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We report the fabrication of a waveguide structure in a near-stoichiometric lithium niobate crystal using 200-MeV argon-ion irradiation at a fluence of 2 × 1012 ions/cm2. Guided modes were detected in the visible and near-infrared wavelength regions, suggesting that the waveguide can be used at fiber communications wavelengths. The refractive index profiles of the waveguide were reconstructed from the effective index functions. Micro-Raman spectra recorded in the waveguide layer and the substrate showed that the Li/Nb ratio was preserved in the waveguide layer after swift argon-ion irradiation.
©2012 Optical Society of America
1. Introduction
Lithium niobate is one of the most important materials for producing commercial optical modulators and switches. Congruent lithium niobate (CLN), in which the Li/Nb concentration ratio is about 48.4/51.6, has been widely investigated [1]. This nonstoichiometry introduces anti-site defects which result in high internal electric fields and strong photorefractive damage in laser applications. Compared to CLN, near-stoichiometric lithium niobate (SLN) crystals have higher nonlinear coefficients and much lower switching fields required for 180° domain reversal [2]. SLN is an ideal candidate for waveguide substrates and for a number of applications including E-O modulation, frequency doubling, and Q-switching.
There are several techniques which have been developed to form waveguide structures in CLN crystals, such as Ti-diffusion [3, 4], proton exchange [5, 6], and ion implantation [7–9]. In the proton exchange process, lithium atoms are replaced by protons. Ti-diffusion is performed at very high temperatures (around 1000 °C) at which lithium out-diffusion takes place. For SLN, these two processes cannot preserve the Li/Nb ratio. Ti-diffused SLN waveguides have also been fabricated by carrying out vapor transport equilibration (VTE) and Ti-diffusion at the same time at 1060 °C for 12 hours [10]. However, the near-stoichiometric layer generated by this process is less than 10 μm in thickness. Unlike proton exchange and Ti-diffusion, ion implantation can be performed at any temperature. In particular, the samples can be cooled by liquid nitrogen and maintained at a very low temperature. For SLN crystals, ion implantation is a convenient method of forming waveguide structures. Planar and channel waveguides in SLN crystals have been fabricated by ion implantation using 0.5-MeV protons and 3-MeV oxygen ions [11–13].
Swift heavy ion (SHI) irradiation has been widely used to modify various physical properties of materials. Waveguide structures have been fabricated in CLN crystals using 22-MeV F-ion and 45.8-MeV Cl-ion irradiation at fluences from 2 × 1012 to 3 × 1015 ions/cm2 [14, 15]. In this paper, a z-cut SLN crystal was subjected to swift argon-ion irradiation at a very low fluence. Optical modes and refractive index profiles were investigated in the visible and near-infrared wavelength regions. The Li/Nb ratio in the waveguide layer was detected using a micro-Raman system.
2. Experiments
A z-cut SLN crystal with dimensions of 10 × 5 × 0.5 mm3 was subjected to 200-MeV argon-ion irradiation at a fluence of 2 × 1012 ions/cm2. Swift argon-ion irradiation was carried out at the Heavy Ion Research Facility in Lanzhou (HIRFL). The initial beam had an energy of 792 MeV, and was slowed down by passing it through a 260-μm-thick Al foil. The sample was then annealed under normal atmosphere condition at 260 °C for 30 minutes.
The optical modes of the Ar-irradiated SLN waveguide were measured by the m-line technique through a Metricon 2010 prism coupler at wavelengths of 632.8 nm and 1539 nm. From the m-line spectra thus obtained, the refractive index profiles of the waveguide structures were reconstructed using an inverse WKB (IWKB) method [16].
The two end facets of the Ar-irradiated SLN waveguide were carefully polished for end-face coupling experiments. Light from a 632.8-nm He-Ne laser was coupled into the waveguide through a 25 × microscope objective lens. Light from a tunable laser diode (1280 – 1620 nm wavelengths) was coupled into the waveguide through a tapered fiber. The light coupled out of the waveguide was then recollimated by a 25 × objective lens and imaged onto charge coupled device (CCD) cameras with sensitivity in the appropriate wavelength regions.
