School of Engineering, University of Glasgow
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Grant, J. P., McCrindle, I. J. H., Cumming, D. R. S. Simulation, Fabrication and Characterization of THz Metamaterial Absorbers. J. Vis. Exp. (70), e50114, doi:10.3791/50114 (2012).
Metamaterials (MM), artificial materials engineered to have properties that may not be found in nature, have been widely explored since the first theoretical1 and experimental demonstration2 of their unique properties. MMs can provide a highly controllable electromagnetic response, and to date have been demonstrated in every technologically relevant spectral range including the optical3, near IR4, mid IR5 , THz6 , mm-wave7 , microwave8 and radio9 bands. Applications include perfect lenses10, sensors11, telecommunications12, invisibility cloaks13 and filters14,15. We have recently developed single band16, dual band17 and broadband18 THz metamaterial absorber devices capable of greater than 80% absorption at the resonance peak. The concept of a MM absorber is especially important at THz frequencies where it is difficult to find strong frequency selective THz absorbers19. In our MM absorber the THz radiation is absorbed in a thickness of ~ λ/20, overcoming the thickness limitation of traditional quarter wavelength absorbers. MM absorbers naturally lend themselves to THz detection applications, such as thermal sensors, and if integrated with suitable THz sources (e.g. QCLs), could lead to compact, highly sensitive, low cost, real time THz imaging systems.
This protocol describes the simulation, fabrication and characterization of single band and broadband THz MM absorbers. The device, shown in Figure 1, consists of a metal cross and a dielectric layer on top of a metal ground plane. The cross-shaped structure is an example of an electric ring resonator (ERR)20,21 and couples strongly to uniform electric fields, but negligibly to a magnetic field. By pairing the ERR with a ground plane, the magnetic component of the incident THz wave induces a current in the sections of the ERR that are parallel to the direction of the E-field. The electric and magnetic response can then be tuned independently and the impedance of the structure matched to free space by varying the geometry of the ERR and the distance between the two metallic elements. As shown in Figure 1(d), the symmetry of the structure results in a polarization insensitive absorption response.
1. Simulation of a Single Band THz Metamaterial Absorber
A 3D view of the simulation set-up is shown in Figure 2.
load("Jove_sim"); # loads the simulation file
f = getdata("r95","f"); # gets the reflection data and frequency data from the r95 monitor
R = transmission("r95"); # defines R as a matrix containing the reflection data
A = 1-R; # calculates the absorption
fthz=f/1e12; # converts frequency to THz
plot(fthz,A,R,"Frequency(THz)","Absorption","Reflection"); # plot data on a single graph
filename = "Jove_sim.csv"; # export data for excel import
rm(filename); # delete file if it already exists
lambda = c/f; # converts frequency to wavelength
write(filename,num2str(f(i)) + "," + num2str(fthz(i)) + "," + num2str(R(i)) + "," + num2str(A(i)) + "," );
} # outputs a .csv file in column format of the frequency, reflection and absorption
2. Fabrication of a Single Band THz Metamaterial Absorber
Figure 3 shows the most important fabrication steps.
3. Characterization of a Single Band THz Metamaterial Absorber
Figure 4 shows the basic FTIR instrument layout.
Figure 5(a) shows the experimentally obtained and simulated absorption spectra for a MM absorber with a 3.1 μm thick polyimide dielectric spacer. This MM structure has a repeat-period of 27 μm and dimensions K = 26 μm, L = 20 μm, M = 10 μm and N = 5 μm. Experimental measurements were also performed on samples with no ERR layer to confirm that absorption was a consequence of the MM structure and not of the dielectric. The 7.5 μm thick polyimide sample with no ERR structure has a maximum absorption of 5 % across the frequency range of interest, see Figure 5(a), thereby verifying that at the resonance frequency absorption is a result of the MM structure. The experimental data shows a resonance peak at 2.12 THz of 77% absorption magnitude. This result is in excellent agreement with the simulated absorption maximum of 81% at 2.12 THz. Figure 5(b) shows the experimental data for MM absorbers with the same ERR geometry for different polyimide thicknesses ranging from 1-7.5 μm and for an absorber where the dielectric is 3 μm of SiO2. As the polyimide thickness increases from 1 μm to 3.1 μm the peak absorption increases, but at polyimide thicknesses greater than 3.1 μm there is a slight reduction in the peak absorption value. A distinct red-shift of 0.25 THz is observed as the polyimide thickness increases from 1 μm to 7.5 μm. Absorbers that had SiO2 as the dielectric instead of polyimide were also studied. A maximum absorption value of 65% at 1.90 THz was measured for such a MM absorber with a 3 μm thick SiO2 dielectric layer.
