Miguel L. Crespillo

Thick optical waveguides in lithium niobate induced by swift heavy ions (~10 MeV/amu) at ultralow fluences

Heavy mass ions, Kr and Xe, having energies in the approximately 10 MeV/amu range have been used to produce thick planar optical waveguides at the surface of lithium niobate (LiNbO3). The waveguides have a thickness of 40-50 micrometers, depending on ion energy and fluence, smooth profiles and refractive index jumps up to 0.04 (lambda = 633 nm). They propagate ordinary and extraordinary modes with low losses keeping a high nonlinear optical response (SHG) that makes them useful for many applications. Complementary RBS/C data provide consistent values for the partial amorphization and refractive index change at the surface. The proposed method is based on ion-induced damage caused by electronic excitation and essentially differs from the usual implantation technique using light ions (H and He) of MeV energies. It implies the generation of a buried low-index layer (acting as optical barrier), made up of amorphous nanotracks embedded into the crystalline lithium niobate crystal. An effective dielectric medium approach is developed to describe the index profiles of the waveguides. This first test demonstration could be extended to other crystalline materials and could be of great usefulness for mid-infrared applications.

Temperature measurements during high flux ion beam irradiations

A systematic study of the ion beam heating effect was performed in a temperature range of -170 to 900 °C using a 10 MeV Au(3+) ion beam and a Yttria stabilized Zirconia (YSZ) sample at a flux of 5.5 × 10(12) cm(-2) s(-1). Different geometric configurations of beam, sample, thermocouple positioning, and sample holder were compared to understand the heat/charge transport mechanisms responsible for the observed temperature increase. The beam heating exhibited a strong dependence on the background (initial) sample temperature with the largest temperature increases occurring at cryogenic temperatures and decreasing with increasing temperature. Comparison with numerical calculations suggests that the observed heating effect is, in reality, a predominantly electronic effect and the true temperature rise is small. A simple model was developed to explain this electronic effect in terms of an electrostatic potential that forms during ion irradiation. Such an artificial beam heating effect is potentially problematic in thermostated ion irradiation and ion beam analysis apparatus, as the operation of temperature feedback systems can be significantly distorted by this effect.

In situ monitoring the optical properties of dielectric materials during ion irradiation

In this work we have used in situ reflectance to study structural modifications in silica and quartz irradiated with swift heavy ions. Quantitative analysis of reflectance spectra allowed us to (i) obtain the detailed kinetics of surface modification and (ii) reconstruct the refractive index profiles created in the irradiated materials. We have shown that in situ reflectance yields very accurate results; for instance, track radii and irradiation threshold in silica and quartz obtained from our measurements are similar to those reported in the literature. In particular, reflectance has several advantages over Rutherford Backscattering in the channeling configuration (RBS-C) because it can be measured in situ (allowing recording of detailed kinetics not attainable by RBS-C), requires less sophisticated equipment and, more importantly, can be used with any material whereas RBS-C is restricted to mono-crystalline materials.

Fabrication of Periodically Poled Swift Ion-irradiation Waveguides in LiNbO3

Periodically poled swift-ion irradiated waveguides have been prepared following two different methods: i) Ion irradiation of two types of periodically poled lithium niobate (PPLN), that is, Czochralski off-centred grown PPLN and external electrical field PPLN. After the ion irradiation, the domain structure of the original PPLN was preserved and the waveguides were also proved functional. ii) Electric periodical poling of previously fabricated swift-ion irradiated waveguides. The periodic polarization of the ion irradiated waveguide was achieved for the first time to our knowledge. In both cases, the combination of PPLN with optical waveguide structures created by swift-ion irradiation, which have good nonlinear and electrooptical properties, and high optical confinement, gives rise to quite good candidates for nonlinear optical devices.

Permanent modifications in silica produced by ion-induced high electronic excitation: Experiments and atomistic simulations

The irradiation of silica with ions of specific energy larger than ~0.1 MeV/u produces very high electronic excitations that induce permanent changes in the physical, chemical and structural properties and give rise to defects (colour centres), responsible for the loss of sample transparency at specific bands. This type of irradiation leads to the generation of nanometer-sized tracks around the ion trajectory. In situ optical reflection measurements during systematic irradiation of silica samples allowed us to monitor the irradiation-induced compaction, whereas ex situ optical absorption measurements provide information on colour centre generation. In order to analyse the results, we have developed and validated an atomistic model able to quantitatively explain the experimental results. Thus, we are able to provide a consistent explanation for the size of the nanotracks, the velocity and thresholding effects for track formation, as well as, the colour centre yield per ion and the colour centre saturation density. In this work we will discuss the different processes involved in the permanent modification of silica: collective atomic motion, bond breaking, pressure-driven atom rearrangement and ultra-fast cooling. Despite the sudden lattice energy rise is the triggering and dominant step, all these processes are important for the final atomic configuration.

Improved high temperature radiation damage tolerance in a three-phase ceramic with heterointerfaces

Radiation damage tolerance for a variety of ceramics at high temperatures depends on the material’s resistance to nucleation and growth of extended defects. Such processes are prevalent in ceramics employed for space, nuclear fission/fusion and nuclear waste environments. This report shows that random heterointerfaces in materials with sub-micron grains can act as highly efficient sinks for point defects compared to grain boundaries in single-phase materials. The concentration of dislocation loops in a radiation damage-prone phase (Al2O3) is significantly reduced when Al2O3 is a component of a composite system as opposed to a single-phase system. These results present a novel method for designing exceptionally radiation damage tolerant ceramics at high temperatures with a stable grain size, without requiring extensive interfacial engineering or production of nanocrystalline materials.

Two-stage synergy of electronic energy loss with defects in LiTaO3 under ion irradiation

ABSTRACT Understanding energy dissipation in electronic and atomic subsystems and subsequent defect evolution is a scientific challenge. Separate and combined effects of electronic and nuclear energy deposition in z-cut LiTaO3 have been investigated. Irradiation of pristine LiTaO3 samples with 2 MeV Ta ions leads to amorphization due to atomic displacement damage, described by a disorder accumulation model. While 21 MeV Si ions do not produce significant damage in pristine LiTaO3, introduction of pre-existing defects sensitizes LiTaO3 to the formation of ion tracks from the electronic energy loss by 21 MeV Si ions that induce a synergistic two-stage phase transition process. Impact statement Experimental study shows that the introduction of pre-existing defects prior to high energy irradiation sensitizes LiTaO3 to ion track formation leading to a synergistic two-stage phase transition process. GRAPHICAL ABSTRACT