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Highly Efficient and Broadband Mid-Infrared Metamaterial Thermal Emitter for Optical Gas

Sensing

YONGKANG GONG,1,2* ZUOBIN WANG,3 KANG LI,1,2 LESHAN UGGALLA,4 JUNGANG HUANG,1,2 NIGEL COPNER,1, 2 YANG ZHOU1, DUN QIAO1, AND JIUYUAN ZHU1

1 Wireless and Optoelectronics Research and Innovation Centre, Faculty of Computing, Engineering and Science, University of South Wales, CF37 1DL, United Kingdom.2 Foshan Huikang Optoelectronics Ltd., B block, Sino-European Center, Foshan 528315, China3 International Research Centre for Nano Handling & Manufacturing of China, Changchun University of Science and Technology, Changchun 130022, China4Faculty of Computing, Engineering and Science, University of South Wales, CF37 1DL, United Kingdom.*Corresponding author: yongkang.gong@southwales.ac.uk

Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX

Development of novel, cost-effective and high efficient mid-infrared light source has been identified as a major scientific and technological goal within the area of optical gas sensing. We have proposed and investigated a mid-infrared metamaterial thermal emitter based on micro-structured chromium thin film. The results demonstrate that the proposed thermal light source supports broadband and wide angular absorption of both TE- and TM-polarized light, giving rise to broadband thermal radiation with averaged emissivity of ~0.94 in mid-infrared atmospheric window of 8 -14m. The proposed microphotonic concept provides a promising alternative mid-infrared source and paves the way towards novel optical gas sensing platforms for many applications. © 2017 Optical Society of America

OCIS codes: (040.3060) Infrared; (160.3918) Metamaterials;(050.6624) Subwavelength structures; (350.4238) Nanophotonics and photonic crystals

http://dx.doi.org/10.1364/OL.99.099999

Gas sensing plays an increasingly important role in numerous industrial applications such as detection of explosive or toxic gases within industrial productions [1, 2], indoor/outdoor air quality monitoring [3, 4], medical diagnosis [5], and food safety analysis etc [6]. Due to the higher demand and vital importance, the gas sensors market is expected to witness higher growth within the sectors over the years to come. Developing novel gas sensing technologies has long been a scientific goal as well as a commercial goal to stay vigorous within a very competitive market. Significant achievements have been made throughout the years in sensing strategies and among them

mid-infrared optical gas sensing is highlighted due to its higher accuracy, compactness, fast speed, and the economic efficiency [1]. The physical mechanism of mid-infrared optical gas sensing is based on the characteristic light absorption of the targeted gas molecules in mid-infrared atmospheric windows of 3-5 and 8-14 microns, where a number of pollutants and toxic gases lines exist. Considering that light sources are the key component of many mid-infrared gas sensing system, to date several different kinds of mid-infrared light sources have been commercially developed. They can be mainly categorized into three types: lasers (such as quantum cascade lasers and fiber lasers [7, 8]), light emitting diodes [9, 10], and thermal emitters. Even though lasers offer higher output power, the operating wavelengths are limited and are not always available for wavelengths of interest. Furthermore, infrared lasers are very costly. LEDs are cheaper compared to lasers, but they generally work at wavelengths <5 m. Mid-infrared thermal emitters (MITEs), typically relying on heating coiled filament (i.e, heating panels) made of refractory metals (such as tungsten, nickel, chromium, platinum, and aluminum and their alloys) to radiate light, are the mostly widely used approach to generate mid-infrared light because of the ultrabroad operating wavelengths and much lower cost. However, the emission power from MITEs drops significantly at long-wave mid-infrared wavelengths due to low emissivity of the heating panels at these wavelengths, making them unsuitable for long-wave mid-infrared gas detection at 8 -14 m.

Attributing to the unprecedented technological development of advanced coating technologies and nanofabrication in recent years, micro- and nanofabricated plasmonic metamaterials have emerged as promising approaches to enhance infrared thermal radiation, hence

have attracted a great attention. Researchers have proposed and developed various micro/nano photonic structures to achieve high emissivity and narrowband MITEs, such as complementary metal-oxide-semiconductor nanoplasmonic crystals [11-13], one-dimensional multilayer thin films [14, 15], SiC gratings [16], stacked gratings [17], semiconductor photonic crystals [18-20], and plasmonic nanostructures [21-23]. Recently, MITEs based on metamaterial technologies [24-26], especially refractory metamaterials [27, 28], have become particularly popular due to their ability to generate narrow bandwidth thermal radiation with near-unity emissivity at arbitrary infrared wavelengths. Despite the progress made toward narrowband MITEs, the realization of broadband MITEs operating at high temperatures are lacking. Recently, infrared emitters working at near-infrared and short-wave mid-infrared wavelengths have been developed based on plasmonic thin films and nanostructures [29-31]. Hyperbolic metamaterial waveguide taper array has been proposed to enhance thermal emission between 3 m and 5 m [32]. By using doped silicon photonic crystals, broadband enhanced thermal emission at 5 -20 m has been demonstrated [33].