The effective permittivity and permeability can be extracted from the simulated data via inversion of the S parameters22. The retrieved parameters for the simulated MM absorber with a 3.1 μm thick polyimide spacer are displayed in Figure 5(c). As can be observed the real parts of the optical constants cross close to zero - a condition required for zero reflection, while whenever the real part of the permittivity is positive the real part of the permeability is negative and vice versa - a condition required for zero transmission. At the frequency of maximum absorption, ω0, there is a peak of the imaginary component of the permeability implying high absorption.
Lumerical FDTD can also be used to establish the location of the absorption within the MM structure. The simulated power absorption distributions for the ERR, dielectric and the ground plane layers are shown in Figures 6(a-c) while a cross-section of the power distribution in the xz plane at y = 3 μm is shown in Figure 6(d). From these plots it is clear that the majority of the energy is dissipated as Ohmic loss in the ERR layer and as dielectric loss in the first 500 nm of polyimide below this layer. The regions of maximum absorption loss occur between adjacent unit cells and around the inner edges of the cross.
Table 1. Geometric parameters of the multi-layer absorber.
Metamaterial absorbers are inherently narrowband devices; the bandwidth typically being no more than 20% of the centre resonant frequency. Several applications, such as THz spectroscopy, require sensors that exhibit broadband THz absorption. We have developed two strategies to realize such broadband absorption. The first, depicted in Figure 7(a-c) is to stack alternating layers of metallic ERRs and dielectric layers on top of a continuous ground plane. In different layers we design crosses of differing lengths (L1 - L3) in order to support several resonant modes closely positioned together in the absorption spectrum. By tuning the dielectric thickness (h1 - h3) the multi-layer structure can be impedance-matched to free space at each resonant frequency and broadband absorption obtained. A standard electron beam registration process is used to align the ERRs on top of one another. Our second strategy is to incorporate four ERRs into a four "colour" super-pixel, see Figure 7(d), onto a single dielectric layer i.e. ground plane/dielectric/metallic ERRs. Such a device is much simpler to fabricate than the multi-layer absorber.
The experimentally obtained absorption spectrum and the simulated data for a multi-layer MM absorber, with dimensions stated in Table 1, are shown in Figure 8(a). Also plotted is the experimentally obtained absorption spectrum for a single ERR of arm length 17 μm and dielectric thickness of 2 μm. The one layer structure has a single resonance peak at 5.42 THz where 78 % of the EM radiation is absorbed. In contrast, the 3-layer device has three resonances at 4.32, 5.31 and 5.71 THz with absorption magnitudes of 66 %, 77 % and 80 % respectively. Owing to these three closely position resonant peaks we obtain a wide frequency band, from 4.08 THz to 5.94 THz, where the absorption is greater than 60 %. Taking the central frequency of the 3-layer structure to be 5.01 THz the full width half maximum (FWHM) of the absorption is 48 % of the central frequency. This is almost two and a half times the FWHM of the single layer structure (FWHM of the single layer is 20 %). The experimental data is in reasonable agreement with the simulated spectrum.
To understand the origin of the spectral characteristics the simulated absorption distributions in the x-z plane of the three resonances are plotted in Figure 9 (a-c). The resonance at 4.84 THz is primarily associated with excitation of the bottom ERR layer while the resonances at 5.16 THz and 5.70 THz are mainly a consequence of excitation of the middle and top ERR layers respectively. These distributions clearly reveal that each ERR contributes to the broadband absorption.
