Nanoboomerang-based inverse metasurfaces—A promising path towards ultrathin photonic devices for transmission operation

The demand for creating desired states of light using fully integrated nanostructures has led to the development of metasurfaces (MSs), which rely on the interference of light from engineered scatterers and are composed of ultra-flat planar metallic nanostructures.^1,2 These structures have nanoscale dimensions and allow engineering of the direction and the relative phase of the scattered light to an unprecedented degree. Based on the Huygens principle, an ensemble of such scatterers can be employed in order to create almost any desired beam shape via superposition, since each MS element can be considered as a dipole-type source of an elementary wave.³ One particular striking feature of MSs is the emergence of a phase-gradient along the MS, leading to generalized forms of the laws of refraction and reflection.³ Due to advances in nanostructuring technology, MSs are used in various fields of photonics, examples of which include sophisticated polarization and beam control,⁴ the observation of the photonic spin Hall effect,^5,6 ultra-flat lenses,^7–10 and the generation of orbital angular momentum states.^11–13 Very recent developments in the field include MSs made from non-metallic materials, leading, for instance, to all-dielectric ultra-flat lenses^14–16 or for optical cloaks¹⁷ or novel geometries such as catenary MSs.¹⁸ 

One class of widely used MSs is nanoboomerangs having identical two arms with subwavelength lengths and widths and defined opening angles. Compared to other MS geometries such as rotated nanoantennas¹⁹ or structures with sophisticated shapes and materials,^20,21 the nanoboomerang geometry presents an excellent compromise between fabrication complexity and plasmonic resonance tunability.² The basic working principle of the nanoboomerang geometry relies on a hybridization of the longitudinal localized surface plasmon resonance (LSPR) of the isolated arm,^4,22,23 resulting in two new non-degenerated eigenstates with antenna resonances at two different wavelengths, whereas the resonance wavelengths are mainly determined by the opening angle and arm length. Associating each eigenstate with a dipole Lorentzian oscillator, the superposition of both oscillators allows engineering phase and amplitude almost independently within the spectral interval between the two resonances, which cannot be achieved in non-hybridized structures such as simple bars.

For many applications, it is important that the MS allows access to phases between −π and +π. This is typically achieved by appropriately designing the elements in order to cover the phase space from −π to 0 and symmetrically rotating half of the elements by 180° or by an appropriate mirror operation to reach the remaining phases. Due to symmetry reasons, the rotation operation only affects the phase of the crossed polarization state, i.e., the state orthogonal to the input polarization. In our samples, the nanoboomerangs are arranged in a square lattice with their symmetry axes oriented 45° relative to the lattice axes (Fig. 1). Simulations and measurements include an input polarization state parallel to one of the lattice axes (defined as x-axis). Due to the scattering of the elements, the output light contains both parallel and crossed polarized contributions, which are defined as x-state and y-state, respectively. The phases of the parallel state, however, remain unaffected by the rotation operation, and as a consequence the full phase space cannot be accessed when operating in parallel polarization. Ideally, all elements should have identical and strong scattering amplitudes for the crossed polarization state and small amplitudes for the parallel state in order to make the crossed state dominant.

For a MS to operate in reflection, the mentioned conditions are typically achieved using bar-type MSs (here referred as positive MS, meaning pMS), thus isolated scattering elements forming a MS with comparably low metal filling fraction. If transmission mode operation is demanded, however, the low metal filling fraction leads to dominant scattering amplitudes of the parallel polarization state which are stronger than those of the crossed state due to the contribution of the light passing through the non-patterned regions (e.g., Figs. 2(a) and 4(b)). Hence, pMS is problematic to operate in transmission, which is the desirable operation mode for many applications.

