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Enhanced four-wave mixing in hybrid integrated waveguides with graphene oxide

Jiayang Wu, Yunyi Yang, Xingyuan Xu, Linnan Jia, Yao Liang, et al.

Jiayang Wu, Yunyi Yang, Xingyuan Xu, Linnan Jia, Yao Liang, Sai T. Chu, Brent E. Little, Roberto Morandotti, Baohua Jia, David Moss, "Enhanced four-wave mixing in hybrid integrated waveguides with graphene oxide," Proc. SPIE 10920, 2D Photonic Materials and Devices II, 109200K (27 February 2019); doi: 10.1117/12.2508120 Event: SPIE OPTO, 2019, San Francisco, California, United States

Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 11 Mar 2019 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use Enhanced four-wave-mixing in hybrid integrated waveguides with graphene oxide

Jiayang Wua, Yunyi Yanga, Xingyuan Xua, Linnan Jiaa, Yao Lianga, Sai T. Chub, Brent E. Littlec, Roberto Morandottid, e, f, Baohua Jiaa, and David Mossa, * aCentre for Micro-Photonics, Swinburne University of Technology, Hawthorn, VIC 3122, Australia bCity University of Hong Kong, Tat Chee Avenue, Hong Kong, China cXi’an Institute of Optics and Precision Mechanics Precision Mechanics of CAS, Xi’an, China dINRS –Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, Québec, Canada eNational Research University of Information Technologies, Mechanics and Optics, St. Petersburg, Russia fUniversity of Electronic Science and Technology of China, Chengdu 610054, China. * Electronic mail: [email protected]

ABSTRACT

Owing to the ease of preparation as well as the tunability of its material properties, graphene oxide (GO) has become a rising star of the graphene family. In our previous work, we found that GO has an ultra-high Kerr nonlinear optical response - several orders of magnitude higher than that of silica and even silicon. Moreover, as compared with graphene, GO has much lower linear loss as well as nonlinear loss (two photon absorption (TPA)), arising from its large bandgap (2.4~3.1 eV) being more than double the photon energy in the telecommunications band. Here, we experimentally demonstrate enhanced four-wave mixing (FWM) in hybrid integrated waveguides coated with GO films. Owing to strong mode overlap between the integrated waveguides and the high Kerr nonlinearity GO films as well as low linear and nonlinear loss, we demonstrate significant enhancement in the FWM efficiency. We achieve up to ~9.5-dB enhancement in the conversion efficiency for a 1.5-cm-long waveguide with 2 layers of GO. We perform FWM measurements at different pump powers, wavelength detuning, GO film lengths and numbers of layers. The experimental results verify the effectiveness of introducing GO films into integrated photonic devices in order to enhance the performance of nonlinear optical processes. Keywords: Four-wave mixing, graphene oxide, integrated photonics

1. INTRODUCTION Without complicated optical-electrical-optical (O-E-O) conversion process, directly processing signal in optical domain is a key approach that can dramatically decrease power consumption and speed up optical telecommunications systems [1- 3]. Four wave mixing (FWM), as an important nonlinear optical effect, has been widely explored to realize all-optical signal processing functions such as wavelength conversions, optical logic gates, optical comb generations, and quantum entanglements [4-11]. To date, efficient FWM in telecommunications band has been successfully demonstrated by using silica fibers, III/V material-based semiconductor optical amplifiers (SOAs), integrated photonic devices based on silicon or silicon compounds, polymer composites, chalcogenide devices, and so forth [5, 12]. Among them, the integrated photonic devices provide a competitive solution to achieve on-chip processing with compact footprint, high stability, mass- producibility, and excellent cost performance [13-20]. Although silicon has been the leading platform for integrated photonic devices [21-24], its high two-photon absorption (TPA) at near-infrared wavelengths poses a fundamental limitation to the FWM performance of silicon photonic devices in the telecommunications band [5]. The quest for high-performance integrated platforms for nonlinear optics has motivated the development of other complementary metal–oxide–semiconductor (CMOS) compatible platforms such as silicon nitride and high-index doped silica glass [25]. Benefiting from the extraordinarily low losses — both linear and nonlinear, high-index doped silica glass has been demonstrated to be a successful integrated platform for nonlinear photonic devices [26-28]. Nevertheless, the relatively low Kerr nonlinearity as compared with silicon and silicon nitride limits the performance of nonlinear photonic devices made from high-index doped silica glass. The implementation of hybrid devices with advanced materials having high Kerr nonlinearity and low TPA effect could open up a way towards

