Description: DESCRIPTION
Technical Field of the invention

The present invention relates to the field of anti-reflective surfaces, more specifically to anti-reflective surfaces of thin stainless-steel sheets fabricated using interference-based ultrafast laser patterning for near-perfect black surfaces which is superhydrophobic with self-cleaning ability .

Background of the invention

Various scientific fields, including particle research/studies, solar energy harvesting, low-light imaging devices, optoelectronic devices, infrared sensing, space applications, and stealth applications require surfaces with low reflectivity for a wide range of wavelengths. Usually antireflective surfaces are constructed using thin-film coatings, but they have limitations such as small bandwidth and degradation over time due to chemical composition changes and thermal variations. Researchers have explored different methods to fabricate antireflective surfaces, including lithography techniques, chemical etching, plasma etching, coating carbon nanotubes, and laser patterning. Laser patterning has emerged as an attractive tool for fabricating nanostructures on various surfaces, offering control over surface properties and stability in harsh environments.

Laser patterning is a writing technique that utilizes a laser to directly modify the material, typically at its focal point. In this process, the laser beam interacts with the material, creating the pattern on it. The laser beam produces micropatterns on the material surface through laser ablation, removing layers in the foot print with micrometer accuracy and seamless repeatability.

Direct laser writing using a simple lens has a limitation of cutting groove width due to the diffraction-limited spot size and the step size employed in laser writing. The reflectivity of the patterned sample depends on the microgroove size, depth, aspect ratio, and types of nanostructures on it. For efficient geometrical trapping of photons, a small micro groove size is advantageous. If we reduce the microcavity to a smaller size compared to the incident wavelength, the trapping of photons' efficiency will increase. A smaller footprint of the laser can be achieved by the laser interference patterning.

Sagnac interferometer utilizes the Sagnac effect, also called Sagnac interference, phenomenon encountered in interferometry that is produced by rotation. the Sagnac interferometer is the most stable one to run over long periods and larger areas. In this interferometer, the beam splitter splits the laser beam into two beams, and the two beams are made to follow the same path but in the opposite direction before they recombine. As a result, two beams are experiencing the same external disturbance, and there is no dynamic path difference between the two beams, which results in a stable interference pattern. The number of interference lines can be varied according to our groove size requirements for patterning. The other experimental parameters, like scanning speed, laser fluence, and the distance between the scanning lines, are optimized for Sagnac interference patterning. The focused laser beams have a Gaussian profile, so the center's interference patterning depth is more profound than the edge lines.

For stray light suppression in space applications, black surfaces should fulfill the stringent requirements such as efficiency over broadband, insensitivity of incident light polarization, absorption independent of incident angles (wide acceptance angle), thin metal sheets with long life, and good mechanical & thermal stability. The design of such surfaces is classified into two groups either by coating the absorbing materials or by modifying the surface structures. In this work, we choose the route of structural modifications, essentially to pattern a particular refractive index profile that minimizes the reflectance of the SS metal substrate.

In sub-wavelength structures, the refractive index (RI) is not just determined by the intrinsic/bulk optical properties but rather by its volume fraction of varying composite structures. Creating a sparse material with lots of air gaps can bring the average RI very close to the RI of air, thus reducing the reflectivity (R = (n-n0)2/(n+n0)2) as per Fresnel’s equation). Recent work has demonstrated; bio-inspired nanostructures that mimic the compound eyes of a moth with low reflectivity over a certain bandwidth.

A few patents on thin stainless-steel sheets fabricated using different techniques are discussed below:
The patent CN104465462B (The preparation method that a kind of laser ablation is used for Magnetron Sputtering Thin Film patterning) discloses the preparation method that a kind of laser ablation is used for Magnetron Sputtering Thin Film patterning. This method of preparation is different from our present invention wherein, our present invention does not include any thin film coating and uses Sagnac-based interferometer.

