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Abstract
We demonstrate tailored thermal radiation in the mid-infrared wavelength range with intriguing capabilities of high emissivity, narrowband spectra, and sharp angular response. The proposed thermal emitter consists of stacking a two-dimensional metallic grating on top of a one-dimensional dielectric Bragg grating (BG). It is interesting to find that the light interaction between the metallic grating and the BG gives rise to impedance matching at wavelengths located in the photonic bandgap of the BG, and allows enhanced polarization-dependent emissivity, selective resonance response, and high radiation directivity. We investigate the plasmonic resonance by exploring various properties and functions including tuning spectral selectivity and tailoring bandwidth at different light polarization and angle of incidence. The developed plasmonic stacked gratings could pave the way towards novel integrated infrared source platforms for various applications such as thermal analysis, imaging, security, biosensing, and medical diagnosis.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-infrared light is a portion of the electromagnetic spectrum that has found extensive applications ranging from biosensing, military defense, medical diagnosis, imaging, thermophotovoltaics to telecommunications. So far, a variety of advanced mid-infrared light source technologies have been proposed and developed. They can be classified into three types: lasers [1,2], light emitting diodes [3,4], and thermal emitters. Lasers offer high light power, but they are costly, and the operating wavelengths are not always available at the desired wavelengths. Although infrared light emitting diodes are cheaper than lasers, they generally work at wavelengths < $5\; \mu \textrm{m}$. Thermal emitters are the most popular approach to generate mid-infrared light attributing to ultrabroad operating wavelengths and much lower cost. In nature, a thermal emitter, such as a black body or an incandescent lamp, is usually incoherent and omnidirectional, which differs fundamentally to a laser that is both temporally and spatially coherent.
Artificial control of thermal radiation that is difficult to attain with natural materials has been a research topic of interest for decades. Plasmonic metamaterials, a class of artificially structured materials, is a promising candidate for manipulating thermal emission properties that are difficult to achieve in natural materials. To date, different types of narrow band metamaterial emitters/absorbers have been reported, such as nano-gratings [5–8], photonic crystals [9–11], thin films [12], and three-layer-metamaterials [13–16]. These proposals have been a strong inspiration for enhancing light-matter interaction and have triggered promising interdisciplinary applications of optical sensors [17–19], hot-electron photodetectors [20], optical modulators [21–25], high-speed switching [26], energy recycling [27–29], image encryption [30,31], to thermal imaging [32]. Among the reported metamaterial thermal emitter technologies, the three-layer metamaterial scheme is the most promising arrangement since they support emission features such as a much higher peak and sharper bandwidth than that of a blackbody [33–13]. However, this strategy suffers from a common disadvantage of poor spatial coherence and relatively large spectral bandwidth, which limits the device performance and real-world applications such as ultrahigh resolution optical sensing [35,18].
In this paper, we propose a new kind of thermal emitters based on plasmonic stacked gratings (PSGs) and demonstrate the realization of temporally and spatially coherent thermal radiation. The PSGs are composed of a two-dimensional (2D) metallic nanostructure and a one-dimensional (1D) dielectric Bragg Grating (BG). It is found that the metallic grating acts like a homogeneous slab with large permittivity and small permeability [36], and the interaction between the metallic grating and the BG gives rise to impedance matching at wavelengths located in the photonic band gap of BG, which allows PSGs to perform high emissivity with narrow spectrum and shape angular response in mid-infrared regime. In addition, the operating wavelength, radiation polarization and angle of the PSGs are highly flexible and can be effectively tuned by varying both the BG bandgap and geometry of the metallic gratings. These features open possibilities to obtain a novel compact mid-infrared source scheme that could have great potential for applications such as thermal analysis, imaging, security, biosensing, and medical diagnoses.
2. Concept and Optical Characteristics of the PSGs
The proposed PSGs structure is schematically illustrated in in Fig. 1(a), where a 2D metallic nanostructure with thickness ${t_m}$ rests on top of a 1D BG arranged in z direction. The BG is composed of two alternately arranged dielectric materials with thicknesses ${t_a}$ and ${t_b}$ and refractive indices ${n_a}$ and ${n_b}$, respectively. The total number of grating periods in the BG is denoted as$\; P$. The metallic nanostructures are consisted of periodic silver patches with pitches of ${P_x}$ and ${P_y}$ along x and y directions. Each silver patch has height of$\; {t_m}$, width of W, and length of L. We investigated absorption properties of the proposed PSGs’. According to the Kirchhoff ‘s law, the absorptivity of a blackbody equals its emissivity in thermodynamic equilibrium. Therefore, we can tailor the proposed nanostructure absorption to achieve the desired emission properties.
