Mechanism of fine ripple formation on surfaces of (semi)transparent materials via a half-wavelength cavity feedback

Abstract

The mechanism of the fine ripples, perpendicular to laser polarization, on the surface of (semi)transparent materials with period smaller than the vacuum wavelength, λ, of the incident radiation is proposed and experimentally validated. The sphere-to-plane transformation of nanoplasma bubbles responsible for the in-bulk ripples accounts for the fine ripples on the surface of dielectrics and semiconductors. The mechanism is demonstrated for 4H:SiC and sapphire surfaces using 800 nm/150 fs and 1030 nm/300 fs laser pulses. The ripples are pinned to the smallest possible standing wave cavity inside material of refractive index n. This defines the corresponding period, Λ = (λ/n)/2, of a light standing wave with intensity, E(2), at the maxima of which surface ablation occurs. The mechanism accounts for the fine ripples at the breakdown conditions. Comparison with ripples recorded on different materials and via other mechanisms using femtosecond pulses is presented and application potential is discussed.

Two-photon polarization-selective etching of emergent nano-structures on diamond surfaces

“…The rich variety of patterns produced as functions of facet and polarization reveal that the ejection probability is a complex function of the local surface structure and beam polarization. Such an ejection mechanism is consistent with the thresholdless and 'cold' nature of the process, and is distinct from the exponential and thermal nature of the periodic structuring observed previously [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] when using higher intensities, either near or above the ablation threshold. Further work is needed to determine whether bond scission results from direct coupling of the ultraviolet with carbon-carbon bonds or is reliant on cooperative processes with electronic states associated with surface reconstructions, radical sites or chemisorbed atoms.…”

“…As the process can be performed in ambient conditions, it provides a straightforward and area-scalable method for tailoring the nano-scale morphology of diamond surfaces. This may be of direct interest in areas such as ultralow friction surfaces 15 , biochemical substrates 14 , and surface-enhanced ultrasensitive molecular sensors 35 . Of more fundamental importance, the evidence for coupling between the light field and localized bonds reveals promise for developing optical tools for bond-specific manipulation in covalent materials.…”

Subwavelength ripples adjustment based on electron dynamics control by using shaped ultrafast laser pulse trains

“…As the pulse delay increases, a single pulse is separated into two delayed subpulses; and neither of them is sufficient to damage the surface. Structures [29,30] on the dielectrics surface are very similar to the HSFL described in this work, which can be attributed to the periodic plasma enhancement of the incident laser field at the surface [29]. During the ablation process, surface plasmons can be resonantly excited by coupling between the surface free electrons and the incident field at a certain electron density.…”

Randomization of gold nano-brick arrays: a tool for SERS enhancement

“…It was found that "hot-spots" where the light field intensity is enhanced up to 2 × 10 4 (numerically predicted) are found on random patterns, and such values exceed those obtained from periodic nano-disk arrays. By nano-texturing of surfaces through laser ablation, quasi-periodic patterns of ripples are formed [17] with a random distribution of nanoscale features, which are responsible for a strong increase of sensitivity when they are used for Raman sensing [14,[18][19][20]. How the degree of randomization of the quasi-periodic patterns affects the formation of hot-spots in the spatial and spectral domains, is a topic in need of further investigation [21].…”