Generation of high-confinement step-like optical waveguides in LiNbO3 by swift heavy ion-beam irradiation

Abstract

We demonstrate a swift ion-beam irradiation procedure based on electronic (not nuclear) excitation to generate a large index jump step-like optical waveguide (Δn0≈0.2,Δne≈0.1) in LiNbO3. The method uses medium-mass ions with a kinetic energy high enough to assure that their electronic stopping power Se(z) reaches a maximum value close to the amorphous (latent) track threshold inside the crystal. Fluorine ions of 20 and 22MeV and fluences in the range (1–30)×1014 are used for this work. A buried amorphous layer having a low refractive index (2.10 at a wavelength of 633nm) is then generated at a controlled depth in LiNbO3, whose thickness is also tuned by irradiation fluence. The layer left at the surface remains crystalline and constitutes the core of the optical waveguide which, moreover, is several microns far from the end of the ion range. The waveguides show, after annealing at 300°C, low propagation losses (≈1dB∕cm) and a high second-harmonic generation coefficient (50%–80% of that for bulk unirradiated LiNbO3, depending on the fluence). The formation and structure of the amorphous layer has been monitored by additional Rutherford backscattering/channeling experiments.

Analysis and optimization of propagation losses in LiNbO3 optical waveguides produced by swift heavy-ion irradiation

“…Finally, good nonlinear optical and photorefractive properties have been recently reported [3,7,8]. However, the preliminary reported values for PL yielded values ranging between 1-10 dB/cm [1,8], which are still high for many applications, leaving much room for improvement and optimization.…”

“…Recently, a method to produce optical waveguides on LiNbÜ3 substrates by ion-beam irradiation [1][2][3] has been proposed and developed. At difference with conventional ion implantation that uses light ions (H, He) and relatively low energies (1-3 MeV), the method involves irradiation with ions of high energy (tens of MeV) and moderate mass, such as F at 20 MeV, often designated as swift-heavy ions (SHI).…”

Femtosecond laser and swift-ion damage in lithium niobate: A comparative analysis

“…An analysis of the damaged regions has been performed by a dark-mode method, which measures the propagating modes and resonances through the generated refractive index profile. [8][9][10] Light from a He-Ne laser ͑633 nm͒ of 5 mW power was coupled into the sample through a standard isosceles rutile prism. Also, the areal fraction of disorder has been obtained through Rutherford backscattering spectrometry [7][8][9][10] under channeling conditions ͑RBS/C͒.…”

“…7,[10][11][12][13] In such electronic regime, individual ions generate amorphous ͑latent͒ tracks of only a few nanometers in diameter [14][15][16] when the electronic stopping power overcomes a certain threshold value, which depends on material and, at a lesser extent, on irradiation conditions. This technique opens new possibilities for nanostructuring of materials and may compete with femtosecond-laser pulse irradiation.…”

“…Therefore, the purpose of this work is to present experimental observations of a number of relevant phenomenological features and explore whether a correlation between laser and ion damage can be reasonably established. We will specifically focus on a relevant photonic material, 18 LiNbO 3 ͑having a gap of around 4 eV͒, where some systematic information on the effect of femtosecond laser [19][20][21][22] and swift-ion irradiation [7][8][9][10][11][12][13][23][24][25] ready available. To the best of our knowledge, only a few systematic comparisons have so far been reported.…”