March 15, 2014 / Vol. 39, No. 6 / OPTICS LETTERS
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Planar broadband and high absorption metamaterial using single nested resonator at terahertz frequencies Yongzheng Wen,1 Wei Ma,1 Joe Bailey,2,3 Guy Matmon,2 Xiaomei Yu,1,* and Gabriel Aeppli2 1
National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, Beijing 100871, China 2 London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, 17-19 Gordon Street, London WC1H 0AH, UK 3 Centre for Mathematics and Physics in the Life Sciences and Experimental Biology, University College London, Physics Building, London WC1E 6BT, UK *Corresponding author: [email protected] Received January 21, 2014; revised February 13, 2014; accepted February 13, 2014; posted February 13, 2014 (Doc. ID 203792); published March 12, 2014 A planar broadband metamaterial absorber with high absorptivity working at terahertz frequencies was designed and fabricated in this work. Two nested back-to-back split-ring resonators (BSRRs) constitute a single resonator, which achieves three strong resonances, with two of them merged into a broadband peak. Cobalt silicide and parylene-C were innovatively applied as ground plane and dielectric spacer. The nested BSRR absorber experimentally realizes a bandwidth of 0.66 THz with the absorptivity above 0.8, and the highest absorptivity reaches 0.97. Taking the central frequency at 2.74 THz, the measured FWHM is 47% and the Q factor is 2.13. © 2014 Optical Society of America OCIS codes: (160.3918) Metamaterials; (300.1030) Absorption; (040.2235) Far infrared or terahertz. http://dx.doi.org/10.1364/OL.39.001589
The advent of metamaterials, which are artificial composite materials being designed to have specific electromagnetic (EM) responses, has provided unique routes to realize some intriguing phenomena, such as invisible cloaking [1] and negative refraction [2–4]. This ability to manipulate incident radiation is particularly important at terahertz (THz) frequencies, which remains the least developed region in the EM spectrum due to the relative paucity of suitable materials [5]. Since they were first demonstrated, metamaterial absorbers (MAs) have attracted considerable attention [6], and the results were soon extended to THz regime [7]. By virtue of their special merits, such as high absorption efficiency, tunable EM response, and ultrathin thickness, MAs are considered as ideal candidates for THz applications, including sensors [8], thermal emitters [9], and imaging devices [10]. The metal resonator/dielectric spacer/metal sandwich structure is widely applied in most MAs. The top metallic subwavelength patterns serve as electric resonators, and the bottom continuous thick metal ground plane blocks transmittance effectively. The coupling between two metallic layers results in a magnetic resonance, depending on the thickness and dielectric constant of the dielectric spacer [11]. Therefore, precisely controlling the spacer thickness is vital in improving the performance of the MAs. On the other hand, the use of metal ground planes may limit the high temperature process in the micro-fabrication of THz detectors. Most MAs exhibit narrowband EM response due to the resonant nature of the metamaterial. Although dual-band and triple-band MAs have been demonstrated with distinct narrow absorption frequencies [12–14], the development of broadband MAs with high absorption remains challenging. Most recent attempts to realize THz broadband MAs combine resonators with little resonance 0146-9592/14/061589-04$15.00/0
frequency separation into one unit cell horizontally or vertically [15–18]. A vertically stacked multilayer MA proposed by Grant et al. in 2011 [17] achieved more than 0.6 absorptivity with a bandwidth of 1.86 THz, where the central resonant frequency is 5 THz. Vertical stacking structures are, however, complicated to fabricate, which limit their applications. In 2012, Huang et al. reported a planar broadband MA [15] with the bandwidth of 0.1 THz beyond the absorptivity of 0.8 at the central frequency of 1 THz. However, it is difficult to realize very broad absorption band for the planar MAs at high frequency due to the coupling among the different resonators in one unit cell. In this Letter, we developed a broadband MA with nested back-to-back SRRs (BSRRs) designed in one unit cell using cobalt silicide (Co–Si) film as the ground plane and parylene-C as the dielectric spacer. The single resonator exhibits three strong inductance–capacitance (LC) resonances with two of them close enough to realize a broad absorption band at THz regime. The schematic diagram of the broadband MA is shown in Fig. 1, which shows the typical sandwich structure. The resonator comprises two BSRRs sharing the same center bar, and the inner BSRR is symmetrically nested in the outer one. The BSRR, which is also known as an
