We report the first time optical planar waveguide fabricated in vanadate laser crystal Nd:LuVO4 by 3.0 MeV oxygen ion implantation with the dose of 6×1014 ions/cm2 at room temperature. After the implantation, an enhanced ordinary refractive index region was formed with a width of about 2.1 µm beneath the sample surface to act as a waveguide structure. The modes were observed at 633 nm, while only one mode was observed at 1539 nm after annealing at 300 °C for 60 min in air. The changes of ordinary refractive index in the guiding region were about 4.42% and 4.07% before and after annealing.
©2005 Optical Society of America
Laser diode pumped solid-state lasers have been found in a wide variety of applications such as the military, industry, medical treatment and scientific research, due to the high level of stability, compactness, efficiency and longevity [1,2]. Among current laser crystals, the zircon-structure neodymium-doped vanadate crystals, such as Nd:YVO4 and Nd:GdVO4, have been proved to be excellent laser crystals, and they are now widely used in low or middle power laser system. Nd:LuVO4 crystal is a new crystal with zircon structure, it is also a vanadate crystal with excellent laser properties. When Nd:LuVO4 crystal was pumped by a cw Ti:sapphire laser, output power of 80mW and slop efficiency of 60–66% can be obtained, and the pumped threshold was about 30mW .
In integrated optical and electric devices, a surface structure is necessary which can enhance the conversion efficiency and hence minimize the light source. Several techniques can be used to form guiding regions in crystal. Ion exchange for instance, has been extensively used to elaborate waveguides in well-known non-linear crystals such as LiNbO3 and KTP. Diffusion and epitaxial growth have been used to fabricate waveguides in LiNbO3 and Si-based substrates. However, recent research reveals that ion implantation may be a universal method for fabricating waveguide structure in most optical materials because it has a superior controllability and reproducibility to other techniques. In the MeV light ion implanted waveguides, a low refractive index optical barrier has happened at the end of the track duo to the damage induced by nuclear energy deposition. And such an optical barrier confines the light between itself and the surface acting as a waveguide. More recently, many results show that heavier ion implanted waveguides may be fabricated efficiently in optical crystals because it needs relatively lower ion doses (1013–1015 ions/cm2) [4,5]. The wavelength at 1540 nm is preferred in optical fiber communication systems due to a minimum of fiber loss. Therefore, the property studies of the waveguides at telecommunication wavelength (~1540 nm) are desired. In this paper, we report for what we believe is the first time, the fabrication of planar waveguide in Nd:LuVO4 crystal by 3.0 MeV oxygen ion implantation with the dose of 6×1014 ions/cm2 at room temperature; and we also investigated the annealing behavior of the waveguide at 300 °C for 60 min in air.
2. Experimental details
The z-cut Nd:LuVO4 crystal sample was grown by the Czochralski method in the state key laboratory of crystal material at Shandong University . The samples, with size of 8×5×2 mm3, were optically polished and cleaned before the ion implantation. The MeV oxygen ion implantations were performed at 1.7 MV tandem accelerator of Peking University. The sample was implanted by 3.0 MeV oxygen ions with the dose of 6×1014 ions/cm2 at room temperature. The ion beam was scanned to ensure a uniform implantation over the samples with a typical intensity at 60 nA. The sample was annealing at 300 °C for 60 min in a furnace to investigate the thermal stability. The prism coupling method was used to observe the waveguide mode with the Model 2010 Prism Coupler (Metricon). A laser beam with 633 or 1539 nm from the lasers struck at the base of the prism, and hence the laser beam was coupled into the waveguide region. A photodetector was used to detect the reflected beam. The prism, waveguide and the photodetector were mounted on a rotary table so that the incident angle of the laser beam could be changed. The intensity of the light striking the photodetector was plotted as a function of the incident angle, where a sharp drop in the intensity profile would correspond to a propagation mode. A computer has controlled the measurement system. The refractive index of the prism used in the measurement was 2.8649, the index resolution was ±0.0005, and the index accuracy was less than 0.001. We had not measured the waveguide loss because the sample was not long enough to be measured.
3. Results and discussion
Figure 1(a) and 1(b) show the relative intensity of the 633 nm light [Transverse electric (TE) polarized] reflected from the prism versus the ordinary effective refractive index of the incident light in the Nd:LuVO4 waveguide formed by 3.0 MeV oxygen ion implantation at the dose of 6×1014 ions/cm2 at room temperature before and after annealing at 300 °C for 60 min in air.
