Study of gold nanoparticles effect on thermal diffusivity of nanofluids based on various solvents by using thermal lens spectroscopy

Dual beam thermal lens technique is used to determine the thermal diffusivity of different solvents in presence of gold nanoparticles. In this technique an Ar⁺ laser (wavelength 514 nm, power 40 mW) and intensity stabilized He-Ne laser were used as the heating source and probe beam respectively. The experimental results showed that thermal diffusivity values of the studied solvents (water, ethanol and ethylene glycol (EG)) were enhanced by the presence of gold nanoparticles.     

Thermal characterization of a liquid resin for 3D printing using photothermal techniques

Thermal properties of a liquid resin were studied by thermal lens spectrometry (TLS) and open photoacoustic cell (OPC), respectively. In the case of the TLS technique, the two mismatched mode experimental configuration was used with a He–Ne laser, as a probe beam and an Argon laser was used as the excitation source. The characteristic time constant of the transient thermal lens was obtained by fitting the theoretical expression to the experimental data in order to obtain the thermal diffusivity (α) of the resin. On the other hand, the sample thermal effusivity (e) was obtained by using the OPC technique. In this technique, an Argon laser was used as the excitation source and was operated at 514 nm with an output power of 30 mW. From the obtained thermal diffusivity (α) and thermal effusivity (e) values, the thermal conductivity (k) and specific heat capacity per unit volume (ρc) of resin were calculated through the relationships k = e(α)^1/2 and ρc = e/(α)^1/2. The obtained thermal parameters were compared with the thermal parameters of the literature. To our knowledge, the thermal characterization of resin has not been reported until now. The present study has applications in laser stereo-lithography to manufacture 3D printing pieces.     

Characterization of thermal diffusivity of micro/nanoscale wires by transient photo-electro-thermal technique

In this work, a transient photon-electro-thermal (TPET) technique based on step laser heating and electrical thermal sensing is developed to characterize the thermophysical properties of one-dimensional micro/nanoscale conductive and nonconductive wires. In this method, the to-be-measured thin wire/tube is suspended over two electrodes and is irradiated with a step cw laser beam. The laser beam induces a transient temperature rise in the wire/tube, which will lead to a transient change of its electrical resistance. A dc current is applied to the sample, and the resulting transient voltage variation over the wire is measured and used to extract the thermophysical properties of the sample. A 25.4-μm thick Pt wire is used as the reference sample to verify this technique. Sound agreement is obtained between the measured thermal diffusivity and the reference value. Applying the TPET technique, one can measure the thermal diffusivity of conductive single-wall carbon nanotube (SWCNT) bundles and nonconductive cloth fibers. For nonconductive wires, a thin (∼ nm) metallic film is coated on the outside of the wire for electrical thermal sensing. The measured thermal diffusivity for the SWCNT bundle is 2.53×10^-5 m²/s, much less than the thermal diffusivity of graphite in the layer direction. For microscale cloth fibers, our experiment shows its thermal diffusivity is at the level of 10^-7 m²/s. PACS 78.20.Nv; 42.62.-b; 65.80+n; 66.30.Xj     

Enhancement of the thermal transport in a culture medium with Au nanoparticles

In this work, it is reported the gold nanoparticles synthesis, their characterization, and their application to the enhancement of the thermal transport in a cellular culture medium. The Au nanoparticles (NPs), with average size of 10 nm, contained into a culture medium (DMEM (1)/F12(1)) (CM) increased considerably the heat transfer in the medium. Thermal lens spectrometry (TLS) was used to measure the thermal diffusivity of the nanofluids. The characteristic time constant of the transient thermal lens was obtained by fitting the theoretical expression, for transient thermal lens, to the experimental data. Our results show that the thermal diffusivity of the culture medium is highly sensitive to the Au nanoparticle concentration and size. The ability to modify the thermal properties to nanometer scale becomes very important in medical applications as in the case of cancer treatment by using photodynamic therapy (PDT). A complementary study with UV-vis and TEM techniques was performed to characterize the Au nanoparticles.     