Fig. 1. (a) A schematic diagram of the proposed PSGs composed of 2D metallic nanostructured surface and 1D BG that contains alternative TiO2 and SiO2 thin films with a period of$\; \; \Lambda $. (b) Emissivity, reflectivity, and transmissivity spectrum of the PSGs under normal incidence at TM polarization, where ${P_x} = {P_y} = 1.63\; \mu \textrm{m}$, $L = 1.1\; \mu \textrm{m}$, $w = 0.7\; \mu \textrm{m}$, ${t_m} = 0.05\; \mu \textrm{m}$, ${t_a} = 0.38\; \mu \textrm{m}$, ${t_b} = \; 0.65\; \mu \textrm{m}$, and $P = 20.$
We consider TiO2 and SiO2 as the dielectric materials in the BG. Three-dimensional finite-difference time-domain (FDTD) method is utilized to design and optimize the proposed PSGs, and the refractive index of all materials are taken from experimental data [37]. The calculated spectrum of the PSGs is plotted in Fig. 1(b). It is noted that a significant resonant dip occurs at a wavelength of $3.86\; \mu \textrm{m}$ in the reflection spectrum, which results in a narrow-band peak as high as 0.99 in the emissivity spectrum. The physical mechanism of the emergence of the strong and sharp resonance originates from that fact that when a plane wave is impinging on the PSGs along z direction, the 2D silver nanostructure can be equivalently described as a homogeneous slab with frequency-dependent permittivity and permeability [36]. The transmission is negligible at all PSG wavelengths due to the existence of photonic bandgap enabled by the dielectric BG. To get more insight of the behavior of the sharp and strong emissivity, we obtain the emissivity spectra for both TE- and TM-polarized light as well as the reflection and transmission spectra of the BG, as depicted in Fig. 2(a) and 2(b). Clearly, the proposed PSGs support polarization-dependent thermal radiation, which is desirable for various potential applications such as polarization-assisted thermal encryption [29]. We note that the emissivity peaks for both polarizations always appear within the BG bandgap.
Fig. 2. Polarization-dependent optical characteristics of the PSGs. (a) Emissivity spectra of the PSGs under TE- and TM-polarized incidence of light, respectively. (b) The reflectivity and transmissivity spectra of the BG. Field distribution ${|E |^2}$ of the PSGs in the central plane (i.e., $x$-$y$ plane) of the metallic nanostructure at resonant wavelength of $3.32\; \mu \textrm{m}$ for the TE-polarized light (c) and at resonant wavelength of $3.86\; \mu \textrm{m}$ for the TM-polarized light (d). Field distribution |E|2 of the PSGs in the $x$-$z$ plane of the metallic nanostructure at resonant wavelength of $3.32\; \mu \textrm{m}$ for TE-polarized light (e) and at resonant wavelength of $3.86\; \mu m\; $for TM-polarized light (f). Design parameters are ${P_x} = {P_y} = 1.63\; \mu \textrm{m},$ $L = 1.1\; \mu \textrm{m}$, $w = 0.7\; \mu \textrm{m}$, ${t_m} = 0.05\; \mu \textrm{m}$, ${t_a} = 0.38\; \mu \textrm{m}$, ${t_b} = 0.65\; \mu \textrm{m}$, and $P = 20$. For the TE- and TM-polarized incidence of light, the electric fields oscillate along the$\; x$ and y directions, respectively.
It indicates that the emissivity wavelength can be flexibly tuned by shifting the BG bandgap by means of simply varying the film thickness of the BG. The field distribution of the PSGs at resonant wavelengths are plotted in Fig. 2(c)-(f). It indicates that light is strongly localized and absorbed near the corners of the metallic nanoparticles (Fig. 2(c) and 2(d)), and there is no transmission of light through PSGs (Fig. 2(e) and 2(f)).
