A low-loss and high-Q Ta2O5 based micro-ring resonator is presented. The micro-ring resonator and channel waveguide with core area of the 700 by 400 nm2 were fabricated on amorphous Ta2O5 thin films prepared by reactive sputtering at 300°C and post annealing at 650°C for 3 hours. The Ta2O5 micro-ring resonator with a diameter of 200 μm was coupled to the channel waveguide with a coupled Q up to 38,000 at a 0.9 μm coupling gap. By fitting the transmission spectrum of the resonator, the extracted loss coefficient inside the ring cavity and transmission coefficient of TE mode were 8.1dB/cm and 0.9923, leading to the estimated unloaded Q of higher than 44,000. In addition, based on the cut-back method, the propagation loss and the coupling loss of Ta2O5 channel waveguide with an inverse taper were 1.5dB/cm and 3.2 dB, respectively. The proposed Ta2O5 technology offers an unique alternative for fabricating high performance guided wave devices, and may well lead to novel applications in photonic integrated circuits.
© 2015 Optical Society of America
To achieve high bit rate signal processing in the integrated optical system, the ultrafast all-optical modulator is regarded as the key element in the modern optical communication system . For example, the dynamically refractive index change induced by free-carrier plasma dispersion effect or nonlinear effect have been utilized to realize the all-optical switching [2–4]. Typically, the response time of the third-order susceptibility is much faster than the free-carrier plasma dispersion effect [5, 6], implying that high nonlinear refraction properties are suitable for high-speed all-optical modulator. Up to date, number of material systems, such as Si, SiO, SiN and so on, have been chosen for nonlinear optical processing in waveguide photonics fields [1, 3, 5]. However, the physical limitation on application is on the inevitable linear or nonlinear absorption performance. For example, Si has only a finite transparency range, restricting its usage in the visible region. Furthermore, Si suffers from significant two-photon as well as free-carrier absorption due to low band gap structure, greatly suppressing the nonlinear optical property, and hence the nonlinear optical application in Si photonics . On the other hand, low nonlinear coefficient in SiO2 and high stress on depositing thick Si3N4 film has posed material limitation on applications [8, 9]. As a result, seeking high-nonlinearity material while maintaining low absorption is always an important issue to enable all-optical photonics. Tantalum Pentoxide (Ta2O5) is regarded as a suitable candidate for the nonlinear waveguide optics due to its low absorption loss, high nonlinear refractive index properties, and especially CMOS compatible process [10, 11]. Through the lager bandgap, Ta2O5 exhibits low optical absorption properties from visible- to infrared- regimes, showing comparative results with Si3N4 material . In addition, the two photon absorption-free characteristic of the Ta2O5 enables high intensity operation and results in the large nonlinear phase shift. By further comparing with Si3N4, Ta2O5 has been shown with larger index, better thermal stability, and higher nonlinearity, suggesting great potential in the nonlinear waveguide photonics . However, up to date, the challenge on the Ta2O5-based waveguide photonics is mostly on the deposition processing toward low loss operation [14, 15]. Large grant boundaries and surface roughness of Ta2O5 is typically formed through dc sputtering technique, detrimental to waveguide processing and then device performance. In this work, an optimized reactive sputtering following by post-annealing treatment is developed to attain crack-free Ta2O5 film. With fabricating Ta2O5 based waveguide and micro-ring resonator, the ultralow loss waveguide and high-Q properties were observed, confirming its feasibility on the photonic application.
2. Ta2O5 deposition and characterization of waveguide fabrication
The Ta2O5 thin films were deposited on quartz substrates by reactive long-throw sputtering at a substrate temperature of 300 °C. The Ta2O5 targets used for the sputtering were commercially available with 99.9% purity. The sputtering chamber was evacuated to less than 5 × 10−6 torr prior to sputtering, and backfilled with a mixture of argon and oxygen. The sputtering pressure and the power density used were 2 mtorr and 5 W/cm2, respectively. Before growth, we presputtered the target for at least 15 minutes to remove contaminants on its surface. After the deposition, the Ta2O5 thin films were post annealed in oxygen environment to improve oxygen deficiency in the as-grown film . In addition, the annealing temperature and the time of annealing were carefully adjusted to prevent possible crystallization and cracking of the films. The oxygen deficiency and crystal grains of the films could result in excess optical absorption and scattering losses for optical wave propagation.
