Electromagnetic fields which carry chirality – in their simplest form, left- and right-circularly polarized plane waves – deserve huge interest as they interact with matter chirality, enabling for instance to discriminate enantiomers in chemistry, which are ultimately connected with key features of living organisms. Indeed, many biomolecules have a specific handedness (homochirality), and it is not yet clear why nature has decided to go in that precise direction . From a more application-oriented point of view, the pharmaceutical industry constantly seeks for effective methods to discriminate stereoisomers, an application where chiral light-matter interaction could prove useful.
To date, the most common technique to prepare and analyze chiral light is to employ birefringent plates and linear polarizers that convert light to and from linear polarization, as the technology of direct sources and detectors of chiral light is still in its infancy , , , . Last advances in nanotechnology are however revolutionizing chiral optical devices , , , , , , , , , , , . Far- and near-field chiral electromagnetic responses have been indeed observed in a variety of artificially structured systems, where the shape of the machined elements must have a three-dimensional character if geometric and electromagnetic chirality has to be attained in its most rigorous form, because of the requirement of the absence of any mirror symmetry plane , , , . Several proposed structures hence rely on volumetric fabrication techniques, which however suffer from either scarce throughput or limited flexibility. Thus, less demanding fabrication technologies – i.e. planar technologies – may also be employed, as witnessed by some reports , , . For instance, a dielectric film with a non-centrosymmetric partially etched planar pattern was proposed as a simple gateway towards strong chiro-optical phenomena . Indeed, there is a wide interest in developing subwavelength-patterned high-index dielectrics, both for applications and for fundamental research: from one side, they enable the synthesis of flat lenses, polarimeters, spectrometers, nonlinear components and computer-generated holograms , , , , , , , ; from another side, they exhibit a variety of intriguing phenomena such as Fano lineshapes, perfect forward scattering, geometric phase effects, and bound states in the continuum resonances , , , , , , .
Most of these effects arise from complex electromagnetic behaviors whose essence can be grasped understanding the interplay between localized and delocalized resonances. In other words, these systems often rely on the co-presence, and on the competition, between guided wave phenomena and antenna-like responses, as highlighted by the prototypical transition between the guided mode filter regime, the photonic crystal regime, and the independent-particle Mie-scattering regime . In this work we have reported on the observation of photonic bands in a chiral metasurface, highlighting that the chiral response shows fingerprints of both guided wave and locally resonant phenomena. The object under investigation is the simplest conceivable 2d-chiral dielectric metasurface: a slab perforated with L-shaped holes . In general, a 2d-chiral object is defined as follows: given the family П of planes perpendicular to a given plane πz (the subscript z refers to the fact that πz is usually chosen to be a constant-z plane), an object is 2d-chiral if it cannot be superimposed with itself after a mirror reflection for any plane belonging to П. A 2d-chiral object is also 3d-chiral if it cannot be superimposed with itself after a mirror reflection for any plane parallel to πz. An object that is 2d-chiral but not 3d-chiral has a mirror symmetry plane of the type πz. 2d-chiral objects may have vanishing or finite thickness along z. Our metasurface has a finite thickness; this property allows it to implement interesting chiral electromagnetic features such as chiro-optical far-field response at normal incidence . To our knowledge, our work is the first report of a 2d-chiral, 3d-achiral, dielectric metasurface; moreover, it is the first report of angularly-resolved spectroscopy of 2d-chiral, 3d-achiral metasurfaces. The metasurface under analysis also exhibits superchiral near-fields with incident unpolarized light. The response at normal incidence will be analyzed by means of a singular-value decomposition approach that reveals the operational capabilities of any 2d-chiral photonic device. In addition, we have shown that the extremely simple L-shaped structure is also a minimal one: by tuning a small number of geometrical parameters, it is possible to access a very wide set of the transmission matrices characteristic of a general 2d-chiral metasurface. In this sense, we approached the solution of the inverse design problem of 2d-chiral metasurfaces .
