While surface microstructures of butterfly wings have been extensively studied for their structural coloration or optical properties within the visible spectrum, their properties in infrared wavelengths with potential ties to thermoregulation are relatively unknown. The midinfrared wavelengths of 7.5 to 14 µm are particularly important for radiative heat transfer in the ambient environment, because of the overlap with the atmospheric transmission window. For instance, a high midinfrared emissivity can facilitate surface cooling, whereas a low midinfrared emissivity can minimize heat loss to surroundings. Here we find that the midinfrared emissivity of butterfly wings from warmer climates such as Archaeoprepona demophoon (Oaxaca, Mexico) and Heliconius sara (Pichincha, Ecuador) is up to 2 times higher than that of butterfly wings from cooler climates such as Celastrina echo (Colorado) and Limenitis arthemis (Florida), using Fourier-transform infrared (FTIR) spectroscopy and infrared thermography. Our optical computations using a unit cell approach reproduce the spectroscopy data and explain how periodic microstructures play a critical role in the midinfrared. The emissivity spectrum governs the temperature of butterfly wings, and we demonstrate that C. echo wings heat up to 8 °C more than A. demophoon wings under the same sunlight in the clear sky of Irvine, CA. Furthermore, our thermal computations show that butterfly wings in their respective habitats can maintain a moderate temperature range through a balance of solar absorption and infrared emission. These findings suggest that the surface microstructures of butterfly wings potentially contribute to thermoregulation and provide an insight into butterflies' survival.

Spectral emissivity control is critical for optical and thermal management in the ambient environment because solar irradiance and atmospheric transmissions occur at distinct wavelength regions. For instance, selective emitters with low emissivity in the solar spectrum but high emissivity in the mid-infrared can lead to significant radiative cooling. Ambient variations require not only spectral control but also a mechanism to adjust the emissivity. However, most selective emitters are fixed to specific wavelength ranges and lack dynamic control mechanisms. Here we show ultraviolet to mid-infrared emissivity control by mechanically reconfiguring graphene, in which stretching and releasing induce dynamic topographic changes. We fabricate crumpled graphene with pitches ranging from 40 nm to 10 μm using deformable substrates. Our measurements and computations show that 140 nm-pitch crumpled graphene offers ultraviolet emissivity control in 200–300 nm wavelengths whereas 10 μm-pitch crumpled graphene offers mid-infrared emissivity control in 7–19 μm wavelengths. Significant emissivity changes arise from interference induced by the periodic topography and selective transmissivity reductions. Dynamic stretching and releasing of 140 nm and 10 μm pitch crumpled graphene show reversible emissivity peak changes at 250 nm and at 9.9 μm wavelengths, respectively. This work demonstrates the unique potential of crumpled graphene as a reconfigurable optical and thermal management platform.

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While most selective emitter materials are inadequate or inappropriate for building applications, here we present a techno-economically viable optical coating by integrating glass bubbles within a polymer film. A controlled glass bubble volume concentration from 0 to 70% leads to a selective solar reflectivity increase from 0.06 to 0.92 while the mid-infrared emissivity remains above 0.85. Outdoor measurements show the polymer coating on a concrete surface can provide a temperature reduction up to 25 °C during the day when conduction and convection are limited and a net cooling power greater than 78 W/m2 at a cost less than $0.005/W. The impact of polymer coating on common buildings is estimated as potential annual energy savings of 2–12 MJ/m2 and CO2 emission savings of 0.3–1.5 kg/m2. More savings are expected for higher surface-area-to-volume-ratio buildings, and the polymer coating is also expected to resolve cooling issues for old buildings with no air conditioning.

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We present the design, fabrication, and experimental characterization of silicon nitride based medium-index contrast gratings on glass substrate for fluorescence enhancement in the yellow to red spectral range with resonances for both incident excitation and fluorescence emission wavelengths under surface normal incidence. A comparison of the design space to realize resonant field enhancement in high-index contrast silicon and medium-index contrast silicon nitride grating structures is presented. The one-dimensional sub-wavelength grating structures studied here are designed with large duty cycle (∼80%) to account for the medium refractive index contrast (n ∼ 0.5) between silicon nitride and the glass substrate to ensure that the device operates in the two-mode regime. The resonant enhancement of fluorescence is experimentally verified using rhodamine-B isothiocyanate dye as the fluorophore of interest. A resonant enhancement of 10.8 times is demonstrated in this sample when compared to un-patterned film for transverse electric-transverse magnetic (TE-TM) polarization combination. We have also performed simulation study with plane wave excitation and incoherent dipole array emission to model the resonant excitation and emission processes, respectively. The simulations corroborate well with the best observed experimental results for the doubly resonant fluorescence configuration. Silicon nitride based medium contrast gratings are a promising platform to fabricate scalable structures for resonant enhancement of light-matter interaction with potential applications in high-sensitivity biological fluorescence assays and as a platform for polarization selective interrogation of light emission from nanoscale emitters attached to the grating.

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Spectral resonances in the mid-infrared region with polarization independence and angle tolerance are useful for filtering applications in infrared spectroscopy and imaging systems, when used with unpolarized light and across a wide field-of-view. Guided mode resonances are particularly attractive for this purpose due to the simple fabrication procedure to realize grating structures and the robust filter characteristics achievable through design. In this paper, the electromagnetic design, fabrication, and experimental characterization of polarization-independent, angle-tolerant mid-infrared spectral resonance using amorphous-germanium two-dimensional fully-etched high index contrast gratings on a calcium fluoride substrate is presented. The resonance, centered at 7.42 µm wavelength, exhibits polarization-independent, notch-type characteristics with minimal change across a 0 to 30° incidence angle. The angle tolerance of such dielectric high contrast grating filters is found to be intermediate between the highly angle sensitive dielectric partially etched grating structures and least angle sensitive metallic nano-aperture structures.