Fabrication and characterization of three-dimensional biomimetic chiral composites

Here we show the fabrication and characterization of a novel class of biomimetic photonic chiral composites inspired by a recent finding in butterfly wing-scales. These three-dimensional networks have cubic symmetry, are fully interconnected, have robust mechanical strength and possess chirality which can be controlled through the composition of multiple chiral networks, providing an excellent platform for developing novel chiral materials. Using direct laser writing we have fabricated different types of chiral composites that can be engineered to form novel photonic devices. We experimentally show strong circular dichroism and compare with numerical simulations to illustrate the high quality of these three-dimensional photonic structures. ©2011 Optical Society of America OCIS codes: (160.5298) Photonic crystals; (160.1585) Chiral media; (160.3918) Metamaterials. References and links 1. T. Kawazoe, K. Kobayashi, and M. 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Introduction
Nature"s ability to self-assemble complex nanostructured materials with superior properties to that of conventional materials, has interested scientists across a range of disciplines [1][2][3][4][5].These biological designs have evolved over the ages to provide materials that are mechanically robust, have useful properties and are adaptable to different environments.Recent discoveries of biological nanostructures have led to the biomimetic engineering of novel nanophotonic devices such as optical nanofountains [1], artificial compound eyes [2] and optical gas sensors [3].
The design of chiral asymmetries within nanophotonics has emerged due to the strong discrimination of circular polarization in light-matter interactions.This unique ability has led to the development of applications such as nanoscale plasmonic motors [6] and ultrasensitive spectroscopy of chiral biomolecules [7].The control of chirality at the nanoscale could also be useful in integrated quantum photonic circuits, where the manipulation of circularly polarized qubits at the micron-scale is required [8].Metamaterials (MMs) have also recently been developed with chiral geometries, demonstrating huge optical activity [9,10], strong circular dichroism [9][10][11][12] and negative refractive indices without requiring a doubly negative medium [9,[13][14][15].Chiral geometries were first introduced to photonic crystals (PCs) with the demonstration that the spiral PC was as a three-dimensional (3D) PC with large complete photonic bandgaps [16].It was then discovered that these spiral-based chiral PCs show strong circular dichroism [11,12] manifesting in the existence of polarization stop bands [17].
However, these chiral nanophotonic designs typically have only uniaxial chirality and are highly anisotropic, greatly limiting their potential applications.The development of novel photonic structures providing complete 3D control of chirality is important for the advancement of photonic devices in a broad range of applications.Recently, the bi-chiral PC [12] was developed which consisted of spirals orientated along all three Cartesian axes forming an interconnected network with both chirality and cubic symmetry.This PC design has the ability to control the chirality by engineering the handedness of the spirals with the handedness of the corners formed by the spiral connections.
Here we demonstrate the fabrication and characterisation of a novel class of 3D photonic microstructures inspired by a recent finding in butterfly wing-scales [18][19][20].These biomimetic photonic chiral composites have cubic symmetry, are fully interconnected, have robust mechanical strength and possess chirality.However, unlike the bi-chiral PC, we control the chirality and hence the photonic properties of these microstructures through the composition of multiple chiral networks that intertwine to form new photonic chiral composites.We experimentally characterize the transmission properties of these structures, demonstrating circular dichroism within different types of chiral composites and compare results with numerical simulations.Finally, we propose the engineering of multiple chiral composites in a single structure to form photonic devices with possible applications such as circularly polarized beam splitters and super prisms.

