dc.description.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. |
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