Mr. Herman Hogstrom Profile

Herman Hogstrom

at Uppsala Univ

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Publications (3)

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SPIE Journal Paper | 1 February 2005

Realization of selective low emittance in both thermal atmospheric windows

Herman Hogstrom, Göran Forssell, Carl Ribbing

OE, Vol. 44, Issue 02, 026001, (February 2005) https://doi.org/10.1117/12.10.1117/1.1839887

KEYWORDS: Reflectivity, Silicon, Medium wave, Cameras, Optical filters, Transmittance, Polarization, Dielectrics, Polarizers, Thermography

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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.

Proceedings Article | 29 December 2003 Paper

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

Herman Hogstrom, Andreas Rung, Carl Ribbing

Proceedings Volume 5184, (2003) https://doi.org/10.1117/12.504710

KEYWORDS: Silicon, Reflectivity, Dielectrics, Photonic crystals, Semiconducting wafers, Multilayers, Metals, Infrared radiation, Refractive index, Oxidation

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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.

Proceedings Article | 29 December 2003 Paper

Interaction between photonic and polaritonic gaps studied with photonic band structure calculations

Andreas Rung, Herman Hogstrom, Carl Ribbing

Proceedings Volume 5184, (2003) https://doi.org/10.1117/12.504717

KEYWORDS: Polaritons, Photonic crystals, Dielectrics, Photon polarization, Sodium, Infrared radiation, Dispersion, Transmittance, Beryllium, Oxides

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The interaction between the two kinds of gaps that appear in the band structure of a photonic crystal has been studied. The structure gap appears as a consequence of diffraction in the periodic structure, if the optical contrast between the the two matrials is sufficiently strong. The width of such gaps increases with the optical contrast and the position, for a given structure, scales with the lattice constant. Secondly, the dielectric function of one of the materials may be such that the photonic crystal exhibits an effective stop band. Metals have a dielectric function with a large negative real part in the visible and infrared wavelength regions. Metallo-dielectric photonic crystals have been intensively studied recently, and interesting results have been obtained. Alternatively, a Reststrahlen band can be used, within which the dielectric function is metal-like. The physical mechanism behind such a band is the excitation of polaritons, i.e. lattice oscillations. Only compounds have Reststrahlen bands, and they appear in the infrared. We refer to the corresponding stopband as a polaritonic gap. Transfer matrix calculations have been used to obtain the photonic bandstructure in the infrared for a 2-D square structure consisting of beryllium oxide cylinders in air. Photonic band structure calculations across a reststrahlen band region are numerically demanding because of the strong dispersion. Calculations were made with different lattice constants and fill factors. We have compared a situation when the two gaps are widely separated, with one where the gaps are close or even on top of each other. We report two kinds of forbidden gap states as a function of the imaginary wave-vector. We use normal incidence transmittance spectra to define phonomenological gaps, and report their variation with linear density and lattice constant.

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