1. Introduction

The exploitation of enhanced electric near-fields in the vicinity of a metallic or dielectric nanostructured surface is a promising approach in biophotonics, optical spectroscopy and solar energy devices. Luminescent materials located nearby or inside the nanostructures can experience strongly enhanced (nonlinear) absorption and emission, hence allowing for a significant reduction of the required excitation irradiance levels and an increase of sensitivity. This mechanism was first described around 1980 in the context of surface enhanced Raman spectroscopy [1], further developed in the field of photonic crystals [2] and plasmonic structures [3,4] and recently attracted large attention in the vibrant field of metasurfaces [5]. Particularly dielectric nanostructured surfaces, such as photonic crystals (PhC) slabs [2,6,7] or periodically arranged nano-discs [5,8,9] can open up practical applications by significantly enhancing excitation and emission of the electromagnetic radiation without the obstacle of parasitic absorption often occurring in plasmonic structures. Such dielectric metasurfaces are known to exhibit leaky resonant electromagnetic modes, which can be extended over the entire photonic structure and excited via external radiation. Recently, dielectric metasurfaces started being applied for up-conversion applications enabling large enhancement factors due to the non-linear dependence on the near-field energy density [10,11]. However, limited areas, expensive fabrication procedures and the lack of adjustability often impede the spreading of metasurfaces in applications on large scale. Further, experiment design often sticks to normal incidence excitation significantly restricting the degree of freedom.

In this study, we engineer large-area metasurfaces based on silicon PhC slabs for surface-enhanced optical sensing and photon up-conversion. We design the metasurfaces such that the frequency and in-plane momentum of the incident light and a leaky mode match, which often comes along with strong near field enhancement effects. Our bottom-up fabrication sequence comprises nanoimprint-lithography (NIL), physical evaporation of silicon and thermal annealing. All processing steps are compatible with large-area, high-throughput, and low-cost fabrication, forming an ideal experimental platform for applications favouring large-area functional surfaces. For adjustment of the leaky mode position we pursue a large-area compatible approach, recently introduced by Ondic et al. [12], by variation of the slab thickness. We experimentally determine the spectral position of the leaky modes by angularly resolved directional transmission (ARDT) measurements. Numerical optical calculations performed with the finite element method (FEM) allow for the correlation of measured resonances with strong near-field enhancement effects. We discuss the benefit of excitation at oblique incidence allowing to couple to resonances with otherwise symmetry-suppressed coupling at normal incidence.

2. Methods

2.1 Fabrication of silicon photonic crystal slabs

Leaky modes of a PhC slab can be tuned by changing a range of parameters. For instance, the refractive index of the slab and substrate, the in-plane geometrical parameters of the slab (lattice constant and hole diameter), and the slab thickness [13]. The limited number of available materials for the production process impedes adjustability via the refractive index. The adjustment of hole diameter or periodicity of the PhC slab often requires costly nanofabrication steps. In contrast, the slab thickness can easily be varied if a bottom-up thin-film fabrication procedure is chosen, e.g. by changing the deposition time [12]. The spectral leaky mode position can hence be systematically tuned by adjusting the slab thickness as a single experimental parameter. The silicon PhC slabs used in this study were fabricated by soft–NIL, a powerful method that enables hundred-fold replication of nanostructures from a master [14]. The stamps for the soft-NIL were fabricated by molding Polydimethylsiloxane (PDMS) by Wacker on two different silicon master structures (Eulitha) consisting of hexagonally arranged nanopillars whose aspect ratio are 3:5 and 4:5 and two different lattice constants p (600 and 1000 nm). The PDMS molds were imprinted on UV-thermally curable sol-gel [15] spin-coated on 5 cm × 5 cm glass substrates (Corning). After UV curing for 500 seconds, thermal curing at 100°C for 8 min was immediately applied in order to evaporate the residual solvent on the surface of the samples. Post-deposition thermal curing at 600°C for 1 hour was done on the samples in order to form silicon oxide (SiO_x) nanopillar arrays on the glass substrates. Silicon was chosen as a dielectric material for our metasurfaces owing to its superior material properties – non-toxicity, abundance and a high dielectric constant. Silicon was deposited in amorphous state with various thicknesses (60 nm to 150 nm) on the imprinted substrate by physical vapor deposition. The layers were subsequently solid phase crystallized by thermal annealing at 600°C under nitrogen flow. Residual amorphous parts of the layers at the flanks of the nanopillars were removed by exposing the samples into a poly-silicon etching solution for approximately 3 seconds. The stripped nanopillars were then able to be mechanically removed, consequently, holes with a diameter of 400 nm for p = 1000 nm and 325 nm for p = 600 nm on the crystallized silicon slabs were formed. More details on the fabrication process are described in Ref. [16]. Figure 1(a) shows a photographic image of silicon photonic crystal slab on glass with a size of 5 × 5 cm² in total. The colorful part on the left-hand side contains the nanopatterned silicon PhC slab. The dark part on the right side of the sample is an unstructured silicon layer for reference. A scanning electron microscopic images of silicon PhC slab with 150 nm slab thickness is shown in Fig. 1(b). The schematic of the silicon PhC slab is illustrated in Fig. 1(c) as side view and Fig. 1(d) as top view. It also illustrates the possible geometrical parameters for adjustment of the metasurface mode positions: the hole diameter d, the period p, and the slab thickness t. In this study, we restrict ourselves to variations in p for coarse adjustment and t for fine adjustment of the leaky mode spectral positions.