According to Babinet’s theorem, inverse metasurfaces (iMSs) consisting of slit-type elements can show identical scattering properties as the respective positive structures with the advantage of blocking the non-scattered fraction of the transmitted light. Inverse structures have been fabricated and investigated with regard to applications such as creation and characterization of orbital angular momentum modes,^10,24 implementation of various kinds of elements for polarization discrimination,^25–27 elements with chiral or holographic properties,^28,29 and devices for focussing and shaping of beams.^29–32 Slot-type nanoboomerangs have been used for electro-optical switching.³³ A comparable approach was used in the terahertz frequency region for planar electric split ring resonators.³⁴ 

Here we show that the individual elements of iMS geometry being composed of slot-type nanoboomerangs with feature sizes of about 100 nm and a high metal filling fraction fulfill the abovementioned conditions for the case of transmission mode operation (Fig. 1(a)). The comparably high metal filling fraction efficiently blocks the undesired contribution of the parallel polarization state, leading to transmission values of the crossed state of the same order as or even slightly higher than that of the parallel state. By conducting a straightforward designing procedure, we show that iMSs being composed of inverse nanoboomerangs allow accessing the entire phase space of 2π, providing identical functionalities as the pMS with substantially improved polarization extinction.

The elements of the iMSs considered in this work are formed by nanostructured thin metallic films (thickness 60 nm) on a silica substrate. The individual iMS element (i.e., nanoboomerangs, Fig. 1(a)) is formed by two identical slots with one joined edge of defined width and length penetrating the entire film. Here we assume the film to be made from gold, which is a widely used plasmonic metal due to several advantages particular from the application and fabrication perspective.^35–37 We assumed a constant slot width of 120 nm, which is straightforwardly accessible with our fabrication technology. In order to desirably tune the transmission and phase of each element, i.e., to control the plasmonic hybridization of the longitudinal LSPRs, each single element of the iMS has an independent arm length L and a defined opening angle α. To optimize the structure for a fixed wavelength (here: $1.55μm$⁠), we performed a scattering analysis using finite element simulations yielding amplitude and phase of the transmitted light (with respect to the input phase) for any combination of the structural parameter in either a crossed or parallel polarization state. The simulations include one unit cell of a single nanoboomerang with periodic and four-port boundary conditions below and above the structure. As only the relative phase between the elements is relevant, we have added an offset value to the phase such as to obtain a zero crossed-polarized phase for an opening angle of $α=180°$⁠. This geometry imposes a plane wave excitation and allows obtaining the transmission and reflection amplitudes of the parallel and crossed polarized light.

The main advantages of using nanoboomerangs are the two main geometric parameters (arm length and opening angle) allowing to adjust amplitude and phase of the crossed polarized wave independently. In detail, we simulated the scattering properties for a number of opening angles from 25° to 180° and arm lengths between 250 and 550 nm (the inset of Fig. 2(b)). From this data set, we obtained the maximum amplitude that is possible for all opening angles (gray dashed line in the inset of Fig. 2(b)). If we select the arm lengths corresponding to this selected amplitude value for all opening angles, we obtain a set of nanoboomerangs having the same amplitude for the crossed state covering a phase range of π.

For the iMS geometry considered here (gold film with thickness 60 nm, air slot width 120 nm), the above outlined optimization procedure reveals that accessing the full half phase space from −π to 0 is indeed possible at a transmission level of 10% (solid pink lines and purple squares in Figs. 2(a) and 2(b)) in the crossed-polarized state. Except for very small opening angles $(α<30°)$⁠, the transmission of the parallel polarization state is roughly of the same order. This situation is dramatically different in the corresponding pMS, which was optimized using again the mentioned two-step process (metal strip width: 120 nm, gold thickness 60 nm). The procedure yields a pMS showing a maximum transmission of 13% in the crossed state (Fig. 1(b)), being roughly equal to that of the iMS. The transmission in the parallel state, however, is substantially higher for almost all opening angles (dashed dark cyan line in Fig. 2(a)), reaching values being up to four times larger than that of the crossed state. This clearly emphasizes that iMSs are favorable if operation in the transmission mode is considered, since this geometry effectively suppresses the parallel polarization state. The crossed polarization state is therefore more pronounced in an iMS, whereas its impact can diminish in pMS structures. In non-periodic MS arrangements (e.g., geometries including phase gradients), the result from the simulation with periodic boundary conditions can be used by assuming that the total scattered field is the superposition of the fields of the elements, with the individual element characterized by its dipole moment. In order to confirm the validity of our results with respect to non-periodic boundary conditions, we calculated the crossed-polarized phase (in the far-field) of single isolated nanoboomerang elements using a simulation volume surrounded by perfectly matched layers yielding a phase distribution similar to that of the periodic boundary simulations (light gray curve in Fig. 2(b)).

