PHYSICAL REVIEW B 87, 205121 (2013) Coherent emission of light using stacked gratings Yongkang Gong, 1,2,* Xianliang Liu, 3 Kang Li, 1,2 Jungang Huang, 1 J. J. Martinez, 1 Daniel Rees-Whippey, 1 Sara Carver, 1 Leiran Wang, 2 Wenfu Zhang, 2 Tao Duan, 2 and Nigel Copner 1 1 Faculty of Advanced Technology, University of South Wales, CF37 1DL, United Kingdom 2 State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China 3 Department of Physics, Boston College, 140 Commonwealth Avenue, Chestnut Hill, Massachusetts 02467, USA (Received 24 December 2012; revised manuscript received 26 April 2013; published 14 May 2013) The possibility of temporally and spatially coherent thermal emission has been demonstrated utilizing stacked gratings. We demonstrate that the metallic grating with narrow air slit behaves like a homogeneous slab with large permittivity and small permeability and find that the interaction between the metallic grating and the Bragg grating gives rise to impendence matching at wavelengths located in the photonic band gap of the Bragg grating, which enables the stacked gratings to perform high emission with ultranarrow spectrum and antenna-like spatial response. This paves the way towards the design of a novel infrared source platform for applications such as thermal analysis, imaging, security, biosensing, and medical diagnoses. DOI: 10.1103/PhysRevB.87.205121 PACS number(s): 78.67.Pt, 79.60.Dp, 65.80.−g I. INTRODUCTION A thermal radiation source, such as a blackbody or an incandescent light bulb, is usually omnidirectional and broadband in nature, in marked contrast to a laser, which is both temporally and spatially coherent. The desire to convert thermal emission from broadband to narrowband has long been a research topic of interest for scientists in the last decades, such as using luminescent bands of rare-earth oxides, 1 and tailoring photon density of states of photonic crystals. 2 It has been recently noted that plasmonic nanophotonics offer exceptional opportunities to tailor absorption of a target material so as to modify its thermal emission spectrum on the basis of Kirchhoff’s law. 3,4 In this scheme, noble metals are usually known to be excellent reflectors but when they are structured, light reflection fades away and absorption occurs because resonant excitation of surface plasmon polaritons (SPPs). This concept can greatly enhance thermal emission at the resonant frequency with a spectrum much narrower than that of a blackbody at the same temperature and has been widely studied in the past few years in diverse structures from metallic photonic crystals, 5 gratings, 6,7 to nanoparticles. 8 More recently, a more promising candidate to manipulate thermal emission is metamaterials—a class of artificially structured materials composed of arrays of subwavelength “meta-atoms” with the ability to exhibit exotic electromagnetic properties, which are difficult to attain with nature materials. In 2008, Padilla et al. proposed a concept of perfect absorber based on a three-layer metamaterial with the physical origin of engineering metamaterials’ electric and magnetic responses to reach impedance matching. 9 This design shows good ability to yield narrowband and near-unity absorption at almost any frequency from visible to near infrared and terahertz (see the recent review paper, 10 and references therein), and triggers a class of applications such as plasmonic sensing, 11 all-optical switching, 12 and hyper spectral-single pixel imaging. 13 The three-layer-metamaterials ideal is particularly promising for designing thermal emitters because of its advantages of supporting emission features with much higher peak and sharper bandwidth than that of a blackbody, and, hence, has attracted a great interest. 14–16 However, emitters based on the three-layer-metamaterials ideal suffer from a common disadvantage of quasi-Lambertian angular pattern (i.e., poor spatial coherence). More unfortunately, the degree of spectral narrowing is quite limited and the bandwidth (generally hundreds of nanometers) is not narrow enough. We propose in this paper a design of stacked gratings (SGs) that allows for thermal emission with ultranarrow spectrum and antenna-like spatial directivity. The SGs are composed of a metallic grating (MG) followed by a Bragg grating (BG). We observe that both the MG and the BG are highly reflective to the light but when they are combined together, interestingly, zero reflection occurs at the wavelengths located in the BG band gap and gives rise to thermal emission in anomalous and intriguing ways. Numerical and analytical results demonstrate that the SGs are able to radiate light within a narrow angular lobe as an antenna and their operating wavelength can be effectively tuned by varying the BG band-gap position. More interestingly, the full width at half maximum (FWHM) of the SG’s emission spectrum is tunable and can be significantly narrowed by narrowing the BG band gap. We also demonstrate that multiwavelength emission with near-unity peak and ultranarrow bandwidth can be achieved. These are the key results that open immense possibilities for a novel infrared source, pertinent to applications such as thermal analysis, imaging, security, biosensing, and medical diagnoses. 17 II. THEORY AND OPTICAL PROPERTIES OF THE SGs The geometry of interest is depicted in Fig. 1, where a metallic film of thickness d m , corrugated by narrow air slits of width w and period of p, is resting on top of a BG composed of two alternately arranged dielectric layers of thickness d a and d b and refractive index of n a and n b with total number of periods of N . When a plane wave of transverse-magnetic (TM) polarization (H -field pointed along the x axis) is impinging on the SGs along z direction, the metallic grating with narrow air slits can be equivalently described as a homogeneous slab with effective permittivity ε eff = ε w p/w and permeability 205121-1 1098-0121/2013/87(20)/205121(5) ©2013 American Physical Society