Multi-spectral Materials: Hybridisation of Optical Plasmonic Filters and a Terahertz Metamaterial Absorber Journal Article

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Multi-spectral imagers can combine several spectrally specifi c cameras into a single system. It is also possible to use image fusion algorithms to provide a composite image of a scene over several wavebands. A single image that combines data from multiple wavebands can convey more information than individual images at each wavelength. [1][2][3][4] The diversity of materials required for imaging separate spectral bands mean that current wide-band multi-spectral imaging systems must use a range of different cameras. For example, a common digital camera uses silicon photodiodes as part of a complementary metal-oxide semiconductor (CMOS) chip, whereas an IR detector requires a different semiconductor material, such as indium antimonide (InSb), to act as a photodiode. [ 5 ] IR imagers can also be made with microbolometer arrays utilising materials such as vanadium oxide (VOx). [ 6 ] VOx has also demonstrated potential at terahertz (THz) frequencies [ 7 ] and through integration with a THz metamaterial (MM) absorber, has been optimised for THz imaging. [ 8 ] Further spectral selectivity within specifi c wavebands can also be desirable in applications such as optical imaging, where dye doped colour fi lters are often used in conjunction with photodiodes to form a full colour image of a scene.
Work to date has shown the potential for co-integration of different wavebands on a single chip. [ 9,10 ] However, because the different wavelength sensors are not coaxial, the visible/IR and THz wavebands must be detected using different regions on the surface of the chip. We propose the use of a synthetic multispectral material (SMM): a single structured material capable of operating over different wavebands simultaneously, thereby maximising the spectral information density of a multi-spectral imager in a coaxial format. SMMs exploit hybridised plasmonic and MM structures to combine multi-spectral functionalities into a single material. [ 11d ] Surface plasmons (SP) are electron density oscillations at the interface between a conductor and a dielectric. SPs can resonantly couple with incident light to form surface plasmon polaritons (SPP) in a process known as surface plasmon resonance (SPR). Periodic subwavelength hole arrays, at optical wavelengths, etched into a thin metal fi lm can excite SPR leading to enhanced transmission and wavelength fi ltering of the incident light. [12][13][14][15] Colour fi lters have been fabricated by etching triangular subwavelength hole arrays of varying periods and hole sizes into a 150 nm aluminium fi lm, sandwiched between two silicon dioxide layers. [ 16,17 ] Plasmonic fi lters have been optimised for digital imaging by engineering transmission spectra corresponding to the 1931 International Commission on Illumination (CIE) colour matching functions [ 18 ] and have also been integrated with CMOS image sensors. [ 19,20 ] Filters fabricated into a metal layer as part of the CMOS process offer a solution to substantial cross-talk between different colours that is expected as CMOS imagers scale to smaller sizes. [ 21,22 ] It is also possible to extend the operation of plasmonic fi lters to near IR (NIR) wavelengths by further scaling of the hole size and period. [ 11 ] At THz frequencies the plasmonic response is diminished due to the high conductivity of the metal, however, a MM with a specifi c unit cell geometry can still exhibit resonant behaviour. [23][24][25][26] A MM absorber consists of a metal ground plane and an electric ring resonator (ERR) array, separated by a dielectric spacer. The ERR couples to the incident electric fi eld and magnetic coupling is provided by the inclusion of a ground plane, as can be observed by anti-parallel currents on the metal surfaces. [27][28][29][30][31][32] At resonance, absorption is maximised by matching the wave impedance of the MM to the wave impedance of free space and engineering a large extinction coeffi cient. [28][29][30] Our SMM consists of plasmonic fi lters fabricated into the ground plane of a THz MM absorber, which uses a hollow cross shaped ERR. The SMM is capable of fi ltering fi fteen optical wavelengths and a NIR wavelength whilst simultaneously absorbing a single THz frequency through exploitation of two unique electromagnetic phenomena. The sparseness of high performance THz detectors means that it is advantageous to include the THz detection medium as part of the SMM. Hybridisation of the MM absorber with plasmonic fi lters yields a signifi cant advancement towards the creation of a coaxial multi-spectral imager operating in visible, NIR and THz wavelength regimes.
The SMM geometry, layer structure and composition were optimised by performing simulations using Lumerical FDTD Solutions. [ 33 ] The plasmonic fi lter structures are signifi cantly smaller than the MM absorber features and required a very fi ne mesh to defi ne the holes. The small mesh resulted in a substantial increase in memory requirements compared with a standalone MM absorber, such that it was impractical to simulate the complete SMM structure at THz frequencies. However, it is possible to consider the ground plane of the SMM as an unperforated metal fi lm at THz frequencies. [ 11 ] Optimisation of the entire SMM was performed by independently simulating the plasmonic fi lters and the MM absorber, with an unperforated ground plane. Details of the simulation procedures are outlined in the experimental section.
