Electromagnetically induced transparency (EIT)-like effects in silver, gold, and aluminum metamaterials consisting of dipole resonators and quadrupole resonators were demonstrated at visible wavelengths. Optical characteristics of the metamaterials could be controlled by the gap distance between the two resonators. EIT-like effects were observed at wavelengths between 603 and 789 nm, 654 and 834 nm, and 462 and 693 nm for the silver, gold, and aluminum EIT metamaterials, respectively. At wavelengths longer than around 650 nm, the silver metamaterials had better EIT-like features. At wavelengths shorter than around 650 nm, on the other hand, the aluminum metamaterials showed promising EIT-like results.
© 2014 Optical Society of America
Metamaterials with a unit structure smaller than operation wavelengths have attracted a great deal of attention as artificial electromagnetic materials having unique optical characteristics such as a negative refractive index, which does not exist in nature [1,2]. Since a wide range of refractive indices from negative to positive values can be obtained by metamaterials, a variety of innovative optical applications to optical communication systems, optical storages, cloaking devices, and solar cells have been expected. Structures of metamaterials currently attracting attention include, for example, split ring resonators [3–7], fishnet structures [8–10], plasmonic structures having Fano resonances [11–16], and electromagnetically induced transparency (EIT) metamaterials [17–33].
EIT metamaterials generate an EIT-like effect in plasmonic circuits, based on the coupling between bright and dark modes arising from surface plasmon polaritons . They can suppress radiative losses over narrow spectrum regions due to a quantum interference effect, in addition to playing a role in slow-light media and phase shifters because of drastic modifications of the dispersive properties. Moreover, since EIT-like effects are regulated by a change in the coupling efficiency between the two modes, the dispersive properties can be controlled according to the coupling efficiency.
As far as we know, there have been no experimental studies on EIT metamaterials at visible wavelengths, although there have been several numerical studies on them [21–24]. At near-infrared wavelengths, on the other hand, there have been few experimental demonstrations [12,17–20]. Liu et al. reported a stacked EIT metamaterial made of gold consisting of a single bar and a pair wire at the resonant frequency of 173 THz, which is 1733 nm in wavelength . Zhang et al. reported a planar EIT metamaterial made of gold consisting of a split ring resonator and a pair wire at the resonant wavelength of 1560 nm . Verellen et al. observed EIT-like effects in gold metamaterials consisting of monomers and dimers at wavelengths around 780 nm, which is the shortest resonant wavelength so far reported . The observed EIT-like features had broad Fano dips in extinction. Although higher frequency EIT metamaterials toward to the visible region have the potential to advance the applications of metamaterials significantly, generally, it is difficult to fabricate EIT metamaterials with a resonant wavelength being shorter.
Silver, gold, and aluminum are candidate materials for metamaterials operating at visible wavelengths, in which optical characteristics depend strongly on constructional materials of EIT metamaterials. In the near-infrared wavelength region, gold is often used as constructional material for EIT metamaterials because of its relatively high electrical conductivity and chemical stability [12,17–20]. In the visible wavelength region, however, since the real part of the relative permittivity of constructional materials gradually approaches to zero due to the wavelength approaching the plasma frequency and electronic interband transitions, these materials gradually lose the optical characteristics as a metal. Hence, it is necessary to pay attention to the selection of the constructional materials in order to obtain high quality EIT-like characteristics. The electrical conductivity and plasma frequency of silver are larger than that of gold. Aluminum has a higher plasma frequency in comparison with silver and gold, although the electrical conductivity is lower. Therefore, experimental revelation of the effect on constructional materials such as silver, gold, and aluminum is highly required for realizing optimum EIT-like features with a sharp spectral response.
In this paper, planar EIT metamaterials consisting of dipole resonators and quadrupole resonators are fabricated. EIT-like effects in EIT metamaterials are observed, for the first time, at visible wavelengths. The constructional material dependency on EIT metamaterials is studied using silver, gold, and aluminum. Optical characteristics dependency on the gap distance between the two resonators is experimentally evaluated. Moreover, the EIT-like features are compared between several EIT metamaterials with different sizes of the unit structure and different gaps between the two resonators.
