A high extinction ratio THz polarizer fabricated by double bilayer wire grid structure A high extinction ratio THz polarizer fabricated by double bilayer wire grid structure Bin Lu, Haitao Wang, Jun S[.]
A high extinction ratio THz polarizer fabricated by double-bilayer wire grid structure , Bin Lu, Haitao Wang, Jun Shen, Jun Yang, Hongyan Mao, Liangping Xia, Weiguo Zhang, Guodong Wang , , , Xiao-Yu Peng , and Deqiang Wang Citation: AIP Advances 6, 025215 (2016); doi: 10.1063/1.4942515 View online: http://dx.doi.org/10.1063/1.4942515 View Table of Contents: http://aip.scitation.org/toc/adv/6/2 Published by the American Institute of Physics AIP ADVANCES 6, 025215 (2016) A high extinction ratio THz polarizer fabricated by double-bilayer wire grid structure Bin Lu,1,2 Haitao Wang,1,2 Jun Shen,2 Jun Yang,2 Hongyan Mao,2 Liangping Xia,2 Weiguo Zhang,2 Guodong Wang,1,a Xiao-Yu Peng,2,b and Deqiang Wang2,c Henan polytechnic university, No.2001 Century Road High TechDistrict, Jiaozuo, China 454000 Chongqing Key Lab of Multi-scale Manufacturing Technology, Chongqing Institute of Green and Intelligent Technology Chinese Academy of Sciences, No.266 Fangzheng Avenue, Beibei District, Chongqing, China 400714 (Received 23 October 2015; accepted 10 February 2016; published online 18 February 2016) We designed a new style of broadband terahertz (THz) polarizer with doublebilayer wire grid structure by fabricating them on both sides of silicon substrate This THz polarizer shows a high average extinction ratio of 60dB in 0.5 to 2.0 THz frequency range and the maximum of 87 dB at 1.06 THz, which is much higher than that of conventional monolayer wire grid polarizers and singlebilayer wire grid ones C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4942515] Terahertz (THz) radiation usually refers to the electromagnetic waves with frequency range from 0.1 to 10 THz (corresponding to wavelength of mm to 30 µm).1 It penetrates non-metal materials but reflects from metals It is suitable for non-destructive detection technique because of its low photon energy.2,3 THz technologies have shown numerous potential applications such as communication, radar, spectroscopy, imaging and so on THz technologies have been or will be used in many basic research domains such as astronomy, physics, chemistry, biomedical and many application fields like homeland security, national defense, postal checking, pharmaceutics, and so on.4–6 In many applications, THz wave polarizer is a key component inside many THz systems, especially for THz imaging or THz spectroscopy For example, the THz polarizers can improve the polarization of the THz beam thus improving the image quality Although THz polarizers like Brewster’s angle polarizers, liquid crystal polarizers and wire grid polarizers have been widely used,7–14 these polarizers have their own shortcomings The Brewster’s angle polarizeris limited by the incident THz beam with specified angle Liquid crystal polarizer has good performance but it works only in the range of 0.2 to THz The commercial wire grid THz polarizer with a simple monolayer metal wire grid structure is shown in Fig 1(c), which is easy to be fabricated, but its extinction ratio (ER) need to be improved In recent year, a single-bilayer polarizer with µm period and high ER of around 60 dB in the 0.6 to 3.0 THz frequency range was reported.15 However, it is difficult to fabricate higher aspect ratio structure to achieve higher ER with thick metal pattern for a single-bilayer polarizer To achieve higher ER and simplify the fabrication process, in this paper we demonstrate a new THz polarizer with double-bilayer wire grids fabricated on both sides of the silicon substrate by using the etching and evaporation coating techniques In 0.5 to 2.0 THz frequency range, our THz polarizer shows a high average extinction ratio of 60 dB, especially the maximum of 87 dB at 1.06 THz, much higher than that of conventional monolayer wire grid polarizers and single-bilayer ones a wgd@hpu.edu.cn b xypeng@cigit.ac.cn c dqwang@cigit.ac.cn 2158-3226/2016/6(2)/025215/6 6, 025215-1 © Author(s) 2016 025215-2 Lu et al AIP Advances 6, 025215 (2016) FIG Schematics structure of the Double-bilayer wire grid (DBWG) polarizer in (a) and its scanning electron micrograph (SEM) image of one surface in (b) For comparison, the monolayer wire grid (MWG) polarizer and thesingle-bilayer wire grid (SBWG) polarizers are shown in (c) and (d), respectively The definition of the parameters include: the wire grid period P, the wire grid width a, the thickness of metal film t, the depth of the slots d, the electric field E, the magnetic field H Fig 1(a) shows the schematic diagram of our THz polarizers We call it double-bilayer wire grid (DBWG) polarizer We fabricated it on the both surfaces of silicon substrate with the thickness of 500 µm For each surface, it has bilayer gold strap on the bottom surface of silicon slot and top of silicon nitride respectively In order to reduce the difficulties of the fabrication processing and avoid high insertion loss, we designed the period of grid P as µm and the fill- factor as 0.5 The distance of double gold films d is µm, and the thickness of gold film t is 100 nm We set the silicon as loss free and the refractive index as 3.4 In order to design a good THz polarizer, achieving a high transmittance of the transverse magnetic (TM) component of an incident THz wave and high ER are desirable The simulation results