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We report the fabrication of a planar waveguide in polycrystalline zinc sulfide by 6.0 MeV C ions implantation with a fluence of 5 × 1014 ion/cm2 at room temperature. The near-field light intensity profiles in the visible and near-infrared bands are measured by the end-face coupling method with different laser sources. Investigation of the Raman spectra demonstrates that the microstructure of the polycrystalline zinc sulfide has no significant change after C ion implantation. The absorption spectra show that the implantation processes have no influence on the visible and infrared bands.
©2013 Optical Society of America
1. Introduction
Zinc chalcogenides have been of interest in various scientific areas and applications since bulk crystals were first obtained Zinc sulfide (ZnS) is a principal window material for airborne FLIT systems [1]. Because of its wide band gap of 3.7 eV, ZnS is an important II-VI compound semiconductor material for potential use in thin film devices and can be used for light-emitting diodes in the near ultraviolet region. The large band gap leads to a decrease in the window absorption losses and results in the transmission of high-energy photons to the junction and in improvements in the short circuit current of the cell [2,3]. ZnS is also an excellent transmissive optical material because of its wide spectral range from 0.4 μm in the visible to approximately 14 μm in the infrared, high refractive index, good refractive index stability and low scattering [4]. Based on these remarkable properties, it can be widely used in the following enumerated devices and systems: optical components in the infrared and visible spectra, photoluminescence devices, windows and domes for lasers, thermal imaging systems, radiometers, infrared pyrometers, and infrared detectors and sensors [5–7].
Considering its advantages, ZnS has potential value for integrated optical devices, especially in the infrared region. Optical waveguides are the basic components of integrated optical devices. The structures can conðne light propagation within very small volumes, and the optical intensities on the micrometer or submicrometer scales can reach a much higher level with respect to the bulk [8]. Several techniques have been developed to fabricate waveguide structures in optical materials, such as diffusion, ion exchange, sol-gel, femtosecond laser inscription, ion implantation, and deposition [9–12]. As reported in ref [13], ZnS waveguides have been formed on oxidized silicon wafer substrates using a cold deposition method. However, the ion implantation technique, with its wide applicability and accurate control of the substrate refractive index, has been successfully used since the technique’s introduction in 1960’s to form waveguide structures in more than 100 materials, including crystals, glasses, and even polymers [10,14]. Thus, it is possible to form waveguide structures on ZnS materials.
In this paper, a planar waveguide structure is fabricated in polycrystalline ZnS by C ion implantation for the first time. The waveguide characteristics are studied in the visible and near-infrared regions. We also have measured absorption spectra and confocal micro-Raman images to evaluate the influence of C ion implantation on the polycrystalline ZnS structure.
2. Experimental details
In our work, the sample is Cleartran ZnS. It is cut to dimensions of 10 mm × 10 mm × 1 mm and is optically polished before ion implantation. C ions at an energy of 6.0 MeV and a fluence of 5 × 1014 ion/cm2 are implanted on one of the sample surfaces using a 2 × 1.7 MV tandem accelerator at Peking University, China. To minimize channeling effects, the sample is tilted by 7° from the beam direction. After implantation, the effective refractive indices of the modes are investigated by the m-line technique, using the prism-coupling method with the Model 2010 Prism Coupler. The end-face coupling method is used to detect the near-field intensity distribution of the guided light in visible band with a linearly polarized He-Ne laser (λ = 633 nm) and in the near-infrared region with a diode laser (λ from 1260 nm to 1620 nm), as indicate in Fig. 1 , the light is coupled into the waveguide through a tapered fiber and is recorded by the CCD camera after the microscope objective lens in the system.
Fig. 1 The schematic of the experimental setup for end-face coupling in the near-infrared region with a tunable diode laser.
The Raman spectra of the bulk and modified layers in the polycrystalline ZnS are recorded with a micro-Raman spectrometer (Horiba/Jobin Yvon HR800). The absorption spectra of the sample before and after implantation were measured using a Jasco U570 spectrophotometer.
3. Results and discussion
Figure 2(a) shows a photograph, which was collected by a metallographic microscope with 500 × magnification using reflected polarized light, of the C ion-implanted polycrystalline ZnS sample. The modified layer is approximately 4.7µm thick. We use the Stopping and Range of Ions in Matter (SRIM) 2010 code [15] to simulate the electronic (Se) and nuclear (Sn) stopping power profiles of the 6.0 MeV C ions implanted on the ZnS sample, as shown in Fig. 2(b). The SRIM 2010 code can effectively simulate the physical process of ion implantation and provide a better understanding of the formation mechanism of the waveguide. From the simulated results, the C ions lose most of their energy by electronic ionizations along their trajectory inside the polycrystalline ZnS sample. During the ion implantation process, the incident C ions lose energies through inelastic collisions of target electrons (electronic stopping power) and elastics collisions (nuclear stopping power). The energy transfer can result in the formation of defects which can modify the refractive index of target in implanted areas. The nuclear collision at the end of the ion trajectory results in a decrease in the physical density; this decrease plays an important role in reducing the refractive index of the implanted layer. From Fig. 2(b), the position of the optical barrier is located at approximately 4.7µm, which is in agreement with the measured thickness of the waveguide layer in Fig. 2(a).
Fig. 2 (a) A photograph, collected by a metallographic microscope with 500 × magnification using reflected polarized light, of the C ion-implanted polycrystalline ZnS. (b) The nuclear and electronic stopping powers (Sn and Se) as a function of the penetration depth for 6.0 MeV C ions in the ZnS material based on the SRIM 2010 code.