Figure 1(a) shows schematics of a designed planar EIT metamaterial array. A unit EIT metamaterial consists of a single wire and a pair wire on a silica substrate. The single wire and the pair wire play roles in a dipole resonator with a bright mode and a quadrupole resonator with a dark mode, respectively. An EIT-like effect occurs at a resonant wavelength when a bright mode excited on the dipole resonator having a low quality factor is coupled to a dark mode excited on the quadrupole resonator having a high quality factor. Figure 1(b) shows a schematic of a unit of the EIT metamaterial. An incident light with the wavenumber k, electric field E, and magnetic field H impinges along the normal direction to the substrate surface from the upper side. The polarization direction is along the y-axis. There is a gap with a distance g between the two wires. When g approaches zero, strong coupling between the two wires occurs and the EIT-like effect can be generated strongly. When g becomes larger, on the other hand, because the coupling between the two wires becomes weaker, each of the wires works separately as each resonator in which only a bright mode arising from the single wire can be resonated. For each material of silver, gold, and aluminum, EIT metamaterials with several different dimensions as listed in Table 1 are designed. In Table 1, SF represents a scaling factor and is set between 1.0 and 0.6.
Figure 2(a) shows the relative permittivity of silver, gold, and aluminum . The real and imaginary parts of the permittivity are related to the stored energy and the loss of energy, respectively, within the medium. For aluminum, the amplitude of the real part of the permittivity shows the highest negative values in comparison with silver and gold, although the imaginary part of the permittivity shows the highest values at this wavelength region. For silver, the amplitude of the real part of the permittivity shows large negative values in comparison with gold and the imaginary part of the permittivity shows the lowest values. Figure 2(b) shows the simulation model of the unit structure of the EIT metamaterial array. To calculate the transmittance spectra, numerical simulation was carried out by using rigorous coupled-wave analysis, which enables calculation of exact solutions for periodic structures based on Maxwell’s equations . Figures 2(c)–2(e) show the transmittance spectra as functions of a wavelength and SF. In the calculations, SF is set between 0.2 and 1.0. The permittivity of the glass substrate is assumed to be 2.11. Dotted lines represent peak positions within the EIT-like transmission. For each constructional material, EIT-like effects are observed and the resonant wavelength becomes shorter according to a decrease of SF. For silver EIT metamaterial, the peak transmission within the EIT-like transmission gradually decreases according to a wavelength being shorter. Also, the EIT-like effect disappears at wavelengths around 570 nm as shown in Fig. 2(c). For gold EIT metamaterial, the EIT-like effect gradually disappears at wavelengths around 660 nm as shown in Fig. 2(d). For aluminum EIT metamaterial, on the other hand, the EIT-like effect occurs at calculated wavelength region, although the peak transmittance within the EIT-like transmission is lower than that of the silver EIT metamaterial as shown in Fig. 2(e).
Figure 3 shows a process flow of the EIT metamaterials. The planar EIT metamaterials were fabricated on a silica substrate with a thickness of 500 μm [Fig. 3(a)]. First, an electron beam (EB) resist (ZEP520A, Zeon) with a thickness of about 120 nm was spin-coated on the silica substrate. Next, arrays of the EIT metamaterials with several gap distances from 15 to 90 nm were patterned by a high-resolution EB lithography system (ELS-G125S, Elionix) operating at the acceleration voltage of 130 kV and the beam current of 100 pA. Patterned EB resist had an inverse tapered cross-sectional shape required for a lift-off process as shown in Fig. 3(b). Each EIT metamaterial array was formed within 100-μm-square. Next, using an EB evaporation system, a 1-nm-thick chromium layer was deposited as an adhesion layer, followed by deposition of a metal layer with a thickness of about 60 nm [Fig. 3(c)]. The metal layer consisted of one of silver, gold, and aluminum. Finally, a lift-off process was performed to remove the EB resist and undesired metals as shown in Fig. 3(d).