The micro-Raman spectra of the Ar-irradiated SLN waveguide were measured using a confocal micro-Raman system. The 473 nm beam from the light source was focused into a 1-μm-diameter spot on the polished end facet of the SLN sample. Two spectra were recorded, one with the focused spot hitting the waveguide layer, and the second with the spot hitting the substrate. A spectrum of a CLN crystal was also recorded for comparison.
3. Results and discussion
The m-line spectra of the Ar-irradiated SLN waveguide measured using TE-polarized 632.8-nm and 1539-nm lasers are shown in Fig. 1 and Fig. 2 , respectively. Each drop in relative intensity corresponds to one mode. The sharp drops at the left side are more likely to represent the guided modes, while the broader ones at the right side are more likely to represent the leaky modes. Near-field intensity distributions of the TE-polarized optical modes exiting the output facet of the SLN waveguide were recorded by two CCD cameras (one for the visible and the other for the near-infrared wavelength). Seven guided modes (up to TE6) were recorded at the wavelength of 632.8 nm. Only four guided modes, TE0 – TE2 and TE6, are shown in the inset of Fig. 1 for clarity. One guided mode (TE0) was recorded at the wavelength of 1539 nm and is shown in the inset of Fig. 2.
Fig. 1 Measured relative intensity of the light reflected from the prism versus the effective refractive index of the incident 632.8-nm laser for the Ar-irradiated SLN waveguide. The inset shows the near-filed intensity distributions of the TE-polarized modes (TE0-TE2, and TE6) recorded using a CCD camera.
Fig. 2 Measured relative intensity of the light reflected from the prism versus the effective refractive index of the incident 1539-nm laser for the Ar-irradiated SLN waveguide. The inset shows the near-filed intensity distribution of the fundamental TE-polarized mode recorded using a CCD camera.
The effective refractive indices of the 19 modes at 632.8 nm wavelength and those of the 10 modes at 1539 nm wavelength as a function of the mode number are shown in Figs. 3(b) and 3(c) by red squares. The thin red lines in Figs. 3(b) and 3(c) are the effective index functions determined by polynomial fit. The refractive index profiles were reconstructed from the effective index functions by the IWKB method [16]. The calculations were implemented by an iterative procedure starting at the surface of the sample. The calculated ordinary refractive index (no) profiles at 632.8 nm and 1539 nm wavelengths are shown in Figs. 3(b) and 3(c) by thick dark lines.
Fig. 3 (a) Electronic and nuclear stopping powers (Se and Sn) for 200-MeV argon ions simulated by the SRIM 2006 program. (b) The reconstructed no profile (solid thick line) and the estimated no profile (dashed line) of the waveguide structure at 632.8 nm wavelength. (c) The reconstructed no profile (solid thick line) and the estimated no profile (dashed line) of the waveguide structure at 1539 nm wavelength. The effective indices of the modes are depicted in (b) and (c) by red squares. The red thin lines in (b) and (c) are polynomial fits.
The electronic and nuclear stopping powers (Se and Sn) for 200-MeV argon ions were simulated by the SRIM 2006 program and are shown in Fig. 3(a) by thick dark lines. It can be seen that Se increases from the surface to a depth of 27.6 μm, while the ordinary refractive indices at 632.8 nm and 1539 nm wavelengths decrease monotonically over the same depth. The dashed lines in Figs. 3(b) and 3(c) are the estimated no profiles beyond the depth of 27 μm and are obtained from the inverted Se profile.
