1. Introduction

Potassium titanyl phosphate (KTiOPO₄ or KTP) is a well known nonlinear optical crystal which combines many excellent features, such as large nonlinear optical coefficients, high optical damage threshold (~15 J/cm²) and broad transparent range [1,2]. This unique combination has made KTP a promising candidate for different nonlinear optical and waveguide applications. KTP has been widely used for frequency doubling and frequency conversion of many Nd³⁺ doped lasers [3,4], particularly at the low or medium power density. KTP is also commonly used as an electrooptic modulator, including phase modulators, amplitude modulators and directional couplers [5–7]. In integrated photonics, waveguide micro-structures, as the basic components, have attracted considerable attention owing to their compact confinement of light propagation inside structures to dimensions of the order of several microns, reaching very high optical intensities [8]. Second harmonic generation (SHG) in waveguides offers multiple conversion channels between different guided modes of fundamental and SH waves, and in principle, with low pump threshold and high SH conversion efficiencies. One of the major problems encountered in producing efficient SHG devices is that the nonlinear coefficients in the guiding region may undergo degradation during the fabrication process. In other words, it is important that, the waveguide fabrication procedure should have minimum influence on the SHG properties of the nonlinear crystal. Waveguides in KTP have been achieved by a few of methods including ion exchange [9], pulsed laser deposition [10], and ion implantation [11–17]. Among these different fabrication techniques, ion implantation, which can accurately control the refractive index profiles of the waveguides through adjusting the species, energies, fluences of the incident ions, has attracted a great attention and widely applied to produce guiding structures in a variety of optical materials [18–25]. For materials implanted by light ions, e.g. H or He, an optical barrier with reduced refractive index is usually generated at the end of the ion trajectory, due to the nuclear collisions of the incident ions with the target nuclei, which confines the waveguide in the region between the cladding air and itself. At the same time, some point defects, which can increase the propagation loss of the waveguide, will be induced along the ion tracks. Thermal annealing is an effective method to reduce the loss and improve the guiding properties. Waveguides in KTP have been realized by the implantation of H [11,12], He [13,14], C [15], N, B [16], or Li [17] ions. The SHG in ion implanted waveguides has been realized in a small number of materials, such as lithium niobate (LiNbO₃) [26], potassium niobate (KNbO₃) [27], and KTP [28]. In this work, we will focus on the influence of the implantation on the SHG of KTP waveguide by applying SH confocal microscopy to elucidate how the nonlinear response of the KTP network has been modified. The spatial variation of the nonlinear conversion efficiency has been compared with the μ-Raman images and with the ion induced refractive index variations. From this comparison, the physical mechanisms for the waveguide formation have been discussed.

2. Experimental details

The KTP crystal wafers were cut into dimensions of 7(x)×5(y)×1.5(z) mm³, and optically polished. By using the standard UV lithography technique, a series of wedged smooth photoresist stripes were deposited on the x-y surface of the sample. For comparison, part of the sample was not covered by the photoresist, allowing planar waveguide formation. The multiple He⁺ ions at energies of (1.9+2.0+2.1) MeV and fluencies of (2.7+2.7+4.5)×10¹⁵ ions/cm² were performed on this surface with photoresist mask at the 2×1.7 MV tandem accelerator at Peking University. Multi-energy ion implantation was carried out in order to make the optical barrier (created at the end of ion track) thicker and, consequently, to reduce the leaky effect of the modal field, thus leading to a better optical confinement [18]. Then, in order to improve the guiding properties, the sample wafer was annealed at 200°C for 30 min in an open oven. We experimentally found that this particular temperature optimized the optical propagation properties of the waveguides because it leads to a relevant reduction in the propagation losses while maintaining the refractive index contrast and preserving the quality of sample’s edge. Further details on waveguide fabrication can be found in Ref [14].

