1. Introduction

Integrated optics [1] provide means for efficient nonlinear interactions thanks to the ability to provide high confinement over long interaction lengths, and it has been a field of intense research since the early days of nonlinear optics [1]. With quasi-phase matching (QPM), χ² and χ³ interactions can be tailored for specific interactions [2]. Practical QPM waveguides were first realised in LiNbO₃ [3,4] and KTiOPO₄ (KTP) by diffusion techniques [5,6]. At the time, the field was driven by the need of a blue laser for reading of optical discs, and power levels in the blue exceeding 100 mW were obtained with KTP waveguides [7], but after the invention of the blue laser diode [8] this technology was essentially discarded. By the development of electric field poling [9] and novel waveguide fabrication methods, waveguide devices have become of interest again and exploited in several contemporary applications like spectroscopy, metrology sensing and quantum optics [10–13].

To reach the higher conversion efficiencies, which is necessary for low power applications like quantum communication [11,12], the modal cross section needs to be small, the overlap high and the losses low. Most of the work in this direction has exploited various LiNbO₃ waveguides, either based on proton exchange [14] or adhered waveguides in which a ridge structure has been diced out [15], or nanophotonic waveguides [16]. KTP is a potential alternative to LiNbO₃ for waveguide fabrication, as it has high nonlinear coefficients, strong resistance to optical damage and a wide transparency [17]. Ion exchange [18], ion implantation [19], diffusion bonding [20] and laser writing [21] have all been successful methods for realization of waveguides in KTP. Recently, Chen et al. [22] and Volk et al. [23] demonstrated that it was possible to obtain low loss ridge waveguides in ion-implanted and ion-exchanged KTP by precise diamond blade dicing. These waveguides were used to demonstrate Type II second harmonic generation (SHG) birefringent phase matching with a normalized efficiency of 0.17%/Wcm² to the green [23]. To further increase the efficiency and flexibility in the nonlinear interaction, one should try to combine this technology with periodically poled structures to obtain QPM of Type I for KTP so the strong d₃₃ coefficient could be exploited [24].

In this work, we have fabricated ridge waveguides by precise diamond dicing of a planar Rb-ion exchanged waveguide in a periodically poled Rb-doped KTP (PPRKTP) sample and used it to generate blue light at 468.8 nm through first order (m = 1) Type I QPM SHG. To the best of our knowledge, this work represents the first demonstration of a waveguide in Rb-doped KTP. We achieved a conversion efficiency of 0.12% with a cw Ti:Sapphire laser, with 5.8 mW of fundamental radiation measured at the waveguide output.

2. Waveguide fabrication and characterization

Bulk Rb-doped KTP (RKTP) was chosen as the crystal material as it can be periodically poled with high quality, and the dense gratings necessary for advanced applications [25–28]. 10$\times$5$\times$1 mm samples (x,y,z) of 0.3% Rb-doped KTP were first periodically poled with a period of Λ = 5.82 µm over a 8 $\times$ 4 mm area by the standard electric-field poling technique [25]. Afterwards, the sample was selectively etched to reveal the domain structure. It was then tested for first order (m = 1) QPM SHG using a cw Ti-Sapphire laser, focused in the crystal with optimum focusing parameters [29]. The sample was found to be well-poled with a normalized conversion efficiency of 1.8%/Wcm at a wavelength of 933.8 nm at room temperature. This value is in close agreement with what is expected from a first order QPM grating with 50% duty-cycle.

Waveguides were fabricated by immersing the samples in RbNO₃ for 2.5 hrs at 345°C degrees, in order to produce a planar waveguide that propagates a single mode at 930 nm. The Rb⁺-ions diffuse into the crystal through the z-faces, replace outdiffusing K⁺- ions during the ion-exchange, forming a Rb-concentration and refractive index profile following an erfc(z/d) where z is the depth coordinate and d is the characteristic diffusion depth [18]. The crystal was then selectively etched again, and we confirmed no alteration in the domain structure due to the ion-exchange process. Subsequently, we characterize the planar waveguide at blue (457 nm), red (633 nm) and near-IR (853 nm) wavelengths by prism coupling. We observed two modes in the blue, while the waveguide guided one mode only in the red and the near-IR. The ion-exchange in RKTP was slower and the refractive index change considerably lower than what is obtained for regular flux-grown, undoped KTP for the same exchange conditions. It is attributed to the fact that the crystal already contains a small amount of Rb in the relatively open channels that these crystals consist of in the z-direction. These ions partly block additional diffusion and slow down the process [17,30].

