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Abstract
The design and fabrication of a ZnSe-based linear polarizer operating in the 8–12 μm infrared region with multilayer nanogratings is demonstrated. The multilayer structure is formed by a low- and a high-refractive index thin layer that are evaporated successively on a ZnSe substrate, followed by a dielectric nanograting that is etched into the high-refractive index thin layer, and then a double-layer metallic nanograting that sits on the dielectric nanograting. Polarization characteristics of the proposed polarizer on structural parameters are investigated and an optimized multilayer structure is obtained. Experimental fabrication of the multilayer nanograting structure using UV lithography and thin-film deposition is conducted in a way in which no ion etching process is needed for the formation of metallic nanogratings. An extinction ratio (ER) of 35 dB and TM-transmission (TMT) of averagely higher than 80% are obtained experimentally in the whole 8–12 μm waveband with 250-nm-period multilayer nanogratings. The ZnSe-based multilayer structure shows the possibility of achieving large-area and high-performance polarization in the 8–12 μm infrared region with relatively easy fabrication.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Polarizer is an important device in optical system and optical measurement [1–3]. With continuous development of nanophotonics and nanotechnology, great progress has been made in high quality and high performance polarizers. Nanowire-grid (period of the grid is smaller than the working wavelength) polarizers that function as a linear polarizer in a wide wavelength range have shown great applications in miniaturization and integration of devices [4–7], polarization imaging [8–10], medical applications [11], and displays [12,13].
The performance of a linear polarizer is represented by the extinction ratio, which is a ratio of the transmitted transverse magnetic (TM) and the transverse electric (TE) polarized light, and the transmission of the TM-polarized light. TM-polarized light transmits the polarizer efficiently while TE-polarized light is highly reflected. A number of studies have been conducted on the design, fabrication and applications of the nanowire grid sub-wavelength metallic gratings. Zhang et al. demonstrated a linearly polarized InGaN/GaN diode emission using an aluminum (Al) nanograting directly on a substrate with a period of 150 nm. An equivalent ER of 8.5 dB was obtained [14]. Ma et al. also demonstrated a similar linear polarizer integrated on emitting surface of a GaInN diode with an Al wire-grid on sapphire substrate, and an ER of 17 dB was obtained [15]. This type of single-layered nano-wire grid structure usually suffer from low TM transmission due to the refractive index mismatch between the substrate and the nanowire grid and also stringent requirement on structural dimensions and complex fabrication techniques and processes, such as E-beam lithography and inductively coupled plasma (ICP) reactive-ion etching (RIE) of metals for high aspect ratio (depth/width) of the metallic lines. A silicon wire grid polarizer operating in ultraviolet (UV) spectral range was also proposed in which a grating period of 140 nm was realized by means of E-beam lithography and dry etching in amorphous silicon film on fused silica. A TM-transmission of 42% and an average extinction ratio of 19.5 dB were measured over a bandwidth of 100 nm [16]. Yamada et al. demonstrated a mid-IR wire-grid polarizer consisting of a 350 nm pitch WSi grating on an Y2O3 ceramic substrate using two-beam interference lithography and dry etching, from which a TM-transmission of ∼70% and ER of ∼20 dB in the 3–7 μm wavelength range was obtained [17]. In 2015, Wang et al. proposed a fabrication method for wire-grid polarizers in the visible and near-IR based on deep-UV interference lithography, nanoimprint, and glancing angle deposition. The performance of the fabricated Al wire grids is, however, limited by the roughness and porosity of the Al film and the underlying SU-8 structure [18]. Very recently, Kang et al. fabricated a 4-in large-area flexible infrared nanowire grid polarizer using a nanoimprint and metal thermal evaporation process. The fabricated nanowire grid structure on IPS substrate generated a TM transmission of ∼70% and ER of ∼20 dB in the 4.5–6.5 μm wavelength range [19].
