Extremely Low-Loss Broadband Thermal Infrared Absorber Based on Tungsten Metamaterial FATHI BENDELALA, 1 ALI CHEKNANE, 1,4 HIKMAT S. HILAL, 2 and SOURAYA GOUMRI-SAID 3 1.—Laboratoire des semiconducteurs et mate ´riaux fonctionnels, Universite ´ Amar Telidji de Laghouat, Bd des Martyrs BP37G, 03000 Laghouat, Algeria. 2.—SSERL, Department of Chemistry, An-Najah National University, P.O. Box 7, Nablus, West Bank, Palestine. 3.—Physics Department, College of Science, Alfaisal University, Riyadh 11533, Saudi Arabia. 4.—e-mail: a.cheknane@lagh-univ.dz An extremely low-loss polarization-insensitive broadband absorber in the thermal infrared region, based on tungsten metamaterial, is proposed. The absorber exhibits almost perfect absorption of 99%, with a broad absorption bandwidth ranging from 56 THz to 117 THz. The broad absorption is attrib- uted to the large effective impedance adaptation of the tungsten metamaterial over a wide angle range in both the transverse electrical (0° to 75°) and transverse magnetic (0° to 80°) field polarizations. Based on these extraordi- nary electromagnetic properties, the proposed system can achieve excellent performance, with a figure of merit of 2.15 9 10 6 , higher than previously re- ported values for similar metamaterial absorbers. Key words: Broadband, absorption, polarization insensitive, tungsten metamaterial, low loss INTRODUCTION Metamaterials (MTMs) with subwavelength-scale unit cells have attracted attention due to their special properties, such as negative permittivity, negative permeability, and negative refractive index. 1 Due to these features, MTMs are being used in various applications, such as miniature anten- nas, 2 solar energy devices, 3 invisibility cloaks, 4 and sensors. 5 Moreover, MTMs are tunable, 6,7 making MTM absorbers suitable for use in different fre- quency ranges, including at microwave, 8,9 mid- infrared, 10,11 terahertz, 12,13 and optical frequen- cies. 14,15 Many applications demand absorption over a wide frequency band, but existing absorption bandwidths are rather narrow. Increasing the absorption bandwidth is thus imperative, and var- ious approaches have been proposed. Qin et al. 16 proposed an absorber in the long-wavelength (in- frared) region using the frequency dispersion of MTM surfaces. An absorption bandwidth in the range from 21 THz to 44 THz, with absorption rate above 97%, was reported. Use of a high-impedance surface was described 17 to design broadband absor- bers in the microwave region, ranging from 3.65 GHz to 13.93 GHz, with absorption above 90%. Other structures have also been proposed, e.g., double-circle rings, loaded with lumped resis- tances. 18 Three-dimensional structures have also been reported in attempts to widen the absorption bandwidth. 19 The infrared frequency range, known as the thermal radiation range, includes three regions: The far-infrared region, from 300 GHz to 30 THz (1 mm to 10 lm), corresponds to very low temperatures (80.15°C to 270.15°C). The mid- infrared region, from 37 THz to 120 THz (10 lm to 2.5 lm), corresponds to intermediate temperatures (89°C to 693°C). The near-infrared region, from 120 THz to 400 THz (2.500 lm to 750 nm), corre- sponds to higher temperatures (693°C to 3591°C). 11 The mid-infrared (thermal infrared) region is widely employed in various applications including biosens- ing, thermophotovoltaic conversion, and thermal devices. Important sources of loss in MTMs for use (Received November 26, 2018; accepted February 20, 2019) Journal of ELECTRONIC MATERIALS https://doi.org/10.1007/s11664-019-07090-0 Ó 2019 The Minerals, Metals & Materials Society