Optical constants for simulations were obtained by R- and T-measurements on TiAlN thin films deposited on
Corning 7059 glass. The model parameterized free carrier effects and an inter-band excitation. The calculations
demonstrated that the colour effects are due to interference and inter-band absorption around 500 nm in a single
layer coating. The peak shifts with the thickness of the thin film which gives a simple way to obtain different
colours. Solar absorptance of 86 % can be reached already for a single TiAlN-film on an Al substrate.
Aluminium nitride films, 0.8-1.8 μm thick, have been deposited by reactive, magnetron sputtring of aluminium in an
argon-nitrogen atmosphere. The sputtered films exhibit a Reststrahlen band in the wavelength range 11-16 μm. We have
investigated the possibilities to use different substrate materials and dielectric coatings to extend this interval to the entire
upper thermal window 8-13 μm and a secondary interference maximum in the 3-5 μm range, i.e. the lower thermal
window. Our results indicate a potential for the use of AlN-films in applications that benefit from wavelength selective
emittance, e.g. IR signature control and frost prevention.
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.
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.
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.
The feasibility of an alternative surface polariton sensor is demonstrated. Existing surface plasmon resonance (SPR-) sensors use the surface of a thin metal film as detector. The critical condition for this excitation is a large negative real part of the dielectric function (epsilon) 1((omega) ), which is characteristic for good metals in the visible and near UV range. This condition is also fulfilled in the infrared region for many compounds with partly ionic binding. Multilayer calculations are used to show that a ceramic film can be used as a sensor surface. In the calculations, the Kretschmer configuration is used to demonstrate the excitation of a surface polarion beyond the critical angle of incidence, sensitive to minute changes in refractive index near the free surface. Optimization with respect to film thickness and excitation wavelength within the Reststrahlen Band region is discussed.
KEYWORDS: Black bodies, Solar energy, Solar radiation, Absorption, Sun, Earth's atmosphere, Astronomy, Reflectivity, Atmospheric physics, Temperature metrology
The problem of scaling the impinging solar radiation spectrum, to the excitance of blackbodies on earth at moderate temperatures is discussed. Typically diagrams with these two kinds of spectra are used to demonstrate wavelength separation as the background for solar, optical selectivity. All spectra can satisfactorily be approximated with Planck curves, but the solar spectrum must be reduced in intensity because of the long distance Earth - Sun. Some difficulties with using astronomical data for this scaling are noted. As an alternative, the T5-dependence of the Planck curve maxima, when drawn on a wavelength axis, is used to demonstrate that the curves can be scaled based on their total energy and solar absorber stagnation. The ratios of the peaks of the impinging solar radiation to the excitance spectra for black surfaces on earth at various ordinary temperatures are reported. It is pointed out that the choice of independent variable effects the positions of the Planck function peaks, as well the shape of the spectra. The peak values increase proportionally to T3 when frequency is used instead of wavelength and the width of the curves grows linearily with temperature. Effects of using a logarithmic wavelength scale are also noted.
The international DVD standard, ECMA-267, for compact discs with improved storage capacity is based on a semi- transmitting mirror. This coating provides a top memory layer that permits reading an opaque second mirror layer underneath. According to the standard, the top layer should have a reflectance in the interval 18-30 percent and simultaneously sufficient transmittance to permit a reflectance in the same interval from the bottom mirror layer, all at the reading wavelength 650 nm. The standard also includes the optical effects of the 550 micrometers polycarbonate disc and the 55 micrometers spacer layer used for protection.
Thermal control is an important issue in small-scale satellite design, and thin film coatings suit the limited mass and volume constraints. Group IVB transition metal nitride films meet the criteria that the satellite surface must be mechanically and chemically stable, and electrically conducting. Thin film TixAlyNz coatings have been investigated and tailored for temperature control. The films were deposited by reactive sputtering on aluminum substrates in N2/Ar-atmosphere. The solar absorptance, (alpha) , and thermal emittance, (epsilon) , were calculated from spectral reflectance measurements. It was found that an optimization of film composition leads to a reduced equilibrium temperature. The composition temperature. The composition Ti0.16Al0.41N0.43 has a flatter reflectance curve than TiN, and was found to be close to optimal. By varying the film thickness, interference effects could further reduce the equilibrium temperature. A 650 nm Ti0.16Al0.41N0.43 film showed a reflectance interference minimum positioned at the maximum of the blackbody spectrum, resulting in an increased emittance. Neglecting internal heat contributions, the lowest calculated equilibrium temperature was 34.6 degrees C for this film.
The measurement of total scatter losses is a major prerequisite for the development, optimization and commercialization of high quality optical components. Especially in laser technology, optical scattering gained of importance in the source of the development of laser system with ever increasing output power and improved beam parameters. Besides its influence on the efficiency of laser systems and the beam steering arrangement, total scattering is an important safety aspect for application of these laser systems in materials processing, medicine and fundamental research. As a consequence of this global trend, working groups of TC 172/SC 9 initialized the development of an International Standard for the measurement of total scattering in optical components.
