Photography-based real-time long-wave infrared scattering estimation technique.

The scattered light distribution of surfaces in the long-wave infrared (λ∼8-12µm) is measured using a small set of thermal camera images. This method can extract scatter patterns considerably faster than standard laboratory bidirectional reflectance distribution function measurements and is appropriate for passive homogeneous surfaces. Specifically, six images are used in this study, each taken with respect to a thermal light source at an angle ranging from 10° to 60° to the normal of the surface. This data is deconvolved with the shape of the light source to estimate the scattering pattern. Both highly specular (black Masonite) and diffuse (painted drywall) surfaces are tested. Errors between the estimated scattering distribution and a directly measured one using a goniometer stage and quantum-cascade laser (QCL) are less than or equal to 3% except for extremely specular surfaces where viable QCL measurements cannot be made due to the increased relative contribution of speckle noise.

Effects of Kolmogorov turbulence on optical cavity spectral response.

An aberrated wavefront incident upon an optical resonator will excite higher order spatial modes in the cavity, and the spectral width and distribution of these modes are indicative of the type and magnitude of the aberration. We apply this concept to atmospheric turbulence modeled by the Kolmogorov distribution. The spectral widths of the cavity transmission spectra are demonstrated via simulations to correspond to the structure constant that characterizes the variation in the optical index of refraction and thus the turbulence strength. Such a relationship can be harnessed to build a sensor for simply and quickly assessing optical turbulence strength.

High-Q diamond microresonators in the long-wave infrared.

High quality factor (Q) photonic devices in the room temperature thermal infrared region, corresponding to deeper long-wave infrared with wavelengths beyond 9 microns, have been demonstrated for the first time. Whispering gallery mode diamond microresonators were fabricated using single crystal diamond substrates and oxygen-based inductively coupled plasma (ICP) reactive ion etching (RIE) at high angles. The spectral characteristics of the devices were probed at room temperature using a tunable quantum cascade laser that was free space-coupled into the resonators. Light was extracted via an arsenic selenide (As2Se3) chalcogenide infrared fiber and directed to a cryogenically cooled mercury cadmium telluride (HgCdTe) detector. The quality factors were tested in multiple microresonators across a wide spectral range from 9 to 9.7 microns with similar performance. One example resonance (of many comparables) was found to reach 3648 at 9.601 µm. Fourier analysis of the many resonances of each device showed free spectral ranges slightly greater than 40 GHz, matching theoretical expectations for the microresonator diameter and the overlap of the whispering gallery mode with the diamond.

Long-wave infrared absorption measurement of undoped germanium using photothermal common-path interferometry.

Germanium is one of the most commonly used materials in the longwave infrared ($\lambda \sim{8 {-} 12}\;\unicode{x00B5}{\rm m}$λ∼8-12µm), but ironically, its absorption coefficient is poorly known in this range. An infrared photothermal common-path interferometry system with a tunable quantum cascade pump laser is used to measure the absorption coefficient of ${ \gt }{99.999}\% $>99.999% pure undoped germanium as a function of wavelengths between 9 and 11 µm, varying between about 0.15 and ${0.45}\;{{\rm cm}^{ - 1}}$0.45cm-1 over this range.

Decomposition of aberrated or turbulent wavefronts into a spatial mode spectrum using optical cavities.

It is shown that an aberrated wavefront incident upon a Fabry-Perot optical cavity excites higher order spatial modes in the cavity and that the spectral width and distribution of these modes is indicative of the type and magnitude of the aberration. The cavities are purely passive, and therefore frequency content is limited to that provided by the original light source. To illustrate this concept, spatial mode decomposition and transmission spectrum calculation are simulated on an example cavity; the effects of various phase delays, in the form of two basic Seidel aberrations and a composite of Zernike polynomial terms, are shown using both Laguerre-Gaussian and plane wave incident beams. The aggregate spectral width of the cavity modes excited by the aberrations is seen to widen as the magnitude of the aberrations' phase delay increases.

Laser acceleration of absorbing particles.

A Yb-doped fiber laser is used to accelerate and evaporate absorbing particles in air. Optical intensities of 1MW/cm2 and 2MW/cm2 illuminate stainless steel particles. These particles are accelerated to velocities of tens of meters per second before evaporating within a few tenths of a millisecond. Position measurements are made using direct imaging with a high-speed camera. A fundamental system of coupled differential equations to track particle momentum, velocity, mass, radius, temperature, vapor opacity, and temperature distribution is developed and shown to accurately model the trajectories and lifetimes of laser heated particles. Atoms evaporating from the particle impart momentum to the larger particle, which accelerates until it is slowed by drag forces. Heat transfer within the evaporating particles is dominated by radiation diffusion, a process that usually only dominates in astrophysical objects, for example in the photospheres of stars.