The single srs network
The 3D microstructures fabricated here are based on the srs-network [21][22][23] that has been observed in butterfly wing scales [18][19][20] such as the Callophrys rubi (Figs.1(a and in zeolites [24].The srs-network (see Figs. 1(c), 1(d) and 1(e)) is a chiral network of cubic I4 1 32 symmetry with three-coordinated nodes that has been theoretically proposed as a PC with large photonic bandgaps [25].The srs-network defines the centers of the two labyrinthine domains separated by the 3 Ia d gyroid minimal surface [26] (Fig. 1(c)); the two domains have opposite handedness, hence the two networks are an achiral pair of interthreaded chiral srs-networks of opposite handedness, with symmetry d Ia3 .This achiral srs/gyroid structure is ubiquitously found in self-assembled soft-matter structures [18,19,21].
Self-assembled copolymers with the d Ia3 double gyroid morphology have shown to possess bandgaps in the ultra-violet and visible frequencies [27,28]; however these structures are achiral.On the contrary, the single srs-network found in the wings of the Callophrys Rubi is geometrically chiral and has recently been theoretically demonstrated to possess strong circular dichroism bands [29].A unique feature of these srs-networks is that they can combine to form complex photonic microstructures with engineered optical properties (Figs.1(f), 1(g) and 1(h)).
Here we employ the direct laser writing (DLW) method (see Section 5 for experimental details) which provides the ability to realize 3D microstructures with arbitrary geometry [12,30]  Scanning electron microscope (SEM) images of a chiral RHD srs-network which is 22 unit cells wide, 4 unit cells high and with a cubic unit cell size of 3 μm are shown in Figs.2(c) and 2(d).The excellent replication of the srs-network geometry illustrates the superior mechanical strength of the structure.The four-screw axis of the srs-network along [100] has been highlighted in blue to illustrate the RHD chirality of the cubic network.In Fig. 2(e) we show experimentally measured transmission spectra of circularly polarized light (see Section 5 for experimental details).The transmission spectra along [100] agree with the recent band structure analysis of chiral gyroid PCs [29] and show good agreement with the numerical simulations given in Section 4 further illustrating the high uniformity of the PC.At wavelengths between 3.25 -3.45 μm there is a circular dichroism band where left circularly polarized (LCP) light has high transmission and right circularly polarized (RCP) light has low transmission.The bandwidth of this circular dichroism stop band is approximately 6% and is comparable with that of the bi-chiral PC [12].
Figure 3 contains scanning electron microscope (SEM) images (Figs.3(a) and 3(b)) of the srs-networks at an oblique angle close to [111] and a comparison with the simulated srsnetwork (Fig. 3(c)), showing good replication of the srs-network design in all three dimensions.However an elongation of the structural features (Fig. 1(b)) along the vertical axis is seen due to the aspherical focal spot used in the DLW method.This engineering asymmetry can be avoided by using a combination of apodization filters [31] and multi-write techniques [32] which is the subject of future work.With a symmetric cross section, the cubic symmetry of the network would be maintained and the optical properties of the PC would be identical in any set of three orthogonal directions, and in particular in all three [100] coordinate axes.It is also interesting to note that the srs-network has a three-screw axis along [111] of opposite handedness to that of the four-screw axis along [100].

Photonic chiral composites
Having demonstrated that these chiral srs-networks are practical designs for fabrication of highly uniform 3D microstructures, we now seek to build up a new set of 3D chiral materials with tailored properties.Geometrically, it is possible to arrange two, three, four or eight likehanded srs-networks into a structure of cubic symmetry [33][34][35].These multiple-networks form what we call chiral composites, new materials based on a combination of chiral constituents.While not observed in spontaneous self-assembly yet, these multiple interthreaded networks exhibit strong circular dichroism [29] and have been realized by DLW here.
We have fabricated two different chiral (Fig. 1(f)) and achiral (Fig. 1(g)) composites, each consisting of two srs-networks.Figure 4(a) shows an SEM image of the achiral composite based on the cubic d Ia3 double srs-network consisting of two srs-nets of opposite chirality, related to the core-shell gyroid phase in copolymers.The experimentally measured transmission spectrum shown in Fig. 4(b) clearly demonstrates that this achiral composite does not provide circular dichroism.In comparison, a chiral composite consisting of two interthreaded like-handed srs nets (Fig. 4(c)) does possess net chirality leading to strong broadband circular dichroism (Fig. 4(d)).These transmission spectra (Figs.3(b) and 3(d)) have good agreement with numerical simulations given in Section 4. Thus, the srs-network makes an excellent building block for the design of chiral composites whose chirality can be controlled, a desirable feature for many applications whose functionality relies on the chiral light-matter interactions.
On another note, it is possible to induce chirality with the RHD and LHD double srsnetwork by changing the filling fraction of one of the srs-networks and breaking the symmetry and thus have I4 1 32 symmetry.This would allow for a continuous variation of the chirality of a c the network and may be of interest for structures where a gradual, continuous variation in chirality is required.To illustrate the possibilities of these chiral composites we have designed a complex 3D photonic structure (see Fig. 1(h)).This conceptual design is based on the srs-network and the fabrication results are shown in Fig. 4(e).This structure contains two srs-networks of opposite handedness that partially overlap, thus creating three distinct RHD (blue), achiral (purple) and LHD (red) regions.Here we have drawn LCP and RCP incident light with blue and red arrows respectively, which in certain quantum optics experiments may represent "ones" and "zeros".At the boundaries of different chiral composite materials we expect interesting reflection properties depending on the polarisation of the incident light and handedness of the two bounding materials.The advantage of designing photonic devices by integrating two composite materials is that one can tailor dispersion and impedance (e.g. by tuning the filling fraction of one the srs-networks) of either side of the boundary to engineer useful functionality.The development of compact circularly polarized beam splitters, or filters, are in principle realizable and are of great interest for the development of integrated quantum optics that use circular polarization for the encoding of qubits.

