ABSTRACT This paper reports the design, the microfabrication and the experimental characterization of an ultra-thin narrow-band metamaterial absorber at terahertz frequencies. The

metamaterial device is composed of a highly flexible polyimide spacer included between a top electric ring resonator with a four-fold rotational symmetry and a bottom ground plane that

avoids misalignment problems. Its performance has been experimentally demonstrated by a custom polarization-maintaining reflection-mode terahertz time-domain spectroscopy system properly

designed in order to reach a collimated configuration of the terahertz beam. The dependence of the spectral characteristics of this metamaterial absorber has been evaluated on the azimuthal

experimental set-up can be applied to investigate arbitrary polarization-sensitive terahertz devices under oblique incidence, allowing for a wide reproducibility of the measurements. SIMILAR

CONTENT BEING VIEWED BY OTHERS POLARIZATION INSENSITIVE AND WIDEBAND TERAHERTZ ABSORBER USING HIGH-IMPEDANCE RESISTIVE MATERIAL OF RUO2 Article Open access 19 August 2024 ALL-DIELECTRIC

POLARIZATION-SENSITIVE METASURFACE FOR TERAHERTZ POLARIMETRIC IMAGING Article Open access 30 March 2024 WIDE-ANGLE METAMATERIAL ABSORBER WITH HIGHLY INSENSITIVE ABSORPTION FOR TE AND TM

MODES Article Open access 12 August 2020 INTRODUCTION Terahertz (THz) technology has recently received growing attention, enabling it to overcome the so-called “ THz gap” thus connecting the

fields of electronics and optics1. While the microwaves and the far-infrared frequency regions immediately below and above the THz band, respectively, have been extensively investigated,

the THz frequencies are still the aim of the research breakthroughs. An intense interaction between the disciplines of physics and engineering has made it possible to generate and detect

broadband coherent THz radiation. In particular, THz time-domain spectroscopy (THz-TDS) has matured into an extremely useful tool for characterizing THz devices, thanks to its

non-destructive and non-invasive features2,3,4. The non-ionizing nature of the THz frequencies has promoted advanced applications in medical diagnoses and in material science, allowing it to

identify different substances through their specific absorption patterns5,6. THz devices have also been successfully applied in security detection7,8,9, because non-conductive and non-polar

materials, such as packaging, plastics, paper and ordinary clothes, show low absorption at these frequencies. In this context, the THz regime represents a fertile area for the development

of metamaterial absorbers (MMAs)10,11,12,13, which are manmade devices designed to absorb specific bands of the incident electromagnetic radiation and generally constituted of periodic

structures with sub-wavelength unit cells14. Their customizable optical characteristics15,16,17 are not observed in their constituent materials, but they can be artificially manipulated by

employing electromagnetic resonators. This resonant spectral feature is of particular interest at THz frequencies, where it is difficult to find natural materials with both narrow absorption

bands and high absorption coefficients18. In addition, since the unit cell sizes necessary for the THz frequencies are of the order of tens of micrometers, they can be reproduced with high

precision through micro-fabrication processes based on standard photolithographic techniques. This can be very useful in the development of various MMAs and applications, such as highly

sensitive biochemical sensors19, microbolometers of thermal detectors20,21, THz stealth technology22, and THz imaging23. By exploiting the principle of scalability of these devices, once

proved their feasibility, it is conceivable to extend their use even at higher frequencies where the tolerances required in manufacturing processes are more critical. A specific-designed MMA

could be also dimensioned to operate over a wide range of frequencies spanning from microwave, THz, to the infrared (IR), and optical spectrum24,25. In order to assess a benchmark for

future MMAs development, it is fundamental to investigate the underlying physics on simple three-layer metal-dielectric-metal devices in which the interplay between the metallizations

through the dielectric layer can be more easily studied as a function of incidence angle and polarization. In general, it is essential to maximize the losses and consequently the absorbance

_A_(_ω_), where _ω_ is the angular frequency, in narrow spectral bands. According to the relation _A_(_ω_) = 1 − _R_(_ω_) − _T_(_ω_)26, this can be obtained through the simultaneous

minimization of the reflectance _R_(_ω_) by matching the impedance of the MMA to that of free space at a desired frequency, and of the transmittance _T_(_ω_) by employing a metallic ground

plane. This achievement normally requires first a preliminary study in which the characteristics of the resonator are defined in connection with the material properties of the middle spacer;

then followed by an optimization phase in which the geometrical features of the resonator and the thickness of the dielectric layer are adjusted for the best performance. Following the

design phase, an accurate experimental characterization of MMA electromagnetic response is needed. This makes the THz spectroscopy fundamental both to demonstrate the functionality of the