that transmittance and ER as function of THz frequency are shown in Fig Here we only described points as 0.5 THz, 0.8 THz, 1.1 THz, 1.4 THz, 1.7 THz and 2.0 THz In Fig 2(a), the transmittance of the transverse magnetic (TM) THz wave through the polarizer was around 50%, and the transmittance of the transverse electric (TE) wave is as low as 10−15 on average Fig 2(b) shows the ER for the polarizer We define the ER as the following equation: ER = 10 log( TransmittanceTM ) TransmittanceTE (1) It can be seen from the Fig that the average ER of DBWG polarizer is 145 dB and the maximum ER reaches 152 dB from our simulation results, much higher than that of reported in Refs and 15 with the same parameters as used in our simulation We observe the similar phenomena that ER slightly decreases with the increase of frequency as reported in Ref 15 To qualify our DBWG polarizer, we compare it with the monolayer wire grid (MWG) polarizer and the single-bilayer wire grid (SBWG) polarizer We designed and fabricated these three polarizers based on same basic parameters and materials Figures 1(c) and 1(d) show the schematic structures of MWG and SBWG polarizers, respectively The simulation results are shown in Fig Obviously, the transmittance of the TM THz wave through the polarizers are close to each other among three styles, but the transmittance of the TE THz wave is 10−4 for the MWG polarizer and 10−8 for the SBWG polarizer, respectively As a result, the ER of the DBWG polarizer is around two times of that of SBWG polarizer, and four times of that of MWG polarizer, respectively Our DBWG polarizer shows a significant improvement of ER over the other styles Apparently, this improvement comes from the special structure of our polarizer with two layers of SBWG structure or the combination of four MWG structures The incident TE THz wave radiates on the first gold 025215-3 Lu et al AIP Advances 6, 025215 (2016) FIG Simulation results of the polarization performance of three polarizers (a) Transmittance dependence on frequency for both TE and TM THz waves (b) Extinction ratio as a function of frequency Here we only described points as 0.5 THz, 0.8 THz, 1.1 THz, 1.4 THz, 1.7 THz and 2.0 THz Lines with solid square, solid circle, and solid triangle denote the monolayer, single-bilayer, and double-bilayer wire grid polarizer, respectively film and reflects partly into the air The remains that gets into the polarizer then reflects partly once again at the second gold film The same stories happens from the third and fourth gold films.15 In addition, we find that the performance of our DBWG polarizer is strongly dependent on the thickness of gold film t Here we only show the results of a THz wave at the frequency of 1.5 THz Fig 3(a) shows the transmittance and ER as a function of t In Fig 3(a), the d is kept at µm and t is changed from 100 nm to 500 nm From this figure, one may observe that the transmittance of the TM and TE THz waves decreases with t, but the later decreases faster, resulting in the increase of the ER The reason for this phenomenon is that the gold film ohmic loss leads to a decrease of the transmittance of the TM THz wave while the transmittance of the TE wave decreases due to the dissipation characteristics related to the thickness of the films.16 Furthermore, we investigated how the depth of the slots affected the performance of our DBWG polarizer We find that our DBWG polarizer is strongly dependent on the depth of the slots d Here we also show the results of a THz wave at the frequency of 1.5 THz Figure 3(b) shows the transmittance and ER as a function of d In the simulation t is fixed into 100 nm, it is worth noting that the ER increased obviously as the d rises in the range from µm to µm It is because that the Fabry-Pe’rotresonant cavity causes the ER increase Our polarizer (DBWG) was fabricated on anintrinsic (100) silicon wafer with 200 nm silicon nitride layer The silicon wafer with its resistivity > 10000 Ω·cm showed low insertion loss in THz 025215-4 Lu et al AIP Advances 6, 025215 (2016) FIG Simulation results of the performance of double-bilayer wire grid polarizers dependent on thickness t of metal film in (a) and depth d of the slots in (b) range and both of its surfaces were polished in order to reduce the loss of transmission and experimental errors.17 The silicon wafer was coated with 200 nm silicon nitride by PECVD The silicon nitride layers play a role of supporting layer for metal structures of the polarizer Ordinary etching technology usually bring the problem of overcut grooves, i.e the side wall of the slits would be covered with gold films completely by evaporation coating, resulting in the extreme low TM transmittance To resolve this problem, we used the silicon nitride for wet etching mask Firstly, we used the lithography etching technique to make the both sides of silicon nitride layer to periodic groove structure, then put the sample into the acidic solution (HF and HNO3 (1:99)) for isotropicwet etching The solution is not able to etch the silicon nitride, but the silicon below the silicon nitride will form a transverse cylindrical groove, the structure looks like a mushroom Finally, we used the evaporation coating to make the gold films on both sides of the wafer, forming a double-bilayer wire grid polarizer The scanning electron microscope (SEM) image of one surface of our polarizer is shown in Fig 1(b) For comparison, we also fabricated the MBG and SWBG