2.1 Device Fabrication
Figure 1(a) shows a schematic of a uniformly GO-coated waveguide polarizer. The waveguides were fabricated from high-index doped silica glass core surrounded by silica via CMOS compatible processes [26, 42] with chemical mechanical polishing (CMP) used as the last step to remove the upper cladding, so as to enable GO film coating on the top surface of the waveguide. GO coating was achieved with a solution-based method that yielded layer-by-layer GO film deposition, as reported previously [40, 41]. Four steps for in-situ assembly of monolayer GO films were repeated to construct multilayer GO films on the desired substrate, with the process being highly scalable.
We uniformly coated waveguides with 1 to 10 layers of GO. Figure 1(b) shows the measured Raman spectra of the waveguides without GO and with 2 layers of GO, confirming the integration of GO onto the top surface by the presence of the D (1345 cm-1) and G (1590 cm-1) peaks of GO. The microscope images of the integrated waveguide with zero and 2 layers of GO are shown in the insets, which illustrate the good morphology of the GO films. Figure 1(c) shows the thickness of GO films versus the layer number characterized by atomic force microscopy. The insets show images of 1 and 10 layers of GO coated on a 2.2 cm × 2.2 cm silica substrate with high uniformity. The dependence of GO film thickness versus layer number shows a nearly linear relationship, with a thickness of ~ 2.18 nm on average for each layer.
In addition to the uniformly coated devices, we selectively patterned areas of GO films using lithography and lift-off processes. Apart from allowing precise control of the size and placement of the GO films, this enabled us to test the performance of the GO-coated waveguide polarizers with shorter GO coating lengths but higher film thicknesses (up to 100 layers). The chip was first spin-coated with photoresist and then patterned using photolithography to open a window on the waveguide. Next, GO films were coated on the chip using the method mentioned above and patterned via a lift-off process.
As compared with the drop-casting method that has a GO film thickness of ~ 0.5 µm and a minimum size of about 1.3 mm for each step [39], the combination of our GO coating method with photolithography and lift-off allows precise control of the film placement, size, and thickness (with an ultrahigh resolution of ~ 2.18 nm), all of which are critical for optimizing the device performance including the polarization figure of merit (FOM) and four-wave mixing conversion efficiency. Further, our solution based GO coating approach, unlike for example, the sophisticated transfer processes employed for coating 2D materials such as graphene [29, 43], is capable of covering large areas (e.g., a 4 inch wafer) on dielectric substrates (e.g., silicon and silica wafers) with relatively few defects. The combination of patterning and deposition control of GO films along with large area coating capability is critical for large-scale integrated devices incorporated with GO.
2.2 Polarization Loss Measurements
We used an 8-channel single-mode fiber (SMF) array to butt couple both TE and TM polarized continuous-wave (CW) light from a tunable laser near 1550 nm into the waveguides. The mode coupling loss between the SMF array and the waveguides was ~ 8 dB/facet, which can readily be reduced to ~ 1.0 dB/facet with mode convertors [26]. The propagation loss of the uncoated 1.5-cm-long waveguides was very low (< 0.25 dB/cm) and so the total insertion loss (TE = − 16.2 dB; TM = − 16.5 dB) of the uncoated devices was dominated by mode coupling loss. The slight PDL of the uncoated waveguides resulted mainly from a slightly different mode-coupling mismatch but possibly also polarization dependent scattering loss from the roughness of the polished top surface.
