We report on the formation and the optical properties of the planar and ridge optical waveguides in rutile TiO2 crystal by He+ ion implantation combined with micro-fabrication technologies. Planar optical waveguides in TiO2 are fabricated by high-energy (2.8 MeV) He+-ion implantation with a dose of 3 × 1016 ions/cm2 and triple low energies (450, 500, 550) keV He+-ion implantation with all fluences of 2 × 1016 ions/cm2 at room temperature. The guided modes were measured by a modal 2010 prism coupler at wavelength of 1539 nm. There are damage profiles in ion-implanted waveguides by Rutherford backscattering (RBS)/channeling measurements. The refractive-index profile of the 2.8 MeV He+-implanted waveguide was analyzed based on RCM (Reflected Calculation Method). Also ridge waveguides were fabricated by femtosecond laser ablation on 2.8 MeV ion implanted planar waveguide and Ar ion beam etching on the basis of triple keV ion implanted planar waveguide, separately. The loss of the ridge waveguide was estimated. The measured near-field intensity distributions of the planar and ridge modes are all shown.
©2012 Optical Society of America
Titanium dioxide (TiO2) has attracted much attention over last three decades and been widely used in applications such as dye-sensitized solar cells [1, 2], electrochemical sensors , photocatalysts , and biomedical implants  due to their high chemical stability, avirulence and strong photo-induced oxidation. In addition to those applications, TiO2’s high refractive index and transparency at visible and near-infrared wavelengths make it a promising medium for integrated optical devices in recent years . In particular, the higher refractive index allows for a greater index contrast with surrounding media, which leads to tighter light confinement and more highly integrated light-guiding structures . Furthermore, the exceptionally high nonlinearity of TiO2 [8–10], could lead to diverse nonlinear nanophotonic devices, such as supercontinuum sources or ultrafast all-optical switches [8, 9].
Optical waveguides are the basic components of integrated optical devices. Such structures are defined as the high-refractive-index regions surrounded by low-index regions and could confine light propagation within small volumes in one dimensional (1D) planar waveguide or in two dimensional (2D) channel or ridge waveguide . Compared with the planar waveguides, the 2D guiding structures can carry higher optical density and are widely applied in electro-optical circuits as the interconnecting elements. In past two decades, microfabrication processes for TiO2 waveguides such as laser-beam lithography, nanoimprint, and electron-beam methods have been developed by several groups [12–16]. However, all of them are almost based on TiO2 film, and there has been no report for the TiO2 waveguides formed on single crystal substrate. Ion implantation is a very promising technique to form photonic guiding structures in various materials [17–19] owing to its accurate control of both the depth and the concentration of dopants at low temperature [20–22]. The implantations of light ions, such as H and He, are often used to fabricate waveguides that are mainly confined by a low-refractive-index optical barrier layer buried at the end of the ion track in which the irradiation-induced nuclear damage plays a key role. To reduce light leakage from the waveguide to the substrate through the barrier wall, multiple-energy implants are often used to broaden the barrier width [23, 24]. Waveguide formation by ion implantation has been realized in more than 100 materials, including single crystals, polycrystalline ceramics, glasses, semiconductors and organic materials .
The refractive index profile is a very important parameter for investigating the optical properties of the waveguide and the application of photonics device based on the waveguide . The well-known methods of reconstructing the index profile include iWKB , Intensity Calculation Method (ICM) , Reflectivity Calculation Method (RCM)  and so on. iWKB is suitable for the index profile of gradual change with the depth, such as indiffused or exchanged waveguides. The method of Intensity Calculation Method (ICM) can be adopted to reconstruct the index profile of the single-mode waveguide, which is based on the beam propagation method (BPM) and image processing. RCM is a promising method to simulate the refractive index profile of the multimode ion-implanted waveguide.
Ion beam etching/milling is almost a physical technique and has been widely used for semiconductor device production, as well as for the microengineering of many insulating optical materials . Pure ion beam etching is used to mill surfaces in a vacuum, and this method has been applied in practice to optical materials for a variety of applications such as ridge waveguides . In addition, recent results have shown that the use of femtosecond laser ablation for etching [31, 32] two grooves on top of an ion implanted waveguide is also successful for ridge waveguide formation.
The purpose of this work is first, to explore the possibility of planar waveguide formed in rutile by He+ ion implantation; second, to use Rutherford backscattering (RBS)/channeling measurement for studying the damage in ion implanted waveguides; third, to demonstrate the properties of the ridge waveguides fabricated by the femtosecond laser ablation and the Ar ion beam etching on the basis of the planar waveguides formed above.
