Generation, control and erasure of dual LIPSS in germanium with fs and ns laser pulses

Abstract

Laser-induced periodic surface structures (LIPSS) can readily be fabricated in virtually all types of materials and benefit from an efficient parallel patterning strategy that exploits self-organization. The wide range of different LIPSS types with different spatial scales and symmetries is continuously growing, addressing numerous of applications. Here, we report on the formation of two fundamentally different types of LIPSS on germanium upon exposure to femtosecond laser pulses (λ = 800 nm, 130 fs), featuring different periods and orthogonal orientations. On the one hand, the well-known low-spatial frequency LIPSS (LSFL) with a period ≈ λ and perpendicular orientation to the laser polarization are formed, which can be extended homogeneously in 2D by sample scanning. Additionally, extremely smooth ripples with a period ≈λ/2 and parallel orientation were generated at lower pulse numbers. We show that this new kind of ripples, named parallel high-spatial frequency LIPSS (HSFL-∥), can be superimposed onto LSFL by increasing the pulse number, forming complex dual LIPSS with nanohill-like morphology. While exposure to multiple nanosecond laser pulses is found to trigger also the formation of LSFL, HSFL-∥ cannot be formed under these conditions, which points out the role of ultrafast excitation in the formation of the latter. By performing time-resolved reflectivity measurements, we are able to resolve the melting and solidification dynamics, revealing melting of a very shallow surface layer (<20 nm) and melt durations of a few ns for both pulse durations pulses at the fluences employed for LIPSS formation. Finally, we demonstrate erasure of both types of LIPSS by exposure to single nanosecond pulses at high fluences, which paves the way for erasable multi-level data storage.

Ten Open Questions about Laser-Induced Periodic Surface Structures

“…In a distinct optical approach, the grating-like spatial characteristics of LSFL can be imprinted in the sub-ablation regime to a phase change material (PCM) such as GeTe [94] or Ge 2 Sb 2 Te 5 [95], allowing the stripes/ridges of the LIPSS to be represented either by the amorphous or by the crystalline phase of the PCM. In this way, by proper choice of the laser irradiation parameters, micron-sized spots with an optically encoded LIPSS patterns can be written and erased again [95,96]. Such optical patterns may be used for information encoding or act as unique safety features.…”

Nanosecond Laser Switching of Phase‐Change Random Metasurfaces with Tunable ON‐State

“…The transient reflectivity changes occurring upon nanosecond laser pulse irradiation were monitored by a cw probe beam and a fast photodiode, similar to the configuration reported in refs. [38,65]. Briefly, the probe beam was a single-mode intracavity laser at 532 nm with vertical polarization; the continuous wave beam was temporally modulated by means of an acousto-optical modulator to produce single rectangular pulses of 10 µs duration at the repetition rate of the nanosecond laser (100 Hz), temporally synchronized.…”

“…[37] However, when a much larger time window needs to be investigated, as in this study, the use of a fast photodiodes is more appropriate, still preserving sub-nanosecond temporal resolution. [38] In the following investigation, we reveal the fast and reversible dynamic optical response in the visible region of Biembedded nanostructures in a dielectric matrix forming a phase-change random metasurface (PCRM). We use the term random metasurface to make reference to metasurfaces in which the nanostructured resonant constituent elements are randomly placed with certain size, density, and orientation distributions, rather than composed of well-ordered elements with a single size, density, and orientation as those, which can be fabricated using lithography techniques.…”

Nonlinear ionization control by temporally shaped fs+ps double-pulse sequence on ZnO

“…As far as for the sequence of two short-long sub-pulses [e.g., fs + microseconds (μs)], their widths differ greatly, so the long pulse far exceeds the lifetime of the electrons excited by the short pulse, leading to the inefficiency of energy deposition. In addition, the long pulse easily causes thermal effects and melts the nanostructures [17] . Based on an idea of taking advantage of direct interaction between two sub-pulses for nonlinear ionization control, we propose using the fs+ps double-pulse sequence (FPDPS) to improve the processing efficiency.…”