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Abstract
Spectral resonances in the mid-infrared region with polarization independence and angle tolerance are useful for filtering applications in infrared spectroscopy and imaging systems, when used with unpolarized light and across a wide field-of-view. Guided mode resonances are particularly attractive for this purpose due to the simple fabrication procedure to realize grating structures and the robust filter characteristics achievable through design. In this paper, the electromagnetic design, fabrication, and experimental characterization of polarization-independent, angle-tolerant mid-infrared spectral resonance using amorphous-germanium two-dimensional fully-etched high index contrast gratings on a calcium fluoride substrate is presented. The resonance, centered at 7.42 µm wavelength, exhibits polarization-independent, notch-type characteristics with minimal change across a 0 to 30° incidence angle. The angle tolerance of such dielectric high contrast grating filters is found to be intermediate between the highly angle sensitive dielectric partially etched grating structures and least angle sensitive metallic nano-aperture structures.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-infrared (Mid-IR) and long-infrared (Long-IR) wavelength range spanning 3 to 15 µm is widely used for imaging, material ablation and spectroscopic sensing in military, healthcare, industrial and scientific application space [1]. Optical filters with broadband or narrowband, band-pass and band-reject characteristics are essential components used in such infrared spectroscopy or imaging systems [2]. Standard dichroic filters comprising of multilayer stacks of quarter wavelength thick low and high refractive index materials achieve such filtering characteristics [3]. In the mid-IR and long-IR range, multilayer stacks generally require complex fabrication procedures due to the scarcity of optical materials with suitable refractive index contrast and low optical absorption. Furthermore, the multilayer stacks can get significantly thick for achieving complex filter characteristics, making such filter coatings prone to stress related reliability issues. This has motivated research into grating based filters, in which repetitive photonic lattices support guided mode resonances in simple single or few layer dielectric waveguide grating structures with sub-wavelength thickness [4]. Previous work on mid-IR filters using sub-wavelength grating structures include silicon nitride-on-glass one-dimensional (1D) gratings for infrared multispectral imaging [5], two-dimensional (2D) silicon nitride-on-calcium fluoride (CaF2) gratings based narrowband notch filter [6], 1D and 2D germanium-on-zinc selenide partially etched grating structures for notch filtering with angle tuning [7,8] and broadband reflective filters in the 8-11 µm region [9]. The 1D gratings and structures with asymmetric unit-cell are inherently polarization-dependent, thus resulting in effective filtering of only one polarization component. Furthermore, partially etched 1D or 2D structures, and low-index contrast structures are found to be inherently angle sensitive, with angle-insensitivity achieved only for 1D conical mounting [10]. Polarization-independence and angle insensitive filter characteristics are desirable for spectroscopy and imaging applications using unpolarized light sources and for imaging across wide field-of-view. Polarization independent resonances can be achieved using symmetric 2D grating structures [6,8,11]. Angle tolerance can be achieved by ensuring that the resonant field is concentrated strongly in the grating unit-cell element. For example, the strong field concentration around the edges of metallic apertures due to localized plasmonic resonance has been leveraged to realize extraordinary optical transmission with angle insensitivity for up to 60° incidence angles [12,13]. Fully etched higher index contrast dielectric grating structures [11] are also suitable for angle insensitive operation when compared to partially etched dielectric zero-contrast gratings due to enhanced field concentration in the photonic lattice elements. Operating in the Mid-IR wavelength range, the dimensions of such grating structures are also more amenable to large area scaling using conventional optical lithography techniques. There are few simulation studies reported previously in similar lines using silicon-on-calcium fluoride (CaF2) arrays [14], and germanium-on-CaF2 mesh-grid structures [15]. In this paper, experimental demonstration of polarization-independent, angle tolerant spectral resonance using amorphous germanium-on-calcium fluoride (aGe-on-CaF2) high contrast gratings for notch filtering application is presented. Electromagnetic simulation studies, structure fabrication, and experimental characterization are discussed. The origin of angle insensitive operation in such structures and a comparison with previous reports of mid-IR notch filters is also presented.
2. Design and simulation results
A schematic layout of the aGe-on-CaF2 grating structures studied here is shown in Fig. 1. aGe is a suitable material for mid-IR applications owing to its low optical absorption and ease of fabrication using standard cleanroom processing [7–9]. The high refractive index of aGe (> 4) with respect to the CaF2 substrate (∼ 1.4) in the infrared wavelength region also widens the design space available in terms of structure dimensions for designing resonant high contrast grating structures [11]. The design parameters of interest here are the pitch (a), the diameter (d), and height (h) of the pillar as illustrated in Fig. 1. The polarization properties of the structures are characterized by using the two orthogonal incident light polarizations; namely transverse electric (TE) and transverse magnetic (TM) along Y and X axes respectively with the propagation wavevector, k along the Z axis for normal incidence. For varying angles of incidence (AOI), the propagation axis is rotated by an angle θ as shown in Fig. 1.(b).
