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Abstract
Applications of subwavelength grating based-polarizers for polarimetric detections are being hindered due to the limited extinction ratio. In this work, the structural effect, including the line edge roughness (LER), of the gratings on the polarizing characteristics was studied by both numerical simulations using finite difference and time domain (FDTD) method and experiments, aiming to figure out the optimal grating profile for achieving high transmittance as well as high extinction ratio. Two different configurations of the gratings, one is dual layer Au lines and the other is parabolic shaped Al lines on structured spin-on-carbon (SOC) films were systematically studied and compared. Nanofabrication of the gratings by electron beam lithography without lift-off process were conducted and optical measurements of their polarization properties demonstrate superior performance of the developed polarizers. The origin of the structural effect was explained by the local surface plasmonic modes, existing in the nano-slits in metallic gratings, which is instructive for further enhancement of the polarization performance.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Polarization imaging is one of the most important detection techniques owing to its unique functionality in enhancing image contrast and signal/noise ratio, allowing broad applications in medicine [1], ocean observation [2] and weak signal recognition [3], etc. To enable sensitive detection, high transmission along with high extinction ratio of incident light is essential for polarizers. Among the vast amounts of polarizers such as calcite prisms, pile of plates, liquid crystal micropolarizing elements [4], nanoparticle polarizers [5], polymer micropolarizers [6], metal nanowire [7–10], plasmonic lenses [11] and photonic crystal polarization splitter [12], etc., subwavelength gratings have demonstrated their attractive advantages in miniaturized dimensions, compatibility to planar process of CMOS circuits, prospects of monolithic integration with semiconductor detectors and broadband applicability [13–15]. However, the advance of this technology is still hindered by limited light transmittance and low extinction ratio such that applications of grating-based polarizers in photoelectronic detectors are rarely reported. Since the polarization performance is directly related to the grating structure (pitch, height and grating shape, etc.) [16], the material and even the process [17], substantial efforts have been made but the progress is still not satisfactory. Broadband nanowire-grid polarizers were designed in both single and dual layer buried in a dielectric material, only numerical simulation results were given [18]. Bilayer Al grating was fabricated and as high as 104 extinction ratio was measured at the cost of the transmission below 50% [19]. Double-layer metallic subwavelength slit arrays were reported with the maximum transmission below 70% in the mid of infrared wavelength [20]. In recent years, encouraging progress has been made by focusing on the structural [16] and process optimization effect [17,21,22] on the polarization property, both light transmission and extinction ratio was enhanced up to over 70% and 100:1, respectively. Unfortunately, when applied onto actual detectors [23] as well as focal plane detector arrays [13–15], the extinction ratio drops to low level for unknown reasons.
Based on the progress as well as the existing problems as mentioned above, this paper reports our further study of structural effects on the polarization performance of sub-wavelength grating based polarizers. Two different grating structures, one is a sub-wavelength dual-layer grating (SDG) and the other is a dual-layer grating with triangular shape, were proposed and systematically studied by using finite difference and time domain (FDTD) method. Nanofabrication by electron beam lithography without lift-off process was developed and optical characterizations of the fabricated polarizers help us to further improve the theoretical model for the depth understanding of the relation between the polarization quality and the grating structure. The knowledge established in this work, is extremely important for us to develop high performance polarizers which will play an important role in many fields such as augmented reality and virtual reality [24], deep space detection [25] and cancer detection [26,27].
2. Theoretical calculations
Sub-wavelength metallic gratings with two different structural configurations on Si substrate, as illustrated in Fig. 1, were theoretically compared by FDTD simulation using Lumerical software. The subwavelength dual-layer grating (SDG), as illustrated in Fig. 1(a), is formed by thermally coating a layer of thin Au film on the top of ZEP520A lines/spaces with the half-pitch of 50 nm on the substrate of Si/220 nm SOC, where SOC stands for spin-on-carbon. The underneath SOC layer was used not only as an anti-reflection layer, but also an interlayer between Si and Au to enhance the transmission [23].
Fig. 1. Schematic diagrams for the proposed subwavelength gratings with specially designed profiles as miniaturized polarizers. (a) A self-aligned dual layer grating in Au formed by deposition of a thin Au film onto pre-patterned gratings in ZEP520A. (b) A sub-wavelength grating in Al with parabolic shape by depositing a thin Al film on the pre-patterned triangular lines of SOC.