Experimental determination of the thermal lens parameters in a broad area semiconductor laser amplifier

Due to the build-up of temperature gradients along their width, semiconductor laser diodes tend to be affected by thermal lensing effects. We propose a simple and easy-to-implement experiment in order to determine the thermal lens coefficient in a broad area semiconductor laser amplifier during operation. The results obtained are compared to simulations of the temperature distribution in the laser structure. In order to further validate our method, we compare the measured M ² value of a free running broad area laser diode with the calculated M ² of such a laser under the influence of a thermal lens as predicted by diffraction theory of first-order optical resonators. The experimental results are seen to be in good agreement with the calculations.     

Thermal and optical properties of femtosecond-laser-structured PMMA

Thermal diffusivity of laser micro- and nano-structured regions in polymethylmethacrylate (PMMA) was measured by the temperature wave method with a lateral resolution reduced to ～10 μm using an array of micro-sensors. The volume fraction of laser modified phase was maximized by implementing tightly focused femtosecond laser pulses inside PMMA and maintaining distance of few micrometers between the irradiation spots. The absolute value of thermal diffusivity of PMMA 1.066±0.08×10^?7 m²/s was reliably determined with the miniaturized sensors. Regions laser structured by single pulses had no trace of carbonization, almost the same thermal diffusivity as the host PMMA, and a stress-induced birefringence Δn～10^?4 modulated with period ～2 μm.     

Metal thin film ablation with femtosecond pulsed laser

Micromachining thin metal films coated on glass are widely used to repair semiconductor masks and to fabricate optoelectrical and MEMS devices. The interaction of lasers and materials must be understood in order to achieve efficient micromachining. This work investigates the morphology of thin metal films after machining with femtosecond laser ablation using about 1 μm diameter laser beam. The effect of the film thickness on the results is analyzed by comparing experimental images with data obtained using a two-temperature heat transfer model. The experiment was conducted using a high numerical aperture objective lens and a temporal pulse width of 220 fs on 200- and 500-nm-thick chromium films. The resulting surface morphology after machining was due to the thermal incubation effect, low thermal diffusivity of the glass substrate, and thermodynamic flow of the metal induced by volumetric evaporation. A Fraunhofer diffraction pattern was found in the 500-nm-thick film, and a ripple parallel to the direction of the laser light was observed after a few multiple laser shots. These results are useful for applications requiring micro- or nano-sized machining.     

Influence of ordering change on the optical and thermal properties of inflation polyethylene films

Changes of thermal diffusivity inside femtosecond laser-structured volumes as small as few percent were reliably determined (with standard deviation less than 1%) with miniaturized sensors. An increase of thermal diffusivity of a crystalline high-density polyethylene (HDPE) inflation films by 10-20% from the measured (1.16 ± 0.01) × 10⁻⁷ m² s⁻¹ value in regions not structured by femtosecond laser pulses is considerably larger than that of non-crystalline polymers, 0-3%. The origin of the change of thermal diffusivity are interplay between the laser induced disordering, voids’ formation, compaction, and changes in molecular orientation. It is shown that laser structuring can be used to modify thermal and optical properties. The birefringence and infrared spectroscopy with thermal imaging of CH₂ vibrations are confirming inter-relation between structural, optical, and thermal properties of the laser-structured crystalline HDPE inflation films. Birefringence modulation as high as Δn ∼ ± 1 × 10⁻³ is achieved with grating structures.     