Fig. 1. Schematic diagram of (a) perspective view and (b) top view of the unit cell. © 2014 Optical Society of America
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electric-field-coupled resonator [19], provides LC resonance at lower frequencies and dipole resonance at higher frequencies [20]. To improve the temperature compatibility of MAs, we replaced the metal ground planes with Co–Si film, which has high melting point and conductivity to block the transmission of EM waves. Furthermore, to address the requirements of both accurate control of spacer thickness and a highly flexible structure we utilized parylene-C as the dielectric spacer. The geometric dimensions of the nested resonator were optimized by commercially available finitedifference time-domain (FDTD) software. The length (l1 and l2 ) and width (w1 and w2 ) of the outer and inner BSRRs were optimized to be 34 and 32 μm, 19 and 22 μm, respectively. The line width (w) and gap (g) are both 3 μm with the lattice constant (p) of 36 μm. Numerical simulations were performed under normal incidence with the polarization shown in Fig. 1(a). The gold was modeled with a conductivity 4.09 × 107 S∕m and a thickness of 100 nm, and the Co–Si was modeled with the measured conductivity of 8.3 × 105 S∕m and a thickness of 100 nm. A permittivity ε 2.42 and loss tan δ 0.04 were used for the parylene-C film [21]. The absorptivity Aω was calculated by the formula Aω 1 − Rω− Tω, where Rω is the reflectance and Tω is the transmittance. The simulated transmission spectrum of the nested BSRR MAs with optimized dielectric layer thickness of 9.3 μm is plotted in Fig. 2. It is obvious that the transmittance is less than 0.01 from 0 to 4.0 THz when using 100 nm Co–Si film as the ground plane. Therefore, the formula used for the absorptivity calculation can be simplified to Aω 1 − Rω with the transmission neglected in the following simulations. Three resonance peaks at 0.89, 2.45, and 2.87 THz are observed from the simulated absorption spectrum of the nested BSRR MA in Fig. 2 with the absorptivities of 0.90, 0.99, and 0.98, respectively. The resonant peaks of 2.45 and 2.87 THz merged with each other to form a broader absorption band, and the bandwidth is 1.23 THz for the absorptivity beyond 0.6, and it is 0.89 THz, even if we define a strict criterion of the absorptivity more than 0.8. Taking the central frequency of the broadband resonance as 2.66 THz, the full width at half-maximum
(FWHM) of the broadband absorption is 55%, and the Q factor is 1.82. For comparison, we also modeled the single BSRR MA with the geometric parameters same as the outer BSRR of the nested resonator MA, including the thickness of the dielectric layer and the lattice constant. As shown in Fig. 2, there are two obvious resonance peaks in the absorption spectrum of the single BSRR MA with the absorptivity of 0.82 at 0.89 THz and 0.73 at 2.60 THz. The two resonances originate from the low frequency LC mode and the high frequency dipole mode of BSRR, respectively. The FWHM of the two peaks are calculated to be 24% and 38%, and their Q factors are 4.2 and 2.63, respectively. It can be seen that there is no absorption above 0.8 and the bandwidth is 0.46 THz for the absorptivity beyond 0.6 at the dipole resonance, which is only 37% of that of the nested BSRR MA at the same frequency regime. To study the origin of the broadband absorption, the simulated surface current distributions of the nested resonator at three resonance peaks are examined, as shown in Fig. 3. It can be clearly observed from Figs. 3(a) and 3(c) that the surface currents, which generate 0.89 and 2.87 THz resonances are largely confined to the outer and inner BSRRs, respectively, similar to what we would see for a single BSRR. On the other hand, the resonance at 2.45 THz mainly results from a third BSRR formed between the outer and inner BSRRs, as revealed in Fig. 3(b). It demonstrates that our single nested resonator realizes three BSRRs in total and produces three individual peaks, as shown in Fig. 2. The current loops and directions of the three resonances are indicated in Fig. 3(d) and labeled with corresponding resonance frequencies for clarity. All the resonances are in LC modes, and the close resonant peaks of 2.45 and 2.87 THz lead to a broadband absorption. We modified the thickness of the dielectric layer to investigate its effect on the performance of the nested BSRR MA. The simulated absorption spectra of the nested BSRR MA with 4.0, 5.9, and 9.3 μm dielectric layer thicknesses are plotted in Fig. 4. With the increase of the
Fig. 2. Simulated transmission spectrum of the nested BSRR MAs, and absorption spectra of the nested and single BSRR MAs.