To characterize the performance of the devices, we introduce two figures of merit (FOMs) – one for the excess propagation loss (FOMEPL) and one for the overall polarization dependent loss (FOMPDL):
FOMEPL = (EPLTE − EPLTM) / EPLTM, (1)
FOMPDL = PDL / EIL, (2)
where the excess propagation losses, EPLTE (dB/cm) and EPLTM (dB/cm), are GO-induced excess propagation losses for the TE and TM polarizations, respectively. PDL (dB) is the polarization dependent loss defined as the ratio of the maximum to minimum insertion losses. The excess insertion loss, EIL (dB), is the insertion loss induced by the GO film over the uncoated waveguide. The EIL only considers the insertion loss induced by GO, while excluding from the overall insertion loss both the mode coupling loss between the SMF array and the waveguide as well as the propagation loss of the uncoated waveguide. In our case, since the TM polarization had the lowest insertion loss, EIL is the excess GO-induced insertion loss for the TM polarization and is given by EIL = EPLTM ∙ L, where L is the GO coating length. Note that FOMEPL only considers the propagation loss difference induced by the GO films, and so is more accurate for the characterization of their material anisotropy, whereas FOMPDL is more widely used for evaluating the device performance [33] since it also includes the background (uncoated) PDL. FOMEPL equals FOMPDL only when the TE and TM polarized insertion losses of the uncoated waveguide are the same.
Figure 2 shows the polarization dependent (TE (in-plane) and TM (out-of-plane)) performance for both the 1.5-cm-long uniformly coated waveguides (0–10 layers), left side (i), and the patterned 2-mm-long devices (10–100 layers), right side (ii). Figure 2(a) shows the polarization dependent insertion loss. The data points depict the average values obtained from the experimental results of 3 duplicate devices and the error bars illustrate the variations for different samples. Figure 2(b) presents the PDL and EIL calculated based on the average insertion loss in Fig. 2(a). Figures 2(c) and (d) show the FOMs calculated based on Fig. 2(b) and the polar diagrams for the insertion loss, respectively.
The TE insertion loss increases much more strongly than TM with layer number, thus yielding a large PDL with low EIL and forming the basis for our high-performance polarization dependent devices. Since GO is a dielectric film, our TM-pass GO-coated waveguide polarizer is quite different from TE-pass metal-clad waveguide polarizers based on a deeper power penetration of the evanescent TM field into a lossy metal cladding [27, 28]. The PDL reached a maximum of ~ 37.4 dB for a 10 layer uniformly coated device and ~ 53.8 dB for a 100 layer patterned device, with a modest maximum EIL of ~ 5.0 dB and ~ 7.5 dB for the two devices, respectively. By optimizing the waveguide geometry to achieve a better mode overlap with the GO films [33], the EIL can be further reduced. Moreover, the PDL was still increasing at a rate of 2–3 dB/cm/layer at 100 layers, and so substantially higher PDL can be obtained using layers thicker than 200 nm. Both FOMEPL and FOMPDL increase with a maximum of FOMEPL and FOMPDL of ~ 8.2 and ~ 8.1, respectively, at about 50 layers with the difference between them subsequently decreasing. This is because the impact of the background PDL (~ 0.3 dB) becomes smaller as the EIL increases for increased GO layer numbers.
2.3 GO Film Properties
Figure 3(a) shows the experimental propagation loss (dB/cm) extracted from Fig. 2(a) for both polarizations of the two devices, along with the TE propagation loss calculated (by means of the Lumerical FDTD commercial mode solving software) using ellipsometry measurements (at 1550 nm) for the refractive index n and extinction coefficient k of two samples having 2 layers (Fig. 3(a-i)) and 20 layers (Fig. 3(a-ii)) of GO. Since the out-of-plane (TM polarized) response of the layered GO films is much weaker [44, 45], we could only measure, via ellipsometry, the in-plane (TE polarized) n and k of the GO films (uncertainty < 3%), which were used (in conjunction with the mode solving software) to calculate the waveguide loss for the TE polarization. The simulations assume constant n, k for different GO layer numbers in each plot. In Fig. 3(a), the experimental TE propagation loss agrees extremely well with simulations for 2 and 20 layers of GO. For other thicknesses the experimental TE loss increased more rapidly with GO layer number, indicating that the intrinsic GO film loss increases with thickness. This is not surprising, and could be due to any number of effects such as increased scattering loss and absorption induced by imperfect contact between the multiple GO layers as well as interactions between the GO layers.
