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Silver is proposed as a useful metal for thin metal-film subwavelength-grating polarizers in both the terahertz and mid-infrared regions. A triangular Ag-film grating for a terahertz-region polarizer fabricated on a resin substrate showed measured TE-wave losses of higher than 45 dB in the frequency range of 0.5–2.2 THz, while TM-wave losses were lower than 0.75 dB in the range of 0.5–3THz. A triangular double Ag-film grating structure on a thin silicon substrate with an anti-reflection layer on its reverse side was fabricated for the polarizer in the mid-infrared region. Measured TE-wave losses were higher than 27 dB in the wavelength range of 16–21 μm, while the minimum TM-wave loss was 3.5 dB at around the wavelength of 19 μm. Silver films are confirmed to be promising candidates for fabricating high-performance polarizers in the terahertz and mid-infrared regions.
© 2016 Optical Society of America
1. Introduction
The metallic wire-grid structure is widely employed for polarizers in the mid-infrared and terahertz regions [1–15]. Most wire-grid polarizers comprise line-and-space thin metal-film stripes with subwavelength period on transparent substrates. Both a high aspect ratio (thickness vs. width) and smooth sidewalls are required for each metal stripe to obtain a high extinction ratio and a low insertion loss. Furthermore, it is recommended to avoid using toxic materials, such as ZnSe and KRS-5, for the substrates employed in most of the conventional mid-infrared wire-grid polarizers. While the winding technique can be used for free-standing-type wire-grid polarizers in the terahertz region [8], the fabrication process requires sophisticated equipment, and the fabricated fine metal-wire grids are fragile. A robust, easy to fabricate, and nontoxic polarizer with a high optical performance in the terahertz and mid-infrared wavelength regions is desirable.
A polarizer employing a thin metal-film subwavelength grating for the mid- and far-infrared regions was proposed [16]. The specific feature of the polarizer is that the metal film has a continuous structure in contrast to that used in wire-grid polarizers, and thus a metal-film patterning process is not required. Experimental performance in the terahertz region using a triangular Au-film grating was presented [16,17]. While the structure and fabrication method are quite simple and robust [17], the requirement for a high-purity Au target for the sputtering deposition of the Au film makes the method expensive. An inexpensive metal is preferable for widespread use in practical applications. The configuration was extended to fabricate mid-infrared polarizers by employing a multiple Al-film grating structure [18,19] fabricated on an Si substrate. Since the refractive index of the Si substrate was high, there was a high Fresnel reflection loss at the reverse surface of the substrate [18]. The use of multiple Al-film gratings on both sides of the Si substrate made the fabrication process complicated and thus unsuitable for mass production [19]. Furthermore, the optical characteristics of Al film are sensitive to deposition conditions and the surrounding environments.
In this paper, we propose Ag as the metal for subwavelength grating polarizers in both the terahertz and mid-infrared regions. Complex refractive indexes of Ag films deposited by using an rf magnetron sputtering method are found to be promising for polarizers in both wavelength regions. In the terahertz region, a single Ag-film subwavelength-grating polarizer fabricated on a resin substrate by using the imprinting method was found to have optical performance as high as that measured with Au-film grating polarizers. A double Ag-film grating fabricated on one side of the Si substrate with an anti-reflecting structure on the other side was fabricated for the mid-infrared polarizer, and its optical performance was measured.
2. Terahertz polarizer employing the Ag-film subwavelength grating
The schematic configuration of the polarizer for the terahertz region is depicted in Fig. 1, showing that the structure is the same as that reported in Refs. 16 and 17 except for the kind of metal. Ag is newly employed for the triangular cross-sectional thin film instead of Au. The substrate is made of Tsurupica®, a specific kind of ZEONEX® resin with a refractive index of 1.53. The period Λ of the grating is chosen to be considerably smaller than the wavelength λ. The aspect ratio of the grating (h/Λ) is set to around unity or larger, where h is the height of the grating, and the thickness of the Ag film (t) is chosen to be in the order of the skin-depth of the light wave. Note that the thickness t is measured in the Z-direction, rather than the surface normal. The input TE-wave (polarized in the Y-direction) is strongly reflected by the metal grating, whereas the TM-wave (polarized in the X-direction) passes through the grating with a low loss. Calculated contour diagrams of TE- and TM-wave losses as a function of the complex refractive index (n ‒ jκ) of a metal are shown in Figs. 2(a) and (b), respectively. The rigorous coupled-wave analysis (RCWA) method [20] was used in the calculation. In the calculation, the structural parameters of the triangular metal-film grating were assumed to be Λ = 25 μm, h = 35 μm, t = 25 nm, and the frequency f was chosen as 2 THz. Roughly speaking, the TE-wave loss increases in proportion to the absolute value of the complex refractive index, while the TM-wave loss decreases with larger absolute value of the index. A metal having a large absolute value of the index is preferable for the polarizer. Complex refractive indexes of various metals at f = 2 THz reported in Ref. 21 are shown in Fig. 2 by small circles, showing that Ag has the largest absolute value of the index among them.