To check the crystal variation with annealing time of Ta2O5, the X-ray diffraction (XRD) analysis is performed. Figure 1 shows the XRD spectra of Ta2O5 with different annealing time. The XRD peaks at 22.9°, 28.3°, 28.8°, and 37° are related to the (001), (110), (200), and (111) crystal orientation of β-Ta2O5 [17, 18]. These XRD peaks become clearer with the annealing time longer than 5 hours. It implies that the amorphous structure of Ta2O5 is dominated when the annealing time is shorter than 3 hours. The diffraction pattern and high-resolution transmission electron microscopy (HRTEM) are also utilized to further confirm the crystallization of Ta2O5. As shown in Fig. 1(b), no discrete diffraction point is observed in the diffraction pattern of Ta2O5 with annealing time shorter than 3 hours. Moreover, no crystal or grain boundaries are observed in the cross-sectional HRTEM image of Ta2O5 with annealing time of 3 hours [referred to Fig. 1(c)]. Although the absorbance of the as-grown Ta2O5 can be suppressed by filling the oxygen deficiencies with long-term thermal annealing , the more crystallized Ta2O5 would result in the significant scattering loss. Therefore, to avoid the significant scattering loss induced by the crystal structure or grain boundary of Ta2O5 film, the annealing time should be controlled as shorter than 3 hours.
To check the surface quality of Ta2O5, the top-view images of Ta2O5 recorded by scanning electron microscope (SEM) are shown in Fig. 2(a). The cracks on Ta2O5 surface are observed when the annealing time is longer than 5 hours. The long-term annealing process would introduce the crystallization or regrowth of Ta2O5. In that case, the stress relaxation induced by the crystallization would result in the inevitable surface cracks. The surface density of cracks on the annealed Ta2O5 is increased up to 3.8 × 105 #/cm2 with the annealing time of 7 hours. Moreover, the minimum surface roughness of 0.13 nm for Ta2O5 with annealing time of 3 hours is observed by using atomic force microscopy (AFM). Based on the aforementioned discussions, the Ta2O5 with annealing time of 3 hours at 650°C is the optimized annealing receipt with low absorbance, scattering loss, and crack-free properties. It can be utilized to fabricate the ultralow loss Ta2O5 channel waveguide. The optical property of Ta2O5 with annealing time of 3 hours is measured by using ellipsometer, see Fig. 2(b). The refractive index at 1550 nm is ~2.1, which is larger than that of Si3N4 and SiO2. That is, the Ta2O5 based waveguide can provide better optical confinement than Si3N4 or SiO2 waveguide. The extinction coefficient is very low at wavelengths ranged from 300 to 1700 nm, indicating low absorption property ranged from visible to near infrared region.
3. Waveguide fabrication and optical properties investigation
3-1. Ta2O5 waveguide fabrication
For waveguide fabrication, the Ta2O5 with thickness of 400 nm is deposited on the Si wafer, which is covered by 3-μm thick thermal oxide. Then, the E-beam lithography is utilized to define the waveguide pattern. The positive resist (ZEP-520A) with thickness of 260 nm is covered on the Ta2O5 by using the spin coating. The width of waveguide in the E-beam layout is set as 700 nm for single mode operation. The beam propagation method suggests that the waveguide becomes multimode operation with the width larger than 800 nm. The length of the waveguide is controlled as 4.8 mm, 9 mm, and 12.6 mm, respectively. Furthermore, the micro-ring resonator is also demonstrated. The core area of bus and ring cavity are set as the same dimension of 400nm × 700nm. The diameter of the ring cavity is designed as 200 μm, and the gap between the ring and bus waveguide is set as 900 nm. The diameter of the ring cavity is designed as 200 μm, and the gap between the ring and bus waveguide is set as 900 nm. After E-beam lithography and develop processes, the Cr with thickness of 80 nm is then deposited on the Ta2O5 by using the E-gun evaporation. The Cr is served as the hard mask to protect the defined region without being etching in the reactive ion etching (RIE) process. The etching gas is CHF3 with fluence of 30 sccm, and the RF power is set as 90 W. After dry etching process, the hard mask is dissolved by using the chromium etchant (CR7). Then, the SiO2 with thickness of 2-μm is deposited on the Ta2O5 waveguide to serve as cladding layer by using plasma-enhanced chemical vapor deposition (PECVD). Finally, both ends of Ta2O5 channel waveguide are diced and polished.
