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We report on the second harmonic generation at ~532 nm of optical waveguides in Nd:GdCOB produced by swift carbon ion irradiation. The fabricated waveguide shows good guiding property. Under pump of ~1064-nm fundamental light, the optical conversion efficiency of the frequency doubling is 0.48% W−1 and 6.8% W−1 for continuous wave and pulsed laser beams, respectively.
©2011 Optical Society of America
1. Introduction
The gadolinium calcium oxoborate (GdCa4O(BO3)3 or GdCOB) is an excellent nonlinear crystal for frequency doubling of infrared light into green and blue wavelength regimes [1,2]. Doped with neodymium (Nd3+) ions, GdCOB becomes promising candidate as gain medium for self-frequency-doubling (SFD) lasers [3–6]. Owing to the combination of outstanding luminescence and nonlinear features of the crystal, green lasers at 545 nm as high as 1.35 W have been realized by using laser diode pumping of the Nd:GdCOB crystals [7]. The efficient SFD of Nd:GdCOB has exhibited intriguing potential applications of miniature green light source. In integrated photonics, optical waveguides can offer compact confinement of light propagation to tiny dimensions of order of a few microns [8]. Consequently, light inside waveguides could reach much higher intensities with respect to the bulks. This will bring out some attractive phenomena, e.g., some features related to the materials could be improved. For example, waveguide lasers are usually with the low pump thresholds and enhanced efficiencies owing to the strongly reduced active volumes, and photorefractive waveguides have much faster response than bulks [9,10]. For nonlinear waveguides, the second harmonic generation (SHG) may be generated at low pump powers and be with multiple conversion mechanism from different guided modes [11]. The SHG in the Nd:GdCOB waveguide is the first significant step for the realization of SFD waveguide lasers in the crystal.
Energetic ion beams could modify the networks of optical materials and induce refractive index changes in certain regions [12–16]. Ion implantation has been successfully used to produce waveguide structures in more than 70 optical materials [13,14]. The normal ion implantation technique utilizes H or He ions with energies from several hundred keV up to 3 MeV, to create a lower-index optical barrier at the end of ion range (mainly depending on the nuclear damage contribution), constructing guiding layer between the barrier and air cladding [17,18]. Recently swift heavy ion irradiation (with electronic stopping power S e of more than 1 MeV/amu) has emerged to be another efficient method to fabricate waveguides in optical materials [19–27]. Different from the normal ion implantation, the swift ion beams modify the original lattices mainly by electronic damage instead of nuclear collisions. Successful examples of swift heavy ion irradiate waveguides include lithium niobate (LiNbO3) and Nd:YAG. In these cases, high-quality waveguides were used to achieve electrooptic modulation or integrated lasers [23,24,27].
Waveguides have been fabricated in GdCOB crystals by He ion implantation for blue light generation [28,29]. However, the SHG conversion efficiency was relatively low, partly due to the decrease of nonlinear coefficients induced by the incident ions. Recently, we found that the nonlinear optical responses may be enhanced in swift heavy ion irradiated waveguides [30]. With such advantage one could achieve better SHG performance of the nonlinear waveguides.
In this work, we report, for the first time to our knowledge, the fabrication of Nd:GdCOB planar waveguides by using 17 MeV C5+ ion irradiation. The SHG at 532 nm has been realized in the waveguides under both continuous wave (cw) and pulsed laser configurations.
2. Experiments in details
The 8 at.% Nd-doped GdCOB crystal was grown by Czochralski method. It was polished and cut to dimensions of 6×4×2 mm3. The crystal was designed to achieve the Type I phase matching of a fundamental light beam at 1064 nm propagating at the x-z principle plane. By using the 3MV tandem accelerator at Helmholtz-Zentrum Dresden-Rossendorf, Germany, 17 MeV C5+ ions at fluence 2×1014 ions/cm2 were irradiated on one of the sample surfaces (6×4 mm2). The ion current density was kept at a low level (around 6-8 nA/cm2) to avoid the heating and charging of the sample.
The m-line measurement was performed to measure the dark-mode spectroscopy of the waveguides (via a prism coupler, Metricon 2010, USA), and an end-face coupling arrangement was utilized to experimentally characterize the modal profiles of the guided modes. The measurements of these guiding properties were all taken with He-Ne lasers at wavelength of 632.8 nm.