MMAs and to study their compliance with the design features. With the aim of implementing a valid THz experimental set-up for an MMA as described above, it is necessary to solve some

significant problems. Due to the presence of a lower metallic ground plate which prevents transmission of THz radiation through the device, measurements should be conducted in

reflection-mode. This configuration needs a higher number of THz optical components as compared to the more easily configurable transmission-mode set-up4. The quality of the experimental

results critically depends on alignment errors of THz optical components which might alter the measured signals due to differences in THz pulses path length and phase errors. In this paper,

we show how it is possible to realize and characterize at THz frequencies an ultra-thin ground-plane-backed MMA27 with an extremely selective absorbance spectrum. A specifically designed

polarization-maintaining reflection-mode THz-TDS set-up with innovative features is employed for its electromagnetic characterization. This three-layer metal-dielectric-metal configuration

has been chosen for the MMA to simplify the fabrication processes and to avoid misalignment problems with respect to other more complex multi-layer configurations already published23,26.

This approach reduces the number of experimental degrees of freedom in the fabrication process, allowing a more straightforward controllability of the prototype stage. The choice of

polyimide, as the dielectric spacer, also takes into account its possible application as a flexible MMA, adaptable to non-planar surfaces. The unit cell of the proposed MMA has been designed

to exhibit a single resonance in the THz frequency range accessible to the custom reflection-mode THz-TDS set-up, providing a well-recognizable experimental absorption signal particularly

useful for comparison with numerical simulations. Furthermore, the wide reproducibility found in the measurements proves our THz-TDS set-up to be potentially effective to accurately

characterize arbitrary THz polarization-sensitive devices in reflection mode under oblique incidence. In fact, in the proposed reflection-mode THz-TDS system, only two identical plano-convex

lenses and a custom aluminum prism reflector have been properly employed, thus avoiding the use of a complex set of parabolic mirrors to manipulate the THz optical path2,28,29, potential

source of disturbance for both polarization and propagation of THz radiation. RESULTS MMA DESCRIPTION The ultra-thin narrow-band MMA27 is realized with a three-layer metal-dielectric-metal

configuration consisting of a patterned frequency-selective surface (FSS)27,30 inspired by ref.23, a highly flexible insulating polyimide spacer, and a metal ground plane on a silicon

substrate. Polyimide PI-2611 from HD MicroSystems31 with nominal dielectric constant _ε′_ = 2.9, loss tangent _tan δ_ = 0.002, and thickness of 5.4 _μ_m, has been chosen as middle dielectric

layer for its high electrical and thermal stability, low refractive index, low absorption, and flexibility32,33,34,35,36,37,38,39,40. It is a well-established material within the

micro-fabrication processes, and it is used in applications of electronics on plastic and in photonic devices41,42. The upper electric ring resonator (ERR)23,27,30 (with optimized dimensions

specified in Fig. 1(a)) and the lower ground plane with 100 nm thickness are both made of lossy gold in order to ensure chemical stability and a high electrical conductivity (_σ_ = 4.09 × 

107 S/m). This ultra-thin MMA with unit cell of 80 _μ_m periodicity presents a significant subwavelength thickness of about _λ_0/49 at the absorbance frequency, where _λ_0 is the free-space

wavelength. The materials choice in the metamaterial design is a key point for the energy dissipation: in order to reach the desired absorbance, one needs to select highly conductive metals

and highly insulating dielectrics, i.e., materials with low losses in the working frequency band. By adopting these material characteristics and the optimized dimensions, we have reached in

the simulations a near unity absorbance at 1.09 THz, supposing a plane wave impinging on the unit cell under 16° oblique incidence in transverse electric (TE) polarization with the electric

field parallel to the ERR arm (_φ_ = 0°). These specific operating conditions have been implemented in the numerical simulations to take into account the restrictions imposed by the

experimental set-up. However, it has been shown in ref.27 how the proposed narrow-band MMA exhibits a wide angular response for both TE and transverse magnetic (TM) polarizations as the

incidence angle _θ_ varies (see Fig. 1(b)), even if the TE case exhibits the more critical working conditions. Indeed, under oblique incidence in TE polarization, the absorbance levels

undergo a monotonic decrease with the increase of the incidence angle, while in TM polarization the resonance peaks maintain high absorbance levels even under 80° incidence. EXPERIMENTAL

