Surface Thermal Lensing Research Articles

Study of UV repetition laser-induced absorption on fused silica surface using a surface thermal lensing technique.

A surface thermal lensing technique is used to study the time evolution of the absorption of a fused silica surface when exposed to UV pulsed laser irradiation in the nanosecond regime. The time evolution of the absorption is characterized as a saturated exponential increase, and the disruptive change indicates that the macrodamage occurs. A time-dependent absorption defect model is used to fit the experimental data, and a single-photon process is proposed to explain the results. This method can be used to estimate the operation lifetime of optical components.

Multi-channel averaging detection for fast imaging of weakly absorbing defects in surface thermal lensing.

Surface thermal lensing (STL) is a high sensitive pump-probe technique for photothermal characterization of weakly absorbing defects in optical materials. Commonly, the frequency modulation is applied in the pump beam, and the STL signal that appears as small modulation on top of an intense background is then detected on the probe beam by lock-in amplification. However, the lock-in detection by filtering the signal in the frequency domain results in low efficiency in the STL, preventing investigations of the large area surfaces. Here it is shown that the multi-channel averaging, which corresponds to temporal integration, is better suited for detection of the STL signal. It is demonstrated that by converting the conventional single-point scanning STL to the line scanning STL and employing suitable width of the time bins and the number of records, the imaging speed of the multi-channel averaging detection can be improved by a factor of 3.7 relative to the lock-in detection at the same signal-to-noise ratio of 6 dB and imaging resolution of 20 μm. Therefore, high-quality photothermal images of weakly absorbing defects can be recorded with higher efficiency and lower laser irradiance, making it possible to investigate large samples.

Optical nonlinearity of CdSe-PMMA hybrid nanocomposite investigated via Z-scan technique and semi-empirical relations

CdSe-PMMA nanocomposite has been synthesized by ex-situ technique. The effect of different Ag doping concentrations on its structural and optical properties has been studied. X-ray diffraction reveals the hexagonal wurtzite structure of the polymer nanocomposites with preferential growth of the nanocrystals along (100) direction. Transmission electron micrograph shows the spherical CdSe nanoparticles embedded in polymer matrix. The nonlinear refractive index of the nanocomposites has been calculated using Tichy & Ticha semi-empirical relations and Z-scan technique. Z-scan results disclose the two photon absorption process in the hybrid nanocomposites with self focussing behaviour. With Ag doping, the nonlinearity is found to be increased up to 0.2% Ag doping concentration due to the confined effect of Surface Plasmon, Quantum confinement and thermal lensing. Above 0.2% Ag concentration, its value decreases due to the declined linear refractive index of the nanocomposites. Maximum two photon figure of merit is 76 for 0.2% Ag doped CdSe-PMMA hybrid nanocomposite. The present results accentuate the possibility of tuning the optical non-linearity of CdSe-PMMA hybrid nanocomposite by adjusting the doping concentration.

Stroboscopic Surface Thermal Lensing for Fast Detection of Thermal Defects in Large-Scaled Coating Films.

A stroboscopic surface thermal lensing (SSTL) system for the fast detection of thermal-induced defects in large-scaled optical coating films was constructed. The SSTL signal was generated by a set of double-modulators and captured by a high speed matrix camera, respectively. The spot size of both pump laser and probe laser expanded for larger detection area was finished in a single step. Based on the STL technique, both the mapping of amplitude and the phase of SSTL signal on the whole area of the coatings can be achieved simultaneously.

Thermal conduction in thin films measured by optical surface thermal lensing

Thermal conduction across thin films is measured optically by using the surface thermal lensing effect. Pump-probe laser measurements combined with numerical modeling are used to study thermal conduction in a variety of materials as thin as 10 nm. The method is relatively simple, robust, rapid, and offers an alternative to current techniques. Thermal conductivity in gold films is found to drop from 300 to 100 W/Km when the film thickness is reduced from 2000 to 100 nm. Results for silver, tin and aluminum films are also presented and compared with results from other studies.

Enhanced surface thermal lensing for absorption evaluation and defect identification of optical films

We used surface thermal lensing (STL) to measure the weak absorption of optical films. Different trends of STL phase signals were observed in films with two types of defect, i.e., absorptive defects and heat-resistant defects. Theoretical analysis was made, and it is in good agreement with the experimental results. Therefore, an enhanced STL technique is proposed to deduce both the magnitude of absorption and the type of defect in optical films, by considering signal intensity with its phase status.

