1. Introduction

As one of the most favorite gain media, neodymium-doped yttrium vanadate (Nd:YVO₄) has been widely used in solid state laser systems owing to its many excellent features, such as high emission cross section, broad absorption bands, excellent thermal and mechanical properties [1,2]. Optical waveguides restrict light propagation in reduced volumes and lead to high intensities inside the structures; as a consequence, waveguide lasers are with lower lasing thresholds than that of the bulk laser systems [3–8]. In addition, the laser elements based on waveguides benefit from the small scales of the structures, which enables further integration for the construction of multiple functional devices for diverse photonic applications.

Waveguide structures have been fabricated in Nd:YVO₄ crystals by several techniques such as ion implantation [9–13], thermal diffusion [14,15], and femtosecond laser inscription [16], and the integrated lasers have been realized in some of the samples [17,18]. Recently, the swift heavy ion irradiation has emerged as a powerful method to modulate the refractive index of optical materials and to form waveguides. In such processes, high-energy (energies larger than 1 MeV/amu) heavy ions (e.g., F, Cl, Ar) are incident into the optical materials to modify the surface properties of the substrates. Differently from the traditional light ion implantation (H and He) where nuclear collisions between incident ions and target atoms play a main role, the electronic excitations are dominant over nuclear damage, which is responsible for the refractive index changes of the irradiated materials. The electronic stopping power (S_e) is one of the key parameters to determine the electronic damage and the induced refractive index change [19,20]. When S_e is close to the material-dependent threshold value, a single ion impact will created partial or complete amorphous volume, and hence induces considerable refractive index changes along ion trajectories [21,22]. There is a remarkable advantage of swift heavy ion irradiation over the normal ion implantation for waveguide formation of optical materials, that is, the conspicuous reduced irradiation fluence. While the fluences generally used in light ion implantation are 10¹⁶~10¹⁷ cm⁻² and 10¹⁴~10¹⁵ cm⁻² for normal heavy ion implantation, fluences at the level of 10¹² cm⁻² are adequate for swift heavy ion irradiation processes to confine light propagation in the irradiation region when the S_e is above the damage threshold [20,23]. The significantly reduced fluence for waveguide formation by using swift heavy ions results in faster fabrication of the guiding devices. Compared with femtosecond laser inscription, the advantage of swift ion irradiation is the ability of easy production of a large-area circuit planar waveguide layer within very short time. This faster processing makes the swift heavy ion irradiation a promising technology for the possible industrial production. As of yet, researchers have used this technique to fabricate waveguides in a few optical crystals such as LiNbO₃, KGW, Nd:YAG and Nd:GdCOB [19–21,23–27]. In this paper, we report on the fabrication of planar waveguide in Nd:YVO₄, using Ar⁸⁺ ions irradiation at energy as high as 180 MeV and low irradiation fluence at 2 × 10¹² cm⁻². The micro-photoluminescence (μ-PL) and the laser performance of guiding structure have also been investigated.

2. Experimental methods

The Nd:YVO₄ crystal (doped by 1mol% Nd³⁺ ions) used in this work was cut into dimensions of 10(a) × 2(b) × 3(c) mm³ and optically polished. One 10 × 3 mm² surface was the irradiated surface. The Ar⁸⁺ ion irradiation process was carried out by using the facility “HIRFL” at the Institute of Modern Physics, Lanzhou, Chinese Academy of Sciences. The accelerating energy was set at a fixed value of 880 MeV and the fluence was at 2 × 10¹² cm⁻². Before the Ar⁸⁺ ions incident into the Nd:YVO₄ crystal, an aluminum foil with proper thickness was used as a mask in order to slow down the incident ions. The ion current density was kept less than 30 nA/cm² to avoid charging and heating effect of the sample. According to our calculation, the practical irradiation energy reaching on the sample surface was 180 MeV.

The micro-photoluminescence (μ-PL) properties of the waveguides were studied through an Olympus BX-41 fiber-coupled confocal microscope equipped with a 488 nm argon laser. The 488 nm excitation laser was focused onto the cross section by using a 100 × microscope objective with numerical aperture N. A. = 0.95. Then, the back-scattered Nd³⁺ fluorescence signals were collected with the same objective and, after passing through a series of filters and a confocal pinhole, were collected by a fiber-coupled spectrometer (SPEX500M, USA). The sample was mounted on an XY motorized stage with a high spatial resolution of 100 nm.

Laser operation experiment was also launched by using a typical end-face coupling system [21] at room temperature. Two mirrors (the input one with transmission of 98% at 808 nm and reflectivity >99% at 1064nm and the output one with reflectivity >99% at 808 nm and ~95% at 1064 nm, respectively) were attached the two end-faces of the Nd:YVO₄ sample by pressure, respectively, to form the Fabry-Perot laser cavity. A Ti:sapphire cw laser (Coherent 110) was used as the pumping source. The generated 808 nm light beam was focused into the cavity using a convex lens (focus length of 25 mm), and the emission waveguide laser was collected with a 20 × microscope objective lens and imaged by an infrared CCD camera.

3. Results and discussion

The energy deposition process of the swift Ar⁸⁺ ions irradiation on Nd:YVO₄ was simulated with the software Stopping and Range of Ions in Matter (SRIM) 2010 code [28]. The curves of the electronic and nuclear stopping powers (S_e and S_n) as functions of penetration depth of the Ar ions into the Nd:YVO₄ are shown in Fig. 1 as dashed and solid lines, respectively. As we can see, the S_e was dominant over S_n in the first 30 μm. The maximum value of S_e was 6.9 keV/nm at depth of ~26 μm. While the S_n remained nearly zero in the first 30 μm, and climbed to a peak of about 0.8 keV/nm at depth of 32 μm. This suggests that the electronic damage plays the main role for the possible refractive index changes for the waveguide formation.