In order to investigate the concept of the iMS from the experimental perspective, we implement slot-type nanoboomerangs using a combination of thin film deposition, electron-beam lithography, and ion beam etching. After depositing a 60 nm thick gold film on a planar silica substrate (including a thin titanium adhesion layer), the pattern was written in a 150 nm thin e-beam resist (AR6200.09, ALLRESIST GmbH, Berlin) using e-beam lithography (Vistec SB350 OS). The SB350 OS system operates at 50 keV, leading to accurately positioned and fast exposures down to sub-100 nm structures on wafer-scale areas.³⁸ After developing (60s in AR600-546 and stopped in isopropyle alcohol), the resist pattern was transferred into the gold film by using Ar⁺-ion beam etching (IBE). The residual resist was removed by solvent (AR600-700), followed by a short oxygen-plasma cleaning step (RIE). Scanning-electron microscopy (SEM) revealed an over-etching of the silica substrate by about 30 nm because of the non-selective nature of the IBE.

The resulting structures consist of arrays of identical nanoboomerangs in a square arrangement (array size: 1 mm × 1 mm, number of identical elements: 1250 × 1250) with an inter-element pitch of 800 nm (examples of various iMS are shown in Figs. 1(b)–1(f)). The elements of the different arrays have identical air slot width (approximately 120 nm) but various discrete opening angles and engineered arm lengths (of the order of 400 nm) in order to obtain high transmission and large phase angle variations in the crossed-polarized state according to the optimization procedure outlined above. We have fabricated five different arrays with the following opening angles/arm lengths (value refers to α / L): 45°/440 nm, 60°/455 nm, 90°/455 nm, 120°/390 nm, 180°/320 nm (widths between 100 nm and 130 nm). To protect the nanostructures from environmental influences, they were covered with a 10 nm thick Al₂O₃ protection layer using atomic layer deposition.³⁹ Preliminary simulations and measurements showed that this layer had negligible influence on the optical properties of the nanoboomerangs, i.e., on the hybridization of the LSPRs. For comparison, we also fabricated elements of a pMS structure using a lift-off process, resulting in nanostructures with a metal strip width of approximately 110 nm and various opening angles and arm lengths.

In order to spectroscopically characterize our nanoboomerangs, we measured the spectral distribution of the transmission between $1μm$ and $2.4μm$ using a Perkin-Elmer dual beam spectrometer (sample oriented normal to light beam). Precise control over the crossed and parallel polarization states was ensured by two adjustable polarizers before and after the sample. The extinction of the first polarizer has no influence on the measurement because of an identical polarizer included in the reference beam at equal orientation. The analyzing polarizer in the sample beam has a polarization contrast between 500 and 800 across the spectral interval of interest. In order to evaluate the influence of the absorption of all optical elements within the beam path of the spectrometer (e.g., polarizers), a baseline corresponding to 100% transmission was measured without the sample for normalization purpose. Furthermore, the dark currents of the detectors were recorded by a measurement with both beams blocked. Intensity fluctuations and the light spectral source characteristic were automatically removed from the data by internal normalization using the intensity of the reference beam. In order to account for the Fresnel reflection at the various interfaces and to obtain the transmission of the arrays only, we normalize the transmission that measured at a non-structured part of the sample (without gold) in parallel polarization configuration.

The crossed polarized measurements revealed that all five arrays have a peak transmission between 10% and 15% (Fig. 3). All curves show a clear indication of two Lorentzian oscillator-type resonances, whereas the resonance on the blue side of the spectrum moves towards shorter wavelength with decreasing resonance amplitude for larger opening angles. Note that for $α=180°$⁠, only one resonance is visible. The resonance on the red side, however, remains at a high level for all geometries, whereas a maximum resonance wavelength is reached for the structure with $α=90°$⁠.

