Laser Calorimetry Research Articles

Laser calorimetric absorptance testing of samples with varying geometry

Laser calorimetry is based on the measurement and evaluation of the temperature increase caused by absorption in the sample exposed to laser radiation. A temperature distribution develops in the irradiated sample as a result of dependence on the thermal diffusivity of the sample. Therefore, finding a correlation between the temperature increase and absorption becomes a complex task. This challenge was met by keeping the sample geometry at a standard size and simulating the thermal distribution for a number of optical materials. Using this method, Laser Zentrum Hannover e.V. (LZH) developed a calorimetric test setup that offers fully calibrated absorptance values for sample diameters of 25 mm (or 1 in.) with a total error of below 13% and a relative measurement error of below 5%. However, this technique is limited to the above-mentioned sample geometry. This work presents an approach to adjust the measurement configuration to numerous sample sizes for standard circular laser components. Finite element analysis and experimental verification are presented for exemplary values of the samples’ diameters. Based on the different sample mount concept, this procedure allows utilizing flexibility in test wavelength and angle of incidence, combined with the sensitivity level sufficient for current optical materials.

Relation of optical properties and femtosecond laser damage resistance for Al2O3/AlF3 and Al2O3/SiO2 composite coatings

We report on the realization of aluminum oxyfluoride thin films and alumina/silica mixture coatings with different ratios by ion beam sputtering. The atomic compositions quantified by energy dispersive x-ray spectroscopy are correlated with the optical properties calculated from spectrophotometry and laser calorimetry measurements. Furthermore, the femtosecond laser damage resistance (τ=400 fs) of single layers is investigated in the infrared at 1030nm and in the ultraviolet at 343nm wavelengths. Experimental results on the wavelength scaling of the laser-induced damage threshold for oxyfluoride and oxide composite coatings are presented.

Attenuation, absorption, and scattering in silver halide crystals and fibers in the mid-infrared

The attenuation and the absorption of silver halide crystal and fibers in the mid-IR were measured. The absorption coefficients of silver halide crystals, deformed crystals, and fibers were measured by laser calorimetry at several different wavelengths in the near-IR and the mid-IR. The absorption coefficients of the fibers were higher than those of the deformed crystals and these were higher than those of the un-deformed crystals. We suggest that the absorption behavior is best described by a weak absorption tail. The total attenuation in fibers was measured using the cut back method. The scattering values were calculated by subtracting the absorption coefficients from the total attenuation coefficients. The scattering in the silver halide fibers was ascribed to pores of estimated size of 0.7 μm. We showed that the main contribution to the attenuation of fibers in the mid-IR range was due to scattering and suggested that reducing the scattering will dramatically improve the transmission of silver halide fibers in the mid-IR.

激光量热法测量深紫外氟化物薄膜吸收

To obtain the relationship between the absorptances of thin-film materials and the incoming ultraviolet laser energy density, it is better to establish a standard measurement method. The principle and measurement process of laser calorimetry is introduced, the dose depending effect, nonlinear absorption, and unrecoverable absorption phenomenon that might influence the measurement results are analyzed, a method based on the laser calorimetry is proposed, which is used to measure the absorptance of fluoride coatings that can be used in 193 nm ultraviolet photolithography systems, and experiments. According to the new proposed method, the dose depending effect, the corresponding laser energy density while the unrecoverable absorption phenomenon appears when the fused silica substrates are irradiated by 193 nm laser are measured; the relationship between absorptances of substrates and the laser energy density are calculated. After that, two substrates are coated with LaF3 and MgF2 separately by boat evaporation; the relationships between absorptances of the coated samples and laser energy density are measured. By the comparison of absorptances before and after coating, the relationships of the two thin-film materials between absorptances and laser energy densities are calculated, and the absorptances of LaF3 and MgF2 in actual working condition are figured out, also the relationship between deposited temperature and absorptance, roughness, waviness of LaF3 is obtained. The experimental results confirm the feasibility of the proposed method, which help to improve the imaging quality and prolong the working life of the objective system that can be used in the photolithography system.

应用于高性能光学薄膜表征的光热光声检测技术

In this paper,the basic principle of the optical-thermal and optical-acoustics detecting methods is reviewed. The common optical-thermal and optical-acoustics detecting methods are described. Combined with specific applications such as absorption test of optical films,characterization of laser irradiation and laser damage to the thin films and characterization of the mechanical properties of thin films,some typical detecting methods are analyzed including laser calorimetry technique,photothermal deflection technique,surface acoustic waves and so on. The advantages and disadvantages of these methods are indicated. Finally,some achievement in the field by these methods are summarized,and the prospect of these methods is presented.