SET-UP FOR POLARIZATION-MAINTAINING REFLECTION-MODE THZ-TDS In order to characterize the electromagnetic behavior of the MMA in the THz range, we devised a THz-TDS system in reflection mode

(see Fig. 2(a)). This set-up is based on a standard Menlo Systems (Germany) TERA K15 configuration43,44,45 and it is designed to minimize the mobile optical and mechanical parts (mirrors,

lenses, and mechanical mountings). This is to avoid alignment errors, which might result in differences in THz pulses path length and phase errors, altering the measured signals and making

difficult the comparison with the theoretical predictions. Terahertz radiation is generated by photoconductive antennae that allow the optical to THz signal conversion4. The antennae are

excited by a femtosecond fiber-coupled laser (Menlo Systems T-Light) with an emission wavelength of 1560 nm, a repetition frequency 100 MHz, and pulse duration of nearly 90 fs. The remote

control station of the THz-TDS system is constituted by an amplifier with the aim of amplifying the detected amplitude signal, an A/D converter and a PC for control and preliminary data

processing (Fig. 2(a)). For the purpose of realizing the reflection configuration, the THz beam line of the system was designed by using a couple of HRFZ-Si plano-convex lenses with

effective focal length _f_ = 25 mm and a custom metallic aluminum prism (Fig. 2(a)). The diverging THz radiation emitted by the emitter antenna is collected and collimated in a beam with a

diameter of about 12 mm by means of the first lens (Lens 1 in Fig. 2(a)). Radiation collimated by Lens 1 impinges onto the prism and it is then reflected onto the sample (see Fig. 2(b)).

Radiation reflected by the sample is reflected by the prism along the THz beam axis, while the second identical lens (Lens 2) permits the THz beam to be focused on the detector antenna. This

realizes a reflection-mode collimated configuration in which the 12 mm diameter collimated beam of TE polarized THz wave impinges obliquely on the sample/reference with an angle _θ_ of

about 16° imposed by the prism geometry. It is to be noted that the THz beam line of the reflection-mode THz-TDS system has been optimized employing only two identical plano-convex lenses

and a custom aluminum prism reflector which determines the incidence and reflection angles on the MMA, thus maintaining the project requirements for a linearly polarized incident plane wave.

Further, the choice of a compact set-up in which the THz radiation interacts with a small number of components (two lenses and a metal prism) is functional to minimize disturbing effects

for both the polarization and the propagation of THz beam, which are instead likely present when one or more parabolic mirrors are used to manipulate the THz optical path. The aluminum prism

shown in Fig. 2(b), has been properly designed to allow the collimation of the THz beam on the sample following the derived relations: $$\chi ={\cot }^{-1}\sqrt{1-\frac{{h}_{p}}{h}}$$ (1)

$$b=\frac{{h}_{p}}{\sqrt{1-\frac{{h}_{p}}{h}}}$$ (2) where _χ_ is the semi-angle of the lower vertex of the prism, _h_ _p_ is its height, _h_ is the distance between the axis of the incident

THz beam on the prism and the sample surface, and _b_ is the basis of the prism. All the optical elements in the THz beam path were embedded in a dry nitrogen purged chamber at a relative

humidity of about 4% in order to reduce the water vapor contribution, particularly present in the frequency band of interest. All spectroscopic measurements were performed at room

temperature. The MMA (sample) and a reference made of an aluminum sheet of 970 _μ_m thickness were placed at about 36.5 mm from the lower vertex of the prism. The electric field of the THz

pulses propagating with the MMA sample and with the aluminum reference was measured. The THz pulses were averaged over 400 scans of the delay line with an overall acquisition time of 50 s

and a delay line scan range of 100 ps. By applying the fast Fourier transform (FFT) to the acquired signals, the sample and reference THz electric field spectral amplitudes and phases were