Figure 3. (a) Experimental waveguide propagation loss (extracted from Fig. 2(a)) for (i) a uniformly coated device with 0 − 10 layers of GO and (ii) a patterned device with 10 − 100 layers of GO. Simulated TE loss estimated by using in-plane (TE polarized) n and k for (i) 2 and (ii) 20 layers of GO obtained from ellipsometry measurements in conjunction with mode solving software (assuming constant n, k for different layer numbers in each plot). (b) Experimental loss per layer given by the experimental loss in (a) divided by the number of layers for both polarizations. (c) Material loss of the GO films for TE (kTE) and TM (kTM) polarizations as well as their ratio (kTE / kTM). The two yellow data points (labelled as “SE”) show the in-plane k measured by spectral ellipsometry (SE) for 2 and 20 layers of GO. (d) Fractional contribution to PDL from mode overlap (ηMO) and material loss anisotropy (ηMA). In (b) − (d), the solid data points refer to the results for a uniformly coated device, whereas the hollow points are for a patterned device.
Figure 3(c) shows the GO film loss (k) extracted from the waveguide loss measurements. We neglect the effect of any variations in GO film refractive index (n) on the mode overlap. The much larger film loss k for TE polarization (kTE) is a reflection of the high intrinsic loss anisotropy of the GO films, which has been previously noted in a qualitative manner [46]. The results presented here are the first quantitative measurement of intrinsic film loss anisotropy in GO films, showing that the in-plane (TE) electric field loss is significantly higher than the out-of-plane (TM) loss. This material anisotropy significantly improves the polarization dependent loss of our devices, which otherwise would derive solely from the polarization dependent mode overlap with the GO films. Both kTE and kTM increase with layer number, with the ratio kTE / kTM reaching a maximum of about 5 at 4 − 10 layers after which it decreases to ~ 3 for very thick films (100 layers). This could arise from a reduction in, for example, anisotropic film stress related effects for very thick films (> 200 nm). The two k data points (marked in yellow) in Fig. 3(c) are obtained from ellipsometry measurements, with kellips2-layers = 0.0050 and kellips20-layers = 0.01165, thus agreeing extremely well with the waveguide propagation loss results − kwg2-layers = 0.0054 and kwg20-layers = 0.01212. Note also that the k’s for the patterned and uniformly coated waveguides agree well for the same number (10) of GO layers.
Figure 3(d) shows the fractional contribution to the PDL from the polarization dependent mode overlap (ηMO) and the material film loss anisotropy (ηMA), where ηMO + ηMA = 1. We calculated ηMO and ηMA by comparing the measured propagation loss in Fig. 3(a) for both polarizations, to the propagation loss calculated assuming isotropic film properties (using the extracted kTE in Fig. 3(c) for both TE and TM polarizations). For low GO layer numbers (< 10) the PDL is dominated by the material anisotropy ηMA at 75%, despite the overall TE loss only being 1 dB/cm/layer (at 1 − 2 layers). The contribution of the GO material anisotropy steadily decreases, becoming comparable to the mode overlap contribution, ηMO, at ~ 20 layers, beyond which, for very thick films (100 layers), is smaller than ηMO, which is about 65%. This could reflect the transition of the film properties slightly towards a bulk (isotropic) material for very thick films. However, it is also interesting to note that even for very thick films the intrinsic film loss anisotropy is still large enough to form the basis for polarization dependent devices. To reconfigure the polarization selection, it would be relatively easy to change the mode overlap, while changing the material loss anisotropy is more challenging. A possible method to implement a TE-pass GO waveguide polarizer would be to conformally coat a high-aspect-ratio waveguide with GO films on the sidewalls. Finally, we note that although our GO-coated waveguide polarizers were based on a CMOS compatible doped silica platform, these GO films can readily be introduced into other integrated platforms (e.g., silicon and silicon nitride) [47–49], offering polarization selective devices with reduced footprint.