Fig. 1. (a) Schematic diagram of the proposed metamaterial MITE (not to scale). (b) Emissivity spectrum of the MITE at normal emission angle. For comparison, the emissivity of MITEs based on single thin film of tungsten (W), chromium (Cr), and nickle (Ni) are also presented. Inset gives the spectral emissivity from 0.4 m to 14 m and shows that these MITEs’s emission drops significantly when wavelength increases from visible to mid-infrared. (c) Magnetic field distribution |Hx+Hy+Hz|2 at XZ plane at impedance-matched wavelengths of 14 m and 8.8 m, and non-impedance-matched wavelength of 6 m, respectively. The red-dotted lines indicate the structure geometry. Here, the geometric parameters are h1=2 m, h2=3.2 m, h3=1 m, h4=0.2 m Px=Py=6 m, and Lx=Ly=2.2 m.

In this paper, we propose and develop a novel, high efficiency and broadband metamaterial thermal emitter for long-wave mid-infrared light generation. The proposed metamaterial consists of micro-structured chromium (Cr) on top of tantalum pentoxide (Ta2O5) and silicon dioxide (SiO2) thin films. In contrast to the existing commercial MITEs based on refractory metals, the proposed metamaterial can significantly increase thermal emissivity over the entire atmospheric transparency window of 8 -14m. The new approach opens possibilities to obtain novel, cost-effective, compact, and high power mid-infrared light source for optical gas sensing.

The Stefan–Boltzmann law states that radiant power of a thermal emitter is proportional to the fourth power of its temperature, so it is crucial to make MITEs to have high emissivity and operate at high temperatures. The proposed metamaterial emitter concept is schematically illustrated in Fig. 1(a), where a Cr layer (with a thickness of h4), a SiO2

layer (with a thickness of h3) and a Ta2O5 layer (with a thickness of h2) are successively coated on a silicon (Si) substrate. The Cr layer acts as a reflector and allows no transmission of light. We choose SiO2 and Ta2O5 thin films as dielectric spacers because of their high absorption properties in mid-infrared and high melting points. A Cr thin film with a thickness h1 is deposited on top of the Ta2O5

layer. It is then patterned into a periodic array of Cr patches along both x and y directions. The reason we use Cr rather than gold (Gold is normally used for plasmonic metamaterials) is that Cr has a higher melting point than gold.

Fig. 2. (a) Emissivity of the proposed metamaterial emitter versus emission angle and wavelength. (b) and (c) demonstrate light absorptivity of TM-polarized light (electric field along x axis) and TE-polarized light (electric field along y axis) at angle of incidence of 00, 150, 300, 450, and 600, respectively.

According to the Kirchhoff’s law, the absorptivity of a blackbody equals its emissivity in thermodynamic equilibrium, thus we can tailor structure absorption to get the desired emission properties. We design and optimize the proposed structure by taking refractive index of all

materials from experimental data [34, 35]. To ensure the accuracy of modelling, simulations are implemented by both Finite-difference time-domain (FDTD) method and Rigorous coupled-wave analysis (RCWA). Comparing the simulations indicates that the results from FDTD and RCWA are very similar to each other, hence proving the accuracy of our modeling. The optimized geometric parameters are h1=2 m, h2=3.2 m, h3=1 m, Px=Py=6 m, and Lx=Ly=2.2 m. The bottom Cr layer acts as a reflector and it works as long as its thickness h4 remains larger than its skin depth. In our design, h4 is 200 nm. Emissivity (red solid line) and reflectivity (red dotted line) of our metamaterial MITE are investigated and illustrated in Fig. 1(b). In contrast to the widely used refractory metal thermal materials such as chromium (Cr), nickel (Ni) and tungsten (W), which provide high emissivity at visible and near-infrared wavelengths but low emissivity at mid-infrared wavelengths (see the inset of Fig 1(b)), our structure offers high emissivity in a broad mid-infrared range. We observe from Fig. 1(b) that the average emissivity of our structure over 8 -14 m is as high as ~0.94, which is ~50 times and ~10 times higher than that of W and Cr thermal emitters, respectively. This characteristic makes the proposed design a novel, highly efficient mid-infrared thermal light source for gas sensing. The physical mechanism of our metamaterial MITEs relies on the plasmonic effect of the Cr gratings and the strong dispersion of the high-loss Ta2O5 and SiO2 layers. The two-dimensional Cr metasurface acts as a dispersive anisotropic thin film with effective permeability and permittivity [17, 25]. The effective thin film has optical impedance matched to that of the highly dispersive and lossy Ta2O5 and SiO2 thin layers, giving rise to broadband absorption. Our structure works in a similar way to the broadband absorber based on doped silicon metamaterials reported in Ref. [33]. Figure 1(c) depicts H-field intensity distribution at XZ plane at impedance-matched wavelengths of 14 m and 8.5 m, respectively. We note that light at 14 m is mainly concentrated at the interface between the Cr grating and the Ta2O5 layer and there is little light reflection from the microstructure, which is the typical plasmonic effect. At 8.5 m, light is mainly confined in the Ta2O5 layer. A small amount of lights penetrates the Ta2O5 layer and enters the SiO2 thin film. Multiple reflection at the interfaces of Ta2O5/SiO2 and SiO2/Cr are then canceled due to the interference effect. Therefore, our structure has little reflected light. We also plot in Fig. 1(c) electromagnetic field distribution at the non-impedance-matched wavelength of 6

