1. Introduction

Terahertz (THz) technology has promising applications in various fields such as biological imaging and material identification. In applications, polarizers with high extinction ratios and low insertion losses are needed to detect fine optical anisotropy with high resolution. Compactness and robustness are also required in actual use. Several kinds of THz polarizer have been developed [1–16]. Wire-grid structures, both free-standing and substrate-based ones, are the most popular polarizers and are commercially-available [1–12]. However, these polarizers have insertion losses of more than 2–3 dB or extinction ratios lower than 40 dB. A prism-type polarizer employing a nematic liquid crystal was shown to have an extinction ratio as high as 50 dB [13], but its insertion loss is higher than 3 dB. Meanwhile, a polarizer made from a stack of silicon wafers at Brewster’s angle has a low insertion loss of 0.46 dB, but an extinction ratio of 38 dB [14]. Both the prism-type and stacked-silicon wafer polarizers are bulky, which limits their applications. A polarizer including aligned nickel nanoparticles has relatively low insertion losses of 0.1 and 1.9 dB at frequencies of 0.2–0.6 and 1.6 THz, respectively [15], but its extinction ratio is as low as 11 dB.

A polarizer employing a triangular thin metal-film subwavelength grating was experimentally demonstrated to have an extinction ratio of higher than 50 dB together with insertion loss of lower than 1 dB in the frequency range of 1.0–2.5 THz [16]. This new configuration is compact and robust, and can have a wide aperture. However, a dicing machine was used to fabricate gratings, and then it took long process time and it was difficult to obtain large-aperture polarizers with high reproducibility. Furthermore, the insertion loss in the frequency range of 0.6–1 THz was still relatively high at 1–1.7 dB. Insertion losses lower than 1 dB (transmittance of higher than 80%) over the wide frequency range of 0.5–3 THz are desirable.

In this paper, we present a metal-film subwavelength grating polarizer with high optical performance over the wide frequency range of 0.5–3 THz. An improved imprinting technique was used to fabricate subwavelength gratings, which greatly simplified the process and enabled high reproducibility and optical high performance.

2. Fabrication method and microstructural parameters

Figure 1(a) shows the polarizer comprising a triangular thin metal-film subwavelength grating. The substrate is made of Tsurupica^®, a specific kind of ZEONEX^® resin with a refractive index of 1.53 [17]. Gold (Au) is chosen as the material for the metal-grating films because of its large absolute value of complex refractive index and long-term chemical stability in practical applications. The period Λ of the grating is chosen to be considerably smaller than the wavelength, and thus no diffracted wave is excited. The aspect ratio of the grating (h/Λ) is set to around unity or larger, where h is the height of the grating, and thickness of the metal film is chosen to be in the order of skin-depth of light wave in metal. Note that the film thickness (t) is measured in the Z-direction rather than the surface normal. The input TE wave is strongly reflected by the grating, whereas the TM wave passes through the grating with a low loss. A simplified model illustrating low-loss transmission of the TM wave is shown in Fig. 1(b). Free electrons shift to a left-hand side surface of each thin-metal film due to input electric field E_xⁱⁿ, and resulting charges produce X-directional electric fields in the substrate. The dependence of theoretical transmission characteristics on structural parameters was described in a previous paper [16].

 

Fig. 1 (a) Schematic diagram of the polarizer and (b) simplified model illustrating TM-wave transmission.

Download Full Size | PDF

A circular blade attached to a dicing machine was used to fabricate gratings in the previous study [16]. Clearly, this mechanical fabrication method makes it difficult to obtain uniform gratings, and also is not suitable for mass production. It is well known that the imprinting technique is a promising method for fabricating and duplicating gratings [18], and some wire-grid polarizers were fabricated by the method [6, 19, 20]. In a previous study, the technique was also used to fabricate a metal-film subwavelength grating polarizer with a parabolic profile, but the insertion loss in the frequency range of 0.6–1 THz was relatively high due to imperfect grating uniformity in the brass mold used [21].

To overcome the problems, we employed an improved metal mold made of nickel-plated STAVAX^®. The grating on the mold surface was fabricated by mechanical grooving with a triangular diamond bit with a vertex angle of 25°, resulting in a grating with a triangular cross-sectional profile of period Λ = 25 μm and height h = 35 μm. The area of the grating is 25 × 25 mm². A photo of the mold and its SEM photomicrograph are shown in Figs. 2(a) and 2(b), respectively. The new mold has a much more regular grating period than that employed before. The grating was subsequently transferred to the surface of the Tsurupica^® substrate in an atmospheric temperature of 190°C for 120 min. A photomicrograph of the cross section of the transcribed grating on the substrate surface is shown in Fig. 3(a), showing that a fine grating structure was obtained. Cross-sectional structural parameters of the grating are shown in Fig. 3(b). Each tip of the triangular grating is rounded with a radius of 4.1 μm, which reflects the profile of the triangular-diamond bit that was used. The aspect ratio of the grating is 1.4, slightly smaller than that reported before (1.6) [16].