An SEM image of a four colour super-pixel THz absorber is shown in Figure 7(d). Figure 8(b) shows the simulated and experimental absorption spectra for a super-pixel with arm lengths of 17 μm, 15 μm, 13 μm and 11 μm and arm widths of 6 μm. The pixel period is 44 μm while the polyimide thickness is 2 μm. Four resonances are observed in both the simulation and experimental data. The disadvantage of such a super-pixel structure is that, as shown in Figure 8(b), there is some polarization dependence. For both polarizations the super-pixel absorber has greater than 50% absorption between 5.08 and 7.27 THz; a range of 2.19 THz. The FWHM for TE polarization is 37% while it is 41% for TM polarization, representing double the FWHM of the single pixel.
Figure 1. (a) Schematic of the ERR of the MM absorber and (b) cross-section of complete MM absorber. A current is induced in the sections of the ERR that are parallel to the E field (direction denoted by blue arrows in (a). An anti-parallel image current flows in the regions of the ground plane imemdiately below the cross, resulting in a resonant response. (c) SEM image of the unit cell and (inset) section of the array. (d) Simulated absorption spectra for different incident polarization angles showing polarization insensitivity of the MM absorber. Each successive plot from 0-90 ° is offset by one major unit of the ordinate axis.
Figure 2. 3D Schematic of the simulation set-up.
Figure 3. Fabrication of single band MM absorber. 1) A 20 nm/100 nm Ti/Au stack is evaporated onto a 15 mm by 15 mm section of silicon. 2) PI2545 is spin coated onto the sample, baked at 140 °C and then cured at 220 °C. 3) A bi-layer of 15% 2010 and 4% 2041 is spin coated and baked at 180 °C. 4) After exposure to a 100 keV electron beam the sample is developed in a solution of MIBK and IPA. The 2010 PMMA, owing to its lower molecular weight, develops faster than the 2041 PMMA. This results in the desired overhang profile required to attain successful lift-off. 5) A 20 nm/150 nm Ti/Au film is evaporated onto the sample. 6) Unwanted regions of metal are lifted-off by immersing the sample in a beaker of warm acetone.
Figure 4. Schematic of a Fourier Transform Infrared Spectrometer27.
Figure 5. (a) Experimental and simulated data of a MM absorber with a polyimide thickness of 3.1 μm. Also plotted is the absorption of a 7.5 μm thick polyimide film. (b) Experimental absorption spectra for MMs with differing dielectric spacer thickness and type. (c) Extracted optical parameters from the simulated 3.1 μm thick polyimide MM absorber. Click here to view larger figure.
Figure 6. Energy dissipation in a MM absorber structure with a 3.1 μm thick polyimide spacer at a frequency of 2.12 THz. Energy dissipation in (a) the ERR layer, (b) the centre of the polyimide, (c) the ground plane and (d) xz plane at y = 3 μm.
Figure 7. (a) Plan view of the 3-layer MM absorber and (b) cross-section of the complete device. (c) SEM image of 9 unit cells of the multi-layer absorber and (d) SEM image of a single 'super-pixel' broadband absorber. The orientation for TE polarization is shown in the inset.
Figure 8. (a) Experimental and simulated (FDTD) data of the multi-layer absorber. Also plotted is the experimental absorption spectrum for a single layer absorber. (b) Absorption spectra for the 'super-pixel' broadband absorber.
Figure 9. (a-c) Absorption distribution in the x-z plane at y = 0 μm at the three resonant frequencies. The horizontal white lines denote Au layers.