Fig. 3. Simulated surface currents distributions at (a) 0.89 THz, (b) 2.45 THz, and (c) 2.87 THz. (d) Current loops and directions at the three resonance frequencies.
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Fig. 5. Microscope images of the fabricated (a) nested BSRR and (b) single BSRR MAs. Fig. 4. Simulated absorption spectra of the nested BSRR MAs with 4.0, 5.9, and 9.3 μm thick dielectric layers.
dielectric layer thickness, the absorption of MA increases, and the resonance frequencies slightly shift to lower frequency. It can be seen that the dip between the two high frequency resonances gets deeper with the spacer thickness decreased, the merged broadband disappeared for the 4.0 μm sample while the 5.9 μm sample still presents a broadband absorption, whose bandwidth with the absorptivity above 0.6 is 1.49 THz, which is 0.26 THz wider than that of the 9.3 μm sample. It demonstrates that by decreasing the spacer thickness within a limited range, the bandwidth of the broadband MA will increase despite of a slight reduction on absorption. These results also demonstrate that the spacer thickness of MAs has great influences on the magnetic coupling with the incident THz wave. The effective capacitance and inductance of the nested BSRR will become different with the change of the dielectric layer thickness, leading to the difference of the EM responses, which, therefore, causes the variations of the resonance frequency and the absorptivity. Both nested and single BSRRs MAs were fabricated by a micro-fabrication technique as the structure shown in Fig. 1. First, a 200 nm thick SiO2 film, used as an insulating layer for the Co–Si alloying process, was deposited on the 400 μm double-polished Si substrate by low pressure chemical vapor deposition (LPCVD). Then, a 100 nm poly-Si film was deposited by LPCVD followed by phosphorus ions implantation and activation at 1050°C for 30 s. After that, a 40 nm cobalt film was sputtered to achieve a Co–Si film with a thickness of 100 nm. To produce a highly conductive Co–Si film, two rapid thermal processes (RTP), which are 600°C for 1 min and 850°C for 1 min, were applied before and after wet etching of the unalloyed cobalt. In the third step, the dielectric spacer of parylene-C layers with two different thicknesses were deposited on the surface of Co–Si film with vapor phase parylene-C monomers at room temperature and a pressure of 22 mTorr. Finally, a Cr/Au resonator array was patterned by a lift-off process with layer thickness of 20 nm∕100 nm. The microscope images of the fabricated nested and single BSRRs MAs are shown in Fig. 5. The chip size is 15 mm × 15 mm for measurement convenience. The thicknesses of parylene-C were measured to be 5.9 and 9.3 μm by a thin film reflectometry system (K-MAC ST2000-DLXn).The square resistance of the Co–Si film
was measured to be 12Ω∕sq by a four-probe meter corresponding to an electrical conductivity of 8.3 × 105 S∕m. The fabricated MAs were characterized by a Fourier-transform infrared (FTIR) spectrometer (Bruker IFS125HR) extended to THz range by a mercury vapor source. Radiation from 0.7 to 4.0 THz is sampled with a resolution of 7.6 GHz. Reflection and transmission spectra were measured with the FTIR beam incident at
1589
Planar broadband and high absorption metamaterial using single nested resonator at terahertz frequencies Yongzheng Wen,1 Wei Ma,1 Joe Bailey,2,3 Guy Matmon,2 Xiaomei Yu,1,* and Gabriel Aeppli2 1
National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, Beijing 100871, China 2 London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, 17-19 Gordon Street, London WC1H 0AH, UK 3 Centre for Mathematics and Physics in the Life Sciences and Experimental Biology, University College London, Physics Building, London WC1E 6BT, UK *Corresponding author: [email protected] Received January 21, 2014; revised February 13, 2014; accepted February 13, 2014; posted February 13, 2014 (Doc. ID 203792); published March 12, 2014 A planar broadband metamaterial absorber with high absorptivity working at terahertz frequencies was designed and fabricated in this work. Two nested back-to-back split-ring resonators (BSRRs) constitute a single resonator, which achieves three strong resonances, with two of them merged into a broadband peak. Cobalt silicide and parylene-C were innovatively applied as ground plane and dielectric spacer. The nested BSRR absorber experimentally realizes a bandwidth of 0.66 THz with the absorptivity above 0.8, and the highest absorptivity reaches 0.97. Taking the central frequency at 2.74 THz, the measured FWHM is 47% and the Q factor is 2.13. © 2014 Optical Society of America OCIS codes: (160.3918) Metamaterials; (300.1030) Absorption; (040.2235) Far infrared or terahertz. http://dx.doi.org/10.1364/OL.39.001589