Based on the refractive index profile at 1539 nm wavelength, we can predict that the SLN waveguide formed by 200-MeV argon-ion irradiation supports guided modes over the whole near-infrared region. Figures 4(a) –4(c) show the near-field intensity distributions of the fundamental TE modes at 1300 nm, 1500 nm, and 1610 nm wavelengths, confirming the existence of the guided mode in the wavelength range for optical fiber communications. The Sellmeier equation of SLN shows that the refractive index decreases slowly and almost linearly with the increase of wavelength after 1000 nm [17]. The refractive index profile (nλ(x)) in the wavelength range for optical fiber communications can be estimated from the refractive index profile at 1539 nm (n1539(x)) by equation nλ(x) = nλ,sub - [n1539,sub - n1539(x)]. Refractive indices of the substrate (nλ,sub) at 1300 nm, 1500 nm, and 1610 nm wavelengths are 2.2221, 2.2146, and 2.2109, respectively. The fundamental modes at the three wavelengths were simulated from n1300(x), n1500(x), and n1610(x) using the finite difference beam propagation method (commercial software BeamPROP). Because the waveguide is very thick, these three mode profiles are almost identical and cannot be differentiated from each other in Fig. 4(d). The effective refractive index (Neff) is a very important parameter in many applications. Neff of the three fundamental modes obtained from the simulations are shown at the bottom in Figs. 4(a)-4(c).
Fig. 4 (a)–(c) Near-filed intensity distributions of the fundamental TE modes at 1300 nm, 1500 nm, and 1610 nm wavelengths. (d) The fundamental mode profiles at the three wavelengths calculated from the estimated refractive index profiles nλ(x). The three profiles are almost identical and cannot be differentiated from each other. The effective refractive indices of the three modes are shown at the bottom in (a)–(c).
Figure 5(a) exhibits the micro-Raman spectra of the Ar-irradiated SLN along with that of a CLN crystal. A photograph of the polished end facet of the Ar-irradiated SLN is shown in Fig. 5(b). The photograph shows a waveguide layer of 33 μm which is consistent with the simulated ion range shown in Fig. 3(a). The two yellow spots indicated the depth at which the focused spot hit the end facet. The corresponding Raman spectra recorded in the waveguide layer and the substrate are shown by spectra (i) and (ii). Spectra (i) and (ii) have the same number of Raman peaks and identical peak positions. The peaks at 280.1 cm−1 and 337.4 cm−1 which are indicated by the left and the right arrows in spectra (i) and (ii) become weak shoulders in spectrum (iii) recorded for the CLN crystal. In addition, the peak at 299.4 cm−1 visible in spectra (i) and (ii) is missing in spectrum (iii). The linewidth of the Raman peaks is known to decreas as the Li/Nb ratio increases [18, 19]. The 156 cm−1 E(TO1) Raman peak, which is very intense and well-separated from the other lines, was chosen for each of the three cases and replotted in Fig. 5(c) with the three spectra being normalized. It can be seen that the E(TO1) peaks of SLN waveguide and SLN substrate have the same linewidth, and this linewidth is smaller than that of the E(TO1) peak of the CLN crystal. This indicates that 200-MeV argon-ion irradiation preserved the Li/Nb ratio in the waveguide layer in the SLN crystal.
Fig. 5 (a) Micro-Raman spectra recorded in the SLN waveguide layer (red thick line), the SLN substrate (dotted line), and the CLN crystal (blue thin line). (b) Optical photograph of the polished end facet of the SLN waveguide. The two yellow spots indicate the depth at which spectra (i) and (ii) were recorded. (c) The normalized 156 cm−1 E(TO1) Raman peaks of the three spectra in (a).
4. Conclusion
A waveguide structure has been fabricated in an SLN crystal using 200-MeV argon-ion irradiation at a fluence of 2 × 1012 ions/cm2. The guided modes are investigated in the visible and near-infrared wavelength regions. Micro-Raman spectra at 156 cm−1 recorded in the SLN waveguide layer and the SLN substrate have narrower peaks than the spectrum of the CLN crystal, which indicates that the waveguide layer is still a near-stoichiometric layer. This demonstrates that swift argon-ion irradiation is an appropriate method of fabricating waveguide structures in SLN crystals.
Acknowledgments
This work is supported by the National Science Foundation of China (Grant No.10975094), the National Basic Research Program of China (Grant No.2010CB832906), IIFSDU, NLHIRFL, and FANEDD of China.
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