1. Introduction

Yttrium gallium garnet (Y₃Ga₅O₁₂, YGG) is a kind of laser crystal. It belongs to cubic crystal. Its unit cell can be seen as a network of dodecahedrons, octahedrons and tetrahedrons. Garnet materials are useful as solid-state laser hosts for their excellent optical and physical properties such as high thermal conductivity and being optically isotropic. As an excellent representative, yttrium aluminum oxide (Y₃Al₅O₁₂/YAG) got a lot of attentions [1]. As another outstanding candidate, YGG has excellent optical prosperities, machinability, chemical stability and physical characteristics, and is regarded as a good luminescent material or laser material. As a laser crystal, Y³⁺ in the YGG is easily replaced by lanthanide ions, because the ionic radius of Y³⁺ is very close to that of lanthanides. The rare earth ions doped into YGG act as activators. In this work, we chose neodymium ion (Nd³⁺) doped into YGG crystal for the salts of Nd³⁺ giving good laser actions [2]. The laser source around 555 nm has potential application value in medical field because it could be used as carbon monoxide detector in blood. As reported in [3], Nd:YGG crystal could emit the laser at 1110 nm with a pump source. Based on this, 555 nm yellow–green laser output could be obtained easily by use of relative phase matched crystal such as LBO. The Nd:YGG crystal makes it easy to the generation of this wavelength laser. The researchers have already reported laser emission characteristics [4] and fluorescence characteristics [5] of Nd:YGG crystal due to its superiority on laser emit. The waveguide structures based on Nd:YGG may also be used as waveguide lasers or amplifiers, etc. There had some of report on Nd:YAG crystal waveguide, which has similar structure with Nd:YGG. Continuous-wave lasers at ∼1064 nm have been achieved in the ridge waveguide in Ref. [6], and the optical amplification based on Nd:YAG waveguide was obtained in Ref. [7].

As the essential component of the integrated optics, waveguide can be processed into various devices, such as optical switches [8], couplers [9], waveguide lasers [10,11] and photonic crystals [12]. Many researchers have formed optical waveguide structures by ion implantation [13,14], ion diffusion [15], femtosecond laser writing [16,17] and molecular beam epitaxy [18] on hundreds of materials, such as polymers [19], laser crystals, glass [20], ceramics [16] and semiconductors [21]. Swift heavy ion (SHI) irradiation is a feasible approach to fabricate waveguide and has good repeatability for the controllable ion species, irradiated energy and fluence. However, the fluence of ion irradiation is much lower than that of traditional ion implantation 2-3 orders of magnitude in respect to ion irradiation (10¹¹-10¹⁴ions/cm2) and ion irradiation (10¹⁴-10¹⁶ions/cm²), respectively. The optical properties could be modified significantly at ultra-low ion fluence due to the different physical mechanism for swift heavy ion irradiation. It is attributed to the electronic excitation (the so-called electronic damage) rather than by the nuclear collisions with the target atoms [22]. In the swift heavy ion irradiation process, the so-called heavy ions, such as C, N, O, Si, Ar [14] and Kr [23], can be implanted into the samples at energies of tens of MeV to GeV. Recently, Kr-ion irradiation is also employed to YAP and YAG [24] to study the effect of swift heavy ion irradiation. However, the optical fabrication and properties on Nd:YGG crystal by use of swift heavy ion irradiation haven’t been reported up to now.

Kr⁸⁺ ion irradiation was used in this paper to fabricate a single mode waveguide in Nd:YGG crystal. The optical properties of the waveguide were investigated by prism-coupling and end face coupling methods. The energy loss was simulated by the stopping and range of ions in matter (SRIM) code 2010 and the results have shown that the electronic damage created by ion irradiation is the major factor for the increase of refractive index in waveguide. Fluorescent and Raman spectrums, before and after ion irradiation, were given to demonstrate the effects of Kr⁸⁺ irradiated on Nd:YGG crystal as well.