The advent of metamaterials, which are artificial composite materials being designed to have specific electromagnetic (EM) responses, has provided unique routes to realize some intriguing phenomena, such as invisible cloaking [1] and negative refraction [2–4]. This ability to manipulate incident radiation is particularly important at terahertz (THz) frequencies, which remains the least developed region in the EM spectrum due to the relative paucity of suitable materials [5]. Since they were first demonstrated, metamaterial absorbers (MAs) have attracted considerable attention [6], and the results were soon extended to THz regime [7]. By virtue of their special merits, such as high absorption efficiency, tunable EM response, and ultrathin thickness, MAs are considered as ideal candidates for THz applications, including sensors [8], thermal emitters [9], and imaging devices [10]. The metal resonator/dielectric spacer/metal sandwich structure is widely applied in most MAs. The top metallic subwavelength patterns serve as electric resonators, and the bottom continuous thick metal ground plane blocks transmittance effectively. The coupling between two metallic layers results in a magnetic resonance, depending on the thickness and dielectric constant of the dielectric spacer [11]. Therefore, precisely controlling the spacer thickness is vital in improving the performance of the MAs. On the other hand, the use of metal ground planes may limit the high temperature process in the micro-fabrication of THz detectors. Most MAs exhibit narrowband EM response due to the resonant nature of the metamaterial. Although dual-band and triple-band MAs have been demonstrated with distinct narrow absorption frequencies [12–14], the development of broadband MAs with high absorption remains challenging. Most recent attempts to realize THz broadband MAs combine resonators with little resonance 0146-9592/14/061589-04$15.00/0
frequency separation into one unit cell horizontally or vertically [15–18]. A vertically stacked multilayer MA proposed by Grant et al. in 2011 [17] achieved more than 0.6 absorptivity with a bandwidth of 1.86 THz, where the central resonant frequency is 5 THz. Vertical stacking structures are, however, complicated to fabricate, which limit their applications. In 2012, Huang et al. reported a planar broadband MA [15] with the bandwidth of 0.1 THz beyond the absorptivity of 0.8 at the central frequency of 1 THz. However, it is difficult to realize very broad absorption band for the planar MAs at high frequency due to the coupling among the different resonators in one unit cell. In this Letter, we developed a broadband MA with nested back-to-back SRRs (BSRRs) designed in one unit cell using cobalt silicide (Co–Si) film as the ground plane and parylene-C as the dielectric spacer. The single resonator exhibits three strong inductance–capacitance (LC) resonances with two of them close enough to realize a broad absorption band at THz regime. The schematic diagram of the broadband MA is shown in Fig. 1, which shows the typical sandwich structure. The resonator comprises two BSRRs sharing the same center bar, and the inner BSRR is symmetrically nested in the outer one. The BSRR, which is also known as an
Fig. 1. Schematic diagram of (a) perspective view and (b) top view of the unit cell. © 2014 Optical Society of America
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OPTICS LETTERS / Vol. 39, No. 6 / March 15, 2014
