This paper demonstrates, for the first time, a method to fabricate optical ridge waveguides in SBN photorefractive crystal, i.e. by first using high-energy carbon ion implantation (forming planar waveguide substrate) followed by Ar+ ion sputter etching (constructing ridged stripes). A two-dimensional (2D) cross-sectional refractive index profile of ridge waveguide is reconstructed by carefully considering the ridged topography as well as the index distributions of the planar waveguide. Based on this profile, the waveguide modes are calculated, in which shows a reasonable agreement with the experimentally observed modal near-field intensity distributions.
©2007 Optical Society of America
SBN (strontium barium niobate) is a well-known photorefractive crystal that has been successfully used to realize optical amplification, holographic storage, and self-pumped phase conjugation, and also exhibits promising potential applications for optical information processing and optical computing [1,2]. Particularly, more recent investigations on various optical solitons (in bulks as well as in versatile waveguides) have made SBN to be the most widely used material in this outstanding research field because of its excellent photorefractive performance [3–6]. Forcing light to propagate through waveguide structures makes possible to achieve high optical densities even at low power (of the order of a few milli-watts), resulting in a considerable improvement on the versatile properties with respect to the intrinsic bulks, e.g. nonlinear harmonic generation, laser actions or photorefractive amplification [7–9]. A number of techniques have been developed to produce waveguide structures in various substrates, e.g. metal ion diffusion , ion exchange , film deposition , and ion implantation [13,14], etc. As a mature material-modification method, ion implantation has shown its wide applicability to fabrication of waveguides in a variety of optical materials, including crystals, glasses, semiconductors, and organic materials, through precise control of refractive index at selective depths inside the substrates [13–20]. Particularly, in some oxide crystals with very low Curie temperatures, for example, SBN, KNSBN, KNbO3 or BaTiO3, ion implantation has been proved to be the most efficient technique to create waveguide structures because it does not require a high temperature processing of the materials [19–23]. Planar waveguides in SBN crystals have been produced by using He+ or H+ ion implantation [9,13,14,21,25], and the photorefractive properties of these waveguides were well investigated in detail by Kip et al . Channel waveguides in SBN can be formed by specially modulated light, e.g. via mechanism of optical soliton writing [3,5,6], or strain technique , however, no ridge waveguides in SBN have been mentioned. Moreover, recent research shows that implantation of medium-mass ions at high energy may create waveguide structures more efficiently at much lower doses with respect to the light ions [14,27]. In this work, we report to our knowledge the first time the ridge waveguides fabrication in SBN crystal by using carbon ion implantation combined with Ar+ ion sputter etching. The modal analysis is also performed by applying a numerical method.
2. Experiments in details
The pure x-cut SBN (Sr0.60Ba0.40Nb2O6, SBN60) crystal with size of 1.5(x)×6(y)×6(z) mm3 is provided by the State Key Laboratory of Crystal Materials, Shandong University, China. The z-axis points the crystalline c direction of the crystal. Figure 1(a)–(c) show the schematic plots of the fabrication process of the ridge waveguide in SBN. First, the crystal is implanted with C3+ ions at energy of 6 MeV and dose of 1×1014 cm-2, onto the facet with size of 6×6 mm2. The beam direction is set to be tilted 7° off the normal plane of the sample surface in order to minimize the channeling effect. With this processing one buried optical barrier layer with lowered refractive index is generated inside the crystal, constructing a planar waveguide structure in the near surface region [Fig. 1(a)]. Second, the standard lithographic technique is used to form specially designed mask stripes onto the planar waveguide surface. In this step, a thick-film positive photoresist is spin-coated onto the sample surface at 5000 rpm for 15s, forming a photoresist-mask with thickness of ~5 µm. After exposure of UV light through a special mask plate, a series of photoresist stripes with width of 10µm and separation space of 40µm between the adjacent channels are deposited on the waveguide surface as the sputtering mask [Fig. 1(b)]. In the last step, the Ar+ ion sputter etching (with beam at energy of 500eV, tilted by 30° off the normal direction along the channels) is performed to etch the planar waveguide sample in the unshielded regions for 90 min [Fig. 1(c)]. After this processing, a series of ridge waveguides are formed on the SBN sample. For comparison, some parts of the sample surface are well protected from being etched, keeping original planar waveguide structures.
The planar waveguide in SBN is characterized by well-known m-line technique (via Metricon 2010 Prism Coupler, USA) and end-face coupling method at wavelength of 632.8 nm. In the latter case, a microscope objective lens (×25) focuses the light beam into the waveguide to excite the guided modes, and another microscope objective lens (×25) collects the light from the output facet of the sample, which is imaged onto a CCD camera. In addition, the cross sections and the top surface of waveguides are imaged by a microscope with a reflected polarized light (Olympus BX51M, Japan) for topography investigation.