Thermal lensing in Nm-cut monoclinic Tm:KLu(WO4)2 laser crystal

The thermal lensing effect is characterized in slab-shaped monoclinic Tm:KLu(WO4)2 crystal cut for light propagation along the Nm optical indicatrix axis (the light polarization is E ∥ Np). The optical power of the thermal lens was measured directly by a modified probe beam technique. Alternatively, it was calculated on the basis of measured material parameters, including the thermal expansion coefficients along the optical indicatrix axes (αp = 3.1, αm = 11.1 and αg = 13.1(10−6 K−1)) and the principal thermo-optic coefficients (dnp/dT =− 13.3, dnm/dT =− 5.9, dng/dT =− 9.0(10−6 K−1) at 1.95 μm). The two methods are in good agreement and confirm the defocusing action of the thermal lens; the ‘effective’ thermo-optic coefficient Δ for Nm-cut crystal is −2.1 ± 0.5 × 10−6 K−1. The fractional head load ηh for slab-shaped Tm:KLu(WO4)2 crystal is 0.33 ± 0.03 (as determined from ISO-standard laser calorimetry).

Absorption measurements on silver bromide crystals and fibers in the infrared

The absorption coefficients of single crystals of silver bromide, deformed crystals and polycrystalline fibers, extruded from single crystals, were measured by laser calorimetry. The measurements were carried out at three wavelengths in the near-infrared and middle-infrared ranges. The absorption coefficients of the fibers were higher than the coefficients of the crystals and deformed crystals, over the entire spectral range. We suggest that this absorption is due to the weak absorption tail.

Simultaneous Absorptance and Thermal-Diffusivity Determination of Optical Components with Laser Calorimetry Technique

The laser calorimetry (LCA) technique is used to determine simultaneously the absorptances and thermal diffusivities of optical components. An accurate temperature model, in which both the finite thermal conductivity and the finite sample size are taken into account, is employed to fit the experimental temperature data measured with an LCA apparatus for a precise determination of the absorptance and thermal diffusivity via a multiparameter fitting procedure. The uniqueness issue of the multiparameter fitting is discussed in detail. Experimentally, highly reflective (HR) samples prepared with electron-beam evaporation on different substrates (BK7, fused silica, and Ge) are measured with LCA. For the HR-coated sample on a fused silica substrate, the absorptance is determined to be 15.4 ppm, which is close to the value of 17.6 ppm, determined with a simplified temperature model recommended in the international standard ISO11551. The thermal diffusivity is simultaneously determined via multiparameter fitting to be approximately 6.63 × 10−7 m2 · s−1 with a corresponding square variance of 4.8 × 10−4. The fitted thermal diffusivity is in reasonably good agreement with the literature value (7.5 × 10−7 m2 · s−1). Good agreement is also obtained for samples with BK7 and Ge substrates.

Photothermal self-phase-modulation technique for absorption measurements on high-reflective coatings

We propose and demonstrate a new measurement technique for the optical absorption of high-reflection coatings. Our technique is based on photothermal self-phase modulation and exploits the deformation of cavity Airy peaks that occurs due to coating absorption of intracavity light. The mirror whose coating is under investigation needs to be the input mirror of a high-finesse cavity. Our example measurements were performed on a high-reflection SiO2-Ta2O5 coating in a three-mirror ring-cavity setup at a wavelength of 1064 nm. The optical absorption of the coating was determined to be α=(23.9±2.0)·10(-6) per coating. Our result is in excellent agreement with an independently performed laser calorimetry measurement that gave a value of α=(24.4±3.2)·10(-6) per coating. Since the self-phase modulation in our coating-absorption measurement affects mainly the propagation through the cavity input mirror, our measurement result is practically uninfluenced by the optical absorption of the other cavity mirrors.

Precision, all-optical measurement of external quantum efficiency in semiconductors

External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent temperature or power-dependent photoluminescence. Time-resolved photoluminescence lifetime and power-dependent photoluminescence measurements are used to evaluate unprocessed heterostructures for critical performance parameters. The crucial importance of parasitic background absorption is discussed.