electric-field-coupled resonator [19], provides LC resonance at lower frequencies and dipole resonance at higher frequencies [20]. To improve the temperature compatibility of MAs, we replaced the metal ground planes with Co–Si film, which has high melting point and conductivity to block the transmission of EM waves. Furthermore, to address the requirements of both accurate control of spacer thickness and a highly flexible structure we utilized parylene-C as the dielectric spacer. The geometric dimensions of the nested resonator were optimized by commercially available finitedifference time-domain (FDTD) software. The length (l1 and l2 ) and width (w1 and w2 ) of the outer and inner BSRRs were optimized to be 34 and 32 μm, 19 and 22 μm, respectively. The line width (w) and gap (g) are both 3 μm with the lattice constant (p) of 36 μm. Numerical simulations were performed under normal incidence with the polarization shown in Fig. 1(a). The gold was modeled with a conductivity 4.09 × 107 S∕m and a thickness of 100 nm, and the Co–Si was modeled with the measured conductivity of 8.3 × 105 S∕m and a thickness of 100 nm. A permittivity ε 2.42 and loss tan δ 0.04 were used for the parylene-C film [21]. The absorptivity Aω was calculated by the formula Aω 1 − Rω− Tω, where Rω is the reflectance and Tω is the transmittance. The simulated transmission spectrum of the nested BSRR MAs with optimized dielectric layer thickness of 9.3 μm is plotted in Fig. 2. It is obvious that the transmittance is less than 0.01 from 0 to 4.0 THz when using 100 nm Co–Si film as the ground plane. Therefore, the formula used for the absorptivity calculation can be simplified to Aω 1 − Rω with the transmission neglected in the following simulations. Three resonance peaks at 0.89, 2.45, and 2.87 THz are observed from the simulated absorption spectrum of the nested BSRR MA in Fig. 2 with the absorptivities of 0.90, 0.99, and 0.98, respectively. The resonant peaks of 2.45 and 2.87 THz merged with each other to form a broader absorption band, and the bandwidth is 1.23 THz for the absorptivity beyond 0.6, and it is 0.89 THz, even if we define a strict criterion of the absorptivity more than 0.8. Taking the central frequency of the broadband resonance as 2.66 THz, the full width at half-maximum
(FWHM) of the broadband absorption is 55%, and the Q factor is 1.82. For comparison, we also modeled the single BSRR MA with the geometric parameters same as the outer BSRR of the nested resonator MA, including the thickness of the dielectric layer and the lattice constant. As shown in Fig. 2, there are two obvious resonance peaks in the absorption spectrum of the single BSRR MA with the absorptivity of 0.82 at 0.89 THz and 0.73 at 2.60 THz. The two resonances originate from the low frequency LC mode and the high frequency dipole mode of BSRR, respectively. The FWHM of the two peaks are calculated to be 24% and 38%, and their Q factors are 4.2 and 2.63, respectively. It can be seen that there is no absorption above 0.8 and the bandwidth is 0.46 THz for the absorptivity beyond 0.6 at the dipole resonance, which is only 37% of that of the nested BSRR MA at the same frequency regime. To study the origin of the broadband absorption, the simulated surface current distributions of the nested resonator at three resonance peaks are examined, as shown in Fig. 3. It can be clearly observed from Figs. 3(a) and 3(c) that the surface currents, which generate 0.89 and 2.87 THz resonances are largely confined to the outer and inner BSRRs, respectively, similar to what we would see for a single BSRR. On the other hand, the resonance at 2.45 THz mainly results from a third BSRR formed between the outer and inner BSRRs, as revealed in Fig. 3(b). It demonstrates that our single nested resonator realizes three BSRRs in total and produces three individual peaks, as shown in Fig. 2. The current loops and directions of the three resonances are indicated in Fig. 3(d) and labeled with corresponding resonance frequencies for clarity. All the resonances are in LC modes, and the close resonant peaks of 2.45 and 2.87 THz lead to a broadband absorption. We modified the thickness of the dielectric layer to investigate its effect on the performance of the nested BSRR MA. The simulated absorption spectra of the nested BSRR MA with 4.0, 5.9, and 9.3 μm dielectric layer thicknesses are plotted in Fig. 4. With the increase of the
Fig. 2. Simulated transmission spectrum of the nested BSRR MAs, and absorption spectra of the nested and single BSRR MAs.