The three transition metal nitrides TiN, ZrN and HfN have remarkably high stability due to their bonding: a mixture of covalent and ionic contributions. The optical properties of these nitride compounds are free-electron like to a surprisingly large extent, in particular in comparison with the corresponding carbides. It is argued that the interband optical excitations of the d-electrons are restricted by selection rules, resulting in a Drude like behavior of these d-electron compounds. Hitherto, one of the main optical applications has been as opaque, wear-resistant replacements for gold-coatings. This review includes the efforts to study, understand and enhance the optical selectivity of group IVB transition metal nitrides as the selective surface in high temperature thermal solar absorbers, as the metal layer in LE-coatings on energy-efficient or solar control windows and as a Langmuir probe coating. The reflectance edge is not in the optimum position for absorber applications and attempts to shift it with alloying were unsuccessful. Recent development of inhomogeneous, cermet- type nitrides hold some promise. The LE-coatings will not reach as high selectivity as the current noble metal based multilayers, but many find use in aggressive environments because of their excellent stability.
The standard equation for the radiometric emission factor as monitored by an infrared radiometer includes radiation emitted and reflected by the sample. The derivation of this equation assumes that the temperature variation of the band emittance is neglible which is only valid if the sample is gray within the band of observation. The radiometric emission factor obtained from the standard equation will therefore not agree with the true average emittance if the spectral emittance exhibits spectral structure. Ceramic beryllium oxide has one emittance edge rising with wavelength in the 3 - 5 micrometers atmospheric window and another edge falling with wavelength in the 8 - 13 micrometers window. Broadband radiometer measurements and model calculations are used to compare the temperature variation of the radiometric emission factors with that of the correct band emittance. Comparing the model calculations with experimental heat- camera data confirm the prediction that the band emittance and the radiometric emission factor decrease with temperature in the lower atmospheric window and increase in the upper atmospheric window. An approximation based on linearizing the temperature dependence of the radiometric data to obtain the correct band emittance is applied to these two cases. The agreement with the integrated results of spectral measurements is very satisfactory for the data from the 8 - 13 micrometers window, but less so for the 3 - 5 micrometers range. In the lower atmospheric window the material is partly transmitting and the atmosphere partly absorbing.
The general problem of extracting the correct emission factor from broadband radiometric measurements on non-gray samples is treated with emphasis on polycrystalline beryllium oxide and BeO with a coating of silicon. These samples exhibit a strong spectral variation in their emittance functions where the Planck function has large weight. Under these circumstances the band-averaged emission factor will be temperature dependent, even if the spectral emittance is temperature independent. The consequences of this for the conventional expression which includes a correction for radiance from the surroundings reflected by the sample are investigated. It is concluded that the observation of a temperature variation in this emission factor not only violates an assumption of the derivation, it is also a criterion indicating that the numerical value is incorrect Two algorithms, based on linearization and iteration of the temperature variation are introduced and applied to an emittance step model and the experimental radiometer values for the reststrahlen band materials. It is found that the emission factors obtained after this correction procedure are in significantly better agreement with values obtained from weighted integration of spectral emittance over the spectral window of the radiometer. The room-temperature value of the upper TIR emission factor is 0.40 and 0.22 for BeO and the Si-BeO double layer respectively. A sand-blasted aluminum sample had almost perfectly gray emittance and the emission factor is 0.39 and temperature independent.
The possibility of using selectively low emission to reduce the thermal signature of radomes is identified. A material can simultaneously have low radiance in the working range of a detector and cool radiatively to the surrounding atmosphere. If the band of low emission is based on lattice excitation, the signature reduction is compatible with electrically insulating properties and radar transmittance. High-density, polycrystalline beryllium oxide is identified as a material with low emittance in the upper atmospheric window at 8 to 13 μm, owing to a strong reststrahlen band. Radiometer measurements and bulk reflectance spectra are reported for ceramic BeO and are used to calculate the thermal infrared emission factors as well as the radiance for a 50°C BeO surface. The results depend heavily upon the short wavelength threshold of the detector. The calculated values have been compared with radiometer measurements in the 3- to 5-μm and 8- to 13-μm ranges. The possibility of reducing the emittance even further with a second material has been investigated with Fresnel calculations. Very favorable calculated and measured results for a 0.8-μm silicon overlayer on BeO are reported.
