Photonic structures created by coupling a narrow resonance to a broad resonance can significantly improve the sensitivity of optical sensors. We investigated a planar metal-insulator-metal (MIM) multilayered structure using attenuated total reflection to couple surface plasmon polaritons with the waveguide (WG) mode. A plasmon-induced transparency (PIT) to plasmon-induced adsorption (PIA) transformation was realized by controlling the coupling strength between the incident light and the WG mode. The results indicated that PIT and PIA have differing coupling strength and reflectance phase at surface plasmon resonance. Moreover, Fano resonance was realized by adjusting the center of the absorption band of the WG mode.
© 2016 Optical Society of America
Since Otto and Kretschmann described methods for exciting surface plasmon polaritons (SPP) with attenuated total reflection (ATR) [1,2], various optical sensors based on surface plasmon resonance (SPR) have been proposed and developed [3–9]. SPR allows SPP in a metal to bind with transverse magnetic (TM)-polarized electromagnetic waves propagating along the dielectric-metal interface. In the typical Kretschmann configuration, the wave vector of a TM wave that propagates along the interface is changed by varying the incident angle, and may coincide with the wave vector of SPP [1,2]. Therefore, the angular distribution of reflectance associated with ATR exhibits a narrow absorption band at resonance condition (i.e., SPR). SPR is extremely sensitive to changes in the refractive index [7,8] and, hence, has been widely applied to optical sensors used for the measurement of biological and chemical quantities. However, planar SPR resulting from the Kreschmenn configuration is usually generated by Au or Ag thin films with optical constants that determine the resonance curves, which limit the performance of SPR-based sensors. The analysis area is also relatively small because short-range surface plasmons are used. Grating coupler-based SPR and an optical planar waveguide-based SPR system have been developed to overcome this drawback [10–14]. However, the structure and fabrication processes of these devices are highly complex. Moreover, grating-coupler-based SPR devices have the same sensitivity as SPR sensors that use an ATR prism coupler .
The development of photonic structures created by coupling a narrow resonance to a broad resonance has recently been aimed at significantly improving the sensitivity of optical sensors. Electromagnetically induced transparency (EIT) and Fano resonance have received considerable attention in this regard. Similar interference phenomena have been observed in dielectric multilayer structures and nanostructure-based SPR [15–17]. When broad and narrow resonances are coupled, the resulting interference yields EIT-like and Fano resonance. In the case of EIT-like resonance, a sharp transmission band occurs in the middle of a broad absorption band; both sides of the absorption band have symmetrical shapes. EIT-like resonance is converted into Fano resonance when off resonance is produced under each resonance condition. Owing to its sharp absorption dip, Fano resonance may significantly improve the sensitivity against the refractive index . EIT-like and Fano resonance are usually observed in optical resonance, but may also occur in mechanical, electrical, and quantum fields. For example, coupled spring resonators or electrical circuit resonators exhibit EIT-like and Fano resonance . Hayashi determined (via calculations) that, with intensity modulation, Fano resonance can increase the sensitivity by two orders of magnitude [15,16] and therefore, the coupling of two resonance systems has attracted considerable interest recently.
This paper describes several possible coupling modes in planar Metal-Insulator-Metal (MIM) multilayer structures, which can be applied to sensor applications. We have investigated photonic structures, which may yield EIT-like phenomena; a planar MIM multilayer structure is a promising candidate for these structures. Figure 1 shows the simulation results obtained by using the finite difference time domain (FDTD) method [Lumerical Solution Inc.] to evaluate a MIM multilayer structure coupled with an ATR prism. Details of the calculated MIM structures are provided in the figures. The top diagrams in Figs. 1(a) and (b) show the reflectance curves from the MIM structure. These curves reveal EIT-like and electromagnetically induced absorption (EIA)-like phenomena. In addition, slight variations in the thickness of each layer lead to significant changes in the reflectance curve. The colored diagrams (lower plots in Figs. 1(a) and 1(b)) show the power distribution as a function of the incident angle. The power was estimated from the square of the electric field and normalized by the square of the incident light. The color bars represent the magnitude of the normalized power. Furthermore, the horizontal and vertical axes represent the incident angle of the TM-polarized He-Ne laser and the multilayer structure, respectively.
Studies of the modes in a MIM structure [19–22] show that the coupling between two SPPs at the two insulator-metal (Au-SiO2 and SiO2-Ag) interfaces generates symmetric and antisymmetric SPP modes, which are TM modes. In addition, MIM structures support transverse electric (TE) modes, which are essentially photonic modes. At an incident angle of 41°, the electromagnetic field energy accumulated in the SiO2 layer (see color diagram in Fig. 1(a)) and, hence, the broad dip in the reflection curve was attributed to the TM mode resonance. At an incident angle of 45°, the electromagnetic field energy accumulated at the Ag-air interface, the energy in the SiO2 layer dissipated, and the reflection intensity increased. This peak in the reflection curve was attributed to a plasmon-induced transparency (PIT), which arose from destructive interference between the two optical pathways: one path bypassed the SPP and the other path became excited and was re-emitted from the SPP. A similar effect is observed in the case of the plasmon-induced absorption (PIA, see Fig. 1(b)).
