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We fabricated one-dimensional periodic multilayered metamaterial structures consisting of Ag and SiO2 alternating layers. Optical responses, such as transmission and absorption, are consistent well within finite difference time domain (FDTD) simulations. Angle dependent real and imaginary dielectric permittivity reflection spectra demonstrate their operational capability in the visible wavelength region. This multilayer metamaterial can be converted into a photonic crystal by manipulating the thickness of SiO2 and we demonstrate that proper filling of SiO2/Ag layers the operating wavelength can be tuned to higher wavelength region. However, absolute value of transmission reduces with increasing number of multilayer pairs due to metal absorption.
© 2014 Optical Society of America
1. Introduction
Noble metal films having dimensions down to the nanometer scale show different and exciting behavior, such as optical, electromagnetic and chemical properties, from its bulk counterpart due to quantum size and surface effects. The interaction of the light wave with the metal surface creates intense optical absorption and scattering effects related to the movement of free electrons on the metal surface which is induced by surface plasmon resonance [1–3]. When the frequencies of the electromagnetic radiation and the plasma oscillation match each other, resonances occur. The oscillation frequency is determined by four factors: the density, mass of electrons and the size and shape of the charge distribution. On the macro scale, this resonance manifests as optical absorption by the metal layer thickness. The metal surface plasmon resonance is the main factor in determining the optical properties of materials, which can be achieved by adjusting the structure, morphology, size, thickness, and composition of the metal [4–7].
Traditionally, metal and dielectric multilayer thin films have been used for optical filtering [8]. Hence, one-dimensional (1D) planar stacks using periodic metal-dielectric multilayer on a dielectric substrate, can be designed to be transparent over a tunable range of optical frequencies [9, 10]. These structures can also be designed to enhance reflection [11] or absorption [12, 13]. Some researchers have reported so-called “transparent metal,” where the periodic metal and dielectric multilayers, have a fairly large optical transparency [9] as they exhibit a transmission band in the visible range filtering of both ultraviolet and infrared light. Transparent metals can be employed for eye protection devices, heat reflecting windows, transparent electrodes for light emitting diodes and liquid crystal displays. It is well known that, the highest transmission of visible light through the metal/dielectric multilayer system is due to the resonant tunneling phenomenon [9]. Here, we designed an efficient uniaxial planar stack composed of alternating layers of silver (Ag) and silicon dioxide (SiO2) having two different dielectric constant values on top of a glass substrate and studied the effects on various optical phenomena, such as absorption, transmission, and angle-dependent reflection, with varying thickness of the SiO2 layers. Ag was chosen as one of the component because it exhibits a negative value of dielectric permittivity [14] in the visible region of the spectrum. Similarly, SiO2 is a potential oxide dielectric material having a positive value of dielectric constant (2.8) and non-absorbing characteristics in the visible regime. Moreover, SiO2 can be deposited with controllable thickness. To obtain a suitable multilayer structure with a homogenous effective medium, it would be important to optimize the thickness of the Ag layer as thin as possible. Based on constraints imposed by deposition of Ag and SiO2 multilayers to maintain film uniformity, maximum thickness of SiO2 was found to be 55nm and further increase in SiO2 thickness might create cracks in the films due to stress induced by the superlattice structure. Similarly, Ag thickness of 15 nm was found optimal. It is noted that, Ag/SiO2 multilayer is more attractive as it would have minimal losses due to the smaller thickness of Ag layer in this structure. It has also proved both theoretically and experimentally in [15], Subramania et al. that a higher dielectric constant material is preferable to that of a lower one in order to better emulate an effective medium but at the cost of a higher metal content, which could potentially increase losses. Simulations were performed with the finite-difference time-domain (FDTD) method [16] incorporating the dielectric function of the metal in order to corroborate our experimental results with theory. The wavelength dependent dielectric constant of the silver layers was determined using the Lorentz-Drude model and the Drude-Lorentz parameters for silver as well as SiO2 were taken [17, 18]. Hence, FDTD simulations of all Ag/SiO2 multilayer structures were performed to corroborate it with the experimental results.
