We experimentally and numerically developed a tunable absorbing nanoscale thin-film system, comprising of dye molecules doped dielectric coatings on reflecting surfaces, the absorption behaviors of which can be flexibly tuned by adjusting the system parameters, i.e. the coating thickness and the doping concentration of dye molecules. Specifically, with appropriate system parameters, our absorbing thin-film system exhibits very directional and polarization dependent absorption properties, which can be significantly altered if applied with different parameters. Calculations demonstrate the unique absorption behaviors are a result of coupling between molecular absorption and Fabry-Perot resonances in the thin-film cavity. In addition, we theoretically show that both the spectral and directional range of the absorption in the thin-film system can be intentionally regulated by doping dyes with different absorption band and setting proper excitation conditions of Fabry-Perot resonances.
© 2014 Optical Society of America
Manipulating light absorption is an important application in nanophotonics, in which nanoscale architectures can manipulate or manage photons at sub-wavelength dimensions to give absorption characteristics not available in conventional materials. Contemporary nanofabrication techniques allow us to fabricate various nanostructures capable of absorbing light, e.g. absorbers comprising of metallic nanoparticles  can trap light by excitation of nanoparticle plasmons. By enhancing light scattering [2,3], complicating light paths , or strengthening localized resonances [5–7], photonic/plasmonic crystals can also be used to enhance the light harvesting efficiency. In addition, absorption enhancement through coherent control of resonant-cavity properties has been studied and demonstrated [8,9].
Recently, Kats et al. [10–12] reported a new type of ultrathin-film system that has broadband light absorption properties based on interference effects, comprising of one or more nanoscale absorbing dielectric coatings on reflecting substrates. The thickness of such optical coatings can be as small as a few nanometres while retaining the interference effects, which, is much thinner than the conventional anti-reflection coatings that usually have a quarter-wave thickness (, where λ is the wavelength and n is the refractive index of the material). This is because the highly absorbing dielectric coatings together with reflecting substrates create a special dielectric environment where FP type resonances can be confined in a very small thickness. As the result, the incident light is mostly localized inside the absorbing dielectric coatings and ultimately absorbed. The fabrication of such novel absorbing architectures is straightforward, requiring just serial evaporation of metallic or dielectric thin films, which is a relatively inexpensive and large-area fabrication process allowing for mass-production.
However, the absorbing behavior of the ultrathin dielectric coatings on reflecting substrates is limited by the intrinsic optical properties of the chosen dielectrics and reflecting surfaces. For example, in  the broad absorption maximum (~300 nm FWHM) of germanium (Ge) coatings on gold (Au) substrates directly relates to the broadband imaginary part of the refractive indices of Ge and Au. Furthermore, in order to achieve the ultrathin thickness, only strongly [10,11] or highly dense  absorbing media were used to make up the dielectric coatings, thus restricting the freedom to the selection of dielectrics, which significantly compromises the applications of such an absorbing thin-film system for real life optical and photonic devices.
To overcome such limitations, it is necessary to develop a thin-film system in which absorbing species are incorporated in a controllable way, allowing for precisely regulating the spectral positions of absorption and flexibly engineering the absorbing behaviors. In this context, organic polymer films doped with absorbing media (e.g. dye molecules , quantum dots ) coated on reflecting substrates would be a good option since the solution-based fabrication of doped-polymer coatings offers high degrees of freedom to choose the dopants, hosts and the doping concentrations. Therefore, it is instructive to design a new type of thin-film system, namely absorbing-media-doped polymer coatings on reflecting substrates, the optical absorbing behaviors of which can be intentionally engineered according to practical needs.