In this work, we propose and demonstrate a high performance multilayer nanogratings structure working in the 8–12 μm wavelength range, one of the most important atmospheric windows for remote sensing detection, night imaging, and temperature measuring of targets [20,21]. The multilayer nanogratings were fabricated on a ZnSe substrate, on which a low-refractive index thin layer (BaF2) and a high-refractive index thin layer (ZnS) is evaporated successively to form an antireflection surface. A dielectric grating is etched in the ZnS layer and then a layer of metal (Al) is evaporated onto the top to form double layered metallic gratings. An ER of 40 dB and TM transmission of higher than 80% were obtained experimentally from the fabricated multilayer gratings of 250-nm period. The experimental fabrication of the multilayer nanogratings are implemented with UV lithography followed by ICP of ZnS and E-beam evaporation deposition of Al, which is capable of making large-area polarizers and more importantly eliminates the ion etching process for metals which is usually required in conventional nanowire grids. The ZnSe-based multilayer nanogratings show the possibilities of implementing high-performance polarizing devices in a fashion of relatively easy-fabrication with significant relaxation on the tolerance of structural parameters.
2. Design of multilayer nanogratings
A schematic diagram of a ZnSe-based multilayer polarizing nanogratings working in 8–12 μm infrared region is shown in Fig. 1. A plane wave light is incident from the bottom of a ZnSe substrate. A BaF2 thin layer (thickness H1) with low-refractive index and a ZnS thin layer (thickness H2 + H3) with high-refractive index is evaporated successively onto the ZnSe substrate to form an antireflection surface. A nanogratings with a period of P and a grid width of W (duty cycle DC = W/P) was etched into the ZnS layer with grid height of H3. A thin layer of metal (Al) with a thickness of H4 was then vertically evaporated onto the top and bottom of the dielectric grating to form a double-layered metallic gratings. The metallic nanogratings allows TM polarized wave to pass through (i.e., high TM transmission, TMT) while TE wave is reflected (i.e., low TE transmission, TET), from which an extinction ratio:
can be defined [22–24]. The physical mechanism of TE/TM selectivity in the transmission (i.e., generation of a linearly polarized transmission) can be understood by the restricted motion of electrons perpendicular to the metallic nanograting. When an unpolarized light is incident onto the metallic nanograting, for polarization component along the grating direction, the conduction electrons are coherently driven along metal lines with unrestricted movement, which is exactly the same as that in the case of a thin metal film. For polarization component perpendicular to grating direction, the electron movement is confined similar to that in the case of a dielectric when the period of nanograting is much smaller than the incident wavelength, resulting in high transmission of this polarization component [14,25]. The performance of the TMT and ER of the subwavelength nanogratings can be calculated and optimized using a numerical finite difference time domain (FDTD) method (Lumerical FDTD solutions, Canada). Two-dimensional calculation is used in the calculation, in which periodic boundary conditions are used in the direction of X, and perfectly matched layers along Z direction (Fig. 1). In the simulation, the mesh accuracy X, Y and Z are all set as 5 nm. The refractive index of the materials in the structure are from data source (kx.lumerical.com). A TM polarized plane wave and a TE polarized plane wave is, respectively, incident vertically along the positive direction of Z axis from the bottom of ZnSe substrate in the wavelength range of 8 to 12 μm. The optimization process is to improve TMT and ER of the multilayer system as much as possible, i.e., maximizing the TMT and minimizing the TET during design process.
Fig. 1. Diagram of a ZnSe-based linear polarizer with multilayer nanogratings. P, period; W, width of grid; H1, thickness of BaF2 layer; H2, thickness of ZnS layer; H3, thickness of dielectric grating; and H4, thickness of metal layer.
Figure 2(a) and (b) is the TM transmission and ER of the gratings shown in Fig. 1 with different thicknesses of the transition layer H1. Other structural parameters are: P = 250 nm, DC = 0.5, H2 = 250 nm, H3 = 100 nm, and H4 = 60 nm.