4. Results and discussion
Figures 4(a)–4(c) show scanning electron microscope (SEM) images of the fabricated EIT metamaterials consisting of silver, gold, and aluminum, respectively. For each material, SF is adjusted to be 1.0, 0.9, 0.8, 0.7, and 0.6. Geometrical dimensions of the EIT metamaterials measured by SEM observation are listed in Table 2. The EIT metamaterials can be fabricated with high accuracy as designed.
Figures 5(a)–5(c) show measured transmittance spectra of the fabricated EIT metamaterials consisting of silver, gold, and aluminum, respectively. Transmittance spectra were measured with a microspectroscope. SF is between 1.0 and 0.6. The gap distance g is measured with an SEM. Each variation range of g is determined according to the states of each EIT metamaterial from the strong coupling to uncoupling. Hence, these different variation ranges of g show that g required for the coupling between the two wires becomes shorter according to SF becomes smaller. In the case of the uncoupling states, optical spectra are not changed even when g increases more. Therefore, the variation range of g shown in Figs. 5(a)–5(c) is different for different SF for all these constructional materials. Table 3 shows measured optical characteristics of the EIT metamaterials at the resonant wavelengths. In Table 3, a quadrupole resonant wavelength λq is a wavelength at the peak transmittance in the EIT-like effect, a dipole resonant wavelength λd is a center wavelength of the dipole resonant dip, and a detuning parameter δ, which is λq - λd, represents the difference of the two resonant wavelengths from the dark mode to the bright mode. For each g in which the peak transmittance is obtained at the wavelength λq, the EIT metamaterial reaches a state of the strongest coupling between the single wire and pair wire. For each g in which transmittance is obtained at the wavelength λd, on the other hand, the EIT metamaterial goes into an uncoupling state. In the case of SF being 1.0, at g of 15 nm and a wavelength of 789 nm, since the single wire and the pair wire are coupled strongly, an EIT-like effect occurs and high transmittance of 70% is observed clearly for the silver EIT metamaterial as shown in Fig. 5(a). At g of 80 nm and a wavelength of 776 nm, on the other hand, since the coupling between the two wires disappears and only the bright mode is excited, an EIT-like effect does not occur and transmittance decreases to 26%. With increase of g from 15 to 80 nm, the EIT-like effect gradually disappears. Therefore, δ of 13 nm is obtained for the silver EIT metamaterials withSF of 1.0. In the case of SF being 0.9, at g of 15 nm and a wavelength of 729 nm, an EIT-like effect occurs and transmittance of 68% is observed. At g of 80 nm and a wavelength of 728 nm, an EIT-like effect does not occur and transmittance decreases to 24%. Therefore, δ of 1 nm is obtained. In the case of SF being 0.8, at g of 15 nm and a wavelength of 692 nm which is a visible wavelength, an EIT-like effect occurs and transmittance of 65% is observed. At g of 69 nm and a wavelength of 684 nm, an EIT-like effect does not occur and transmittance decreases to 26%. Therefore, δ of 8 nm is obtained. In the case of SF being 0.7, at g of 12 nm and a wavelength of 647 nm, an EIT-like effect occurs and transmittance of 61% is observed. At g of 61 nm and a wavelength of 641 nm, an EIT-like effect does not occur and transmittance decreases to 32%. Therefore, δ of 6 nm is obtained. In the case of SF being 0.6, at g of 18 nm and a wavelength of 603 nm, an EIT-like effect occurs and transmittance of 53% is observed slightly. At g of 50 nm and a wavelength of 608 nm, an EIT-like effect does not occur and transmittance decreases to 41%. Therefore, δ of −5 nm is obtained. Figure 5(d) shows the measured quadrupole resonant wavelengths λq as a function of SF. λq depends on materials due to a difference of the permittivity between silver, gold, and aluminum. In our study, resonant wavelengths of a dipole resonator and a quadrupole resonator are mainly determined in accordance with the length of these resonators, the permittivity of the constructional materials of theses resonators, and the surrounding permittivity. In this case, the lengths of the resonators and the surrounding permittivity are the same between the silver, gold, and aluminum EIT metamaterials at each SF. On the other hand, each of the constructional materials has a specific permittivity ε as shown in Fig. 2(a). Therefore, the aluminum EIT metamaterials show the shortest λq due to the large negative amplitude of the real part of the permittivity. Also, λq shifts linearity according to SF for each material. For the silver EIT metamaterials, λq shifts from 789 to 603 nm according to SF from 1.0 to 0.6 and the rate of a change of λq with respect to SF is 465 nm/SF. For the gold EIT metamaterials, λq shifts from 834 to 654 nm according to SF from 1.0 to 0.6 and the rate of a change of λq with respect to SF is 450 nm/SF. Also, for the aluminum EIT metamaterials, λq shifts from 693 to 462 nm according to SF from 1.0 to 0.6 and the rate of a change of λq with respect to SF is 578 nm/SF. Therefore, the aluminum EIT metamaterials have the shortest λq at each SF and have the most sensitive λq to a change in a scale of the structures.