 

Fig. 1. (a) Photographic image of a large-area silicon photonic crystal slab on glass. The colorful area on the left contains the nanopattern, while the darker area on the right shows an unstructured silicon reference layer. (b) Scanning electron microscopic images of a photonic crystal slab with 150 nm thickness. (c) Finite element mesh used in the simulations, for clarity parts of the layers are removed; (d) Top view of a mesh comprising of the unit cell and its first nearest neighbours. Only the fundamental unit cell (indicated in light grey) must be simulated due to the periodic boundaries in the x-y plane. The relevant geometrical parameters are indicated, namely the angle of incidence θ, the slab thickness t, the periodicity p, the hole diameter d, and the high symmetry directions of the hexagonal lattice Γ → M and Γ → K.

Download Full Size | PDF

2.2 Electromagnetic simulations

Angular resolved transmittance and electric field energy enhancement were calculated via finite element simulations using the commercial software JCMsuite [17]. A hexagonal computational domain was used with periodic boundaries in the x-y-plane. In the z-direction, transparent (perfectly matched layer) boundary conditions were used. The period and hole diameter of the photonic crystal were chosen to match those of the experimental sample. Above and below the silicon photonic crystal slab, 100 nm thickness planar layers of air and glass, respectively, were included in the simulation. The refractive index for silicon was taken from literature [18], while for glass a constant value of 1.53 was assumed. The transmittance was determined via integration over all propagating k-vectors of the Fourier transform of light scattered from the computational domain in the –z direction. The electric field energy enhancement was calculated by integrating the electric field energy density in the air region of the computational domain and normalizing to the integrated electric field energy density in the same region for a plane wave. The air region consists of the hole inside the photonic crystal, as well as 100 nm above the surface. For more details on the electric field energy density enhancement see Ref. [19].

3. Results and discussion

A common way to examine the spectral distribution and angular dispersion of leaky modes of PhC slabs is to perform angularly resolved far-field measurements such as zeroth order transmittance or reflectance measurements [20,21]. Due to the extra momentum provided by the periodic lattice, light may be scattering into diffraction orders of the Si PhC slab. External radiation can then be efficiently coupled to leaky modes of the slab [22]. As a result, resonance patterns, which are dependent on the symmetry direction and polarization, appear in transmittance and reflectance measurements. We measure zeroth order (directional) transmittance as it is convenient way to examine the spectral and angular position of the leaky modes even at normal and oblique incidence conditions.