2. Experimental

The Nd:YGG crystals used in this work were provided by State Key Laboratory of Crystal Materials, Shandong University of China. The crystal grown by optical floating zone method had a cubic structure with the dimension of 3mm×2mm×1.5 mm, the sample's density was 5.7 g/cm³ and the Nd-doping concentration was 1 at.% (in the melting condition). The surface of Nd:YGG crystal was optically polished and cleaned before the ion irradiation. The swift Kr⁸⁺ ion irradiation experiment performed at the Heavy Ion Research Facility in Lanzhou (HIRFL) at the Institute of Modern Physics, Chinese Academy of Sciences. In the ion irradiated process, we have used Al foil with the thickness of 288 µm to slow down the speed of ions. The initial beam energy was 2.1 GeV, but when they pass through the Al foil the beam energy was about 30 MeV calculated by stopping and range of ions in matter (SRIM) code 2010. The ion beam was irradiated over the Al foil vertically, but the sample was tilted 7° off the incident Kr⁸⁺ ion beam to prevent channeling effect. The ion current density and the ion irradiation fluence were set to 10-30 nA/cm² and 2×10¹² ions/cm² respectively.

We simulated the transport process of Kr⁸⁺ in the Nd:YGG crystal at energy of 30 MeV by SRIM2010. We were only concerned with the energy loss for convenience and to avoid confusion. The energy loss profile determined the formation mechanism of Nd:YGG planner waveguide in swift heavy ion irradiation process according to previous reports [25,26]. The guided mode effective refractive indices (n_eff) versus relative reflected light intensity distribution was measured by the prism coupling method with a Metricon prism coupler (Model 2010) at wavelength of 633 nm. After polishing the end face of the sample, the end face coupling equipment was utilized to investigate the near-field light intensity of the guide mode at the wavelength of 633 nm which was a visualized way to judge the qualification of the fabricated waveguide structure. The beam propagation method (BPM) was used to simulate the light propagation process of Nd:YGG waveguide. Both the micro-luminescence emission spectra and the Raman spectra were measured at the School of Chemistry and Chemical Engineering, Shandong University with a laser excitation wavelength of 633 nm and the focused laser spot diameter of 1µm. The confocal micro-fluorescence experiments were performed to measure the emission spectra of Nd ions at ⁴F_3/2-⁴I_9/2 transition, measuring the substrate and the waveguide respectively through adjust the location of the focused laser spot at the sample end face. To study the structure of the original and irradiated Nd:YGG sample, the Raman spectra has been measured on a JYT64000. All of these measurements were operated at room temperature.

3. Results and discussion

The prism coupling measurement has performed on the sample at Metricon 2010 prism coupler with the wavelength of 633 nm and 1539 nm and the resolution was better than 0.0002. During the prism-coupling measurement, a dark mode will be observed when the laser beam is coupled into the experimental sample because a CCD is arranged to collect the reflected light. For this, a lack of reflected light will result in a dip of the dark mode spectrum. The narrow and deep dip (sharp dip) in the dark-mode profile predicates that the planar waveguide could carry a real propagation mode. Figure 1 shows the measured relative intensity of the transverse magnetic (TM) polarized light versus the effective refractive index of the Nd:YGG planar waveguide fabricated by swift heavy ion irradiation at the energy of 30 MeV with the influence of 2×10¹² ions/cm². The substrate refractive index of the Nd:YGG crystal is labeled (n_sub=1.9353) also for comparison. As shown in Fig. 1(a), we can learn that there are two dark modes (dip in the reflected intensity), and the effective refractive index (n_eff=1.9366) of the first sharp dark mode (TM₀ mode) is higher than the substrate refractive index of the Nd:YGG crystal. The surface refractive index (n_sur=1.9389) of the Nd:YGG sample is enhanced after our ion irradiation process. There is only one broad dip in Fig. 1(b). The measured results presented in Fig. 1 indicate that the waveguide could carry TM₀ mode only at the wavelength of 633 nm and there has no real mode at the wavelength of 1539 nm correspondingly.

 

Fig. 1. Reflective light intensity (TM polarized) reflected from the prism versus effective refractive index profile (a) 633 nm; (b) 1539 nm.