The refractive index profile in the waveguide is an important parameter and plays a role in devices with practical applications. From the m-line measurement of the TE guided mode at 633 nm, we reconstruct the refractive index profile of the planar waveguide by applying the reflectivity calculation method (RCM), shown in Fig. 3(a) . Many well-known methods have been used to reconstruct the refractive index profile, including the inverse Wentzel-Kramers-Brillouin (iWKB) method [16,17] and parameterized index profile reconstruction [18]. The RCM has been shown to be particularly suitable for analysis of waveguides fabricated by ion implantation [19]. The errors between the experimental and calculated values for the effective refractive indices of the dark modes are within 10−4 in the calculation. We can see that the experimental and calculated values of the effective refractive indices of the TE mode are also shown in Table 1 and match well. The refractive index profile of the TE mode is a typical barrier-confined distribution, which has a maximum negative index change for the barrier from the surface to the depth of 4.7 μm inside the wafer.
Fig. 3 (a) The reconstructed refractive index profile from the TE mode at 633 nm. The near field light intensity profile of the TE0 mode at 633 nm collected by the CCD camera (b) and simulated by the FD-BPM method (c).
Table 1. Experimental and Calculated Values of the Effective Indices of the TE Modes at 633 nm
The near-field light intensity profile of the planar waveguides is obtained by the end-face coupling method at 633 nm. Figure 3(b) shows the TE0 mode profile, which is collected by a CCD camera at the output face of the waveguide, of the waveguide formed by C ion implantation. The measured result shows that light can be well confined in the waveguide layer. For comparison with the measured result, we simulate the near-field light intensity profile of the waveguide using the finite difference beam propagation method (FD-BPM) software, as shown in Fig. 3(c). The FD-BPM is an established numerical technique in integrated photonics. One can conclude that there is a reasonable agreement between the calculated and the experiment results.
The refractive index in substrate of the sample is 2.2714 at a wavelength of 1539 nm, and a sharp decrease couples into the waveguide layer with the TE-polarized m-line technique; this decrease presents the possibility of communications in the near-infrared band. According to the Sellmeier equation for Cleartran ZnS,
the refractive index of the substrate is 2.2784 at 1300 nm at 290K. Some parameters for Cleartran ZnS are given in ref [20], Si are the strengths of the resonance features in polycrystalline ZnS at wavelengths λi. AndBased on the Sellmeier equation, we estimate the refractive index profile at 1300 nm and 1539 nm, as shown in Fig. 4(a) and 4(b). The fundamental mode profile at 1539 nm is simulated using the FD-BPM method based on the estimated refractive index profile, shown in Fig. 4(e). Figure 4(c) and 4(d) present the near-field intensity distributions, which are measured using the experimental setup in Fig. 1, at 1300 nm and 1539 nm, respectively. The results confirm the existence of the guided mode in the polycrystalline ZnS waveguide in the wavelength range for optical fiber communications.Fig. 4 Reconstructed refractive index profile from TE mode at 1300 nm (a) and 1539 nm (b). The near field light intensity profile in the near-infrared band of TE0 mode collected by CCD camera at 1300 nm (c), 1539 nm (d), and calculated by FD-BPM from the estimated refractive profile at 1539 nm (e).
Infrared absorption spectroscopy is extensively used to detect and identify hazardous and greenhouse gases in the atmosphere [21]. The absorption spectra of the 1 mm thick sample are measured by a spectrophotometer before and after implantation at room temperature, given in Fig. 5 . To obtain exact results, both the upper and lower faces of the sample are optically polished. We can see that the absorption spectrum of polycrystalline ZnS has slightly increased absorption, which may be due to damage in the waveguide layer, at the near-edge optical absorption (λ≈360nm) after ion implantation. As shown in Fig. 5, the high transmittance ratio exhibits good transparency in the visible and infrared band, and is almost the same after the ion implantation. Thus, we can conclude that the absorption spectra in the visible and infrared bands are barely affected by C ion implantation, and this lack of change provides the possibility for applications of ion implanted waveguide structures.
To obtain a better understanding of the microstructural changes induced by C ion implantation, the Raman spectra of the bulk and the waveguide layer in the polycrystalline ZnS are investigated by confocal micro-Raman spectrometry of the cross section at room temperature. Raman spectra rely on inelastic scattering, or Raman scattering, of monochromatic light from a laser. The energy of the laser photons is shifted by the interaction of the laser light with phonons or other excitations, which gives information about the phonon modes in the system [22]. Figure 6 shows the confocal Raman spectra at room temperature obtained from the bulk and the waveguide layer using an excitation beam at 473 nm with the size of 1μm. As shown, the LO mode is located at 352 cm−1. To see this mode clearly, the 352 cm−1 position is magnified in the upper right corner of Fig. 6. We can see from Fig. 6 that by comparing the Raman spectra from the bulk and the waveguide layer, the peak positions and widths exhibit no obvious changes. The results for the Raman spectra of the bulk and the waveguide layer in polycrystalline ZnS reflect the absence of microstructural changes, which may a result of the low irradiation fluence of the C ions being unable to change the number of Raman peaks and peak positions in the waveguide layer.
4. Conclusion
A planar waveguide structure has been fabricated in polycrystalline ZnS using 6.0 MeV C ion implantation at a fluence of 5 × 1014 ion/cm2. The near-field light profiles of the TE guided modes in the visible (633 nm) and near-infrared (1300 nm and 1539 nm) band are measured with different laser sources. Through the absorption spectra, we find that optical transmission in the visible and infrared bands is barely affected by C ion implantation. The confocal micro-Raman images prove that there is little microstructural change in the polycrystalline ZnS after C ion implantation. A waveguide on polycrystalline ZnS fabricated by ion implantation could be of interest for optical waveguide devices in the visible and infrared bands.
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), and the State Key Laboratory of Nuclear Physics and Technology at Peking University.
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