Figure 2 shows the angular resolved directional transmittance (ARDT) of a 100 nm thick silicon photonic crystal (PhC) slab for wavelengths between 400 nm and 2000nm for both, TE (upper row) and TM (lower row) polarized light, under rotation along the high symmetry directions Γ → M (left) and Γ → K (right). Several resonant features can be observed and associated with coupling of the incident light to a leaky mode. Please note the resonances at a wavelength of around 1600 nm near the Γ point (normal incidence, θ = 0°). While these resonances are clearly distinguishable at oblique incidence illumination, they vanish when approaching the Γ point. In contrast, light with a wavelength of around 1450 nm can couple to a metasurface resonance at normal incidence as indicated by the transmittance transitioning from zero to near unity. From the pattern of spectrally narrow resonances, a strong near-field enhancement can be inferred. The presence of a large number of leaky modes spanning the whole near infrared region brings flexibility to the choice of modes when engineering for the desired application. However, for systematic adjustment of modes to a specific excitation wavelength a simple and applicable experimental procedure is desirable, ideally avoiding time consuming and financially costly electron beam lithography steps.

 

Fig. 2. Angular resolved directional (zeroth order) transmittance of a silicon PhC slab on glass with slab thickness t = 100 nm, periodicity p = 1000 nm, measured with TE (upper row) and TM (lower row) polarized light and rotation along the Γ → M (left) and Γ → K (right) direction.

Download Full Size | PDF

Figure 3 summarizes the angular resolved directional transmittance (ARDT) measurements for the fabricated silicon PhC slabs with various thicknesses t (60 nm – 150 nm) for two different periodicities of p = 1000 nm and p = 600 nm (Fig. 3(a) and 3(b), respectively), as an example for TE polarized light and rotation along the Γ → M direction. It is clearly seen that the leaky mode pattern shifts to larger wavelengths with increasing slab thickness. In the case of a silicon layer thickness of only 60 nm, leaky mode resonances are observable down to a wavelength of around 500 nm. With increasing thickness the border for visibility of distinct features shifts up to wavelengths of around 800 nm. This is likely due to absorbance in the crystalline silicon. Therefore, a metasurface based on silicon photonic crystals is optimally suited for applications with excitation in the near infrared region where the absorption coefficient of silicon is low. As an example, three typical excitation wavelengths (808 nm, 980 nm and 1550 nm) applied for photon up-conversion with lanthanide ion doped crystals in biophotonics and photovoltaics are included as dashed lines. As photon up-conversion is a nonlinear optical process usually requiring high excitation intensities, metasurface enhanced excitation is an appealing option.

 

Fig. 3. Angular resolved directional (zeroth order) transmittance of silicon PhC slabs measured with TE polarized light and rotation along the Γ → M direction for lattice periods of (a) d = 1000 nm and (b) d = 600 nm. From left to right the PhC slab thickness is increased from t = 60 nm to 150 nm (as indicated). The dashed red lines indicate three typical excitation wavelengths applied for photon up-conversion using lanthanide ion doped crystals (808 nm, 980 nm and 1550 nm).

Download Full Size | PDF

The silicon PhC structures with 1000 nm periodicity feature a prominent leaky mode at normal incidence, which can be red-shifted to a wavelength of around 1550 nm by adjustment of the slab thickness to above 100 nm. This mode could be blue-shifted to the spectral range covering wavelengths of 980 nm and 808 nm by decreasing the slab thickness far below 60 nm. However, the effectiveness of surface enhancement provided by the leaky modes might be diminished if the thickness of the slab is reduced under a critical value. Below the critical thickness value, dielectric contrast originating from periodically arranged air holes becomes less significant when considering air holes on the silicon slab as a perturbation of a planar dielectric surface. Consequently, the leaky mode assisted near field enhancement might be significantly reduced. Since in the left image of Fig. 3(a) it is already difficult to discern the upper-most resonance in the transmission pattern, further decreases in the slab thickness were not considered for this study. In order to achieve distinct isolated resonances at 808 and 980 nm we also designed silicon PhC slabs with a lattice constant p = 600 nm and a hole diameter of 325 nm while keeping the other parameters the same. The spectral displacement of the leaky modes to shorter wavelengths according to scale-invariance of Maxwell equations is shown in the ARDT measurements presented in Fig. 3(b). The results show explicitly that the leaky modes of the silicon PhCs were successfully blue-shifted to the spectral region of interest, particularly the range covering 980 and 808 nm for the biophotonic applications employing lanthanide ion doped materials. Previous work supports the claim that resonant features in the ARDT measurements as shown in Fig. 3 are accompanied by electric field energy enhancement in the vicinity of the nanostructured surface [16].