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.

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.

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.

Midwave thermal infrared detection using semiconductor selective absorption.

The performance of thermal detectors is derived for devices incorporating materials with non-uniform spectral absorption. A detector designed to have low absorption in the primary thermal emission band at a given temperature will have a background-limited radiation noise well below that of a blackbody absorber, which is the condition typically assessed for ultimate thermal detector performance. Specific examples of mid-wave infrared (ʎ ∼ 3-5 µm) devices are described using lead selenide as a primary absorber with optical cavity layers that maximize coupling. An analysis of all significant noise sources is presented for two example room-temperature devices designed to have detectivities up to 4.37 × 10(10) cm Hz(1/2) W(-1), which is a factor 3.1 greater than the traditional blackbody limit. An alternative method of fabricating spectrally selective devices by patterning a plasmonic structure in silver is also considered.

Optical coatings in microscale channels by atomic layer deposition.

High-aspect-ratio channels may be coated using atomic layer deposition (ALD) due to the unique self-limiting nature of the process, and this has been often demonstrated using deep reactive-ion etched trenches in silicon. However, for optical and microfluidic applications, many channels are centimeters deep with diameters of tens to hundreds of micrometers, and the relatively large area exposes more difficult problems of temperature and gas flow uniformity. To quantify the uniformity of optical coatings deposited by ALD under those conditions, an air wedge has been created between two square wafers of silicon approximately 7 cm on a side, with the air gap varying linearly from 0-1560 microm. ALD aluminum oxide uniformity is astounding, while hafnium oxide shows a need for process optimization, but still exceeds the capability observed in other deposition techniques. A six-layer Fabry-Perot optical cavity with fixed 500 nm resonance was deposited inside a wedge, and the measured resonant wavelength closely matched predictions, except at the deepest regions of the wedge.

Photon diffusion in microscale solids.

This paper presents a theoretical and experimental investigation of photon diffusion in highly absorbing microscale graphite. A Nd:YAG continuous wave laser is used to heat the graphite samples with thicknesses of 40 µm and 100 µm. Optical intensities of 10 kW cm-2 and 20 kW cm-2 are used in the laser heating. The graphite samples are heated to temperatures of thousands of kelvins within milliseconds, which are recorded by a 2-color, high speed pyrometer. To compare the observed temperatures, differential equation of heat conduction is solved across the samples with proper initial and boundary conditions. In addition to lattice vibrations, photon diffusion is incorporated in the analytical model of thermal conductivity for solving the heat equation. The numerical simulations showed close matching between experiment and theory only when including the photon diffusion equations and existing material properties data found in the previously published works with no fitting constants. The results indicate that the commonly-overlooked mechanism of photon diffusion dominates the heat transfer of many microscale structures near their evaporation temperatures. In addition, the treatment explains the discrepancies between thermal conductivity measurements and theory that were previously described in the scientific literature.

Encapsulation of low-refractive-index SiO(2) nanorods by Al(2)O(3) with atomic layer deposition.

Thin films composed of SiO(2) nanorods or nanoporous SiO(2) (np- SiO(2)) are attractive for use as a low refractive index material in various types of optical coatings. However, the material properties of these films are unstable because of the high porosity of the films. This is particularly apparent in dry versus humid atmospheres where both the refractive index and coefficient of thermal expansion (CTE) vary dramatically. In this article, we demonstrate that np-SiO(2) can be encapsulated by depositing Al(2)O(3) with Atomic Layer Deposition (ALD), stabilizing these properties. In addition, this encapsulation ability is demonstrated successfully in a 4-pair distributed Bragg reflector (DBR) design. It is hoped that this technique will be useful in patterning specific regions of a film for optical and mechanical stability while other portions are ambient-interactive for sensing.

Thermoluminescent microparticle thermal history sensors.

While there are innumerable devices that measure temperature, the nonvolatile measurement of thermal history is far more difficult, particularly for sensors embedded in extreme environments such as fires and explosions. In this review, an extensive analysis is given of one such technology: thermoluminescent microparticles. These are transparent dielectrics with a large distribution of trap states that can store charge carriers over very long periods of time. In their simplest form, the population of these traps is dictated by an Arrhenius expression, which is highly dependent on temperature. A particle with filled traps that is exposed to high temperatures over a short period of time will preferentially lose carriers in shallow traps. This depopulation leaves a signature on the particle luminescence, which can be used to determine the temperature and time of the thermal event. Particles are prepared-many months in advance of a test, if desired-by exposure to deep ultraviolet, X-ray, beta, or gamma radiation, which fills the traps with charge carriers. Luminescence can be extracted from one or more particles regardless of whether or not they are embedded in debris or other inert materials. Testing and analysis of the method is demonstrated using laboratory experiments with microheaters and high energy explosives in the field. It is shown that the thermoluminescent materials LiF:Mg,Ti, MgB4O7:Dy,Li, and CaSO4:Ce,Tb, among others, provide accurate measurements of temperature in the 200 to 500 °C range in a variety of high-explosive environments.