2. Experimental details
The samples of rutile TiO2 single crystals are obtained from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, with dimensions of 10 mm(x) × 10 mm(y) × 1 mm(z), and polished optically. They are either implanted by 2.8 MeV He+ ions at a fluence of 3 × 1016 ions/cm2 or irradiated by He+ ions at triple energies of (450, 500, 550) keV with fluences of (2, 2, 2) × 1016 ions/cm2 at room temperature through a 1.7MV tandem accelerator at Peking University, China and Institute of Semiconductors, Chinese Academy of Sciences, separately. During the implantation, the samples are tilted by 7° off the beam direction to minimize the channeling effect . After the implantation, the samples were annealed in the ambient air at 200 °C for 1 h to remove the color centers and improve the waveguide quality and thermal stabilities [21, 22]. In order to observe the damage in ion implanted waveguide, RBS/channeling measurement has been carried out in ion implanted waveguide by 2.1 MeV He ions at 1.7 MeV tandem accelerator of Shandong University.
The waveguide properties are investigated by prism coupling as well as end-face coupling methods. In this paper, we only concerned with TM modes for convenience and avoiding confusion. We use a Model 2010 Prism Coupler (Metricon) to measure the mode lines (dark line spectra) of the waveguide. We performed the measurement at wavelength of 1539 nm, (here the wavelength of 632.8 nm are not available). Since in the measurement, silicon prism is utilized which is not transparent in the visible light. During the measurement, the polarized laser beam strikes the base of a Si prism, being reflected and measured by a germanium photo-detector. A rotary table holds the prism, sample and the photo-detector so that the light incident angle could be changed to reach the coupling conditions, resulting in some “dips” (mode lines) in the intensity spectrum of the reflected light. We also perform end-face coupling measurement to directly investigate the transmission properties of the planar waveguides. Experimentally, a 40 × microscope objective lens is used to couple the incident polarized light (at wavelength of 632.8 nm from a He–Ne laser) into the waveguides, and another 40 × lens collects it from the rear facet of the crystal. Finally, the crystal’s output facet is imaged onto a charge coupled device camera. The objective lenses are located on two three-dimensional optical stages, respectively, while the waveguide sample is placed on a six-dimensional optical stage, which makes it moveable along the three axes as well as rotatable within the perpendicular three planes. The crystal's output facet is imaged onto a CCD camera. In addition, the cross sections of the waveguides are imaged by a microscope (Olympus BX51M, Japan), which somehow shows the implantation induced structural changes on the samples.
The ridge waveguides were fabricated by using two methods in our work. For triple keV He+ ion implantation waveguide, we utilized Ar ion etching technology. Firstly, the standard lithographic technique was applied to form a photoresist-mask with thickness of ~3μm on one x-y face of the sample. After exposure of UV light through a special mask plate, a series of photoresist stripes with width of 4 μm and separation space of 50 μm between the adjacent stripes are deposited on the waveguide surface as the sputtering mask. After post baking, Ar ion bean etching was performed on the uncovered regions for 7 hours. The energy of the Ar ion beam was about 500 eV. The beam current was about 2 mA/cm2. The sample was tilted by 40° off the incident beam direction. Eventually, the photoresist mask was removed and a series of ridge waveguides were formed. And for 2.8 MeV He+ ion implantation waveguide, the ridge waveguides were fabricated by a regeneratively amplified femtosecond Ti: sapphire laser (Spectra-Physics Ltd.), which produced 1 kHz, 120 fs, mode-locked laser pulses centered about a wavelength of 800 nm. The laser pulses with 5 mW average power were focused by a 100 × objective with a numerical aperture of 0.8 onto the sample surface, resulting in spot diameter about 10 μm. The periphery of the Gaussian beam was blocked by an aperture in order to achieve uniform pulse deposition. The sample, with its surface perpendicular to laser beam and monitored by a CCD camera, was mounted on a computer-controlled three-dimensional translation stage at the scanning rate of 100 μm/s.
3. Results and discussion
3.1 Planar waveguide
The arrangement of atoms determines the properties of materials. The RBS/channeling technique is extensively used in the detection of the crystal damage (defect). The RBS/channeling spectra of the implanted waveguide by 2.8 MeV He ion implantation (before annealing and after annealing) are indicated in Fig. 1 . The virgin and random spectra are also measured from the pure rutile wafer for comparison. The minimum yield is around 2% for pure rutile. It means that the rutile is a very good quality crystal. The spectra of the samples after annealing almost coincide with that of the virgin rutile TiO2, indicating that the lattice damage induced in the implantation process were recovered after annealing.