2.4 Optical Bandwidth and Power Dependence
Figures 4(a-i) and (a-ii) illustrate the PDL of both uniformly coated and patterned waveguides versus wavelength from 1500 to 1600 nm, showing a variation less than 2 dB and confirming the broadband operation of the polarizers. Table I compares the performance of a range of silicon photonic polarizers and 2D material based optical polarizers. The bandwidth of the material anisotropy of GO thin films is very broad [37] − several hundred nanometers, even extending to visible wavelengths (Fig. 4(a-i) insets). This is a distinct advantage of GO-coated waveguide polarizers that is extremely challenging to achieve with silicon photonic polarizers [3, 21, 24]. Moreover, GO based polarizers have simpler designs with higher fabrication tolerance as compared with silicon photonic polarizers. Indeed, the latter require precise design and control of the dimensions [2, 3]. It is also interesting to note that the PDL slightly increases at longer wavelengths for both the uniformly coated and patterned devices. This is probably a result of the excitation of high-order modes in the doped silica waveguides at shorter wavelengths, which reduces the strength of the interaction between the GO films and the evanescent field leaking from the waveguides, thus leading to a degradation in PDL.
The insertion loss versus CW power is shown in Figs. 4(b-i) and (b-ii), for both samples and polarizations, indicating only a slight increase in loss for the thicker layers and only for the TE polarization, possibly due to photo-thermal reduction of the GO film at higher powers [50]. This might also result from self-heating and thermal dissipation in the multilayer GO film, being the subject of on-going research. In contrast, since the TM polarized absorption was very low, it did not show any significant variation with power. The increase in loss for TE polarized light was reversible − indicating that the optically induced changes were reversible. Note that this slight reversible increase in loss for the TE polarization with power actually enhances the device performance. Finally, we have shown previously [38, 51] that the material properties of GO can also be permanently changed by laser-induced photo-reduction but at significantly higher power levels than those used here, with femtosecond laser pulses. This is different from the reversible photo-thermal reduction observed here.
Table I.
Performance comparison of silicon photonic polarizers and 2D material based polarizers. PDL: polarization dependent loss. IL: insertion loss. FOM: figure of merit. ER: extinction ratio. N/A: not applicable.
Device | Length [mm] | PDL [dB] | ILa) [dB] | FOM =PDL/IL | Bandwidth [nm] (≥ 20 dB ER) |
Shallowly etched silicon waveguide [21] | 1 | 25 | 3 | 8.3 | > 100 |
Silicon subwavelength grating waveguide [24] | 0.009 | 27 | 0.5 | 54 | ~ 60 |
Silicon nanoplasmonic slot waveguide [22] | 0.001 | 16 | 2.2 | 7.3 | ER < 20 |
Graphene-coated side polished fiber [29] | 2.1 | 27 | 5 | 5.4 | ~ 1000 |
Graphene-coated polymer waveguide [30] | 7 | 19 | 26 | 0.73 | N/Ab) |
Chalcogenide glass-on-graphene waveguide [33] | 0.4 | 23 | 0.8 | 29 | ~ 450 |
MoS2-coated Nd:YAG waveguide [52] | 10 | 3 | 0.4 | 7.5 | ER < 20 |
GO-coated polymer waveguide [39] | 1.3 | 40 | 6.5 | 6.1 | > 490 |
GO-coated doped silica waveguide (this work) | 2 | 54 | 7.5 | 7.2 | > 540c) |
a) The ILs exclude the fiber-to-chip coupling losses. |
b) The device was only characterized at a single wavelength. |
c) We cannot precisely characterize the bandwidth due to the lack of suitable lasers. In our measurements, we achieve high PDLs of over 52.4 dB from 1500 to 1600 nm (with a variation less than 2 dB) and also a PDL of ~ 25.2 dB at 1064 nm. |