Fig. 3. Simulated surface currents distributions at (a) 0.89 THz, (b) 2.45 THz, and (c) 2.87 THz. (d) Current loops and directions at the three resonance frequencies.
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Fig. 5. Microscope images of the fabricated (a) nested BSRR and (b) single BSRR MAs. Fig. 4. Simulated absorption spectra of the nested BSRR MAs with 4.0, 5.9, and 9.3 μm thick dielectric layers.
dielectric layer thickness, the absorption of MA increases, and the resonance frequencies slightly shift to lower frequency. It can be seen that the dip between the two high frequency resonances gets deeper with the spacer thickness decreased, the merged broadband disappeared for the 4.0 μm sample while the 5.9 μm sample still presents a broadband absorption, whose bandwidth with the absorptivity above 0.6 is 1.49 THz, which is 0.26 THz wider than that of the 9.3 μm sample. It demonstrates that by decreasing the spacer thickness within a limited range, the bandwidth of the broadband MA will increase despite of a slight reduction on absorption. These results also demonstrate that the spacer thickness of MAs has great influences on the magnetic coupling with the incident THz wave. The effective capacitance and inductance of the nested BSRR will become different with the change of the dielectric layer thickness, leading to the difference of the EM responses, which, therefore, causes the variations of the resonance frequency and the absorptivity. Both nested and single BSRRs MAs were fabricated by a micro-fabrication technique as the structure shown in Fig. 1. First, a 200 nm thick SiO2 film, used as an insulating layer for the Co–Si alloying process, was deposited on the 400 μm double-polished Si substrate by low pressure chemical vapor deposition (LPCVD). Then, a 100 nm poly-Si film was deposited by LPCVD followed by phosphorus ions implantation and activation at 1050°C for 30 s. After that, a 40 nm cobalt film was sputtered to achieve a Co–Si film with a thickness of 100 nm. To produce a highly conductive Co–Si film, two rapid thermal processes (RTP), which are 600°C for 1 min and 850°C for 1 min, were applied before and after wet etching of the unalloyed cobalt. In the third step, the dielectric spacer of parylene-C layers with two different thicknesses were deposited on the surface of Co–Si film with vapor phase parylene-C monomers at room temperature and a pressure of 22 mTorr. Finally, a Cr/Au resonator array was patterned by a lift-off process with layer thickness of 20 nm∕100 nm. The microscope images of the fabricated nested and single BSRRs MAs are shown in Fig. 5. The chip size is 15 mm × 15 mm for measurement convenience. The thicknesses of parylene-C were measured to be 5.9 and 9.3 μm by a thin film reflectometry system (K-MAC ST2000-DLXn).The square resistance of the Co–Si film
was measured to be 12Ω∕sq by a four-probe meter corresponding to an electrical conductivity of 8.3 × 105 S∕m. The fabricated MAs were characterized by a Fourier-transform infrared (FTIR) spectrometer (Bruker IFS125HR) extended to THz range by a mercury vapor source. Radiation from 0.7 to 4.0 THz is sampled with a resolution of 7.6 GHz. Reflection and transmission spectra were measured with the FTIR beam incident at