In this letter, we report for the first time the manipulation of light absorption on such a thin-film system. Specifically, we use polyvinyl alcohol (PVA) thin films doped with Rhodamine 6G (R6G) molecules coated on opaque silver (Ag) substrates (PVAR6G-Ag) as a prototype system (Left-hand schematic in Fig. 1). We demonstrate that with proper system parameter, e.g. R6G doping concentrations (CR6G) and the thickness (d) of the PVAR6G coating, the absorbing-media doped thin-film system exhibits very directional and polarization-dependent absorption properties, which can be significantly altered by further adjusting the system parameters. For example, in our experiment, under certain system conditions (, CR6G = 0.5 mM), at the spectral position of R6G molecular absorption, only the s-polarized illumination at an oblique angle () is absorbed by the PVAR6G-Ag sample, whereas the p-polarized illumination is almost fully reflected. Yet further adjusting the parameters (, CR6G = 1 mM) leads to the inversion of polarization dependence, i.e. only p-polarization is absorbed while s-polarization is reflected. In addition, if we increase the doping concentration to CR6G = 13.6 mM and reduce the thickness down to , the PVAR6G-Ag sample exhibits a high (>90%) and identical absorption magnitude for both s- and p-polarized illumination at nearly normal incidence (). Transfer matrix calculations agree with the experiment very well and the calculation by partial waves shows the highly controllable absorbing behaviors are the result of interplay between the R6G molecular absorption and FP resonances in the PVAR6G-Ag cavity. Furthermore, based on this configuration, we theoretically designed a new resonant absorbing thin-film system, whose dielectric coating is densely doped with ATTO 594, i.e. dyes with different absorption band from R6G. It turns out such a new absorbing system with a relatively small coating thickness () shows high absorption magnitudes (>95%) in a very broad incident angle range at the spectral position of ATTO 594 molecular absorption, suggesting our thin-film system can be used to flexibly tune the spectral and directional range of absorption by doping with proper absorbing media and setting appropriate excitation conditions of FP resonances.
2. Experiments and simulations
In our experiments, aqueous PVA solutions doped (or undoped) with R6G molecules were spin-coated on optically thick Ag and glass substrates. The Ag substrates were prepared by thermally evaporating 130nm Ag on smooth silicon wafers. The dopant concentration CR6G can be tuned by adjusting the quantity of R6G in the PVA solutions and the thickness d of the PVAR6G coatings is controlled by changing either the spinning speed or the density of PVA in water solution. The same PVAR6G film and the bare PVA film with the same thickness were coated on a glass substrate and a Ag substrate respectively, as reference samples. The angle-resolved transmission (T) and reflection (R) spectra with a resolution of were acquired on these samples with different incident angles from ( in the case of reflection) to using a custom setup as reported elsewhere . The samples were illuminated by white light from a tungsten lamp with a collimated beam of ~1 mm in diameter. Spectra were acquired using s- and p-polarized light, the electric field vectors of which are oriented perpendicular and parallel to the plane of incidence (Left-hand schematic in Fig. 1), respectively. The net absorption spectra were acquired by or in the case of Ag substrates (no transmission measured for the opaque Ag samples).
For simulations of the spectra, the dielectric constant of Ag was taken from the literature  and the wavelength dependent dielectric constant of PVAR6G with different R6G concentrations was described by a sum of Lorentzian functions ,18]. Applying the refractive indices retrieved by Eq. (1), the transfer matrix method  was used to calculate the absorption spectra of the dyed PVA coatings on Ag and glass substrates over a wide incident angle range from to with an angle resolution of , and the partial wave method was used to model phasor diagrams. These calculations were used for comparison with experimental results.
