Lucas N. Taylor

Continuous-wave laser damage of uniform and nanolaminate hafnia and titania optical coatings

The laser-damage thresholds of single material and nanolaminate thin films were compared under continuous-wave (CW) illumination conditions. Nanolaminate films consist of uniform material interrupted by the periodic insertion of one or more atomic layers of an alternative material. Hafnia and titania were used as the base materials, and the films were deposited using atomic-layer deposition. The nanolaminates were less polycrystalline than the uniform films, as quantified using x-ray diffraction. It was found that the nanolaminate films had reduced laser-damage thresholds on smooth and patterned substrates as compared to uniform single-material films. This behavior is unusual as prior art indicates that amorphous (less polycrystalline) materials have higher laser-damage thresholds under short-pulse excitation. It is speculated that this may indicate that local thermal conduction affects breakdown more strongly under CW excitation than the dielectric properties that are important for short-pulse excitation.

Ring-like damage morphologies produced by continuous-wave laser irradiation

The surface damage morphologies produced by continuous-wave laser irradiation of coated optics were measured and analyzed. A few laser damage morphologies were observed to be bull’s-eye patterns. It is noted that the bull’s-eye pattern has some similarities to Bessel distributions of the form found in solutions of basic heat transfer or surface acoustic wave problems, which may indicate a relationship. If these morphologies are truly thermal phenomenon, then the Bessel ring diameter would be a function of thermal diffusivity. This might indicate that the ring diameter could be used to assess the resistance of a film to laser damage.

Reduced blackbody microheaters for measuring high temperature thermoluminescent glow curve peaks

Infrared-transparent microheaters have been constructed to reduce the background blackbody radiation produced by the heater. Among other applications, such heaters allow one to probe the high temperature peaks of thermoluminescent(TL) materials. The microheater consists of peripheral platinum heating elements on a mid-infrared transparent alumina platform. Alumina has a relatively low blackbody signal at high temperature for wavelengths less than 8μm. To test the reduced blackbody emission, an aperture was placed over the heating coils and then the transparent center of the microheater. The amount of infrared light transmitted through the aperture was reduced by 90% as the aperture moved from the highly emissive heater coils at 450°C to the largely transparent center at the same temperature.

High-power laser testing of 3D meta-optics

3D Meta-Optics are optical components that are based on the engineering of the electromagnetic fields in 3D dielectric structures. The results of which will provide a class of transformational optical components that can be integrated at all levels throughout a High Energy Laser system. This paper will address a number of optical components based on 2D and 3D micro and nano-scale structures and their performance when exposed to high power lasers. Specifically, results will be presented for 1550 nm and 2000 nm spectral bands and power densities greater than100 kW/cm2.

Subsampling phase retrieval for rapid thermal measurements of heated microstructures

A subsampling technique for real-time phase retrieval of high-speed thermal signals is demonstrated with heated metal lines such as those found in microelectronic interconnects. The thermal signals were produced by applying a current through aluminum resistors deposited on soda-lime-silica glass, and the resulting refractive index changes were measured using a Mach-Zehnder interferometer with a microscope objective and high-speed camera. The temperatures of the resistors were measured both by the phase-retrieval method and by monitoring the resistance of the aluminum lines. The method used to analyze the phase is at least 60× faster than the state of the art but it maintains a small spatial phase noise of 16 nm, remaining comparable to the state of the art. For slowly varying signals, the system is able to perform absolute phase measurements over time, distinguishing temperature changes as small as 2 K. With angular scanning or structured illumination improvements, the system could also perform fast thermal tomography.

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.

High-speed optical interferometry of micro and nanothermal wave propagation induced by current flow in metal interconnect lines

Thermal deformation waves in a soda-lime-silica glass substrate were measured using high-speed Hilbert-phase interferometry during interconnect heating and Q-switched Nd:YAG irradiation. 100-microsecond thermal events were captured with 256×256 resolution over a 100 micron square area.

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.

Laser Damage of Nanolaminate HfO2 and TiO2 Optical Coatings

We compared the laser damage performance of uniform versus nanolaminate thin films for hafnia and titania-based coatings. Our experiments showed that the films with higher crystallinity appeared to have better performance for CW systems.