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Abstract
Tantalum pentoxide (Ta2O5) is a promising material for optical waveguide applications of photonics integration due to its excellent linear and nonlinear optical properties, such as high refractive index, large bandgap, and high nonlinearity. The quality of thin film deposition will then be critical for realizing optical waveguide devices and modules. In this work, an ion-assisted electron-beam evaporation system has been used to deposit such thin film. As low as 0.73 nm thickness roughness has been demonstrated in a 700 nm thick film, indicating it as a candidate for fabricating a low-loss waveguide. An optical waveguide-based ring resonator was made for examining the optical waveguide performance. Through the flat surface morphology, a low propagation loss of 1.4 dB/cm with an unloaded quality factor of 3 × 105 ring resonance has been realized. The nonlinear index of refraction (n2) in as-deposited Ta2O5 film was found to be in an order of magnitude of 10−14 cm2/W, which was also confirmed by both Z-scan technique and all-optical modulation technique. By such high bandgap properties, a nonlinear absorption threshold of few TW/cm2 was also observed for the first time. The measured device performances are comparable to the state-of-the-art results from up-to-date counterparts.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nonlinear optical effects such as nonlinear optical frequency conversion [1,2], self-phase modulation (SPM) [3–5], optical switching [6], two photon absorption (TPA) [7,8], parametric amplification [9,10], cross-phase modulation (XPM) [11,12], and stimulated Raman scattering (SRS) have been successfully demonstrated on a chip scale of waveguide devices [13]. Nonlinear optical frequency conversion has been applied to many important applications, including all-optical switching, correlated photon pair generation, and narrow line width or multi-wavelength sources [14,15]. Among the nonlinear optical phenomena, four-wave mixing (FWM) is a common approach to frequency conversion, which is based on a third-order nonlinear process and inevitably sensitive to the third-order susceptibility χ(3) of the nonlinear medium. Therefore, the coherent and multiple wave natures of FWM approach has been developed for several advanced and crucial photonic technologies [16–20]. Several groups have reported FWM-related works using different materials, for examples, III-V [18], chalcogenide [21], silicon [22–24], graphene [25,26], silica [27], and silicon nitride [28,29], showing the potential. However, the low nonlinear property of silica and the TPA effect of silicon both degrade the signal quality and then limit the conversion efficiency of FWM. Also, the tensile stress in depositing thick silicon nitride film is typically an obstacle to realize the low loss waveguide fabrication. As a result, the employment is one of the most crucial issues for developing the advanced nonlinear optical applications. To further enhance the nonlinear optical applications, an alternative material, Ta2O5, has been investigated recently [30–32] due to its large bandgap (4.2 eV), low optical loss, high refractive index and high nonlinearity. The following is discussed: The linear and nonlinear optical loss, such as TPA effect, can thus be significantly reduced [33]. In general, direct growth by sputtering and thermal oxidation of tantalum thin film have been reported for fabricating Ta2O5 film [34,35], where the micro-ring resonator is typically one kind of devices to test the nonlinear and linear properties of thin-film material. A high quality (Q) value of 45,000 was demonstrated from the Ta2O5-film micro-ring resonator by oxidation of Tantalum film [35], but the high temperature processing was needed. That means, the sputter grown Ta2O5 film is one of the most popular ways due to the simple processing. Furthermore, to date, using the reactive sputtering method, the Ta2O5 channel waveguide and micro-ring resonator with low propagation loss have been successfully fabricated [36,37]. The large nonlinear refractive index of Ta2O5 was also investigated with order of 10−14 cm2/W, which is higher than that of silica and silicon nitride [38]. Besides, the optical damage threshold of Ta2O5 is much higher than other optical materials [39]. The sputter grown Ta2O5 channel waveguide was also used to realize the nonlinear FWM process with a conversion efficiency of −50 dB at the coupled pump power of 40 mW into an optical waveguide [40]. Moreover, the Ta2O5-based micro-ring resonator with an unloaded Q of 182,000 has been verified to realize efficient FWM conversion of approximately −30 dB with a pump power of 6 mW [41]. Recently, to be noted that a high-efficient super-continuum generation from visible to near infrared wavelengths further suggests that the great potential of Ta2O5 thin film on on-chip applications [42]. In spite the fact that degenerated FWM-based optical parametric oscillation (OPO) has been performed, the nondegenerate FWM-OPO from Ta2O5 is still under the development due to the lower reported Q value in the micro-rings. As a result, comparing to the SiN which is the most common material used in thin film, the Ta2O5 based nonlinear waveguides are still somehow less competitive even though the reported property is an order of magnitude higher in optical nonlinearity. Typically, the Ta2O5 film quality is a critical factor to realize the low-loss waveguide and nonlinear optical applications. Therefore, improvement of Ta2O5 thin film quality for improving the waveguide devices have become an inevitable issue for the related researches.