3. Results and discussion
For a better understanding of the mechanism of the planar waveguide formation in SBN, we use stopping and range of ions in matter (SRIM code, version 2006) to simulate the implantation process of 6 MeV C3+ ions into SBN crystal . Figure 2 shows the curves of electronic and nuclear energy loss of the incident C ions versus their penetration depth inside the crystal, respectively. In most cases, although the electronic energy deposition is much larger than that of the nuclear one, the electronic excitations only create removable point defects, which have very slight effects on the refractive index of the substrates. This means that in the electronic dominant regimes, i.e. during most parts of trajectory of the incident ions, the refractive index may not be modified significantly. In a different way, the nuclear collisions can generate a buried barrier layer at the end of ion track, where it is dominant over the electronic damage, with considerably reduced index through volume expansion, constructing waveguide structure between the barrier and the sample surface (air).
Table 1 lists the measured effective refractive index values of the both TE- and TM-polarized modes by m-line technique. As one can see, all the values of the observed modes are less than the corresponding refractive index of the substrate (n e=2.2888, n o=2.3146), which implies that the C3+-ion-implanted SBN waveguide is a typical barrier-confined structure.
We use reflectivity calculation method (RCM)  to reconstruct the refractive index profiles of the SBN planar waveguide, see Fig. 3 for both the extraordinary (n e) and ordinary (n o) cases. The profiles of both n e and n o indicate the presence of a low index optical isolation barrier. The C3+ ion implantation induces ~0.015 and ~0.021 for n e and n o at the barrier peak position, respectively, whilst almost no changes of these two indices at the sample surface. Therefore the waveguide (still remaining well optically anisotropic) is sandwiched between the barrier layer and the cladding air.
Figure 4 depicts the microscopic images of a) the ridge waveguide cross section and b) the top-view of the sample surface. As one can see, the Ar+ ion sputter etching removes 1.5µm-thick planar waveguide in the unshielded regions, whilst in the shielded regions the waveguide is well protected from being etched; consequently, a series of ridge waveguide stripes are produced on the SBN planar waveguide substrate.
The propagation modes of the ridge waveguide are characterized by the end-face arrangement. Figure 5 shows the (a) 2D and (b) 3D plot of the near-field intensity distribution (quasi-TE00 mode) of the SBN ridge waveguide, respectively. It is much helpful to simulate the light propagation in the waveguide so that the practical guided device could be therefore well designed. In this work, we use the finite-difference beam propagation method (FD-BPM)  to calculate the waveguide modes according to a reasonable 2D refractive index profile, which carefully considers not only the ridged shape of the waveguide cross section but also the 1D refractive index distribution (n e) of the planar waveguide [Fig. 5(c)]. The modal intensity profile of quasi-TE00 mode is also shown in Fig. 5(c), in case of a Gaussian beam with transverse width of 7 µm (FWHM) launching into the waveguide. With a comparison of the modal distribution in Figs. 5 (a) and (c), we may conclude that there is a reasonable agreement between the experimental and the calculation results.
The propagation loss of the channel waveguide is determined to be ~8 dB/cm at the wavelength of 632.8 nm, whilst, before the etching, for the planar waveguide this value is estimated to be ~2 dB/cm. Further improvement of the sputter etching technique on SBN planar waveguide for smoother ridge sidewalls with less roughness may be helpful to obtain lower-loss ridge waveguides.
The first ridge waveguide in SBN crystal is successfully produced by combination of 6 MeV C3+ ion implantation and Ar+ ion sputter etching. The waveguide is characterized by either m-lines or end-fire coupling methods. The 2D refractive index profile of the ridge waveguide is constructed by considering the ridged shape of the waveguide cross section as well as the 1D refractive index distribution of the planar waveguide. Based on this profile, we carry out FD-BPM simulation of the guided mode, which shows a reasonable agreement with the experimental near-field mode observations. It is also expected that, with special designed masks, various waveguide arrays may be manufactured in photorefractive SBN crystals by carbon ion implantation combined with Ar+ sputter etching, which is helpful to investigate versatile interesting nonlinear phenomena in discrete optical systems.
This work is carried out with the financial support of the National Natural Science Foundation of China (under grant No. 10505013), and the associated NSFC-RFBR joint international project (under grant No. 10711120169). FC also thanks SRF for ROCS, SEM.
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