Fig. 1. Schematic illustration of (a) top-view of the 2D lattice and (b) perspective-view of the aGe-on-CaF2 grating structures. The coordinate axes and polarization/ propagation directions are also shown.
The aGe-on-CaF2 grating structures support guided mode resonances resulting in wavelength selective in- and out-coupling of the incident light to the guided modes of the effective waveguide formed by the aGe grating structure. The wavelength selective out-coupled wave constructively interferences with the incident wave resulting in spectral resonances. In the context of fully-etched high index contrast gratings, the resonant phenomenon can also be explained based on the coupled-mode theory of longitudinally propagating waveguide modes excited in the composite aGe-air structure and their interaction with the top and bottom interfaces [11]. The high contrast grating structures considered here were designed using Finite-difference time-domain (FDTD) method [16], by calculating the transmission spectrum as a function of varying structure dimensions. The pitch of the 2D grating structures was fixed at 4.5 µm, which resulted in a cut-off wavelength between the zeroth order and higher order diffraction region (= a × nsubstrate) of ∼ 6.3 µm for normal incidence. This ensures that the spectral resonances under consideration are located in the zeroth order diffraction regime for wavelengths longer than the cut-off wavelength. Figure 2 shows the transmission spectra of the resonances for varying height (0.5 to 1 µm in steps of 0.1 µm) and diameter (2 to 3.2 µm in steps of 0.4 µm) of the aGe pillars. The range of heights and diameters used for simulations were chosen considering the ease of deposition/ etching of the aGe film and ease of patterning the circular features using optical lithography respectively. It is found that with increasing pillar diameter or height the resonance dip in the transmission spectra shifts to longer wavelengths. For a fixed height, there is an optimum diameter at which the notch characteristic becomes nominally symmetric with noticeable asymmetry for other diameters. The high extinction edge is also found to shift from longer to shorter wavelength side of the spectral resonance with increasing diameter. The asymmetry observed in the resonance shape is a characteristic of guided-mode resonance phenomenon, [4] especially pronounced for structures with different substrate and cover refractive indices. With the criteria of achieving large extinction on either side of the resonance dip with reasonable symmetry to the spectral shape, nominal dimensions of h = 0.7 µm, d = 2.8 µm, and a = 4.5 µm were chosen.
Fig. 2. Simulated transmission spectra for varying aGe pillar heights and diameter. Pillar height considered are: h = (a) 0.5 µm, (b) 0.6 µm, (c) 0.7 µm, (d) 0.8 µm, (e) 0.9 µm, and (f) 1.0 µm. Pillar diameter considered are: d = 2.0 µm, 2.4 µm, 2.8 µm and 3.2 µm with the respective colors shown above. The polarization considered here is TE.
3. Grating structure fabrication
Calcium fluoride (CaF2) substrates were first cleaned in piranha (H2O2 : H2SO4) solution followed by plasma enhanced chemical vapor deposition of aGe using GeH4 chemistry. The thickness of the aGe layer was measured to be ∼ 0.71 µm using cross-section scanning electron microscopy (SEM). The grating structures were patterned using a direct laser-writer based lithography (Heidelberg µPG 501) using AZ 5214E as the photoresist. The patterns were subsequently transferred onto aGe film using reactive ion etching with SF6 and C4F8 gas chemistry. A photograph of the fabricated structures on the CaF2 wafer is shown in Fig. 3(a) with the patterned area of 2 × 2 mm2. The top-view, perspective view SEM image and corresponding atomic force microscopic (AFM) profile are shown in Figs. 3(b)–3(d) respectively. The average dimensions of the fabricated grating structure are h = 0.71 µm, d = 2.88 µm and a = 4.47 µm with vertical sidewall profile observed in Fig. 3(c).
Fig. 3. (a) Photograph showing the fabricated sample (patterned area is 2 × 2 mm2) in comparison to standard measuring scale dimensions, SEM image of (b) aGe pillars illustrating the square lattice (scale bar is 5 µm) (c) individual aGe pillar having nearly vertical side-wall profile (scale bar is 300 nm), (d) AFM image (image size is 20 µm x 18 µm), (e) Experimental and (f) simulated transmission spectra are shown for both TM and TE incident polarizations.