3-2. Low loss Ta2O5 channel waveguide with inverse taper structure
In order to enhance the coupling efficiency between the lensed fiber and waveguide facet, the inverse taper structure is introduced into our waveguide designing . The coupling efficiency is simulated based on the beam propagation method. The width of the inverse taper is varied from 300 nm to 700 nm within 200-μm taper length. The waveguide height is set as 400 nm. The refractive index of the Ta2O5 is set as 2.1, and the surrounding is set as SiO2 with refractive index of 1.46. The input optical filed is set as Gaussian beam with beam diameter of 3-μm at wavelength of 1550 nm, and the reference optical field is the fundamental TE0 mode for the Ta2O5 channel waveguide. The working principle of the inverse taper is described below. For the inverse taper structure, the weak guiding condition exists at the tip of the inverse taper and most of the optical power is spreading in the cladding layer. When optical field is propagating through the inverse taper region, the gradual increased effective index along the inverse taper would result in the enhanced optical confinement in the Ta2O5 core layer. Therefore, the coupling efficiency between the waveguide facet and lensed fiber can be significantly enhanced by using the inverse taper structure. In our simulation results, the coupling efficiency is increased from 41% to 92% by introducing the inverse taper into the channel waveguide.
In experiment, we have demonstrated the Ta2O5 channel waveguide with and without inverse taper for comparison. The waveguide length is controlled as 4.8 mm and the sidewall angle is around 84° for both cases. The waveguide illustrations and SEM images of Ta2O5 channel waveguide end facet with and without inverse taper structure are shown in Fig. 3(a). The core area of Ta2O5 channel waveguide with inverse taper is 400nm × 300nm at the end face, and the core area is inversely tapered from 400nm × 300nm to 400nm × 700nm within 200-μm taper length. For the channel waveguide without inverse taper, the core area of channel waveguide is remained as 400nm × 700nm without deformation. In order to characterize the insertion loss of the waveguide devices, the wavelength of 1550 nm outputted from tunable laser is seeding to the waveguide. By calculating the power difference between the transmitted power with and without devices, the insertion loss of the device is obtained. The insertion losses at 1550 nm for 4.8-mm long Ta2O5 channel waveguide with and without inverse taper is −4.0 and −7.8 dB, which are shown in Fig. 3(b). Due to the same propagation length for both cases, the 2-dB enhanced coupling efficiency (each facet) by using the inverse taper structure is experimentally obtained. It shows good agreement with the simulation result of 3-dB coupling enhancement by using the inverse taper structure.
To characterize the propagation loss of Ta2O5 channel waveguide, the insertion losses of Ta2O5 channel waveguide with different waveguide lengths are demonstrated. The relation between the insertion loss and waveguide length is shown in Fig. 3(b). To carefully estimate the insertion loss of the Ta2O5 channel waveguide, the insertion loss is recorded with error bar by sampling 6 trials of different waveguides with same length. The insertion loss is increased from −4 to −5.4 dB by increasing waveguide lengths from 4.8 mm to 12.6 mm. By fitting the insertion loss variation with the waveguide lengths, the propagation loss of −1.5 dB/cm (0.35 cm−1) and total coupling loss of −3.2 dB for the Ta2O5 channel waveguide with inverse taper structure are determined. In addition, we observed that the variation of the insertion loss is increased when measuring the waveguide with longer propagation length. It is mainly resulted from the misalignment of the E-beam lithography when stitching different patterns to form long waveguide. The misalignment caused by the E-beam lithography is ~50 nm. Therefore, the quality of the waveguide becomes degraded when fabricating the waveguide with length longer than 1 cm. If the E-beam stitching error can be furthered modified, the propagation loss of the Ta2O5 channel waveguide is expected to be lower than 1 dB/cm.
