We report on the fabrication of ridge waveguides in zinc selenide (ZnSe) crystal by using swift Kr8+ ion irradiation and precise diamond blade dicing. The ridge waveguides operating at mid-infrared wavelength of 4 μm support multi-mode guidance. The minimum propagation loss of the ridge waveguide is measured to be as low as ~1.1 dB/cm. The simulated modal profiles of the ridge waveguides are in good agreement with the measured near-field intensity distributions. The micro-Raman spectra indicate that there is no significant lattice change in the ZnSe waveguides after the Kr8+ ion irradiation.
© 2015 Optical Society of America
Optical waveguides, as the basic elements in integrated photonics and modern optical communication systems, could confine the light propagation within very small volumes. Consequently, light propagating inside the waveguides structures could reach relatively higher optical intensities with respect to the bulk geometries [1, 2]. Actually two-dimensional (2D) waveguides (with ridge or channel configurations) possess more compact geometries and exhibit stronger spatial confinement of light fields compared with one-dimensional (1D) waveguides (with planar or slab configurations) [3, 4]. Owing to the superior guiding properties, 2D waveguides connecting to the optical fibers are more effective to construct integrated photonic devices . Several techniques have been utilized to fabricate optical waveguides in a number of materials, such as metal diffusion , ion exchange , ion implantation/irradiation [8–12] and femtosecond (fs) laser inscription [13–16]. Recently, diamond blade dicing has become an increasingly fascinating technique to fabricate high-quality ridge waveguides owing to the ability of precise cutting and surface polishing. Diamond blade dicing has been successfully applied to manufacture ridge waveguide structures on the surface of LiNbO3  and Nd:YAG  crystals.
The mid-infrared (MIR) wavelength region (i.e. λ = 2-25 µm) has attracted a significant amount of interest as it contains strong and specific absorption features of various molecules and gases. Photonic devices operating in this wavelength region can thus be widely applied in the fields of bio-chemical sensing, trace gas detecting, environmental monitoring and free-space optical communication [19–21]. ZnSe is one of the most important materials for various photonic applications in mid-infrared region owing to its uniquely high nonlinearity and extremely large transmission window . The transparency of ZnSe crystal ranges from 500 nm to 21 µm, covering bands from yellow (visible) to far IR , which enables wide applications in medical and industrial areas. As a dominant transition-type semiconductor with a wide band gap of 2.7 eV, ZnSe is also used as a promising material for the blue light-emitting devices . Recently, ZnSe waveguides with feature of low propagation losses and thermal stability have attracted much attention due to the possible MIR applications in many aspects such as astrophotonics [25, 26]. As of yet, a few techniques have been developed for waveguides fabrication in ZnSe crystals, including metal organic vapor phase epitaxy (MOVPE) [27, 28], ultrafast laser inscription  and proton implantation .
In this work, we have fabricated optical ridge waveguides in ZnSe crystal by using swift Kr8+ ion irradiation combined with precise diamond blade dicing. The refractive index profile and guiding properties of the ridge waveguides at MIR wavelength of 4 µm and micro-Raman spectra of the sample are measured and investigated in details.
The ZnSe crystal used in this work was cut with dimensions of 10 × 8 × 2 mm3 and optically polsihed. Figure 1 depicts the schematic plot of the ridge waveguides fabrication process. One surface (10 × 8 mm2) of the sample was irradiated with Kr8+ ions at energy of 1.2 GeV and fluence of 2 × 1011 ions/cm2 by using the Heavy Ion Research Facility in Lanzhou (HIRFL) at Institute of Modern Physics, Chinese Academy of Sciences, to construct a planar waveguide layer with a thickness of 100 μm beneath the crystal surface. During the irradiation, the ion current density was kept in the range of 10-30 nA/cm2 to avoid the heating and charging of the sample. After that, we utilized a rotating diamond blade, which moved along the x-axis of the sample, to manufacture parallel air grooves on the planar waveguide surface. We set the rotating velocity and cutting speed of the diamond blade to be 10.000 rpm and 0.02 mm/s, respectively. The width of the ridge structure could be determined by adjusting the separation of two adjacent air grooves. Consequently, two ridge waveguides with widths of 20 μm and 50 μm were constructed in ZnSe crystal.