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overcoming this limitation [29-31]. Owing to the ease of preparation as well as the tunability of its material properties, graphene oxide (GO) has become a rising star of the graphene family [32, 33]. In our previous research [33, 34], we have found that GO has a giant Kerr nonlinear response, which is 4~5 orders of magnitude higher than that of high-index doped silica glass. Moreover, as compared with graphene, GO has much lower loss, a very large bandgap (2~3.5 eV) for inefficient TPA effect in the telecommunications band [35], and better capability for large-scale fabrication, which are all key attributes for the implementation of high-performance nonlinear photonic devices. In this paper, improved FWM performance of GO coated integrated waveguides made from high-index doped silica glass is theoretically analysed and experimentally demonstrated. By designing and fabricating integrated waveguides with strong mode overlap with GO, we achieve greatly enhanced FWM efficiency in the GO hybrid integrated waveguides. A maximum conversion efficiency (CE) enhancement of ~9.5 dB is experimentally achieved for a 1.5-cm-long integrated waveguide with 2 layers of GO. FWM measurements are also carried out for different pump power, wavelength detuning, GO coating lengths, and GO layer numbers. The results confirm improved FWM performance of integrated photonic devices incorporated with GO.

2. DEVICE FABRICATION AND CHARACTERIZATION Figure 1(a) shows the GO-coated integrated waveguides made from high-index doped silica glass, with a cross section of 2 μm × 1.5 μm. The integrated waveguide is surrounded by silica except that the upper cladding is removed to enable coating the waveguide with GO films. The GO films, with a thickness of about 2 nm per layer, were introduced on the top of the integrated waveguide in order to introduce light-material interaction with the evanescent field leaking from the integrated waveguide. The Kerr coefficient of GO is on the order of 10-15~10-14 m2/W [33, 34], which is slightly lower than that of graphene (~10–13 m2/W) [36, 37], but still orders of magnitude higher than that of high-index doped silica glass (~10–19 m2/W) and silica (~10–20 m2/W) [25]. The waveguides were fabricated via CMOS compatible processes [38-40]. First, high-index doped silica glass films (n = ~1.60 at 1550 nm) were deposited using standard plasma enhanced chemical vapour deposition (PECVD), then patterned using deep UV photo-lithography and etched via reactive ion etching (RIE) to form waveguides with exceptionally low surface roughness. After that, silica glass (n = ~1.44 at 1550 nm) was deposited via PECVD and the upper cladding of the integrated waveguides was removed by chemical mechanical polishing (CMP). Finally, the GO film was coated on the top surface of the chip by a solution-based method that yields layer-by-layer deposition of GO films. As compared with graphene that is typically prepared by mechanical exfoliation or chemical vapour deposition, both needing sophisticated transfer processes, GO can be directly coated on dielectric substrates (e.g., silicon and silica wafers) via a solution-based approach. This approach is capable of coating large areas (e.g., a 4-inch wafer) with relatively few defects, which is critical for the fabrication of large-scale integrated devices.

Fig. 1. (a) Schematic illustration of hybrid waveguides integrated with GO. (b) Image of the hybrid integrated waveguide with two layers of GO. (c) Raman spectra of GO on the integrated chip. (d) Measured refractive indices and extinction coefficients of GO and graphene. (e) Measured insertion loss of a 1.5-cm-long integrated waveguide with different numbers of GO layers. (f) Additional propagation loss of the integrated waveguide with different numbers of GO layers.