The resonant wavelength, λ SPP , for a plasmonic fi lter triangular hole array is given by Equation ( 1 ) :

Multi-Spectral Materials: Hybridisation of Optical Plasmonic Filters and a Terahertz Metamaterial Absorber
where P is the period of the hole array, ε m is the dielectric constant of the metal, ε d is the dielectric constant of the dielectric, and i and j are the scattering orders of the array. [ 15,16 ] The sixteen colour plasmonic fi lter set was designed by scaling the hole size and the array period to shift the resonant wavelength, whilst maintaining similar bandwidths and transmission magnitudes. It is possible to increase the transmission magnitude by using larger holes or a thinner metal, however this also results in an increased bandwidth. [ 16 ] A cross section of the simulated plasmonic fi lter structure is shown in Figure 1 a; the simulation region (enclosed by a solid purple line) and the unit cell (enclosed by a dashed purple line) are shown in Figure 1 b. The MM component geometry was inspired by an established THz MM absorber. [ 31 ] The constituent materials and layer thicknesses were altered to optimise the MM absorber for integration with optical plasmonics. The designed MM absorber consisted of a 150 nm aluminium ground plane and a 150 nm hollow cross aluminium ERR, separated by a 3 μ m silicon dioxide spacer. Silicon dioxide was chosen as the dielectric spacer material as it exhibits low loss at optical wavelengths and aluminium was chosen because it exhibits high conductivity at THz frequencies whilst still being suffi ciently low loss at optical wavelengths to be used for optical plasmonics fi lters. into the ground plane of the MM absorber. The MM absorber unit cell that was simulated is as is shown in Figure 1 c-d, but with an unperforated ground plane. The absorption spectrum, A( ω ) , was calculated using Equation ( 2 ) : where R( ω ) is the refl ection spectrum and T( ω ) is the transmission spectrum, as determined by simulations. [ 27 ] The procedure for fabricating the SMM is outlined at the end of the article. The MM absorber ground plane consisted of sixteen plasmonic fi lters. Each fi lter was 1 mm × 1 mm in size and was separated from neighbouring fi lters by 0.33 mm; the sixteen fi lters extended to a square area of 5 mm × 5 mm. The hole diameters of each of the fi lter arrays were measured prior to deposition of silicon dioixde on to the ground plane. A SEM image of a red plasmonic fi lter ( P = 430 nm) is shown in Figure 2 a,b.   respectively. Figure 2 f shows a tessellated picture of the SMM surface. The ERR structures are not resolved in Figure 2 f. The spectral characteristics of the SMM were investigated in detail using sources and detectors appropriate to the wavelengths of interest and the measurement procedure is described in the experimental section at the end of the article. The transmission spectra from the plasmonic fi lter regions of the SMM are shown in Figure 3 a-c. The legend denotes the hole period and the measured hole diameters of each fi lter. Fabry-Perot oscillations appear in the transmission spectra due to the silicon dioxide dielectric spacer. The peaks are consistent over all colour fi lters and correspond to a 3 μ m resonator cavity with refractive index: n = 1.48. [ 34 ] The transmission and refl ection spectra of the SMM at THz frequencies were measured and, using Equation ( 2 ) , it was possible to calculate the absorption spectrum, as is shown in Figure 3 d. It was observed that the MM absorber component of the SMM exhibits 67% absorption at 1.93 THz.
Our transmission measurements have been made over a large area and we observe a drop in light intensity over the visible range due to the ERR array. The ERR geometry and plasmonic fi lter positioning can be optimised to maximise the fi ltered light transmitted over a desired region, whilst still retaining frequency selective THz absorption of the MM component of the SMM. The reduction in transmission magnitude due to the ERRs can be quantifi ed and accounted for by scaling the transmission spectra with respect to the ERR metal fi ll factor on the surface of the SMM (33.6%), thereby approximating the standalone plasmonic fi lter spectra. The scaled spectra for three of the plasmonic fi lter on the SMM, and the corresponding simulation results, are shown in Figure 4 a. Palik aluminium and silicon dioxide models were used to classify the simulated materials. [ 35 ] It should be noted that the simulated structure only used a 200 nm cap layer and therefore did not exhibit Fabry-Pérot oscillations in this wavelength range.
The measured absorption spectra from the SMM and from a MM absorber of the same design, but without a perforated ground plane, are shown in Figure 4 b. It can be observed that both structures exhibit similar spectral characteristics in this frequency range, thereby demonstrating the negligible impact that the plasmonic fi lters have on the MM response. Simulations of the MM absorber were repeated to account for the optical properties of the fabricated materials. The silicon dioxide layer was modelled using the complex The ERR array that was patterned after silicon dioxide deposition covered an area of 12 mm × 12 mm over the plasmonic fi lters.
Transmission microscope images of the completed SMM are shown in Figure 2 c-f. Figure 2 c-e show the ERR array above blue, green and red plasmonic fi lter ground plane sections,  material with a CMOS imaging chip to create a multi-spectral camera. SMMs have the potential to maximise the spectral information density of an optical system. By using a single material with multiple engineered optical properties over the same surface area we open up the possibility of creating high resolution multi-spectral imagers.