The relationship between the resonant peaks and the structural parameters of ${t_{m,\; }}\; {t_a},\; \; {t_b}$, $\textrm{and}\; {P_x},{P_y}$ is investigated and plotted in Fig. 3. As observed, the resonant wavelength of the PSGs shifts when ${P_x},{P_y}$ rises from 1.63 to 2.03 $\mu \textrm{m}$, and the emission peak becomes smaller when the grating period becomes larger than 1.8 $\mu \textrm{m}$. The emission wavelength is slightly tunable by varying the BG period, as shown in Fig. 3(a) and (b). We observed the thickness of TiO2 $({t_a})$ and SiO2 $({{t_b}} )\; $layers in the Bragg gratings plays a crucial role in the emission peak as well as emission wavelength. When thickness ${t_a}$ varies from $0.33$ to $0.43\; \mu \textrm{m}$, the emission peak slightly drops to ${\sim} \; 80\%$. The BG bandgap position is determined by Bragg condition of ${\lambda _{Bragg}} = 2({{t_a}{n_a} + {t_a}{n_b}} )$, where ${\lambda _{Bragg}}$ is the wavelength of BG bandgap. There, the emission wavelength of the proposed PSGs almost shifts linearly with ${t_a}$ and ${t_b}$, as indicated by Fig. 3(a) and 3(b). The dependence of light emissivity on the thickness of silver film ${t_{m\; }}$is shown in Fig. 3(d). As ${t_m}$ increases, the resonant wavelength of the PSGs obviously shift, when the thickness increase from 0.05 to 1$\; \mu \textrm{m}$. The dependence of transmission and emission at P = 5, 10, and 15 are demonstrated in Fig. 3(e) and 3(f), respectively. Hence, the total number of grating periods in the BG $(P )\; $would affect the impedance matching condition and cause more transmission through the stacked gratings, only when the number of grating periods of the stack are less than 15. In Fig. 3 (e), when we use a period stack of $P\; \le \; 15$, there is very little transmission, and it does not affect the emission wavelength except the emission intensity. When P is less than 15, there are observed transmission components within the structure, causing less emissivity of the proposed PSGs structure. Therefore, increasing P leads to reduced light transmission through the structure and emission intensity increases.
Fig. 3. Evolution of the emissivity spectra to the structure parameters of the PSGs. (a) Dependence of the emissivity spectra on the thickness of $Ti{O_2}$ when ${P_x} = {P_y} = 1.63\; \mu \textrm{m}$, $L = 1.1\; \mu \textrm{m}$, $w = 0.7\; \mu \textrm{m}$, ${t_m} = 0.05\; \mu \textrm{m}$, ${t_b} = 0.65\; \mu \textrm{m}$ and $P = 20$. (b) Dependence of the emissivity spectra on the thickness of $Si{O_2}$ when ${P_x} = {P_y} = 1.63\; \mu \textrm{m}$, $L = 1.1\; \mu \textrm{m}$, $w = 0.7\; \mu \textrm{m}$, ${t_m} = 0.05\; \mu \textrm{m}$, ${t_a} = 0.38\; \mu \textrm{m}$, and $P = 20$. (c) Dependence of the emissivity spectra on the pitch length when $L = 1.1\; \mu \textrm{m}$, $w = 0.7\; \mu \textrm{m}$, ${t_m} = 0.05\; \mu \textrm{m}$, ${t_a} = 0.38\; \mu \textrm{m}$, ${t_b} = 0.65\; \mu \textrm{m}$, and $P = 20$ (d) Dependence of the emissivity spectra on the thickness of silver film when ${P_x} = {P_y} = 1.63\; \mu \textrm{m},$ $L = 1.1\; \mu \textrm{m}$, $w = 0.7\; \mu \textrm{m}$, ${t_a} = 0.38\; \mu \textrm{m}$, ${t_b} = 0.65\; \mu \textrm{m}$, and $P = 20$. $\textrm{Transmissibity}$ (e) and emissivity (f) at P = 5, 10, 15, when ${P_x} = {P_y} = 1.63\; \mu \textrm{m},$ $L = 1.1\; \mu \textrm{m}$, $w = 0.7\; \mu \textrm{m}$, ${t_m} = 0.05\; \mu \textrm{m}$, ${t_a} = 0.38\; \mu \textrm{m}$, and ${t_b} = 0.65\; \mu \textrm{m}$.
