We report on the preparation and characterization of the planar and ridge waveguides in TGG crystals. The planar waveguide was formed by the 6.0-MeV Si-ion implantation with a dose of 2.0 × 1015 ions/cm2 and the precise diamond blade dicing technique was applied to manufacture the surface of the planar waveguide to construct the ridge structure. The images of planar and ridge cross-sections were photographed by a metallographic microscope. The SRIM 2013 was utilized to achieve the energy loss profiles of electrons and nuclei. The dark-mode spectra and the guiding properties were separately measured by the prism coupling and end-face coupling methods. The simulation processing on the refractive index profile and the near-field mode distribution were operated by the RCM and the BPM software, respectively. In consideration of the high-quality optical propagation performances, the TGG planar and ridge waveguides can serve as superior candidates in the magneto-optical devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Since Faraday effect was acknowledged, magneto-optical materials have been catching substantial attentions and accelerating the development of plentiful optical devices, such as magneto-optical isolators, magneto-optical modulators and magneto-optical switches [1–4]. As the desirable magneto-optical material, the terbium gallium garnet crystal (TGG) is equipped with a large Verdet constant (35 RadT−1m−1 at 1.064 μm), low optical loss (< 0.1%/cm), high thermal conductivity (7.4 Wm−1K−1), and high laser damage threshold (> 1 GW/cm2) [5,6]. Furthermore, TGG can be applied in the wavelength range of 400-1100 nm, while yttrium iron garnet (YIG) counterparts manifest transparency in the region of 1200-5000 nm and can no longer satisfy the demand of the research on the visible and near-infrared magneto-optical devices . Although traditional magneto-optical devices can avoid optical feedback, they are bulk structures and hence cannot be integrated in micro-scale optical circuits. Therefore, new Faraday-rotating devices with properties of miniaturization, high sensitivity and integration call for urgent exploration in the field of integrated optics. Especially, the magneto-optical waveguides with promising application prospect have been increasingly favored by more and more researchers, which is the first step for the realization of waveguide isolators [8,9].
An optical waveguide plays an indispensable role in the manufacture of optical devices with its high integration, low cost of scale production and other distinct characteristics. By restricting light to propagate in the narrow regions, the waveguide structure improves the optical density and cuts down the pump thresholds of the laser materials. Numerous preparation methods have been developed in different materials based on significant and practical application of the guiding structures [10–13]. As a mature and efficient technology, ion implantation has been adopted on over 100 optical materials to fabricate waveguide configurations, which is also the first selection in this paper to prepare the planar waveguide (one-dimensional (1D) waveguides) because of its favorable controllability and no pollution on the sample surface [14–16]. In practice, two-dimensional (2D) waveguides possess superior properties in geometrical features, space constraints and guiding characteristics. Moreover, benefit from the nature of high coupling efficiency and connected simplification to the optical fiber, high-quality 2D guiding configurations with large optical density perform excellent integration on the micro-optical circuits. As of yet, technologies such typically as lithography , diamond cutting  and femtosecond laser ablation  have been utilized to prepare ridge optical waveguides (2D waveguides), based on a planar waveguide structure. In terms of the preponderances of low-cost, simple-procedure, high-precision and surface-polishing, diamond blade dicing has emerged in this experiment to construct an unprecedented TGG crystal ridge structure on the ion-implanted planar waveguide. In general, this fantastic method with great application prospects has fabricated abundant ridge structures in Nd:LGS  and KTiOPO4  crystals.
In this work, we firstly fabricate ridge waveguides in TGG crystals by combining Si ion implantation at 6.0 MeV and diamond blade dicing technique. Some enhanced characteristics such as guiding-mode propagation and light limitation have been investigated at a wavelength of 632.8 nm in detail.
A terbium gallium garnet crystal with a size of 10.0 × 5.0 × 2.0 mm3 was utilized in this work to fabricate planar and ridge waveguides. Figure 1 presents the schematic diagram of the preparation procedure. The first step was to form planar waveguides in the TGG crystal. After polishing and cleaning, the 10.0 × 5.0 mm2 surface was irradiated equally by 6.0 MeV Si ion beam with a fluence of 2.0 × 1015 ions/cm2 on the 2 × 1.7 MV tandem accelerator at the institute of heavy ion physics (Peking University), as shown in Fig. 1(a). The angle between ion beam and the direction perpendicular to the implanted surface was 7° to control the depth of the implantation and minimize the channeling effect. On the other hand, the current of the ion beam was controlled in a lower range to avoid the sample being heated and charged. Afterwards, we took the second step to prepare ridge waveguides, which was precise diamond blade dicing, as shown in Fig. 1(b). The rotating blade at the speed of 30000 rpm moved at 0.1 mm/min on the surface of the planar waveguide to conduct series of air grooves and then ridge waveguides with 10.0-μm width were formed. For purpose of enhancing the thermal stability and propagation performances, the sample was annealed at 260 °C in air for 1 h in the last step.
