We demonstrate a rapid, accurate, and convenient method for tailoring the optical properties of diamond surfaces by employing laser induced periodic surface structuring (LIPSSs). The characteristics of the fabricated photonic surfaces were adjusted by tuning the laser wavelength, number of impinging pulses, angle of incidence and polarization state. Using Finite Difference Time Domain (FDTD) modeling, the optical transmissivity and bandwidth was calculated for each fabricated LIPSSs morphology. The highest transmission of ~99.5% was obtained in the near-IR for LIPSSs structures with aspect ratios of the order of ~0.65. The present technique enabled us to identify the main laser parameters involved in the machining process, and to control it with a high degree of accuracy in terms of structure periodicity, morphology and aspect ratio. We also demonstrate and study the conditions for fabricating spatially coherent nanostructures over large areas maintaining a high degree of nanostructure repeatability and optical performance. While our experimental demonstrations have been mainly focused on diamond anti-reflection coatings and gratings, the technique can be easily extended to other materials and applications, such as integrated photonic devices, high power diamond optics, or the construction of photonic surfaces with tailored characteristics in general.

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.

Direct laser writing of amorphous lines in crystalline silicon has the potential for becoming a flexible alternative to silicon‐on‐insulator technology for photonic integrated circuits. Yet, the maximum amorphous layer thickness achieved is 60 nm, which is below the requirements for waveguiding at telecom wavelengths. Here, the authors report on different strategies to push the layer thickness beyond today's limit. To this end, irradiation with femtosecond laser pulses covering an extremely broad wavelength range (515 nm–4 µm) up to the yet unexplored near‐ and mid‐infrared region of silicon transparency is investigated. The results show that much thicker amorphous layers can be obtained upon multipulse irradiation at 3‐µm wavelength. The deepest amorphization is achieved in silicon wafers covered with a thick silicon dioxide layer that strongly assists the heat extraction, yielding steep index profiles with a maximum amorphous layer thickness of 128 nm. This superior thickness is compatible with single mode waveguiding for a symmetric waveguide configuration. This study also contributes to a better understanding of the mechanisms involved in laser‐induced amorphization.

Plasmonic metasurfaces based on the extraordinary optical transmission (EOT) effect can be designed to efficiently transmit specific spectral bands from the visible to the far-infrared regimes, offering numerous applications in important technological fields such as compact multispectral imaging, biological and chemical sensing, or color displays. However, due to their subwavelength nature, EOT metasurfaces are nowadays fabricated with nano- and micro-lithographic techniques, requiring many processing steps and carrying out in expensive cleanroom environments. In this work, we propose and experimentally demonstrate a novel, single-step process for the rapid fabrication of high-performance mid- and long-wave infrared EOT metasurfaces employing ultrafast direct laser writing. Microhole arrays composing extraordinary transmission metasurfaces were fabricated over an area of 4 mm2 in timescales of units of minutes, employing single pulse ablation of 40 nm thick Au films on dielectric substrates mounted on a high-precision motorized stage. We show how by carefully characterizing the influence of only three key experimental parameters on the processed micro-morphologies (namely, laser pulse energy, scan velocity, and beam shaping slit), we can have on-demand control of the optical characteristics of the extraordinary transmission effect in terms of transmission wavelength, quality factor, and polarization sensitivity of the resonances. To illustrate this concept, a set of EOT metasurfaces having different performances and operating in different spectral regimes has been successfully designed, fabricated, and tested. Comparison between transmittance measurements and numerical simulations has revealed that all the fabricated devices behave as expected, thus demonstrating the high performance, flexibility, and reliability of the proposed fabrication method. We believe that our findings provide the pillars for mass production of EOT metasurfaces with on-demand optical properties and create new research trends toward single-step laser fabrication of metasurfaces with alternative geometries and/or functionalities.

Monitoring the laser-induced melting and solidification dynamics of Ge upon laser irradiation is an enormous challenge due to the short penetration depth of its liquid phase. In this work, real-time pump-probe experiments in combination with finite element calculations have been employed to investigate the melting and solidification dynamics of germanium upon ns and fs laser pulse irradiation (λ=800nm). Excellent agreement between experiments and simulations allowed us to indirectly determine additional time- and depth-dependent information about the transformation dynamics of germanium, including the thickness evolution of the molten layer, as well as its melting and solidification velocities for the two pulse durations for different fluences. Our results reveal considerable differences in the maximum thickness of the molten Ge superficial layers at sub-ablative fluences for ns and fs pulses, respectively. Maximum melt-in velocities of 39m/s were obtained for ns pulses at high fluences, compared to non-thermal melting of a thin layer within 300fs for fs pulses already at moderate fluences. Maximum solidification velocities were found to be 16m/s for ns pulses, and up to 55m/s for fs pulses. Weak signs of amorphization were observed for fs excitation, suggesting that the lower limit of solidification velocities for a complete amorphization is above 55m/s. In addition, we show high precision measurements of the melt-in velocities over the first 20 nm by means of fs microscopy with sub-ps temporal resolution. Here, differences of the melt-in process of several orders of magnitude were observed, ranging from virtually instantaneous melting within less than 2 ps even for a moderate peak fluence up to 200 ps for fluences close to the melting threshold.