Sangho S. Kim

Measurement of Rapid Temperature Profiles Using Thermoluminescent Microparticles

The thermal history of a material with initially filled trap states may be probed using thermoluminescence. Since luminescent microparticles are composed of robust oxides, they are viable candidates for sensing temperature under conditions where all other types of direct-contact sensors fail. Mg2SiO4: Tb, Co particles with two thermoluminescent peaks have been heated using micromachined heaters over a 232°C to 313°C range on time scales of less than 200 ms. The effect of maximum temperature during excitation on the intensity ratio of the two luminescent peaks has been compared with first-order kinetics theory and shown to match within an average error of 4.4%.

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.

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.

Sensing of thermal history using thermoluminescent microparticles

The thermal history of a material with initially filled trap states has been probed using the thermoluminescence of microparticle sensors. Mg2SiO4∶Tb,Co particles with two thermoluminescence peaks have been heated using microheaters over a 230°C to 310°C range for durations of less than 200ms. The effect of maximum temperature during excitation on the intensity ratio of the peaks is compared with first-order kinetics theory and shown to match within an average error of 4.4%.

Thermal history sensing inside high-explosive environments using thermoluminescent microparticles

Thermoluminescent LiF:Mg,Ti (TLD-100) microparticle sensors are demonstrated to record the thermal history of the region near a detonated high explosive. Microparticles were gamma-irradiated to fill their charge-carrier traps and then exposed to the detonation of 20 g of a plastic bonded explosive formulation containing HMX and Al particles at a test distance of approximately 22 cm from the center of the detonation. The thermal history was reconstructed by measuring the thermoluminescent signature of the traps and matching it to appropriate models. The trap populations derived from luminescence measurements and modeling indicate that the particles experienced a maximum temperature of 240 °C, then cooled to 1 °C above ambient temperature within 0.4 seconds. The resulting glow curve intensity is calculated to match the observed post-detonation signal to 3% averaged over the comparison values used for reconstruction.

Depth-dependent temperature effects on thermoluminescence in multilayers

It is well known that thermal gradients penetrating deep into a material can preserve a memory of the temperature history of the surface. To date, this concept has been largely applied in the earth sciences, but there are many applications where a memory of rapid thermal events would be useful. For example, multiple layers of thermoluminescent films could serve as temperature sensors that indicate temperature versus depth in a microfabricated structure. As an advance toward this goal, this paper examines the effect of nonuniform temperature profiles on the thermoluminescence of heterogeneous multilayers. A Nd:YAG laser is used to create a known thermal event and apply pulses of heat energy of varying duration to a metalized thermoluminescent multilayer composed of LiF:Mg,Ti and CaF2:Dy. The thermoluminescence of the system is measured before and after the applied laser pulse. To model the process, a finite-difference time-domain method is used to calculate the dynamic heat transfer, and the temperature di...

Mechanical Design of Thermally Invariant Mirrors Coated by Atomic Layer Deposition

Thermal expansion mismatch between the layers of an optical coating and its substrate can cause changes in the shape of a mirror or optical element. It is shown that with knowledge of the linear stress-temperature relationship for each film, one can design coatings to have a specific thermal deformation characteristic, although the requirements for thin film repeatability are high. Such coatings are demonstrated using atomic layer deposition (ALD), and thermally invariant ALD coatings are investigated through optomechanical simulation.

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.

Early thermal damage in optical coatings identified by infrared spectral signatures

Laser induced damage can take several forms. Perhaps the most common 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 infrared 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 O-H stretching mode decrease after laser treatment, indicating a reduction in the number of film hydroxyl groups.

SiO2 Nanorod Thin Film Encapsulated by Al2O3 with Atomic Layer Deposition and its Optical Application

Low refractive index materials such as SiO2 nanorods or nanoporous SiO2 (np-SiO2) and MgF2 have been attractive for use in various types of optical coatings. Due to the high porosity of the films, however, the material properties of these films are unstable. This phenomenon is confirmed in dry versus humid ambient where both the coefficient of thermal expansion (CTE) and refractive index change dramatically. We report here that the properties of np-SiO2 can be stabilized by depositing a cap layer of Al2O3 using atomic layer deposition (ALD) on top. This stabilizing ability is demonstrated successfully for single layer and distributed Bragg reflectors (DBR). With this method, one can pattern nanoporous sensors so that some regions are stable for packaging while others are exposed to show enhanced interactions with environment for sensing.