obtained as a function of frequency. The acquisition parameters of the THz set-up determines the frequency spectrum resolution Δ_f_ = 1/(_N_Δ_t_) = 10.02 GHz, being the time resolution of

the THz pulses Δ_t_ = 33.36 fs and the measured data points _N_ = 2992. In the measurement set-up, the correct placement of the surfaces of the MMA sample and of the aluminum reference at

the same distance from the lower vertex of the aluminum prism is fundamental in order to avoid differences in THz pulses path length and phase error. For this reason, a specific sample

holder allowing longitudinal and perpendicular micrometric displacement with respect to the lower vertex of the aluminum prism was employed. Finally, in order to obtain the complex spectral

reflection _S_11, i.e., the transfer function of the system which reports on the amplitude and phase changes due to absorbance and refraction of the MMA for a far-field characterization, we

have divided the sample spectrum _E_ _sam_ by the reference spectrum _E_ _ref_ : $${S}_{11}=|{S}_{11}|{e}^{j\angle {S}_{11}}=\frac{|{E}_{sam}|}{|{E}_{ref}|}{e}^{j(\angle {E}_{sam}-\angle

{E}_{ref})}.$$ (3) The absorbance can be, therefore, obtained by: _A_(_ω_) = 1 − |_S_11(_ω_)|2 = 1 − _R_(_ω_), where reflectance is defined as _R_(_ω_) = |_S_11(_ω_)|2. This because the THz

waves cannot be transmitted through the Si substrate due to the lower gold ground plane thicker than the penetration depth and, consequently, the transmittance of the MMA _T_(_ω_) = 

|_S_21(_ω_)|2 is negligible. SIMULATION AND EXPERIMENTAL RESULTS One of the main aims of this work was to experimentally characterize the MMA in view of the optimizations of its performances

under its more critical operating conditions, i.e., under oblique incidence in TE polarization27. In this case, the absorbance levels of the investigated MMA are typically lower, because

the four-fold rotational symmetry46,47 of the resonator about the propagation axis cannot be fully exploited. Due to the presence of the lower metallic ground plate which prevents

transmittance of the THz wave, we properly devised a compact polarization-maintaining THz-TDS set-up in reflection-mode able to guarantee a collimated configuration of the THz beam for

oblique incidence analysis in TE polarization as described in the previous section. Therefore the electromagnetic response of the ultra-thin MMA has been measured as a function of the

azimuthal angle _φ_ to verify the polarization insensitivity, rotating the sample of 0, 22.5, 45, 67.5, and 90 degrees with respect to the horizontal arm of the crossed-shaped ERR. As

expected, thanks to the four-fold rotational symmetry of the MMA, the absorbance levels for the different values of _φ_ are nearly equal and comprised between 67% and 74%, even under oblique

incidence, as shown in Fig. 3(a) (see Supplementary Figs S1–S6). The measured absorbance _A_(_ω_) differs from the near-unity value of the one simulated, and this discrepancy is mainly due

to the values of the material parameters used in the simulation for describing the MMA. A possible reason for the mismatch between the simulations and the measurements can be attributed to

higher Ohmic losses of the metal layers, being the electrical conductivity of thin Au films at THz frequencies much smaller than its bulk dc value13,48. Relying on the published results in

ref.48 and having performed the spectroscopic measurements at room temperature, we have considered a halved electrical conductivity _σ_ = 2 × 107 S/m for the 100 nm-thick metallizations in

the full-wave simulations. However, this has produced only a slight reduction of the absorption level (considering an almost lossless polyimide dielectric layer) and a slight broadening of

the bandwidth (see Supplementary Fig. S7). Instead, the dielectric losses of the polyimide spacer have been found to have a pronounced effect on the simulated results; indeed, the

permittivity and loss tangent selected in the simulations were based on the datasheet reported in ref.31. In the literature, there are many published papers33,34,35,36,37,38 in which the

polyimide is treated as a dielectric with an a priori assumed permittivity value at THz frequencies. In the light of the foregoing considerations, we have determined a different value for

the loss tangent of the polyimide, through parametric sweeps in order to match the measurements. We have found an estimated loss tangent _tan δ_ = 0.086 to which corresponds a complex

relative permittivity _ε_ _r_  = 2.9 − 0.25 _j_ for the polyimide layer. Figure 3(b) shows the comparison between the simulations with the modified permittivity and the experiments: a better