 

Fig. 2 (a) Bird’s-eye view and (b) SEM photomicrograph of the metal mold used in the experiments.

Download Full Size | PDF

 

Fig. 3 Cross sections of the grating fabricated on the surface of the substrate; (a) photomicrograph and (b) structural parameters.

Download Full Size | PDF

The polarizer can be obtained by coating a thin metal film on the grating by using conventional deposition methods, such as vacuum evaporation and sputtering. However, the metal-film thickness distribution across the grating was not known. To investigate the distribution, a triple layer consisting of a-Si (100 nm) / Au (30 nm) / a-Si (500 nm) was deposited on the grating by using the rf magnetron sputtering method. After cleaving the substrate along the X direction, an end face was etched slightly with a reactive ion etching (RIE) equipment employing mixed gas plasma of SF₆ and O₂. The etching depth for the a-Silayers corresponds to 100nm. Figure 4 shows an SEM photomicrograph of the end face after the RIE. A continuous Au layer, even at the bottom, is clearly shown. Note that the Au layer shown represents the Y-directionally 100nm-long Au-film image, and so looks thicker than the real one.

 

Fig. 4 Cross-sectional SEM photomicrographs of the triple layer deposited on the sub-wavelength grating.

Download Full Size | PDF

The Au-film thickness distribution was estimated by using the thickness distribution of the upper a-Si layer obtained from the SEM photomicrograph. Figure 5(a) shows the estimated Au-film thickness distributions as a function of the distance from the bottom of the grating, assuming that the Au-film thickness is 25 nm if Au was deposited on a flat substrate normally. The thicknesses at the bottom and the top of the grating are 14 and 23 nm, respectively. The Z-directional thickness t has the maximum value of 57 nm at Z = 30 μm, whereas the surface normal thickness t_n has the minimum value of 5.5 nm at a slope near the bottom. The influence of lack of uniformity in the Au-film thickness distribution on transmission losses for the TE and TM waves was calculated by using the rigorous coupled-wave analysis (RCWA) method [22]; the results are shown in Fig. 5(b). In the calculation, the complex refractive index of Au given in [23] was employed. The solid and dashed lines correspond to the estimated Au-thickness distribution shown in Fig. 5(a) and the uniform Au-film thickness of t = 25 nm across the grating, respectively. The difference between the solid and dashed curves is negligibly small for the TM waves, and is less than 7% for the TE waves, meaning that non-uniformity of the thickness distribution does not have much effect on the transmission losses.

 

Fig. 5 (a) Estimated Z-directional (t) and surface normal (t_n) Au-thickness distributions. (b) Calculated transmission losses for polarizers with uniform and non-uniform Au-thickness distributions.

Download Full Size | PDF

3. Measurement of optical properties

Polarizer samples were fabricated by coating an Au-film of the same thickness as that shown in Fig. 5(a) with the rf magnetron sputtering method on the grating shown in Fig. 3. Figure 6 shows a bird’s-eye view of the fabricated polarizer. Transmission characteristics weremeasured with a terahertz time-domain spectrometer. The custom-made spectrometer, constructed by Tochigi Nikon Corp., utilizes a collimated beam as a probe light that passes through a sample normally to avoid defocusing effect at a semiconductor detector due to insertion of the sample. Figure 7(a) shows time-domain output waveforms of the spectrometer. In the TE-wave measurement, a pair of identical polarizer samples under the crossed nicols condition was employed because the extinction ratio of the light beam in the spectrometer is less than 30 dB. Transmission losses in the frequency range of 0.5−3 THz obtained from Fig. 7(a) are shown in Fig. 7(b). Insertion losses (TM-wave losses) lower than 0.5 dB were obtained, including the reflection loss at the reverse side of the substrate. The unwanted relatively high insertion loss characteristic in the 0.6–1 THz range that appeared in the previous work [16, 21] has disappeared. The extinction ratios are 50 dB in the frequency range of 0.5−2.3 THz, while they decrease to 40 dB at around 3 THz. This decrement is due to degradation in the dynamic range of the spectrometer used as shown in the figure.

 

Fig. 6 The fabricated polarizer.

Download Full Size | PDF

 

Fig. 7 Measured transmission characteristics of the polarizer: (a) time-domain detector output waveforms and (b) transmission losses for the TM and TE waves.

Download Full Size | PDF

4. Conclusion

A greatly simplified fabrication method using the imprinting technique was presented for obtaining a metal-film subwavelength-grating polarizer with low insertion losses and high extinction ratios. Microstructural parameters were investigated in detail. Insertion losses lower than 0.5 dB in the wide frequency region of 0.5–3 THz and high extinction ratios of 50 dB in the range of 0.5−2.3 THz were obtained. Thus, thin metal-film sub-wavelength grating polarizers fabricated by the imprinting method offer excellent potential as low-loss, high-extinction ratio, wide-aperture, and robust polarizers for the terahertz region.