This protocol describes the simulation, fabrication and characterization of THz metamaterial absorbers. It is essential such sub-wavelength structures are accurately simulated before any effort is committed to costly fabrication procedures. Lumerical FDTD simulations provide information on not only the MM absorption spectrum but also the location of the absorption, essential knowledge to aid placement of a transducer and obtain the maximum response. In addition the optimization algorithm in Lumerical can be implemented to rapidly establish an appropriate absorber structure for a pre-defined figure of merit (e.g. frequency position, absorption maximum, absorption minimum, bandwidth etc). Simulation, fabrication and characterization of a single band MM absorber can be completed in less than 24 hr allowing rapid prototyping of any design. Our multi-layer broadband absorber consists of three separate electron beam write steps (two registration steps) and could be realized in less than 4 days. We have also fabricated absorbers which have SiO2 and Si3N4 insulating regions between the ERR and the ground plane. These layers were deposited by PECVD and ranged in thickness between 0.6 and 3 μm. The absorption magnitudes were similar to devices with polyimide dielectric layers however there was a red shift in the frequency position for absorbers of the same thickness.
The beauty of metamaterials is their inherent scalability - absorber structures have been demonstrated from the mm23 region through to infrared and optical frequencies24. These devices consist of the standard metallic ERR/insulator/metallic structure with the appropriate ERR feature size and insulator type and thickness. In our design the resonant frequency position is mainly dependent on the period, cross arm length of the structure and insulator type while the absorption magnitude is determined by the thickness of the insulating layer. The resonant frequency position of our cut-out cross design is blue shifted compared to more traditional whole cross designs (no-cut out sections). This allows the pixel period to be reduced for a particular targeted resonant frequency (e.g. 2.52 THz) and has important implications for THz imaging applications. A major advantage of our device is that in contrast to more complex and computationally intensive ERR geometries our ERR geometry is simple to understand and computationally undemanding. While we use effective medium theory to describe our metamaterial absorbers, a different explanation centering on interference theory has recently been proposed25.
Research into THz radiation, with wavelengths between 30 μm and 3 mm, has burgeoned in the last decade. This interest has been stimulated by the unique properties of THz rays; they can penetrate materials such as plastics, paper and many organic compounds, including human tissue, without the hazards or potential dangers associated with ionising radiation such as x-rays. Furthermore, THz may be used to identify specific materials via their characteristic spectra, including explosives, hazardous chemicals, drugs and DNA, as molecular rotations and vibrations occur in this wavelength range. Accordingly THz imaging has found applications in areas such as security, healthcare, pharmaceuticals, automotive, materials science and non-destructive testing.
However there are many unfulfilled opportunities owing to the lack of low-cost, compact and easily deployable equipment. Present THz imaging systems cost >£250k, use mirrors for optics and mechanically raster a single pixel. A further limitation of existing commercial systems is the time taken to produce an image from the mechanically rastered single pixel detector, taking minutes to hours to compile detailed images. IR focal plane arrays, typically comprising array sizes of 640x320 pixels read out at 30 Hz, have been used for THz imaging applications26 however these sensors have less than 5% absorption in the THz region and do not provide sensitive enough detection. Integration of our single band or broadband THz metamaterial absorber with a thermal sensor, such as a pn diode or resistive bolometer, into a focal plane array would realize a device capable of absorbing 80% of the THz radiation at the resonance frequency. Such a device would provide a highly sensitive, frequency selective, real time, compact, room temperature THz imaging sensor.
No conflicts of interest declared.
This work is supported by the Engineering and Physical Sciences Research Council grant number EP/I017461/1. We also wish to acknowledge the contribution played by the technical staff of the James Watt Nanofabrication Centre.
|Silicon wafer||IDB technologies||Single sided polished|
|Plassys 450 MEB evaporator||Plassys Bestek|
|Methyl Isobutyl Ketone||Sigma-Aldrich|
|Plasmaprep5 barrel Asher||Gala Instrumente|
|VB6 UHR EWF electron beam writer||Vistec|
|Tanner L-Edit||Tanner Inc.|
|Layout Beamer||GenISys Inc.|
|Polymethyl methacrylate (PMMA)||Sigma-Aldrich||293261 Sigma-Aldrich|
|IFV 66v/s FTIR||Bruker|
|Pike 30spec reflection unit||Pike Technologies|
|Hg arc lamp||Bruker|
|Au mirror||Thor Labs||PF05-03-M01|
|Leica INM20 Optical Microscope||Leica microsystems|
|6 mm Mylar Beamsplitter||Bruker|