Fig. 2 Contour diagrams of (a) TE-wave losses and (b) TM-wave losses as a function of the complex refractive indexes n and κ reported in Ref. 21.
Absolute values of complex refractive indexes for various kinds of sputter-deposited metal films were compared by measuring transmission losses of deposited thin metal films in the terahertz region. When the thickness of the metal film is very small and the absolute value of the refractive index is large enough, the transmission loss α of the film is mainly determined by reflection at both surfaces of the film. In this case, the loss is approximated by
meaning that the comparison can be done by measuring the transmission loss. Four kinds of metal, Ag, Au, Al, and Cu, were sputter-deposited to form 20-nm-thick films on Tsurupica substrates by using the conventional rf magnetron sputtering method. Transmission characteristics of the films were measured with a terahertz time-domain spectrometer; the results are shown in Fig. 3. In the measurements, a custom-made spectrometer was employed that utilizes a collimated beam as a probe light that passes through a sample normally to avoid the defocusing effect at the semiconductor detector due to insertion of the sample. The Ag film has the highest transmission loss among them, which confirmed that Ag has the largest absolute value of complex refractive index among the films.The polarizer was fabricated by coating a 32-nm-thick Ag film on the Tsurupica grating with a triangular cross section duplicated from a metal mold by using the imprinting method [17]. A 20-nm-thick a-Si film was deposited on the Ag film as a passivation layer. Figure 4 shows a bird’s-eye view of the fabricated THz polarizer. Transmission characteristics were measured with the same terahertz spectrometer. Figure 5(a) shows time-domain output waveforms of the spectrometer. Transmission losses in the frequency range of 0.5−3 THz obtained from Fig. 5(a) are shown in Fig. 5(b). Overall insertion losses (TM-wave losses) lower than 0.75dB in the frequency range of 0.5−3 THz were obtained, in which the reflection loss at the reverse side of the substrate was estimated to be 0.19 dB. The extinction ratios are higher than 45 dB in the frequency range of 0.5−2.2 THz, while they decrease to 40 dB at around 3 THz. The transmission characteristics are roughly the same as those obtained in the polarizer employing the Au film [16,17]. Although the measured characteristics are not explicitly higher than those measured in polarizers employing Au film, Ag is confirmed to be a promising metal for terahertz polarizers. It is noteworthy that the measured optical characteristics of the Ag-film polarizer with the 20-nm-thick a-Si passivation layer did not change after immersion in 80°C hot water for 2 hours, confirming that the optical characteristics of the polarizer employing the passivation layer are stable against high temperature and humidity.
Fig. 5 Measured transmission characteristics of the polarizer: (a) time-domain detector output waveforms and (b) transmission losses for the TM and TE waves.
3. Mid-infrared polarizer employing a double Ag-film subwavelength grating
The multiple subwavelength metal-film configuration [18,19] was proposed to overcome the problem that the absolute values of the complex refractive indexes of metals in the mid-infrared region are much smaller than those in the terahertz region. Figure 6 depicts the basic structure of the polarizer, showing a triangular double Ag-film grating. Two thin Ag films with a transparent intermediate a-Si layer between them are deposited on a triangular cross-sectional subwavelength grating formed on a non-doped single-crystal Si-wafer substrate that also has the same grating on the reverse side. A 50-nm-thick a-Si layer (not shown in the picture) is coated on the upper Ag-film surface as a passivation layer. Note that the thicknesses of each Ag film t and intermediate layer T are measured in the Z-direction, rather than the surface normal.