The patent RU2760694C2 (Method and systems for generating images in coherent radiation and controlling with feedback for modifying materials) relates to methods and systems for generating images in coherent radiation and controlling the method with feedback for modifying the material; This method is different from our current invention.

Thus, there exists a need for an inexpensive, scalable method of fabrication of a broadband ultralow reflective material with sufficient strength, light weight, durability with superhydrophobic properties. Our present invention addresses this need by using a Sagnac-based interferometric wavefront modulation used in micromachining to fabricate the anti-reflective stainless-steel material with the required properties.

Objective of the invention

The main objective of our present invention is to provide a method of preparing a broadband ultralow reflective stainless-steel sheet which is of sufficient mechanical strength, thermal stability, and rugged nanostructures.

Another objective of our present invention is to provide a material with a complex structure so that the reflectance over broadband is reduced.

Another objective of the present invention is to provide an anti-reflective surface with super hydrophobic properties, prepared using an inexpensive method that can be easily scaled to the industrial level.

Another objective of the present invention is to provide an anti-reflective surface fabricated without using a different material coating/deposition method, so that the material does not lose its anti-reflective, super light absorbing properties with thermal degradation, or wear and tear.

Brief Summary of the invention

The following presents a simplified summary of the disclosure in order to provide a basic understanding to the reader. This summary is not an extensive overview of the disclosure and it does not identify key/critical elements of the invention or delineate the scope of the invention. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

The present invention relates to a broadband ultralow reflective stainless-steel sheet obtained by a Sagnac-based interferometric wavefront modulation used in micromachining exhibiting both low reflective and water repellent nature. Multiscale hierarchical micro and nanostructures were prepared in a single-step process, this process is inexpensive and can be easily scaled to the industrial level. These structures can trap the photons incident on them by multiple reflections and absorb nearly 99% of the photons. An average total reflectance of 1.2% was achieved for 400 to 2000 nm optical bandwidth, with an average value is less than 0.02% of specular reflectance in the same spectral range. This anti-reflective surface is fabricated by laser structuring or without using a different material coating/deposition method on stainless steel. A laser ablation on a thin 100µm SS sheet is shown to display sufficient mechanical strength, thermal stability, and rugged nanostructures. The laser causes a change in the surface roughness of the Stainless steel leading to a change in its wetting behaviour and becoming the super hydrophobic surface with 155.55° contact angle.

These types of black thin sheets are of great value to general or space-based low-light imaging optical systems to block stray light as they do not add excess weight to the system. Our present invention can be of significant utilization in industrial applications for defence, space, Straylight control in optical devices, solar energy absorbers, solar heaters, thermoelectric generations, and artworks.

Further objects, features, and advantages of the invention will be readily apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.

Brief Description of the Drawings

The invention will be further understood from the following detailed description of a preferred embodiment taken in conjunction with an appended drawing, in which:

Fig. 1 illustrates the schematic diagram of Sagnac interference-based direct laser interference patterning (DLIP), in accordance with an exemplary embodiment of the present invention.

Fig. 2 illustrates the schematic representation of Sagnac direct laser interference patterning (DLIP), in accordance with an exemplary embodiment of the present invention.

Fig. 3 illustrates the FESEM images of the Sagnac interference pattern, subfigures (i), and (ii) represent the single line with three grooves pattern, and an interference pattern with overlapping of multiple scanning lines, respectively, in accordance with an exemplary embodiment of the present invention.

Fig 4 illustrates the images showing the surface morphology of the laser-patterned substrates, in accordance with our present invention;

Fig 5 illustrates the image showing the Cross-sectional view of laser patterned substrate, in accordance with our present invention;

Fig 6 illustrates the specular reflectivity for s polarized source of the laser patterned surface for broadband range (400 nm to 2500 nm), in accordance with our present invention;

Fig 7 illustrates the reflectivity of s and p polarized light on the sample, in accordance with our present invention; wherein the red and black lines represent the reflectivity of p and s- s-polarizations, respectively.