3. Results and discussions
Turning to the details of the results, the experimentally acquired absorption spectra of the PVAR6G-Ag sample (solid thick line) and two reference samples, i.e. PVA coatings on Ag substrates (PVA-Ag, solid thin line) and PVAR6G coatings on glass substrates (PVAR6G-glass, dashed thin line), under oblique s-polarized incidence () are shown in Fig. 1(a). The thickness of all PVA coatings (dyed and undyed) is nm and the R6G concentration for PVAR6G coatings is CR6G = 0.5 mM. As a result of molecular absorption of R6G, the absorption spectrum of the PVAR6G-glass sample peaks at 532 nm with a maximum value of 7% under s-polarized illumination. While in the absence of R6G molecules, the PVA-Ag sample with the same thick coating also shows an absorption band with a maximum value of 18% at 541 nm, which is induced by FP resonances  supported in PVA-Ag cavity (Note the PVA coating has a quarter-wave thickness d ≈λ/4n). In other words, both mechanisms, i.e. molecular absorption and FP resonances, can give rise to the light absorption in thin-film structures but with small magnitudes. This is because in the PVAR6G-glass sample the R6G doping concentration is very low, and in the case of PVA-Ag sample the only lossy element is Ag substrates. However, when coated on Ag substrates, the PVAR6G film with the same thickness can absorb 85% of s-polarized incidence at nm, indicating under the same illumination conditions the light absorption in the PVAR6G-Ag sample is substantially enhanced as compared to reference samples (~13-fold, 7% to 85% with respect to the PVAR6G-glass sample; and ~4.7-fold, 18% to 85% with respect to the PVA-Ag sample). On the other hand, if under p-polarized illumination with [Fig. 1(b)], the absorption spectrum of the PVAR6G-glass sample holds a maximum value of 8% at nm, similar to the absorption under s-polarized illumination. However, the p-polarized absorption spectrum of the PVA-Ag sample demonstrates no signs of resonances. In addition, the p-polarized absorption spectrum [solid thin line in Fig. 1(b)] of the PVAR6G-Ag sample does not differ significantly from that of the PVAR6G-glass sample. Comparing the results in Figs. 1(a) and 1(b) we can easily find the relevance between light absorption in PVAR6G coatings on Ag substrates and FP resonances, i.e. the light absorption can only be significantly enhanced in the presence of FP resonances. The simulation spectra [Figs. 2(c) and 2(d)] demonstrate similar results but with more pronounced features. For example, under similar conditions (nm,, and ΔεR6G = 0.005), the simulated difference between s- and p-polarized absorption of PVAR6G-Ag samples is larger than that in the experimental case. In particular, according to modelling, the absorption of the PVAR6G-Ag sample for s-polarization can reach a value of 100%, while the maximum value of absorption for p-polarization is only 2.5%. Undoubtedly, together with the Ag substrate, the lightly doped PVAR6G coatings provide a special dielectric environment that can give rise to the highly enhanced absorption for s-polarization and the unchanged absorption for p-polarization.
The absorption of PVAR6G-Ag samples also exhibit very strong dependence on incident angles θ. Figures 2(a) and 2(b) experimentally demonstrates the absorption spectra of a PVAR6G coating (nm, mM) on a Ag substrate as a function of θ. The absorption at nearly normal incidence () for both s- and p-polarization shows a maximum at nm with a value of 25%. The absorption maximum for s-polarization shows an exponential increase with more oblique illumination, reaching as high as 85% at , whereas the absorption maximum for p-polarization [Fig. 2(b)] undergoes a damping towards higher incident angles, finally reducing down to 8% at . The simulated spectra [Figs. 3(c) and 3(d)] demonstrate exactly the same trend. In addition, we note that the spectral width () of the absorption band is always less than 50 nm irrespective of θ, which is apparently inherited from the molecular absorption of R6G and is much narrower than those [10–12] reported in the ultra-thin resonant absorbing system using materials with intrinsic absorption as coatings. It is well known that light absorption modulated by resonant-cavity effects is very sensitive to the change of incident angles . The strong dependence on incident angles θ suggests that the highly enhanced, polarized and directional absorption of the PVAR6G-Ag sample is the result of interplay between R6G molecular absorption and FP resonances.
To further explore the mechanism resulting in the unique absorbing behaviors of such a thin-film system, we expanded the reflection coefficients of PVAR6G coatings on Ag and glass substrates for s- and p-polarization to partial waves  to getFig. 3(a), the light reflection from such a 2-layer system (Ag or glass substrates) can be viewed as a multiple-reflection process, where the final reflectivity is the accumulation of all partial wave reflections. These partial waves have complex magnitudes, thus allowing us to plot them as vectors in a complex-plane phasor diagram [Fig. 3(a)].
