The near-infrared waveguide properties of an LGS crystal formed by swift Kr8+ ion irradiation

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Abstract

In this work, we report on the optical properties in the near-infrared region of a LGS crystal planar waveguide formed by swift heavy ion irradiation. The planar optical waveguide in a LGS crystal was fabricated by 330 MeV Kr8+-ion implantation at a fluence of 1 × 1012 cm−2. The initial beam had an energy of 2.1 GeV and was slowed down by passing it through a 259 μm thick Al foil. The guided mode was measured using a prism coupler at a wavelength of 1539 nm. The near-field intensity distribution of the mode was recorded by a CCD camera using the end-face coupling method. The FD-BPM was used to simulate the guided mode profile. The lattice damage induced by SHI irradiation in the LGS crystal was studied using micro-Raman spectroscopy. The Raman spectra are consistent with the stopping power distributions of the Kr8+ ions simulated by SRIM and with the micro-photograph of the waveguide taken by a microscope using polarized light.

Introduction

Single-crystalline La3Ga5SiO14 (LGS) has been considered one of the most interesting piezoelectric crystals due to its promising applications in acoustic wave devices [1], [2]. An LGS crystal has a reasonably large piezoelectric coefficient (three times larger than that of quartz) and belongs to the point group 32 and to the space group p321 [3]. Additionally, LGS displays relatively large electromechanical coupling coefficients, and its stable structural phase enables applications at high temperatures. These favorable properties suggest the utilization of LGS for acousto-electronic applications, such as in devices based on surface acoustic waves (SAW) operated at high temperatures. The LGS crystal has also been studied for laser applications and has been reported as an excellent laser host material for rare-earth ions such as Nd3+, allowing multi-watt CW laser operation using laser diode pumping. LGS doped with rare-earth ions has been grown for the aforementioned applications [4], [5], [6]. Recently, it has been confirmed that LGS also possesses good electro-optic features, so it could be used as Q switches for lasers [7].

The waveguide structure is a basic component in integrated photonics. Integrated photonics offers a unique platform to realize multiple functions in small-size circuits in many aspects of modern photonics and telecommunication systems [8]. LGS waveguide provide a new way to apply the LGS crystal to photonics devices of micro- or nano-dimensions. There are several techniques for fabricating waveguides in optical materials, including ion in-diffusion [9], ion exchange [10], ion implantation [11], ultrafast laser writing [12] and the deposition of epitaxial layers [13]. Recent research has revealed that ion implantation could be a universal method for creating waveguide structures in most optical materials because it has superior controllability and reproducibility relative to other techniques [14]. Moreover, ion implantation allows for the burying of a waveguide barrier at different depths below the surface of a substrate by using different ion species and implantation energies during the implantation. In the waveguides formed by various ion implantations, e.g., He, H, C, O or Si, an optical barrier was built up at the end of the track due to the damage induced by stopping power. Recently, Klenmenz reported on LGS films grown by liquid phase epitaxy (LPE) for use as oscillators and resonators [15]. The fabrications of planar waveguides in LGS crystals and channel waveguides in Nd:LGS crystals were reported by Wang [16] and Ren [17] separately. To our knowledge, research on the waveguide preparation and optical properties of an LGS crystal at the wavelengths used in near-infrared communications and in infrared remote sensing has not been extensive.

In recent years, swift heavy ion irradiation has been widely used to modify various physical surface properties of materials. Generally, the path of swift heavy ions (SHIs) through a target material primarily leads to electronic excitation and ionization. In the traditional ion implantation process, the nuclear stopping power (the nuclear collisions between the implanted ion and the target lattice) plays the main role in implantation and in producing the changes in material properties. Thus, a large fluence (1016–1017 cm−2 for light ions, 1014–1015 cm−2 for heavy ions) of implanted ions is necessary to generate a sufficient number of nuclear collisions [13]. However, in the high-energy (larger than 1 MeV/amu) irradiation process using heavy ions (e.g., Ar or Kr), the electronic stopping power Se (electronic excitation) is dominant over the nuclear collision. The theory is that a single ion impact creates a partially or completely amorphous volume when the Se (the value of the electronic stopping power) is above a certain threshold, Set [18], [19], and with the increase of Se the amorphous region will increase (along the ion track, Se increases with depth up to a certain maximum and then decreases). Because the fluence is very low (approximately 1012 cm−2), the irradiation did not cover the whole surface. As an effect of the no-overlapped tracks, the material remains crystalline near the surface, whereas an optically low-refractive-index (amorphous-like) region is generated inside the crystal around the depth with the largest Se. The high-index crystalline layer at the surface constitutes the core of an optical waveguide while the low-refractive-index region plays a role of optical barrier [19], [20]. Optical waveguides have been fabricated in many crystals, such as LiNbO3 [18], [19], [20], [21], Nd:YVO4 [22], Nd:YCOB [23], BGO [24] and Nd:YAG [25], [26], using this technique. These characteristics indicate that the SHI irradiation method has obvious advantages in manufacturing waveguides for the infrared and near-infrared wavelength regions.

Section snippets

Experimental details

The Z-cut LGS crystal samples were grown by the State Key Laboratory of Crystal Materials at Shandong University. The optically polished sample was cut into a wafer with dimensions of 6 × 5 × 1.5 mm3 before implantation. The swift heavy ion irradiation process was carried out at the Heavy Ion Research Facility in Lanzhou (HIRFL) at the Institute of Modern Physics, Chinese Academy of Sciences. The sample was implanted using 330 MeV Kr8+ ions. The fluence was 1 × 1012 ions/cm2. The initial beam had an

Results and discussion

We used the SRIM code to effectively simulate the energy deposition process of swift Kr8+ ions in the irradiated crystal. The nuclear Sn and electronic Se stopping power of 330 MeV Kr8+ ions versus their trajectories in the LGS crystal are shown in Fig. 1. The results provided a better understanding of the formation mechanism of the LGS waveguide. As clearly seen in this figure, Se was overwhelmingly dominant over Sn at depths of 0–20 μm and decreased until its value reached zero at the depth of

Conclusions

We have reported the fabrication of waveguides in an LGS crystal using swift Kr8+ irradiation at low fluence. Using an SRIM simulation and micro-Raman spectra, we assessed the structural changes along the ion track and the lattice disorder distribution at the surface of the LGS crystal. The SHI generated a large irradiated region. The damage level at the end of the ion track induced by the nuclear stopping power was obviously higher than the damage level at the beginning of the ion track

Acknowledgements

This work is supported by the National Science Foundation of China (Grant No. 11275117), the National Basic Research Program of China (Grant No. 2010CB832906), NLHIRFL of China.

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