2 Chiral response at normal incidence
The metasurface under investigation is illustrated in Figure 1A. It consists of a 220 nm thick gallium arsenide membrane, patterned with L-shaped holes arranged over a square lattice. The geometric parameters of the holes are reported in Figure 1B, superimposed to the scanning electron microscope (SEM) image of a fabricated sample. While in forthcoming analysis the parameters a, f1…f4 are allowed to vary, the first part of the article deals with a specific choice of parameters, which correspond indeed to the SEM image. In detail, we have a=1134 nm, f1=0.76, f2=0.58, f3=f4=0.327. The sample fabrication process, whose details are reported in the Supplementary Materials, allows to obtain a frame-supported membrane, freely accessible from both sides to perform optical measurements with a moderately focused (≈50 μm spot) near-infrared (1–2 μm wavelength) beam. The beam originates from a supercontinuum source, filtered by means of an acousto-optic tunable filter yielding a spectral bandwidth of ≈2 nm. The system is controlled through an automated software developed in C++. The polarization state is prepared with a Glan-Taylor polarizer followed by a λ/4 superachromatic waveplate; no polarization analysis was performed on the beam transmitted after the sample. The sample was mounted on a rotating support that enables to measure the angularly-resolved transmittance, as illustrated in Figure 1A. A first set of measurements has been collected at normal incidence (α=0). The experimental data are reported in Figure 1C, and are compared with the outcome of a numeric model (rigorous coupled wave analysis, RCWA – see the Supplementary Materials for details). The spectra consist of a series of quite narrow dips, some of them having different shapes and depths depending on the polarization state of the incident light. From the transmittances for left (right) circularly polarized light, respectively
The data reported so far indicate that a 2d-chiral metasurface with a very simple design exhibits strong circular polarization differential transmittance in a narrow band close to the optical telecommunication window. It should be emphasized that the observed CPDT is not accompanied by absorption; rather, it originates from a redistribution of the incident energy among the transmitted and reflected beams (no diffraction is present, thanks to the subwavelength dimensions of the pattern). In this sense, the presented object is rather a beam splitter sensitive to the circular polarization, and the observed CPDT is inherently different from the absorption circular dichroism (or circular diattenuation, CD), properly occurring, for instance, in isotropic solutions of chiral molecules. However, from an observer point of view who can only access the transmitted beam, CPDT is indistinguishable from CD. Hence, our metasurface can be employed to mimick the CD of target objects, such as naturally occurring molecules.
3 Chiral response at oblique incidence and band structure
In the transmission spectra reported in Figure 1, a rich structure can be noticed, whose physical origin deserves attention per se and in view of applications. In order to get further insight into the nature of the resonances leading to CPDT peaks, a powerful method is to measure the metasurface transmittance at different angles of incidence (i.e. for different orientations of the incident wavevector). Angularly-resolved measurements have been recently employed to reveal special features of polarization phenomena like magneto-optic effects in quasi-ordered structures  and asymmetric transmission in low-symmetry plasmonic hole arrays . With angularly-resolved measurements, one may distinguish between dispersive and non-dispersive resonances, which originate from different physical phenomena: guided mode resonances and antenna-like resonances, respectively. In Figure 2A and B, we have reported the CPDT mapped in the frequency-wavevector space for our metasurface. Here, the in-plane wavevector is the projection of the incident light wavevector on the metasurface plane. In the experiment only positive angles have been studied, while the model data are available for both positive and negative angles. The good matching between experimental and model data allows to acquire confidence in the model as a whole, which shows interesting features. First, strongly dispersive bands, with positive and negative slope, suggest that the metasurface optical response has important contributions from guided mode resonances (also known as quasi-guided modes); this picture is supported by an empty-lattice band-folding model illustrated in the Supplementary Material. These resonances are responsible for the narrow dips observed in the transmission spectra and for the sharp oscillations in the CPDT spectra.
