Thermal Mirror Research Articles

Time-resolved thermal mirror technique with top-hat cw laser excitation

A theoretical model was developed for time-resolved thermal mirror spectroscopy under top-hat cw laser excitation that induced a nanoscale surface displacement of a low absorption sample. An additional phase shift to the electrical field of a TEM(00) probe beam reflected from the surface displacement was derived, and Fresnel diffraction theory was used to calculate the propagation of the probe beam. With the theory, optical and thermal properties of three glasses were measured, and found to be consistent with literature values. With a top-hat excitation, an experimental apparatus was developed for either a single thermal mirror or a single thermal lens measurement. Furthermore, the apparatus was used for concurrent measurements of thermal mirror and thermal lens. More physical properties could be measured using the concurrent measurements.

Nanoscale surface displacement detection in high absorbing solids by time-resolved thermal mirror

Nanoscale surface displacement is used to determine thermo-optical–mechanical properties of high absorbing solids by means of the time-resolved thermal mirror method. The thermoelastic equation for the surface displacement and an expression for the probe beam intensity at the detector plane were derived. Experiments were performed in a high absorbing TiO2-doped low silica calcium aluminosilicate glass, and obtained the valuable values of the fluorescence quantum efficiency and thermal properties. The results indicate that this method is reliable for the characterization of semitransparent, high absorbing, and opaque materials.

Theoretical research on InnoSlab output beam quality with hybrid resonator

Output beam quality of InnoSlab laser with hybrid resonator is analyzed in this paper for the thermal lens effect and mirror misalignment. Many aspects, such as far-field light intensity distribution, far-field divergence angle, beam waist radius, beam quality factor M2 and PIB curve, etc, are discussed. Especially, in the off-axis unstable resonator direction, the influence of beam quality is discussed with respect to mirror misalignment by M2 and PIB curve. This gives valuable reference to unstable resonator beam quality evaluation, for which a unified standard has not been set up as yet.

Time-resolved thermal mirror for nanoscale surface displacement detection in low absorbing solids

A time-resolved thermal mirror method for measurements of absolute thermo-optical-mechanical properties of low absorbing solids is presented. The thermoelastic equation for the surface displacement and an analytical expression for the probe beam intensity at the detector plane were derived. Experimental proofs were performed in an optical glass and the fitted parameters are in good agreement with previous literature data for thermal, optical, and mechanical properties, suggesting that the method is a useful tool for the characterization of a wide range of transparent materials.

Stability Effects of Reflective Mirror on GaN LED/Mirror/Si Via Wafer Bonding and Laser Lift-Off Techniques

GaN vertical-conducting light-emitting diodes (LEDs) with high thermal stability mirror on Si substrate have been fabricated using a combination of wafer-bonding and laser lift-off techniques. The thermal stabilities of NiO/Ag and NiO/Ag/Ni mirrors in vertical GaN/mirror/Si structure were studied. It was found that the NiO/Ag/Ni mirror presents the best performance, where the specific contact resistance and the reflectivity can achieve 8.37 x 10-3 Ω-cm2 and 92 % at 470 nm after oxidation annealing (500oC for 10 min). The top Ni layer could protect the Ag mirror from degradation of the high temperature annealing. The output powers of GaN/sapphire LED and LEDs with NiO/Au/Ag and NiO/Ag/Ni mirrors show 4.5, 4.3 and 13 mW, respectively.

“Thermal Mirror” Method for Measuring Physical Properties of Multilayered Coatings

In this study theoretical principles underlying the photothermal displacement (“thermal mirror”) method for measuring physical properties of opaque multilayered and functionally graded coatings with low thermal conductivity are analyzed. In this method, the specimen is locally heated by a power laser beam, and a two-dimensional transient temperature field is formed in a specimen. The physical basis for the photothermal displacement method is the non-stationary buckling and displacement of an irradiated surface due to a non-uniform thermal expansion. The surface is monitored by a low-power probe beam of a second laser, which is reflected from the specimen, i.e., the system operates as a convex “thermal mirror.” The photoinduced displacement varies with time, and the probe beam is reflected at a different angle depending on the slope of the displacement. The deflection angle is measured as a function of time by a position sensor, and the results of these measurements are compared with the theoretical dependence of the deflection angle on time and physical properties of a coating. This dependence was determined analytically from the solution of the two-dimensional thermal elasticity problem. It is shown that for the specimen composed of a substrate and a coating it is feasible to determine the properties of the coating, e.g., the thermal diffusivity and coefficient of linear thermal expansion provided that the analogous properties of the substrate are previously measured or otherwise known.

