Fast microstructuring of silica glasses surface by NIR laser radiation

Optics and Lasers in Engineering 68 (2015) 16–24

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Fast microstructuring of silica glasses surface by NIR laser radiation G.K. Kostyuk, M.M. Sergeev, R.A. Zakoldaev n, E.B. Yakovlev ITMO University, 197101 Saint Petersburg, Russian Federation

art ic l e i nf o

a b s t r a c t

Article history: Received 18 August 2014 Received in revised form 26 October 2014 Accepted 1 December 2014

The glass surface microstructuring technology using laser radiation with NIR wavelength (λ ¼1.064 μm) was revealed in this work. Glass plates were placed on the cellular graphite surface. Focused laser radiation passed through the glass plate and interacted with cellular graphite. The radiation heated the graphite surface and thus the high temperature influenced the back side of the glass plate. After consecutive laser scans, having certain periods and interruptions of laser radiation, the microstructures with depth  0.5 μm were formed. Besides, in this work we suggested the method to calculate optical characteristics of formed elements. It was experimentally shown that these microstructures could be used to form phase diffraction gratings (PDGs) and random phase plates (RPPs). We experimentally demonstrated the possibility of these elements being used as RPPs which are suitable for multimode laser radiation homogenization and as PDGs which are suitable for laser simultaneous processing of metal films. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Laser micromachining MOEs Fused silica LIBBH.

1. Introduction One of the trends in the development of modern laser technology is the glass surface structuring and properties changes. Micro-optical elements (MOEs) creation is based on these technologies. MOEs are used in optical communication systems [1,2], image multiplexing and object identification [3,4], high-quality image building systems [5,6], as well as small sections of the wavefront detection systems in adaptive optics [7]. In laser systems, such MOEs are widely used for laser beams homogenizing [8–11], for laser beam splitting into several equal beams [12,13], for power control of laser radiation [14,15], as well as in medical equipment [16–18]. It is important, that MOEs structures are divided into two types: periodical MOEs based on the Fraunhofer diffraction principle, and typical MOEs based on the laws of light refraction and reflection. Periodical MOEs may be used as phase diffraction gratings (PDGs) able to create a high and equal intensity of the first diffraction orders [19,20]. It is necessary for the simultaneous laser processing of materials [13]. Besides, they are used as random phase plates (RPPs) known as homogenizers for multimode laser radiation with the inhomogeneous structure of the beam [10,11]. More than 30 years ago, first attempts to form MOEs on the glass surface influenced by CO2 laser radiation [21,22] were taken. The MOEs formation in this case is based on thermal effects, which were caused by laser heating of the glass surface [23–27]. The MOEs size in this technology is not less than 100 μm. Nowadays,

n

Corresponding author. E-mail address: [email protected] (R.A. Zakoldaev).

http://dx.doi.org/10.1016/j.optlaseng.2014.12.004 0143-8166/& 2014 Elsevier Ltd. All rights reserved.

the physical processes based on CO2 laser micromachining have been sufficiently studied, what made it possible to create different structures on the glass surface. Laser radiation with a wavelength of the UV range (λ o0.3 μm) is usually used [28] to reduce the size of MOEs. This wavelength is fundamentally absorbed by the majority of siliceous materials. Furthermore, lasers with nanosecond pulse duration are also used [29,30]. In this case, the ablation of the glass surface takes place [29–31]. An interference scheme [29] was used to form MOEs sized 0.1–0.3 μm. The sample was heated up to the glass softening temperature that is 480 1C, to decrease the possibility of cracking [30]. The lasers with femtosecond pulse duration are an integral part in the glass surface structuring technology [31–33] based on the ablation process. Laser processing in combination with the chemical etching (HCl, HF, HNO3, etc.) may also be used to create MOEs with micrometer precision [34]. Another technology is based on strong absorption of laser radiation with aqueous solution [35,36] or organic material [37–57], and molten metal [38] placed in contact with the back side of the glass plate. It is used to create microstructures on silicate glass surface as well as other optical transparent materials. Basic principles of these technologies are united under the title LIBWE (laser-induced backside wet etching) and are described in works [42–46] by the research group headed by H. Niino, etcetera. At the same time, the following scientists: G. Kopitkovas [47–51], K. Zimmer [37,38,52–54] and J.Y. Cheng [55–57] were also involved in the development of LIBWE technology. Physical process is shown in this work [58]. The key feature of this process is a high temperature and pressure jump appearing while in contact with glass-liquid due to the strong absorption of the incident laser beam. As a result, shock waves take