m for comparison. We can see that light at this wavelengthμ is highly reflected by the top Cr grating. Light reflected by the Ta2O5 layer, the SiO2 layer and the bottom Cr layer cannot deconstructive interfere and leaves the structure, resulting high reflection. As we can see from Fig. 1(c) that the broadband absorption behavior of our structure mainly arises from the interaction between Cr grating and Ta2O5

layer. The SiO2 layer is necessary as it can offer more flexibility and can further improve absorption at wavelengths of interested. In our current design, emissivity at 12.5 m -14 m is not very high. This may be solved by adding more dielectric layers to achieve impedance

matching in this wavelength range to enhance absorption [29, 30]. Figure 1 demonstrates the high emissivity of the proposed metamaterial MITEs at the normal emission angle. To further investigate the performance of our MITE, we study its directional spectral emissivity ε( ,θ ) by averagingλ spectral absorptivity of both TE- and TM-polarized light, where is θ

Fig. 3. Spectral radiance of the proposed metamaterial MITE operating at temperatures of (a) 600 K, (b) 800 K, and (c) 1000 K, respectively. (d) Emission power versus operating temperature. The inset gives thermal radiation enhancement at different temperatures. Thermal emission of ideal blackbody and tungsten emitter are given here for comparison. Ideal blackbody is thought as a perfect absorber with 100% emissivity at any wavelengths and any emission angles.

polar angle. We calculate the spectral emissivity/absorptivity at different angles of incidence from 0 to 90 deg (see Fig. 2(a)). Figure 2(b) and (c) show light absorptivity for TM- and TE-polarized light at 0, 15, 30, 45, and 60 deg, respectively. Absorptivity of both TE- and TM-polarized light is high over an ultrabroad bandwidth at wide angles, providing increased spectral emissivity over 8 m-14 m. We study total radiant emittance E(T) by integrating emissivity over both angles and wavelengths, i.e.,

(1)where h=6.626×10-34 J⋅s, k=1.38×10-23 J⋅K−1, c, and T are Planck constant, Boltzmann constant, light velocity, and temperature in Kelvin, respectively. λmin and λmax are 8 mμ and 14 m, respectively. μ Spectral radiances at different operating temperatures are calculated and plotted in Figs. 3(a)-(c). We can see that spectral radiance of the proposed metamaterial emitter approaches that of ideal blackbody radiation, and is much higher than that of tungsten thermal emitter. The temperature dependent radiant power is plotted in Fig. 3(d). We also depict thermal radiation enhancement by calculating the radiant power ratio between the proposed metamaterial emitter and tungsten

emitter. Figure 3(d) clearly shows that our novel design can significantly enhance mid-infrared thermal radiation, comparing to the traditional infrared thermal sources based on refractory materials such as tungsten. For example, at the operating temperature of 700 K our structure has radiant emittance of ~2373 W/m2, which is ~34 times higher than that of the tungsten-based emitter.

In conclusion, we have proposed a new scheme to obtain broadband and high efficiency MITE. We designed and optimized the proposed MITE based on calculating the angular emissivity spectra, electromagnetic field distribution, spectral radiance and radiant emittance. The RCWA and FDTD simulation results demonstrated that the MITE can significantly enhance the thermal emissivity over the entire optical gas sensing window of 8 -14 m. The radiant emittance of our structure is >30 times stronger, comparing to the traditional tungsten thermal emitter operating at temperature between 500 K to 1000 K. The proposed concept provides a promising alternative mid-infrared thermal emitter strategy for novel optical gas sensing. Finally, it is worth mentioning that the proposed concept can be further improved, for example, by combing the proposed microstructured metamaterial with indium tin oxide [36], thermochromic materials [38, 39], phase change materials [40, 41], and graphene [42, 43], to achieve fast tuning of emission intensity in a broad wavelength range.

Funding. 2015 Foshan City Technology Innovation Group Project (Advanced Solid State Light Source Application and Innovation Team).

Acknowledgment. The authors acknowledge the discussions with Prof. Yiqi Zhang in Xi’an Jiaotong University and Dr. Hua Lu in Northwestern Polytechnical University.

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