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Enhanced light absorption in all-polymer biomimetic photonic structures by near-zero-index organic matter

Natural photosynthetic photonic nanostructures can show sophisticated light matter-interactions including enhanced light absorption by slow light even for highly pigmented systems. Beyond fundamental biology aspects these natural nanostructures are very attractive as blueprints for advanced photonic devices. But the soft-matter biomimetic implementations of such nanostructures is challenging due to the low refractive index contrast of most organic photonic structures. Excitonic organic material with near zero index (NZI) optical properties allow overcoming these bottlenecks. Here we demonstrate that the combination of NZI thin films into photonic multilayers like the ones found in nature enables broadband tuneable strong reflectance as well as slow light absorption enhancement and tailored photoluminescence properties in the full VIS spectrum. Moreover, it is shown that this complex optical response is tuneable, paving the way towards the development of active devices based on all polymer and near zero index materials photonic structures.


Introduction
The control over light absorption is crucial for photosynthesis, the process by which light energy is transformed into chemical energy by phototrophic organisms. However, the lack of detailed optical descriptions for such complex natural photonic systems has made it difficult to address the photonic phenomena in photosynthetic membranes 1 . Natural photonic structures provide a mechanism by which higher plants and other living organisms are able manipulate light at the nanoscale beyond the intrinsic properties of the organic compounds. It was recently discovered that some higher plants present sophisticated hierarchical photonic multilayer structures in their photosynthetic membranes (thylakoids), known as iridoplasts, that can serve to adapt light absorption capabilities to the spectral distribution available in their environment 2 . Recent developments have demonstrated that the narrowband absorption spectrum of the pigments in photosynthetic membranes is key to achieving the near unity light-harvesting quantum efficiency 3 of photosynthesis. In addition, iridoplasts present a photonic crystal with a hierarchical distribution of thylakoids, providing simultaneously and in the same photosynthetic system photonic modes and narrow absorption bands.
In natural systems the narrow absorption bands of thylakoids are provided by molecular aggregates embedded within the photosynthetic light harvesting apparatus 4 . In order to mimic the excitonic and photonic properties of photosynthetic membranes it is necessary to realize hierarchical photonic structures with similar strong narrow absorption bands. Replicating these structures with soft matter materials with similar optical properties has not yet been achieved. In this context, J-aggregates have recently attracted much attention 5 due to their narrow absorption and emission properties with properties closer to the natural counterpart in photosynthesis than oher non-aggregating dyes. It was recently demonstrated that, with high levels of aggregation, J-aggregates present narrow absorption and emission bands 6,7 that, as we show in this work, correspond to refractive indices that are close to zero Interestingly, the refractive index properties shown by thin films of J-aggregates embedded in polymeric matrices can approach values suitable for Near Zero Index materials (NZI) 8 . NZI can be achieved by materials with a Lorentzian dispersion profile 9 in which their real component of the refractive index ( ) shows values close to zero for particular wavelengths. Their inherent strong dispersion in a narrow spectral range provides these materials with unique features such as strong phase velocity divergences or large electric field enhancements 10,11 making them very attractive in a wide range of applications, particularly as emission control and extreme non-linear interactions 12 . Most realizations of NZI materials make use of metal nanostructures showing strong optical resonances to electric field oscillations through engineered plasmonic properties 9 . These building blocks can be integrated in more complex structures such as forming an effective medium with tunable NZI properties in the visible to infra-red range 8 . Mixing NZI materials with a positive permittivity material (e.g, a dielectric) in a periodic arrangement has been used to form a photonic structure with enhanced NZI properties. This approach has recently been proposed to be capable of providing a super-absorbing close-to-darkest material in simple multilayer structures formed by ITO/TIO pairs 13 . However, NZI implementations with highly processable, low-loss or loss-free and largely tunable materials are still to be developed.
Here we propose and show experimentally that iridoplasts can work as a blueprint for the implementation of a photonic structure in which NZI properties are enhanced by the photonic multilayer design to tailor absorption to wavelengths well off-resonance of the narrow absorber maximum. We do this by doping polymer layers with J-aggregates and similarly forming a Bragg stack as the distribution of photosynthetic pigment in iridoplasts. We demonstrate that the combination of the NZI properties of the J-aggregate layers with the biomimetic photonic structuring allows for enhanced absorption at particular wavelengths. These wavelength can be selected by choosing the appropriate multilayer periodicity, allowing tunability, whilst the NZI material provides photonic bandgap splitting close to the resonance. We also demonstrate that the slow light effects 14 described for iridoplasts in the natural system can be engineered in an all-polymeric NZI system to obtain strong tunable off-resonance absorption which opens the road for the use of all organic NZI materials in several fields. Finally, we show that photoemission of the biomimetic iridoplast is tailored by the photonic dispersion bands of the heterostructure.