Second, by comparing the CPDT observed at opposite angles, it can be noticed that in the region at the center of the map (−20°<α<20°, 190 THz<ν<210 THz) the CPDT is not symmetric with respect to the exchange α↔−α. The effect can be better noticed looking at the curves in Figure 2C, where the CPDT as a function of the incidence angle has been reported for three fixed values of light frequency (labels i–iii in the figure). We attribute this effect to the presence of a spectrally and angularly broad chiral resonance, which we referred to as L-hole resonance. This resonance has a behavior that contrasts in two ways that of the guided mode resonances: first, the broad angular response of the L-hole resonance suggests that it has a spatially localized nature, as opposed to the delocalized, traveling-wave nature of the guided mode resonances (which instead follow a dispersive energy-wavevector curve). Second, the L-hole resonance induces the property CPDT(α)≠−CPDT(−α) (observed in the central region of the map, around point A in Figure 2B), while the guided-mode resonances display CPDT(α)=−CPDT(−α) (dispersive bands, point B).
Seeking for further confirmation of this view, we have calculated (see the Supplementary Materials for details) the optical near fields of the metasurface unit cell, upon the illumination conditions that excite the two types of resonances. The data are plotted in Figure 2D, where both the energy density (color map) and the Poynting vector (arrows) are reported. While the energy density distributions do not show remarkable behaviors, the Poynting vector field shows interesting properties that are consistent with the picture sketched above. When the metasurface is illuminated with the energy-wavevector pair labeled as “A” in Figure 2B, the Poynting vector in the unit cell has a strongly inhomogeneous distribution: for left polarized excitation, it even “winds up” around the L-shaped hole. For right-polarized excitation the Poynting vector has a different, yet irregular, distribution. We have interpreted these irregular distributions as arising from the L-hole resonance: a localized resonance dominated by multipoles, similar to the reports of . On the contrary, when the metasurface is illuminated with the energy-wavevector pair labeled “B”, the Poynting vector shows much more regular distributions, typical of a guided mode that propagates parallel to the slab. Noticeably, the direction of the Poynting vector is opposite with respect to that of the in-plane wavevector, consistently with the negative dispersion of the photonic band where point B lies on.
4 Superchiral near-fields
Besides far-field angularly-dispersive chiro-optic response, which is of interest – for instance – for filters, holograms, and multiplexers, near-field optical chirality plays a fundamental role in the interaction with chiral matter. First relegated to the role of pure mathematical curiosity, electromagnetic (e.m.) chirality is now regarded as a crucial quantity to be taken care of when the optical detection of chiral molecules is under investigation , , , , . The ability of a photonic structure to enhance the e.m. chirality is quantified by the normalized e.m. chirality, defined as
5 Singular value decomposition analysis of the metasurface operation
We will now focus back to the far-field metasurface response, in the attempt to understand in a comprehensive way its amplitude, phase, and polarization response. This analysis relies on the properties of the T-matrix (transmission matrix, or Jones matrix), where the device response concerning light transmission is completely encoded.1 The T-matrix can be written over different bases; the most common choices being that of linearly or circularly polarized waves. The following Section identifies the T-matrix written in these bases as TL and TC, respectively. For instance, TC operates over Jones vectors whose elements are the right- and left-handed circularly polarized components of incident/transmitted light: t=TCi, with (t=tR,tL)and i=(iR,iL).