Figure 1 shows the schematic plot of the experimental setup for SHG in Nd:GdCOB waveguide. The waveguide laser was excited by utilizing a typical end-face coupling system. As the fundamental waves, a polarized continuous wave (cw) or a pulsed laser beam (pulse width of 11.05 ns, pulse energy of ~80 μJ, frequency of ~6 kHz, maximum average power of 480 mW) at ~1064 nm was focused and coupled into the waveguide using a convex lens (focal length f = 25 mm). The SH signals at ~532 nm was collected with a 20× microscope objective lens (N.A. = 0.4). After separating from the leaked fundamental laser beam by a mirror with high-reflection at 1064 nm (HR, R>99%) and high-transmission at 532 nm (HT, T = 70%), the generated green light was aggregated with another convex lens and then detected by the spectrometer, CCD camera and powermeter.
Fig. 1 Experimental set-up for SHG experiments: P, polarizer; L1 and L2, convex lens; Obj, microscope objective lens; M, mirror (HR at 1064 nm, HT at 532 nm).
3. Results and discussion
Figure 2(a) shows the microscope photograph of the swift C5+ ion irradiated Nd:GdCOB sample. The modified layer is of ~11 μm thick. From the m-line measurement, there are several modes observed (for brevity the dark-mode spectroscopy was not shown). Based on the spectrum, we reconstructed the refractive index profiles of the waveguide by reflectivity calculation method (RCM) [31] [Fig. 2(b)], which has been successfully applied for ion implanted waveguides in various materials [12–15]. As is indicated, the C5+ ion irradiation induced a positive index change (Δnw ≈ +0.0012) in the near-surface region and an optical barrier with negative index change (Δnb ≈-0.014) at the end of the incident ion track (∼11 μm beneath the surface of the crystal). The waveguide region and optical barrier were demonstrated in Fig. 2(a), which were consistent with the refractive index profiles very well. With the index distribution, we obtain the modal profile of the waveguide [see an example shown in Fig. 2(c)]. Such a profile shows a good agreement with the experimental measured near-field intensity distribution [Fig. 2(d)] from the end-face of the waveguide, which proves that the reasonability of our index reconstruction. Besides, we used the stopping and range of ions in matter (SRIM) 2010 code [32] to obtain the electronic and nuclear stopping powers (S e and S n) profiles of 17 MeV C5+ ions in Nd:GdCOB crystals [Fig. 2(e)]. The calculated mean projected average range R p ≈11.5 μm, which is in good agreement with the waveguide layer thickness.
Fig. 2 (a) The microscope photograph of the 17 MeV C5+ ion irradiated Nd:GdCOB waveguide. (b) The refractive index profile of the waveguide reconstructed according to the m-line spectrum. The experimental (c) and simulated (d) near-field modal profiles (TM0) of the waveguide. The electronic stopping power (red line), nuclear stopping power (blue line) curves as a function of the depth from the sample surface.
Figures 3(a) and (b) show the typical spectra of the fundamental waves centered at 1064 nm and the SH signals centered at 532 nm in the waveguide for the cw and pulsed configuration, respectively, showing clearly the nonlinear process of SHG. In both cases, the 1064-nm fundamental waves are with TM polarizations at zero-order mode and the SH waves are with the first TE one, i.e., the SHG process occurs under TM0 ω→TE0 2ω. From the inset of Figs. 3(a) and (b), one can see clearly the images of the SH waves at the fundamental modes (i.e., TE0 2ω). This is reasonable since the light fields in lowest-order modes for both fundamental and SH waves have maximum overlap, usually resulting in the highest conversion efficiency. It is noted that in the bulk, the fundamental and SH waves are also with TM and TE polarizations, respectively, which is as same as those in the waveguides. From the previous work, the He ion implanted GdCOB waveguides may have opposite polarizations for the fundamental and SH waves with respect to the bulk [28]. From this point of view, it seems that the swift C ion irradiated GdCOB waveguides preserve more original features of the bulk for SHG that the He implanted one.