A local increase in the electric field energy density should lead to an increase of luminescence when luminescent particles are directly placed on the metasurface. Therefore it is crucial for the functioning of an optical sensing device to verify that transmittance resonances lead to enhanced near fields. Figure 4(a) shows again the ARDT measurement for a PhC slab with a lattice constant of p = 1000 nm and thickness of 100 nm. The measurement was performed with rotation along the Γ → M direction under TE polarization. The same geometry and light source were used for the simulation of the transmission spectrum using the finite element method (Fig. 4(b)). Comparing the distinctive resonance fingerprint shown in part (a) to the simulation in (b), we see that the two are in extremely good agreement. Figure 4(c) shows the simulated electric field energy enhancement on a logarithmic scale. Resonances in the enhancement factor closely follow the spectral positions of the transmittance resonances seen in Fig. 4(a) and 4(b). Enhancement factors are typically in the range of 10-100 for the resonances shown, i.e. values between 1 and 2 in the logarithmic scale in Fig. 4(c). These results clearly show that resonant features of ARDT transmittance measurements can be used to predict the excitation conditions necessary for strongly enhanced electric near fields. Please note that considerable enhancement factors are present down to a wavelength of 800 nm, i.e. below the absorption onset of silicon at around 1100 nm. For a layer thickness of only 100 nm absorbance losses still don’t dominate the behavior.

 

Fig. 4. Comparison of experimental and simulation results of the silicon PhC slab with d = 1000 nm and t = 100 nm; (a) ARDT measurement, (b) the simulated ARDT, and (c) the simulated electric field energy enhancement in a 100 nm layer above the metasurface with rotation along the Γ → M direction under TE polarization.

Download Full Size | PDF

For many applications, strongly enhanced near fields at normal incidence are of a great value. Having established the correspondence between the resonances in transmission resonances and enhanced near fields, we now use simulations to find the optimal slab thickness for normal incidence near field enhancement. We previously observed that certain modes vanish at normal incidence, which we attribute to symmetry protected bound state in the continuum modes. Therefore, we also simulated the near field enhancement for near normal excitation.

The simulated dependency of the electric field energy enhancement on the slab thickness for a silicon PhC slab with periodicity p = 1000 nm is presented in Fig. 5(a) for the three different example excitation wavelengths 808 nm, 980 nm and 1550 nm for normal (solid lines) and for near-normal (dashed lines) incidence excitation conditions. ‘Near-normal’ excitation corresponds to 4° tilt in Γ → M direction using TE polarized light. The calculated electric field energy enhancement for 980 nm (red curves) and 808 nm (black curves) do not exceed a factor of 4 in the whole range of investigated PhC slab thicknesses from t = 0 nm to 250 nm, neither for normal incidence nor for 4° incident excitation. Interestingly, for slightly oblique incidence some resonances appear that are not observable at normal incidence. This effect is particularly evident for 1550 nm excitation: The simulations yield electric field energy enhancement factors up to about 17 when the metasurface thickness is around either 100 nm or 220 nm at normal incidence (solid blue curve). In contrast, at slightly oblique incidence new resonances with much higher enhancement factors occur (dashed blue curve). More than 500-fold enhancement appears e.g. for an incident angle of 4° with rotation along the Γ → M direction with TE polarization for a slab thickness of about 195 nm. Comparing to Fig. 2 and 4, a mode with similar properties can be identified at a wavelength of 1630 nm: It exhibits a huge enhancement factor above 3000 at 4° incident angle, which is completely vanishing at normal incidence. The simulated electric field energy enhancement for the silicon PhC slab with periodicity p = 600 nm is shown in Fig. 5(b). For excitation wavelengths of 808 nm and 980 nm the maximum electric field energy enhancement factors at normal incidence are slightly improved with respect to the previous design with p = 1000 nm from around 3-4 to around 9. We again see resonances appearing at 4° oblique incidence (red and black dashed lines) that are not present in the normal-incidence case (red and black solid lines). Up to 22-fold enhancement occurs for a slab thickness of t = 144 nm and excitation at 980 nm. At 1550 nm wavelength no resonances and hence no significant enhancement is present for a periodicity of p = 600 nm.

