Most practically used optical isolators are based on the magneto-optical Faraday effect. There also exist isolators based on different physical phenomena. In this work, we have experimentally demonstrated for the first time in our knowledge the new type of optical isolator based on Sagnac effect.
Metal films on transparent substrates are widely applied for mask production in lithography, and lasers are frequently
used for their patterning. Quality of the patterning is limited by fundamental phenomena taking place close to edges of the laser ablated area. We experimentally and numerically investigated transformations in metal films during their
irradiation with the nanosecond laser beam with fluence above the ablation threshold. Ridges of the resolidified metal with non-uniform thickness were always formed on edges of the cleaned area. Instabilities during the ablation process forced the molten metal in the ridges to break up into droplets with the periodicity predicted by the Plateau–Rayleigh instability. The droplets on ridges were starting points for formation of self-organized lines of metal film by irradiation with partially overlapping laser pulses. The initial droplets and later the self-organized parallel lines of chromium metal were heat sinks that cooled down the metal in their close proximity. Temperature modulation along the laser irradiation spot was high enough to initiate the Marangoni effect which resulted in movement of the molten metal from hot to colder areas.
Ablation characteristics of chromium thin film on a glass substrate by nanosecond laser pulses were investigated. The
laser beam was tightly focused through the glass substrate to a stripe-like spot using the acylindrical lens. The metal was
removed only by the central part of the laser beam, where local laser fluence exceeded the well-defined ablation
threshold. Formation of a wide area cleaned by the series of laser pulses caused some side effects. The stripe ablated by a
single laser pulse had sharp edges on both sides, while the partially overlapping pulses formed a wide stripe with a
complicated structure made of the remaining metal. Regular structures, ripples, were developed when laser fluence was
slightly above the threshold and the shift between pulses was less than a half width of the line ablated by a single laser
pulse. The ripples were located periodically (~4 &mgr;) and were orientated perpendicularly to the long axis of the beam
spot (in parallel to the laser pulse shift direction). Their direction did not depend on the laser beam polarization.
Different models of the ripple formation in the thin metal film were considered, and instability of the moving vapor-liquid-
solid contact line during evaporation of thin liquid films appear to be the most appropriate process responsible for
the observed phenomena. Regular gratings with the unlimited line length can be produced by using the technique.
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