Other than the sputtering deposition system, electron-beam (e-beam) evaporation has been known as one of the common vacuum systems for mass-production thin-film deposition. With the local heating through injecting an electron beam, the low and stable deposition rate of the material can be realized with well controllable beam current. Dense and uniform film with good surface morphologies can also be attained under a high-vacuum and long-throne chamber. In order to fabricate low-loss waveguide with dense and flat Ta2O5 film for linear and nonlinear optical applications, e-beam evaporation system is thereby one of the best choices. However, during the e-beam assisted thermal evaporation under high-vacuum condition, the stoichiometric property will generally be deteriorated. Building an oxygen- and argon-assisted ion gun into the e-beam evaporation system with following the consecutive thermal annealing treatment under oxygen atmosphere will thus be employed to improve the film quality. It suggests that the usage of such Ta2O5 thin film preparation could have a potential for photonic device application. But using e-beam evaporation for growing high quality Ta2O5 films is still less discussed and e-beam based Ta2O5 nonlinear waveguide applications are consequently less performed.
In this paper, a Ta2O5 channel waveguide has been realized through an oxygen and argon ion-assistant e-beam evaporation scheme, where an optical ring resonator was used as the tested element. By taking the advantages of e-beam evaporation scheme, as low as 0.73 nm of surface roughness with high quality of thin film material has been attained. High-quality film has been examined by X-ray diffraction (XRD) analysis and refractive index measurement. The optical transmission analysis and all-optical modulation in the Ta2O5 micro-ring resonator at ∼1550 nm has been obtained, further extracting the low propagation loss of 1.4 dB/cm and the nonlinear refractive index of Ta2O5 thin film. The low-loss propagation and high optical nonlinearity of the Ta2O5 channel waveguide are expected to be utilized in nonlinear waveguide applications with low demanding optical power.
2. Ta2O5 film deposition and analysis
The Ta2O5 thin film with a thickness of 700 nm was deposited on the substrate by an e-beam evaporation system, where an ion-gun equipment was installed aside the evaporation, setting up a co-evaporation system. The substrate is a Si wafer, which is covered by a 3 μm thick thermal oxide for optical waveguide fabrication. In order to improve the film quality, the deposition of Ta2O5 was carried out in a proper oxygen and argon mixed ion atmosphere by simultaneously operating ion gun. Typically, the roughness of the film can be optimized by properly adjusting the argon ion gun due to the increasing mobility during the deposition. In addition, by oxygen ion gun, the vacancy of the oxygen will be also properly filled, leading to the denser thin film. After the deposition, the Ta2O5 thin films were post annealed in oxygen environment to further reduce oxygen deficiency of the as-grown film. In addition, the annealing temperature and time were carefully adjusted to prevent possible crystallization and crack within the films. Otherwise, the oxygen deficiency and crystal grains of the films could result in excess optical absorption and scattering losses in the optical wave propagation [43]. In this experiment, the Ta2O5 film was annealed at 580°C for two hours. To identify the structure, the XRD spectra (blue line) was first characterized, as seen in Fig. 1(a). Comparing with reported XRD result (red line) of sputtering growth Ta2O5, the profiles clearly all behave similarly. Additionally, for ensuring the crystallization of the film, an e-beam grown Ta2O5 which was annealed at 580°C for seven hours was also investigated (green line). Meanwhile, as shown in Fig. 1(a), it should be noted that the XRD peaks which are related to the (001), (002), (021), (110), (111), (200), (201) and (311) crystal orientation of β-Ta2O5 were all observed in the long-time annealing films [44]. It implies that the amorphous structure of Ta2O5 is dominated when the annealing time is shorter than 3 hours. To check the surface morphology of Ta2O5, the top-view image of Ta2O5 was recorded by atomic force microscope (AFM). Figure 1(b) shows the AFM picture of Ta2O5 film. As low as 0.73 nm of root-mean-square roughness Rrms on a 700 nm thick film material was found, implying that ion-assistant e-beam evaporation system with post annealing treatment could realize a flat and good quality of film material. Moreover, to further exam the effect of the ion-assistant scheme on the film, the optical property of Ta2O5 film is measured by an ellipsometer and shown in Fig. 1(c). The depositions of Ta2O5 film with/without oxygen and argon ion-assistant were compared. As shown in the inset of the Fig. 1(c), apparently, the ion-assistant film reveals higher refractive index, further confirming the function, i.e. the denser film quality can be attained through this scheme. The refractive index is ∼2.07 at 1 μm wavelength regime, which is larger than that of Si3N4 and SiO2. It also can be seen that the extinction coefficient is very low at the wavelengths ranged from 500 to 1700 nm, indicating low material absorption property of the deposited Ta2O5 film.