a. Theoretical description
Iridoplasts are natural 1D photonic crystals, also known as Distributed Bragg Reflectors (DBR), where the thylakoid membranes containing photosynthetic chromophores are evenly separated by the stroma, a homogenous non-absorbing protein-rich aqueous medium 15 . The distance between thylakoids is appropriate to create a photonic stopband in the visible range. To mimic the iridoplast we consider here a 1D photonic crystal composed of 16 unit cells, each formed by a pair of polymer thin films (i.e. 32 layers in total), as shown in Fig. 1a. One of the polymer thin films is non-doped polystyrene (PS), which will play the role of the stroma, the homogeneous layers of the natural system. PS was selected due to its ease of processing and well established optical properties as a dielectric (Fig.1b). The second constituent of the unit cell is an organic excitonic dye-doped polymer film. We selected the cyanine dye known as TDBC that can be assembled into a J-aggregate form in which the exciton de-localization makes them the most suitable candidates as biomimetic photosynthetic chromophores 16 . Moreover, since TDBC is stable in aqueous solutions, polyvinyl alcohol (PVA) is used as a polymer host-matrix.
J-aggregates are supramolecular structures with a narrow absorption band of around 10 nm in width.
We can therefore consider a generic description of a material doped with J-aggregates as a highly dispersive material modelled by a Lorentz oscillator with a strong exciton transition at the central vacuum frequency 7 : where ω is the working frequency, f is the Lorentz oscillator strength, 0 is the damping rate and ∞ is the permittivity at high energy, a term that considers the dielectric constant of the material hosting the excitonic species. Therefore, for off-resonance frequencies, the dielectric constant tends towards the constant value ∞ and it is strongly dispersive near the resonance. The optical properties of films resulting from embedding TDBC into PVA 7 are well known and can show very large oscillator strengths, as high as = 0.4 which provides a strong dispersive material. Fig. 1b  In this work, we study how the photonic structure modifies the absorption of TDBC-PVA films when they are structured as a DBR, similarly to the natural system 17  Using a Transfer Matrix Method 18 simulation (see Methods for more detail), we calculated the reflectance at normal incidence for a fully undoped DBR (16 pairs of PVA and PS, thickness 53 nm and 90nm, respectively) and a doped DBR with the same structure but using TDBC-PVA with oscillator strength 0 = 0.4 (Fig. 1c). The reflectance of the undoped structure shows a strong reflectance peak that marks the spectral position of the photonic stopband 18 given by the phase-matching condition (supplementary Note II). However, in the case of the doped DBR, two high reflectance bands are seen which can be associated with two different stopbands. We coin the terms + and − to identify the central wavelength of the stopband depending on whether it is in the spectral range below or above respectively of the anomalous dispersion region (550 < < 600 ). These regions are represented in Fig. 1C by the red and blue colours respectively. Moreover, it is also remarkable that, for the − region, the refractive index of TDBC-PVA film is below that one of the PS while, for the + region, the opposite is true. In this example, for the doped DBR, the − the stopband is far from the anomalous dispersion band and, due to the increase in refractive index contrast = − , the absolute reflectance gets enhanced when compared to the undoped case, achieving over 80% reflectance.
However, for the doped DBR in the + stopband, the Bragg reflection is close to the excitonic resonance where absorptance is high which reduces the reflectance peak through damping. By tuning the thickness of the PS layer (hence modifying period ), it is possible to shift the + stopband of the DBR away of the anomalous dispersion band while pushing the − stopband across the excitonic wavelength = 590 . Fig. 1d shows a contour plot of the reflectance at normal incidence for a DBR with 16 layers of 53 nm of TDBC-PVA while the PS thickness is varied from 60 to 200 nm. We observe a clear dependence of the reflectance on the periodicity of the structure, with an anomalous reflection region near the resonance. Interestingly, for periods with 80 < < 180 , the photonic stopband is strongly modified by the highly anomalous dispersion. Under these conditions, the highly dispersive nature of TDBC creates a narrow band of strong refractive index contrast on both sides of the exciton band, allowing therefore simultaneously existence of + and − . This phenomenon is corroborated by solving the dispersion equation (supplementary Note II) of the photonic bands for the 1D photonic crystal considered here, represented by the black line in Fig. 1c. Note that the calculations show that, for periods of 90 < < 160 , a third stopband is obtained near resonance ( * ≈ ). This stopband is not shown in reflectance as a result of the strong damping due to the narrowband absorption of the TDBC-PVA but it plays an important role in absorption as discussed in this work. For comparison, the dispersion relation for an undoped DBR with the same structure was plotted as a dashed red line in Fig. 1c. As expected, the regions where + ≫ or − ≪ the stopband is tuned away from the anomalous dispersion band and therefore the photonic properties will approximate those of a DBR formed by two non-absorbing homogenous media with a refractive index contrast ∆ = − .