It can be seen from Fig. 2(a) and (b) that with the increase of the thickness of the transition layer, the peak of the TM transmission moves towards the long wave direction. When H1 = 400 nm, the TMT is greater than 90% and the ER is higher than 55 dB in the entire wavelength band of 8 to 12 μm. If H1 = 0, i.e., no BaF2 layer is added, the TM transmission is then decreased to ∼80% and also the ER is decreased by ∼3 dB. It is noted that a period of 250 nm (i.e. ∼1/40 wavelength only in the 8–12 μm waveband), is selected in this work considering both factors of functional performance and experimental fabrications.
Figure 3(a) and (b) shows the effect of thickness of the ZnS (H2) on TMT and ER of the device. Parameters used in the simulation are the same as those in Fig. 2, in which H1 = 400 nm. From Fig. 3(a) and (b), it can be seen that the effect of H2 on ER is negligible. The TMT can be greater than 90% and ER is higher than 55 dB in the 8–12 μm range when H2 is 250 nm.
Figure 4(a) and (b) show the effect of thickness of the dielectric grating (H3) on TMT and ER. The structure parameters: P = 250 nm, DC = 0.5, H1 = 400 nm, H2 = 250 nm, H4 = 60 nm, same as those used in Fig. 2. From Fig. 4(a) and (b), it is seen that the height of dielectric grating has little effect on ER while the TMT varies with height H3 because of the effective refractive index of the dielectric grating layer. Considering the facilitation of the device fabrication, H3 can be optimized at 100 nm.
The effect of thickness (H4) of metallic Al grating on TMT and ER is shown in Fig. 5(a) and (b). The structural parameters are the same as those in Fig. 2 with H1 = 400 nm, H2 = 250 nm, and H3 = 100 nm. It is seen that both TMT and ER are sensitive to the thickness of the metallic gratings. The TMT decreases with the increase of the thickness of the metal grating while the ER increases, which is understandable because of the increased optical absorption with the increased metallic thickness. It is also noticed that when H4 is larger than 100 nm, i.e., Al film continuously covers the whole surface, TMT becomes zero (yellow line), which is expected.
The effect of DC (DC = W/P) on TMT and ER is shown in Fig. 6(a) and (b). The structural parameters are P = 250 nm, H1 = 400 nm, H2 = 250 nm, H3 = 100 nm, and H4 = 60 nm. It can be seen that TMT and ER show small sensitive to the duty cycle of the grating, which is in significant contrast to those single layered grating structures [26,27]. This is expected because DC in the conventional single-layer metal grating affect directly the width of the metallic ridge of the grating while the total width of the metallic ridges is not affected by DC in the proposed double-layer metallic gratings.
The incident angle dependence of performance of the optimized device is shown in Fig. 7. The incident angle is defined as the angle of incident light in X-Z plane with respect to Z-axis (seen in Fig. 1). It is seen that performance of the device can be well maintained within an angle of ± 50°, similar to those reported previously [28,29].
Fig. 7. Incident angle dependence of performance of the optimized device. Simulation parameters: P = 250 nm, H1 = 400 nm, H2 = 250 nm, H3 = 100 nm, H4 = 60 nm, DC = 0.5
3. Experimental fabrication and characterization
Experimental fabrication of the proposed multilayer nanograting structure is conducted and the performance of the device is characterized. Based on the polarizing characteristics of the multilayered structure discussed above, an optimized set of parameters of the ZnSe based multilayer nanogratings for working wavelength in 8–12 μm can be obtained as follows: P = 250 nm, DC = 0.5, H1 = 400 nm, H2 = 250 nm, H3 = 100 nm, and H4 = 60 nm, from which a TMT of greater than 90% and an ER of larger than 55 dB can be obtained theoretically. These parameters are then set as the target parameters in the experimental fabrications. It should be mentioned that an optimized DC of 0.5 is chosen, rather than DC = 0.7 as shown in Fig. 6(a) (b) based on the following two factors: one is the effect of DC on the TMT and ER, which is relatively small when DC is around 0.5 (seen in Fig. 6(a)(b)), in which TMT changes maximally at 8 μm from 0.91 to 0.93 and ER changes from 55.7 dB to 56.3 dB only when DC changes from 0.5 to 0.7. Moreover, considering the experimental fabrication of the proposed double-layer structure in which UV interference lithography is used. It is well known that the typical DC of the UV interferometric pattern is around 0.5. Therefore, considering the factor of fabrication process (while the TMT and ER performance of the device is almost not compromised), an optimized DC of 0.5 is chosen.