agreement is evident, with a remarkable resonance at around 1.092 THz. DISCUSSION We have simulated and microfabricated an ultra-thin narrow-band MMA in the THz regime: its performance has

been experimentally characterized by using a custom collimated configuration reflection-mode THz-TDS. The realization of the MMA needs five main fabrication steps with only one lithographic

phase: because the bottom layer is a continuous metal film, no alignment between the metal layers is needed. Therefore, the fabrication process is noticeably simplified with respect to other

available solutions, for example that presented in ref.23 which implies the control of misalignment errors between the two stacked crossed-shaped metallizations, resulting in lower

production time and costs. In addition, the lower ground plane avoids the influence of the support Si substrate that produced in ref.23 uncertainty in the definition of the unit cell, as

well as a non-negligible asymmetry in the propagation direction due to the bianisotropy of the ERR design. In order to experimentally characterize the MMA, we have modified a commercial

Menlo Systems TERA K15 standard transmission configuration by inserting a fixed angle aluminum reflector in the collimated THz beam path. This permits the collimation of THz radiation on the

sample/reference with a specific incidence angle, allowing investigating the MMA under oblique incidence. The devised reflection-mode THz-TDS set-up is polarization-maintaining because it

does not use parabolic mirrors to manipulate the THz beam, allowing the characterization of polarization-sensitive devices and ensuring a wide reproducibility of the measurements. The

experimental results, carried out as a function of the azimuthal angle, have proved the high polarization insensitivity of the MMA even under 16° oblique incidence with absorbance levels

comprised between 67% and 74% at 1.092 THz and a full-width-at-half-maximum (FWHM) of the absorption band of about 0.2 THz. The observed mismatch of the experimental FWHM with the full-wave

simulations has been found to chiefly depend on the value of dielectric losses of the polyimide spacer. A good agreement with the experimental results has then been obtained by adjusting the

value of the loss tangent of the polyimide spacer to tan _δ_ = 0.086. Furthermore, the proposed ultra-thin MMA has been realized with a highly flexible polyimide middle layer which expands

its uses in non-planar applications. The MMA can be made actually flexible by spin-coating a further layer of polyimide on the Si substrate39,40, which represents only a mechanical support,

not actively contributing to the absorption mechanism. The MMA can thus be regarded as a flexible metasurface which can be wrapped around non-planar surfaces and exploited to cover objects

of considerable size. These features make the device particularly suited for spectroscopic applications in several fields such as detection of explosive materials, THz imaging,

electromagnetic cloaking, medical diagnoses, and spectrally selective security detection. The ultra-thin profile, the narrow spectral band of absorption and the polarization insensitivity,

as well as the potential flexibility, open up also interesting prospects in the design of wearable THz MMAs. METHODS NUMERICAL SIMULATION The proposed MMA was simulated by applying the

finite-element method (FEM), using the RF module of the commercial software package COMSOL Multiphysics. To this end, a single unit cell has been considered by applying Floquet periodic

boundary conditions for the side boundaries perpendicular to the plane of the MMA to model a periodic structure. Perfectly matched layers (PMLs) are employed on the top and bottom of the

unit cell to absorb the excited mode from the source port and any higher-order modes generated by the periodic MMA. Port boundary conditions are applied on the interior boundaries of the

PMLs. For the excitation at the source port, an obliquely incident plane wave illumination with transverse electric polarization (i.e., the electric field is parallel to the MMA interface)

and 1 W input power, constant over the THz band of interest, were used. FABRICATED PROTOTYPE In order to manufacture the device, we adopted the top-down fabrication approach using standard

microfabrication techniques. The MMA sample was fabricated on a high-resistivity 360 _μ_m-thick silicon substrate that also serves as a support for the structure. The manufacturing procedure

consisted of five main fabrication steps: first (Fig. 4(a)), on the silicon substrate cleaned with a buffered HF solution, a metal film was deposited by gun evaporator (10 nm of Cr adhesion

layer and 100 nm of Au metal mirror); subsequently a 5.4 _μ_m-thick polyimide (PI-2611, HD MicroSystems) middle layer was deposited onto the Au film by spin-coating and cured in vacuum oven

at 250 °C (Fig. 4(b)). Then (Fig. 4(c) and (d)), on the polyimide layer, the geometry of the unit cell, a matrix consisting of 6 cm × 6 cm of total active area (750 × 750 unit cells), was

patterned with a UV-inverted resist (AZ 5214 E, MicroChemicals) using standard lithography process (Mask Aligner Karl Suss MA160); finally (Fig. 4(e)), a second metal layer was evaporated