Fig 8 illustrates the reflectivity of the sample for the incident angle of 6 degrees, 15 degrees, 30 degrees, 45 degrees, and 60 degrees, in accordance with our present invention;

Fig 9 illustrates the diffused reflectivity of the laser un-patterned surface and diffused reflectivity of the Sagnac interference pattern-based sample (100 µm) , in accordance with our present invention.

Figure 10 illustrates the less reflectance using the thick (1.5 cm) SS sheet, in accordance with our present invention.

Figure 11 illustrates the Physical view of a work piece size of 30 mm *30 mm, in accordance with our present invention.

Figure 12 illustrates the method of measuring the contact angle of a single water droplet, in accordance with our present invention.

Reference Numerals
100 – 350 fs LASER, 200 KHz
101 – Neutral Density Filter (NDF)
102 –Mirror 1
103 –Mirror 2
104 – Mirror 3
105 –Mirror 4
106 – Mirror 5
107 – Focusing Lens
108 – Vacuum Mount
109 – Ø
110 – XPS
111 – Interference Patterns
112 – Beam Splitter
200 – 3-Line Sagnac Interference Pattern
201 – Deposited micro and nano-structures
202 – Microgrooves by interference pattern
203 – Stainless Steel Substrate

Detailed Description of the invention

It is to be understood that the present disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The present disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. In addition, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

The use of “including”, “comprising” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Further, the use of terms “first”, “second”, and “third”, and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another.

The embodiments disclosed herein can be expressed in different forms and should not be considered as limited to the listed embodiments in the disclosed invention. The various embodiments outlined in the subsequent sections are constructed such that it provides a complete and a thorough understanding of the disclosed invention, by clearly describing the scope of the invention, for those skilled in the art.

Throughout this specification various indications have been given as to preferred and alternative embodiments of the invention. It should be understood that it is the appended claims, including all equivalents, which are intended to define the spirit and scope of this invention.

The present invention relates to complex hierarchical micro–nanostructured surfaces which are fabricated by the use of an ultrafast laser on a thin SS sheet. Essentially a grid of 3D pyramid structures is fabricated these contain nanodots, flowers, and micro- and nanostructures. If the structures are subwavelength, they create a cavity to maximize the internal reflections of the photons and eventually get absorbed.

To reduce the reflectance over broadband, a more complicated structure is required. Here, we suggest two methodologies: (i) Boost interfacial absorption by producing a graded refractive index using a sub-wavelength structure, and (ii) Maximize internal reflection utilizing a light trapping structure, such as a hierarchical structure. In our present invention, complex hierarchical micro–nanostructured surfaces are fabricated by the use of an ultrafast laser on a thin SS sheet. Essentially a grid of 3D pyramid structures is fabricated these contain nanodots, flowers, and micro- and nanostructures. If the structures are subwavelength, they create a cavity to maximize the internal reflections of the photons and eventually get absorbed.

In one embodiment of our present invention, a 5 W fiber-based femtosecond laser (100) (Satsuma, Amplitude) with a central wavelength of 1030 nm, 200 kHz pulse repetition rate, and 350 fs pulse width is used to pattern the thin sheet of stainless steel (SS304). The patterning of the substrate was performed on three-dimensional (x, y, and ? (109)) Newport nano-positioner stages, which were controlled by an XPS (110) motion controller using a LabVIEW program, and the focusing lens was mounted on a vertical linear stage. The vacuum suction method (108) was used to mount thin samples perpendicular to the laser beam such that there would not be any surface modulations induced due to mounting.

In one embodiment of our present invention, the high-resolution, field emission scanning electron microscope is used as a tool to examine the surface morphology of materials (FESEM, Zeiss Ultra 55). The performance of the sample's surface antireflection was investigated using an Agilent Carry 6000 UV-Vis-NIR spectrometer and its UMA (universal measurement accessory) and DRA G9831A. Also, total reflectivity is measured with a 150 mm integrating sphere of LAMBDA 950, PerkinElmer. The contact angle measurements are performed using a commercial Goniometer (ADCAM-02, Apex instruments).