Numerical characterization
Here we present the results of numerically simulated transmission spectra for RCP (blue) and LCP (red) light through the PCs discussed in Sections 2 and 3. Numerical simulations were performed using the commercial finite element method software (CST Microwave Studio).The numerical simulation assumes periodic boundary conditions laterally and 4 unit cell repetitions along the propagation direction i.e. along [001].The simulated structure takes into account the elongation of the features due to the non-spherical focal spot, assuming an aspect ratio of 3 (i.e. the rods are three times wider in the vertical direction that in the lateral directions).A refractive index of 1.52 is used for the polymer network.The spectra have been filtered by using a moving average filter to account for the finite opening angle used in the experimental setup that causes a broadening and weakening of the observed bandgaps.
Figure 5(a) shows the transmission spectra through a RHD srs-network.As seen with the experimentally measured transmission spectra above, the RHD srs-network causes RCP light to reflect more than LCP light at wavelengths between 3.25 and 3.45 μm.The achiral composite consisting of RHD and LHD srs-networks does not show any circular dichroism (the LCP and RCP curves overlap) (Fig. 5(b)).The RHD chiral composite consisting of two RHD srs-networks shows broadband circular dichroism (Fig. 5(c)).All spectra are in good agreement with the experimental results discussed above, illustrating the high quality of the fabricated structures.It is important to note that the spectra shown in Figs.5(a) and 5(b) both have negligible polarization conversion, i.e. incident RCP (LCP) light is transmitted as almost fully RCP (LCP) light.For these structures with cubic symmetry the polarization conversion is typically less than 0.1% and has a maximum of 3.5% for the single srs network (see Fig. 6(a)) and 1.5% for the chiral 2-srs network (see Fig. 6(b)).However for the chiral 2-srs network the cubic symmetry is broken due to the translation of the second srs-network along [100].This causes strong polarization conversion (see Fig. 6(c)) of around 20% at the centre of the circular dichroism band and up to 50% at shorter wavelengths.This could be avoided by choosing other chiral composites such as the 3-srs and 4-srs [29] networks which possess cubic symmetry and thus should not possess any strong polarisation conversion.However, these more complex chiral composites would require materials that provide greater index contrast to achieve the formation of bandgaps or polarisation stop bands.
Note that whilst the srs-network contains a 3-screw axis of opposite chirality along [111], our numerical simulations have not shown the formation of bandgaps or polarisation stop bands in this direction.

Experimental setup
The fabrication of these 3D networks was achieved by the DLW method.A beam of femtosecond pulses (~150 fs) operating at a wavelength of 580 nm was focused by an oil immersion objective (Olympus, N.A. 1.4, 100X) in the commercial photoresist IP-L (Nanoscribe Gmbh).The 3D networks were written by the 3D translation of the photoresist mounted on a piezoelectric translation stage (P-562, Physik Instrumente).The srs-networks were built starting from the substrate using a layer-by-layer approach to ensure mechanical stability of the network at all times during fabrication.
Experimental characterization of the transmission spectra were performed by using a Thermo Nicolet Fourier-transform infrared spectrometer in conjunction with an infrared microscope (Continuum).Circular polarization analysis was achieved by using a combination of a ColorPol MIR polarizer and a Bernhard Halle achromatic MgF 2 quarter-wave plate.Transmission spectra were normalized relative to the transmission through the silica substrate.A pinhole was used in front of the microscope objective to reduce the full opening angle of incident light to 10° and the sample was mounted such that the light was incident along the [100] axis (in the vertical direction).