Materials with ionic bonding exhibit high reflectance somewhere in the infrared spectral range: the reststrahlen band. This arises because of lattice vibrations that are excited by the incident light. For some materials, e.g. the alkali halides NaF, NaCl, KCl and polycrystalline samples of BeO, SiC, secondary minima in the reststrahlen band have been observed, which have previously not been fully understood. We show, theoretically and experimentally, that small inhomogeneities on the surface are sufficient to cause a measurable absorption and a substructure in the reststrahlen band. The electric field of the incident light beam excites surface mode resonances in the inhomogeneities which absorb light. Inhomogeneities with different shapes have surface modes with different resonance frequencies because of the strong dispersion in the reststrahlen band region. The position and width of the absorption band is determined by the details of the surface roughness: in particular the mixture of spherical, ellipsoidal and disk-shaped irregularities which can be used to describe the actual tomography. IR reflectance spectral can be used to monitor the departure from a perfect smooth surface and to determine the concentration density and shape of topographic defects on ceramic surfaces. Light scattering measurements over the wavelength range 5 - 20 micrometers for bulk samples of polycrystalline beryllium oxide and 5 - 20 micrometers for silicon carbide powder are reported in verify this interpretation.
The possibility of using selectively low emission to reduce thermal signature of domes is pointed out. A material can simultaneously have low radiance in the working range of a detector and cool radiatively to the surrounding atmosphere. If the band of low emission is based on lattice excitation, the signature reduction is compatible with electrically insulating properties and radar transmittance. High density, polycrystalline beryllium oxide is identified as a material with low emittance in the primary atmospheric window 8 - 13 micrometers , owing to a strong reststrahlen band. Bulk reflectance spectra are reported for ceramic BeO of three grades and are used to calculate the average, bulk near normal emittance over various possible detector ranges as well as the radiance for a 50 degree(s)C BeO surface. The results vary: 0.3 - 0.6 and 10 - 37 W/m2, sr respectively, depending upon the short wavelength threshold of the detector. The calculated values have been compared with radiometer measurements in the 3 - 5 and 8 - 13 micrometers ranges. The possibility to reduce the emittance even further with a second material has been investigated with Fresnel calculations. Very favorable calculated and measured numerical results for a 0.8 micrometers silicon overlayer on BeO are reported.
The Drude-like behavior of the group IVB metal nitrides: TiN, ZrN and HfN furnishes the physical basis for the use of these hard, inert materials as replacement for noble metals in optically selective multilayers. A low value of the refractive index, n, in the visible region and rapidly increasing extinction coefficient, k, when the wavelength increases into the infrared, is characteristic for these nitrides, although to a lesser extent than for the noble metals. A screened Drude model can be fitted to the experimental dielectric function over the near infrared and at least part of the visible spectrum to determine the parameters: plasma resonance energy hvp and relaxation time (tau) . Systematic studies of TiN and ZrN films show that n increases with decreasing film thickness below 60 nm when the film transmits. This increase can be modelled with a increasing Drude parameter (tau) and has previously been explained as an extrinsic effect from defects etc. It is argued that most of this change can be understood as an effect of diffuse scattering against the back surface of the film and is therefore not cured by improvements in deposition technology.
Opaque and semi-transmitting film of ZrN have been reactively magnetron sputtered and their optical performance for solar control coating applications evaluated. The optical constants of the opaque films were determined from near normal reflectance measurements and Kramers- Kronig calculations over the interval 0.23 - 3.0 micrometers . The thin film optical constants were determined with combined R- and T-measurements. Systematic variations with growth conditions were observed. Thin films were observed to have higher n-values in the visible, a change that could be partly compensated by substrate heating. For the triple layers: glass / ZrO2 / ZrN / Zro2 it was noted that the crystallinity of the first oxide did not influence the refractive index of this oxide, but improved significantly the selectivity of the nitride growing upon it. Triple layers for solar control applications with 57% luminous transmittance and close to 1/3 each of solar transmittance, reflectance and absorption have been realized.
Triple layer structures of TiO2/TiN/TiO2 and quadruple layer structures of TiO2/Al/TiN/TiO2 have been sputtered on glass substrates at temperatures ranging from room temperature to 300 degree(s)C. The reflectance and transmittance were measured in the visible and the near infrared wavelength regions. Accelerated degradation tests with respect to high temperature and acid exposure have been performed with these laboratory samples of low-e coatings and the degradation has been compared with that of commercial silver based window coatings. As expected the durability of the nitride based coatings is far superior to the stability of those based on noble metals. Furthermore, the nitride coatings with an aluminum sacrificial layer have been found to resist aging at elevated temperatures (as high as 350 - 400 degree(s)C) far better than similar coatings without the aluminum. It has also been shown that the aluminum layer protects the nitride film during deposition of the top oxide layer. The effects of high temperature annealing have been modeled with optical multilayer calculations. Comparison of two degradation mechanisms demonstrates that the silver layers fail by agglomeration while the nitride suffers successive oxidation. This explains the effectiveness of the aluminum layer which forms a dense oxide during the initial stages of TiO2- deposition.
An empirical relationship between the long wavelength edge of the residual ray band in the IRreflectance
of partly ionic compounds and bulk hardness is demonstrated. The group of materials studied
includes alkali halides, semi-conductors and some hard compounds, mostly with cubic structure and a few
with hexagonal. The correlation is shown for the Young modulus, the melting temperature and also to
some extent the indentation hardness. The Young moduli cover a range from 10 to almost 500 GPa and the
melting temperatures vary from 600 to 3000°C.
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