Before the Si ion implantation, we used the SRIM 2013 (Stopping and Range of Ions in Matter) to simulate the irradiation process and ensure the appropriate irradiation conditions (energy and dose). In terms of the existence of reflect polarized light, the end-face microstructures of the planar and ridge waveguides were recorded by a metallographic microscope. A scanning electron microscopy (SEM) was taken to measure the roughness and estimate the width for the ridge waveguide. The m-line spectrum in the Si-irradiated TGG crystal was measured by a prism coupler (Metricon Model 2010, USA) with a polarized light at a wavelength of 632.8 nm from a He-Ne laser. The spectrum also contained the effective refractive indices of the guided modes. Based on the measured parameters, the RCM (Reflectivity calculation method) was employed to reconstruct the refractive index distribution of the TGG crystal multi-mode waveguide. Additionally, the BPM (Beam Propagation Method) was utilized to simulate optical-field propagation properties in the TGG waveguide.
The energy deposition process of the 6.0 MeV Si ion implantation on the TGG crystal was simulated by the SRIM 2013 software. Figure 2(a) depicts the relevance of electronic and nuclear energy depositions versus the penetration depth, which reflects the refractive index modification of different layers varying with the depth induced by the ion implantation. The electronic energy loss appears a gradual declined trend. It climbs to a maximum near the surface of the sample and approaches zero near the optical barrier. Nevertheless, the nuclear stopping power remains in a constant state within the range of 0-1.5 μm and reaches a peak value of 0.78 keV/nm at a depth of 2.1 μm, while the corresponding electronic energy distribution is negligible. It is in evidence to suppose that the nuclear energy deposition contributes to the formation of the optical barrier and even conclusively reconstructs the refractive-index profile.
To explore the structural transformation of the sample after the ion implantation and the roughness of the lateral walls after the precise diamond blade dicing, two cross-sectional photographs were imaged with reflected polarized light by a metallographic microscope. In Fig. 2(b), the representations with different colors at 1000 × magnification indicate that the sample has been apparently divided into multiple structural layers. The darkest stripe is the optical barrier caused by the lattice distortion. The thickness of the planar waveguide was measured to be ~2.0 μm, which is considerably identical to the recorded range of the 6.0 MeV Si ion implantation in the TGG crystal calculated by the SRIM 2013 software. In addition, owing to the drawbacks of the optical polish, the end-face appears some noticeable dents and scratches beneath the barrier.
Figure 3 sketches out a curve of the relative intensity fluctuating as the effective refractive index, which is obtained from the prism-coupling measurement for TE polarization. The method implements optical excitation in the waveguide by matching the incident light with the guided mode under some conditions. The first sharpest and narrowest descending dip with an effective refractive index of 1.9544 in the m-line curve usually called a propagation mode. As we can appreciate, the index is lower than the substrate refractive index (nsub = 1.9660). From the perspective of view, we can deduce that the refractive index decreases in the near-barrier region after the bombardment of Si ions into the TGG crystal. Homogeneously, the second and third dips also manifest relatively narrow decreased dips, which are two other guided modes. Conversely, the fourth boarder dip indicates a leaky mode formed by the multiple optical reflections. In brief, it demonstrates that an optical-barrier appears at the end of the ion track and the waveguide reveals a better light-limiting effect on the lower-order mode.