Calculated TE- and TM-wave transmission losses of the double metal-film grating polarizer as a function of the complex refractive index of a metal are shown in Figs. 7(a) and 7(b),respectively. The RCWA method was used in the calculation. We chose parameters Λ = 2.4 μm, h = 1.55 μm, t = 10 nm, T = 1 μm, and λ = 20 μm. Refractive indexes of the substrate, intermediate layer, and protective layer were all assumed to be 3.42. Roughly speaking, the TE-wave loss increases in proportion to the absolute value of the complex refractive index, while the TM-wave loss decreases with larger absolute value of the index (note that the vertical scales are larger than those of the horizontal ones). A metal having a large absolute value of the index is preferable for mid-infrared polarizers, much the same as for terahertz polarizers. Small filled circles represent complex refractive indexes of three metals that have relatively large absolute values given in Ref. 21. Since Al has the largest absolute value in the mid-infrared region, we employed Al-film gratings for the polarizer in the previous papers [18,19]. Optical constants of Al film, however, are sensitive to deposition conditions, and thus the absolute value of Al film deposited with a standard sputtering system will be considerably decreased from that of the bulk one [22].
Fig. 7 Contour diagrams of (a) TE-wave losses and (b) TM-wave losses as a function of the complex refractive indexes n and κ. Filled and empty circles represent the indexes from Ref. 21 and measured ones, respectively.
To investigate the decrement of the absolute values due to deposition processes we measured the complex refractive indexes of Al-, Au-, and Ag-films with thickness of 100 nm, thick enough not to form an island structure, deposited by using the conventional rf magnetron sputtering method. The deposition conditions were a target diameter of 100 mm, rf power of 100 W, and Ar-gas pressure of 0.3 Pa. The purity of the target of Ag, Au, and Al was 99.998, 99.998, and 99.999%, respectively. The complex refractive indexes of metal films at λ = 20 μm were obtained by measuring complex reflectivity with a spectroscopic ellipsometer (IR-VASE, J. A. Woollam Co. Inc.), and are shown in Figs. 7(a) and (b) by the small empty circles. Measured absolute values of the indexes of Ag, Au, and Al are 123, 115, and 109, respectively, showing that Ag has the largest absolute value of the index among them, and thus Ag is a promising candidate as a metal that is useful not only in the terahertz region but also in the mid-infrared region.
Very thin metal films tend to form island structures that make the optical properties of the film shift from metallic- to dielectric-like. Thus, it is important to find the critical thickness that determine whether the Ag film has an island structure or a continuous one. Metal films with island structures show specific transmission characteristics in the wavelength range from the visible to near-infrared region [22]. Figure 8 shows transmission characteristics of sputter-deposited Ag films on glass substrate for different mass thicknesses tm as a function of the wavelength. It is clear that the film with mass thickness 3 nm has an island structure, and those with thicknesses of more than 10 nm have a continuous structure. Thus, the thickness of each Ag film must be more than 10 nm for the double Ag-film grating polarizers.
The cross-sectional grating profile deforms from the original triangular to a parabolic cross section during the deposition process of the a-Si intermediate layer [19]. To investigate the change of grating profile, a triple layer consisting of Al (20 nm), a-Si (1.5 μm), and Al (20 nm) was deposited on the Si triangular subwavelength grating with the rf magnetron sputtering system. The triangular grating on the Si-substrate surface was fabricated by using the conventional photolithography-patterning technique together with the preferential etching property of the single-crystal Si substrate. After depositing and cleaving the films along the X-direction, an end face was etched slightly with a reactive ion etching (RIE) equipment by employing a mixed gas plasma of SF6 and O2. The inset in Fig. 9 shows an SEM photomicrograph of the end face after the RIE. Since Ag has a high etching rate, Al was used for the metal layers instead of Ag to observe distinct metal layers. It is clearly shown that the original triangular cross section becomes a parabolic one as the deposition progressed. The influence of the deformation of the upper metal-film layer on the transmission characteristics of the double metal-grating polarizer was numerically evaluated with the RCWA method as shown in Fig. 9, where t = 10 nm, T = 1.5 μm, and the measured complex refractive index of Ag was assumed. The reflection loss at the reverse side of the substrate and the absorption loss in the substrate are ignored in the calculation. The influence of the deformation on lossesis noticeable in the shorter wavelength region. The transmission loss of the TE-wave slightly decreases due to the deformation, whereas the TM-wave loss increases considerably at wavelengths shorter than 12 μm. The deformation is smaller for the thinner a-Si intermediate layer, and so T was set to 1.0 μm in the following polarizer fabrication.