4. Experimental characterization
The transmission spectra of the fabricated high contrast gratings were experimentally characterized using PerkinElmer Frontier Fourier transform infrared spectroscopy (FTIR), at a resolution of 4cm-1 with a polarizer placed before the sample to select TM or TE polarization. The illumination spot at the sample is restricted to ∼ 1mm using an iris aperture just before the sample. At the collection end no aperture is placed to select diffraction orders, as the resonant structure is designed to operate in the zeroth order diffraction regime. Transmission ratio is obtained by normalizing the transmission measurements with and without the sample using the FTIR software. A comparison of the experimentally measured and simulated transmission spectra considering the fabricated structure dimensions, at normal incidence is shown in Figs. 3(d) and 3(e), respectively. Good agreement is obtained between the simulation and experimental measurements, with clear observation of polarization-independence and spectral resonance with notch-type characteristic at a center wavelength of 7.42 µm. The simulated and experimentally measured 3dB bandwidths of the spectral resonance are 0.36 µm and 0.56 µm respectively, with a maximum extinction ratio of 14 dB and 8 dB respectively.
The angle dependence of the resonance feature was measured by acquiring transmission spectra as a function of varying AOI. These measurements were performed in the FTIR by precisely tilting the structures placed on a rotating sample holder and acquiring the transmission spectra for both TM and TE incident polarizations. The angle dependent transmission spectra are shown in Fig. 4. It is observed from Figs. 4(a)–4(d), that the center wavelength of the resonance dip shifts slightly from 7.42 µm to 7.56 µm for TM polarization and from 7.42 µm to 7.7 µm for TE polarization for increasing AOI from 0° to 30°.
Fig. 4. Transmission spectrum for different AOI for TM and TE polarizations as indicated in the figures. (a) and (c) correspond to experimental measurements, (b) and (d) correspond to simulation results.
The spectral width is found to remain constant up to 20° AOI and broaden subsequently while maintaining similar filter characteristics for both polarizations. Good agreement is obtained between the experimental and simulated angle-dependent transmission spectra, shown in Fig. 4. The measured angle-dependent transmission spectra show that the central wavelength shift is about 27% and 50% of the spectral width for TM and TE polarizations respectively. This indicates that the spectral resonance remains fairly angle-insensitive to within a fraction of the resonance spectral width, showing the potential for use in notch filtering application across 0-30° AOI of the incident light. Beyond 30° AOI the resonance dip is found to depart considerably from the desired notch-type response. The polarization independence characterized here by considering only TE and TM incidence polarization is strictly valid only for normal incidence may not hold good for larger AOI. This would require a more rigorous treatment by considering the transmission matrix and computing the eigenvalues/ eigenfunctions to estimate the polarization independent directions [17]. Nonetheless, for the high-index contrast, circularly symmetric structures considered here, the approximate polarization independence with respect to TE/TM directions still holds good over AOI of interest, as observed in Fig. 4.
5. Discussion
To explain the observed angle tolerant resonance feature, simulated transmission spectra contour map for the grating structure are shown in Figs. 5(a) and 5(b) as a function of normalized wavevector and normalized x-component of wavevector along the y and x-axis for TM and TE polarizations respectively. These simulations were performed using S4 -Rigorous coupled-wave analysis (RCWA) package [18]. To acquire this contour map, the AOI is varied by tilting the wavevector about the z-axis, as shown in Fig. 1. Similar contour maps have been used previously to compare high contrast grating with photonic crystals with good correspondence observed between the transmission spectra contour map and photonic bandstructure calculations [11]. The white dashed lines in the figure denote the transmission spectra for different AOI with the light line shown at θ = 90°. The blue arrows shown in the y-axis indicates the transmission dip at a center wavelength of 7.42 µm under normal incidence condition. The minimum transmission window extends horizontally in the contour map intersecting the light lines at almost the same normalized wavevector, resulting in angle insensitive filter characteristics up to ∼ 30° AOI. The white circles in the figure denote the onset of higher-order diffraction for different AOI, which shifts to longer wavelengths with increasing AOI. The crossing of the white circles below the blue arrow indicates that higher-order diffraction does occur in the fabricated grating structures for AOI larger than 10°. Nonetheless, the notch-type spectral resonance characteristics are closely maintained, indicating that a strong fraction of energy is still present in the zeroth-order.
Fig. 5. Simulated transmission spectra are shown as a function of normalized wavevector and x-component of the wavevector for (a) TM and (b) TE polarizations. The light lines for 90° AOI are shown by the white dashed lines. Blue arrow indicates the notch filter center wavelength and the white circles indicate the onset of higher order diffraction for varying AOI.