The form of T, and hence the possible operations that a metasurface can implement, are dictated by geometrical symmetry properties of the pattern lattice and unit cell. For instance, the L-shaped hole structure belongs to the more general category of objects that have a single mirror symmetry plane πz (see Section 1); these objects are usually said to belong to the Mx,y symmetry class. Under normal incidence, the TL matrix of such metasurface is symmetric:
Algebraic manipulations (see the Supplementary Material) leveraging on the symmetry of TL imply that the unitary matrices V and W appearing in the SVD of TC, i.e. TC=VΣW†, must fulfil
We now pay attention to understand the meaning of the parameters entering the matrices introduced above. To this end one should explicitly describe the operation of TC over a generic incident field vector i. By decomposing it over the basis defined by the columns w(1,2) of the unitary matrix W, one has
To complete the picture one should also consider the phase response of the metasurface, which is described by the parameters
The algebraic and parametric analysis performed so far is valid at each individual wavelength. More information can however be obtained by studying the spectral dispersion of the SVD parameters, whose frequency-dependent response can be eventually correlated with quantities of more direct experimental access. In Figure 5A we have plotted the calculated spectral dispersion of the singular values for the L-shaped hole array described in the previous sections. The data follow from numerical SVD of the Jones matrix calculated from RCWA (see Supplementary Material); an experimental measurement of the SVD parameters is possible, in principle, relying on Müller-Jones polarimetry (exception made for the determination of ϕ that would require interferometry). In the analysis of Figure 5A singular values are not sorted in decreasing order, rather, they are sorted such as their trend with respect to the wavelength is smooth. The spectrum reveals a structure with narrow dips, mostly occurring in pairs, which stand out of a background where both σ’s are close to 1 (for λ<1.45 μm) or where one σ is close to 1 and the other is close to 0.5 (λ>1.45 μm). Noticeably, these dips occur at the same wavelengths where the spectra of transmission, circular polarization differential transmittance, and near-field chirality also show peaks or dips. It is also interesting to notice that the dips reach zero at those wavelengths, the metasurface does not transmit the radiation which is incident with the polarization dictated by the corresponding right singular vector. Figure 5B reports the spectral dependence of the third Stokes parameter of the right singular vectors. Here it can be noticed that the
6 Inverse problem and metasurface minimality
Provided with the SVD formalism, and motivated by the conclusions of the previous Section, an intriguing question is whether it is possible to solve the inverse problem: given a target metasurface function (i.e. a target T matrix, or better a set of target parameters σ1,2,
In conclusion we have reported the observation of various chiro-optical phenomena occurring in a 2d-chiral patterned dielectric slab, i.e. in a patterned slab that exhibits a single mirror symmetry plane parallel to the slab itself. Circular polarization differential transmittance is arranged in dispersive bands, and shows the fingerprints of localized and delocalized photonic resonances. It is in particular the effect of a localized resonance that induces an intrinsic chiral response on the metasurface, with consequences on the near-field chirality that shows superchirality and other anomalous behaviors. Relying on the singular value analysis of the transmission matrix, we identified the key parameters describing in full the operation of a 2d-chiral slab. We also noticed connections between the singular value spectrum and the above cited phenomena. Finally, we showed that the L-shaped hole structure, i.e. the most intuitive 2d-chiral pattern, is also a minimal one, as it allows to implement arbitrary transmission matrices with a minimal number of parameters. This result about inverse design might open the way towards advanced space-variant metasurfaces that exploit in full explore the phase, amplitude, and polarization degrees of freedom of light.
We like to acknowledge Alberto Bordin, who participated in an early stage of the project, Stefano Luin (Scuola Normale Superiore, Pisa) for the support on data fitting, Francesca Bontempi (Scuola Superiore Sant’Anna, Pisa) for ellipsometric measurements of the dielectric thin film, Sara Nocentini and Lorenzo Pattelli (LENS, Firenze) for their useful discussions and precious support for the spectroscopic measurements. This work was in part supported by the European Commission through the project PHENOMEN (H2020-EU-713450).
Author contributions: S.Z. has conceived the research and performed the electromagnetic simulations and the SVD analysis. S.Z. has fabricated the metasurface, from a thin film grown epitaxially by G.B. The spectroscopic measurements have been performed by S.Z. with the support of G.M. and F.R. The data have been interpreted by all the authors. The manuscript has been written by S.Z., with comments by all the authors.
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The online version of this article offers supplementary material (https://doi.org/10.1515/nanoph-2019-0321).
We recall that we are limiting ourselves to the regime where the wavelength is larger than the pattern periodicity, i.e. to the metasurface regime, where no diffraction takes place.