Effects of deposition parameters on laser-induced damage threshold of Ta 2O 5 films

Ta 2O 5 films were deposited on BK7 substrates by e-beam evaporation with different deposition parameters such as substrate temperature (323–623 K), oxygen pressure (0.5–3.0×10 −2 Pa) and deposition rate (0.2–0.5 nm/s). Absorption, scattering and chemical composition were investigated by surface thermal lensing (STL) technique, total integrated scattering (TIS) measurement and X-ray photoelectron spectroscopy (XPS), respectively. The laser-induced damage threshold (LIDT) was assessed using pulsed Nd:YAG 1064 nm laser at a pulse length of 12 ns. The results showed that optical properties, absorption and LIDT were influenced by the deposition parameters and annealing. However, scattering was little correlated with the deposition parameters. On the whole, the LIDT increased with increasing substrate temperature and oxygen pressure, whereas it increased firstly and then decreased upon increasing deposition rate. After annealing at 673 K for 12 h, the LIDT of films improved significantly. The dependence of possible damage mechanism on deposition parameters was discussed.

Properties of HfO 2 thin films prepared by dual-ion-beam reactive sputtering

HfO 2 thin films were prepared in dual-ion-beam reactive sputtering (DIBRS) method. Spectrophotometer, surface thermal lensing (STL) technique, Rutherford backscattering (RBS), and X-ray diffraction (XRD) were employed in measuring the transmittance, absorptance, stoichiometry, and microstructure, respectively. Experimental results indicate that the peak transmittance value of the sample is about 90%. Weak absorptance measurement for 1064 nm wavelength laser by STL technique investigated that the absorption is 180 ppm for as-grown sample, which is larger than expected. Substoichiometry is the main cause for larger absorptance, which could be proved by RBS and annealing test results. XRD result shows that the films are polycrystalline, and the monoclinic is the dominant phase.

Single laser beam measurement of thermal diffusivity

Thermal diffusion properties of interfaces are measured using self-induced surface thermal lensing with a single laser beam. The time evolution of the reflected beam reveals information on heat diffusion away from the interface. Unambiguous correlation between measured signal and thermal diffusivity is shown, theoretically and experimentally, from which calibration curves are obtained. Being simpler and less sensitive to vibrations and misalignments, the technique offers definite advantages over standard two-beam (pump-probe) methods.

Investigation of annealing effects on the laser-induced damage threshold of amorphous Ta 2O 5 films

Ta 2O 5 films were deposited using the conventional electron beam evaporation method and then annealed at temperatures in the range 373–673 K. Chemical composition, scattering and absorption were examined by X-ray photoelectron spectroscopy (XPS), total integrated scattering (TIS) measurement and the surface thermal lensing (STL) technique, respectively. The laser-induced damage threshold (LIDT) was assessed using the output from an Nd:YAG laser with a pulse length of 12 ns. The results showed that the improvement of the LIDT after annealing was due to the reduced substoichiometric and structural defects present in the film. The LIDT increased slightly below 573 K and then increased significantly with increase in annealing temperature, which could be attributed to different dominant defects. Moreover, the root mean square (RMS) roughness and scattering had little effect on the LIDT, while the absorption and the LIDT were in accord with a general relation.

Influence of oxygen post-treatment on laser-induced damage of antireflection coatings prepared by electron-beam evaporation and ion beam assisted deposition

Antireflection coatings at the center wavelength of 1053 nm were prepared on BK7 glasses by electron-beam evaporation deposition (EBD) and ion beam assisted deposition (IBAD). Parts of the two kinds of samples were post-treated with oxygen plasma at the environment temperature after deposition. Absorption at 1064 nm was characterized based on surface thermal lensing (STL) technique. The laser-induced damage threshold (LIDT) was measured by a 1064-nm Nd:YAG laser with a pulse width of 38 ps. Leica-DMRXE Microscope was applied to gain damage morphologies of samples. The results revealed that oxygen post-treatment could lower the absorption and increase the damage thresholds for both kinds of as-grown samples. However, the improving effects are not the same.

