Scheme of near-wavelength periodic surface structures induced by ultrasort laser irradiation

Polarization ripples are parallel oscillations which have been observed since the 1960s on the bottom of pulsed laser irradiation of semiconductors. They have the property to be very dependent to the orientation of the laser electric field. Since the wide availability of

femtosecond laser Mode locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12 s) or femtoseconds (10−15 s). A laser operated in this way is sometimes r ...

s, such structures have been observed on

metals A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

semiconductors A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...

, but also on

dielectrics In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mater ...

. Moreover, the ripples can reach far sub-wavelength periodicities until 100 nm as recently observed in titanium. The "cumulative" changes occurring from pulse to pulse in the material properties are still under investigation.

Formation mechanisms

The formation mechanisms are still under debate. However, two types of formation mechanisms can be underlined: * the resonant mechanisms, which are based on electromagnetic aspects, as periodic energy deposition due to roughness, as surface plasmon polariton excitation during the laser illumination; * the non-resonant mechanisms, more related with thermal consequences of the irradiation of the target by the laser, like

capillary waves A capillary wave is a wave traveling along the phase boundary of a fluid, whose dynamics and phase velocity are dominated by the effects of surface tension. Capillary waves are common in nature, and are often referred to as ripples. The wav ...

formed in the melted layer. The set of resonant mechanisms leading to formation of ripple is defined by the strong link between ripple periodicity and laser wavelength. It includes the excitation of surface electromagnetic wave such as surface plasmon polariton, and surface waves excited by an isolated defect or surface roughness, especially under femtosecond irradiation An alternative mechanism that assumes the synergy of electron excitation and capillary wave solidification has been also proposed to explain both the formation of ripples and the observed ripple periodicity. An extension of the mechanism was also proposed to account for the development of periodic structures with periodicity larger than the laser beam's wavelength (i.e. grooves) that are formed perpendicularly to the subwavelength-sized ripples; the proposed physical mechanism assumes the erasing of periodic energy deposition followed by the formation of hydrothermal convection rolls that propagate parallel to the electric field polarisation. The analogy of the structure shape with the solution o
Kuramoto-Sivashinsky equations
is often mentioned to support different theories such as defect accumulation, or ultrafast modification of the atomic lattice.

Applications

Their interest is about potential applications in building microfluidic channels, changing the color of materials, modifying local electrical properties, and building sub-diffraction-limit optical

diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structura ...

s. They also constitute the first stage of the

Black Silicon Black silicon is a semiconductor material, a surface modification of silicon with very low reflectivity and correspondingly high absorption of visible (and infrared) light. The modification was discovered in the 1980s as an unwanted side effect ...

formation process by femtosecond irradiation.

References

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