In order to check the possible deterioration of the KTP nonlinear response due to ion implantation, μ-SH images of the planar and channel waveguides were obtained through an Olympus BX-41 fiber-coupled confocal microscope using a mode locked Ti:Sapphire laser providing 100 fs pulses at a repetition rate of 80 MHz. The 800 nm fs laser was focused onto the cross sections by using a 100× microscope objective with numerical aperture N. A. = 0.8. The back-scattered SH signals were collected with the same objective and, after passing through a series of filters and a confocal aperture, were collected by a fiber-coupled spectrometer. The sample was mounted on an x-y motorized stage with the spatial resolution of 100 nm. The μ-Raman measurements were performed with the same setup but with another 100 × microscope objective (N. A. = 0.9). In this case, the excitation laser was a 488 nm Argon laser and a set of notch filters were placed between the focusing objective and the confocal aperture.

3. Results and discussion

Figures 1(a) and (b) show the optical transmission photographs of the cross sections of the planar and channel waveguides, respectively. We focus on an area including waveguide and bulk volumes (red squares) to investigate the spatial variation of the SH efficiency. Figures 1(c) and (d) are the corresponding µ-SH images of the selected regions in (a) and (b), respectively. It is clear that the nonlinear responses (the back-scattered SH signal intensity) at the waveguide active volumes are not strongly deteriorated in respect to the bulk, in both planar and channel waveguides. At the same time, there is a strong reduction in the SH efficiency in the boundary of the waveguide and bulk. This boundary, the so-called optical barrier, is located at the end of ion range and leads to the largest refractive index change due to the nuclear collisions of the incident ions and target nuclei. We note that the SH intensity is decreased by ~60% compared with the bulk. Figure 2 shows the SH spectra obtained from the waveguide, barrier and bulk. The intensities are above 90% in the waveguide volumes and 40% in the barrier regions in respect to the intensity of the bulk. Thus, the SH intensity at the waveguide differs less than 10% in respect to bulk. This variation is close to the variations we found in the SH caused by surface imperfections or by slight changes in the surface-to-objective distance as a consequence of a non-perfect parallelism of the sample, which can be considered to be within the uncertainty range in our µ-SH setup. Therefore, we consider that the nonlinear properties are well preserved in the waveguide’s volume (they are not drastically affected by the implantation process). The fact that nonlinear properties are not strongly deteriorated in the waveguide is, indeed, an advantage for the potential application of KTP waveguide for efficient frequency doubling.

 

Fig. 1 Optical transmission photographs obtained from the cross sections of (a) planar and (b) channel waveguides. (c) and (d) are the corresponding confocal µ-SH cross sectional images of the planar and channel waveguides after annealing 200°C for 30 min in the air, respectively. Scale bars are 10 μm in (a) and (b), and 5 μm in (c) and (d), respectively.

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Fig. 2 Comparison of the SH spectra obtained after a femtosecond 800 nm laser excitation in waveguide (blue line), barrier (red line) and bulk (green line) after annealing 200°C for 30 min.

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Figure 3 shows the normalized SH intensity profile (blue line) and the n _z refractive index profile (red line reconstructed by reflectivity calculation method (RCM), see Ref [14].) as a function of the in-depth distance in the KTP planar waveguide. As we can see, the maximum change of n _z takes place at a depth of ~5.5 μm, whereas the refractive index no changed in the waveguide volumes. The insert is the calculated defect distribution (relative atom displacement) by using SRIM 2010, with the maximum atomic displacement of ~11% at a depth of ~5.5 μm, which demonstrates that the nuclear collisions are the main reason for the structural modification. This well agrees with the SH intensity profile: the nonlinear features are well preserved in the waveguide’s volumes (from the surface to ~4 μm below the edge, a region with a very low density of defects), and then decreased to the minimum value at the barrier regions (close to an in-depth distance ~5.5 μm, where the density of defects reaches its maximum). The SH intensity reduction observed in the barrier is attributed to a partially lattice damage induced by the nuclear collisions. Lattice damage could cause a decrease in the SH efficiency due to several causes including breaking in the local symmetry, scattering induced self-trapping of SH photons and bond-breaking. In previously studied ultrafast laser inscribed nonlinear waveguides, in which severe (or catastrophic damage was induced), the SH back scattered signal was found to increase at damage regions [29,30]. We state that this different behavior is due to the different damage degree (being much lower in ion implanted waveguides in respect to laser written waveguides) so that in presence of severe damage makes defect induced back scattering to be dominant over other mechanisms.