A direct comparison revealed a very close match between simulation and experiment for all nanostructures across the spectral domain of interest (Figs. 3(a) and 3(b)). Particularly remarkable is the close overlap between the resonance wavelength and the associated transmission, indicating the high quality of the fabricated nanostructures. The appearance of two spectrally separated resonances clearly demonstrates the plasmonic hybridization effect within the nanoboomerang geometry (two exemplary intensity distributions are shown in the right-handed images of Fig. 3). It is important to note that the simulations include the real physical dimensions of the nanoboomerangs (given in the above text) and the mentioned 30 nm over-etching into the substrate. The over-etching in fact leads to a blue shift of the hybridized resonances, as otherwise the simulated resonance would be located about 30-80 nm towards longer wavelength (depending on the opening angle).

In order to confirm the superior operation of slot-type nanoboomerang-based iMSs in transmission, we compare the transmission values of both crossed and parallel polarization states at a fixed wavelength of $1.5μm$ (crossed (parallel) state: pink (green) curve in Fig. 4). The simulations were performed by taking into account the mentioned 30 nm over-etching into the substrate glass and using the exact geometrical dimensions of the fabricated samples, which are slightly different from those of the nanoboomerangs discussed in Fig. 2. Except for very small values of α, the transmission in the crossed polarization state lies within the interval between 5% and 10% and is higher than that of the parallel state, with the transmission values being fully reproduced by simulations. The increase of the transmission in the parallel state for $α=45°$ which we attribute to fabrication inaccuracies, as a detailed SEM analysis showed that the inside-section of the nanoboomerang connecting the two arms is particularly vulnerable to such fabrication-induced uncertainties. A direct comparison to the corresponding experimental data of optimized arrays of pMS elements (arm width approximately 100-120 nm, the inset of Fig. 4 showing SEM images) shows the superior property of the iMS geometry of having a higher transmission in the crossed polarization state, which yields a four times improvement in the polarization extinction.

In this work, we show the superior properties of inverse metasurfaces compared to their positive counterparts in case the transmission mode operation is considered. The key advantage of such slot-type nanostructures is the strong suppression of transmitted light in the parallel-polarization state, making the crossed-polarization state, which is the relevant state for metasurface operation, highly visible. In the example of nanoboomerangs, we present an optimization procedure to simultaneously obtain high and constant transmission levels of the various metasurface elements while accessing the full phase space. An up to four times improvement in polarization extinction is observed for various elements of the inverse metasurface geometry. We experimentally verified our approach by implementing a series of nanostructures with feature sizes of the order of 100 nm and determined their spectral characteristics to be polarization-sensitive. The fabricated metasurface elements show the plasmonic hybridization effect, i.e., two spectrally separated resonances, which is the key for the phase and transmission engineering properties of metasurfaces in general. The experiments closely match corresponding simulations and indeed confirm the fourfold improvement in the polarization extinction of inverse metasurface geometry.

Since our presented metasurface concept operates at $1.5μm$⁠, a large number of ultra-flat monolithic photonic elements for applications in telecommunications can be envisioned. Here, the inverse metasurface concept can provide a solid solution to form nanoscale elements such as lenses or devices with ultra-small footprints to generate, e.g., sophisticated states of polarization and beam shapes. To extend the application range of inverse metasurfaces even further, future studies will aim to reduce feature sizes to reach the visible spectral domain for the implementation of, e.g., metalenses⁴⁰ or holographic ultra-flat photonic elements,^41,42 which both are of particular interest for biophotonics, sensing, or spectroscopy. In the next step, we plan to directly measure the predicted phases of the fabricated nanoboomerangs (e.g., as conducted by Huang et al.⁴³) to verify the presented concept of transmission mode operation using inverse metasurface structures.

This work was funded by the Thuringian State Ministry for Economics, Labor, and Technology (Project No. 2012FGR0013) supported by the European Social Funds (ESF).