Kyle D. Olson

Absorption to reflection transition in selective solar coatings.

The optimum transition wavelength between high absorption and low emissivity for selective solar absorbers has been calculated in several prior treatises for an ideal system, where the emissivity is exactly zero in the infrared. However, no real coating can achieve such a low emissivity across the entire infrared with simultaneously high absorption in the visible. An emissivity of even a few percent radically changes the optimum wavelength separating the high and low absorption spectral bands. This behavior is described and calculated for AM0 and AM1.5 solar spectra with an infrared emissivity varying between 0 and 5%. With an emissivity of 5%, solar concentration of 10 times the AM1.5 spectrum the optimum transition wavelength is found to be 1.28 µm and have a 957K equilibrium temperature. To demonstrate typical absorptions in optimized solar selective coatings, a four-layer sputtered Mo and SiO₂ coating with absorption of 5% across the infrared is described experimentally and theoretically.

High power laser heating of low absorption materials

A model is presented and confirmed experimentally that explains the anomalous behavior observed in continuous wave (CW) excitation of thermally isolated optics. Distributed Bragg Reflector (DBR) high reflective optical thin film coatings of HfO2 and SiO2 were prepared with a very low absorption, about 7 ppm, measured by photothermal common-path interferometry. When illuminated with a 17 kW CW laser for 30 s, the coatings survived peak irradiances of 13 MW/cm2, on 500 μm diameter spot cross sections. The temperature profile of the optical surfaces was measured using a calibrated thermal imaging camera for illuminated spot sizes ranging from 500 μm to 5 mm; about the same peak temperatures were recorded regardless of spot size. This phenomenon is explained by solving the heat equation for an optic of finite dimensions and taking into account the non-idealities of the experiment. An analytical result is also derived showing the relationship between millisecond pulse to CW laser operation where (1) the heating is proportional to the laser irradiance (W/m2) for millisecond pulses, (2) the heating is proportional to the beam radius (W/m) for CW, and (3) the heating is proportional to W / m ⋅ tan − 1 ( t / m ) in the transition region between the two.

Band Gap Dependence of Continuous-Wave Laser Induced Damage Threshold of Optics with Absorptive Contamination

The laser induced damage threshold of optical coatings with differing band gaps was measured using a high power 1070 nm continuous-wave laser. High reflectivity distributed Bragg reflectors of niobia-silica, tantala-silica, and hafnia-silica were tested in addition to half-wave coatings of titania, tantala, hafnia, and alumina. Absorbing contamination in the form of 20–50 µm carbon particles was added to the surface of the optics prior to exposure to test for particle induced damage. For both half-wave and high reflectivity coatings, the minimum damage thresholds were found to increase for larger band gap materials. Low band gap niobia reflectors and titania half wave coatings damaged at 20 kWcm−2 and 155 kWcm−2 respectively while larger bandgap hafnia reflectors failed at 1 MWcm−2 and alumina half-wave coatings failed at 9.7 MWcm−2. Most laser induced damage was catastrophic with the film and underlying substrate being damaged and the optic uncontrollably hearting. Damage effects differed for high band gap half-wave coatings that did not damage the substrate or thermally runaway with additional laser exposure.

Microheater multilayer interference to reduce thermal emission for low photon number luminescence measurement

Under low light conditions, high temperature measurements of luminescence are limited by the overlap of the thermal emission spectra and the luminescent emission spectra being measured. A solution to this is to have a heat source that can be designed not to emit in a certain wavelength range(s) by coating it with an interference multilayer. The multilayer effectively changes the emissivity of the heat source. Microheaters made from aluminum oxide platforms with platinum heating elements were coated with aluminum oxide and titanium oxide multilayers. This multilayer structure was used to measure the thermoluminescence of CaSO4:Ce,Tb up to 420°C. They also showed a thermal emission background 800 times lower at 600°C than the same microheater with no multilayer structure.