In one embodiment of our present invention, a 5 W fiber-based femtosecond laser (100) (Satsuma, Amplitude) with a central wavelength of 1030 nm, 200 kHz pulse repetition rate, and 350 fs pulse width is used to pattern the thin sheet of stainless steel (SS304). The patterning of the substrate was performed on three-dimensional (x, y, and ? (109)) (10) Newport nano-positioner stages, which were controlled by an XPS (110) motion controller using a LabVIEW program, and the focusing lens was mounted on a vertical linear stage. The vacuum suction method was used to mount thin samples perpendicular to the laser beam such that there would not be any surface modulations induced due to mounting. Figure 1 shows the Sagnac interference-based DLIP experimental setup. We can achieve the minimum groove size cut of ~13 µm groove cut by the DLIP method. The laser beam power is controlled via neutral density (ND) filters (101) and variable attenuators. Additionally, the polarization and energy of the laser beam are precisely controlled by combining a polarizer with a quarter wave ( ?/4) plate. On polished 100 µm thick SS sheets (203), the laser interference modulated beam (200) is used to pattern the surface. Using a convex lens (1.5 cm focal length), the fs laser beam is focused perpendicular to the substrate surface.

In one embodiment of our present invention, a scanning speed of 5mm/s, 15µm step size, 70.4 J/cm2 fluence, and double scanning (orthogonal scanning) method. By double scanning, we achieved an inverted cone kind of structure which has the best light absorption capability.

In one embodiment of our present invention, a 1.5 cm CaF2 convex lens is used as a focusing lens and ~37 µm focused footprint was obtained as seen from FESEM images, and has three grooves in it.

In one embodiment of our present invention, the step size between the successive lines was optimized to compensate for the depth difference between the center and the edges to get a uniform surface patterning over a large area.

In another embodiment of our present invention, Figure 4 illustrates the surface morphology of patterned micro-grooves and nanostructures on the SS by using Sagnac DLIP. With a 15 µm step size and 5 mm/s, the scans in orthogonal directions are performed to get the best grid-like structures, as shown in Figure 4. The noticeable thing is that this structure has more depth and a lot of space between both the nano and microstructures, which is essential to trapping the incident photons effectively.

Some of the properties of the present embodiment of our current invention are discussed below:
Depth of the patterned substrate: The grove width and depth play an essential role in suppressing the reflectivity of the incident photons by multiple reflections inside the micro and nanostructures and also on the mechanical strength of the thin sheets. The depth of the laser irradiation from the cross-sectional view of the FESEM images are analyzed for single-scan laser writing. For interference-based pattering with a restriction of three lines in the laser beam profile, the depth of the groove varying from 7 to 10 µm are observed. The average depth of ablation, in one embodiment of our present invention, is about 8.5 µm. and Sagnac interference-based patterning can do lesser damage to the material compared with single-spot ablation, naturally due to the energy distribution in the interference lines. Most area of the material thickness is intact (>95%) when we used the Sagnac interference beam profile patterning, hence the mechanical strength of the material will be superior. As these structures are manifestations of the same material having the same thermal coefficient, they can survive any number of thermal cycles of variation of temperatures, and their antireflection properties remain intact. This is not true in the case of coating with other materials on a substrate, which may degrade their performance with time, and coatings can fail their bonding strength after several thermal cycles of variations.

Specular reflectivity:
The specular reflectivity of the patterned substrate is recorded and shown in the figure 6. In one instance of the present invention, the reflectivity of the S-polarized beam is measured at an angle of incidence of 45°. Ultralow specular reflection <0.02% for the broadband spectrum from UV-Vis-NIR (400 nm to 2000 nm) are also observed.