3. Angular and tunability properties of the PSGs
Attributing to the PSGs’ unique property of the emission peak always being located within the band gap of the BG, the FWHM of the PSGs can be flexibly tuned and significantly narrowed by means of decreasing the BG band gap such as by minimizing the refractive index difference of the BG. According to our observations, the emission peak is always located in the BG bandgap. Therefore, when the BG bandgap is narrower, the FWHM of resonant peak would be smaller. Narrowing the BG bandgap relies on minimizing refractive index difference of the BG [36]. In this case, the wavelength range of impedance matching condition becomes narrower. Another intriguing property of the PSGs is spatial directivity. We investigate this property by calculating emission at different angle. Figure 4(a) and (b) illustrate the dependence of the resonant peaks on the radiation angles from 5 deg to 20 deg. The resonant peaks’ wavelength varies greatly with angle and the angle bandwidth is quite narrow. As a result, we can observe emission of a single wavelength only at a certain direction. To put it simply, the resonant emission of the proposed PSGs is highly directional. The physical origin for the behavior arises from the fact that the emission wavelengths appear in the BG band gap that is sensitive to the angle of light incidence.
Fig. 4. Angular response of the PSGs. Dependence of the emissivity spectra on the radiation angle for TM-polarized (a) and TE-polarized light (b), respectively. Structure parameters are ${P_x} = {P_y} = 1.63\; \mu \textrm{m}$, $L = 1.1\; \mu \textrm{m}$, $w = 0.7\; \mu \textrm{m}$, ${t_m} = 0.05\; \mu \textrm{m}$, $\; {t_a} = 0.38\; \mu \textrm{m},$ $\; {t_b} = 0.65\; \mu \textrm{m}$, and $P = 20$.
Finally, we demonstrate that the resonance peaks can be flexible tuned by adjusting the structure parameters. For example, when the nanostructure width w varies from $0.3\; \mu \textrm{m}$ to $0.7\; \mu \textrm{m}$, the emissivity peak wavelength shifts $3.86$ to $4.10\; \mu \textrm{m}$ for the TM-polarized light and $3.32\; \mu \textrm{m}$ to $3.15\; \mu \textrm{m}$ for the TE-polarized light, as depicted in Fig. 5(a) and (b). The dashed line in Fig. 5 (b) indicates the photonic boundary limitation in the spectrum to separate the area that possess zero transmission (right) and non-zero transmission (left) due to polarization changes.
Fig. 5. Tailoring thermal radiation of TM- (a) an TE-polarized light (b) by tuning geometric parameter w from 0.3 µm to 0.7 µm. Other structure parameters are ${P_x} = {P_y} = 1.63\; \mu \textrm{m}$, $,\; \; L = 1.1\; \mu \textrm{m}$, $w = 0.7\; \mu \textrm{m}$, ${t_m} = 0.05\; \mu \textrm{m}$, $\; {t_a} = 0.38\; \mu \textrm{m}$, ${t_b} = 0.65\; \mu \textrm{m}$, and$\; P = 20$.
4. Conclusion
In summary, we have proposed a novel PSGs strategy for mid-IR thermal radiation and demonstrate the possibility of enhanced and coherent thermal emission. 3D-FDTD method is utilized to study the spectral and angular emission response of the PSGs. An intriguing characteristic of the thermal emitter scheme is that the emission resonant peaks appear within the bandgap of the BG. Therefore, the FWHM of the emission spectrum can be significantly narrowed by reducing the BG band gap. Replacing the BG with a metallic film could also give rise to a strong resonant emission peak, but the FWHM would be much larger than that of our proposal. In addition, the operating wavelength of the thermal emitter can be flexibly tuned by shifting the bandgap of the BG. These features open possibilities to obtain a cost effective and compact mid-IR source platform. Due to its tunable and narrow spectral and angular response, the PSGs can find potential applications of thermal sensing, security, and infrared diagnoses etc.
Funding
Petroleum Technology Development Fund.
Disclosures
No conflicts of interest
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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