Fig. 1. Material characterization of the Ta2O5 film: (a) the XRD pattern, (b) the AFM room-mean-square image and (c) the n&k data.
3. Ring resonator fabrication and characterization
To identify the low-loss characteristics of Ta2O5 waveguide, a micro-ring resonator has been fabricated. The cross-sectional dimension of the bus waveguide was set as 700 × 700 nm2, and the diameter of the ring resonator was controlled as 100 μm. The gap between the bus and ring cavity was made as 700 nm. The waveguides of resonator and bus line were defined by an e-beam lithography, then a reactive ion etching (RIE) system, and the final SiO2 deposition for top cladding. As shown in Fig. 2(a), the top-view and side-view SEM images of micro-ring resonator were taken before depositing 2-μm thick SiO2 cladding. The normalized optical transmission spectrum of the Ta2O5 micro-ring resonator is shown in Fig. 2(b). Two modes, including TE0 and TM0, are observed in the normalized transmission spectrum. In order to extract the resonance properties and the loss of the waveguide, the transmittance of the micro-ring resonator is simulated by using the transfer function of the all-pass ring resonator [45]. The model can be formulated as:
Fig. 2. (a) Top view(left) and side view(right) of the fabricated Ta2O5 micro-ring. (b) The normalized transmittance spectrum of Ta2O5 micro-ring resonator and the corresponding fitting curves.
In addition, one can accordingly estimate the loaded quality factor (Q) of the fabricated micro-ring resonator. The Q values of 43,708 for TE0 and 21,961 for TM0 has been found. In the past report [41,46,47], using RF sputtering as deposition method, the loaded Q value in the order of 50,000 (unloaded Q 182,000) has been reported, where the same ring diameter and also thickness of Ta2O5 but the wider waveguide width (1,500 nm in the ring) was designed as comparing with the e-beam method (this work). Both works show the same order magnitude of loaded Q value, however, the smaller propagation loss ($\alpha $ = 0.3 cm-1) for e-beam grown Ta2O5 has been obtained, implying the potential in using this method. Once neglecting the coupling contribution in the micro-ring resonator, the unloaded Q of 3×105 is determined from the group index ng, wavelength λ and propagation loss α (Qunloaded = 2πng/αλ) [48], which is larger than the reported data by RF sputtering one. The degraded Q of the ring cavity is probably originated from the gap distance or imperfect sidewall. As a result, the Q of the Ta2O5-based micro-ring resonator is expected to be higher than 105 if the fabrication is further improved with ultralow cavity loss. Nonetheless, the e-beam Ta2O5 growth scheme shows a quite potential for optical waveguide engineering. Based on our primary results, the low-loss and high-Q Ta2O5-based micro-ring resonator is expected to exhibit ultrafast all-optical switching or FWM based optical parametric oscillator [41,46,47].
