1. Introduction

The neodymium doped yttrium aluminum garnet (Nd:Y₃Al₅O₁₂ or Nd:YAG) is one of the most favorable gain media for solid state lasers owing to its excellent fluorescence and thermal properties. Optical waveguides are basic components which can offer efficient confinement of light propagation to dimensions of order of a few microns [1]. As a result, light inside the guiding structures reaches much higher intensities, and some improved performances, in respect to bulk, can be achieved [1–3]. For laser materials, the advantage of waveguides is that the low pump thresholds and enhanced efficiencies may be available owing to the strongly reduced active volumes [4].

Integrated lasers have been realized in Nd:YAG waveguides by few techniques, such as ion implantation [5–8] and ultrafast laser inscription [9–13]. Generally, the energetic ion beams can be used to modulate the refractive index of the optical crystals, which is closely correlated to the energy transferring from the incident ions to the target lattices. The normal light ion implantation technique uses H or He ions with energies from several hundred keV up to 3 MeV. In these conditions, the refractive index changes leading to the formation of waveguides are mainly given by the nuclear damage contribution [14,15]. In cases of the normal heavy ion (such as N, O, F etc.) implantation, the typical energy of the used ions is of no more than 10 MeV, and the index modification caused by the synergy of electronic excitation and nuclear collisions [15–17]. Recently, swift heavy ion irradiation has emerged as a powerful and efficient method to alter the refractive index of optical materials and hence to form waveguide structures [17,18]. With higher energy (more than 1 MeV/amu), the electronic stopping power (S _e) plays an extremely important role for determination of the electronic damage created by the incident ions. Partial or complete amorphous volume can be created either by a single ion impact (high S _e) or by the overlap of the tracks corresponding to a few ions (low S _e). The correlation of index change vs. electronic damage by the swift ion beams has been well referenced for LiNbO₃ crystals [17–20]. For the case of Nd:YAG system, there are still many open questions concerning the damage evolution process induced by the swift ions. Nevertheless, in the past authors already reported on the fabrication of optical laser waveguides in Nd:YAG crystals by swift heavy ion irradiation using 60 MeV Ar⁴⁺ ions at an ultralow fluence of 2×10¹² ions/cm² [21]. In that case the maximum S _e achieved (at a depth of 4 µm) was close to 7.5 keV/nm, so that a single ion may induce a relevant lattice distortion inside the crystal through the electronic excitation. Micro luminescence experiments carried out on these waveguides revealed that this distortion was very weak when compared to other fabrication methods (the hyper-sensitive transition lines of Nd³⁺ ions did not show any relevant distortion). This, in turns, leads to low a refractive index modulation (close to 2×10⁻³). Thus, new approaches to increase the lattice modification and, hence, to improve the refractive index modulation created by swift ion implantation in Nd:YAG crystals should be explored. These new approaches would involve the use of different implanted ions as well as different implantation energies and doses. In this work, we use 20 MeV N³⁺ ion irradiation to form Nd:YAG waveguides at a relatively high fluence (2×10¹⁴ ions/cm², i.e. two orders of magnitude larger than that previously used). Thus, by choosing these parameters, the refractive index change caused in the Nd:YAG crystal as a consequence of ion irradiation can be correlated to defect accumulation of a certain number of ions with lower S _e rather than a single ion with high S _e.

2. Experiments in details

The 1mol% Nd-doped YAG crystal was cut to dimensions of 10×4×2 mm³ and optically polished. By using the 20 MV tandem accelerator at Tokai Research and Development Center, Japan Atomic Energy Agency, 20 MeV N³⁺ ions at fluence 2×10¹⁴ ions/cm² were irradiated on one of the sample surfaces (10×4mm²). The beam direction was tilted by 7° off the surface normal plane to minimize channeling effect, and the ion current density was kept less than 300 nA/cm² to avoid the heating and charging of the sample. After irradiation, the sample was annealed at 200°C for 60 min in air to improve the guiding properties.

The m-line technique was used to measure the surface refractive index of the sample and possible surface waveguide mode via a prism coupler (Model 2010, Metricon). An end-face coupling arrangement was utilized to experimentally characterize the modal profiles of the guided modes. Both of the measurements were taken with He-Ne lasers at 633 nm.

An Olympus BX-41 fiber-coupled confocal microscope was used to measure the confocal micro-photoluminescence (μ-PL) properties of the waveguide, during which 488 nm laser radiation was focused onto the sample by using a 100 × objective with numerical aperture N.A. = 0.95, exciting the transition of Nd³⁺ ions through ⁴I_9/2 →²G_3/2. Then, the Nd³⁺ fluorescence emission spectra from the waveguide and bulk corresponding to the ⁴F_3/2→⁴I_9/2, ⁴I_11/2 emissions were collected with the same objective lens and analyzed with a fiber coupled high resolution spectrometer (SPEX500M, USA). Meanwhile, the in-depth variation of the Nd³⁺ fluorescence shift and line-width was measured with the emission line at around 945 nm.