Reduction of thermal emission background in high temperature microheaters

High temperature microheaters have been designed and constructed to reduce the background thermal emission radiation produced by the heater. Such heaters allow one to probe luminescence with very low numbers of photons where the background emission would overwhelm the desired signal. Two methods to reduce background emission are described: one with low emission materials and the other with interference coating design. The first uses platforms composed of material that is transparent to mid-infrared light and therefore of low emissivity. Heating elements are embedded in the periphery of the heater. The transparent platform is composed of aluminum oxide, which is largely transparent for wavelengths less than about 8 μm. In the luminescent microscopy used to test the heater, an optical aperture blocks emission from the heating coils while passing light from the heated objects on the transparent center of the microheater. The amount of infrared light transmitted through the aperture was reduced by 90% as the aperture was moved from the highly emissive heater coils at 450 °C to the largely transparent center at the same temperature. The second method uses microheaters with integrated multilayer interference structures designed to limit background emission in the spectral range of the low-light luminescence object being measured. These heaters were composed of aluminum oxide, titanium dioxide, and platinum and were operated over a large range of temperatures, from 50 °C to 600 °C. At 600 °C, they showed a background photon emission only 1/800 that of a comparison heater without the multilayer interference structure. In this structure, the radiation background was sufficiently reduced to easily monitor weak thermoluminescent emission from CaSO4:Ce,Tb microparticles.

Microheater controlled part-per-million-level absorption measurements using Photothermal Common Path Interferometry

Free carrier absorption is a major component of the laser-induced breakdown of optical materials. Free carriers mediate intense absorption of incident laser light, and the resulting heat transfers to the lattice, generating more carriers, creating a catastrophic runaway process. Unfortunately, measurements at very low levels of free carriers relevant to laser breakdown are very difficult, particularly when measurements must be taken at highly elevated temperatures. This paper describes a photothermal common path interferometer (PCI) system that is enhanced by use of micromachined heaters to control local substrate temperatures. The temperature dependence of the free carrier absorption of aluminum oxide and silicon are measured. The aluminum oxide is shown to have essentially no variation in absorption with temperature up to 700K, but silicon shows an exponential increase as would be expected by the relative sizes of their bandgaps.

High-speed quantitative phase imaging of dynamic thermal deformation in laser irradiated films

We present a technique for high-speed imaging of the dynamic thermal deformation of transparent substrates under high-power laser irradiation. Traditional thermal sensor arrays are not fast enough to capture thermal decay events. Our system adapts a Mach-Zender interferometer, along with a high-speed camera to capture phase images on sub-millisecond time-scales. These phase images are related to temperature by thermal expansion effects and by the change of refractive index with temperature. High power continuous-wave and long-pulse laser damage often hinges on thermal phenomena rather than the field-induced effects of ultra-short pulse lasers. Our system was able to measure such phenomena. We were able to record 2D videos of 1 ms thermal deformation waves, with 6 frames per wave, from a 100 ns, 10 mJ Q-switched Nd:YAG laser incident on a yttria-coated glass slide. We recorded thermal deformation waves with peak temperatures on the order of 100 degrees Celsius during non-destructive testing.

Optimal lateral splitting of the AM1.5 solar spectrum for a mono-Si and CdTe two cell PV array

An analysis of the optimal spectral splitting for a lateral two-cell solar photovoltaic system is described and tested experimentally. A diffraction grating with arbitrary but known transmission efficiency versus wavelength was used to split the spectrum. An AM1.5 spectrum was produced using a Xe-arc lamp with 5mm pinhole, AM1.5 optical filter for Xe-arc lamps, and a collimating lens. Detailed balance simulations for solar p-n junctions were performed on the AM1.5 spectrum weighted by the optical transmission efficiency of the diffraction grating. When the optical efficiency is included in the detailed balance calculations, an accurate prediction for the performance of the experiment is obtained. Using the simple diffraction grating, we calculate the optimal efficiency of the system under no concentration to be ∼23.2% with cells of band gaps 1.14 and 1.82eV. Using mono-Si and CdTe with band gaps of 1.12 and 1.46eV respectively results in an optimal spectral split at 849nm and a total efficiency of 21.1%. Experimentally the result is proven using a mono-Si and CdTe PV array by splitting the spectrum over many wavelengths and finding the optimal split.