The effect of cooling rate on the solidification of INCONEL 718

The superalloy INCONEL 718 (IN718) is a commonly used material in aerospace and turbine components. The advantage of this type of material with sluggish precipitation-hardening kinetics is that IN718 is readily weldable. Both wrought and cast parts are used and welded together. While the alloy has been studied previously, new production processes such as laser treatment demand better knowledge of the solidification process in IN718, especially at high cooling rates. In this investigation, the solidification process was studied over a wide range of cooling rates by three different experimental techniques: differential thermal analysis (DTA), mirror furnace (MF), and levitation casting. The solidification sequence and the reaction temperatures were identified. The microstructure and the change in growth morphology were also studied. Segregation measurements were performed, and the distribution of Nb was analyzed in detail for the different types of samples, because of its strong impact on the solidification sequence and microstructure. New observations are that the latent heat decreases and the effective partition coefficient increases with increasing cooling rate. The diffusion rate also seems to be enhanced in the first part of primary solidified dendrites. It is suggested that the new observations can be explained by an increased number of lattice defects formed in the solid as the cooling rate increases.

The thermal focusing mirror of the ESRF Troika beam line:: principle, design and first results

We describe a focussing mirror system that is bent by differential thermal expansion due to a temperature gradient perpendicular to its surface. Temperature control is achieved through a side cooling arrangement on the top, and a heating device at the bottom of the mirror body. We present measurements of the optical properties obtained by speckle interferometry on a test system, and under actual operating conditions in the white undulator beam.

An idea of half-space thermal mirror

The paper describes a theoretical discussion of the optical reflecting properties of an isotropic thermoelastic half-space subjected to an instantaneous heat pulse at a point of its surface (such a system is called the half-space thermal mirror). The fundamental optical properties of the mirror so obtained (i.e.—its aberration characteristic, including optical power and focal length), and their time evolution are derived in the paper. The half-space thermal mirror represents an optical device suffering rather high aberrations; the paraxial optics approximation, and criteria of its applicability for such a mirror are given. Possibilities of an application of the thermal mirror for determining the temperature conductivity of a material are considered.

Open Confinement Systems and the D-3He Reaction

In this paper, the special synergisms between open-ended magnetic confinement systems and the D-{sup 3}He fuel cycle are discussed, both in general terms and through examples. Properties of open systems that make them especially suited for this fuel cycle include their ability to stably confine high-beta plasmas, their compatibility with electrostatic direct converters, and their linear geometry, which can greatly simplify the practical attainment of high magnetic fields. The example systems given include a linear collider and thermal barrier tandem mirror systems for both terrestrial and space travel applications. It is concluded that satisfying the demanding physics requirements posed by the D-{sup 3}He fuel cycle may be more readily possible through the use of open-ended magnetic systems than it will be through the use of closed systems of the tokamak genre.

Thermal effects and mirror surface figure requirements for a diagnostic beamline at the advanced light source

Abstract An imaging beamline based on a Kirkpatrick-Baez mirror configuration has been designed to image the electron beam in the ALS storage ring, to measure its size and shape. The electron beam emittance will be small ( ϵ h = 3.4×10 −9 m rad) and the quality of the image is extremely sensitive to surface figure distortion of the mirrors. Thermal distortions and surface temperatures have been calculated for radiatively cooled mirrors of various materials in a search for a simple design which avoids water cooling. The choice of mirror material and the thermal and mechanical design is discussed.

Dynamics of reflection of signal pulses from a thermal self-pumped four-wave mirror

Dynamics of reflection from a thermal self-pumped four-wave mirror with a YAG:Nd amplifier in a feedback loop is modeled mathematically. It is shown that an allowance for the transient nature of the process, for magnification by telescopes, for different transmissions of waves with orthogonal polarizations by a dielectric polarizer and a nonreciprocal element, and particularly for depletion of the population inversion in the amplifier makes it possible to describe this mirror and to optimize its operation in accordance with real experimental conditions even when the plane-wave approximation is used.