 

Fig. 3 1D spatial scan of the SHG intensity (blue line) and refractive index profile (red line) of the planar waveguide as a function of the depth below the surface after annealing 200°C for 30 min in the air. Inset shows the relative atom displacement distribution of the planar waveguide.

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In order to obtain a deeper knowledge on the microstructural changes induced during the He⁺ implantation, μ-Raman spectra were analyzed. Figure 4 depicts a typical confocal Raman spectrum in the Stokes range (0-1200 cm⁻¹) at room temperature obtained from the bulk of KTP crystal excited by a cw 488 nm laser. For micro-structural imaging we have focused our attention to the Raman mode at the 690 cm⁻¹. Figures 5 (a), (b), and (c) are the corresponding 2D spatial distributions of the intensity, induced energy shift and width of this particular Raman mode, respectively. As it can be observed from the first Raman image, the Raman intensity is preserved at the waveguide’s volume. This fact, together with the absence of any induced shift or broadening of the Raman modes, suggests that the waveguide is constituted by an almost unperturbed KTP network. This is, indeed, in agreement with the in-depth defect distribution shown in the inset of Fig. 3. This also explains the preservation of the nonlinear properties at the waveguide conclude from the SH images of Fig. 1. The simultaneous Raman induced broadening and intensity decrease observed to take place at barrier unequivocally indicate that the lattice as been partially damaged and disordered at the nuclear collision area. This damage/disordering is accompanied by a slight red-shift of Raman modes. This last fact can be attributed, in a first order approximation, to a slight local dilatation of the KTP network. Thus at the barrier we have a partially damaged, disordered and dilated KTP volume. Since this is accompanied by a refractive index reduction, we believe that the dominant mechanism is damage induced refractive index reduction. Otherwise, there is almost no variation in the waveguide volumes. Thus we infer the structural changes for the He⁺ implanted KTP crystals appear at the end of the ions trajectory due to nuclear collisions (barrier regions), the contribution of excitation interactions along the ions path was negligible. As a final remark it is important to note that neither the original lattice properties nor the nonlinear response have been modified at the waveguide’s active volume. This is, indeed, an outstanding property since it ensures that KTP waveguides will have the same properties of bulk KTP.

 

Fig. 4 Confocal micro-Raman spectrum obtained from the bulk of the KTP.

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Fig. 5 Spatial dependence of the intensity (a), the induced spectral shift (b), and the induced spectral broadening of the Raman mode at around 690 cm⁻¹ as obtained from the cross-section of the channel waveguides in KTP crystal after annealing 200°C for 30 min in the air.

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4. Conclusion

In summary, we report on the μ-SH and μ-Raman images of planar and channel waveguides in KTP crystal fabricated by He⁺ implantation. The results suggest that the nonlinear and micro-structural properties have not been strongly deteriorated in the waveguide volumes compared to the bulk material. This fact makes the produced structures good candidates for integrated laser frequency conversion components. Both nonlinear response and structural properties have been found to occur at the low refractive index optical barrier created by nuclear collisions. We have identified these volumes are constituted by a slightly disordered, damaged and dilated KTP network. Both μ-SH and μ-Raman data conclude that at the waveguide’s volume the KTP network has been almost not perturbed whereas the ion-induced defects and modifications are concentrated at the barrier, where nuclear collisions take place.