The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

If properly designed, terrestrial structures can passively cool themselves through radiative emission of heat to outer space. For the first time, we present a metal-dielectric photonic structure capable of radiative cooling in daytime outdoor conditions. The structure behaves as a broadband mirror for solar light, while simultaneously emitting strongly in the mid-IR within the atmospheric transparency window, achieving a net cooling power in excess of 100 W/m2 at ambient temperature. This cooling persists in the presence of significant convective/conductive heat exchange and nonideal atmospheric conditions.

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Ultrabroadband Photonic Structures To Achieve High-Performance Daytime Radiative Cooling

Thermal radiation is one of the most universal physical phenomena, and its study has played a key role in the history of modern physics. Our understanding of this subject has been traditionally bas...

Radiative Heat Transfer

We show that a perfect absorber/thermal emitter exhibiting an absorption peak of 99.9% can be achieved in metallic nanostructures that can be easily fabricated. The very high absorption is maintained for large angles with a minimal shift in the center frequency and can be tuned throughout the visible and near-infrared regime by scaling the nanostructure dimensions. The stability of the spectral features at high temperatures is tested by simulations using a range of material parameters. Since the beginning of the last century it is known that a perfect thermal emitter follows Planck’s law of blackbody radiation. 1 Realistic structures, however, generally do not follow Planck’s law but exhibit a smaller emission. The properties of these emitters strongly depend on the materials and their shapes. From the absorption spectra of a structure the emission properties can be deduced since Kirchhoff’s law directly relates the absorption with the emissivity. The emission is then determined by multiplying the emissivity with the blackbody radiation spectrum. Using photonic crystals, 2,3 it has been shown that this approach is also valid for periodically structured materials. For a number of applications such as thermophotovoltaic converters, it is necessary to control the spectral properties to achieve, e.g., selective emitters in a narrow frequency band corresponding to the band gap of solar cells. 4 In the case of structured metallic surfaces, the changes in the emission spectra are based on surface waves coupled to the external radiation through the periodic surface. 5,6 Alternatively, microcavity resonances can also be used to create narrow-band thermal radiation. 7 Unfortunately, most of the recent designs 6,8 for perfect absorbers/ emitters only work for one incident angle and one polarization. So, there is a need for wide-angle perfect absorber/ emitter nanostructures. In this Brief Report, we suggest a structure which exhibits a large absorption in the terahertz regime for a wide range of angles with respect to the surface. We show that the absorption characteristics are maintained even if the uncertainties in the estimated changes in the material parameters, due to high temperatures, are considered. The proposed structure can be easily manufactured with today’s planar microfabrication techniques. We also comment on the impact of deviations in the geometrical parameters caused by fabricational tolerances. The small size of the structure, in comparison to the wavelength together with the relatively straightforward fabrication, allows for easy integration into various devices, such as perfect thermal emitters, perfect absorbers, bolometers, and very effective light extraction light-emitting diodes LEDs. The suggested structure is shown in Fig. 1. It consists of a metal back plate black with a thickness larger than 200 nm. This is much larger than the typical skin depth in the terahertz regime and avoids transmission through the structure. In this case the reflection is the only factor limiting the absorption. The thickness of the back plate can be adjusted to the specific needs of the final application, e.g., to obtain good heat transport to sensors or to obtain a better stability. On top of the metal plate a spacer layer of silicon nitride SiN is deposited with a thickness Dt. The structure is terminated by an array of metallic stripes with a rectangular cross section. Their arrangement is described by a lattice constant a and their shape is given by a width Ww and a thickness Wt. In this setup a strong resonance with a large field enhancement in the dielectric spacer layer and in between the stripes can be obtained, as will be shown later. Adjusting the size of the metal stripes on the top, the coupling to this resonance can be tuned and the reflection can be minimized. Due to the scalability of Maxwell’s equations, in principle, the structure can be simulated using dimensionless units by dividing all sizes by the lattice constant and using =a / as frequency. However, the Drude model used to describe the metal requires frequencies in terahertz and therefore the lattice constant must be assigned in the simulation. If a shift in the frequencies of the spectral features by adjusting the lattice constant is intended, a different simulation must be done since changes in the dielectric constant would not be considered. In the simulation frequency-dependent material parameters are required. We calculate those using standard methods and adjust their values to take into consideration the high temperatures. The tungsten parts plate and stripes are described by a Drude model