2. Fabrication of multilayers
Alternating layers of high purity Ag and SiO2 are deposited on the glass substrate periodically in order to produce a 1D superlattice structure. The thickness of Ag layer is chosen to be 15 nm since the skin depth for visible light of Ag is in the range of 12–20 nm, where the thicker films will block light transmission. We fabricated 5 and 8 pairs of alternate deposition of Ag and SiO2 layers by controlling the thickness of SiO2 from 15 nm to 55 nm and one sample consists of 8 pairs of Ag/SiO2 having thickness of each layer 20nm using the method of ultrahigh vacuum electron beam evaporation system on a chemically cleaned glass substrate starting with Ag layer. This enables SiO2 to be the topmost layer thereby protecting Ag from oxidation being exposed to air after fabrication. In order to study the optical properties of fabricated multilayer slabs, optical transmission (T), absorption (A) at normal incidence and reflection (R) spectra at different angles of incidence are measured with a halogen white light source in a Perkin-Elmer 950 UV-VIS-IR spectrophotometer. The transmission spectrum is normalized with a bare glass substrate and the reflection spectrum is calibrated with Ag mirror.
3. Results and discussion
Figure 1 shows the typical scanning electron microscopy (SEM) image of a metal-dielectric multilayer structures having uniaxial anisotropy consisting of both positive permittivity correspond to the dielectric component and the negative term to the metallic component which makes an effective medium in visible wavelength region. It is known that such multilayer structures have strong optical non-local effects [19, 20]. In order to take into account the optical non-localities, multilayer structures can be considered as a one-dimensional photonic crystal along the x-direction, which leads to the dispersion relation. The cross-section SEM images clearly reveal the alternating distinct 5 pairs of multilayers of Ag (white) and SiO2 (gray) having a thickness of 15 and 55 nm, respectively. Each pair consists of one layer of Ag and SiO2 and the thicknesses appear to be uniform for all the pairs.
Fig. 1 Cross-section field emission scanning electron microscope image of 5 pairs of Ag/SiO2 structures with a total thickness of ~350 nm with each pair approximately 70 nm thick consisting of ~15 nm Ag and ~55 nm SiO2 fabricated on top of a glass substrate.
Figure 2 shows the X-ray reflection (XRR) measurement of 5 pairs Ag (15 nm) and SiO2 (55 nm) sample together with a fit. The resulting curves were fitted using the simulation X'Pert Reflectivity software from PANalytical in order to determine the multilayer period and its thickness, which agree well with the experimental data.
Fig. 2 XRR spectra of 5 pairs of Ag/SiO2 multilayer structure composed of ~15 nm Ag and ~55 nm SiO2 fabricated on top of a glass substrate.
Figures 3 and 4 show the experimental and FDTD simulation results absorption (A), transmission (T) spectra at normal incident as well as the experimentally observed angle dependent reflection (R) spectra of Ag and SiO2 multilayers comprising of 5 pairs with each Ag and SiO2 layer thickness of 15 nm and 8 pairs with each Ag and SiO2 layer thickness of 20 nm. From the transmission spectra, it is seen that in the shorter wavelength region both samples show a higher value of transmission, which decreases with increasing wavelength. Hence, 5 pairs sample containing 15nm of each Ag and SiO2 layer thickness showed 47% transmission at 331 nm as shown in Fig. 3(b). It is observed that, the Ag layer has a very low absorption capacity, resulting in a higher transmittance in the visible spectrum range, when Ag layer is thinner than its optical skin depth. The skin depth of Ag depends on the substrate and the deposition conditions, and is mostly in the range between 10 and 20 nm in the visible region [21]. However, an increase in the number of multilayers as well as the metal and dielectric thickness to 20 nm causes transmittance value drops to 21% for 8 pairs of Ag and SiO2 multilayers [Fig. (4b)]. It is clearly mentioned here that, although our multilayer containing a total thickness of Ag much greater than its skin depth at visible wavelengths, the incorporation of dielectric layer of proper thickness in between Ag layers favors resonant tunneling. This opens up transmission windows that allow for a high transmission to be achieved in regions where metals are typically opaque [21]. The decrease of optical transmittance is evidently due to the increase of the absorption with respect to the increase in thickness of the embedded metal layers in between the dielectric layers. Absorption spectra of both samples show reverse trend to the transmission spectra and the absorption values increases in the higher wavelength region. In order to corroborate our experimental results with theory, transmission and absorption spectra from FDTD simulations are shown in Figs. 3(a) and 4(a). The transmission values and its behavior observed experimentally are well agreed to our simulation.