Conclusion
In conclusion, we have fabricated and characterized a range of novel biomimetic photonic chiral composites, inspired by a recent finding in butterfly wing-scales.These 3D srsnetworks have cubic symmetry, are geometrically chiral, fully interconnected and have robust mechanical strength providing an excellent platform for the design of chiral metamaterials, chiral PCs, integrated quantum optical chips and ultrasensitive biosensors.We have experimentally and numerically characterized the transmission spectra of these microstructures showing strong circular dichroism (or lack of) within these chiral (achiral) composites.Further engineering of the composition of these networks will lead to novel photonic devices such as circularly polarized beam splitters and superprisms that could be integrated onto an optical chip.
The srs-network is the simplest chiral network; in fact it is the only degree-three network with symmetrically identical vertices and edges [36].Thus the srs-network is suitable important for the scaling down of these complex cubic networks for shorter wavelength operation.Scaling of the unit cell size to achieve active wavelengths in the telecommunications regime of 1.5 µm should be achievable with standard high-resolution DLW methods.

Fig. 1 .
Fig. 1.Chiral composites derived from biomimetic designs.(a) Photograph of Callophrys rubi.(b) SEM image of the chiral srs-network found within the Callophrys rubi.(c) The gyroid minimal surface and its two complementary left handed (LHD) & right-handed (RHD) chiral srs-networks.(d) LHD srs-network.(e) RHD srs-network.(f) Achiral composite consisting of RHD and LHD srs-networks.(g) Chiral composite consisting of two RHD srs-networks.(h) A multifunctional photonic device, designed from a combination of chiral composites.
. The limitations are (1) the minimal feature size given by the diffraction-limited focal spot, and (2) the geometries are constrained to those with sufficient mechanical stability for practical use.A key feature of our structural design shown in Figs.2(a) and 2(b) is that the overall microstructure is in the shape of a pyramid with a flat top, analogous to cleaving the boundaries along the crystallographic planes [100] and [110].This pyramid design, along with the interconnectivity and cubic symmetry of the unit cell make these structures resistant to distortions typically associated with DLW of 3D microstructures.

Fig. 2 .
Fig. 2. Images and transmission spectra of the chiral srs-network.(a) The pyramid-like design of the chiral srs-network from the side view and (b) top view.(c) SEM image of the microstructure possessing a pyramid-like shape to enhance the mechanical strength; the scale bar is 10 μm.(d) A close up view of the same structure showing excellent replication of the srsnetwork topology.The scale bar is 1 μm and a blue arrow shows the direction of the RHD 4screw axis.(e) Experimentally measured transmission spectra of RCP (blue) and LCP (red) light at normal incidence.

Fig. 3 .
Fig. 3. Views of the chiral gyroid srs-network along [111].(a) SEM image of the chiral gyroid srs-network.(b) A close up view showing the asymmetry induced by the aspherical focusing conditions of the DLW method.The scale bars are 10 μm (a) and 1 um (b).(c) View of the underlying srs network model.

Fig. 4 .
Fig. 4. SEM images and transmission spectra of the photonic chiral composites consisting of two srs networks.(a) The achiral composite with blue and red arrows to illustrate the opposite chirality of the two srs-networks, the scale bar is 1 μm.(b) Experimentally measured transmission spectra of RCP (blue) and LCP (red) light through the achiral composite.(c) The chiral composite.Blue arrows illustrate the same chirality of the two srs-networks, the scale bar is 1 μm.(d) Experimentally measured transmission spectra of RCP (blue) and LCP (red) light through the chiral composite.(e) SEM image of a multifunctional chiral microstructure, consisting of LHD and RHD srs-networks partially overlapping to form three distinct regions, the scale bar is 20 μm.

Fig. 5 .
Fig. 5. Simulated transmission spectra for RCP (blue) and LCP (red) light along [100].The unit cell size is 3 μm and the filling fraction of a single network was approximately 15%.(a) Chiral single RHD srs-network.(b) Achiral composite consisting of a RHD and a LHD srs-network.(c) Chiral composite consisting of 2 RHD srs-networks.

Fig. 6 .
Fig. 6.Simulated polarisation conversion spectra for RCP (blue) and LCP (red) light incidence along [100].The unit cell size is 3 μm and the filling fraction of a single network was approximately 15%.(a) Chiral srs-network.(b) Achiral composite consisting of a RHD and a LHD srs-network.(c) Chiral composite consisting of 2 RHD srs-networks, with broken cubic symmetry causing significant polarisation conversion.