G.K. Kostyuk et al. / Optics and Lasers in Engineering 68 (2015) 16–24

place, forming the relief on the glass surface. LIBWE technology allows to create refractive and diffractive optical elements sized less than 1 μm and nanometer rough surface. Besides, the NIR lasers sources have been used in this technology lately [59]. This source does not require a special lens that is transparent to UV radiation. Another technology is laser-induced backside dry etching (LIBDE) of transparent materials. It is based on strong absorption of laser radiation by solid material, so it is close to LIBWE technology [38]. Opportunities of LIBDE technology to surface microstructuring of transparent materials are not inferior to LIBWE technology. The development of new and improved technologies for the creation of microstructures on glass materials is important. In this article, we reported a new laser technology for surface glass modification – lased-induced black-body heating (LIBBH). It allows using any required laser with wavelength, which is transparent for the glass. The technology is based on heating the glass by cellular graphite plate being in direct contact with the back glass surface. In previous years, we used the LIBBH technology to prevent the “ageing” process of porous glass (PG). We used PG as a glass material to create MOEs arrays on the surface [22–24] and in the bulk of it [60]. Unfortunately, the “ageing” process is quite typical to these types of glass. As a result, the optical and the physical-chemical properties change. It happens due to the absorption of water, carbon dioxide and organic compounds from the air. The densification layers were designed to protect the PG structure from ageing process. Besides, the stabilization of PG optical characteristics is important to create the matrices for nanocomposites [61–64]. The creation of densification layers was carried out through sequential scanning of laser beam in contact with PG-graphite plate. In this article we used LIBBH for optical glass surface microstructuring: fused silica (KU-1), which has a high transparency in a wide wavelength range of 0.2–2.5 μm, considerable beam stability and maximum thermal and chemical resistance of all known glass types [65]; borosilicate glass (BK-7) and uviol glass (UV-1).

plate (6) and the cellular graphite plate (7). The glass plate and the cellular graphite plate were on a table (8). The galvanometer scanator (4) and the parameters of laser radiation were controlled by a personal computer (9). It is important to note, that the all types of glass plates using in the experiment are completely transparent τ 0.96 for the incident radiation and the absorptivity of cellular graphite in a wide range of wavelengths is close to 1.0 [66]. That's why, focused laser radiation passed through the glass plate and interacted with cellular graphite. The hollows were wormed on the back side of the glass plate. After consecutive laser scans having certain periods and breakings of laser radiation, PDGs and RPPs were formed. Details of laser processing with various MOEs are listed in Table 1. In these conditions MOEs were formed without cracks and furrows. After laser proceeding graphite particles covered the glass plate. Thus, it was decided to clean it by laser radiation on the same experimental setup. The laser cleaning process was implemented in the following way. The glass plate was turned over and thin water layer was deposited on the dirty area. Laser radiation was focused on the place of water layer with contaminated area. The glass plate with MOEs was cleaned after successive scans of laser beam in contact with water-graphite particles. The optimal characteristics of laser irradiation were determined in experimental studies. The best cleaning result was achieved by the progressive scan of laser beam at a speed 3500 mm/s, pulse repetition 20 kHz, average power 8 W, pulse duration of 200 ns and the overlap factor was 25%. The PDGs with the following characteristics were formed: period p ¼30–100 μm, ratio S/p ¼ 0.30–0.55 (where S is the size of the modified lines) and the depth hPDG ¼0.5–1.25 μm were formed. The RPP with elements of a square shape with a size de ¼0.2–0.7 μm were created as well. In order to achieve the phase shift for passing through the laser beam with wavelength λ the RPPs depth was calculated [10]: hλ 