A microscope (Axio Imager, Carl Zeiss) was utilized to photograph the cross sections of the ridge waveguides in ZnSe crystal. We further used scanning electron microscope (SEM) to study the roughness of the sidewalls of the ridge waveguide structures in ZnSe crystal. An end-face coupling arrangement, as shown in Fig. 2, was utilized to characterize the waveguide properties. The probe light at the wavelength of 4 μm was generated from a Tunable Laser System - MIRTM 8025 (Daylight Solutions, Inc.) and the polarization of the light was controlled by a polarizer. The linearly polarized incident light was focused by an MIR microscope objective lens (N.A. = 0.13) and then was coupled into the ridge waveguide. Another MIR microscope objective lens (N.A. = 0.13) was utilized to collect the output light from the other facet of the sample and finally the near-field modal profile of the output light beam was recorded by an MIR CCD camera. We obtained the propagation loss of the ridge waveguide by measuring the powers of in-coupled and output light using an MIR optical power meter. Based on the end-face coupling arrangement, we used a cubic K9-Glass after the polarizer to alter the incident angle of the laser, realizing modulation of the N.A. of the waveguide for determining the maximum refractive index contrast of the waveguide. In order to couple the light field entirely into the waveguide, the N.A. of the objective lens (0.13) was slightly less than the measured N.A. of the waveguide.
In order to investigate the damage to the ZnSe crystal lattice induced by the 1.2 GeV Kr8+ ion irradiation, we measured the micro-Raman spectra of the sample utilizing a confocal micro-Raman spectrometer (Horiba/Jobin Yvon HR800) at room temperature. The laser beam at the wavelength of 473 nm (with a spot diameter of 1 µm) was focused onto the cross-section of the ZnSe crystal at different ion-irradiated positions to measure the Raman intensity distributions along the ion track. The Raman spectra were carried out with a wavenumber precision of 2 cm−1 in the frequency range from 100 cm−1 to 300 cm−1.
3. Results and discussion
In order to investigate the refractive index modification of the ion irradiated region of the sample, we calculated the energy deposition process of the 1.2 GeV Kr8+ ion irradiation on ZnSe crystal through the software Stopping and Range of Ions in Matter (SRIM) 2010 code . The electronic (Se) and nuclear (Sn) stopping powers as function of penetration depth of the irradiated Kr8+ ions inside the ZnSe crystal is shown in Fig. 3(a). We can find that Se is completely dominant over Sn within the first depth range of 0-90 μm and the peak value of Se is about 14.4 keV/nm at the depth of 80 μm. Meanwhile, the value of Sn remains nearly zero within the first depth range of 0-70 μm, and climbs to the maximum value of ~1.7 keV/nm at the depth of 100 μm. It suggests that compared to the nuclear damage, the electronic damage plays the main role of the refractive index change for the waveguide formation.
In order to estimate the maximum value of the refractive index contrast of the waveguide layer, we measured the numerical aperture N.A. of the waveguide and assumed a step-like refractive index profile through the formula Fig. 3(b).
