Synergistic effects assessment between nuclear damage and electronic energy dissipation in LiTaO3 under heavy ion irradiation using optical waveguides properties and the irradiation angle of incidence

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Abstract

High energy heavy ion irradiation has been performed in LiTaO3 in a broad set of experimental conditions to study the synergistic effects in the damage generation between the nuclear and the electronic energy loss mechanisms. Optical waveguides have been fabricated in LiTaO3, using 20–25 MeV F and 40 MeV Si ions, and their refractive index profiles were used as a very accurate method for the in-depth damage profile determination and its correlation with the energy loss curves. In-situ optical reflectance of low energy irradiations (500 keV F and 300 keV Si ions) has been performed to estimate the nuclear damage kinetics of the buried regions of the high energy irradiations that are not optically accessible with the optical waveguide characterization in the fluence regime when the electronic damage is intense. The angle of incidence has been varied to enhance the damage and further ascertain the existence of a synergistic mechanism.

Introduction

LiTaO3 (LT) is one of the most attractive materials for photonic applications due to its outstanding ferroelectric and optical properties. Its nonlinear optical (NLO) properties make it a very promising material for integrated optics applications and an alternative to LiNbO3 (LN) [1], offering some clear advantages such as wider transparency range into the UV and better resistance to optical damage [2], [3]. It has been, so far, less studied mainly since its lower Curie temperature complicates the application of some traditional waveguide fabrication methods like Ti in-diffusion. Interestingly, LT has been shown to offer better ferroelectric domain reversal patterning, expecting to generate smaller ferroelectric domains than those achieved in LN, which is necessary for the fabrication of NLO devices in the UV [4], [5], [6].

High energy heavy ion irradiation, also called swift heavy ions (SHI) for energies higher than around 1 MeV/amu, has been shown to be a powerful method to modify the structure and optical properties for the efficient fabrication of good quality optical waveguides in LiNbO3 [7], [8] and other optical materials [9], [10], [11], [12]. The ion fluences needed for waveguide fabrication using SHI are several orders of magnitude lower (1011–1015 at/cm2) than those required by light ion implantation (1016–1017 at/cm2) [12], [13]. The damage generation under SHI irradiation is different in most materials since it has been found that the energy loss by inelastic collisions between the ions and target electrons (electronic stopping power, Se) plays a more relevant role, compared to the damage caused by the elastic nuclear collisions (nuclear stopping power, Sn). At these high energies, the maximum value of Se is located a few microns underneath the surface, as required to produce a buried optical barrier due to the structural damage created in most crystals. Although the origin of the nuclear damage is reasonably well understood, the damage mechanisms due to electronic excitation are not yet fully established. On the very high energy loss regime, using the heavier ions, the energy loss per ion track increases above the so called amorphization threshold (Se,th, characteristic of each material); each ion creates along its trajectory an amorphous latent track, whose diameter (a few nm) is proportional to the energy loss and, therefore, is increasing along the track following the energy loss curve for the energy region where E > 1 MeV/amu and further enhanced due to the velocity effect. Thermal spike models are the most recognized ones to explain the observed phenomena in this high energy regime [14], [15], [16]. This type of high energy irradiation creates, by keeping the fluence in the isolated track regime (i.e. at ultralow fluences of the order of 1011 at/cm2), an effective nanostructured medium that behaves like an optical waveguide [8]. For medium mass ions (like O, F, Si) moderate electronic damage is created per ion track that accumulates and, after track overlapping given enough fluence, finally creates buried thick homogeneous amorphous layers with quite sharp boundaries and high refractive index change (decrease > 10%) that allows for high optical confinement as shown in several crystals [7], [9], [10]. Being clear that the electronic energy deposition of SHI plays a fundamental role in the damage generation and the subsequent structural changes of the diverse irradiated materials, the rigorous modeling and understanding of the basic mechanisms of the damage process are still under debate in the community, as discussed in several references [14], [15], [16], [17], [18], [19], [20].

Given the interest in the potential combination of the excellent optical and ferroelectric properties of LiTaO3 with the capabilities of SHI irradiation structuring, recently a first study on the fabrication and characterization of optical waveguides in LT by means of using SHI irradiation has been performed [21], [22]. Aside from the interest in potential technological achievements, taking the broad knowledge available in closed oxides materials, cited above as a reference, can significantly help to improve the understanding of the fundamental aspects of SHI damage generation. In fact, recently, various types of synergy in the damage generation and accumulation between the nuclear and electronic energy loss in various materials (i.e. enhancing or healing the damage depending on the material and particular values Se and Sn) are getting attention [23], [24], [25], [26], including the specific case of LT [27], [28], [29], [30]. In this work, by using a combined analysis based on optical waveguide characterization and optical reflectance, we present a more systematic study of the damage of LiTaO3 under SHI irradiation using a broad set of experimental conditions, regarding the key parameters under SHI irradiation, the value of electronic (Se) and nuclear damage profile. F and Si ions at high and low energies and at a broad set of fluencies have been used. The synergistic effect between the electronic and nuclear damage has been addressed. We show how the angle of incidence used in the irradiations helps to enhance the synergy of both damage mechanisms.

Section snippets

Experimental methods

Z-cut 1 mol% MgO doped stoichiometric LiTaO3 (SLT), provided by Oxide Corporation, and Z-cut congruent LiTaO3 (CLT) wafers, provided by Roditi Corporation, of 1 mm thickness were used along this study. The samples were cut in pieces of about 5 × 5 mm2 for the systematic irradiations and subsequent optical characterization. The irradiations were carried out using the 5 MV Tandem accelerator facility at the Centro de Micro-Análisis de Materiales (CMAM) [31], [32] in Universidad Autónoma de Madrid

Characterization of fluorine irradiations: optical waveguides and nuclear damage analysis.

The measured modes effective refractive indices (Nm) and their corresponding refractive index profiles of the irradiations of SLT crystals made with 25 MeV F ions at an incidence angle of 45° are shown in Fig. 3, for the ordinary polarization. A broader fluence range than in the previous work [21] was explored in order to understand the complex damage kinetics found. As it can be seen, for the lowest fluence range required to produce an optical waveguide, (3.5–20) × 1013 at/cm2, the refractive

Conclusions

We report a detailed study of the kinetics of damage in LiTaO3 under swift heavy ion irradiation, specifically addressing the topic of the potential synergistic effect between nuclear damage and electronic excitations. To this end, the in-depth characterization of the structural damage provided by the properties of the optical waveguides fabricated under 20–25 MeV F and 40 MeV Si irradiations have been used. In addition, the analysis has been combined with surface in-situ optical reflectance

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work has been supported by the Spanish Ministry of Economy and Competitiveness through the project MAT-2011-28379-C03-02, and by Madrid Community through the Program TECHNOFUSION(III)-CM(S2018/EMT-4437). We thank the Technical Staff of the CMAM-UAM center for their support with the irradiations.

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      It also has a wide range of possible applications in various radiation environments, such as tritium breeding in fusion reactors, nuclear waste immobilization, and radiation monitoring [22,23]. Recently, many different types of ions, such as light ions, heavy ions, and swift heavy ions, have been utilized to irradiate LiTaO3 for performance regulation, waveguide manufacturing, and damage effect investigations [17,24–33]. Amorphization of LiTaO3 induced by ion irradiation has always been an intriguing phenomenon, which often greatly depends on the irradiation conditions [34,35].

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