A distinct type of coloration found in nature comes as a result of the analysis of the incident light to individual wavelengths realized by sophisticated periodic or pseudoperiodic constructions present within the body of natural species
Opto-Electronic Advances, Volume. 3, Issue 5, 190035-1(2020)
Omnidirectional iridescence via cylindrically-polarized femtosecond laser processing
We report the femtosecond (fs) laser fabrication of biomimetic omnidirectional iridescent metallic surfaces exhibiting efficient diffraction for practically any angle of light incidence. Such diffractive behavior is realized by means of multi-directional low-spatial-frequency, laser-induced periodic surface structures (LSFL) formed upon exploiting the cylindrical symmetry of a cylindrical vector (CV) fs field. We particularly demonstrate that the multi-directional gratings formed on stainless steel surface by a radially polarized fs beam, could mimic the omnidirectional structural coloration properties found in some natural species. Accordingly, the fabricated grating structures can spatially disperse the incident light into individual wavelength with high efficiency, exhibiting structural iridescence at all viewing angles. Analytical calculations using the grating equation reproduced the characteristic variation of the vivid colors observed as a function of incident angle. We envisage that our results will significantly contribute to the development of new photonic and light sensing devices.
Introduction
A distinct type of coloration found in nature comes as a result of the analysis of the incident light to individual wavelengths realized by sophisticated periodic or pseudoperiodic constructions present within the body of natural species
Owing to their unique optical properties, bioinspired diffractive surfaces with multiple periodic structures are commonly used as diffracting elements for a plethora of scientific and industrial applications, such as color and holographic displays, telecommunication
Nevertheless, none of the methods reported to date, including the laser-based ones, is capable to produce multidirectional spatial frequency patterns, in a single step. As a consequence, the produced iridescence strongly depends on the viewing angle, thus it exists only for specific angles of incidence and is absent upon tilting the diffraction element
In this paper we present, an effective, simple and single-step technique for the fabrication of diffractive surfaces, exhibiting iridescence with great efficiency for any angle of incidence. This is realized via large-area processing of steel with radially polarized cylindrical vector (CV) fs laser beams
Materials and methods
Figure 1.The experimental setup used for surface structuring.The laser fluence was tuned via the use of a
where R is the Gaussian beam radius, while for CV beams, R is the outer and r the inner radius. In the case of area scanning at constant velocity v, repetition rate f and distance between successive scanning lines (δ), Neff-area can be calculated from:
The morphology of the periodic surface structures has been characterized by scanning electron microscopy (SEM, JEOL JSM-7500F). In order to determine the LSFL periodicity and directionality, SEM images were analyzed via two-dimensional fast Fourier transform (2D-FFT), using the Gwyddion software.
To investigate the influence of the angle of an incident white light on the structural color formation, a diffraction characterization setup was built, as shown in
Results and discussion
The generation of LSFL with the best possible uniformity depends strongly on the irradiation parameters. For this purpose, a comprehensive parametric study was conducted via line scanning, to investigate the effect of F and Neff on the topographical features induced. The parametric study led to the creation of optimized, well-organized and uniform LSFL structures under specific irradiation conditions for both polarization states.
Figure 2.LSFL periodicity dependence on the laser fluence (a), and the effective number of pulses (b), for linearly (squares) and radially (circles) polarized fs beams. Top-view SEM images of areas produced upon irradiation using linearly (c, d) and radially (e, f) polarized fs beams.The images shown in (c) and (e) correspond to non-uniform areas, whereas those in (d) and (f) to areas obtained using optimized irradiation conditions. The red arrows depict the electric field polarization state. The 2D-FFT patterns corresponding to each area are shown.
It is observed that the LSFL produced via CV beams exhibit lower periodicities, which is in accordance to our former findings and interpretation
Large areas of 5 mm × 5 mm were also fabricated via scanning the fs beam onto the sample surface using the optimized conditions found during the line scanning experiments.
Figure 3.Top-view SEM images of areas produced upon irradiation using linearly (a–c) and radially (d–f) polarized fs beams.The images shown in (b, c) and (e, f) are higher magnifications of the images (a, d) respectively. The areas in (a–c) were fabricated at
The average period calculated by the FFT pattern for the linear-polarized case equals to 870±80 nm. On the other hand, areal scanning with CV beams gives rise to more complex LSFL structures exhibiting multi-directional orientation (
Following irradiation, all the processed surfaces exhibited vivid coloration, which is expected, considering that the calculated LSFL periodicities are close to the visible wavelengths
where λ is the diffracted wavelength, Λ corresponds to the LSFL period, and m is the order of diffraction, in our case m=±1. The angle φ denotes the white light incidence, while ω corresponds to the sample rotation within the sample plane. Assuming that θ is the sample tilt angle, the grazing angle equals to φ+θ. The ability of the surface to act as a diffraction grating was tested for eight different angles ω, starting from 0° to 360° with a step of 45°. For each angle ω, the grazing angle was changed accordingly and the corresponding coloration was captured by a camera, placed along the z-axis. The angle β was also recorded for each measurement.
Figure 4.(
Figure 5.Schematic illustration of the structural colors observed the S1 (a) and the S2 (b) sample series respectively; 2D-FFT patterns corresponding to the S1 (c) and the S2 (e) sample series respectively. The corresponding intensity plots of the 2D-FFT patterns in four different directions (denoted as '1' to '4') are depicted in (d) and (f) respectively.Inset in (f) displays the periodicity values of LSFL structures for a series of four cross-sections taken in the Fourier image of the S2 surface.
As mentioned above, the diffraction properties and respective structural colors are quite different for the S2 series of surfaces fabricated by a radially polarized CV beam. This is due to the formation of multi-directional LSFL structures, giving rise to reflective diffraction, regardless of the rotational angle ω. As a result, the surface is iridescent for practically any viewing angle, although the vividness and sharpness of the diffracted colors are slightly different depending on the angle ω. Specifically, for the angles ω = 0°, 90°, 180°, and 270°, the diffracted colors appeared to be most vivid and exhibit the maximum rendering capacity, compared to the angles ω
= 45°, 135°, 225°, and 315°. This effect is also indicated by the respective FFT pattern as shown in
Figure 6.(
Conclusions
We have presented a novel technique for the fabrication of large areas of multi-directional laser-induced periodic surface structures exhibiting omnidirectional diffraction properties. It is based on LSFL formed upon exploiting the cylindrical symmetry of CV fs field. The diffraction and structural coloration properties are by far different than those exhibited by LSFL areas fabricated via linearly polarized Gaussian beams. Our results show that it is possible to efficiently tune the diffraction and structural coloration properties via surface processing strategies employing femtosecond CV beams. We envisage that this work will pave the way for the realization of customized photonic components and devices.
Acknowledgements:
This work was supported by MouldTex project-H2020-EU.2.1.5.1 (GrantAgreement No. 768705).
Competing interests
The authors declare no competing financial interests.
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Nikolaos Livakas, Evangelos Skoulas, Emmanuel Stratakis. Omnidirectional iridescence via cylindrically-polarized femtosecond laser processing[J]. Opto-Electronic Advances, 2020, 3(5): 190035-1
Category: Original Article
Received: Sep. 16, 2019
Accepted: Nov. 26, 2019
Published Online: Aug. 10, 2020
The Author Email: Stratakis Emmanuel (stratak@iesl.forth.gr)