Thermoluminescence measurement technique using millisecond temperature pulses.

A measurement technique, pulsed thermoluminescence, is described which uses short thermal pulses to excite trapped carriers leading to radiative recombination. The pulses are obtained using microstructures with approximately 500 micros thermal time constants. The technique has many of the advantages of pulsed optically stimulated luminescence without the need for optical sources and filters to isolate the luminescent signal. Charge carrier traps in alpha-Al(2)O(3):C particles on microheaters were filled using 205 nm light. Temperature pulses of 10 and 50 ms were applied to the heaters and compared with a standard thermoluminescence curve taken at a ramp rate of 5 K s(-1). This produced curves of intensity verses temperature similar to standard thermoluminescence except shifted to higher temperatures. The luminescence of single particles was read multiple times with negligible loss of population. The lower limit of the duration of useful pulses appears to be limited by particle size and thermal contact between the particle and heater.

Infrared absorption signature on laser-damaged optical thin films.

Optical coating degradation under laser irradiation can take several forms. Perhaps the most common that is not due to particulates is thermal breakdown, caused by heating of the coating to a catastrophic failure induced by local melting, delamination, evaporation, or some other change. We demonstrate that micromachined dielectric membranes show strong differences in their hydroxyl signatures as measured by Fourier-transform IR spectroscopy. The changes correspond to regions of high fluence (3200 J/cm2) from a Nd:YAG laser. It is found that the absorption peaks associated with OH decrease after laser treatment, indicating a reduction in the number of film hydroxyl groups.

Control of thermal deformation in dielectric mirrors using mechanical design and atomic layer deposition.

A mechanical design technique for optical coatings that simultaneously controls thermal deformation and optical reflectivity is reported. The method requires measurement of the refractive index and thermal stress of single films prior to the design. Atomic layer deposition was used for deposition because of the high repeatability of the film constants. An Al2O3/HfO2 distributed Bragg reflector was deposited with a predicted peak reflectivity of 87.9% at 542.4 nm and predicted edge deformation of -360 nm/K on a 10 cm silicon substrate. The measured peak reflectivity was 85.7% at 541.7 nm with an edge deformation of -346 nm/K.

Stress-mapping sensors for high-power adaptive micro-optics.

Continuous deformable membrane mirrors are becoming more attractive for use in adaptive optics because they cause no diffraction in the reflected beam and ensure smooth and continuous phase variations across the mirrors. However, when such mirrors are used to correct a high-power incident wave front, the absorption in the coatings causes the temperature of the membrane to increase, thereby creating in-plane thermal stress due to the rigidly clamped boundaries. We present a technique to measure thermal stress in such nondeforming membrane structures. The directional stress and temperature effects are simultaneously measured and decoupled in micromachined membrane mirrors by using a group of three ion-implanted silicon resistors with different orientations. In stress measurements made with incident power, the sensors measure changes in compressive thermal stress to within 80-90 kPa.

Coupled absorption filters for thermal detectors.

A resonant absorption cavity that couples long-wavelength infrared (LWIR) light into a movable plate has been demonstrated for thermal detectors, especially microbolometers. Each device is continuously tunable over 8.7-11.1 microm by using electrostatic actuation with voltages from 0 to 42 V. The width of the resonance is relatively broad, approximately 1.5 microm, to match the large widths of many spectral features in the LWIR. At an actuation voltage of 45 V, the device switches into a broadband mode with an absorption width of 2.83 microm. This latter mode is used to enhance sensitivity in low-light situations in which little spectral information is present.

Current-controlled curvature of coated micromirrors.

Precise control of micromirror curvature is critical in many optical microsystems. Micromirrors with current-controlled curvature are demonstrated. The working principle is that resistive heating changes the temperature of the micromirrors and thermal expansion induces a controlled curvature whose magnitude is determined by coating design. For example, for wide focal-length tuning, the radius of curvature of a gold-coated mirror was tuned from 2.5 to 8.2 mm over a current-induced temperature range from 22 degrees to 72 degrees C. For fine focal-length tuning, the radius of curvature of a dielectric-coated (SiO2/Y2O3 lambda/4 pairs) mirror was tuned from -0.68 to -0.64 mm over a current-induced temperature range from 22 to 84 degrees C. These results should be readily extendable to mirror flattening or real-time adaptive shape control.