 

Fig. 5. Electric field energy enhancement above the silicon PhC slabs versus slab thickness for a lattice constant (a) 1000 nm and (b) 600 nm at three wavelengths (808–980 – 1550 nm) typical for photon up-conversion applications. Plane wave irradiation is incident at normal incidence (solid lines) and a slight oblique incidence of 4° rotated along the Γ → M direction under TE polarization (dashed lines). Volume plots of the local electric field energy density |E₊| in the air above the silicon PhC slab with lattice constant 1000 nm and excitation at a wavelength of 1550 nm with TE-polarized light are shown in part (c) for normal incidence and slab thickness 221 nm and in part (d) for slight oblique incidence and a slab thickness 196 nm, both also indicated by blue stars in part (a). Please note the different scales in (c) and (d). The simulation mesh of the substrate and part of the Si structure are included as a guide to the eye.

Download Full Size | PDF

In order to further investigate the potential of these resonances the local electric field energy of two selected excitation conditions is shown in Fig. 5(c) and 5(d), also marked by two blue stars in Fig. 5(a). The structure shown has a lattice period of 1000 nm, hole diameter of 400 nm and slab thickness 220 nm (part c) and 196 nm (part d), respectively. In Fig. 5(c) light is incident normally from above with 1550 nm wavelength. The electric field energy is concentrated in two lobes that lie on the plateau of the silicon PhC, with a small contribution coming from the bottom of the hole. Figure 5(d) shows the field energy volume plot for near normal incidence. Please note the change in scale bar - absolute values of the field energy are an order of magnitude larger than in Fig. 5(c). The field energy pattern exhibits six lobes on the plateau of the silicon PhC. This can be advantageous for applications since luminescent particles or other emitters do not have to enter inside the metasurface hole in order to obtain the enhancement. This relaxes the requirement for a high filling fraction inside the hole for these two particular resonances. Additionally, the enhancement is spread over a relatively large fraction of the unit cell, this can allow more luminescent material to interact with the increased electric field energy. This is in contrast to other resonances, which are more spatially localized but may offer higher enhancement factors in their limited volume.

The lack of coupling to resonances at normal incidence excitation is in line with the currently actively researched topic of bound states in continuum (‘BICs’) [22–24]. At normal incident excitation, the system exhibits a high symmetry. If the bound state, in our terminology referred as a metasurface resonance, has a different symmetry to that of the external radiation, coupling is forbidden. This is the so-called symmetry protected BIC. Although the presence of the glass substrate breaks the mirror symmetry in the vertical (z) direction (see Fig. 5(c)), the D6 symmetry of the hexagonal lattice in the x-y plane can produce the symmetry protected BIC. Irregularities on the silicon PhC slab surface inherited by the nature of nanoimprinting lithography locally destroy the symmetry. Hence, these modes partially can couple to free space, meaning that small values of field enhancement are still possible under normal incidence radiation. In summary, we find that slightly oblique incident excitation can circumvent symmetry-suppressed coupling between the external radiation and the metasurface.

4. Conclusion

We developed a production procedure for large-area metasurfaces based on silicon photonic crystal slabs enabling easy spectral adjustment of resonances. We shifted the resonance wavelengths of leaky modes of silicon PhC slabs over several hundreds of nanometers by changing only one easily accessible experimental parameter, namely the silicon slab thickness. Angular resolved transmittance measurements allowed us to identify the spectral position of the metasurface resonances at various incident angles. We optimized the metasurfaces computationally for excitation at three wavelengths as examples – 808 nm, 980 nm and 1550 nm –, which are typically used for up-conversion with lanthanide ion doped crystals. Considering the geometrical parameters investigated in these computations, the metasurfaces exhibited electric near-field energies with up to a 17-fold enhancement at normal incidence, but an over 500-fold enhancement at slightly oblique angle excitation, when comparing with the energy density of the incident plane wave in free space. We explained the large increase in enhancement factors at slight oblique incidence via coupling to bound state in the continuum type modes that were previously forbidden at normal incidence due to symmetry. In summary, the produced silicon-based metasurfaces are a promising platform to decrease the required irradiance level for large-area optical sensing applications at near infrared excitation, e.g. in biophotonics. The benefit of slightly off-normal incident excitation is discussed enabling the coupling to otherwise symmetry-protected modes.