Fig. 3 Optical transmission response with corresponding absorption spectra at near normal incidence 5pairs of Ag/SiO2 multilayer structure consists of 15 nm Ag and 15 nm SiO2 (a) FDTD simulation (b) Experimental (c) angle dependent reflection spectra.
Fig. 4 Optical transmission response with corresponding absorption spectra at near normal incidence for 8pairs of Ag/SiO2 multilayer structure consists of 20 nm Ag and 20 nm SiO2 (a) FDTD simulation (b) Experimental (c) angle dependent reflection spectra.
Angle dependent (incident angles ranging from 15° to 75°) reϕλection spectra of both samples were taken with an intervals of 5° and are shown in Figs. 3(c) and 4(c). Both samples show a peak at 331nm the sample shows a high-reflectivity surface towards all higher wavelengths beyond the plasma frequency of the metamaterial that acts as a high pass filter. With varying incident angles, the λ/d ratio is sufficiently large to represent the metamaterial as an effective medium causing slight shifts in the measured reflected spectra. However, the metamaterial cut-off filtering effect is the dominating and persistent effect observed for all the spectra. The plasma frequency for the metamaterial can be further tuned into the optical regime by increasing the material density of the sample i.e. by increasing the dielectric layer thickness.
Figures 5, 6, and 7 show both FDTD simulations and corresponding experimental results of transmission and absorption behavior of 5pair multilayers keeping the Ag layer thickness fixed at 15 nm, while the dielectric SiO2 layers was varied between 25nm to 40 nm. The transmission and absorption spectra of all samples significantly showed different characteristics as compared to our previous equal thickness metal and dielectric multilayers structure. The experimental and simulation transmission spectra (red) exhibit very interesting behavior with the formation of more ripples and a better transmission response in the visible wave length region. Therefore, transmission characteristics of these structures are propagating towards longer wavelengths with an increase in the thickness of SiO2 layers from 25 nm to 40 nm as shown in Figs. 5(a), 5(b), 6(a), 6(b), 7(a), and 7(b). The observation of this phenomenon is due to the existence of both propagating and evanescent waves in our metal-dielectric multilayer structures and contributes to its transmission [22]. The propagating modes are the consequence of the travelling wave interference across the multilayer structures as previously reported [10, 23]. Therefore, the interference between the incident and reflected beams, which normally originates from the phase change, caused upon reflection from each Ag interface as well as the phase advance due to the propagation of the light in the dielectric layer. Transmission increases when the wavelength and angle of the light satisfy the constructive interference condition. Conversely, destructive interference reduces transmission in the higher wavelength region.
Fig. 5 Optical transmission response with corresponding absorption spectra at near normal incidence 5pairs of Ag/SiO2 multilayer structure consists of 15 nm Ag and 25 nm SiO2 (a) FDTD simulation (b) Experimental (c) angle dependent reflection spectra.
Fig. 6 Optical transmission response with corresponding absorption spectra at near normal incidence 5pairs Ag/SiO2 multilayer structure consists of 15 nm Ag and 30 nm SiO2 (a) FDTD simulation (b) Experimental (c) angle dependent reflection spectra.
Fig. 7 Optical transmission response with corresponding absorption spectra at near normal incidence 5pairs of Ag/SiO2 multilayer structure consists of 15 nm Ag and 40 nm SiO2 (a) FDTD simulation (b) Experimental (c) angle dependent reflection spectra.
We also measured the absorption spectra (blue) and corresponding simulations of the Ag/SiO2 multilayer structures. The absorption increases from the blue to the red region and gradually reaches stable values in sufficiently long wavelength are shown in Figs. 5(a), 5(b), 6(a), 6(b), 7(a), and 7(b). All absorption spectra show an oscillation comprising of maxima-minima-maxima with an increase in the number of peaks between 300 to 450 nm wavelength regions as the thickness of SiO2 is increased from 25 nm to 40 nm. When the wavelength is larger than 450 nm, the absorption of the Ag/SiO2 multilayer structures increases gradually and reaches a considerably high value due to the photonic band structure which is a critical factor causing the absorption enhancement effect.
It is known that, in periodic photonic crystal structures, the transmission band and band gaps are formed due to the multiple Bragg scatterings, resulting considerable transmission of light in the multilayer structure. When the thickness of the metallic layer is lower than the relevant skin depth of the corresponding metal, some of the electromagnetic waves will travel through the metallic layer and propagate in the 1D photonic crystal structures.