Influence of SiO 2 additional layers on the laser induced damage threshold of Ta 2O 5 films

A series of Ta 2O 5 films with different SiO 2 additional layers including overcoat, undercoat and interlayer was prepared by electron beam evaporation under the same deposition process. Absorption of samples was measured using the surface thermal lensing (STL) technique. The electric field distributions of the samples were theoretical predicted using thin film design software (TFCalc). The laser induced damage threshold (LIDT) was assessed using an Nd:YAG laser operating at 1064 nm with a pulse length of 12 ns. It was found that SiO 2 additional layers resulted in a slight increase of the absorption, whereas they exerted little influence on the microdefects. The electric field distribution among the samples was unchanged by adding an SiO 2 overcoat and undercoat, yet was changed by adding an interlayer. SiO 2 undercoat. The interlayer improved the LIDT greatly, whereas the SiO 2 overcoat had little effect on the LIDT.

Influences of CO 2 laser annealing on mechanical and laser-induced damage properties of ZrO 2 thin films

Zirconium dioxide (ZrO 2) thin films were deposited on BK7 glass substrates by the electron beam evaporation method. A continuous wave CO 2 laser was used to anneal the ZrO 2 thin films to investigate whether beneficial changes could be produced. After annealing at different laser scanning speeds by CO 2 laser, weak absorption of the coatings was measured by the surface thermal lensing (STL) technique, and then laser-induced damage threshold (LIDT) was also determined. It was found that the weak absorption decreased first, while the laser scanning speed is below some value, then increased. The LIDT of the ZrO 2 coatings decreased greatly when the laser scanning speeds were below some value. A Nomarski microscope was employed to map the damage morphology, and it was found that the damage behavior was defect-initiated both for annealed and as-deposited samples. The influences of post-deposition CO 2 laser annealing on the structural and mechanical properties of the films have also been investigated by X-ray diffraction and ZYGO interferometer. It was found that the microstructure of the ZrO 2 films did not change. The residual stress in ZrO 2 films showed a tendency from tensile to compressive after CO 2 laser annealing, and the variation quantity of the residual stress increased with decreasing laser scanning speed. The residual stress may be mitigated to some extent at proper treatment parameters.

Influences of oxygen partial pressure on structure and related properties of ZrO 2 thin films prepared by electron beam evaporation deposition

ZrO 2 thin films were prepared by electron beam evaporation at different oxygen partial pressures. The influences of oxygen partial pressure on structure and related properties of ZrO 2 thin films were studied. Transmittance, thermal absorption, structure and residual stress of ZrO 2 thin films were measured by spectrophotometer, surface thermal lensing technique (STL), X-ray diffraction and optical interferometer, respectively. The results showed that the structure and related properties varied progressively with the increase of oxygen partial pressure. The refractive indices and the packing densities of the thin films decreased when the oxygen partial pressure increased. The tetragonal phase fraction in the thin films decreased gradually as oxygen partial pressure increased. The residual stress of film deposited at base pressure was high compressive stress, the value decreased with the increase of oxygen partial pressure, and the residual stress became tensile with the further increase of oxygen pressure, which was corresponding to the evolution of packing densities and variation of interplanar distances.

Experimental research of laser-induced damage mechanism of the sol-gel SiO2 and IBSD SiO2 thin films

This paper investigates the high-power laser-induced damage of two types of single-layer SiO2 thin films on K9 substrate, which are respectively deposited by IBSD technique and sol-gel technique, and have the same substrate parameters and the same film thickness. They were tested by surface thermal lensing technique to obtain the thermal absorbance and the dynamic response. The results show that the laser-initiated damage threshold of Sol-Gel SiO2 thin film is far higher than that of IBSD SiO2 thin film. And combined with threshold measurement and the microscopic observation, this paper well explains in detail the threshold difference between Sol-Gel SiO2 and IBSD SiO2 thin films.

Laser-induced damage of Ta2O5/SiO2 narrow-band interference filters under different 1064 nm Nd:YAG laser modes

The laser-induced damage (LID) behavior of narrow-band interference filters was investigated with a Nd:YAG laser at 1064nm under single-pulse mode and free-running mode. The absorption measurement of such coatings was performed with surface thermal lensing (STL) technique. The damage morphologies under the two different laser modes were also studied in detail. It was found that all the filters exhibited a pass-band-center-dependent absorption and laser-induced damage threshold (LIDT) behavior, but the damage morphologies were diverse. The explanation was given with the analysis of the electric field distribution and the operational behavior of the irradiation laser.

