method at 532 nm wavelength. The highest values were obtained for the sample with gold nanoparticles annealed for 52 h. Second harmonic generation (SHG) measurements were also performed and showed low conversion efficiency.

Introduction

Materials with large optical nonlinearities have recently become the topic of a broad scientific interest, mostly because of their possible application in high-speed optical switching devices, which have become increasingly important and are more commonly used [1], [2], [3], [4], [5], [6], [7], [8]. Compounds which may vary in their nonlinear optical response using external laser are of particular interest. The large interest in nonlinear optical materials has been motivated by their potential use in the fabrication of all-optical photonic devices. Among several interesting candidates for photonic applications, glasses receive special attention because of their chemical and mechanical stability and wide compositional range that can be explored to tune the desired property for a specific application. Despite the relatively low nonlinear optical (NLO) efficiencies, their applications have a principal advantage due to possible applications of the photoinduced treatment to operate by the NLO susceptibilities. Furthermore, glasses may present fast response time and high nonlinearities, features of foremost for photonic devices [9]. Glasses containing highly polarized atoms present good nonlinear optical properties on account of their easily deformable electron clouds. Heavy metal oxide glasses have shown to be promising materials for all-optical devices due to large hyperpolarizabilities of their heavy metals [10], [11]. Glasses containing lead oxide are of particular interest; they show significant laser-induced NLO effects [12], [13]. They are also adequate hosts to rare earth incorporation and have demonstrated important technological applications.

These glasses are a convenient technology for incorporation of rare earths [14]. They exhibit a large transmittance window from the visible to the infrared region, low cutoff phonon energy 700 cm⁻¹ and high refractive index (2.0). Their high third-order nonlinearities, are fundamental to the development of all-optical devices [15]. Particularly, the presence of lead oxide was investigated in tellurite, germanate, silicate and borate glass [16], [17], [18], [19], [20]. Studies on binary systems have shown an enhancement of the third-order nonlinearities with increases in lead content due to Pb²⁺ ions [17], [19]. It is known, for instance, that glasses presenting high nonlinear index of refraction also exhibit strong two-photon absorption, which is deleterious for all-optical switching applications. The knowledge of both properties, the nonlinear refraction and absorption, can help select, for example, the optimum operational wavelength for an optical device using a certain sample. Metal dielectric composites have attracted much attention from the point of view of nonlinear optics for their large nonlinear susceptibilities and fast response time [21]. One of the promising ways to enhance their NLO efficiency is to add nano-sized metallic particles. Third-order susceptibilities of dielectric materials with noble metal are considerably enhanced around the surface plasmon resonance (SPR) frequency [21]. Optical materials containing metallic nanoparticles have been attracting a great deal of attention because they may contribute to the enhancement of the optical nonlinearities as well [22]. Such improvement is significant for the development of all-optical photonic devices, such as optical switches and limiters. So the role of plasmonics in nonlinear optics was exploited in bulk and thin films. The results were interesting in that they showed the enhancement of nonlinear optical properties caused by local field enhancement because of collective oscillation of the free electron on the metallic nanoparticle surface, under laser excitation with wavelengths larger than their size. Also, it was shown that smaller nanoparticles and those with sharp edges are more interesting for nonlinear effects as the local field effects enhance the hyperpolarizabilites for these cases [22], [23]. Besides the hyperpolarizability of the glass constituents, addition of metallic nanoparticles is also expected to improve the higher-order optical nonlinearities of glasses. So, when heavy metal oxide materials are doped with metallic nanoparticles, there may be even higher nonlinear effects caused by the local field enhancement in the vicinity of the metallic nanoparticles. PbO-GeO₂ glasses with and without silver nanoparticles demonstrated to be suitable for all-optical switching at telecom wavelengths (700–1400 nm). Besides, at the visible range, a PbO-GeO₂ glass with silver nanoparticles, annealed for 3 h, behaves either as a saturable absorber or as an optical limiter [24]. Also, the third-order nonlinear properties and the response time of GeO₂–Bi₂O₃ glass, as well as the effect of gold nanoparticles on these properties was reported in the reference [15]. The results show that the presence of gold nanoparticles causes a saturable absorption effect that is overcome by the two-photon absorption process at higher light intensities, for wavelengths within the SPR band.

Besides results of nonlinear refractive index and nonlinear absorption coefficient for PbO-GeO₂, thin films with gold nanoparticles were also reported recently [25]. A large value for the nonlinear absorption coefficient was obtained at 532 nm, in the presence of gold nanoparticles, indicating possible applications for optical limiting in the nano and picosecond regime. The nonlinear refractive indices at 532 nm (15 ps pulse duration) and at 800 nm (100 fs pulse duration), are larger than those measured under the absence of gold nanoparticles, by more than one order of magnitude. The potential applications of nonlinear optical (NLO) materials in wavelength conversion, generation of ultra-short pulses, optical signal processing and ultrafast switching has motivated several studies that have been reported in the literature.

This paper presents the second- and third- order nonlinear properties of Er³⁺/Yb³⁺ -doped PbO-GeO₂-Ga₂O₃ glasses with Au nanoparticles. The NLO parameters were investigated using classic Z-scan and second harmonic generation (SHG) techniques. We report results of the nonlinear absorption and refractive index as well as the SHG efficiency. The correlation between the NLO parameters and the annealing for the nucleation of gold nanoparticles is presented, and it shows that the sample annealing process affects the NLO values. Understanding the influence of metallic nanoparticles on the NLO properties of metal-dielectric composites may be of crucial importance in designing nanoparticle -based components for special applications.