The fabrication process is given in Fig. 8 as follows: (1) A layer of 400 nm BaF2 thin film and 250 nm ZnS thin film were deposited by E-beam evaporation. These thin films serve as the transition layer and the basis of dielectric grating, respectively [Fig. 8(a)]; (2) A photoresist with a thickness of 180 nm is spin-coated on the surface of ZnS film [Fig. 8(b)]; (3) Two-beam interference exposure technology is performed and a photoresist mask plate is then obtained [Fig. 8(c)]; (4) The ICP is conducted (conventional gases Ar, O2 and F2 are employed) and nanowire gratings with a depth of about 100 nm inside the ZnS is obtained [Fig. 8(d)]; (5) The remaining photoresist is removed by putting the etched sample into acetone solution for 2 minutes with ultrasonic cleaning [Fig. 8(e)]; (6) Finally, a 60-nm-thick Al layer is deposited onto the dielectric nanograting structure by E-beam evaporation [Fig. 8(f)].
Figure 9 shows the scanning electron microscope (SEM) of the experimentally fabricated multilayer nanogratings. Figure 9(a) and (b) shows the surface and the cross section of the photoresist grating on the top of ZnS layer, respectively. The photoresist grating has a period of 250 nm, a line width of 125 nm and a thickness of 180 nm. Figure 9(c) and (d) shows the surface and cross section of the ZnS dielectric grating after ICP etching. It is seen that the period, linewidth, and thickness of the ZnS grating is 250 nm, 125 nm and 100 nm, respectively. The ZnS grating shows an excellent rectangle shape. Figure 9(e) and (f) shows the surface and cross section of the multilayer nanogratings after deposition of Al film. The thickness of Al layer is 60 nm, the line width of Al grating on the top of dielectric grating is 130 nm, and the period of Al nano-grating is 250 nm (exactly the same as that of dielectric ZnS grating). The whole multilayer Al-grating structure inherits a very good rectangle shape of the dielectric grating.
Fig. 9. SEMs of the fabricated multilayer nanogratings structure. (a) and (b): Surface and cross section of a patterned photoresist on top of ZnS; (c) and (d): surface and cross section of ZnS dielectric grating after ICP; (e) and (f): surface and cross section of the multilayer nanogratings after deposition of Al thin film.
Figure 10(a) is a schematic diagram of the measuring setup used in our experiment. A broadband infrared (IR) light source (Zolix, LSH-SiN40) is modulated by a chopper (Stanford, SR540) and is then incident into a monochrometer. The modulation frequency used in the experiment is 135 Hz. The monochromatic light passes through a collimation system and a standard linear polarizer to form the measuring system. An IR liquid nitrogen-cooled HgCdTe detector (EG&G Judson) was used to detect the transmitted polarizing light, from which electrical signal are firstly amplified by a preamplifier and was then input to a lock-in amplifier (Stanford, SR830). The employment of a lock-in amplifier can effectively suppress the noises associated with the weak light signal at each monochromatic wavelength and TM and TE polarization spectrum with high signal-to-noise ratio can be extracted smoothly. Figure 10(b) shows the measured polarizing transmission spectra of the fabricated multilayer nanogratings under two different linear polarization incidences: TM-polarized light (red curve, i.e., rotating the fabricated sample nanogratings so that a maximum transmitted intensity was obtained) and TE-polarized light (green curve, i.e., rotating the fabricated sample nanogratings so that minimum transmitted intensity was obtained). To verify the repeatability of experiments, two measurements are performed for both TM and TE polarized light, respectively (labelled as TM1, TM2 and TE1, TE2 in Fig. 10(b)). In order to obtain the TMT and ER of the fabricated nanogratings, the spectral energy of light source without fabricated nanogratings (black curve) and the baseline noise of the measuring system (blue curve) are also given in Fig. 10(b). The inset of Fig. 10(b) shows the magnified plots in which a factor of 104 is multiplied to TE1, TE2 and the baseline noise.