(10 nm Cr adhesion layer and 100 nm of Au) obtaining the final structure after lift-off procedure in acetone (Fig. 4(f)). In Fig. 5(a) and (b), the optical microscope images of the completed

device are reported, while in Fig. 5(c) and (d), the Atomic Force Microscopy (AFM) inspection of the unit cell shows that we reached a good control in the process fabrication flow obtaining

a good definition of the tips and a sharp vertical wall of the final metal film. DATA AVAILABILITY The datasets generated during and/or analyzed during the current study are available from

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metal films. _Appl. Phys. Lett._ 93, 051105 (2008). Article  ADS  Google Scholar  Download references ACKNOWLEDGEMENTS M. Missori acknowledges funding from the Ministero dell’Istruzione,

dell’Università e della Ricerca under Progetto Premiale “ THEIA”. M.D. Astorino acknowledges funding from “ La Sapienza” University of Rome under “ Progetti per Avvio alla Ricerca 2016”.

AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Department of Information Engineering, Electronics and Telecommunications, “ La Sapienza” University of Rome, Via Eudossiana 18, 00184, Roma,

Italy Maria Denise Astorino, Fabrizio Frezza, Marco Muzi, Nicola Tedeschi & Andrea Veroli * Dipartimento di Fisica, “ La Sapienza” University of Rome, Piazzale Aldo Moro 5, 00185, Roma,

Italy Renato Fastampa * Istituto dei Sistemi Complessi, Consiglio Nazionale delle Ricerche, Unità Sapienza, Piazzale Aldo Moro 5, 00185, Roma, Italy Renato Fastampa & Mauro Missori *

Istituto per la Microelettronica e Microsistemi - Consiglio Nazionale delle Ricerche, Via Fosso del Cavaliere 100, 00133, Roma, Italy Luca Maiolo & Marco Marrani * Engineering

Departmental Faculty, Campus Bio-Medico University of Rome, Via Alvaro del Portillo 21, 00128, Roma, Italy Marco Muzi * Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche “

Enrico Fermi”, Piazza del Viminale 1, 00184, Roma, Italy Andrea Veroli Authors * Maria Denise Astorino View author publications You can also search for this author inPubMed Google Scholar *

Renato Fastampa View author publications You can also search for this author inPubMed Google Scholar * Fabrizio Frezza View author publications You can also search for this author inPubMed 

Google Scholar * Luca Maiolo View author publications You can also search for this author inPubMed Google Scholar * Marco Marrani View author publications You can also search for this author

inPubMed Google Scholar * Mauro Missori View author publications You can also search for this author inPubMed Google Scholar * Marco Muzi View author publications You can also search for

this author inPubMed Google Scholar * Nicola Tedeschi View author publications You can also search for this author inPubMed Google Scholar * Andrea Veroli View author publications You can

also search for this author inPubMed Google Scholar CONTRIBUTIONS M.D. Astorino conceived, designed the study, and performed the numerical simulations. L. Maiolo, M. Marrani, and A. Veroli

produced the MMA sample. M. Missori, R. Fastampa, N. Tedeschi, M.D. Astorino, and A. Veroli conceived and conducted the experiment. M. Muzi analyzed the results. R. Fastampa, M. Missori, N.

Tedeschi, and F. Frezza supervised the whole study. All authors wrote and reviewed the manuscript. CORRESPONDING AUTHORS Correspondence to Maria Denise Astorino or Mauro Missori. ETHICS

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copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Astorino, M.D., Fastampa, R., Frezza, F. _et al._

Polarization-maintaining reflection-mode THz time-domain spectroscopy of a polyimide based ultra-thin narrow-band metamaterial absorber. _Sci Rep_ 8, 1985 (2018).

https://doi.org/10.1038/s41598-018-20429-7 Download citation * Received: 07 September 2017 * Accepted: 18 January 2018 * Published: 31 January 2018 * DOI:

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