High-purity TeO 2–WO 3–(La 2O 3, Bi 2O 3) glasses for fiber-optics

TeO 2–WO 3, TeO 2–WO 3–La 2O 3 and TeO 2–WO 3–La 2O 3–Bi 2O 3 glasses were produced by high-purity oxides mixtures melting in crucibles of gold or platinum in purified oxygen atmosphere. The total content of Cu, Mn, Fe, Co and Ni impurities was not more than 0.1–2 ppm wt in glasses and below 0.1 ppm wt in the initial oxides. The hydroxyl groups absorption coefficients of the tellurite glasses at the maximum of the absorption band ( λ ∼ 3 μm) lay in the region of 0.01–0.001 cm −1. Stability to crystallization characterized by T x − T g = 150–190 °C was demonstrated for high purity 75TeO 2–25WO 3 glasses by DSC-measurements. There were no thermal effects of crystallization and crystallized phases fusion in case of La 2O 3-containing glasses. The optical absorption losses, measured by the laser calorimetry method at λ = 1.06, 1.56 and 1.97 μm, did not exceed 40–120 dB/km for glass samples. The multimode optical fibers with optical losses of 50–250 dB/km at 1.2–2.2 μm were produced from high-purity TeO 2–WO 3–(La 2O 3, Bi 2O 3) glasses.

Production and properties of high purity TeO 2− WO 3−(La 2O 3, Bi 2O 3) and TeO 2− ZnO−Na 2O−Bi 2O 3 glasses

TeO 2–WO 3 (TW), TeO 2–WO 3–La 2O 3 (TWL), TeO 2–WO 3–La 2O 3–Bi 2O 3 (TWLB) and TeO 2–ZnO–Na 2O–Bi 2O 3 (TZNB) glasses were produced by high-purity oxide mixtures melting in platinum or gold crucible at 800 °C in the atmosphere of purified oxygen. The total content of Cu, Mn, Fe, Co and Ni impurities was not more than 0.1–0.5 ppm wt in the initial oxides and glasses. The stability of TZNB glasses to crystallization, characterized by (T x − T g) value more than 150 °C, was demonstrated by DSC measurements at heating rate 10 K/min. In the case of La 2O 3-containing glasses the thermal effects of both crystallization and fusion of the crystallized phases were not observed. The hydroxyl groups absorption coefficients of pure tellurite glasses at the maximum of the absorption band (λ ~ 3 μm) were in the region of 0.012–0.001 cm −1. The optical absorption losses, measured by the laser calorimetry method at λ = 1.56 and 1.97 μm, did not exceed 100 dB/km.

Accurate temperature model for absorptance determination of optical components with laser calorimetry

In the international standard (International Organization for Standardization 11551) for measuring the absorptance of optical components (i.e., laser calorimetry), the absorptance is obtained by fitting the temporal behavior of laser irradiation-induced temperature rise to a homogeneous temperature model in which the infinite thermal conductivity of the sample is assumed. In this paper, an accurate temperature model, in which both the finite thermal conductivity and size of the sample are taken into account, is developed to fit the experimental temperature data for a more precise determination of the absorptance. The difference and repeatability of the results fitted with the two theoretical models for the same experimental data are compared. The optimum detection position when the homogeneous model is employed in the data-fitting procedure is also analyzed with the accurate temperature model. The results show that the optimum detection location optimized for a wide thermal conductivity range of 0.2-50W/m·K moves toward the center of the sample as the sample thickness increases and deviates from the center as the radius and irradiation time increase. However, if the detection position is optimized for an individual sample with known sample size and thermal conductivity by applying the accurate temperature model, the influence of the finite thermal conductivity and sample size on the absorptance determination can be fully compensated for by fitting the temperature data recorded at the optimum detection position to the homogeneous temperature model.

Optical absorption losses in metal layers used in thin film solar cells

AbstractWe apply optical transmittance and reflectance spectroscopy, photothermal deflection spectroscopy (PDS) and laser calorimetry (LC) to evaluate optical absorption losses at rough interface between thin conductive oxide (TCO) and metal films used as backreflectors and electrical contacts in thin film solar cells. The paper proposes a simple method how to model the dielectric function of rough metal layers used in thin film solar cells. We show that the rough metal layer optically behaves as a semi‐infinite layer with modified dielectric function calculated by the Landau–Lifshitz–Looyenga (LLL) model from the dielectric function of a smooth metal, the dielectric function of TCO and just one free parameter that needs to be found by fitting the total optical absorptance. This approach can be used to simplify the modelling of the optical properties of thin film solar cells.

High optical quality nanocrystalline diamond with reduced non-diamond contamination

It is important for optical applications of nanocrystalline diamond to achieve low optical absorption as well as optical scattering. We discuss the optical and photocurrent spectra measured by the transmittance and reflectance, photothermal deflection spectroscopy, laser calorimetry and dual beam photocurrent spectroscopy of low non-diamond content nanocrystalline diamond films grown by microwave plasma enhanced chemical vapor deposition on fused silica glass substrates.