Polarization insensitivity: The reflectivity of one embodiment of our present invention with s and p-polarized light, at the angle of incidence of 45° is studied. The polarization of the incident beam with respect to sample orientation is an important factor for reflection, especially for asymmetric systems. It is essential to understand the reflection of laser-patterned samples with different incident polarizations. For practical sunlight applications, ideally, the surfaces should show isotropic reflectivity with respect to incident polarization. Figure 7 shows the reflectivity dependence on the polarization of incident lights. The dashed lines and solid lines represent p and s-polarization reflectivity measurements, respectively. This shows these surfaces are isotropic and good for ordinary sunlight applications.

Angle dependence:
The laser-patterned surface of our present invention is studied for its reflectance spectrum for various angles of incidence. Figure 8 shows the reflection spectrum of the sample at incidence angles of 6°, 15°, 30°, 45°, and 60° when a P-polarized incident beam is used. The plot shows that the strength of the specular reflection does not change much up to 60°. Overall reflectance is practically less than 0.05% of the incident cone angle is less than 60°. For a wide cone angle of incidence of ±60°, the surface maintains its antireflective qualities, which is ideal for numerous practical applications.

Total reflectivity:
Diffused reflectivity measurement is necessary to quantify the performance of antireflective surfaces for practical applications. The high-resolution spectrometer with integrating sphere accessories is used to measure the diffused reflectivity. Reflectivity can be tuned by the microgroove width, depths, and types of nanostructures formed on the surface. Groove periodicity and aspect ratio play a prominent role in trapping incident light photons. The groove depth can be increased by decreasing the scanning speed or by increasing the fluence of the laser beam. Groove width can be altered by changing the spot size and distance between consecutive scanning lines. Figure 9, shows the integrating sphere measurement for the laser unpatterned stainless steel and the best-optimized sample. Over a broad bandwidth covering UV-visible-NIR (400 nm to 2000 nm), we achieved an average of 1.2% total reflectivity. Further, this average total reflectivity in the spectral region of 400 to 650 nm is 1.1% only, where many visible optical devices work. Figure 10 represents less reflectance using the thick (1.5 cm) SS sheet. The total reflectivity from 250 to 2200 nm is 1.022 %, while the average reflectivity from 400 to 800 nm is 0.85%.

Super-hydrophobic properties of micro/nanostructure surface:
Fig. 11 shows the water contact angle on the femtosecond laser treated. The contact angle of the surface is 155.55° at the 0° tilt angle. The wetting properties of the femtosecond laser-written metal surfaces do evolve with time, they generally have low contact angles initially. Due to the oxidation or chemical evolution of the surface structural properties, they slowly attain higher contact angles, typically in a couple of weeks they reach super hydrophobic. With a very small tilt angle the water droplet rolls from the surface, which indeed acts like a self-cleaning surface. This process is like mimicking the lotus leaf effect on artificially fabricated nanostructures.

This setup of our present invention is extremely stable and can be used for long hours to fabricate large areas without losing the phase relation between the interfering beams, guaranteeing a high-quality output, repeatable and reliable process.

Thus, our present invention can trap the photons incident on them by multiple reflections and absorb nearly 99% of the photons. An average total reflectance of 1.2% was achieved for 400 to 2000 nm optical bandwidth, with an average value is less than 0.02% of specular reflectance in the same spectral range. On demonstration, using laser ablation on a thin 100µm SS sheet (one embodiment of our present invention) displayed sufficient mechanical strength, thermal stability, and rugged nanostructures. The laser causes a change in the surface roughness of the Stainless steel leading to its super hydrophobic surface with 155.55° contact angle.

The patterned surface of one embodiment of our present invention is insensitive to the incident light polarization of s and p polarizations. This ultra-black surface of one embodiment of our present invention can work for a wide range of incident light cone angles of: ±60°. This thin sheet of antireflective surfaces of one embodiment of our present invention has good mechanical strength and thermal stability for practical applications, and also, they are self-cleaning superhydrophobic surfaces. These black surfaces can retain high temperatures up to 300°C without losing their performance.