2. Experimental The formation of microstructures was carried out on the planeparallel plates of polished KU-1, BK-7, thickness 1.5 mm, and on UV-1 ( 75% SiO2), thickness 1.7 mm. All glasses plated used in the experiment have the surface roughness equal Ra ¼10 nm. Fig. 1 gives the scheme of experimental setup. We used ytterbium fiber laser with wavelength of 1.064 μm, pulse duration: 100–200 ns, pulse repetition rate: 20–100 kHz as a radiation source. Laser beam was scanned with a two-coordinate galvanometer scanner, based on G325DT «GSI Lumonics» drivers. Laser radiation (1) goes through optical fiber (2) and leads to the collimator (3) expanding the laser beam from 0.05 mm till 8 mm. A galvanometer scanator (4) scanned laser beam with the speed of 100–3500 mm/s. Then a telecentric scan lens (5) focused laser beam to the contact between the glass

17

λ ; 2ðn  1Þ

ð1Þ

where n is the refractive index of glass. For the detailed study we chose the PDG with the period p¼100 μm, the ratio S/p¼0.5 and the depth hPDG ¼0.45 μm. In Table 1 Characteristics of laser processing for forming of different microstructures. Average radiation power Pav, W

Type

Mark of Pulse glass repetition frequency ν, kHz

Scan speed υ, mm/sec

Phase diffraction gratings (PDG) Random phase plates (RPP)

KU-1 BK-7 UV-1

50 50 20

100–300 6–15 150–500 5–13 150–300 5–12

KU-1

50

100–300 6–10

1–5

BK-7

50

150–400 7–13

1–5

UV-1

20

100–300 6–12

1–3

Overlap Number of factor, % sequential scans

98

1–5 1–5 1–3

Table 2 Parameters of RPP on the surface of fused silica.

Fig. 1. Experimental setup for glass surfaces microstructuring.

№

dRPP, μm

hλ ¼ 635 nm

hλ ¼ 1064 nm

1 2 3 4

250 340 500 680

695 690 685 700

1195 1190 1200 1185

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addition, we chose four RPPs with parameters listed in Table 2. In Table 2 the value hλ depends on the wavelength that will be passed through the RPP, that is, hλ ¼ 635 for laser beam with wavelength of λ¼0.635 μm and hλ ¼ 1064 for wavelength λ¼ 1.064 μm correspondingly. PDGs and RPPs were made on fused silica as the best optical material possible. We studied the MOEs on the glass surface using an optical microscope Carl Zeiss Axio in transmitted and linearly polarized light with crossed polarizer and analyzer. The morphology of the PDG was investigated using an atomic-force microscope (Solver PRO-M) with maximum measured area of 165 μm. The depth of PDG was investigated using a profilometer (Hommel Tester T8000). The PDG and RPP optical characteristics were investigated by laser module with λ¼ 0.635 μm. The intensive distribution of the laser beam was registered by CCD camera (Gentec Beamage). 3. Results and discussion Cellular graphite is a high-carbon compound, characterized by low density. It appeared during all experiments of PDGs and RPPs formation. Fig. 2 shows the appearance of cellular graphite during the glass micromachining process. It is known that the appearance of cellular graphite shows that the temperature in the area of radiation could reach more than 1000 1С [66]. Evaluation of maximum temperature in the center of the graphite surface in the irradiation zone, with the assumption that all of the absorbed laser energy was converted into the heat, can be assessed by the expression [67]: pffiffiffiffiffiffiffi 2Ac q0 ac τ pffiffiffi þ T H ; Tc ¼ ð2Þ kc π where Ac – graphite absorbability (Ac  1.0), ac – graphite heat diffusivity (ac ¼ 1.24∙10-4 m2/s), kc - graphite thermal conductivity (kc ¼2000 W/(m∙K)), τ – laser pulse duration (τ ¼200 ns) and q0 – laser power density in the focal spot (q0  7,7∙107 W/cm2 at average laser power Pav ¼8 W and pulse repetition rate ν ¼20 kHz). At pointed data Tc can reach up to  2∙103 К and even more. The calculated maximum surface temperature values of cellular