As indicated in Fig. 1(a), three sharp dips and two broader dips were observed, their effective refractive indices of the first three modes were higher than the index of the virgin Nd:LuVO4 crystal (n o=2.0292 at 633 nm), this means that the first three dips may correspond to real propagation modes, where the light could be well confined. The remaining dips should represent the leak modes, the refractive indices were both lower than that of the virgin Nd:LuVO4 crystal, which is the result of optical interference between the multiple reflections occurring at the interfaces in the waveguides . This phenomenon was somewhat different from the normal ion implanted waveguide. The implantation could also excite extraordinary modes, but the refractive index profile of extraordinary index is similar to that of Ni+-implanted KTA crystal waveguide . In Fig. 1(b), three sharp dips were also observed. Compared with Fig. 1(a), it was found that the effective refractive indices of the propagation modes decreased, and the index of the third modes became lower than that of the virgin Nd:LuVO4 crystal. This may duo to the partial recrysatllization induced by the thermal treatment.
It is especially important to get the refractive index profile of the waveguide for application, reflectivity calculation method (RCM) was used to successfully to characterize the non-stationary waveguide . A least-squared fitting program based on RCM was available to calculate the refractive index profile by adjusting certain parameters until the theoretical modes indices match the experimental ones within a satisfactory error. We used the RCM to analyze the refractive index profile of oxygen ion implanted Nd:LuVO4 crystal waveguide. Figure 2 shows the reconstructed refractive index profiles at 633 nm for the waveguide in 3.0 MeV oxygen ion-implanted Nd:LuVO4 crystal with the dose of 6×1014 ions/cm2 before and after annealing. The ordinary refractive index of the virgin Nd:LuVO4 crystal was marked in Fig. 2. As indicated in Fig. 2, a positive index (ordinary index n o) change occurred in the guiding region while a small negative index change happens at interface between the waveguide and the substrate, the position beneath the surface about 2.1 µm, usually called “optical barrier”. Therefore, the region between the surface and the barrier becomes a waveguide layer. The positive index change values of the guiding region are about 4.42% and 4.07%, and negative change values of the optical barrier are 0.83% and 0.79% before and after annealing, respectively. This is in accordance with the analysis of the Fig. 1.
Figure 3 shows the relative intensity of the 1539 nm light [Transverse electric (TE) polarized] reflected from the prism versus the ordinary effective refractive index of the incident light in the Nd:LuVO4 waveguide formed by 3.0 MeV oxygen ion implantation after annealing. As indicated in Fig. 3, only one sharp dip was observed, the effective refractive index was higher than the index of the virgin Nd:LuVO4 crystal. Because the dip was very sharp, this means that the dip corresponds to real propagation mode, where the light could be well confined. A waveguide is frequently characterized by its normalized width V, which can determine the total number of waveguide modes . V is found from the following equation:
where λ is the wavelength of light, d is the waveguide width, and n 1 and n 2 are the refractive index in both of the waveguide and substrate, respectively. The larger the value of V is, the more mode number becomes. For a finite waveguide structure, the longer wavelength λ is, and the less the number of modes which can be excited becomes.
The lattice damage produced by ion implantation is considered to be the main reason for refractive index change in the Nd:LuVO4 waveguide. In most of the reported ion-implanted waveguide, the lattice damage distribution induced by nuclear collisions leads to a decrease in physical density and hence to a reduced refractive index at the end of the ion track, forming an optical barrier . However, it is different to some extent in the present work. Here we used TRIM‘98 (Transport of Ions in Matter) code to simulate the process of the implantation in order to get some information of energy depositions and ion range distribution . Figure 4 shows, for 3.0MeV oxygen ion implantation in Nd:LuVO4 crystal, the energy deposition by the electronic excitation and nuclear energy deposition, as well as the concentration profile of the implanted ions. As indicated in Fig. 4, similar to some previous publications mentioned, the width of ion implanted waveguide may be determined by mean projected range of the implanted ions. But it seems reasonable that the electronic energy deposition may cause a positive change of the ordinary refractive index, and the nuclear energy deposition occurred during the implantation result in the formation of the index decrease at the end of the ion track. Nevertheless, a detailed understanding of such a property still needs further investigation.
Optical planar waveguide has been fabricated in laser crystal Nd:LuVO4 by 3.0 MeV oxygen ions implantation with the dose of 6×1014 ions/cm2 at room temperature. The propagation modes were measured by using the prism coupling method before and after the annealing at 300 °C for 30 min in air, respectively. The refractive index profiles were obtained by RCM. It was found that a positive change of ordinary refractive index happened in the guiding region, the change values were about 4.42% and 4.07% at 633 nm before and after annealing. The TRIM’98 code is used to simulate the process of the oxygen ions implantation. The lattice damage produced by ion implantation is considered to be the main reason for refractive index change in the Nd:LuVO4 waveguide.
This work is supported by the National Natural Science Foundation of China (Grant No. 10035010) and the Key Laboratory of Heavy Ion Physics (Peking University), Ministry of Education, China.
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