Fig. 10. (a) Schematic of measurement setup of polarization performance of the fabricated device, (b) experimental results of the measured transmitting intensity spectra of TM-polarized and TE-polarized incident lights, (c) measured TMT spectrum, and (d) measured ER spectrum.
The measured spectral TMT and ER of the fabricated nanogratings and the detailed comparison between the experimental results and theoretical predictions is given in Fig. 10(c) and (d), respectively. The experimental TMTs are calculated by the ratio of the measured spectral TM1 (TM2) and the spectral energy of light source shown in Fig. 10(b) and the ERs are calculated by the ratio of the measured TM1 (TM2) and TE1 (TE2) in Fig. 10(b) using Eq. 1. It is seen that the average TMT and ER of the fabricated polarizer are higher than 80% and 35 dB in the whole wavelength range of 8 to 12 μm. The measured TMT and ER in the experiment are slightly lower than those theoretically calculated as shown in Fig. 6 and also the red line in Fig. 10(c) and (d). The differences between experimental measurements and theoretical simulations of fabricated device can be mainly attributed to the following factors: (1) a natural oxide thin layer of Al film considering the fact that metal Al film can be easily oxidized when directly exposed to air; (2) a thin Al film coverage on the side wall of the dielectric ZnS grating during the coating process by E-beam evaporation; and (3) the uniformity and defects of the fabricated structure, and the possible small difference between the material parameters used in simulation and those used in experiment. The green line in Fig. 10(c) and (d) is the TMT and ER simulation results when a thin layer of 6nm-thick Al2O3 inside totally 60-nm-thick deposited Al film and the coverage of Al film on the side-wall are considered. It is seen that while the calculated TMT is getting closer to the experimental result, a large difference in ER remains between the experiment and the simulation [Fig. 10(d)]. This difference can be well explained by the experimental baseline noise of the system. By definition, the ER is obtained by the ratio of TMT and TET. It is seen from the inset of Fig. 10(b) that the TE transmitted intensity (blue line) is essentially on the same level as the baseline noise of the system (yellow line), i.e., the measured TE transmission is essentially limited by the system noise, which is about 4 orders of magnitude lower than that of TM transmitted intensity (red line). This explains a maximum ER of a 40 dB can be measured in our experiment.
4. Conclusions
We have designed and fabricated a ZnSe based multilayer polarizing nanogratings working in 8 to 12 μm IR wavelength region, which is rarely seen so far. The structure not only can effectively improve the efficiency of TM transmission, but also effectively improve the extinction ratio of the device, and furthermore, it does not need an etching process for the metals, which greatly reduces the difficulty of the fabrication. Fabrication of the proposed polarizing nanogratings has been successfully implemented and experimental results show that a high TMT (>80%) and ER (>35 dB) in 8–12 μm infrared region can be obtained. The demonstrated nanograting polarizing structure is of advantages of great tolerance to structural parameters, capability of large-area and relatively-easy fabrication and scalability to other wavelengths in infrared region, such as 3-5 μm.
Funding
National Natural Science Foundation of China (NSFC) (61775154); Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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