 

Fig. 1 The electronic (dashed line) and nuclear stopping power (solid line) curves of 180 MeV Ar ions in Nd:YVO₄ crystal as functions of penetration depth from the irradiated sample surface.

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Figure 2(a) shows the microscope image of the end-face of the 180 MeV Ar ion irradiated Nd:YVO₄. As we can see, the waveguide vertical depth was about 31μm, which is consistent with the mean projected range of the Ar ions based on the SRIM calculation. It should be noted that the rough surface remains after the polishing, which may not significantly influence the guiding properties since the waveguide layer was adequate thick. We also measured the modal profile of the waveguide at wavelength of 632.8 nm through a conventional end-face coupling system, using a He-Ne laser. The profile (near-field intensity distribution of the light from the exit face of the waveguide) of a TM mode (TM₂) was shown in Fig. 2(b). From the near-field image, we can see that an obvious multi-mode waveguide was formed. However, the majority of the light field was limited in the fundamental mode (TM₀). We estimated the maximum refractive index increase to be about + 0.015 in the guiding region, by measuring the numerical aperture of the fabricated waveguide [29].

 

Fig. 2 (a) The microscope image of the end-face of the 180 MeV Ar ion irradiated Nd:YVO₄ sample, and (b) the near-field modal profile through end-face coupling system, captured by a CCD camera. White dashed line indicates the boundary of the sample surface and the air. The waveguide mode is TM₂.

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In order to obtain a better understanding of the physical mechanism of the Ar⁸⁺ ion irradiated Nd:YVO₄ waveguide formation, we performed the confocal microscopy to analyze the fluorescence properties. Figure 3(a) shows the typical confocal luminescence at room temperature obtained from the bulk of the Nd:YVO₄ crystal excited by a cw 488 nm argon laser. We focused on this hyper-sensitive 913.6 nm emission line. Figures 3(b), (c) and (d) depict the 1D μ-PL profiles based on the spectral intensity, spectral shift and the spectral broadening of this line, respectively. As we can see, the intensity has a reduction from 7 to 40 μm below the ion irradiated surface of the crystal and the minimum is around 50% of the amplitude of bulk; however, the fluorescence features are well preserved in the first 7 μm depth compare to the bulk. The fluorescence intensity reduction is mainly attributed to lattice damage (lattice defects and imperfections etc.) induced by the electronic collision during the Ar⁸⁺ ions irradiation process. As for the spectral broadening, it starts to grow from the beginning of the edge and reaches the maximum of 25 cm⁻¹ at the in-depth distance of 30 μm. The remarkable line broadening suggests the presence of lattice disorder along the ion trajectory. This is in agreement with the lattice defects and imperfections concluded from the luminescence intensity quenching. The position shift profile clearly denotes that the emission line has been shifted to larger energies during the whole ion trajectory. In the first 20 μm, the blue shift is only around 0.6 cm⁻¹ in respect to the bulk; subsequently, the value increases slowly and reaches the maximum of 1.8 cm⁻¹ at the range of 27 μm to 30 μm below the crystal edge.

 

Fig. 3 (a) Typical luminescence spectrum of Nd:YVO₄ crystal, (b) 1D spatial scan of the emitted intensity, (c) spectral shift, and (d) spectral broadening of the hyper-sensitive 913.6 nm Nd³⁺ emission line.

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Figure 4(a) depicts the waveguide laser spectrum (cw) of the sample, carried on with TM mode. Differently from some previous reports on Nd:YVO₄ waveguide lasers [16,17], in the present work, the wavelength of the laser emission from the waveguide was at 1067 nm. The similar experiment was also performed for the bulk crystal, in which both 1064 and 1067 nm (dual-wavelength) laser oscillations could be generated. The FWHM of the emission spectrum was of ~0.3 nm, which clearly denotes the laser generation. The modal file of the waveguide laser emission is shown as an inset of Fig. 4(a). Figure 4(b) shows the dependence of the output laser power at 1067 nm as a function of the absorbed pump power at 808 nm. The absorbed pump power is calculated by considering the coupling efficiency between the pump beam and the waveguiding mode, the transmittance of the optical elements in the system, and corrected by the waveguide parameters (such as propagation loss, absorption coefficients of the crystal at pump beam wavelength). The pump threshold power was at 9.3 mW, and slope efficiency of waveguide laser system was ~8.5%. As the absorbed pump power increase, the output laser power climbed to a maximum at 3.1 mW at pump power of 47 mW, corresponding to an optical-to-optical conversion efficiency of 6.6%. Compared with the laser performances of femtosecond laser inscribed Nd:YVO₄ channel waveguide, the 180 MeV Ar⁸⁺ ion irradiated planar waveguide possesses lower lasing threshold, whilst the lower slope efficiency and maximum output power are the disadvantages. This may be improved by constructing a channel waveguide configuration and/or reducing the propagation losses by further annealing treatment.

 

Fig. 4 (a) Laser emission spectrum (cw) from the 180 MeV Ar ion irradiated Nd:YVO₄ planar waveguide (inset shows the near-field modal profile at TM polarization) and (b) the dependence curve of the output laser power as a function of absorbed pump power.

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4. Summary

We have reported on the fabrication of optical planar waveguides in Nd:YVO₄ laser crystals, using swift Ar⁸⁺ ion irradiation at energy of 180 MeV and fluence of 2 × 10¹² cm⁻². The waveguide was formed in the electronic damage region, with a maximum ordinary index increase of about 1.5 × 10⁻². The μ-PL properties have been modified to some extent, suggesting lattice disorder happened within the ion trajectory. The cw waveguide laser at 1067 nm was realized with a low lasing threshold of 9.3 mW.