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When we performed the microscope measurement, the end face of the Nd:YGG crystal have been polished after SHI irradiation process. If the light shines on the sample, the intensity of reflected light controlled by refractive index will affect the color of image. The microscope image of the polished end face of swift Kr⁸⁺-irradiation Nd:YGG crystal is shown in Fig. 2 Obviously, it shows the clear distinction among air, waveguide and substrate region by different colors. In the Fig. 2, we can see the ruler has been presented by the blue line. The depth of waveguide region between the substrate edge and air is about 3.7 µm. For further analysis the waveguide formation mechanism in Na:YGG crystal the electronic and nuclear energy loss of Kr⁸⁺ irradiated into Nd:YGG crystal at energy of 30 MeV was simulated by SRIM 2010 and the results are shown in Fig. 3(a). In the process of simulation, we have taken the effect of Al foil into consideration for its non-negligible role during ion irradiation. Therefore, the energy deposition process we obtained is irradiated over Al foil and Nd:YGG sample. As one can see, within the penetration depth range of 0-5.64 µm, the electronic stopping power (S_e) is monotonically decreasing. We have found that S_e is dominated over the nuclear stopping power (S_n) in first 4.3 µm and S_n is achieved its maximum value (S_n,_max ≈ 2.7 keV/nm) at a depth of ∼4.82 µm. From the analysis of the measured metallographic microscope image of Fig. 2, one can determine that the waveguide (first 3.7µm) is formed in the surface region for S_e dominated. Indeed, this suggests that the electronic damage plays a key role in inducing surface refractive index increase in the Nd:YGG crystal. Although more detailed investigations are required, it is reasonable to suppose that partial damage has been induced along the S_e profile with Kr⁸⁺ ion irradiation [23], producing a microstructural modification in the Nd:YGG network that results in a refractive index increment profile in the surface region. The specific value of the refractive index at any depth cannot be measured directly, but we supposed that the refractive index profile is a semi-Gauss curve and set the refractive index is 1.9389 and 1.9353 at the depth of 0 and 3.7 µm respectively based on the prism coupling and metallographic microscope measurement results. The simulated result is shown in Fig. 3(b). As one can observe the region of refractive index increment is narrower than S_e profile, we suggest that a “critical value” of electronic damage exists under our irradiation condition. It could produce enhancement of refractive index when S_e reaches a “critical value” (about 3.0 keV/nm). In other words, the S_e is insufficient after the depth of 3.7 µm for refractive index increment in Nd:YGG crystal. This is the critical factor for S_n has no effect on the variance of refractive index. However, the specific cause of the crystal lattice modification corresponding to “critical value” in surface region is still not clear. Further investigation needs to be done.

 

Fig. 2. Microscopic photograph of the cross-sections of the Nd:YGG waveguides with Kr⁸⁺ irradiation at a influence of 2×10¹² ions/cm²

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Fig. 3. (a) Electronic and nuclear energy loss simulated by the stopping and range of ions in matter (SRIM) 2010 program for Kr⁸⁺ ions irradiated into the Nd:YGG crystal; (b) calculated refractive index profile.

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The Fig. 4(b) shows the simulated waveguide modal profile based on the assumed refractive index profile in Fig. 3(b) by use of BPM. The end face coupling method was used to investigate the light propagation properties of the Nd:YGG planar waveguide. In the end face coupling system, the laser with wavelengths of 633 nm has been used. The light moved into and out of the polished end face of the sample. At the output facet, a CCD camera connecting a PC with a laser beam analyzer is adhered to collect the light and image the propagation profile. The experimental result of the near-filed light intensity profile at the wavelength of 633 nm is shown in Fig. 4(a). It is obvious that a good agreement between the simulated mode and the experimental one shown in Fig. 4(a) has been achieved. These facts indicate that the Kr⁸⁺ irradiation has formed a “well” type singe mode planar waveguide (with positive refractive index change) between the air and substrate layer, and the light can be confined in the Nd: YGG planar waveguide region very well. This is at variance with previous reported Nd:YGG waveguides, in which a “barrier” was induced at the end of the ion range [27–29]. Furthermore, we measured the propagation losses by end face coupling method; the loss value is about 0.8 dB/cm [30]. As reported in [20], an index enhanced “well” confined waveguide has advantages for obtaining lower propagation loss because it could avoid the tunnelling effect.

 

Fig. 4. The near-filed light intensity profile of Nd:YGG planner waveguide measured by end face coupling method at the wavelength of 633 nm (a) experimentally and (b) simulated by assuming the refractive index profile (Fig. 3(b)).