Specifically, Fig. 3(a) shows the reflection coefficients of PVAR6G coatings (nm and ΔεR6G = 0.005) on glass or Ag substrates under highly oblique incidence () at nm (absorption maximum). All partial waves begin at the origin (0,0), then move away. It is noted that the first phasor of s-polarization is the same for both glass and Ag samples corresponding to reflection at the air-PVAR6G interface. The phasor trajectory for the glass sample quickly converges to a position (−0.88, 0.0084) in the vicinity of 100% reflectance [green dashed line in Fig. 3(a)], indicating a low absorption at this wavelength, while the trajectory for s-polarization on the Ag sample (black line) moves to high reflection () in the first place, then makes a sharp turn (), and finally returns back to the origin. The sharp turn of the trajectory is the result of adding , meaning light experiences a near-zero phase shift  when reflected from the Ag substrate back into the PVAR6G coating followed by consecutive reflections taking place between air-PVAR6G and PVAR6G-Ag interfaces. As the result, the accumulated phase shifts, i.e. the complex sum of the secondary partial waves, lead to partial or total cancelation of , hence suppressing reflected waves to air. In the case of p-polarization [red line in Fig. 3(a)], the trajectory keeps moving towards high reflection and finally terminates at (−0.95, 0.085) which is close to the 100% reflectance, caused by constructive interference between reflected waves from interfaces on both sides of the PVAR6G coating.
The physical nature of this absorption process can be further revealed by assuming the Ag substrate acts only as a reflecting mirror without contribution to phase shifts. This effect can be mimicked by doubling the thickness of the PVAR6G film with air being its superstrate and substrate, as illustrated in the right-hand schematic in Fig. 3. The simulated s- and p-polarized T/R/A spectra of the 2d thick PVAR6G film under highly oblique illumination () are shown in Figs. 3(b) and 3(c), respectively. Regardless of the polarizations, both T (dashed line) and R (dotted line) spectra show a clear resonance feature at 425 nm, that is the result of FP resonance, which can also be found in the absorption spectra (solid line), though the FP resonance feature in s-polarized absorption spectrum is narrower and stronger than that in the p-polarized one. Most importantly, there is a ~110 nm spectral difference between the FP resonance (425 nm) and the molecular absorption of R6G at 532 nm, indicating the FP resonance fails to couple with the molecular absorption if no phase shifts occur at the PVAR6G-Ag interface.
Due to the low concentration of R6G molecules, the PVAR6G coating is just a weakly absorbing layer as compared to the Ag substrate (specifically, according to Eq. (1), the imaginary part k of refractive index of Ag is as ~120 times higher than that of lightly doped PVAR6G, CR6G = 0.5 mM, at nm) and it may be reasonable to expect that most of the incident light is absorbed by the Ag substrate. However, this is not the case; most of the light is actually absorbed in the PVAR6G, highlighting this effect is truly an enhancement of the absorption in a low-absorbance layer rather than being enhanced due to coupling into a strong absorber. Illustrating this, we calculated the fraction of light being absorbed in each layer of the thin-film structure using transfer matrix method. The calculation shows that if the PVAR6G coating on a Ag substrate is illuminated with s-polarized oblique incidence (), at the absorption maximum, 91% of incidence is absorbed in the PVAR6G coating, while only 9% is absorbed in Ag substrate.
Previous studies [10–12,22–24] show that both thickness and optical constants of the coating can influence the absorbing behaviors of the resonant absorbing thin-film system. As mentioned before, the relatively easy fabrication process in our experiment enables the convenient adjustment of the thickness and the doping concentration for the polymer coatings, thus allowing us to flexibly engineer the absorbing behavior of the thin-film system. Here we show several examples that demonstrate the manipulation of absorption based on parameter adjustment. Figure 4 shows the experimental and simulated absorption spectra for PVAR6G coatings with different film thickness and R6G concentration under s- and p-polarized illumination. In the case of nm and CR6G = 1 mM, the experimental results [Fig. 4(a)] show that, at , the magnitude of absorption maximum for p-polarized illumination can be as high as 86% while the absorption for s-polarization at the same spectral position is only 7%, reversing the polarization dependence compared with the previous example [Figs. 1(a)-1(d)]. If we significantly increase the R6G concentration, e.g. up to CR6G = 13.6 mM [Fig. 4(c)], the high absorption magnitude (>90%) show up at nearly normal incidence () for both polarizations. In this case, the thickness of PVAR6G coatings can be reduced down to nm (~λ/8n), manifesting that coatings made of highly dense absorbing media contribute to the thickness reduction in the resonant absorbing thin-film system, as reported in earlier studies [10–12]. However, as pointed out in , we also note that in our experiment increasing the doping concentration doesn’t always result in a thickness reduction. More details will be explored and reported elsewhere. The simulations [Figs. 4(b) and 4(d)] agree very well with the experimental results in these cases, in which the adjusted parameters completely alter the excitation conditions of FP resonances in the PVAR6G coating, causing the modification of absorption behaviors.