Fig. 3 The spectra of the fundamental laser beam at ~1064 nm and the second harmonic generation at ~532 nm in the waveguide for the cw (a) and pulsed (b) configuration, respectively. The insets are the photograph of the generated green light in the Nd:GdCOB waveguide .
Figures 4(a) and (b) depict the generated SH wave power (P2ω) at 532 nm as a function of the absorbed pump power (Pω) of the 1064-nm fundamental waves for the 17 MeV C ion irradiated Nd:GdCOB waveguide under the cw and pulsed model, respectively. The solid circles and the lines are the experimental data and the linear fit, respectively. For cw waves, the maximum output power of the SH light is ~0.53 mW with absorbed pump power of ~334.6 mW, resulting in a conversion efficiency of η ≈0.48%W−1. This value is comparable to that of the He implanted GdCOB waveguides for blue light SHG (η ≈0.5% W−1). Note that the above calculation of the conversion efficiency does not consider the linear absorption of Nd:GdCOB for 532 nm green laser (absorption coefficients of ~0.8 cm−1), which is mainly caused by the high doping of Nd3+ ions and reduce the output SH wave power. From this point of view, for 17 MeV C5+ ion irradiated GdCOB waveguide, the conversion efficiency is therefore expected to be 0.78%W−1 because the absorption of 532-nm laser is significantly lower for the un-doped crystal sample. In addition, the conversion efficiency of the C irradiated Nd:GdCOB is comparable to those of He implanted KNbO3 waveguides, since the related effective nonlinear coefficient (d eff > 10 pm∙V−1) of KNbO3 [33] is one order larger than Nd:GdCOB (d eff ~1 pm∙V−1) [34]. As for a pulsed beam is applied, the maximum output power of the 532-nm SH waves is ~0.72 mW with absorbed pump power of ~102.7 mW, from which a conversion efficiency of η ≈6.8% W−1 could be determined. Similarly, the un-doped GdCOB waveguide produced by 17 MeV C ion irradiation may possess higher SH conversion efficiency of 11.2%W−1. The higher conversion efficiency of pulsed SH waves than the cw light is reasonable because of the much higher optical intensity of the pulsed beams, which brings out much more strong nonlinear process.
Fig. 4 Second-harmonic power (P2ω) at ~532 nm as a function of the fundamental pump power (Pω) for 17 MeV C ion irradiated Nd:GdCOB waveguide (as-irradiated, with propagation loss of 8dB/cm) under the cw (a) and pulsed (b) model. The dashed lines show the possible P2ω vs. Pω curves when the propagation loss reduces to 1dB/cm.
In addition, the waveguide loss has been estimated to be ~8dB/cm by using the back-reflection method [35]. After the annealing treatment at 260°C for 30 min the waveguide loss values have no obvious changes. If the propagation loss could be reduced to 1dB/cm by optimization of the post-annealing treatment conditions, the conversion efficiency η for the cw and pulsed SHG from the Nd:GdCOB waveguides could be increased to be 1.3%W−1 and 18%W−1, respectively. Nevertheless, further investigation could be focused on the self-frequency-doubling of the Nd:GdCOB waveguides with acceptable guiding qualities.
4. Summary
The first Nd:GdCOB optical planar waveguide has been fabricated by swift C5+-ion irradiation. By using 1064-nm fundamental wave pump, we have observed the waveguide SHG at 532 nm through the Type I phase matching of TM0 ω→TE0 2ω. The maximum output powers of SH signals are ~0.53 mW (at pump of 334.6 mW) and ~0.72 mW (at pump of ~102.7 mW) for the cw and pulsed beams, resulting in the conversion efficiencies of 0.48%W−1 and 6.8%W−1, respectively. The obtained data suggest that the swift C ion irradiated Nd:GdCOB waveguides may be potential candidates as the integrated self-frequency-doubling light sources.
Acknowledgments
This work is supported by the National Nature Science Foundation of China (No. 10925524), the Program for New-Century Excellent Young Talents in Universities of China (No. NCET-08-0331), and the 973 Project (No. 2010CB832906). S.Z. acknowledges the funding by the Helmholtz-Gemeinschaft Deutscher Forschungszentren (HGF-VH-NG-713).
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