Thermal lens spectrometry in pyroelectric lithium niobate crystals

In this work, the light-induced lens effect due to thermal and/or photorefractive processes was studied in pyroelectric (undoped and Fe²⁺-doped) lithium niobate crystals (LiNbO₃) using thermal lens spectrometry with a two-beam (pump–probe) mode-mismatched configuration. The measurements were carried out at two pump beam wavelengths (514.5 and 750 nm) to establish a full understanding of the present effects in this material (thermal and/or photorefractive). We present an easy-to-implement method to determine quantitative values of the pyroelectric coefficient (dP _s/dT), its contribution to the thermal effect and other thermo-optical parameters like thermal diffusivity (D), thermal conductivity (K) and temperature coefficient of the optical path length change (ds/dT). These measurements were performed in LiNbO₃ and LiNbO₃:Fe (0.1 ppm Fe²⁺) crystals with c axis along the direction of laser propagation.     

Laser reflectance relaxation spectroscopy: Determination of thermal diffusivity in thin films of gold

The technique of thermal modulation is applied to the study of transient thermal properties of thin films of Au. Relaxation spectra as seen in the reflected laser light give a value for the thermal diffusivity of Au as κ ? 1.2 cm²/sec, comparable to previous data for the bulk metal. Versatility and potentials of the laser reflectance relaxation spectroscopy are discussed.     

微变形镜补偿激光器腔内热畸变的研究

大功率激光器中由增益介质热畸变产生热透镜效应,导致激光器的性能下降.采用基于MEMS技术加工的微变形反射镜作为激光器谐振腔的一部分,能实时补偿热透镜效应,提高激光器的输出功率.推导了热畸变与微变形镜镜面形变量、反射镜驱动单元上电压的关系并进行了实验.实验结果表明,在初始功率为287 mW、383 mW、482 mW和800 mW的情况下使用微变形镜进行补偿,可以将功率提高到437 mW、710 mW、894 mW和993 mW,在理论和实验上证明了使用微变形镜可以改善激光器腔内热畸变.     

低功率激光热透镜效应的模拟及研究

与高功率激光相比,低功率激光更安全,其热透镜效应对激光功率变化更敏感,更适合用于实验教学.本文利用有限元法模拟低功率激光的热透镜效应,模拟介质温度场和等效热透镜焦距,定量探究低功率激光热透镜焦距与激光功率以及介质热吸收率、厚度的关系.模拟结果与实验数据符合良好,为探究低功率激光热透镜效应实验提供理论依据.     

Output performance of thermally-insensitive passively Q-switched Nd:YAG laser

We study a Cr⁴⁺:YAG Q-switched Nd:YAG laser with a thermally-insensitive corner-cube-prism cavity where the corner cube prism is the key element. The corner cube prism is insensitive to misalignment in any direction. We demonstrate experimentally that a laser cavity with such a prism provides stable laser performance under violent changes of ambient temperature and the effect of the laser crystal thermal lens. The laser mode properties are analyzed by numerical simulations. We show that the numerically simulated results agree well with the experimental ones.     

Model of pulsed Nd:GSAG laser at 942?nm considering rate equations with cavity structure

The laser resonator structure and beam diameter change with the pump power due to the thermal lens effect. This influences the output power. With increasing pump power, the output power sharply decreases when the cavity becomes unstable. In order to describe this situation, rate equations considering the cavity structure and the thermal lens effect are presented; numerical methods are used to solve them. This modeling can be used in repetitively pulsed laser systems when the thermal lens depends on the average pump power, while the peak value of the output power and pump power are considered by the rate equations. Experiments are performed with 50 Hz LD diode pumped Nd:GSAG laser crystal operating at the quasi-three-level laser transition with 942 nm wavelength. Fitting the calculated results to experimental data allows to determine the cavity loss. PACS  42.55.Xi; 42.60.Da     

Thermal conductivity and thermal diffusivity of SiO<Subscript>2</Subscript> nanopowder