Moreover, the second order stopband is also observed as a narrowband reflectance at low wavelengths and high . Interestingly, it was recently proposed that natural iridoplasts might undergo self-tuning of the photonic properties to avoid photodamage. In this process, the period of the 1D photonic structures is modified to de-tune the photonic stopband from the absorption band of photosynthetic pigments 17 . The artificial iridoplast described above could be considered a simplified scenario of the natural system where the use of an NZI building block is enabling to i) experimentally tune the photonic properties, ii) study light-matter interaction enhancement at different layers of the material and iii) shift two stopbands simultaneously by simply varying the period of the system. Moreover, it is remarkable the higher refractive index contrast caused by the doping allows for a stronger reflection (~90%) than in the non-

b. Experimental
To demonstrate the feasibility of the NZI biomimetic iridoplast described above, a methodology was developed for the fabrication of excitonic all-organic multilayers at different periods (see methods).  To analyse the absorptance of the biomimetic iridoplast, we measured reflectance ( ) and transmission ( ) over the same area of the samples. The absorptance at normal incidence can be calculated by = 1 − − assuming that scattering in other directions is negligible. To compare the absorptance quantitatively, we use the wavelength-dependent absorptance enhancement coefficient ( ( )) used in other studies 2,17 : ( ) 1,2,3… = ( ) 1,2,3… / 0 ( ), (2) where is the absorptance at normal incidence and the subscript numbers represent the samples with periods 0 , 1 , 2 , etc. As a control for the absorptance of effective NZI material without photonic stopband, we consider the reference sample. The parameter is a powerful tool to understand the modifications that a particular photonic environment might induce over absorptance properties of the bulk materials. In the case of the NZI biomimetic iridoplast analysed here, it allows for the comparison between the absorptance of a non-structured TDVC-PVA thin film with a DBR containing exactly the same material thickness but distributed within the photonic DBR structure. Under this definition, one could find that a given DBR can present > 1 (enhanced absorptance) at a particular wavelength or < 1 (reduced absorptance).
By the naked eye, the samples fabricated present high uniformity as shown in supplementary Fig. 3. A change in colour with the period between samples is visible not only to the naked eye but also in high magnification images ( Fig. 3 and supplementary Fig. 4). Reflectance measurements at normal incidence corroborate the differences in the reflectivity colour between samples. The theoretical reflectivity corresponds well to the experiments but with a wider bandwidth for + , explaining why the photonic bandgap centre in this range is slightly redder. The analysis of the reflectance over four points on the samples (see supplementary Fig. 5) showed minimal variation in the central wavelength of the photonic bandgap and absolute reflectance. This result highlights the robustness to disorder and homogeneity of the all-polymer excitonic DBRs. Moreover, as the period increases, the + stopband shifts away from the excitonic resonance. When the + stopband is outside the absorption region, the maximum absolute reflectance is achieved, with values close to 90%. In the case of the − stopband, when the period decreases it shifts away from the excitonic resonance in the region where the absorptance is low; therefore the absolute reflectance also reaches values close to 100%. However, as + and − get closer to there is a damping on the absolute reflection associated to the absorptance peak of TDBC. The  Next, the DBR structures were analysed by angle-resolved reflectance, measured by Fourier Image spectroscopy system coupled to a high magnification microscope that allowed us to evaluate the reflectance for incident angles < 48° over areas of a few µm 2 (see methods). Fig. 4 shows the spolarised reflectance spectra as a function of the incident angle for three different DBRs 2 , 4 and 6 and the reference sample (see supplementary Fig. 6 for all samples). The reflectance dependence with the incidence angle allows us to analyse the mode dispersion for both + or − stopbands in a wide spectral range for each period. As can be observed in Fig. 4, all samples with Δ ≠ 0 show a strong blue shift for both + and − , characteristic of the dispersion curves of the photonic bandgap for a DBR 18 .