For comparison, an alternative configuration of the sub-wavelength grating with parabolic/triangular feature, as illustrated in Fig. 1(b), was also studied. The grating was formed by thermally coating a 50 nm thick Al on the triangularly shaped SOC layer with 110 nm thickness, which was spin-coated on a Si wafer.
In the simulation, the boundary condition along the x- and the y-axis was set to be periodic, and perfectly matched layers were used along the propagation directions. A light source with plane wave was placed above the polarizer, propagating along z-axis. The polarization perpendicular to the gratings was defined as TM mode and that parallel to the grating as TE mode. A transmission monitor was placed underneath the Si substrate. The electromagnetic field and the Poynting vector were monitored from the cross section of the polarizers.
Figure 2 presents the simulation results for the overall polarization characteristics of the designed SDG grating. From Fig. 2(a), the transmissions around 70% in short infrared wavelengths of 1.1-1.6 µm and the extinction ratio from 200 (λ = 1.1 µm) up to 2000 (λ = 1.6 µm) are obtained. The geometry dimensions of the SDG grating in Fig. 2(a) are: the pitch (p) is 100 nm, the spacing (d) between the top grating and the bottom one is 110 nm, the thickness of ZEP520A is 160 nm. A more comprehensive results by simulation are given in Fig. 2(c)-(h) for both the transmissions (TM and TE) and the extinction ratio (TM/TE) in the Spacing-Wavelength plane as well as the Pitch-Wavelength. Clearly, for a fixed wavelength within 1.1-1.6 µm, the shorter pitch gives rise to the higher transmission as well as the higher extinction ratio. Considering the difficulty in nanofabrication, this work fixes the pitch to be 100 nm and the space between the two grating layers to be 100-200 nm.
Fig. 2. Simulation results from three different Au gratings. (a) The numerical simulation results for the transmission and extinction ratio of the three different configurations as illustrated in (b). (c)-(e) The systematic simulations of polarization properties as the function of wavelength and spacing. In the simulations, the thickness of Au, the pitch (p) and the duty cycle are fixed to be 50 nm, 100 nm and 0.5, respectively. (f)-(h) The systematic simulations of polarization properties as the function of wavelength and pitch. In the simulations, the thickness of Au, the spacing (d) and the duty cycle are fixed to be 50 nm, 110 nm and 0.5, respectively.
To find out the role of each Au wire in the grating, the whole SDG polarizer is broken down into two different structures, one with the golden wire on the top and the other with two wires on the bottom beside the ZEP520 ridge, as schematically illustrated in Fig. 2(b). As shown in Fig. 2(a), the grating on the top of the ZEP520A ridge exhibits the highest transmission among all the configurations but low extinction ratio, meaning that such a single layer grating fails to eliminate the TE transmission. Such a low extinction ratio also occurs to the other grating with the wires on the trench bottom by the ZEP520A ridge, which possesses relatively low transmission as well. By thus far, it can be concluded that the grating on the top of the ZEP520A ridge helps to enhance the transmission of TM wave and the cavity formed by the dual layer Au grating helps to eliminate the TE transmission, giving rise to both high transmission and high extinction ratio.
The same simulation procedure was repeated on the parabolic/triangular SDG grating of the Al layer, which should be very close to the real shape, as will be discussed later. Figure 3(a) presents the simulation results. The extinction ratio still remains at high levels of 104∼105 for the wavelength from 1.1 µm to 1.6 µm, and the transmittance undergoes a dip (30%) at the wavelength of 1.35 µm, and then goes up. Especially at 1.6 µm, both the transmission and the extinction ratio reach the peak of 70% and 2 × 104, respectively.
Fig. 3. Simulations and analysis of polarization properties for the parabolic/triangular SDG grating. (a) The simulated transmittance and the extinction ratio for the grating with rectangular and parabolic profile of the lines, respectively. (b)-(c) Spatial distributions of the electric field and the Poynting vector of the triangular SDG with parabolic profile at 1.35 and 1.6 µm, respectively. In the simulations, the height (h) of the SDG grating, the pitch (p), the thickness of SOC layer and the duty cycle are fixed to be 110 nm, 100 nm, 110 nm and 0.36, respectively.