The refractive index distribution is decisive for the formation of a waveguide and the light propagation, which is considered as a crucial reference to the waveguide characteristics. Based on the effective parametric values obtained in the dark-mode curve, we selected the appropriate analytic function and simulated the refractive index behavior by the RCM software that has practically succeeded in the reconstruction of refractive index profile. Some parameters, such as the half-width of two half-Gaussian curves and the lowest refractive index of the optical-barrier region, should be numerically optimized until the experimental values are close to the calculated ones. Figure 4 illustrates the simulation results in a simple and vivid way. It is toilless to assume that a typical barrier-confined structure is formed from the pattern of the curve. The decreased variation (∆nb = 0.028) occurred in the last-ditch range of the Si ion track, which was in agreement with the maximum depth of the nuclear energy loss. Furthermore, there was a slight reduced index (∆nw = 0.0065) in the near-surface region within a width of 1.75 μm. Therefore, a waveguide structure was fabricated between the low-index optical barrier layer and the air surface. Additionally, the volume expansion caused by the structural damage and the molar polarization induced by the ion beam effect can be determined to explain the reduction of negative refractive index. Table 1 describes the comparison of the calculated indices with the experimental ones for the silicon-implanted TGG waveguide. Theoretically, the difference between the two values must be limited to the order of 10−4 ~10−3, which can certify the rationality of the RCM calculated results. It is undoubted that the subtle difference of the four modes in Table 1 is considered as a reasonable change.
The quality of propagation determines the performances of a waveguide, so we collected the near-field mode profile by the end-face coupling system. We did not obtain the intensity distribution of the transmitted-light in the planar waveguide before annealing, which may attribute to the structural damage of the waveguide layer caused by the ion irradiation and the faint light projected on the CCD screen. However, in Fig. 5(a), the propagation property of the annealed counterpart (260 °C for 1 h) presented a bright and continuous line horizontally on the screen, although the camera was saturated to some extent during the measurement process. It indicated the excellent competence of confining transmitted light beam perpendicular to the waveguide layer without light leakage at two surfaces. It is because that the annealing treatment procedure reinstates partial structural damage in the implanted region. According to the simulated results from the RCM, we calculated the modal profile by the FD-BPM software that is a numerical simulation method based on an optical wave equation, as shown in Fig. 5(b). It is worth noting that the two fundamental mode distributions act a high similarity on the measured and simulated outcomes, which also proves the rationality of reconstructed refraction index distribution by the RCM.
Figure 6(a) describes the top-face of the ridge waveguide with 10.0-μm width and Fig. 6(b) shows the cross section of the ridge structure at 500 × magnification. The side walls are all smooth, which is ascribed to the remarkable optical polished performance of the precision diamond blade dicing technique. The bottom of the air groove seem to be incline to some extent. It is because that the diamond blade chatters during the dicing process. Some black spots and slight cracks attribute to the chipping effect by the polishing process.
The guided mode characteristics were perceived by the typical end-face coupler, as shown in Fig. 7. With the linearly polarized light beam launched from a He-Ne laser at a wavelength of 632.8 nm, a couple of 25 × microscope objective lenses (N.A. = 0.5) gathered the light to couple in and out the polished end-face of the sample, which was vertical to the direction of light propagation on the platform. The output light was focused on the CCD that presented the near-field guide mode distribution. By employing the end-face coupling system, the inset of Fig. 7 describes the light guiding mode at TE polarization, which is corresponding to the TE00 mode. It shows good performance for the beam propagation in the ridge waveguide, while the slight leakage phenomenon in the measured intensity distribution is due to the defects on the end face during the polishing process. Propagation loss is a key parameter for a waveguide and measured based on the end-face coupling system. The attenuation values in the optical planar and ridge TGG waveguides are estimated to be 3.1 dB/cm and 4.3 dB/cm, respectively. From the perspective of confining light transmission, planar and ridge waveguides in the TGG crystal all possess an intriguing feature and a prospective future.
In conclusion, we have prepared a planar waveguide by 6.0 MeV silicon ion implantation and ridge waveguides by diamond blade dicing process, which was to our best perception the first time for the fabrication of two-dimensional waveguides in the TGG crystal. The m-line curve of relative light intensity was achieved to demonstrate the existence of the guided modes. A certain low refractive index optical barrier was formed in the nuclei collision region, which was confirmed in the reconstructed refractive index profile. Light restrictions were obtained in both the planar and ridge waveguides after the annealing treatment, which all presented considerable enhancement in the propagation of guided modes at TE polarization. The planar and ridge TGG waveguides have the potential to act as waveguide isolators.
Postgraduate Research and Practice Innovation Program of Jiangsu Province (Grant No. SJCX18_0291); National Natural Science Foundation of China (Grant Nos. 11405041 and 51502144).
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