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We will analysis and discuss the experiment results in three aspects compared to previous related reports [27–29]. Firstly, we discuss the refractive index profile. The waveguide formation attributed the index decrease “barrier” in He-implanted and there is an index enhanced “well” in formed by 5 MeV C ion-implanted and swift Kr ion irradiation. Secondly, the Nd:YGG waveguides have distinct DPA (displacement per atom) values fabricated by different implantation condition. The maximum (DPA) values were 0.818 [27], 2.0 [28], 0.15 [29] and 0.0012 in this work. Thirdly, we presented the comparison of waveguide quality which characterized by propagation loss. The loss value in this work could match the results in Ref. [29] (0.8 compared to 0.83 dB/cm) while the loss values of He-implanted waveguide is relative higher than the demand of practical application field. Therefore, in the case of similar propagation loss requirements, C implantation or Kr irradiation can be selected according to the actual situation. Except the outstanding waveguide quality, this work has one more important advantages compared with the other kinds of ion implantation. The waveguide formed by swift Kr⁸⁺ ion-irradiation has very slight DPA value attributed to the three orders magnitude lower fluence (2×10¹² ions/cm² contrast to 1×10¹⁵ ions/cm²). This gives the better preserve of crystal surface quality and the waveguide formed in this work has undoubtedly superiority of being applied to practical devices with high requirements for crystal quality. Based on these analyses, the waveguide formation condition we selected could be a competitive way for waveguide fabrication on Nd:YGG crystal.

We analyzed the fluorescence emission spectra of Nd³⁺ transited from ⁴F_3/2 to ⁴I_9/2, before and after ion irradiation, shown in Fig. 5, which was measured from the Nd:YGG end face. It can be seen from the Fig. 5 that the main peak is located at about 876 nm. The fluorescence intensity of the waveguide region is lower than that of the substrate, for the reason that the different of refractive index will change the intensity of PL spectrum certainly. The waveguide, in this work, the refractive index changes are mainly caused by electronic damage in the irradiated region. In other words, this change can be attributed to the electron energy loss generated by irradiation process in the near surface region. After the Kr⁸⁺ irradiation, the position of the Nd³⁺ fluorescence peaks haven’t been changed, which means that there is no serious lattice disorder inside the irradiated Nd:YGG crystal. The normalized result is shown in Fig. 5(b) to explore the broadening of the fluorescence peaks. There appears to be slightly broadening which possibly related to a decrease in the excitation lifetime as a result of the increase in implantation damage. Overall, the fluorescence characteristics of Nd:YGG crystals are mostly retained after swift heavy ion irradiation. This result can provide a valuable reference for the research of various integrated laser devices on Nd:YGG waveguides.

 

Fig. 5. The micro-luminescence emission spectra of Nd³⁺ ions for transition⁴F_3/2–⁴I_9/2 at room temperature, (b) is normalized spectra by use of (a).

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Raman scatting is a rapid, sensitive and non-destructive analytical measurement method. In the study of Raman characteristics, it can be utilized to investigate the lattice vibration, impurity defects and stress effects [31]. The Raman scatting spectra of Nd:YGG crystal both before and after swift Kr⁸⁺ ion irradiation are shown in Fig. 6. As can be seen, both Raman spectra are similar, showing the same Raman shift. However, a decrease of waveguide region in relative intensity has been found; this is likely due to the partial electronic damage inside the Nd:YGG sample formed by the swift heavy ion irradiation process, which consistent with the previous analysis.

 

Fig. 6. The Raman spectrum of Nd:YGG with a laser excitation wavelength of 633 nm.

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4. Conclusions

The light propagation measured by end face coupling method, showed that the ion irradiation condition “30 MeV Kr⁸⁺ ion irradiation at the fluence of 2×10¹² ions/cm²” is feasible for formed a high quality index enhanced “well” confined waveguide on Nd:YGG crystal. The propagation loss value is about 0.8 dB/cm. The well structure has advantages for obtaining lower propagation loss because it could avoid the tunnelling effect. Furthermore, the irradiated waveguide in this work has relatively low displacement per atom (DPA) value attributed to the ultra-low fluence (∼10¹² ions/cm² contrasts to ∼10¹⁵ ions/cm²). This gives the better preservation of crystal surface quality and makes it potential useful in application devices with high crystal quality. Based on these analyses, the waveguide formation condition we selected could be a competitive way for waveguide fabrication on. The photoluminescence and the Raman measurement results suggest that the features of the bulk Nd:YGG crystal could preserve mostly in the waveguide layer.