Fig. 9 Calculated transmission losses of polarizers with and without deformed grating structures as a function of the wavelength. The inset shows a cross-sectional SEM photomicrograph of the double metal(Al)-film gratings and the a-Si intermediate layer.
The Si substrate has absorption losses in the mid-infrared region [18]. To reduce the absorption, a 0.3-mm-thick wafer was employed in this work instead of the previous 0.5-mm-thick one. Measured transmission spectra of the non-doped single-crystal Si wafer with thickness of 0.3 mm are shown in Fig. 10. There is an absorption peak at around thewavelength of 16 μm. The transmission losses are mainly due to reflection at the surfaces of the Si wafer. The reflection loss is 1.55 dB at the surface of the flat Si wafer. Since it is known that a subwavelength grating has an effect of reducing reflection losses at the surfaces of the Si wafer [23], the transmission characteristics of the Si wafer with triangular cross-sectional gratings were measured as shown in the figure. The structural parameters of the gratings were Λ = 2.4 μm and h = 1.55 μm, the same as those mentioned above. By employing the gratings, the transmittance increased by more than 30% and 10% in the shorter and longer wavelength regions, respectively, while the reflection reduction effect depended on the polarization direction. To further reduce the reflection loss, the conventional λ/4 anti-reflection (AR) technique was employed. A 1.7-μm-thick ZnS layer was deposited on one side of the Si wafer with gratings by using the rf sputtering method. The thickness corresponds to the optical thickness of λ/4 at the wavelength of 15 μm, where the refractive index of ZnS is 2.2. Measured transmittance characteristics are given in the figure, showing that the wafer with both gratings and the AR layer has the highest transmittance in the wavelength region of 10–22 μm.
Fig. 10 Transmission spectra of the non-doped 0.3mm-thick single-crystal Si wafer with and without subwavelength gratings.
To fabricate polarizers two 10nm-thick Ag films and a 1.0-μm-thick a-Si intermediate layer were deposited on the Si substrate sequentially without opening the vacuum chamber of the rf magnetron sputtering equipment. The 0.3-mm-thick Si substrate with gratings on both sides was used. After deposition of the upper Ag-film layer, a 50-nm-thick a-Si film was deposited on the Ag film as the passivation layer. A 2.3-μm-thick ZnS AR layer, which corresponds to an optical thickness of λ/4 at λ = 20 μm, was then deposited on the reverse side of the substrate. The fabricated polarizer is shown in the inset of Fig. 11. Transmission losses of the fabricated polarizers were measured with an FTIR spectrometer (4100, JASCO Co. Inc.), and are shown in Fig. 11. The TE-wave losses are higher than 27 dB in the wavelength range of 16–20 μm. The measured losses are restricted by the dynamic range of the FTIR for the wavelength range longer than 20 μm. A relatively flat characteristic was obtained for the TM-wave loss curve in the wavelength region longer than 15μm, where the minimum overall loss was low as 3.5 dB at around the wavelength of 19 μm.
Fig. 11 Measured transmission losses for the TE- and TM-waves for the polarizer comprising the double Ag-film grating as a function of the wavelength. The inset shows the fabricated polarizer with an effective area of 8 × 8mm2.
4. Conclusion
Silver is proposed as a useful metal for thin metal-film subwavelength-grating polarizers in both the terahertz and mid-infrared regions. In the terahertz region, a single Ag-film subwavelength-grating polarizer fabricated on a resin substrate by using the imprinting method was found to have optical performance as high as that measured with Au-film grating polarizers. High-performance and low-cost terahertz polarizers have been realized by utilizing Ag. A double Ag-film grating fabricated on one side of the Si substrate with an anti-reflecting structure on the other side was proposed for the mid-infrared polarizer, and its fabrication conditions were discussed in detail. Relatively low insertion-loss characteristics with high extinction ratios in the mid-infrared region have obtained. Thus, Ag films are promising candidates for offering robust and high performance metal-film subwavelength-grating polarizers suited to mass production in both the terahertz and mid-infrared regions.
Funding
JSPS KAKENHI (26420297).
Acknowledgments
The authors would like to thank H. Ohno of Utsunomiya University for his technical assistance.
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