To gain the insight of how the grating structure governs the polarization status, numerical simulations of spatial distributions of both the electric field and the Poynting vector were carried out for the wavelengths at 1.35 µm and 1.6 µm, respectively, as presented in Fig. 3(b)-(c). At 1.35 µm, the electric field (Fig. 3(b)(i)) is concentrated in the narrow gaps between the top and bottom Al layer. But the simulated Poynting vector (Fig. 3(b)(ii)) shows a lot of energy actually do not flow into the substrate so that the transmission give rise to the minima at 30%. On the other hand, at 1.6 µm, relatively strong electric field localized in the gap between the top and bottom Al line is observed, as presented in Fig. 3(c)(i), leading to highly dense energy flow through the gaps (Fig. 3(c)(ii)) such that both the transmission and the extinction ratio reaches to the maximum of 70% and 2 × 104, respectively, as presented in Fig. 3(a), which is exactly what this work is pursuing for. Using the simulation results as guide, experiments were carried out to testify the simulated performance of the proposed polarizers.
3. Methods
3.1 Fabrication of subwavelength dual-layer grating (SDG) based polarizers
The two designed SDG polarizers were fabricated through lift-off free EBL process. A 220-nm thick spin-on-carbon (SOC) was first spin coated on a Si substrate. Then, a 160 nm thick positive resist ZEP520A was coated, followed by a soft-bake on a hot plate at 180 °C for 10 min. After the e-beam exposure by a state-of-the-art beam-writer, JBX 6300FS with the beam spot size of 7-10 nm and the beam current of 500 pA, the exposed resist was developed in O-xylene at 23 °C for 60 s, finished by a 30 s rinse in IPA solution. Gratings with 50 nm lines/spaces in ZEP520A were replicated, as presented in Fig. 4(a).
Fig. 4. SEM micrographs of fabricated SDG gratings. (a) The replicated 50 nm lines/spaces in ZEP520A; (b) The fabricated SDG grating in Au and (c) the fabricated parabolic/triangular SDG grating of Al standing on the spin-on-carbon grating with triangle profile.
As for the dual layer gratings, the patterned samples were subsequently subject to the deposition of a 50 nm thick Au at the rate of 0.2A/s such that the rectangular SDG polarizers were fabricated without the lift-off process. Figure 4(b) shows the micrograph by scanning electron microscope (SEM) of the fabricated polarizer. For the parabolic/triangular SDG polarizer, in order to transfer the grating pattern on to the SOC layer, reactive ion etch (RIE) was carried out in oxygen-based plasma using the 50 nm lines/spaces of ZEP520A as the etching mask, followed by the removal of the ZEP520A in N-methylpyrrolidone (NMP) solution at 40 °C for 5 min. Triangular profile in SOC was formed after the dry etch process. In the end, a 50 nm Al was deposited on the triangular SOC gratings at a rate of 0.1A/s to create another subwavelength grating with parabolic cross-sectional shape in Al, as shown by the SEM micrograph in Fig. 4(c).
3.2. Optical measurement techniques
To measure the transmittance and the extinction ratio of the fabricated SDG polarizers in the wavelength range of 1.1-1.6 µm, a microscopical optical measurement system, as illustrated in Fig. 5, was used with a HL2000 tungsten-halogen lamp (Ideaoptics) as the light source. Using a commercial polaroid and reflector, the polarized light was focused on the sample. After passing through an optical fiber, the emergent light was received by an NIR17s near-infrared spectrometer (Ideaoptics) equipped with a BX53 microscope (Olympus).
4. Results and discussion
Figure 6(a) is the close view of the fabricated dual layer grating standing on the Si/220 nm SOC substrate by the SEM microscope under high magnifications. The transmittances of both TM and TE wave through such a grating were measured, as presented in Fig. 6(b). Transmittance of the TM wave as high as 80% in the wavelength range of 1.1-1.6 µm is observed, and that of the TE wave is below 1%, giving rise to the extinction ratio over 200 in the measurement wavelength range and up to 500 at the wavelength of 1.4 µm. However, comparing with that by simulation as shown in Fig. 2(a) (the blue dot-line), it is found that the measured transmittances are higher than the simulated ones. For the ease of the comparison, the calculated transmittance spectrum in Fig. 2(a) is replotted in Fig. 6(b) (the blue dot-line). Obviously, such a difference should be caused by the structural deviation between the fabricated gratings and the modeled ones (Fig. 1(a)). Careful inspection of the fabricated grating structure, as shown in Fig. 4(b) as well as that in Fig. 6(a), indicates that the profile of the bottom metal grating is actually parabolic instead of rectangular. Taking the parabolic profile into account, the new simulated transmittance plotted in Fig. 6(b) (the red dot line) agrees well with measured ones for the TM polarization, which is about 10% higher than the measured one. The simulated spatial distributions of the electric field strength and Poynting vector (Fig. 6(d)-(e)) help us to understand that the square shaped grating populate plasmonic modes between the rectangular Au lines on the bottom (Fig. 6(d)), converting partial energy into Joule heat. While in parabolic gratings (Fig. 6(e)), which is the real case, the energy is concentrated on the edge of the parabolic line, allowing energy flowing into the SOC layer, as shown by the simulated Poynting vectors. Figure 6(f) presents the transmittance changing as the polarization angle of the TM wave, obtained by both simulation and experiment at the wavelength of 1.4 µm, showing perfectly sinusoidal shape as a high-quality polarizer.