The waveguide laser was excited by utilizing a typical end-face coupling system. During the experiment, the laser cavity was formed by two mirrors (input one with transmission of 98% at 808nm and reflectivity >99% at 1064 nm and the output one with reflectivity >99% at 808 nm and ~95% at 1064 nm, respectively) that were adhered on the two end facets of the sample. A cw Ti:sapphire laser operating at 808 nm (Coherent MBR 110) was used as the pump beam and coupled into the waveguide using a 20 × microscope objective lens (N.A. = 0.4) or a convex lens (f = 25 mm). The generated laser beam was collected with another 20 × microscope objective lens and imaged by an infrared CCD camera.

3. Results and discussion

The energy deposition process of 20 MeV N³⁺ ions when implanted onto Nd:YAG was simulated with the stopping and range of ions in matter (SRIM) 2010 code [22]. The resulting electronic and nuclear stopping powers (S _e and S _n) profiles of incoming N³⁺ ions in Nd:YAG are shown in Fig. 1(a) . As can be appreciated, within the range of 0-10 µm, the values of S _e are non-vanishing and peak at about 2.5 keV/nm at approximately 7 µm beneath the sample surface, overwhelmingly dominant over those of S _n, associated with nuclear collisions, which are about zero within the first 7 µm and climbs to the maximum (~0.25 keV/nm) at around 9.2µm. Thus, it is reasonable to suppose that the waveguide is induced by electronic damage and the width of the waveguide area is about 7 µm. From the m-line measurement, it was found that the surface refractive index was the same as that of the bulk. In addition, very clear modal profiles were observed during end-coupling experiments (see Fig. 1(b)). The waveguide is a multimode one, but in practice, the main energy is confined in the fundamental mode (TM₀) of the waveguide, as showed in Fig. 1(b). The combination of these two experimental evidences indicates that a planar buried optical waveguide has been created after ion irradiation. In other words, these experimental evidences suggest that the waveguide is constituted by a buried layer with positive-index changes located between the surface air and the substrate. Similar phenomena were observed in the 60 MeV Ar ion irradiated Nd:YAG waveguide, but differs from the normal ion implanted samples, in which the surface has a slight increment of refractive index.

 

Fig. 1 (a) The electronic stopping power (dashed line), nuclear stopping power (solid line) curves as well as the refractive profile of the waveguide (dotted line) as a function of the depth from the sample surface. The experimental (b) and simulated (c) near-field modal profiles (TM₀) of the 20 MeV N ion irradiated Nd:YAG waveguide.

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The maximum index increase created at the buried layer was estimated by measuring the numerical aperture of the fabricated waveguide to be Δn ≈ + 0.006 [8]. Considering the S _e profile as well, we reconstructed the refractive distribution of the waveguide (short dashed line in Fig. 1(a)). Based on this index profile, the modal distribution (Fig. 1(c)) of the waveguide was obtained by the well-known BPM method (Rsoft© BeamProp 8.0) [23]. From the comparison of two distributions in Fig. 1(b) and (c), one can conclude that the there is a good agreement between the experimental and simulated modal profiles, which suggests that it is reasonable to suppose that the waveguide is induced by electronic damage and the maximum refractive index increment is 0.006. Note that the refractive index modulation generated is almost three times larger than that found for the case of 60 MeV Ar⁴⁺, 2×10¹² ions/cm² ion implantation [21]. We attribute this larger refractive index modulation to the presence of accumulation effects due to the much larger fluence used in this work. If this is the case, then larger modifications of the YAG network should be observed in the waveguides object of this work when compared to those previously detected when lower doses were used. In order to verify this, μ-PL experiments were carried out.

Figure 2(a) shows the two fluorescence emission lines of Nd³⁺ ions at around 937 nm and 945 nm (corresponding to the ⁴F_3/2→⁴I_9/2) as obtained from the Nd:YAG planar waveguide and bulk. As can be observed, there is only slight change in the shape, peak position and intensity between the emission spectra obtained from the waveguide and the bulk. In addition, since these two fluorescence transitions show an hyper-sensitivity to slight changes in the Nd:YAG network [24], we investigate the in-depth variation of both the spectral peak position and full width at half maximum (FWHM) of the luminescence line at around 945 nm, which are showed in Figs. 2(b) and (c), respectively. It can be observed that, for both lines, slight blue shifts and remarkable broadenings occur at in-depth distances below 10 μm, reaching maximum at about 7 μm under the surface (i.e. at the waveguide region). This also matches the in-depth distance at SRIM code calculations positioned the maximum of S _e and also the in-depth distance in which we assumed the maximum refractive index increment has been produced. At this depth, the induced fluorescence line broadening has been found to be close to 1.2 cm⁻¹ which is close to two times larger than the fluorescence line broadening observed after 60 MeV Ar⁴⁺, 2×10¹² ions/cm² ion implantation [21]. This reveals a larger lattice modification (disordering) that we again attribute to accumulation effects caused by the larger implantation fluence used (in agreement with the larger refractive index modulation).The simultaneous broadening and blue shift of Nd³⁺ fluorescence lines suggest that the positive refractive index change buried layer is constituted by a buried layer in which the YAG lattice has been slightly dilated and disordered. As in Ref [21]. we state also in this case that these modifications lead to a local refractive index increment due to slight bond polarizability changes. At this point we should state that the values of the energy shifts as well as the line broadening are very small, and are only accompanied with negligible reduction of the fluorescence intensity. All of these facts imply that the N³⁺ ions irradiation only slightly affect the original lattice structures of the Nd:YAG crystals. As a consequence, the emission performances of the Nd³⁺ ions in the waveguide region are well preserved with no luminescence quenching.

