The Diagnostic and Analysis of Optical, Thermal and Electrical Properties of Laser Ablated Materials

Abstract

The investigations are performed on thermal, optical and electrical response of UV and IR laser irradiated materials. Changes in structural, morphological, electrical and optical parameters for four transition metals, platinum (Pt), gold (Au), silver (Ag) and copper (Cu) are explored. Experiments are performed in two series. First 4N pure, annealed and fine polished samples are exposed to Nd:YAG laser (1064nm, 9-14ns, 10mJ) for different number of shots (25, 50, 75, 100) in air as well as under vacuum (10 -3 torr and 10 -6 torr). Gaussian profile laser power density at tight focus is 3 10 15 Watt/m 2 where the spot size is ~12 m. In second series of experiments, the samples are exposed to KrF Excimer laser (248nm, 20ns, 50mJ) under vacuum ~10 –6 torr at different laser fluences (0.5J/cm 2 to 2.5 J/cm 2 ).The focal spot size at the tight focus is .02 cm 2 . Irradiated target materials are then characterized for surface morphology and topography, structural, optical and electrical analysis using the diagnostics; SEM , SPM/AFM , XRD , Rotating Compensator Auto-Aligned Ellipsometer and four-point probe respectively. Motic digital microscope is employed for droplet and spot size measurements. IR and UV irradiation of metals, both cause changes in diffracted X-rays intensity and grain sizes consequently changing the dislocation line densities and strain in the target materials. In most of the IR irradiated targets, X-rays diffracted intensity is maximum for (111) and (200) planes. For plane (111) the maximum X-ray diffracted intensity found for irradiated gold (1638.79 counts) and minimum for irradiated platinum (123.77 counts). The maximum change in grain size takes place in gold (~7.43nm). In UV laser irradiated samples, the intensity is found maximum for platinum (21528 counts) for (111) plane. The maximum change in grain size takes place in platinum (~10nm), whereas gold and silver exhibit minimum variation in grain sizes for UV irradiation. Both types of irradiation produce weak stresses on the target surfaces so unable to cause any change in d-spacing. Surfaces of the target metals are modified by craters, cones, molten material, hillocks and redeposited material nearly for both types of irradiation. Splashing, exfoliation and hydrodynamic sputtering are the dominant ablation mechanisms. Non-uniform heat conduction takes place on the surfaces in the form of channels. IR irradiation especially in the presence of air produced laser induced periodic surface structures (LIPSS) mostly in ripples form. Ripple spacing is strongly dependent on the number of laser shots up to a saturation value. In UV (iv)irradiated targets the particle sizes vary from maximum value ~3 m in platinum at fluence 0.5J/cm 2 to 20nm in Cu at fluence 2.5 J/cm 2 . UV irradiation changes optical constants namely the absorption coefficient, refractive index, absorptivity, reflectivity and optical band gap energies of the target materials. For the incident light ranging from 500nm to 1000nm in irradiated Pt, the absorption coefficient changes from 9×10 7 m -1 to 8×10 7 m -1 , the refractive index from 1.2 to 1.5, the absorptivity changes from 70% to 43% and optical band gap energy from 0.85eV to 0.75eV. For UV irradiated gold exposed to light ranging from 500nm to 1000nm, absorption coefficient changes from 1.6 10 7 m -1 to 2.2 10 7 m -1 , refractive index from 1 to 0.8, absorptivity changes from 90% to 65% and optical band gap energy from 0.2eV to 0.02eV. In UV irradiated Ag exposed to light wavelength range from 500- 1000nm absorption is almost constant i.e. 2.5 10 7 m -1 , refractive index changes from 0.85 to 0.7, absorptivity changes from 75% to 40% and optical band gap energy changes from 0.25eV to 0.13eV. The reflectivity shows inverse trend as that of absorptivity. The UV irradiation also changes the electrical conductivity of target metals. For all the four transition metals used, electrical conductivity decreases non-linearly when the laser fluence increases (0.0J/cm 2 to 2.5J/cm 2 ). In Pt and Cu the reduction in electrical conductivity follows an exponential decrease, whereas in Au and Ag, the decrease is in accordance with Boltzmann function, which exhibits a sigmoidal curve.