The electric field intensity profiles inside the micropillars for varying AOI and incident polarization are shown in Fig. 6, at the resonance wavelength for the notch filter. The field profiles resemble anapole modes observed in isolated high aspect ratio (diameter-to-width ratio) dielectric scatterers [19]. For a fixed incident polarization, the field profiles inside the aGe pillars look very similar for increasing AOI. This indicates to the excitation of the same guided-mode resonance with increasing incidence angle resulting figin relatively angle insensitive spectral resonances. There is however a reduction in field strength observed for increasing AOI, especially pronounced for the case of TE polarization. This can explain the larger deviation in the resonance dip observed for TE polarization with increasing AOI in experiments and simulations, as shown in Figs. 4(c) and 4(d). A comparison of the present work with previous reports of mid-IR notch filters is shown in Table 1. The previous demonstrations can be broadly classified as dielectric sub-wavelength grating and metallic nanoaperture-based filters. 2D arrayed structures are employed to achieve polarization independence in these filters. The dielectric partially etched grating structures can achieve narrow spectral profiles with good extinction, however exhibiting strong angle dependence. The partially etched structures benefit from an additional degree of freedom to design narrower resonance by choosing optimum etch depth. The large angle sensitivity is attributed to the guided-mode resonance profile spread within the unetched high refractive index medium.
Fig. 6. Electric field intensity profiles along YZ plane for TM polarization at (a) θ = 0°, (b) θ = 10°, (c) θ = 20°, and (d) θ = 30° and TE polarization at (e) θ = 0°, (f) θ = 10°, (g) θ = 20°, and (h) θ = 30°.
Table 1. Comparison of various experimental reports of GMR based mid-IR notch-filtersa
Though such filters are good for angle tuning application they are generally not suited for wide field-of-view application. In the other extreme are the metallic nano-aperture structures which rely on localized surface plasmon resonance to achieve extraordinary light transmission through the nanoscale metallic aperture with strong field concentration close to the metal aperture. This results in angle insensitive performance (across ∼ 60° AOI), however accompanied by wider spectral widths. The metallic aperture dimensions of hundred nanometers would require precision electron-beam or stepper lithography techniques, which may limit the scalability of such structures to larger areas. The spectral characteristics of dielectric high contrast grating structures as discussed in the present work generally falls in between the above two structures with the benefit of using micrometer-scale dielectric features with reduced angle sensitivity. Such polarization independent high contrast grating filters with angle insensitive operation over a moderate angular range of up to 30° AOI can still find potential application as notch filters in infrared thermal imaging in combination with imaging lenses that support a numerical aperture of up to 0.5 or f/# as large as 1 [20]. Even though the filter characteristics reported here are asymmetric, as reported previously with other GMR based filters [5,8,13], such filters have found applications in infrared multi-spectral imaging [5] and sensing [21] applications. Further improvements to the filter characteristics can be achieved by reducing the filter bandwidth and increasing the extinction ratio. This can be achieved with the present design by improving the fabrication process, especially by controlling the aGe film uniformity and by minimizing etch process induced roughness. Other narrower spectral width resonance with higher quality factors can also be explored during design, especially by increasing the pillar height to explore narrower spectral features near anti-crossing type resonances [11]. This would require fabrication optimization to etch thicker aGe films.
6. Conclusion
In conclusion, the experimental demonstration of polarization-independent, angular tolerant mid-IR spectral resonances for notch filtering applications using aGe high index contrast gratings structures is discussed in this paper. The electromagnetic simulations and experimental results show good agreement with center wavelength of the resonance dip at 7.42 µm with measured spectral width of 0.56 µm and extinction of 8 dB. The angle dependence of the resonance characteristic reveals largely angle insensitive operation over 0-30° AOI with spectral shifts much smaller than the width of the resonance under consideration. The demonstrated polarization-independent, angle-insensitive filters can find potential applications in imaging applications utilizing un-polarized incident light, while at the same time having moderate field-of-view imaging capability of up to 30° incidence angle. Further reduction in filter bandwidth and increase in extinction can be achieved by improving the fabrication process to minimize scattering related losses and by choosing higher quality factor resonant structures during design
Funding
Nano Mission Council, Department of Science and Technology; Ministry of Electronics and Information technology; Science and Engineering Research Board.
Acknowledgments
The fabrication work and characterization were carried out at the National Nanofabrication Centre (NNFC) and Micro Nano Characterization Facility (MNCF) respectively, located at the Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore.
VR and SKS acknowledge financial support from NNetra program funded by DST Nanomission and MeitY. VR acknowledges SERB - Early career award. SKS acknowledges Visvesvaraya young faculty research fellowship by MeitY.
Disclosures
The authors declare no conflicts of interest.
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