3-3. High-Q Ta2O5 based micro-ring resonator
To further identify the low loss characteristics, the Ta2O5 based micro-ring resonator has been also fabricated. The cross-sectional dimension of the bus waveguide is set as 700 × 400 nm2, and the diameter of the ring resonator is controlled as 200-μm. The gap between the bus and ring cavity is set as 900 nm. As shown in Figs. 4(a) and (b), the top-view and side-view images of micro-ring resonator were taken before depositing 2-μm thick SiO2 cladding. The normalized transmission spectrum of the Ta2O5 micro-ring resonator is shown in Fig. 4(c). Two modes, including TE0 and TM0, are observed in the normalized transmission spectrum. In order to extract resonator properties, the transmittance of the micro-ring resonator is simulated by using the transfer function of the all-pass ring resonator . The model can be formulated as:Eq. (1), the group index, transmission coefficient, and loss coefficient for TE0 mode are determined as 2.088, 0.9923, and 1.9 cm−1 (8.1dB/cm), respectively [referred to Fig. 4(d)]. Regarding TM0 mode in the Ta2O5 based micro-ring resonator, the group index, transmission coefficient, and loss coefficient are 1.966, 0.9930, and 2.1 cm−1 (9.0dB/cm), respectively. Because the absorption coefficient of Ta2O5 at infrared region is very small and can be neglected, the loss mechanism inside the ring cavity is mainly attributed to the scattering and bending loss of waveguide sidewalls. Due to the different transverse mode distributions of TE and TM modes from the non-symmetric dimension (400nm × 700nm) in the Ta2O5 waveguide, the loss coefficients of TE and TM modes induced by scattering are slightly different. Generally, with increasing the aspect ratio of waveguide structure (width/height), the higher polarization dependent characteristic is obtained. In addition, high aspect ratio of waveguide structure leads to TE-dominated confinement and thus higher loss in TM modes.
The extracted loss coefficient inside the ring resonator is larger than that of the obtained in the straight waveguide. The degraded loss coefficient inside the ring cavity is probably originated from the imperfect sidewall since the sidewall of the ring cavity is slightly roughness, as shown in Fig. 4(b). This might be induced by the lift-off and RIE process. Nevertheless, the quality factor for the TE0 mode in the Ta2O5 based micro-ring resonator is 3.91 × 104. Once neglecting the coupling contribution in the micro-ring resonator, the unload Q of 4.48 × 104 is determined , which is comparable to the previous work reported by P. Rabiei and associates . One should be noticed that propagation loss of Ta2O5 channel waveguide is around 1.5dB/cm, which is determined from the cut-back method. As a result, the quality factor of the Ta2O5 based micro-ring resonator is expected to be higher than 105 if the fabrication of micro-ring resonator is further improved with ultralow cavity loss. Based on our primary results, the low-loss and high-Q Ta2O5 based micro-ring resonator is expected to be utilized to demonstrated ultrafast all-optical switching or four-wave-mixing based optical parametric oscillator.
The low-loss Ta2O5 channel waveguide has been successfully demonstrated. By carefully setting the annealing time of post-annealing process, the Ta2O5 film with low absorbance, ultra smooth and crack-free properties are obtained. The annealing time of 3 hours at 650°C for Ta2O5 is an optimized receipt for filling oxygen deficiency in the as-grown sample without crystallization. The propagation loss of 1.5 dB/cm for Ta2O5 channel waveguide is determined by fitting the insertion loss variation with different waveguide lengths. Moreover, the total coupling loss of 3.2 dB is realized in the Ta2O5 channel waveguide by introducing the inverse taper structure. Furthermore, the Ta2O5 based micro-ring resonator has been demonstrated. The quality factor of 3.91 × 104 for Ta2O5 based micro-ring resonator with diameter of 200 μm is realized. The ring loss coefficient of 1.9 cm−1 is obtained by analyzing the transmission spectrum of Ta2O5 based micro-ring resonator, which corresponds to the unload Q of 4.48 × 104. Our results indicate the Ta2O5 has great potentials in demonstrating low-loss, high Q micro-cavity device, and it can be utilized in the ultra-compact integrated optical circuit in the near future.
The authors thank the Ministry of Science and Technology, Taiwan, for financially supporting this research under grants NSC 101-2112-M-110-008-MY3, and NSC 102-2221-E-110-072-MY3. In addition, the authors appreciate Dr. M. H. Shih for his assistant in n-k measurement.
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