Herman Högström

Realization of Selective Low Emittance in Both Thermal Atmospheric Windows

The infrared reflectance and emittance of a double layer of silicon and silicon dioxide have been investigated by optical multilayer calculations and spectral and wavelength-integrated measurements. Low emittance in the interval 0.2 to 0.4 can be obtained simultaneously in both thermal atmospheric windows: 3 to 5 and 8 to 13 µm. These results are relevant for IR signature control. The sample consisted of a 0.9-µm Si and a 2.45-µm SiO2 layer on a Si wafer. The layers were grown by standard microelectronic chemical vapor deposition techniques. The key mechanism for lowering the emittance is the interaction between the SiO2 molecular reflectance band, around 9 µm, and interference effects in the double layer. Interference gives one peak in the 3- to 5-µm window, and a widening and strengthening of the SiO2 molecular reflectance band in the 8- to 13-µm window. The calculated spectra are in very good agreement with measured near-normal incidence reflectance spectra in the range 2.7 to 12.5 µm. The emittance of the samples heated to 61 °C was determined in the atmospheric windows using two heat cameras filtered for the respective intervals and equipped with polarizers. Emittance values for the sample in the two windows and the two main polarizations were determined as a function of emission angle from 10 to 60 deg. Qualitative agreement with values calculated from tabulated optical constants was obtained.

Studies of polaritonic gaps in photonic crystals.

In a photonic band structure two kinds of gaps with different origins can be observed. Photonic gaps are determined by the symmetry of the photonic crystal, the lattice constant, and the contrast of the dielectric functions for the two components. Polaritonic gaps originate from the bulk optical properties of one of the components. Excitation of ionic components in the lattice results in a photon energy interval in which the dielectric function is negative. Here we investigate the interaction between photonic gaps and polaritonic gaps in one-dimensional and two-dimensional photonic structures. In particular, we show that by such interactions the polaritonic gap can be made wider and stronger, be left unchanged, or be made to vanish.

Experimental observation of photonic and polaritonic gaps in a silica opal

Experimental observations of the simultaneous presence of a polaritonic and a photonic gap in a three-dimensional photonic crystal is reported, to the best of our knowledge, for the first time. The photonic crystal was made of monodispersed silica microspheres sedimented into a face-centered-cubic structure. Silica has a polaritonic gap for wavelengths between 8 and 9.35 microm. Four different sphere sizes were used, with diameters of d=0.49, 0.73, 0.99, and 1.57 microm. The photonic crystals were studied by normal incidence infrared reflectance measurements in the wavelength interval 0.8-12 microm. Four peaks with the a magnitude of approximately 0.6, originating from the periodicity of the crystal, were recorded in the interval between 1 and 4 microm. Another peak, the polaritonic reflectance peak (approximately 0.4), is observed for wavelengths around 9 microm for all four crystals.

Studies of polaritonic gaps in photonic crystals

Polaritonic gaps are caused by lattice excitations in one of the components in a photonic crystal. The interaction with ordinary photonic gaps, caused by interference, is discussed in 1- and 2-D cases.

Si/SiO2 multilayer: a one-dimensional photonic crystal with a polaritonic gap

The silicon-silicondioxide system is used to illustrate the effect of interaction between a photonic gap in a periodic structure and a polaritonic gap originating from one of the constituent materials. Si is a near ideal dielectric material in the infrared region with a high refractive index and modest dispersion for λ>4 μm. Amorphous SiO2 has lattice absorption in the infrared, with a strong Reststrahlen band covering the wavelengths 8-9.3 μm. Optical multilayer calculations of reflectance spectra for Si/SiO2 double- and multilayers have been made. The results illustrate the effect of the metal-like optical properties of SiO2 in the Reststrahlen region. The high reflectance band persists in thin double layers and combines with conventional interference in the dielectric Si-film. From conventional optical coating technology it is known since long that a dielectric coating can be used to broaden and strengthen a Reststrahlen band, but this has not previously been applied to photonic crystals. For the experimental part, the Si/SiO2-system was prepared using standard microelectronic fabrication technology. Polycrystalline Si (poly-Si) and amorphous SiO2 (a-SiO2) were both deposited by CVD processes. Si from silane, and SiO2 from decomposition of tetra-ethoxy-silane (TEOS). a-SiO2 is also grown by wet- and dry oxidation of a Si wafer. The calculated and the measured reflectance spectra for Si/SiO2 double-layers are compared, and the overall agreement is very satisfactory. In particular, we can observe the Reststrahlen band of high reflectance and the interaction between this material stop band and the designed stop band, defined by the layer thicknesses.