A 3ω approach for the simultaneous determination of the effective thermal conductivity and thermal diffusivity of nanopowder materials was developed. A 3ω experimental system was established, and the thermal properties of water and alcohol were measured to validate and estimate the accuracy of the current experimental system. The effective thermal conductivity and thermal diffusivity of the SiO₂ nanopowder with 375, 475, and 575 nm diameters were measured at 290–490 K and at different densities. At room temperature, the effective thermal conductivity and thermal diffusivity of the SiO₂ nanopowder increased with temperature; however, both values decreased as the particle diameter was reduced. An optimum SiO₂ powder density that decreased with decreasing diameter was also observed within the measurement range. The minimum effective thermal conductivity and maximum effective thermal diffusivity were obtained at 85 × 10⁻³ kg/L, when the particle diameter was 575 nm. The optimum densities of the particles with 375 and 475 nm diameters were less than 50.23 × 10⁻³ and 64.82 × 10⁻³ kg/L, respectively.     

Thermal Diffusivity of Film/Substrate Structures Characterized by Transient Thermal Grating Method

Transient thermal grating method is used to measure the thermal diffusivity of absorbing films deposited on transparent substrates. According to periodically modulated dielectric constant variations and thermoelastic deformations of the thin films caused by the transient thermal gratings, an improved optical diffraction theory is presented. In the experiment, the probing laser beam reflectively diffracted by the thermal grating is measured by a photomultiplier at different grating fringe spaces. The thermal diffusivity of the film can be evaluated by fitting the theoretical calculations of diffraction signals to the experimental measured data. The validity of the method is tested by measuring the thermal diffusivities of absorbing ZnO films deposited on glass substrates.     

反射式热透镜时变信号测量固体热扩散率的研究

介绍了采用反射式热透镜时变信号测量固体热扩散率的一种简便方法,该方法结合表面热透镜技术原理,采用表面透镜的动态变化和反射的探测光发散的幅度来确定固体的热扩散率,相对于传统的热扩散率测量方法要快捷简单。对较大范围的一系列物质的热学特性进行了实验研究,证明了其实用性。     

Transient thermal lensing measurement in a laser diode pumped NdxY1−xAl3(BO3)4 laser using a holographic shearing interferometer

A new technique for measuring a thermal lens, using a holographic shearing interferometer is presented. This technique was used to measure transient thermal lensing in a laser diode pumped NYAB laser. The measured thermal lensing power was proportional to the pumping laser diode intensity, with a gradient of 1.1 × 10⁻¹ m⁻¹ mm²/W. The transient response time of the thermal lens was 1.5 s, this value being consistent with the temporal decline of the second harmonic power.     

Study of thermal diffusivity of nanofluids with bimetallic nanoparticles with Au(core)/Ag(shell) structure

The thermal diffusivity of Au/Ag nanoparticles with core/shell structure, at different compositions (Au/Ag = 3/1, 1/1, 1/3, 1/6), was measured by using the mismatched mode of the dual-beam thermal lens (TL) technique. This study determines the effect of the bimetallic composition on the thermal diffusivity of the nanofluids. In these results we find a lineal increment of the nanofluid it thermal diffusivity when the Ag shell thickness is increased. Our results show that the nanoparticle structure is an important parameter to improve the heat transport in composites and nanofluids. These results could have importance for applications in therapies and photothermal deliberation of drugs. Complementary measurements with UV-vis spectroscopy and TEM, were used to characterize the Au(core)/Ag(shell) nanoparticles.     

High average power and short pulse duration continuous wave mode-locked Nd:GdVO<Subscript>4</Subscript> laser with a semiconductor absorber mirror

Passive mode locking of a solid-state Nd:GdVO₄ laser is demonstrated. The laser is mode locked by use of a semiconductor absorber mirror (SAM). A low Nd³⁺ doped Nd:GdVO₄ crystal is used to mitigate the thermal lens effect of the laser crystal at a high pump power. The maximum average output power is up to 6.5 W, and the pulse duration is as short as 6.2 ps. The optic-to-optic conversion efficiency is 32.5% and the repetition rate is about 110 MHz.