The structural simplicity of this thin-film system allows us to easily predict its optical behaviors as well as facilitates the design of new coatings. For examples, in Fig. 5, we demonstrate the calculated absorption spectra of the PVA coating doped with a fluorescent dye ATTO 594 on a Ag substrate. The ATTO 594 has an absorption maximum at nm [Fig. 5(a)]. In this case, we increased the transitional strength to ΔεATTO594 = 0.5, which corresponds to a high doping concentration (~50mM), and this concentration should be achievable due to the excellent water solubility of ATTO 594 dyes . As the result, the dyed PVA coating with only 30 nm thickness () on a Ag substrate can give >95% absorption at 594 nm over a wide incident angle range [0 - 60o for s-polarized illumination as shown in Fig. 5(b), while 0 - 80o for p-polarized illumination as shown in Fig. 5(c)]. Additionally, similar to the case of R6G doping, the ATTO 594 doped PVA coatings on Ag substrates show an absorption band with spectral width less than 50 nm (at the full angle range for s-polarization), indicating the absorbing-media doped polymer system allows for a better absorption spectral resolution than those [10–12] using materials with intrinsic absorption as the coatings. In a word, adjusting the excitation conditions of FP resonances and selecting the proper absorbing dopants enables flexibly regulating the spectral and directional range of the light absorption in such a thin-film system.
In conclusion, we have reported a thin-film system consisting of absorbing-media doped dielectric coatings on reflecting surfaces, the spectral and directional absorption behavior of which can be flexibly manipulated by properly setting the system parameters. Using PVA coatings doped with R6G on opaque Ag substrates as a test platform, we have experimentally and theoretically examined the manipulation of light absorption. Particularly, with proper doping concentration and coating thickness, the PVAR6G coatings on Ag substrates can show an enhanced, polarized and very directional light absorption. The directionality and polarization dependence of the absorption in this thin-film system can be significantly altered by further adjusting the system parameters. In addition, we theoretically proved that the spectral position of absorption can also be regulated by doping proper absorbing-media into our thin-film system. The highly tunable light absorption behavior is the result of coupling between molecular absorption and FP resonances in dielectric coatings. Therefore, by adjusting the excitation conditions of FP resonances and choosing the appropriate dopants, we can manipulate light absorption using such a thin-film system. In addition, the better absorbing performance given by the simulations suggests that better structural quality, e.g. more precise thickness and doping concentration, would further improve the capability of manipulating light absorption.
The high degree of flexibility makes such a thin-film system suitable for many applications, e.g. light absorbers with tunable spectral and directional ranges; thin-film polarizers if coupled with a prism . Additionally, if embedded with proper fluorophores and plasmonic elements, the dye-doped dielectric coatings on reflecting surfaces can be a good test ground for light emitting devices [16,22,28,29] and plasmonic nanolasers [30,31].
The authors would like to acknowledge Mr. Levi Bourke and Mr. Xingxing Chen for discussions. This work was financially supported by New Zealand’s Marsden Fund through contract UOO-1214, the National Natural Science Foundation of China (grants 61275030, 61205030, and 61235007), the Opened Fund of State Key Laboratory of Advanced Optical Communication Systems and Networks, the Swedish Foundation for Strategic Research (SSF) and the Swedish Research Council (VR).
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