Several ridge waveguides of width varying from 8 to 12 µm were then fabricated on the polar face, i.e, z-face, of the sample by diamond blade dicing using a dicing saw DFD 640, DISCO Corp., and blade P1A863 SD6000N100BR50. The dicing parameters were 30000 rpm and 0.25 mm/s. An example of the diced waveguide is seen in Fig. 1, where Fig. 1(a) shows one of the end faces and Fig. 1(b) shows the top view. Note that the domain structure can be clearly seen in Fig. 1(b) with a duty-cycle close to 50%. Compared to waveguides diced in LiNbO₃ we found that RKTP is slightly more sensitive and more care must be taken to avoid chipping and/or cracking of the sample. We believe that this has to do with difference in crystal structure and the stress induced by ion exchange for the KTP family. While Rb-exchanged channel waveguides [18], as well as diamond diced Rb-waveguides [23], can have very low loss, our waveguides typically transmitted from 5% to 20% of the incident light. Indeed, when light was launched in the waveguide substantial scattering could be seen when looking from atop with a microscope. Furthermore, one can see slight chipping on the polar faces along the waveguide, which should be the main reason for scattering, see Fig. 1(b). We hence believe that further optimization of the dicing process should be possible.

 

Fig. 1 (a) Photo of the end-face of a 10.8 µm diced waveguide and (b) top picture of ridge waveguide where the domain structure as well as the surface roughness can be seen. (c) Corresponding near field distribution of the TM₀₀ mode measured at 937 nm. Simulated intensity distribution at (d) the fundamental and (e) SH wavelengths.

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The waveguides were then evaluated for SHG by launching the Ti-sapphire laser using an objective lens with 10X magnification and 0.25 numerical aperture. The measured fundamental TM₀₀ mode can be seen in Fig. 1(c). Simulations of the waveguide mode profile were carried out using COMSOL, where we assumed an erfc-function for the refractive index profile. Figure 1(d) shows the simulated fundamental TM₀₀ mode and Fig. 1(e) shows the SHG TM₀₀ mode profile. From the overlap between the focused Gaussian input beam and the simulated TM₀₀ waveguide mode, we calculate the coupling efficiency to be 84%. Three distinct SH modes were generated, TM₀₀, TM₁₀ and TM₀₂, when tuning the fundamental wavelength from 930 nm to 940 nm, see Fig. 2. The near field photographs of the generated blue modes are shown together with their respective peaks. The highest blue power, 6.7 µW, was obtained for the TM₀₀ mode with 5.8 mW of fundamental power at 937.3 nm measured at the waveguide output. This corresponds to a conversion efficiency of 0.12%. To compare it to other nonlinear waveguide experiments it is common to calculate a normalized conversion efficiency:

 

Fig. 2 Wavelength scan showing the SH phase matching for the TM₀₀, TM₁₀ and the TM₀₂ modes, where the SHG power has been normalized to unity of the highest peak. Their intensity profiles are shown as insets.

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The overlap area between the simulated modes at the fundamental and SH wavelengths, displayed in Fig. 1(d) and 1(e), is $A_{ovl}$ 50 µm². Considering a lossless waveguide, the normalized conversion efficiency would then be [31]:

To identify the phase matching peaks, we have plotted the waveguide dispersion for the fundamental and SH wavelengths together with the corresponding modes for the waveguide in Fig. 3. Here the modal refractive index for the blue has been adjusted with the QPM condition:

 

Fig. 3 Calculated dispersion for the fundamental (in red) and SH (in blue) modes. The dotted lines represent the bulk refractive indices and the vertical lines mark the phase matching wavelengths from right to left, the TM₀₀ mode in IR to TM₀₀ and TM₁₀, as well as dotted the bulk phase matching.

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3. Summary

In conclusion, we have fabricated a 10.8 µm wide diamond-blade diced ridge waveguide in Rb-exchanged PPRKTP with a period of 5.82 µm and generated blue SH light at 468.8 nm in the TM₀₀ mode. To the best of our knowledge this is the first Type I QPM diamond-diced waveguide SHG in a crystal from the KTP family. As these crystals can be poled with much shorter periods, one should be able to study several novel processes, as mirrorless optical parametric oscillators [34] and entangled sources with signal and idler photons in the opposite direction in such waveguides. Furthermore, by improving the dicing process the chipping could be eliminated and the loss thereby much reduced. With a more homogeneous waveguide the phase-matching bandwidth would be narrowed, further improving the efficiency.