For short periods, such as 2 , the dispersion curves of the − stop band follows the dispersion behaviour of the undoped DBR but shows a higher reflectance close to 100% even with just 32 polymer layers due to the increase of the refractive index contrast. When the period of the DBR is large enough ( 4−6 ) a third stopband appears with central wavelength * . Interestingly this new stopband only appears for samples with periods where the stopband of the undoped DBR cross the excitonic resonance. For small period samples, both + and * stopbands are degenerated making them indistinguishable. However, as the period increases and the − stopband gets closer to the excitonic resonance, + and * split and the two stopbands become evident as in 4 (Fig. 4). When the period of the DBR is large enough ( 5 and 6 ) − and * start merging, particularly at small angles. For 5 , at angles > 40°, the blueshift of − causes a split between this stopband and * . Moreover, because p polarised reflection is more sensitive to angle and has a smaller full width at half maximum, this split becomes evident at lower angles (see also supplementary Fig. 6). The reflectance of * is barely visible in the experimental reflectance due to the strong narrow band absorption of the TDBC-PVA layers. However, we would like to remark that the third stopband condition is only reached in a narrow wavelength window, where large values of the refractive index of TDBC-PVA are achieved ( ≈ 3, see Fig. 1). That wavelength restriction affects to the angular response of the dispersion curve of this third band with i) modes only supported at small angles (case 4 ) or ii) modes only supported at larger angles ( 5 and 6 ). The experimental data is corroborated by the theoretical dispersion lines calculated numerically according to ref. 18 for each of the stopbands (black lines - Fig. 4).
We have shown that the strong dispersive optical properties of the excitonic molecular material can strongly modify the photonic crystal properties when the biomimetic iridoplast morphologies are considered. Moreover, these strong dispersive materials can achieve NZI properties acting as candidates for the fabrication of exotic absorbing materials with super-absorbing capabilities 8 . For example, Fig.   2 shows that, for a DBR with a response on the NZI wavelength region, the field can be strongly localized in the active TDBC-PVA layers at the edges of the stopband. On the other hand, it has been proposed that iridoplasts in high plants can present photosynthetic tissue structured as DBRs to induce slow light phenomena 2 and produce absorptance enhancements at wavelengths of light available in their environment. It is predicted that slow light is actively tailored within these organelles by modifying the period of the natural DBRs hence allowing for their photonic stopband to be tuned depending on the plant needs 17 . Given the optical properties of the biomimetic NZI iridoplast presented above, it is worth investigating whether we can observe similar phenomena in the artificial counterparts.  To further explore the slow light effects over the NZI organic material, we performed reflection and absorptance analysis for the different DBR periods (represented here as PS thickness variation, ).
On the blue edge of the high energy photonic bandgap ( − ), we observe a pronounced absorptance enhancement for all , reaching values as high as = 1.6. In contrast, on the low energy gap ( + ) this enhancement occurs on the red edge, reaching values of > 2. Absorptance enhancement ( > 1) occurs in these areas since light is concentrated in the absorptive material. This is opposed to what happens in the red edge of − and the blue edge of + where light concentrates in the homogeneous material, reducing light absorptance ( < 1).  Note that the reflectance plot is distinct from Fig. 1c as the refractive index used here was the one obtained from ellipsometry measurements, whilst in Fig. 1c  in Fig. 1c) shows the same behaviour in this region. This therefore reinforces our hypothesis that the absorptance enhancement close to resonance is correlated to the presence of the third stopband of the DBR due to the strong anomalous dispersion of the NZI material. In addition, we have demonstrated that the closer the photonic bandgap is to the excitonic band, the higher the effect of the damping of the dipole oscillator which eventually reduces the photonic strength of the structure. Therefore, it is expected that a redshift of the stopband towards the exciton maximum absorptance will reduce . The opposite happens to the increased values for in the long wavelength band-edge of + since, as the stopband is redshifted by the period increase, it departs from the molecular absorption band.
