We developed a fast detector (patent pending) based on the Laser Induced Transverse Voltage (LITV) effect. The advantage of detectors using the LITV effect over pyroelectric sensors and photodiodes for laser radiation measurements is the combination of an overall fast response time, broadband spectral acceptance, high saturation threshold to direct laser irradiation and the possibility to measure pulsed as well as cw-laser sources. <p> </p>The detector is capable of measuring the energy of single laser pulses with repetition frequencies up to the MHz range, adding the possibility to also measure the output power of cw-lasers. <p> </p>Moreover, the thermal nature of the sensor enables the capability to work in a broadband spectrum, from UV to THz as well as the possibility of operating in a broad-range (10<sup>-3</sup>-10<sup>2</sup> W/cm<sup>2</sup> ) of incident average optical power densities of the laser radiation, without the need of adopting optical filters nor other precautions.
Ultra-short pulsed laser sources, with pulse durations in the ps and fs regime, are commonly exploited for cold ablation. However, operating ultra-short pulsed laser sources at fluence levels well below the ablation threshold allows for fast and selective thermal processing. The latter is especially advantageous for the processing of thin films. A precise control of the heat affected zone, as small as tens of nanometers, depending on the material and laser conditions, can be achieved. It enables the treatment of the upper section of thin films with negligible effects on the bulk of the film and no thermal damage of sensitive substrates below. By applying picosecond laser pulses, the optical and electrical properties of 900 nm thick SnO2 films, grown by an industrial CVD process on borofloat®-glass, were modified. The treated films showed a higher transmittance of light in the visible and near infra-red range, as well as a slightly increased electrical sheet resistance. Changes in optical properties are attributed to thermal annealing, as well as to the occurrence of Laser- Induced Periodic Surface Structures (LIPSSs) superimposed on the surface of the SnO<sub>2 </sub>film. The small increase of electrical resistance is attributed to the generation of laser induced defects introduced during the fast heating-quenching cycle of the film. These results can be used to further improve the performance of SnO<sub>2</sub>-based electrodes for solar cells and/or electronic devices.
Large areas of regular diffraction nanogratings were produced consisting of so-called laser-induced periodic surface structures (LIPSS) on thin molybdenum layers (<400 nm) deposited on a borosilicate glass substrate. The aim was to produce these structures without ablating nor cracking the molybdenum layer. Ultra short laser pulses were applied using a focused Gaussian beam profile. Processing parameters such as laser fluence, pulse overlap, number of overscans, repetition frequency, wavelength and polarization were varied to study the effect on periodicity, height, and especially regularity of the obtained LIPSS. It was found that a careful choice of the correct laser parameters is required to avoid detrimental mechanical stresses, cracking, and delamination during the laser processing of the layer in order to remain in its correct range of ductility as well as to ensure regular LIPSS. A possible photovoltaic application of these nanogratings could be found in texturing of thin film cells to enhance light trapping mechanisms.
In this work, we studied the feasibility of surface texturing of thin molybdenum layers on a borosilicate glass substrate
with Ultra-Short Laser Pulses (USLP). Large areas of regular diffraction gratings were produced consisting of Laserinduced
periodic surface structures (LIPSS).
A short pulsed laser source (230 fs-10 ps) was applied using a focused Gaussian beam profile (15-30 μm). Laser
parameters such as fluence, overlap (OL) and Overscans (OS), repetition frequency (100-200 kHz), wavelength (1030
nm, 515 nm and 343 nm) and polarization were varied to study the effect on periodicity, height and especially regularity
of LIPSS obtained in layers of different thicknesses (150-400 nm). The aim was to produce these structures without
cracking the metal layer and with as little ablation as possible.
It was found that USLP are suitable to reach high power densities at the surface of the thin layers, avoiding mechanical
stresses, cracking and delamination.
A possible photovoltaic (PV) application could be found in texturing of thin film cells to enhance light trapping