Development of Thermophysical Property Measurement for Liquid Fe Using Noncontact Laser Modulation Calorimetry

Newly developed noncontact modulation laser calorimetry has enabled us to measure accurate thermophysical properties such as thermal conductivity and heat capacity of high temperature liquid metals with suppressing chemical reactions and convections. However, in spite of the demand for accurate thermophysical properties as input parameters in precise numerical simulation for casting and welding processes, it is still difficult to measure those of liquid iron (Fe). The difficulty is caused by the high density and electrical resistivity of liquid Fe. In this study we designed a new levitation coil having high lifting force with moderate heating for liquid Fe. Furthermore, a superconducting magnet, which has a maximum dc magnetic field of 10 T, was equipped to the measurement system to suppress the convection in the liquid Fe. The isobaric molar heat capacity of liquid Fe was successfully measured at lower dc magnetic field (3 and 4 T). This can be explained by the accomplishment of the semi-adiabatic condition caused by the higher thermal conductance of the convection. The isobaric molar heat capacity of liquid Fe was determined as follows; cp=45.5±4.1 J·mol−1·K−1 [1816–1992K]. On the other hand, the apparent thermal conductivity of liquid Fe decreased with increasing dc magnetic field, and then finally converged to be 38.7±2.5 W·m−1·K−1 [1794–2050 K] above 9 T due to the suppression of convection.

Noncontact modulated laser calorimetry in a dc magnetic field for stable and supercooled liquid silicon

Thermal conductivity of liquid silicon is necessary for numerical process modeling. It is also of scientific interest. However, measuring thermal conductivity is a difficult task because of convections in the liquid and contamination from contact materials. To overcome these experimental difficulties, we have developed noncontact modulated laser calorimetry in a dc magnetic field to measure heat capacity, thermal conductivity and emissivity of high-temperature liquid metals. In this study, through improvement in temperature measurements, we considerably reduced the experimental uncertainty of measurements. Furthermore, the thermal conductivity and heat capacity of supercooled liquid silicon were measured. Thermal conductivity of liquid silicon agrees with the values calculated assuming the Wiedemann–Franz law near the melting point. This result suggests that free electrons play a dominant role in the thermal transport process in liquid silicon.

Fabrication and characterization of chalcogenide glass for hollow Bragg fibers

Low- and high-refractive-index chalcogenide glasses are studied for their potential use in the fabrication of one-dimensional hollow Bragg fibers. The low-index glasses are based on the GeSe-glass systems with indices varying from 2.0 to 2.5, while the high-index glasses are formed from the AgAsSe glasses with indices ranging from 2.8 to 3.8. High-purity elemental starting materials are distilled and the surface oxides removed prior to mixing in a rocking furnace. The refractive indices of the AgAsSe glasses, measured using a CO2 laser reflectometer, were near 3.10 for the compositions most compatible with the low-index Ge20Se80 glass (n = 2.46). Spectral measurements show impurity absorption bands between 12 and 16 microm. The loss at 10.6 microm for the Ag25As40Se35 glass measured using CO2 laser calorimetry was 1.16 x 10(-3) cm(-1).

Accurate determination of subnanoscale deformation with combined laser calorimetry and surface thermal lens technique

Laser-induced surface deformations of optical components are measured with a combined laser calorimetry (LC) and surface thermal lens (STL) technique. The deformation is calculated from the STL amplitude by employing a simple STL model. It is also directly calculated with a rigorous deformation model by measuring the absorptance of the optical component with LC. Experimentally, the laser-induced surface deformation values of a BK7 coating sample measured by STL and Hartmann wavefront sensing techniques are in excellent agreement with that calculated with the rigorous deformation model. The measurement of subnanometer deformation with the combined LC and STL technique is demonstrated by a fused silica coating sample. The deformation measurement sensitivity and error are estimated to be 10 pm and below 10%, respectively.

Optical Monitoring of Nanocrystalline Diamond with Reduced Non-diamond Contamination

AbstractPreviously, the nanocrystalline grain boundaries were often contaminated by the “non-diamond phase” with the photo-ionization threshold at 0.8 eV. Here, we present the optical spectra of the NCD films grown on transparent substrates by the microwave plasma enhanced chemical vapor deposition (CVD) at a relatively low temperature below 600°C. The transmittance and reflectance spectra are useful to evaluate the film thickness, the surface roughness and the index of refraction. The direct measurement of the optical absorptance by the laser calorimetry and photothermal deflection spectroscopy (PDS) provides high sensitive methods to measure the weak optical absorption of thin films with rough surface. The optical measurements indicate the high optical transparency of our standard, nominally undoped 0.2-0.3 μm thick NCD film with low non-diamond content. However, the optical scattering is rather high in UV and needs to be reduced.