Omnidirectional iridescence via cylindrically-polarized femtosecond laser processing  


Structural color is a distinct type of coloration found in nature that arises as a result of the analysis of the incident white light to individual wavelengths due to the existence of sophisticated periodic structures formed on the surface of natural species[1]. 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[2], cosmetics[3], anti-counterfeiting[3–5], spectroscopy and color analysis systems, solar cells[6] and biomedical devices. In order to create omnidirectional iridescent metallic surfaces exhibiting efficient diffraction for practically any incident angle of illumination, the plane direction of light must always be perpendicular to locally formed LSFL, hence reflective diffraction can take place. Efforts have been made in order to overcome the above restrictions with the generation of LSFL in variable orthogonal directions by utilizing the polarization dependence of the ripple orientation. In particular, a grid can be fabricated where each laser scanning direction employs different polarization state such as p and s polarization respectively[7,8]. Therefore, the surface can produce diffraction in two different planes of incidence. Also, it has recently been reported that irradiation with circular polarization pulses[9] can form triangular periodic surface structures that could potentially and significantly increase the range of incident light-illumination rotating angles. Despite these efforts, omnidirectional iridescence has not yet been realized via a single-step irradiation process.

The research group of Dr. Emmanuel Stratakis from IESL-FORTH demonstrates a method to address this exact restriction, presenting here an effective, simple and single-step technique for the fabrication of diffractive surfaces, exhibiting iridescence with great efficiency for any angle of incidence. This work is published in Opto-Electronic Advances in Vol. 3, No. 5, entitled “Omnidirectional iridescence via cylindrically-polarized femtosecond laser processing”. This is realized via large-area processing of steel with radially polarized cylindrical vector (CV) fs laser beams leading to the formation of LSFL, which are spatially oriented in multiple directions. It is observed that for practically every possible incident angle of illumination, the plane direction of light is always perpendicular to locally formed LSFL, hence reflective diffraction can take place. The ability of the obtained structures to act as diffraction gratings was systematically studied and compared against the diffraction originating on the conventional ones fabricated using linearly polarized laser pulses. Particularly, it is demonstrated that the multi-directional gratings formed on the surface by a radially polarized fs laser pulses, could mimic the omnidirectional structural coloration properties found in some natural species. As shown in Figure 1a, the grating structures (S2) fabricated via utilized radially polarized fs laser pulses can spatially disperse the incident light into individual wavelength with high efficiency, exhibiting structural iridescence at all viewing angles in contrast with the structures obtained with linearly polarized beam (S1).

Figure 1. Schematic illustration of the structural colors observed
the S1 (a) and the S2 (b) sample series respectively

About The Group

The Ultrafast Laser Micro- and Nano- processing (ULMNP) laboratory ( of IESL research is focused on the development of novel ultrafast pulsed laser processing schemes for controlled structuring at micro- and nano- scales of a variety of solid materials. By applying ultrashort UV, VIS and IR laser pulses novel surface structures with sub-micron sized features are produced while the physical properties of semiconductor, dielectric and metallic surfaces are significantly modified. Developed methods include laser micro/nano surface structuring performed in different media, direct laser writing with variable laser polarization states and combination of those. Further control over the surface topology is achieved by proper functionalization of the 3D structures obtained with well-defined nanostructures. The unique characteristics of ultrashort pulsed lasers, emitting light pulses shorter than a few tens of picoseconds, have paved the way for new avenues in photonic applications. Our aim is the exploitation of ultrashort laser pulses for material independent, micro/nano structuring, synthesis and functionalization as well as diagnostics for a variety of applications, including biomimetic structures, microfluidics, flexible optoelectronics and tissue engineering. The application of focused fs laser pulses to fundamentally change materials offers great potential for exciting emerging avenues for innovation and exploitation in the photonics industry.


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 Livakas N, Skoulas E, Stratakis E. Omnidirectional iridescence via cylindrically-polarized femtosecond laser processing. Opto-Electron Adv 3, 190035 (2020).