As seen in Figs. 5(c)–7(c), the angle dependent reflection spectra of all three samples show similar trend by the application of electromagnetic wave below the wavelength of 520 nm. However, the number of reflection peaks increases with increase in dielectric thickness of the same sets of samples due the phase shift increases with the layer thickness.
Figures 8(a)–8(d) show the experimental and simulation results of both absorption and transmission spectra for 55 nm thick SiO2 and 15 nm of Ag layers comprising of 5 and 8 pairs of alternating layers, respectively. It is clear that more numbers of prominent absorption peaks appear with an increasing number of pairs. The transmission spectra measured experimentally [Fig. 8(d)] in our samples coincide with the simulated results. The highest transmission values were achieved when the number of transmission peaks increases in the multilayers. However, the sample with 8 pairs showed transmission value slightly less, but with more peaks, compared to the 5pairs sample. Hence, from all graphs it is noticed that, dielectric layers in multilayers are mainly used to minimize the effect of light reflection.
Fig. 8 Optical transmission response with corresponding absorption spectra at near normal incidence 5 and 8 pairs of Ag/SiO2 multilayer structure consists of 15 nm Ag and 55 nm SiO2 (a) FDTD simulation of Absorption (b) Experimental Absorption (c) FDTD simulation of Transmission (d) Experimental Transmission.
With proper control of the refractive index and thickness of the layers, the light beams reflected from the front and back surface of each of the dielectric layers show a phase and intensity interference effect suppressing the light reflection and achieve a maximum light transmission of the multilayer film structure in the visible region. Here, increasing the dielectric thickness shifted the transmission peak towards a longer wavelength region, increased the transmittance across the band pass, and decreased the height of the ripples associated with the resonance peaks. This decrease in transmittance is due to the fact that the reflectivity of Ag increases towards longer wavelength region. The increase in reflectivity decreases the coupling of the Ag layers, such that the transmission of light through the structure is impeded due to partial absorption of light in this wave length region.
However, oscillations in the experimental data are slightly enhanced relative to the simulations. Nonetheless, the maximum experimental transmission values are within a few percent of the simulation values. Certain deviation between the experimentally measured and simulated values are to be expected due to reasons such as differences between the actual thickness and dielectric constant values of the Ag and SiO2 layers and those used in simulations.
Figures 9(a) and 9(b) showed the angle dependent reflection spectra of 5 and 8 pairs of Ag (15 nm) and SiO2 (55 nm) multilayer structures. The reflection spectra for the 5pairs sample showed multiple reflection peaks from the metal and dielectric interfacial layers, which is enhanced towards a higher wavelength region. It is reported by other groups that, refractive index of the materials is small dependent on the thickness as well as density of the layer / medium [24, 25]. So, it is expect that by increasing the thickness of SiO2, the refractive index of the dielectric will lead to a smaller index mismatch between the dielectric and metal, resulting in greater reflectance at each metal/dielectric interface beyond 550 nm. However, by increasing the number of pairs from 5 to 8, more closely spaced reflection peaks are formed. Moreover, it is also seen that with an increase in the angle of incidence, the reflection peaks of the light gradually shift towards shorter wavelength region.
Fig. 9 Angle dependent reflection spectra of Ag/SiO2 multilayer structure consists of 15 nm Ag and 55 nm SiO2 (a) 5pairs (b) 8 pairs.
From all transmission and reflection spectra it is seen that by varying the thickness of SiO2 we can tune the multilayer structure. Transmission peaks are red shifted with an increasing number of pairs and favor the formation of more ripples in transmission spectra. When the thickness of SiO2 is kept constant at 55 nm in a sample with 5 pairs, four distinct ripples in transmission are observed in the visible (from 300 to 550 nm) region.
From the above experimental results it is clear that the thickness of the dielectric layer plays a crucial role designing a tunable multilayer structure towards higher wavelength in the visible region. It is found that the sample having equal thickness and pairs of metal and dielectric layers in the multilayer structures show single peak only and behaves as a bulk as compared to rest of the samples. The basic reason for observation of multiple ripples in T, A, and R is that the metal and dielectric multilayer can be considered as a series of resonant cavities in the dielectric layers with weak coupling through the metal layers. If two cavities with identical resonant frequencies are joined with a weak coupler, then in the combined response the two cavities share resonance peak, which will split into two adjacent peaks. The distance between the peaks is governed by the amount of loss in the coupler. This effect scales with the number of cavities that are joined together, such that; each cavity introduces a new peak. This structure is considered as photonic crystal and distinguishes from the metamaterial structure having no ripples.