These types of black thin sheets are of great value to general scientific research on light, particle physics, or space-based low-light imaging optical systems to block stray light as they do not add excess weight to the system. We believe these surfaces are of great value to many industrial applications in defense, space, straylight control in optical devices, solar energy absorbers, solar heaters, thermoelectric generations, and artworks.

The mechanical strength, thermal stability, and self-cleaning ability (superhydrophobic) of the fabricated structures of one embodiment of our present invention make them suitable for industrial applications and solar energy harvesting.

The patterned surfaces of one embodiment of our present invention are insensitive to incident polarization and have a wide range of acceptance to the incidence cone angle, making them ideal for practical applications of using sunlight.
, Claims:CLAIMS
I/We Claim:
A broadband ultralow reflective stainless-steel sheet, wherein complex hierarchical micro–nanostructured surfaces are fabricated by the use of an ultrafast laser on a thin SS sheet by creating a grid of 3D pyramid structures which contain nanodots, flowers, and micro- and nanostructures, wherein the process of fabrication of the ultralow reflective stainless-steel sheet is given by:
5 W fiber-based femtosecond laser (100) (Satsuma, Amplitude) with a central wavelength of 1030 nm, 200 kHz pulse repetition rate, and 350 fs pulse width is used to pattern on a 100-micron thin stainless-steel sheet (SS304).
The patterning of the substrate was performed on three-dimensional (x, y, and ?) Newport nano-positioner stages, which were controlled by an XPS (110) motion controller using a LabVIEW program, and a 1.5 cm CaF2 convex lens (focusing lens (107)) was mounted on a vertical linear stage
The vacuum suction method (108) was used to mount thin samples perpendicular to the laser beam.
Sagnac-based interferometer based DLIP is used to create three fringe patterns (200) in the beam profile to achieve a groove cut of 13 µm.
The laser beam power is controlled via neutral density (ND) filters (101) and variable attenuators.
The polarization and energy of the laser beam are precisely controlled by combining a polarizer with a quarter wave ( ?/4) plate.
On polished 100 µm thick SS sheets, the laser interference modulated beam is used to pattern the surface (202).
Using a convex lens (107) (1.5 cm focal length), the fs laser beam is focused perpendicular to the substrate surface (203).

The broadband ultralow reflective stainless-steel sheet, as claimed in claim 1, wherein no external coatings are involved in the fabrication process, and surface modifications (201) are done by groove cutting with the depth of the groove under 10 µm in stainless steel surface (203).

The broadband ultralow reflective stainless-steel sheet, as claimed in claim 1, wherein double scanning (orthogonal scanning) method with a scanning speed of 5mm/s, 15µm step size, 70.4 J/cm2 is used in the fabrication process.

The broadband ultralow reflective stainless-steel sheet, as claimed in claim 1, wherein the fabricated anti-reflective stainless-steel sheet shows ultralow specular reflection <0.02% for the broadband spectrum from UV-Vis-NIR (400 nm to 2000 nm).

The broadband ultralow reflective stainless-steel sheet, as claimed in claim 1, wherein the fabricated anti-reflective stainless-steel sheet shows reflectance less than 0.05% of the incident cone angle is less than 60° and for a wide cone angle of incidence of ±60°, the stainless-steel sheet surface maintains its antireflective qualities.

The broadband ultralow reflective stainless-steel sheet, as claimed in claim 1, wherein the fabricated anti-reflective stainless-steel sheet shows average of 1.2% total reflectivity for broad bandwidth covering UV-visible-NIR (400 nm to 2000 nm), and an average total reflectivity of 1.1% in the spectral region of 400 to 650 nm.

The broadband ultralow reflective stainless-steel sheet, as claimed in claim 1, wherein the fabricated anti-reflective stainless-steel sheet has a super hydrophobic surface with 155.55° contact angle. These surfaces exhibit self-cleaning properties similar to the lotus leaf’s in the nature.