Figures 4(a) and 4(e) exhibit the microscope images of the cross sections of the ridge waveguides WG1 (with the width of 20 μm) and WG2 (with the width of 50 μm) in ZnSe crystal, respectively. As one can see, the Kr8+ ion beam modified region has a thickness of ~100 μm. That is indeed in a good agreement with the calculated project range of the 1.2 GeV Kr8+ irradiated ions in ZnSe crystal through the software Stopping and Range of Ions in Matter (SRIM) 2010 . Figures 4(b) and 4(f) show the topographic SEM images of the sidewalls of the ridge waveguides WG1 and WG2 in ZnSe crystal, respectively. We could estimate the roughness of the sidewalls of the ridge waveguides WG1 and WG2 to be 565 nm and 387 nm, respectively. The roughness of the sidewall of the ridge structure could be further improved by additional processing (e.g., ion beam sputtering ). Figures 4(c) and 4(g) depict the measured near-field intensity distributions (TE mode) of the ridge waveguides WG1 and WG2 in ZnSe crystal by employing end-face coupling arrangement at the wavelength of 4 µm, respectively. Because the depth of the ion-irradiated waveguide region is relatively large, both the near-field modal profiles of the waveguides WG1 and WG2 exhibit multi-mode behaviors. It also can be clearly noticed that as the width of the ridge structure enlarges, a higher-order mode is guided. The single mode waveguide may be generated by further optimization of the irradiation parameters and the waveguide geometries. The minimum propagation losses of the ridge waveguides WG1 and WG2 are measured to be 1.3 dB/cm and 1.1 dB/cm, respectively. We can find that with the width of the ridge structure decreasing, the roughness of the side walls of the ridge waveguide structure increases. Consequently, much more scattering occurs during the light propagation within the waveguide region, and this is the reason why the propagation loss becomes larger. In addition, we have found that the ridge waveguides support isotropic guidance for both TE and TM polarizations.
Based on the reconstructed refractive index profile, 2D near-field modal profiles for the ridge waveguides WG1 and WG2 were calculated by using the software Rsoft Beam-Prop  through Finite Difference Beam Propagation Method (FD-BPM) , respectively, as shown in Figs. 4(d) and 4(h). As one can see, the calculated near-field modal profiles of the ridge waveguides WG1 and WG2 are in good agreement with the experimental results, which indicates that the reconstructed refractive index profile of the ridge waveguide is reasonable and the simulations through the software Rsoft BeamPROP are successful.
Figure 5(a) shows the micro-Raman spectra of the ZnSe crystal at different irradiated positions (at depths of 20 μm, 60 μm, 100 μm, 120 μm, and in the substrate, respectively) along the ion tracks in the frequency range from 100 cm−1 to 300 cm−1. As the Raman intensity change may reveal the lattice disorder degree induced by the ion irradiation [35,36], we magnified Raman active mode at 141 cm−1 and studied the Raman intensity distributions of this mode along the ion tracks in detail, as shown in Figs. 5(b) and 5(c). As one can see, within the depth range of 0-100 μm the Raman intensity persistently drops to the minimum value at around 100 μm, which is consistent with the peak position of Sn simulated by the SRIM 2010 code , and after that it climbs rapidly till the substrate intensity. By analyzing Figs. 5(a), 5(b) and 5(c), the peak positions and widths of the Raman spectra have no significant change along the ion tracks, while the maximum Raman intensity reduction is about 10% between the substrate and waveguide region. It implies that the low-fluence Kr8+ ion irradiation does not modify the ZnSe crystal lattice significantly.
We have reported on the fabrication of optical ridge waveguides in ZnSe crystal by using combination of swift Kr8+ ion irradiation and precise diamond blade dicing. The guiding properties of the ridge waveguides at MIR wavelength of 4 μm have been investigated, exhibiting multi-mode behaviors. The refractive index contrast of the waveguides was ~0.008. The modal profiles of ridge waveguides at 4 μm were in good agreement with the experimental results. The minimum propagation loss of the ridge waveguide is measured to be ~1.1 dB/cm at 4 µm. The micro-Raman spectra indicate that there is no significant microstructural change along the ion tracks after 1.2 GeV Kr8+ ion irradiation. This work suggests that the combination of ion irradiation with diamond blade dicing may be an efficient technique to fabricate high-quality ridge waveguides in a number of optical materials and the present results show the potential applications in nonlinear MIR integrated devices. Further work will concentrate on optimizing the ZnSe waveguide fabrication parameters to realize single-mode transmission and to reduce the propagation losses.
The work is supported by the National Natural Science Foundation of China (NSFC) (No. U1332121). The authors thank L. L. Pang and Z. G. Wang for help of ion irradiation processing of the sample.
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