These results indicate that the electric field in SiO2 changes significantly at approximately the incident angle for the SPR condition and the MIM wave guided (WG) mode interferes with SPR. This interference results in PIT and PIA of these MIM multilayer structures. In this paper, we demonstrate (via experiments that the MIM structure allows coupling of the SPP with the WG mode. The MIM structures fabricated in this work exhibit PIT, PIA, and Fano resonance. This is especially true of PIA, which exhibited a sharp absorption band owing to the interference of both optical modes.
The MIM multilayer structure, used in our experiments, has two types of resonator. Electromagnetic waves can propagate in the WG mode, which consists of an insulator layer sandwiched by metal layers and yields a broad resonance. However, the interface of the thin metal film and the air side operate in the SPP mode, which yields a narrow resonance. The dependence of the reflectance, from the MIM structure, on the incident angle is measured via the ATR method. During these measurements, the incident TM-polarized laser is introduced to the MIM multi-layers through a triangular quart prism. A He-Ne laser, with a wavelength of 632.8 nm, is used as the incident light. The exact structures for realizing PIT, Fano resonance, and PIA were determined from the calculated angular distribution of reflectance by using a conventional optical transfer matrix method . Silver (Ag) and gold (Au) are used as the metallic materials, and silicon dioxide (SiO2) is used as an insulator. Poor adhesion of the deposited Au or Ag film to the SiO2 surface is reduced by inserting a thin titanium (Ti) film at the surface between the metal and SiO2 layers. However, owing to its relatively large optical constant (nTi = 2.143 + i2.923), this film has a significant effect on the reflectance and, hence, its thickness is limited to 1 nm. Values of nSiO2 = 1.457, nAg = 0.134 + i3.986, and nAu = 0.196 + i3.255 [24,25] are used for the optical constants of SiO2, Ag, and Au, respectively. The ordering of the MIM structure, based on the ATR prism, is defined as follows: quartz triangle prism, Ti(1 nm), Au, Ti(1 nm), SiO2, Ti(1 nm), Ag, and air, as shown in Fig. 2.
A 1-mm-thick quartz plate is used as the substrate for fabrication of the MIM structure. After ultrasonic cleaning in acetone and ethanol for 5 min and 3 min, respectively, and blow drying with nitrogen, metallic films of designated thicknesses are deposited on the plate; this deposition is performed via high vacuum thermal evaporation under vacuum of <5 × 10−4 Pa. The thickness is carefully controlled by using a crystal oscillator film-thickness monitor. An electron cyclotron resonance sputtering system is used to deposit the SiO2 film. The thickness of this film is controlled by the sputtering processing time, and the thickness of each layer is measured by a step profiler. The quartz substrate, with the prepared MIM multilayer structure, is affixed to the quartz prism by using a liquid with a refractive index of 1.457. Subsequently, the substrate is placed on the θ-2θ rotation stage of the ATR measurement system. When the angular distribution of reflectance is evaluated, the reflected light is measured by a silicon p-i-n photo diode. The signal voltage of the diode is used as the unit of the ordinate for the experimental results presented in this paper.
3. Transformation from PIT to PIA spectra
To observe the PIT spectrum, the peak of the narrow resonance associated with SPR must coincide with that of the broad resonance associated with the WG mode. The Au, SiO2, and Ag layers of PIT have designated thicknesses of t(Au) = 36, t(SiO2) = 190, and t(Ag) = 38 nm, respectively. Figure 3(a) shows the measured reflectance distribution from the fabricated MIM structure. A narrow transmission band formed in the middle of the broad absorption band and both absorption bands, on each side of the transmission band, have symmetrical shapes. The incident angle of the absorption band peaks associated with the SPR and WG modes, in this structure, depends mainly on the thickness of the SiO2 and Ag layers.
A broad and a narrow resonance may also couple via PIA [26–28]. In contrast to PIT, under PIA conditions, the shape absorption peak occurs in the middle of the broad absorption band. In this case, the Au, SiO2, and Ag layers in the designed MIM structure have thicknesses of t(Au) = 15, t(SiO2) = 185, and t(Ag) = 65 nm, respectively. A plot of the PIA spectrum (associated with reflectance) as a function of the incident angle from the MIM multilayer structure (see Fig. 3(b)) reveals a sharp absorption dip in the middle of the broad absorption band. Previous studies have considered PIT in several configurations [15,16,29,30] in addition to the present ATR configuration. However, Fig. 3(b) represents the first demonstration of PIA. In this structure, as in the case of the PIT condition, the absorption peaks of the SPR mode coincide with those of the WG mode, although the coupling strength differs from that of the PIT condition. Our calculation results revealed that the first metal layer from the incident side has a significant effect on the coupling strength of both modes. For example, when the thickness of the Au layer is increased to 30 nm, the absorption curve of PIT can be clearly distinguished from that of PIA.