graphite depending on glass types are show in Table 3. The values of A, R, k, a for each glass and typical values of Pav, υ, τ are shown in the same table. One impulse of absorbed radiation energy taken from the range of possible values used in the experiment showed enough energy to heat the graphite surface up to the evaporation temperature. This brings us to the conclusion that the part of the absorption energy was spent to kinetic energy of graphite particles flying with finite rate during plasma plume formation. It was suggested to calculate the particles separation velocity through the energy conservation law. The suggestion was made that the absorbed radiation energy spent at graphite heating up to the evaporation temperature and the total kinetic energy of all flying particles removed from the surface with average speed. Take into account the pulse duration we supposed that the material mass in the area of radiation broke away in form of the graphite particles during the ablation process. Having done the assumption, we could calculate the graphite particles average speed from the area of radiation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   8P av υp   2Lu ð3Þ 2 ν2 τρπd0 h where ρ – graphite density (ρ¼ 2265 kg/m3), d0 – radiation area (d0 ¼55∙10-6 m), h – the removable depth in graphite plate (h¼5 μm) and Lu – graphite vaporization heat (Lu¼ 50∙103 J/kg) [66]. Having the average power of Pav ¼8 W, the graphite particles average speed could reach 7.0∙103 m/s. The temperature was above the glass softening temperature, so the change of surface topography was ensured by mechanical treatment of the graphite particles flying with high speed and temperature on the glass surface. The process of microstructures formation on the glass plates is determined by not only laser radiation parameters but the physicalthermal characteristics of glass as well. This implies that the main glass physical-thermal characteristics (Table 4) such as its transparency, optical homogeneity, thermal stability, density and melting temperature were influenced on the microstructuring process. The Table 4 shows the main properties of different glass types used in the experiment. From the results shown in Table 3 it is evident that the microstructuring of different types of glasses occurred under the influence of different laser power. For fused silica average power Pav ¼8 W and scan speed υ300 mm/s were optimal. In this regime, microstructures were formed without cracks and furrows. If we used other types of glasses (BK-7 or UV-1) and such regime, a lot of cracks and furrows were formed along the microstructure and sometimes it led to complete destruction of the glass plates. Fig. 3 shows photographs of fragments: (a) – PDG (p ¼100 μm, ratio s/p ¼0.5, hPDG ¼ 0.45 μm), (b) – RPP (de ¼500 μm, hRPP ¼ 0.685 μm) were made by optical microscope. Fig. 3 «c» and «d» are photographs of the same fragments made in a linearly polarized light with crossed polarizer and analyzer. Dark background of both images indicates of thermomechanical stresses absence around microstructures. As to the MOEs surface roughness the central part was not influenced by the laser radiation and as a result has the roughness equal to polished glass plate (Ra ¼10 nm). The lateral area roughness

Fig. 2. Photo of cellular graphite appearing during laser micromachining. Table 4 The main physical-thermal characteristics of glasses used in this experiment. Table 3 Types of glass and regimes of laser micromachining. № 1 2 3

Mark of glass KU-1 BK-7 UV-1

A 0.01 0.01 0.01

R 0.035 0.041 0.055

Pav, W 8 7 6

ν, kHz

τ, ns

q, W/cm2 7

20

200

7.75∙10 6.68∙107 5.56∙107

Density, g/cm3

Tmax, K

№ Mark of glass

Thermal conductivity, W/(m  K)

Melting temperature, 1C

Refraction index, at λ¼ 1.064 μm

Heat capacity, J/(kg∙1C)

2.4∙103 2.1∙103 1.8∙103

1 2 3

2.20 2.43 4.15

1.35 1.13 1.04

1665 657 800

1.4496 1.5066 1.6941

728 870 760

KU-1 BK-7 UV-1

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Fig. 3. The light microscope image of PDG with p ¼100 μm, s/p ¼ 0.5 (a, c) and RPP with de ¼ 250 μm (b, d) formed on the fused silica surface. The pictures made in transmitted light (a, b) and linearly polarized light with crossed polarizer and analyzer (c, d).

