Christopher H. Granier

Optimized aperiodic highly directional narrowband infrared emitters

Bulk thermal emittance sources possess incoherent, isotropic, and broadband radiation spectra that vary from material to material. However, these radiation spectra can be drastically altered by modifying the geometry of the structures. In particular, several approaches have been proposed to achieve narrowband, highly directional thermal emittance based on photonic crystals, gratings, textured metal surfaces, metamaterials, and shock waves propagating through a crystal. Here we present optimized aperiodic structures for use as narrowband, highly directional thermal infrared emitters for both TE and TM polarizations. One-dimensional layered structures without texturing are preferable to more complex two- and three-dimensional structures because of the relative ease and low cost of fabrication. These aperiodic multilayer structures designed with alternating layers of silicon and silica on top of a semi-infinite tungsten substrate exhibit extremely high emittance peaked around the wavelength at which the structures are optimized. Structures were designed by a genetic optimization algorithm coupled to a transfer matrix code which computed thermal emittance. First, we investigate the properties of the genetic-algorithm optimized aperiodic structures and compare them to a previously proposed resonant cavity design. Second, we investigate a structure optimized to operate at the Wien wavelength corresponding to a near-maximum operating temperature for the materials used in the aperiodic structure. Finally, we present a structure that exhibits nearly monochromatic and highly directional emittance for both TE and TM polarizations at the frequency of one of the molecular resonances of carbon monoxide (CO); hence, the design is suitable for a detector of CO via absorption spectroscopy.

Optimized aperiodic multilayer structures for use as narrow-angular absorbers

In this paper, we investigate aperiodic multilayer structures for use as narrow-angular absorbers. The layer thicknesses and materials are optimized using a genetic global optimization algorithm coupled to a transfer matrix code to maximize the angular selectivity in the absorptance at a single or multiple wavelengths. We first consider structures composed of alternating layers of tungsten and silicon or silica, and find that it is not possible to achieve angular selectivity in the absorptance with such structures. We next consider structures composed of alternating layers of silicon and silica, and show that when optimized they exhibit high angular selectivity in absorptance. In addition, as the angular selectivity in absorptance increases, the wavelength range of high angular selectivity also decreases. Optimizing the material composition of the multilayer structures, in addition to optimizing the layer thicknesses, leads to marginal improvement in angular selectivity. Finally, we show that by optimizing the absorptance of the multilayer structures at multiple wavelengths, we can obtain structures exhibiting almost perfect absorptance at normal incidence and narrow angular width in absorptance at these wavelengths. Similar to the structures optimized at a single wavelength, the wavelength range of high angularly selective absorptance is narrow.

Optimized aperiodic broadband visible absorbers

We present optimized aperiodic structures for use as broadband thermal incandescent emitters which are capable of increasing the emittance by nearly a factor of two over the visible wavelength range when compared to bulk tungsten. These aperiodic multilayer structures are designed with alternating layers of tungsten and air or tungsten and silicon carbide on top of a tungsten substrate. We investigate the properties of these structures for use as lightbulb filaments. We find that these structures greatly enhance the emittance over the visible wavelength range, while also increasing the overall efficiency of the bulb and could lead to a decrease in incandescent lightbulb power consumption by nearly 50%. OCIS codes: (350.4238) Nanophotonics and photonic crystals; (260.0260) Physical optics; (290.6815) Thermal emission.

Optimized mid-infrared thermal emitters for applications in aircraft countermeasures

We introduce an optimized aperiodic multilayer structure capable of broad angle and high temperature thermal emission over the 3 μm to 5 μm atmospheric transmission band. This aperiodic multilayer structure composed of alternating layers of silicon carbide and graphite on top of a tungsten substrate exhibits near maximal emittance in a 2 μm wavelength range centered in the mid-wavelength infrared band traditionally utilized for atmospheric transmission. We optimize the layer thicknesses using a hybrid optimization algorithm coupled to a transfer matrix code to maximize the power emitted in this mid-infrared range normal to the structure’s surface. We investigate possible applications for these structures in mimicking 800–1000 K aircraft engine thermal emission signatures and in improving countermeasure effectiveness against hyperspectral imagers. We find these structures capable of matching the Planck blackbody curve in the selected infrared range with relatively sharp cutoffs on either side, leading to i...

Wideband and wide angle thermal emitters for use as lightbulb filaments

We present optimized aperiodic structures for use as broadband, broad-angle thermal emitters which are capable of drastically increasing the efficiency of tungsten lightbulbs. These aperiodic multilayer structures designed with alternating layers of tungsten and air or tungsten and silicon carbide on top of a tungsten substrate exhibit broadband emittance peaked around the center of the visible wavelength range. We investigate the properties of these structures for use as lightbulb filaments, and compare their performance with conventional lightbulbs. We find that these structures greatly enhance the emittance over the visible wavelength range, while also increasing the overall efficiency of the bulb.

Multiwavelength Resonant Absorption Enhancement and Highly Directional Absorption with Aperiodic Multilayer Structures

We show that a graphene monolayer between two aperiodic multilayer structures can achieve near total resonant light absorption at multiple tunable wavelengths, and also that aperiodic multilayer structures can lead to highly directional absorption.

Multilayer structures with highly directional absorptivity for solar thermophotovoltaics

We explore an approach to enhance the efficiency of solar cells using photonic nanostructures for solar ther-mophotovoltaics. Our focus is on designing photonic nanostructures that can provide broadband absorption in a narrow angular range for solar thermophotovoltaic systems which do not employ sunlight concentration. We consider structures consisting of an aperiodic multilayer stack of alternating layers of silicon and silica on top of a thick tungsten layer. The layer thicknesses are optimized to maximize the angular selectivity in the absorp-tivity for both TE and TM polarizations. Using such an approach, we design structures with highly directional absorptivity for both polarizations.