polarizers with same parameters We measured the spectral responses of these polarizers by THz-TDS system and obtained the transmittance waveform in the time-domain After the time-domain THz signal is Fourier transformed, we obtained the THz spectrum as shown in Fig It is clear that the average ER is ∼60 dB for our DBWG polarizer, much higher than that of the MWG (25 dB) and SBWG (40 dB) polarizers The THz source in our THz-TDS system shows not a rigid linear polarization but a flattened elliptical polarization Current commercial THz polarizers can only improve the linear polarization to some extent because of their limited extinction ratio The deviation between the simulation and experimental results could be attributed to the alignment error and the not a rigid linear polarization of our THz-TDS system Because the angle dependence is important for evaluating the performance of a polarizer, we also study the influence of incident angle theta (the angle between incident THz wave and the polarizer’s plane) In Fig 5, the simulation results at different incident angles from 10 to 90 degrees 025215-5 Lu et al AIP Advances 6, 025215 (2016) FIG Measured transmittance in (a) and extinction ratio in (b) of three polarizers as a function of frequency Here we only described points as 0.5 THz, 0.8 THz, 1.1 THz, 1.4 THz, 1.7 THz and 2.0 THz However, we find that the extinction ratio decreases only less than 1%, 5% and 30% as the incident angle reduces 10, 40 and 80 degrees, respectively These results indicate that our designed polarizer is not too sensitive to the incident angle This could be a very good quality for easy practical use FIG Extinction ratio versus frequency at different incident angles Here we only described points as 0.5 THz, 0.8 THz, 1.1 THz, 1.4 THz, 1.7 THz and 2.0 THz 025215-6 Lu et al AIP Advances 6, 025215 (2016) For commercial MWG polarizers, the ER is dependent on the wire grid period and the thickness of gold films.18,19 The MWG structure needs larger aspect ratio to achieve higher ER However, it is too difficult to fabricate For a SBWG polarizer, thicker gold films or deeper silicon slots are also necessary to achieve above 60 dB average ER On the other hand, although the thick gold films can attain higher ER, it would bring the more loss of TM transmittance, affecting the practical application of the polarizer For the DBWG polarizer, the higher ER over 60 dB can be achieved easily even under small t and d, making the production process simple and economical Due to the limitation of our THz-TDS system, the performance of our polarizer cannot be characterized in the frequency over 2.0 THz According to our simulation results, the ER could be reached 170 dB if we optimize the t as 500 nm and d as µm, but our current fabrication conditions limit us to obtain higher ER However, both our simulation and experimental results indicate the advantages of the DBWG polarizer over other two kinds of polarizers What’s more, most of the polarizers are difficult to integrate with other planar optical components in practice However, our techniques are compatible with CMOS fabrication processes and the critical dimensions of this device are around micro-meters Currently, we could fabricate polarizers on two or four-inch wafer The clear area of patterns is larger than the size of THz beam So this kind of devices could be achieved in optical integration system with current fabrication techniques In conclusion, we demonstrate a new style of THz polarizer with extremely high extinction ratio with double-bilayer wire grid structure fabricated on both sides of silicon substrate The experimental results agree with the simulation ones approximately The average extinction ratio reaches to 60 dB in the frequency range of 0.5 to 2.0 THz and the maximum of 87 dB at 1.06 THz The double-bilayer wire grid polarizer with high performance and easy fabrication would make it widely used in various THz systems ACKNOWLEDGMENTS The authors would like to thank Haofei Shi, Yuechuang Zhang, YanxiaoFeng and Dun Liu of the Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences for their helpful discussions This work is partially supported by the National Natural Science Foundation of China (Grant No 61471336, 61504146), an award of the Joint-Scholar 2015 of West Light Foundation of the Chinese Academy of Sciences to D.W and Innovative Research Fund (No Y52A010V10) of Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences T Nagatsuma, IEICE Electron Express 8(14), 1127 (2011) M Tonouchi, Nat Photonics 1(2), 97 (2007) B Ferguson and X C Zhang, Nat Mater 1(1), 26 (2002) P H Siegel, IEEE Trans Microw Theory Tech 50(3), 910 (2002) M C Kemp, P F Taday, B E Cole, J A Cluff, A J Fitzgerald, and W R Tribe, in Terahertz for Military and Security Applications, edited by R J Hwu and D L Woodlard (Spie-Int Soc Optical Engineering, Bellingham, 2003), Vol 5070, p 44 E Pickwell and V P Wallace, J Phys D-Appl Phys 39(17), R301 (2006) F Yan, C Yu, H Park, E P J Parrott, 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THz frequency range, our THz polarizer shows a high average extinction ratio of 60 dB, especially the maximum of 87 dB at 1.06 THz, much higher than that of conventional monolayer wire grid polarizers... single-bilayer wire grid (SBWG) polarizer We designed and fabricated these three polarizers based on same basic parameters and materials Figures 1(c) and 1(d) show the schematic structures of MWG and