CW high power single-lobed far-field operation of long-cavity AlGaAs-GaAs single-quantum-well laser diodes grown by MOCVD

Data on long-cavity 100- mu m-wide broad-stripe laser diodes that lase with a barrow single-lobed far-field pattern in continuous room-temperature operation are presented. Diodes with a cavity length of 1250 mu m emit a power of 200 mW per facet into a 2.5 degrees lobe (full width at half maximum). Short-cavity devices (cavity length of 350 mu m) lase with a continuously increasing number of lateral modes right from threshold, and exhibit a far-field divergence that is over three times greater than that of 1250- mu m diodes. Explanations for the effect of increasing cavity length on the field patterns of these devices are proposed, based on the measured increase in injected carrier diffusion length in long-cavity diodes and the influence of thermal waveguiding and mirror losses on intermodel discrimination. >

Observation of scaling laws of ion confining potential versus thermal barrier depth and of axial particle confinement time in the tandem mirror GAMMA 10

In the thermal barrier tandem mirror GAMMA 10, the scaling law governing the enhancement of the ion confining potential, ϕc, resulting from thermal barrier formation, is obtained experimentally, and is consistently interpreted in terms of the weak and strong ECH theories set up by Cohen and co-workers. The scaling law on the axial particle confinement time, τp‖, related to this ϕc formation, is also demonstrated in detail; it is in good agreement with the Pastukhov theory as modified by Cohen and co-workers. This scaling is verified at any radial position in the core plasma region and at any time through the various stages of a discharge; this indicates a scaling with drastic improvement of τp‖, due to the potential formation in the tandem mirror plasma.

Hα laser fluorescence diagnostic on the Tara Tandem Mirror experiment

A laser fluorescence diagnostic has been used for measuring the neutral hydrogen density in the central cell of the Tara thermal barrier tandem mirror. Experiments have been performed using laser-induced, resonance fluorescence detection of Hα (6563-Å) radiation. Measurements were made at a number of radial positions with 1-cm resolution, from the magnetic axis to near the plasma limiter. Stray laser light contributions to the signal were eliminated with a double-pulse technique. For comparison, the chord-averaged plasma Hα radiation was analyzed under the identical conditions for which laser fluorescence data were taken.

Emission from a YAG:Nd laser with a four-wave thermal mirror in a ring resonator

Hard excitation of a YAG:Nd laser emitting nanosecond pulses was investigated experimentally. Such excitation was achieved in the presence of an external signal in a ring resonator with a self-pumped four-wave mirror based on a thermal nonlinearity of ethyl alcohol. The threshold energy of the external signal needed for lasing was ~ 10 mJ and the maximum energy reflection coefficient reached ~ 30%.

Plasma potential formation and measurement in TMX-U and MFTF-B (invited)

Tandem mirrors control the axial variation of the plasma potential to create electrostatic ‘‘plugs’’ that improve the axial confinement of central cell ions and, in a thermal barrier tandem mirror, control the electron axial heat flow. Measurements of the spatial and temporal variations of the plasma potential are, therefore, important to the understanding of confinement in a tandem mirror. In this paper we discuss potential formation in a thermal barrier tandem mirror and examine the diagnostics and data obtained on the TMX-U device, including measurements of the thermal barrier potential profile using a diagnostic neutral beam and charged-particle energy spectroscopy. We then describe the heavy ion-beam probe and other new plasma potential diagnostics that are under development for TMX-U and MFTF-B.

Particle exchange in the plug of a thermal barrier tandem mirror

The collisional fluxes of trapped electrons escaping from the plug, and the center cell electrons passing over the thermal barrier and becoming trapped, are evaluated for a simple thermal barrier model appropriate to present tandem mirror experiments.

Continuous high-power uv laser

An Ar low-pressure high-current discharge (length=1.7 m, diameter=12 mm) was used for continuous high-power operation of uv laser lines (λ=351.1 nm, 363.8 nm). The measured maximum output power of 18 W for both lines combined was restricted by thermal mirror degradation. Based on gain measurements, a maximum power of 35 W is predicted. It appears that this value is limited by discharge instabilities.

Interferometric measurement of the elongation of a pulsed diode laser

An interferometric technique was developed for detecting fast displacements of small areas in beam direction by less than \lambda/8 . It utilizes a modified Michelson interferometer and a He-Ne laser light source. The technique was applied to measure the temporal and spatial evolution of the thermal mirror deformation of a GaAs-Ga 1-x Al x As single heterostructure laser diode. A small spot on the facet of the laser diode formed one mirror of the interferometer. The other mirror was moved linearly with time by applying a ramp voltage to a piezoelectric transducer. The diode was pulsed during this motion. The local elongation of the laser diode was evaluated from the interferometer output signal. The measured elongation of the active region at a time 1 μs after a 150-ns pulse with current density 48 000 A/cm2was between 4 and 20 nm, indicating a temperature rise of 2-11\deg C. The smallest elongations detected were in the range of 0.5-1 nm. The measured values are in substantial agreement with theoretical expectations from thermal model calculations.