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An image of the integrated waveguide incorporating two layers of GO is shown in Fig. 1(b), which illustrates that the morphology is good, leading to a high transmittance of the GO film on top of the integrated waveguide. The integration of GO onto the waveguide is confirmed by Raman spectroscopic measurements (Fig. 1(c)) that show the representative D (1345 cm-1) and G (1590 cm-1) peaks of GO. Figure 1(d) shows the in-plane refractive index (n) and extinction coefficient (k) of the GO film including 5 layers of GO measured by spectral ellipsometry. For comparison, the refractive index and extinction coefficient of graphene are also shown in Fig. 1(d). The GO film exhibits a high refractive index of ~ 2 in the telecommunications band. On the other hand, due to the existence of oxygen functional groups (OFGs), the GO films also exhibit an ultra-low extinction coefficient in the telecommunications band, which leads to much lower material absorption as compared with graphene. This property of GO could be of benefit for FWM devices, where low loss is always desired for improved efficiency. It should be noted that, unlike graphene where the bandgap is zero, GO has a distinct bandgap of 2.4 to 3.1 eV, which results in low linear and nonlinear light absorption in spectral regions below the bandgap, in particular featuring greatly reduced TPA in the telecommunications band. This represents a significant advantage over graphene. However, GO still has significantly higher propagation loss than silicon or silica at 1.55 μm due to higher scattering loss and absorption by impurities and defects. To couple light into the hybrid integrated waveguides we employed an 8-channel single-mode fibre (SMF) array for butt coupling. The propagation loss of the uncoated doped silica waveguide was ~24 dB/m, which was negligible for the length (~1.5 cm) used here, and so the total insertion loss for bare (uncoated) waveguides was dominated by the coupling loss, which was ~8 dB/ facet. This can be reduced to ~1.5 dB/facet by using mode convertors between SMF and the waveguides. We chose the transverse electric (TE) field polarization for the experiments because it supports in-plane interaction between the evanescent field and the thin GO film, which is much stronger than out-of-plane interaction due to the large optical anisotropy of 2D materials, as is the case for graphene [41, 42]. Figure 1(e) depicts the total insertion loss of the integrated waveguides with different numbers of GO layers measured with continuous-wave (CW) light at a wavelength of 1550 nm. The measured insertion loss did not show any significant variation with power (up to ~30 dBm) and wavelength (1500~1600 nm) of the CW light. We characterized four duplicate integrated waveguides with the same length of ~1.5 cm. The GO layers only affected the propagation (not coupling) loss, which is shown in Fig. 1(f) as a function of the number of layers. The overall propagation loss of the GO hybrid integrated waveguides was on the order of a few dB/cm, which is much lower than that of graphene hybrid integrated waveguides [43, 44] and confirms the low material absorption of GO in the telecommunications band. Another interesting phenomenon is that the rate of increase in propagation loss with layer number increases (i.e., becomes super-linear) for higher numbers of GO layers. This might be attributed to interactions among the GO layers as well as possible imperfect contact between the multiple GO layers in practical devices.

3. FOUR-WAVE MIXING EXPERIMENT We used the experimental setup shown in Fig. 2 to perform FWM measurements in the GO hybrid integrated waveguides. Two CW tunable lasers were separately amplified by erbium-doped ﬁber ampliﬁers (EDFAs) and used as the pump and signal sources, respectively. In each path, there was a polarization controller (PC) to ensure that the input light was TE- polarized. The pump and signals were combined by a 50:50 fiber coupler before being injected into the GO hybrid integrated waveguide. The signal output from the waveguide was sent to an optical spectrum analyser (OSA) with a variable optical attenuator (VOA) inserted before the OSA to prevent the high-power output from damaging it.

Fig. 2. Experimental setup for testing FWM in the GO hybrid integrated waveguide. EDFA: erbium-doped ﬁber ampliﬁer. PC: polarization controller. DUT: device under test. VOA: variable optical attenuator. OSA: optical spectrum analyser. Figure 3 shows the FWM experimental results. The FWM spectra of a 1.5-cm-long integrated waveguide without GO and with 2 layers of GO are shown in Fig. 3(a). For comparison, we kept the same pump power of ~30 dBm before the input of the waveguide, which corresponded to ~22 dBm pump power coupled into the waveguide. It can be seen that although the hybrid integrated waveguide had additional propagation loss (~2.6 dB), it clearly shows enhanced idler output powers as compared with the same waveguide without GO. The CE (defined as the ratio of the output power of the idler to the output power of the signal, i.e., Pout, idler/Pout, signal) of the integrated waveguide with and without GO were ~-47.1 dB and ~-56.6 dB, respectively, corresponding to a CE enhancement of ~9.5 dB for the hybrid integrated waveguide. After