Experimental Section
Electromagnetic Simulations : The plasmonic fi lter simulations are set up as follows: a 150 nm aluminium layer was placed between a semiinfi nite silicon dioxide layer and a 200 nm silicon dioxide cap layer. The aluminium was patterned with two glass holes and the silicon dioxide cap layer was patterned with two etch holes to account for the cap layer topography after deposition of silicon dioxide. A mesh grid with a maximum cell size of 5 nm was defi ned in the vicinity of the holes. Symmetric and anti-symmetric boundary conditions were used to form the triangular hole array structure, and perfectly matched layers (PML) were used in the z boundaries. The aluminium surface was illuminated by a 400 nm to 1 μ m plane-wave source and the transmission spectra were recorded by a monitor placed on the opposite side of the aluminium.
A similar simulation method was used for the MM absorber in which the structure was illuminated from above by a plane-wave source. Symmetric and anti-symmetric boundary conditions were once again used in x and y ; PML boundary conditions were used in the z -direction. The maximum mesh step size in the region of the MM was 75 nm in x and y , and 25 nm in z . Refl ection and transmission monitors were placed above and below the MM unit cell, respectively.
Fabrication of the Synthetic Multi-Spectral Material : The plasmonic fi lter ground plane was fabricated by evaporating a 150 nm aluminium fi lm on to a glass slide. 50 nm of silicon nitride was deposited on to the aluminium to improve the adhesion of spin coated ZEP520A electron beam resist with the material surface. The sixteen colour plasmonic fi lters were patterned into the resist using a Vistec VB6 electron beam lithography (EBL) tool. The nanohole patterns were developed in o-xylene at 23 °C for 35 s and were etched using standard dry etch processing. The silicon nitride layer was etched using trifl uoromethane / oxygen (CHF 3 /O 2 ) in an Oxford Instruments RIE80+ and the aluminium was etched using silicon tetrachloride (SiCl 4 ) in an Oxford Instruments RIE100. The ZEP520A and silicon nitride mask were then removed from the aluminium surface. The sample was then cleaned and 3 μ m of silicon dioxide was deposited on top of the aluminium layer using plasma enhanced chemical vapour deposition (PECVD). As the plasmonic fi lters are unaltered for use as a ground plane in the SMM, the fabrication procedure outlined here can be used to make standalone plasmonic fi lters. [ 16,20 ] The SMM was completed by fabricating the ERR array on the silicon dioxide surface. Fabrication of the ERR array was carried out by spin coating a bilayer of poly(methyl methacrylate) (PMMA) on to the silicon dioxide surface. EBL was used to defi ne features in the PMMA and the PMMA was developed using isopropyl alcohol: methyl isobutyl ketone (IPA: MIBK). A 150 nm layer of aluminium was deposited on top of the device and a lift off process utilising hot acetone was used to remove the remaining resist and unwanted aluminium.
Experimental Characterization of the Synthetic Multi-Spectral Material : The optical and NIR spectra from the plasmonic fi lter sections of the SMM were measured using a TFProbe MSP300 microspectrophotomer with a white light halogen lamp source and a detector with a spot size of 100 μ m. The transmission spectra were normalised to the white light source.
Characterisation of the MM absorber component of the SMM was performed using a Bruker IFS 66v/S Fourier transform infrared spectrometer (FTIR). The sample was illuminated by a mercury arc lamp and the transmission spectrum at normal incidence, T( ω ) , and refl ection spectrum at 30°, R( ω ) , were measured. The transmission spectrum was refractive index: n = 2.04 + i 0.16 and refractive index parameters, defi ned by Rakić, [ 36 ] were used to characterise the aluminium at THz frequencies. The observed simulated absorption magnitude of 71.2% at 1.94 THz is in good agreement with the measured SMM absorption spectrum. The simulation results imply that the silicon dioxide we have deposited has a higher absorption coeffi cient than has previously been documented. [ 37 ] In this article we have demonstrated the design, fabrication and characterisation of a new type of multi-spectral material that hybridises optical plasmonic fi lters with a THz MM absorber to combine multiple functionalities as well as multispectral capabilities into a single material. Our device exhibits multiple functionalities by combining the THz absorber material within the colour fi lter structure and therefore eliminates the need for a separate material for THz detection.
We have fabricated a SMM that combines fi fteen colour plasmonic fi lters, a single NIR plasmonic fi lter and a frequency selective THz MM absorber. Our device demonstrates that the presence of plasmonic fi lter hole arrays in a THz MM absorber ground plane have negligible impact on the THz absorption spectrum associated with the MM.
The plasmonic fi lters can be used in conjunction with photodiodes and the THz MMs can be combined with a detector material, for example a microbolometer, therefore demonstrating a clear path for integrating a multi-spectral . Experimental and simulated spectral characteristics of the synthetic multi-spectral material (SMM). (a) Experimentally measured transmission spectra from the blue, green and red plasmonic fi lters sections of the SMM, scaled to account for the presence of the electric ring resonators (ERRs). Also shown is the simulated transmission spectra for the same periods and hole sizes for standalone fi lters with a 200 nm cap layer. (b) Experimentally measured terahertz (THz) absorption spectra of the SMM compared with a standalone metamaterial (MM) absorber of the same design, but without a perforated ground plane. Also shown are the simulation results for the standalone, unperforated MM absorber.