Figure 2(a) shows the dark-mode spectrum (for transverse magnetic modes, TM) of the 2.8MeV rutile planar waveguide at telecommunication wavelength of 1539 nm. Figure 2(b) shows the dark-mode spectrum after thermal annealing at 200 °C for 1 h. The corresponding extraordinary index ne(sub) of the substrate is marked as well. As we can see, all the effective refractive index (neff) of the TM modes are less than the refractive index of substrate, which means that the ne profile has a typically “barrier-confined” shape.
The refractive index profile of the photonic guiding structure is an important parameter for investigating the waveguide properties and its applications in integrated optics. The information on refractive index profile of the rutile waveguide by He ion implantation is needed. In this work, we adopt the RCM to reconstruct the extraordinary refractive index profile. Figure 3 (solid blue line) illustrates the refractive index profiles of the 2.8 MeV He+ ion implanted waveguides in the rutile TiO2 crystal, which is reconstructed by using a reflectivity calculation method based on the data from the m-line measurement. The errors between the experimental and calculated values of the effective refractive indices of the dark modes are within 10−4 in the calculation. As we can see, the ne profile of He+ implanted waveguide has a typical barrier index shape: a buried barrier layer with a maximum index decrease Δne = ‐0.046 starts from the surface to the depth of 6.3μm inside the wafer. Figure 3 (dash red line) shows the damage depth distribution simulated by SRIM 2010  in rutile crystal induced by 2.8 MeV He ion implantation with a fluence of 2.8 × 1016 ions/cm2 at room temperature. The damage peak is located at around 6.3 μm, which is in good agreement with RCM result.
To investigate the guiding properties of the samples, the end-face coupling measurement is performed with a He-Ne laser at a wavelength of 632.8 nm. Figure 4(a) shows the microscope image of the 2.8 MeV He+ implanted waveguide. It can be clearly seen that the modified region by ion implantation is roughly with depth 6 μm and width of barrier 1.5μm, which is in good agreement with the result calculated by the SRIM 2010 code. Figure 4(b) depicts the near-field intensity distribution of the TM mode from the output facet of the rutile waveguide.
3.2 Ridge waveguide
For practical applications, the ridge waveguide is useful. The ridge waveguide has been fabricated by Ar ion beam etching on the basis of the triple He+ ion implantation planar waveguide after annealing. According to SRIM simulation, we obtain the barrier depth of the planar waveguide is 1.25 μm, and thickness is 0.45 μm. The etching depth of the ridge waveguide is about 1.5 μm. To investigate the propagation property, the end-fire coupling was performed with He-Ne laser at wavelength 632.8 nm. In Fig. 5(a) , the transverse cross section image of the ridge waveguide is shown by SEM. We can see that there is acceptable quality for the ridge waveguide. The measured end-fire intensity distribution of TM00 is depicted in Fig. 5(b). It can be seen that the light is guided well. In the figure, only one transverse propagation modes can be supported in the ridge waveguides because of the narrow width of each ridge waveguide (3.5μm).
We also have fabricated the ridge waveguide by femtosecond laser ablation technology on the basis of the planar waveguide implanted by 2.8 MeV He+. In Fig. 6 , the top view (a) and the transverse cross section image (b) of the ridge waveguide are shown by SEM. Figure 6(c), 6(d) and 6(e) are the measured 3D end-fire intensity distributions of TM00, TM10 and TM20, separately. In the figure, three transverse propagation modes can be supported in the ridge waveguides because of the large index difference and the broad width of each ridge waveguide (10 μm).
We measured the loss of the waveguide using end-face coupling method, and obtained a propagation loss of more than 10 dB/cm for TM polarized light at wavelength of λ = 632.8 nm. We could reduce this value by further process, such as Ar+ ion smoothing the sidewall roughness of the ablated ridges .
In summary, the planar waveguides in rutile have been fabricated by high energy (2.8 MeV) He ion implantation with the fluence of 3 × 1016 ions/cm2 and by triple low energy (450, 500, 550) keV He+ irradiation at all doses of 2 × 1016 ions/cm2. RBS/channeling measurement has been carried out in waveguide in order to observe the atomic displacement from lattice site. The refractive index profile in ion implanted waveguide in rutile with low-index barrier type has been obtained based on Reflection Calculation Method. Also we have fabricated the ridge waveguides by Ar ion beam etching on low energy ion implanted waveguide and femtosecond laser ablation on high energy ion implanted waveguide. The loss of the femtosecond laser ablated ridge waveguide was estimated. We have displayed the near-field intensity distributions of TM modes of the ridge waveguides.
This work is supported by the National Natural Science Foundation of China (Grant No. 10735070) and 973 program (Grant No. 2010CB 832906), and the State Key Laboratory of Nuclear Physics and Technology, Peking University, China. We desire to express our thanks to Prof. F. Chen for his discussion and to H. J. Ma and M. Chen for their help in the process of ion implantation and RBS.
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