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Wide-angle perfect absorber/thermal emitter in the terahertz regime

A metamaterial-based approach in making a wide-angle absorber of infrared radiation is described. The technique is based on an anisotropic perfectly impedance-matched negative-index material PIMNIM .I t is shown analytically that a PIMNIM that is subwavelength in all three dimensions enables absorption close to 100% for incidence angles up to 45° to the normal. A specific implementation of such frequency-tunable PIMNIM based on plasmonic metamaterials is presented. Applications to infrared imaging and coherent thermal sources are described.

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Wide-angle infrared absorber based on a negative-index plasmonic metamaterial

We present a useful framework within which we can understand some of the physical phenomena that drive thermal emission in 2D-periodic metallic photonic crystal slabs, emphasizing phenomenology and physical intuition. Through detailed numerical calculations for these systems, we find that periodicity plays a key role in determining the types of physical phenomena that can be excited. We identify two structures as good candidates for thermal design, and conclude with a discussion of how the emissive properties of these systems can be tailored to our needs.

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Thermal emission and design in 2D-periodic metallic photonic crystal slabs.

We present a useful framework within which we can understand some of the physical phenomena that drive thermal emission in one-dimensional periodic metallic photonic crystals, emphasizing phenomenology and physical intuition. We perform detailed numerical calculations for these systems and find that polarization and periodicity play key roles in determining the types of physical phenomena that can arise. Two promising structures are identified as good candidates for thermal design. We conclude with a discussion of how the emissive properties of these systems can be tailored to our needs.

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Thermal emission and design in one-dimensional periodic metallic photonic crystal slabs.

We perform direct thermal emission calculations for three-dimensionally periodic photonic crystal slabs using stochastic electrodynamics following the Langevin approach, implemented via a finite-difference time-domain algorithm. We demonstrate that emissivity and absorptivity are equal, by showing that such photonic crystal systems emit as much radiation as they absorb, for every frequency, up to statistical fluctuations. We also study the effect of surface termination on absorption and emission spectra from these systems.

Direct calculation of thermal emission for three-dimensionally periodic photonic crystal slabs.

Through detailed numerical and analytical studies, we establish that the significant enhancement of thermal emission via Q matching, which has been possible in one-dimensional (1D) systems only, can be extended to two-dimensional (2D) systems by means of Fano resonances in the 2D system. In particular, we show the existence of essentially 1D behavior in a 2D system--a case of reduced dimensionality. Moreover, we show how properties of these spectra can be controlled by changing the geometrical parameters of the 2D system.

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Emulating one-dimensional resonant Q-matching behavior in a two-dimensional system via Fano resonances

We study point defect geometries in inverted opal photonic crystals that can be easily fabricated by means of colloidal self-assembly. Two broad classes of defects are considered: substitutional and interstitial. Substitutional point defects are found to introduce a usable defect band into the photonic band gap. This can be done by using a silica sphere of radius between 0.33a and 0.35a (where a is the lattice constant). The state is triply degenerate. Reflectance and local density of states calculations are performed to verify the existence and frequency of this defect. The point defect can be made by precoating shrunk silica spheres with a thin layer of silicon. Such a defect can be used as a microcavity for localizing light at a point, with a quality factor Q that is limited primarily by the proximity of the defect to the surface of the photonic crystal and other such defects.

David L. C. Chan

Papers

Thermal emission and design in 2D-periodic metallic photonic crystal slabs.

Thermal emission and design in one-dimensional periodic metallic photonic crystal slabs.

Direct calculation of thermal emission for three-dimensionally periodic photonic crystal slabs.

Emulating one-dimensional resonant Q-matching behavior in a two-dimensional system via Fano resonances

Point defect geometries in inverted opal photonic crystals