In the past reports, the optical nonlinearity of sputtering growth Ta2O5 has been investigated by using various techniques, such as FWM OPA/OPO, SPM [38,40,41]. In addition, the applications for modulation or super continuum generation (SCG) have revealed its promising potential. Here, to characterize the optical nonlinearity, the conventional Z-scan measurement was employed. The Z-scan technique has been widely used for determining nonlinear properties of film material, where the optical transmittance of the film in the far field is measured for analysis. Here, the laser source was a femtosecond regenerative amplifier system with a wavelength of 800 nm, pulse duration of 35 fs, and pulse repetition rate of 5 kHz. The laser beam is then focused by a lens (focal length: 500 mm), generating a beam waist of around 30 μm at the focal point. The Z-scan schematic diagram is plotted in Fig. 3(a). In order to characterize the measurement setup, the Z-scan measurement of a three-layer graphene was first performed with n2 value of 1.15×10−8 cm2/W, indicating the accuracy of the measurement system [49]. In measuring the Ta2O5 thin film, Fig. 3(b) shows the open aperture and close aperture of Z-scan results for a focusing peak intensity of 1,702 GW/cm2. The focusing peak intensity here is the peak intensity of the z-scan measurement. The value is estimated from the focusing beam spot size, repetition rate of the pulse and the average power used in the experiment. Clearly, the normalized transmittance remains unchanged along the Z-axis from -60 to 60 mm, indicating the ignorable nonlinear absorption. This suggests the nature of free nonlinear absorption in Ta2O5 thin film based on the discussion in [38]. Additionally, the nonlinear refractive index can be further obtained by the value of the closed aperture results divided by the open aperture results. As shown in Fig. 3(c), the black line is the extracted vales, where the red line is the fitting curve. Based on the work of Ref. [49], the simulation equation can be expressed as:
Fig. 3. (a) The Z-scan schematic diagram; (b) The open aperture and close aperture of Z-scan results for a focusing peak intensity of 1702 GW/cm2; (c) Experimental and fitting results of the closed / open aperture Z-scan.
Furthermore, peak intensity dependent optical nonlinearity was characterized. Figure 4(a) shows the open aperture result for the incident focusing peak intensity increasing from 43 GW/cm2 to 7,941 GW/cm2. Clearly, the behavior of the normalized transmittance changes while the focusing peak intensity increasing to 7,941 GW/cm2. By estimating the corresponding peak intensity from varying beam waist along the Z-axis from -60 to 60 mm, the nonlinear normalized absorption (-(1-normalized transmittance)) can be accordingly plotted, as seen in Fig. 4(b). Apparently, the absorption appears and exhibits the strong peak intensity as the intensity is larger than thousands of GW/cm2 (or few TW/cm2), indicating the nonlinear absorption threshold of around few TW/cm2. In general, Ta2O5 shows its potential of nonlinear application due to its nature of wide bandgap and the reported work also reveal the characteristic of free nonlinear absorption [38]. However, one should be noted that the reported results are all based on the highest coupled peak intensity of around tens GW/cm2. The facet of the waveguide will be damaged as the coupled power further increasing. Here, by using low-repetition rate laser for avoiding thermal damage, the nonlinear absorption can be therefore realized. The mechanism of the nonlinear absorption can be attributed to multi-photon absorption. In the past, W. Rudolph et. al has discussed femtosecond laser pulse induced breakdown in Ta2O5 thin films and reported the three-photon absorption coefficient [39]. It is worthy to note that the nonlinear absorption threshold property of Ta2O5, which is important for application, can be investigated for the first time. Despite of that, the Ta2O5 still shows the nearly nonlinear absorption free for which the peak intensity is less than TW/cm2. This indicate the promising application potential since the typical peak intensity is much small than nonlinear absorption threshold.
Fig. 4. (a) The open aperture result for the incident focusing peak intensity increasing from 43 GW/cm2 to 7,941 GW/cm2; (b) the peak intensity dependent nonlinear normalized absorption.
4. Nonlinear optical measurement
To further investigate the nonlinear function of e-beam growth Ta2O5 and its application feasibility, an all-optical modulation was performed to confirm the material property and the device application. The schematic diagram of the all-optical switch is shown in Fig. 5. The operation principle is based on the Kerr effect to induce the refractive index change inside the micro-ring cavity [46,47]. As injecting an optical pump pulse into the ring cavity to bring out a timely varied refractive index, the transmittance of the ring resonator can therefore be controlled to achieve a timely all-optical modulation at the wavelength of the injected probe light. Previously, using sputtering growth Ta2O5, tens of GHz all-optical modulation has been realized [53]. The nonlinear refractive index of Ta2O5 at 1.55 μm was estimated as 3.4 × 10−14 cm2/W by using the Kerr effect.