To analyze the coupling strength, we calculated the complex reflection coefficient via the transfer matrix method . Figures 4(a) and 4(b) show the trajectories of the complex reflection coefficient from MIM structures designed for PIT and PIA spectra, respectively. The trajectories are plotted in a complex plane where vertical and horizontal axes are taken as real and imaginary parts, respectively, in the main frame. These trajectories appeared when the incident angle was changed from 0 to 90° and were characterized by a double spiral structure . The inner (trajectory B-C-D) and outer (trajectories A-B and D-E) spirals were associated with the SPR and WG modes, respectively.
When the MIMWG mode was under-coupled to the incident light and the third layer (Ag) was prepared such that the SPP mode was critically coupled to the MIMWG mode, the reflection spectrum exhibited a PIT spectrum. Under PIT conditions (Fig. 4(a)), the spiral approached closest to the origin twice (trajectory points B and D). The farthest point from the origin in the inner spiral (point C) was associated with the SPR condition. A thin first layer (Au) resulted in over-coupling of the MIMWG mode to the incident light; under this condition, and when the third layer was prepared such that the SPP mode was weakly coupled to the MIMWG mode, the reflection spectrum exhibited trends consistent with PIA (Fig. 4(b)). In other words, the outer spiral increased in size and swallowed the origin indicating that the coupling strength between the incident light and MIMWG mode transitioned from under-coupling (Fig. 4(a)) to over-coupling (Fig. 4(b)) conditions. The inner spiral approached closest to the origin only once (point C). This point was associated with the SPR condition and, in the case of PIT, was 180° out of phase with its counterpart (i.e., point C) that occurred in the case of PIA. The trajectories for reflectance indicated that the coupling strength and phase at the SPR condition associated with PIT differed from those associated with PIA.
In addition, the Fano resonance occurs when the MIM multilayer structure is slightly modulated from the exact PIT condition and the off resonance between each incident angle (for resonance of both modes) is increased [15,18]. The resonance peak of the WG mode is shifted to small or large angles from the SPP resonance-band peak by modulating the thickness of SiO2, and this shifting yields the Fano resonance rather than the EIT. Figures 5(a) and 5(b) show the angular distribution of absorption for t(SiO2) = 181 and 196 nm, respectively; the Au and Ag layers had fixed respective thicknesses of 36 and 48 nm. When t(SiO2) is increased, the incident angle for the shape absorption dip switches from the larger angle side of the transmission band to the smaller angle side.
Phenomena associated with PIT have occurred in various systems, including nanowire arrays , nanoparticles [33–37], disk/ring nano cavities [38–40], and metal-insulator-metal waveguides . The sharp spectral structure of Fano resonances has a wide range of applications in optical sensors. Recently, a narrow Fano line shape was demonstrated in planar multilayer structures; in the Ag/Cytop/PMMA (metal-insulator-insulator) structure, SPP and the waveguide mode in the insulator acted as the broad resonance mode and narrow resonance mode, respectively. Calculations [15,16] revealed that the steep slope of the Fano resonance curve leads to a 103-larger figure of merit value compared with that of the conventional SPR sensor that has a single layer of Au. The final layer of this MII structure consisted of an insulator, and the transparent window was induced by the WG mode. The performance, of this structure, as a sensor is determined by the WG resonance in the insulator. In contrast to the MII structure, in the present MIM structure, the MIMWG mode and SPP acted as the broad resonance and narrow resonance, respectively. Furthermore, the final layer consisted of the metal layer (Ag), which sustained the SPP. Therefore, the performance of this system (as a sensor) retains all the advantages of a plasmon sensor and, hence, may improve the slope sensitivity of plasmon sensor applications.
Photonic structures consisting of a planar MIM multilayer were investigated by using an ATR prism to couple the WG mode (which leads to a broad resonance band) with an SPR, which leads to a narrow resonance band. We demonstrated that various reflection spectra can be realized in the ATR spectrum by controlling the layer thickness of the multi-layer metal insulator system. Specifically, PIA in the plasmon system was demonstrated for the first-time ever. The reflectance spectra exhibited trends consistent with transformation from PIT to PIA, and Fano resonance. Our results indicated that the difference between PIT and PIA depends on the thickness of the Au layer. Trajectory analysis indicated that the WG mode changes from over- to under-coupling, when the spectra change from PIA to PIT. Furthermore, under SPR conditions, the reflectance phase of PIT was 180° out of phase with PIA. These PIT, Fano resonance, and PIA may yield a sharp absorption or transmission dip in the reflectance curve [15,16]. More importantly, the present photonic structure, created by coupling a narrow resonance to a broad resonance, may also contribute to improved sensitivity of SPR-based optical sensors.
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