Figure 10 showed optical response of permittivity as a function of wavelength of equal ratio metal dielectric samples such as Ag (15nm)/SiO2 (15nm)-5pairs, Ag (20nm)/SiO2 (20nm)-8pairs and Ag(15nm)/SiO2(15nm)-5pairs samples, respectively by Variable Angle Spectroscopic Ellipsometry (VASE) measurement technique. It is a popular tool that can be used to study the optical properties of thin films. Light is directed to a sample at multiple angles and is either absorbed, transmitted or reflected. The reflected light is analyzed for intensity and polarization; from that many film properties can be determined such as film thickness, index of refraction, permittivity, absorption, etc [26]. In Figs. 10(a) and 10(b), the real part of the permittivity for multilayered samples with equal ratio showed negative values throughout a wide wavelength range without any observation of crossover from its positive to negative quadrant. However, the corresponding imaginary part was close to zero within same wavelength range. This trend of permittivity is associated with a lower filling factor between Ag and SiO2 fabricated samples and behaves as a pure Ag film and this filling percentage has no effect to make the sample an effective medium. However incorporation of a thicker SiO2 (55nm) layer in between the Ag layers completely changes the permittivity behavior and favors the real permittivity from positive to negative, which cross over at a wavelength close to 500nm. Similar behavior is also shown for the imaginary permittivity of the sample [Fig. 10(c)].
Fig. 10 Real and imaginary permittivity of Ag/SiO2 multilayer structure (a) 15 nm Ag and 15 nm SiO2 for 5pairs (b) 20 nm Ag and 20 nm SiO2 for 8pairs (c) 15 nm Ag and 55 nm SiO2 for 5pairs.
In general, transmission spectrum of a metal/dielectric superlattice system is controlled by the transmission of the coupled resonant electromagnetic (EM) waves. Hence, in our case each metal/dielectric multilayer structure for higher pairs Ag and SiO2 sample (8 pairs) acts as a metallic Fabry-Perot cavity and the finite thickness of the Ag layers makes cavity modes overlap. Therefore, a periodic metal/dielectric photonic crystal can be considered as a coupled system of several identical Fabry-Perot effects of multilayer structures as reported by the other groups [9, 27–29], especially for more number of pairs. The coupling between cavity modes is caused by the overlapping of evanescent waves of cavity modes confined in the cavities. When the thickness of the metal layer is larger than its skin depth δ (e.g. δ~15 nm for Ag in the visible range) between the individual cavities, the cavities are weakly coupled. Consequently, one can expect that the EM modes confined in such a weakly coupled cavity system will maintain the characteristics of the confined mode in a single cavity. It can be said that metallic films with thickness of the order of the skin depth, when arranged as a 1D stack, exhibit photonic effects that can be used to tailor the spectral absorptivity/emissivity. Alternatively, the ripples in the transparent region depend on the whole dielectric thickness as the response corresponds to the interferences in the whole multilayer structures. This happens due to the fact that the light wave tunneling through the Ag layers increases the shift in phase in the visible region.
4. Conclusion
In conclusion, we have designed metallic-dielectric multilayer structures constructed with Ag and SiO2 layers with varying thickness and studied their optical properties experimentally to corroborate the results with FDTD simulation in the visible spectrum region. It is found that the incorporation of thicker dielectric layers favors the enhancement of transparency with the formation of more peaks over a tunable range of visible wavelength. As a result, the structure of our multilayer metamaterial is converted in to photonic crystals. These structures can also be designed to enhance reflection or absorption by the proper filling of SiO2 and Ag. We have also found that the high index contrast in these structures makes is possible to tailor the spectral characteristics of the band pass window by moving away from the structures with thickness distribution that are strictly periodic.
Acknowledgments
This work is supported by the DoD (CEAND) Grant Number W911NF-11-1-0209 (US Army Research Office), NSF-CREST (CNBMD) Grant number HRD 1036494.
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