We observed the spontaneous formation of periodic nano-structures in both femtosecond laser ablation and deposition.
The former involved 400-nm femtosecond pulses from a 250-KHz regenerated amplified mode-locked Ti:sapphire laser
and periodic nanocracks and the nano-structure are in the form of periodic nanocracks in the substrate, the latter applied
an 80-MHz mode-locked Ti:sapphire oscillator with pulse energy less than half nanojoule in a laser-induced chemical
vapor deposition configuration and tungsten nanogratings grow heterogeneously on top of the substrates. These two
observed periodic nanostructures have opposite orientations respecting to laser polarization: the periodic nanocracks are
perpendicular to, whereas the deposited tungsten nanogratings are parallel to laser polarization direction. By translating
the substrate respecting to the laser focus, both the periodic nanocrack and tungsten nanograting extend to the whole
scanning range. The deposited tungsten nanogratings possess excellent uniformity on both the grating period and tooth
length. Both the attributes can be tuned precisely by controlling the laser power and scanning speed. Furthermore, we
discovered that the teeth of transverse tungsten nanogratings are self aligned along their axial direction during multiple
scanning with appropriate offset between scans. We demonstrate the feasibility of fabricating large-area one-dimensional
grating by exploiting such unique property. These distinct phenomena of nanocracks and tungsten nanogratings indicate
different responsible mechanisms.
We demonstrate deposition of periodic tungsten nanoripple on different substrate using a single 400nm femtosecond
laser beam at room temperature. The laser beam generated by frequency doubling the output from mode-locked 80MHz
Ti: sapphire oscillator was applied in a laser-induced chemical vapor deposition configuration, in which tungsten
hexacarbonyl was used as precursor. The deposition strongly depended on laser polarization. With linearly polarized
light, periodic ripple structure with willow-leaf shape was formed inside the exposure area. The ripple orientation was
found parallel to the laser polarization direction. Affects of laser power and exposure time on ripple formation were
investigated. By translating the substrate with respective to the laser beam, longitudinal or transverse grating structure
was observed. The period of this grating structure is about 150nm on sapphire, and the orientation is parallel to laser
polarization. Simply by programming the translation of the substrate, large area patterns and other patterns such as circle
and characters were formed. Similar ripple and grating structures observed on all the substrates we investigated,
including insulators, semiconductors and metals, implies that ripple formation might be a universal phenomenon.
Considering the simplicity of this process and material flexibility of laser CVD, this technique may provide a novel costeffective
patterning method to produce periodic subwavelength nanostructures of a wide range of materials on many
substrates.
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