Ripple-patterned substrates for light enhancement applications

Abstract

We report on surface structuring of sapphire, silicon carbide, and silicon by femtosecond laser pulses in multipulse irradiation mode. The formed ripples on the flat surface or on the vertical walls with hierarchical structures whose feature sizes are ranging from the irradiation wavelength down to similar to 50 nm are prospective templates for surface enhanced Raman scattering after coating with plasmonic metals. We study complex patterns of fine ripples with periods Lambda(r), as small as lambda/Rp, where Rp similar or equal to 3 - 5. The mechanisms suggested for such Rp values are discussed: temperature and density of breakdown plasma, angle of incidence, mechanism of second harmonic generation (SHG) and absorption. Predictions of the surface and bulk models of ripple formation are compared with experimental values of Rp-factor. We propose a model of ripple formation on the surface, which is based on the known in-bulk sphere-to-plane formation of breakdown plasma in the surface proximity. In semiconductor 4H:SiC normal ripples with periods 190 and 230 nm were recorded with 800 nm and 1030 nm fs-laser pulses respectively. We show that the period of ripples is defined by the dielectric properties of crystalline (solid) phase rather than the molten phase in the case of silicon. Generation of SHG on the surface of sample and plasma nano-bubbles are discussed: surface-SHG is found not important in ripples' formation as revealed by comparative study of periods on Al2O3 and TiO2 at 800 nm wavelength of irradiation. We propose that ripple periodicity is pinned to the smallest possible standing wave cavity (lambda/n)/2 inside material of refractive index n

Nano-groove and 3D fabrication by controlled avalanche using femtosecond laser pulses

“…However, ripples on surfaces of dielectrics are less well behaved and periods in the range Λ = λ 0 /(2 − 20) have been reported especially for excitation with fs-laser pulses. Most of them scales closely to the empirical Λ = λ 0 /n/2 where n is the refractive index of an unperturbed material [28] and dielectric permittivity of the ambient medium [29]. However, since n is dependent on intensity via strong electronic excitation during the pulse, n(I,t p ), this transient permittivity can imprint ripples of different periods.…”

“…Lines fabricated by single scan of the laser beam contain several grooves -ripples ( Fig. 2(b)) which can then be patterned over larger area by scanning [28]. Ripples on surfaces of high melting temperature substrates such as sapphire and SiC showed almost intensity independent period λ 0 /n/2 [28,30].…”

“…Apparently, the plasma density created for the -ripples is larger than that for ⊥ ones. Larger plasma density is expected from the regions where larger number of structural defects is accumulated, material is preheated, amorphous or noncrystalline regions are created on a crystalline substrate [28]. In all of those situations stronger absorption is expected.…”

Laser fabricated ripple substrates for surface‐enhanced Raman scattering

“…Here we demonstrate a simple direct laser write approach to create nano-textured surfaces, which can perform as SERS substrates over a spectrally broad 0.15 to 5 µm range of the sapphire transmission window. Details of the fabrication and mechanism of ripple formation on dielectric surfaces was described previously [13,14]. Due to the high melting temperature of sapphire the ablated surfaces do not have melt recast and are nano-rough with feature sizes of 20-30 nm, when ablation is carried out by ultra-short (sub-1 ps) laser pulses.…”

Fabrication of nanostructures on silicon carbide surface and microgroove sidewall using 800-nm femtosecond laser

“…1,2 For example, Deki et al 3 reported the enhancement of local conductivity in silicon carbide (SiC) with nanoripples. 1,2 For example, Deki et al 3 reported the enhancement of local conductivity in silicon carbide (SiC) with nanoripples.…”