Fig. 6. Optical measurement results and analysis. (a) The close-up view of the fabricated dual layer grating in Au. Parabolic profile rather than square one can be clearly observed on the lower layer. (b-c) Measured and simulated transmittance and the extinction ratio, respectively. All the parameters used in the simulations are 100 nm for the pitch (p), 110 nm for the spacing (d) and 50 nm for the Au thickness. (d)-(e) The simulations of the spatial distributions of the electric field and Poynting vector in the Au/ZEP520A interface with rectangle and parabola profile, respectively, at 1.4 µm for TM mode. (f) Measured and simulated transmittance of fabricated SDG polarizer as a function of polarization angle at 1400 nm. During the simulation and measurement, the influence of the silicon substrate is eliminated.
Optical measurement was also carried out for the triangular SDG grating, as presented in Fig. 7(a). Once again, the measured transmittance, which is beyond 60% in the major range of 1.1 µm – 1.6 µm, is higher than that by simulation (the blue dot-line from Fig. 3(a)). But the extinction ratio is other way round, indicating that there is still some hidden difference between the real grating structure and the modeled one apart from the parabolic shape in the real Al grating.
Fig. 7. Optical measurement and systematic simulations of both the transmittance (a) and the extinction ratio (b) for the Al grating with parabolic shape on the SOC substrate. The influences by the line edge roughness (LER) on polarization properties are also calculated and included in the figures for comparison.
Careful inspection of the fabricated parabolic/triangular Al grating presented in Fig. 4(c), one can see that the line edge roughness (LER) of the Al grating line is significant and should not be ignored. Here, the LER is defined as the root mean square (RMS) of the difference between the actual boundary and the mean boundary along y-axes. Through a random profile generating script as shown in Code 1 [28], three 3D-models with different LERs were generated for comparison. Investigations of the LER effect on the polarization performance were then carried out by FDTD simulations and the results are presented in Fig. 7. It is rather interesting to see that the transmittance of TM wave increases with the line edge roughness of the Al grating but the extinction ratio goes down at the same time due to the increase of the TE wave. When the LER is around 3 nm, which is in the same order of magnitude as that in actual gratings, the simulated transmittances are in good agreement with the measured ones. It is commonly understood that the LER in Al film is mainly due to the granular feature as grains. These nanoscale metallic grains accommodate various local surface plasmonic resonant modes, leading to the increase of both TM/TE wave and the reduction of the extinction ratio, as shown by the simulated results in Fig. 7(b).
5. Conclusions
In this work, we have investigated how nanostructures in subwavelength metallic gratings determined the polarization performance (the transmission and the extinction ratio), when they were applied as polarizers in the wavelength range of 1.1-1.6 µm. First, self-aligned dual layer grating of Au in structured ZEP520A exhibits TM transmittance close to 80% with the extinction ratio beyond 200. Second, the subwavelength grating with parabolic shape of Al lines, grown on the sidewalls of pre-patterned SOC ridges, demonstrates the TM transmission over 60% with the extinction ratio of 200. Third, in both of the cases as mentioned above, nano-slits existing between the grating lines are essential for obtaining highly polarized transmission. Finally, the lift-off free EBL process applied in fabrications of the proposed polarizers ensures good stability and reliability in the fabrication of dense metallic gratings. Since both ZEP520A and spin-on-carbon were used as the dielectric materials in this work, further work to apply high refractive index materials such as SiNx should enable us to achieve higher and more reliable performance of the polarizers than ever, which is still under investigations in our lab. Nevertheless, the progress made in this work is absolutely encouraging for breaking through the bottleneck to the limited extinction ratio of subwavelength grating based-polarizers.
Funding
Science and Technology Commission of Shanghai Municipality (19142202700); National Natural Science Foundation of China (61927820).
Acknowledgments
The work is partially supported by Shanghai STCSM project, National Natural Science Foundation of China and Zhangjiang Laboratory.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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