Fig. 5. Schematic diagram of the all-optical switch. Based on the given probe wavelength, the probe signal can be directly or inversely modulated by the pumped pulse laser TL, MZM, EDFA, OBPF, Ring, PD, AWG are tunable laser, high-speed Mach-Zehnder modulator, erbium-doped fiber amplifier, optical band pass filter, Ta2O5 high-Q ring, photo-detector and arrow-waveguide, respectively.
Here, following the similar experimental steps reported in [46,47,53], the all-optical modulation of e-beam growth Ta2O5 ring is accordingly demonstrated. The pumped pulse laser and continuous wave (CW) probe laser were simultaneously injected into the ring cavity. The pumped pulse was generated by an external modulated tunable laser in a high-speed Mach-Zehnder modulator (MZM), where the pulse width and period of the pulse laser were set as100 ps and 100 ns, respectively. The pump power was then amplified through an erbium-doped fiber amplifier (EDFA). The ring for linear optical properties investigated aforementioned was used for all-optical modulation. One can therefore estimate the actual optical magnification power inside the ring cavity, which is crucial for evaluating Kerr effect. The buildup factor (B) of the micro-ring resonator is described by Eq. (3), which indicates the optical magnification power inside the ring cavity by comparison with the bus waveguide [53] The B value of 52.4 is therefore calculated.
Fig. 6. (a) Probe modulation traces at different probe wavelengths in the Ta2O5 micro-ring resonator. (b) Evolution of modulated probe amplitude in the Ta2O5 micro-ring resonator as probe wavelength is changed. (c) Normalized transmittance spectra of Ta2O5 micro-ring resonator with and without optical pump.
Up to date, the generation of broadband coherent light source by nonlinear waveguide has been realized as a plausible approach for realizing the practical applications, such as metrology [56]. In addition, a nonlinear performance, such as dual frequency comb generation has been also demonstrated with on-chip waveguide technology [57], implying that the mass-production waveguide technology could be engaged in such field. In addition, high-Q ring resonator has become one of the most important and critical elements for the further on-chip applications. Although the ring performance relies on its structure and fabrication method, the material properties defined for waveguide, such as high n2, and flat surface template will definitely be of extra advantage since the lower threshold can be realized for comb generation and optical modulation from ring, broadband coherent light source, such as SCG, from waveguide. Especially, the aforementioned experiment has suggested that the high nonlinear absorption threshold (up to TW/cm2) has imposed the Ta2O5 based nonlinear devices to be capable of operating at high saturation power level. That is to say, within nonlinear absorption threshold, the higher nonlinearity, such as n2, will be more important for SCG or other nonlinear application because lower power excitation with high efficiency will be welcome for avoiding damage issue [42]. Therefore, the experimental results indicate that e-beam grown Ta2O5 thin films thus shows great potential for use in nonlinear on-chip waveguide applications.
5. Conclusion
In summary, tantalum pentoxide (Ta2O5) of a large bandgap material has shown its potential for Si photonics due to its low absorption coefficient from visible to infrared regions, high Kerr coefficients, and free nonlinear absorption. Development of large scale high optical quality thin film for integrated photonics is therefore desired. In this work, by using e-beam evaporation accompanied with ion-assisted treatment, Ta2O5 thin film growth was performed. Beside the measurement of XRD for identifying the structure, the AFM shows the high-quality thin film with as low as roughness of 0.73 nm. In addition, a low-loss and high-Q Ta2O5-based micro-ring resonator was fabricated and characterized. The propagating loss of 0.3 cm-1 was obtained and unloaded quality as high as 3 × 105 was accordingly estimated. Furthermore, the nonlinear refractive coefficient (n2) was investigated by using Z-scan technique and all-optical modulation technique. All of the extracted n2 shows the same order of the magnitude, 10−14 cm2/W. It should be noted the nonlinear absorption threshold of around few TW/cm2 was characterized and discussed for the first time. Compared to the conventional materials for Si photonics, such as SiN, SiO2 and so on, the larger n2 value also reveal the potential for application.
Funding
Ministry of Science and Technology, Taiwan (106-2112-M-110-006-MY3, 106-2221-E-110-050-MY3, 107-2112-M-110-004).
Disclosures
The authors declare no conflicts of interest.
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