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excluding the addtional propagation loss, the net CE enhancement (defined as the improvement of the output power of the idler for the same pump power coupled to the waveguide) is 6.9 dB. For the integrated waveguide coated with 1 to 5 layers of GO, zoom-in spectra of the generated idlers for the same pump power coupled to the waveguide (~22 dBm) are shown in Fig. 3(b). For the integrated waveguide coated with 1 and 2 layers of GO, there were positive net CE enhancements. When the number of GO layers was over 2, the net change in CE was negative. This is mainly due to the super-linear increase in propagation loss for increased numbers of GO layers as noted above. The output powers of the idler for various pump powers coupled to the waveguide without GO and with 2 layers of GO are shown in Fig. 3(c). The pump power was varied while the signal power remained constant. One can see that as the pump power increased, the idler power increased with no obvious saturation for both samples, which reflects the low nonlinear absorption of both the high-index doped silica glass as well as the GO layers in the telecommunications band. The slope of the curve for the uncoated waveguide is about 2, as expected from classical FWM theory, while the slope of the GO hybrid waveguide is slightly larger than 2. This indicates that the material properties of the GO film (e.g. n2 and absorption) could be a function of pump power, as noted previously.34,48 The physics of this phenomenon is the subject of on-going research. The net CE enhancement for various pump powers coupled to the waveguide with 1 to 5 layers of GO is shown in Fig. 3(d). There were positive net CE enhancements for all pump powers when the waveguide was coated with either 1 or 2 layers of GO. There was a slight increase in CE enhancement for high pump powers. This may also be caused by the slightly different material properties of the GO films at high light powers. The variation in idler power when the pump wavelength was fixed at 1550 nm and the signal wavelength detuned around 1550 nm is shown in Fig. 3(e). There is obvious power degradation of the output idler when the wavelength detuning is beyond ±10 nm. When the wavelength detuning was beyond ±15 nm, we could not observe any idler above the noise floor. The output powers of idler light for a 1.5-cm-long integrated waveguide coated with different lengths of GO are depicted in Fig. 3(f). We used three GO coating lengths of ~0.5 cm, ~1.0 cm, and ~1.5 cm. For the waveguide coated with 2 layers of GO, the output idler power increased with the GO length, whereas for the waveguide coated with 5 layers of GO, there was an opposite trend. This phenomenon further confirms the trade-off between FWM enhancement and propagation loss in these hybrid integrated waveguides.

Fig. 3. FWM experimental results. (a) FWM spectra of the integrated waveguide without GO and with 2 layers of GO. (b) Zoom in spectra of the generated idlers after FWM in the waveguide with 0 to 5 layers of GO. (c) Output powers of idler for various pump powers coupled to the waveguide without GO and with 2 layers of GO. (d) Net CE enhancements for various pump powers coupled to the waveguide with 1 to 5 layers of GO. (e) Power variations of the output idler when the pump wavelength was fixed at 1550 nm and the signal wavelength was detuned around 1550 nm. (f) Output powers of idler for the waveguide with different coating lengths of GO. The GO coating length in (a), (b), (c), (d), (e) is ~1.5 cm. The pump power coupled to the waveguide in (a), (b), (e), (f) is ~22 dBm. WG: waveguide.

4. CONCLUSION In summary, we experimentally demonstrate enhanced FWM in integrated waveguides coated with GO films. Owing to strong mode overlap between the integrated waveguides and GO films that have a high Kerr nonlinearity and low loss, FWM efficiency in the hybrid integrated waveguides can be significantly improved. Experimental results show that up to ~9.5-dB enhancement of conversion efficiency can be achieved for a 1.5-cm-long hybrid integrated waveguide with 2

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layers of GO. Detailed FWM measurements for different pump powers, wavelength detuning, GO coating lengths, and GO layer numbers are also performed.

ACKNOWLEDGEMENTS This work was supported by the Australian Research Council Discovery Projects Program (No. DP150102972 and DP150104327). RM acknowledges support by Natural Sciences and Engineering Research Council of Canada (NSERC) through the Strategic, Discovery and Acceleration Grants Schemes, by the MESI PSR-SIIRI Initiative in Quebec, and by the Canada Research Chair Program. He also acknowledges additional support by the Government of the Russian Federation through the ITMO Fellowship and Professorship Program (grant 074-U 01) and by the 1000 Talents Sichuan Program in China.

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