To compare the EIT-like characteristics between silver, gold, and aluminum EIT metamaterials, modulation depths are introduced as shown in Fig. 6. The modulation depth represents a rate of change in transmittance based on that at the widest g in the fabricated EIT metamaterials with the same SF and constructional material as shown in a following equation.Figure 6(d) shows maximum modulation depths MDp as a function of SF. The EIT-like characteristics of the silver EIT metamaterials show the highest MDp compared with that of the gold and aluminum EIT metamaterials. Especially, at SF of 0.9, MDp of 1.89 is achieved. Although high MDp characteristics are obtained by the silver EIT metamaterials at near-infrared wavelengths, MDp decreases to 0.28 at SF of 0.6 and a visible wavelength of 605 nm. For EIT-like characteristics of the gold EIT metamaterials, although MDp is larger than that of the aluminum EIT metamaterials, MDp gradually decreases from 1.23 to 0.27 according to SF from 0.9 to 0.6. For EIT-like characteristics of the aluminum EIT metamaterials, although MDp is comparatively small, MDp is impervious to the effects of SF in comparison with the silver and gold EIT metamaterials.
The decrease of MDp according to the decrease of SF is mainly caused by the EIT metamaterials operating at wavelengths being shorter close to the plasma frequency of each constructional material. Figure 7 shows a relationship between maximum MDp and their wavelengths λm in order to clarify the effect of the operating wavelengths on the modulation depths. At λm longer than around 650 nm, the silver EIT metamaterials show higher MDp values. The gold EIT metamaterials having superior chemical stability also show applicable MDp values at near-infrared wavelengths, although MDp is lower than that of the silver EIT metamaterials. EIT-like effects in both the silver and gold EIT metamaterials nearly disappear at wavelengths shorter than around 650 nm. On the other hand, EIT-like effects in the aluminum EIT metamaterials can be observed at wavelengths shorter than around 650 nm. Therefore, aluminum should be a candidate material for EIT metamaterials at visible wavelengths.
In conclusion, we fabricated silver, gold, and aluminum EIT metamaterials and experimentally demonstrated their EIT-like effects at visible wavelengths, for the first time. The EIT metamaterial mainly consisted of a single wire and a pair wire which play roles in a dipole resonator with a bright mode and a quadrupole resonator with a dark mode, respectively. The optical characteristics depending on the constructional materials and the gap distance between the two wires were measured. EIT-like effects were observed at wavelengths between 603 and 789 nm, 654 and 834 nm, and 462 and 693 nm for the silver, gold, and aluminum EIT metamaterials, respectively. A wavelength at the peak transmittance in the EIT-like effect shifted linearity according to SF of the EIT metamaterials. In the case of wavelengths longer than around 650 nm, the silver EIT metamaterials showed higher modulation depth. In the case of wavelengths shorter than around 650 nm, on the other hand, the aluminum EIT metamaterials showed the better EIT-like characteristics. Control of the EIT-like effects will be a promising technology for realization of optical filters, phase shifters, low-loss metamaterials and slow-light media in the visible wavelength region.
A part of this work was supported by MEXT KAKENHI 23109503 and 25109702, JSPS KAKENHI 252945, and MEXT Nanotechnology Platform, and was performed in the CINTS and MNC, Tohoku University, Japan.
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