Laser-induced damage threshold of ZrO2 thin films prepared at different oxygen partial pressures by electron-beam evaporation

Zr O 2 films were deposited by electron-beam evaporation with the oxygen partial pressure varying from 3×10−3Pa to 11×10−3Pa. The phase structure of the samples was characterized by x-ray diffraction (XRD). The thermal absorption of the films was measured by the surface thermal lensing technique. A spectrophotometer was employed to measure the refractive indices of the samples. The laser-induced damage threshold (LIDT) was assessed using a 1064 nm Nd: yttritium–aluminum–garnet pulsed laser at pulse width of 12 ns. The influence of oxygen partial pressure on the microstructure and LIDT of ZrO2 films was investigated. XRD data revealed that the films changed from polycrystalline to amorphous as the oxygen partial pressure increased. The variation of refractive index at 550 nm wavelength indicated that the packing density of the films decreased gradually with increasing oxygen partial pressure. The absorptance of the samples decreased monotonically from 125.2 to 84.5 ppm with increasing oxygen partial pressure. The damage threshold values increased from 18.5 to 26.7J∕cm2 for oxygen partial pressures varying from 3×10−3Pa to 9×10−3Pa, but decreased to 17.3J∕cm2 in the case of 11×10−3Pa.

Microstructure and laser-induced damage threshold of ZrO 2 coatings dependence on annealing temperature

ZrO 2 coatings were deposited on different substrates of Yb:YAG and fused silica by electron beam evaporation. After annealed for 12 h at 673 and 1073 K, respectively, weak absorption of coatings was measured by surface thermal lensing (STL) technique, and then laser-induced damage threshold (LIDT) was determined also. The crystalline phase of ZrO 2 coatings and the size of the crystal grain were investigated by X-ray diffraction. It was found that microstructure of ZrO 2 coatings was dependent on both annealing temperature and substrate structure, and coatings containing monoclinic phases had higher damage threshold than others. Due to the strong absorption of Yb:YAG, damage threshold of coatings on Yb:YAG was much less than that on fused silica.

Laser conditioning of ZrO 2:Y 2O 3/SiO 2 mirror coatings prepared by E-beam evaporation

Laser conditioning effects of the dielectric mirror coatings with different designs were investigated. Simple quarter-wave ZrO 2:Y 2O 3/SiO 2 mirrors and half-wave SiO 2 over-coated ZrO 2:Y 2O 3/SiO 2 mirror coatings were fabricated by E-beam evaporation (EBE). The absorbance of the samples before and after laser conditioning was measured by surface thermal lensing (STL) technology and the defects density was detected under Nomarski microscope. The enhancement of the laser damage resistance was found after laser conditioning. The dependence of the laser conditioning on the coating design was also observed and the over-coated sample obtained greatest enhancement, whereas the absorbance of the samples did not change obviously. During the sub-threshold fluence raster scanning, the minor damage about defects size was found and the assumption of pre-damage mechanism, based on the functional damage concept, was put forward. The improvement of the laser induced damage threshold (LIDT) was attributed to the benign damage of the defects and the dependence on the coating design owed to the damage growth behavior of different coating designs.

Annealing effects on structure and laser-induced damage threshold of Ta 2O 5/SiO 2 dielectric mirrors

The effects of annealing on structure and laser-induced damage threshold (LIDT) of Ta 2O 5/SiO 2 dielectric mirrors were investigated. Ta 2O 5/SiO 2 multilayer was prepared by ion beam sputtering (IBS), then annealed in air under the temperature from 100 to 400 °C. Microstructure of the samples was characterized by X-ray diffraction (XRD). Absorption of the multilayer was measured by surface thermal lensing (STL) technique. The laser-induced damage threshold was assessed using 1064 nm free pulsed laser at a pulse length of 220 μs. It was found that the center wavelength shifted to long wavelength gradually as the annealing temperature increased, and kept its non-crystalline structure even after annealing. The absorbance of the reflectors decreased after annealing. A remarkable increase of the laser-induced damage threshold was found when the annealing temperature was above 250 °C.