Despite differences in absolute values, the agreement between theoretical and experimental data is outstanding, validating the results. In , we again see that the photonic bandgap in the longer wavelengths ( + ) has a wider bandwidth in the theory than in the experiments. At + , the absolute values of are very similar even at infra-red wavelengths ( > 700 ). It is worth mentioning that the theoretical narrowband absorptance enhancement shows values of ≈ 1.2 at ≈ 580 . This absorptance enhancement region is related to the the third stopband * not visible in reflectance due to strong narrowband absorption. Also, this spectral region corresponds to the lowest refractive index values of the TDBC-PVA, that is, is the region where the material presents a larger NZI character and therefore the interaction with the third possible stopband becomes more intense. We do not experimentally observe > 1 at those wavelengths. Yet, an increase in γ is observed compared to the bandgap region. The reason for this mismatch is most likely related to the fact that is not possible to produce a perfect slab with the same number of molecules as a reference. In this range, ≈ 1.5 which will induce enhanced absorption even for short periods like the pseudo-slab with  shows an asymmetric spectrum with maximum emission at ≈ 620 nm which is consistent with emission properties of few-molecules layers of TDBC-PVA reported in literature 23 . The slightly stronger asymmetry towards longer wavelengths is most likely due to weak Fabry-Perot optical modes supported by the thick structure. Interestingly, for increasing periods of the Bragg reflector, a continuous reduction of intensity takes place. Such intensity reduction is consistent with the position of the photonic stopbands for each period shown in Fig. 5. Beause the − stopband is redshifted at increasing periods, we will expect that samples 4,5,6 will show − stopband close to the excitation wavelength, producing a strong reflectance at = 550 nm and preventing excitation illumination from reaching the inner TDBC-PVA layers of the structure which reduces the overall intensity of the emission.
At the same time, an increase in period promotes the redshift of + (Fig. 5). Fig. 6 shows the same trend with the maximum emission peak redshifting as the period increases (supplementary Note IX for set up and all samples) and the matching of the stopband with the emission bands leads to a re-shaping of the emission spectrum. This apparent red-shift is the result of the convolution of the TDBC-PVA emission with the photonic stopbands in the range 590 < < 700 nm. As described above (Fig. 3) when the period of the DBR is large enough ( 4−6 ) two stopbands, + and * , can be found in that spectral region reducing the emission in the corresponding spectral range. It is also worth analysing the evolution of the emission maximum intensity upon excitation with different wavelengths. Fig. 6b shows photoluminescence-excitation (PLE) measurements for three samples ( 2,4,6 ) while the contour plot in Fig. 6c shows the results for all the periods considered in this work. These spectra were obtained by fixing the collection at maximum emission for each period and varying the excitation wavelength between 400 and 600 nm. The reference ( 0~0 nm) emission shows a uniform increase of the intensity for excitation wavelengths <520 nm while it decreases for longer excitation wavelengths, as expected from the increased reflectance of the pseudo slab due to the NZI dispersive region of the TDBC-PVA (see Fig. 1b). Overall, the biomimetic iridoplasts show a reduction of the emission intensity in agreement with Fig. 6a. It is worth noting that the periodic structures ( 1−6 ) also show a narrowband spectral region where emission intensity is highly reduced. This reduction in emission matches the position of the photonic stopband for each period ( 2 = 473 nm and 6 = 575 nm in Fig. 6b, see supplementary Fig. 9 for all periods). Fig. 6c shows that the central wavelength of the high energy stopband − at 45° follows the same pattern as the absorptance dip for an incident angle of 45° as shown in Fig. 6d. The reason for this decrease in PLE signal is the match between stopband and excitation wavelength which prevents excitation of inner TDBC-PVA molecules and hence reduces absorptance. These measurements corroborate the measurements in photonic band-edge. This analysis is out of the scope of this work and will be analysed elsewhere.

Conclusions
In conclusion, our work demonstrates that by using photosynthetic photonic systems as inspiration it is possible to tailor light-matter interactions in soft excitonic materials beyond their intrinsic properties.
Our work also shows that J-aggregates are a suitable building block for NZI nanophotonics; these properties can be used to generate new phenomena in photonic structures such as multi-stopband photonic crystals or slow-light-based multi-wavelength absorptive materials with broad spectral separation. Our implementation also supports the hypothesis that the photonic structure of the natural Moreover, more elaborate nanostructuring could add further control over light-matter interaction within the structure, providing therefore a highly processable and low contaminant method for the production of NZI excitonic metamaterials.

Methods
Numerical calculations. Reflectance, transmission and absorptance for all structures were obtained using an inhouse implementation of the Transfer Matrix Method (TMM) 18  highlights the exciton band resonance where absorption is maximum. Red shadowed areahighlights the strong reflection range (553 < < 590) caused by the negative permittivity of the material which provides it with meta-like optical properties that range. Fabry-Perot interference oscillations are visible off-resonance.

Supplementary Note II: Photonic band gap dispersion
Phase matching condition at photonic stopband fulfills: where and represent the wavevectors and and are the real part of the refractive index for the excitonic and the homogeneous medium respectively.
is an integer representing the order of the stopband, which in our case is = 1 since we consider only the first order.