 

Fig. 2 (a) The luminescence emission spectra of Nd³⁺ ions at ⁴F_3/2→⁴I_9/2 transition obtained from the waveguide (blue line) and the bulk (red line). Two emission peaks are indicated by two arrows. 1-D spatial scan of the spectral positions (b) and line-width (at FWHM) (c) of the Nd³⁺ emission line at around 945 nm as functions of the in-depth distance.

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Figure 3(a) compares the room temperature micro-luminescence spectra of Nd³⁺ ions in the N³⁺ ions irradiated Nd:YAG waveguide and bulk corresponding to the ⁴F_3/2→⁴I_11/2 laser channel. It can be appreciated that the outstanding fluorescence property of Nd³⁺ ions are not deteriorated by ion implantation, thus, the fabricated waveguide shows promising potential for efficient waveguide laser operation. In order to verify these good perspectives, laser experiments on the fabricated waveguide were carried out. The spectral distribution of laser radiation was found to be independent on optical coupling system (i.e., no difference in cases of convex or microscope lens coupling system) and it is shown on Fig. 3(b). Lasing was obtained in a single line centered at 1064.2 nm with a FWHM of 0.4 nm.

 

Fig. 3 (a) Comparison of the room temperature micro-luminescence emission spectra correlated to Nd³⁺ ions at ⁴F_3/2→⁴I_11/2 transition obtained from the planar waveguide (red line) and the bulk (blue line). (b) 1064.2 nm laser emission spectrum from the Nd:YAG planar waveguide.

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The laser curves obtained by using the two optical coupling systems are shown in Fig. 4 . When the pump laser beam (at 808 nm) is coupled into the waveguide with the microscope objective lens (Fig. 4(a)), output power of 5 mW was obtained with laser thresholds and slope efficiencies of ~38.3 mW and ~6.6%, respectively. On the other hand, when using the convex lens the laser curve denotes a laser threshold of 79.6 mW and an improved laser slope efficiency of 16% (Fig. 4(b)).

 

Fig. 4 Output laser power as a function of absorbed pump power obtained from the Nd:YAG waveguide.(a) and (b) shows the laser curves obtained when a microscope objective and a single convex lens have been used as the coupling system. The solid lines are the linear fit of the experimental data (solid dots). The insets show the near-field light intensity distributions of the output laser beams.

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The different laser performances observed obey to the different pump spot sizes achieved at input face with these two optical systems. The convex lens and the microscope objective focus the 808-nm beam down to a spot of ~18 μm and 1.3μm, respectively. The smaller-scale pump beam is with higher intensity, realizing laser generation at lower threshold. On the other hand, the overlap between the pump beam and the waveguide modal field is much lower (i.e., field mismatch) in configuration of small pump beam and large waveguide dimension, resulting in less absorbed power of the pump light and the lower efficiency and output waveguide laser power. This phenomenon has been well demonstrated by the theoretical modeling and analysis reported previously [25,26]. When the laser performance of the waveguide fabricated in this work is compared to that obtained from the other swift ion implanted Nd:YAG waveguide reported up to now (60 MeV Ar⁴⁺, 2×10¹² ions/cm² ion implantation) [21], we found that both showed a very similar behavior in terms of both threshold and slope efficiencies [5–7,19]. It should be noted that further improvement of the laser performance is expected if the focusing system is designed to provide a pump spot matching the mode field diameter of the waveguide (close to 10 µm).

4. Summary

We report on the fabrication, optical, spectroscopic and laser properties of Nd:YAG optical planar waveguides fabricated by swift N³⁺-ion irradiation. The fabricated waveguides simultaneously show good guiding and fluorescence properties. Micro-photoluminescence experiments have revealed that ion implantation has been found only to slightly modify the fluorescence properties of Nd³⁺ ions being these modifications associated to the appearance of a buried layer in which the Nd:YAG lattice has been slightly dilated and distorted. Based on the good preservation of the Nd:YAG fluorescence properties at waveguide’s volume, the fabricated waveguides were demonstrated to be capable of laser generation at 1064.2 nm with lowest threshold of 30 mW and highest laser slope efficiency of 16%.