Fig. 5 shows the profile of PDG which has been shown in Fig. 3. The received PDGs have small depth (hPDG ¼0.05–1.25 μm) due to short pulse duration (τ¼200 ns) and to the fact that the heating of the glass was caused only by the heat moving from the graphite plate into the glass plate. It is possible to improve the MOEs quality by treatment heat process in a furnace at the temperature of glass melting. Such way was used in work [12] to receive improvement in quality and defuse tension of MOEs. Another way to improve the MOEs quality was used in work [34]. Chemical etching after laser micromachining was used to reduce. Fig. 4. Experimentally measured PDG morphology using an atomic-force microscope.

4. The PDG investigation

was changed due to the action of plasma and flying graphite particles on the fused silica surface. In this case the lateral area roughness value could reach Ra ¼30–50 nm (Fig. 4). The figure shows the scanning probe microscope result. The roughness value of microstructures, which were created by LIBBH is a bit higher than the value of Ra, which were reported in LIBDE or LIBWE technology.

Determination of PDG intensity distribution was performed on the experimental setup and is schematically shown in Fig. 6 «a». Experimental setup consisted of: a laser module (λ¼0.635 μm, P¼0.1 W) (1), a diaphragm (2), a telescopic system (3) with 4x increase, as a result the radiation beam was expanded to diameter D 10 mm. The PDG (4) was placed behind the telescopic system (3).

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Fig. 5. Experimentally measured surface profiles of PDG using a profilometer: «a» with p ¼100 μm and «b» with p¼ 50 μm.

Fig. 6. Experimental setup for measure of PDG (a) and RPP (b) optical characteristics.

immediately behind the diaphragm (5). A CCD camera (7) was posed in the focal plane of the collective lens (6). Fig. 7 shows the intensity distribution of few first orders of the PDG in the focal plane of the collective lens. The figure shows that the intensity of 71 order is of 0.98 intensity of zero order. It is evident, that tested PDG created three equal diameter spot size (on level 1/e2). The measured diameter is dspot 60 mm and the distance between them is l  1200 μm. Calculated in accordance with the theory of Fraunhofer diffraction [68] (d’spot ¼ 48 mm and l’ ¼ 1210 μm), the diameters of the spots and the distance between them showed satisfactory agreement coincidence with the experimental results.

Fig. 7. Intensity distribution in the focal plane of a collective lens for the first few orders of the PDG.

A square aperture (5) with size of dD ¼ 2 mm was placed behind the PDG (4), what allowed to put out higher diffractive orders with low intensity. Collective lens (6) (dl ¼50 mm, f¼ 190 mm) was located

5. The RPP investigation Homogenization of inhomogeneous laser beam using RPP was held on the experimental setup, which is shown in Fig. 6 «b». The beam of laser module (λ¼ 0.635 μm, P¼0.1 W) (1) was expanded by a telescopic system (2) with an increase 4x, that increased the laser beam up to D  10 mm. A thin glass plate with a thin wire (dwire ¼ 1.5 mm) (3) was placed behind the telescopic system (2).

G.K. Kostyuk et al. / Optics and Lasers in Engineering 68 (2015) 16–24

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Fig. 8. Intensity distribution of the RPP in the focal plane of a collective lens (a) and at a distance of 4.25 mm from focal plane (b).

Fig. 9. Intensity distribution in the focal plane of a collective lens of the RPPs with different size of element de  250 μm (a), de  340 μm (b) and de  680 μm (c).

The thin wire was used to break intensity distribution over the beam cross-section. The RPP (4) was placed after the glass plate (3). Collective lens (5) (dl ¼50 mm, f ¼190 mm) was located immediately behind the RPP (4). The CCD camera (6) was placed so that it could be moved along the optical axis with an accuracy of 71 μm. Fig. 8, «a» shows the intensity distribution in the focal plane of the collective lens in the position z0 as in Fig. 6 «b». The RPP with de 500 μm was used. Fig. 8 «b» shows the intensity distribution in the positions z1 and z2. The difference between those positions (z1 ¼-z2) and the positions of the lens focal plane z0, was 74.25 mm. Distance of Δl¼z1 – z2 ¼8.5 mm, where the intensity distribution had a uniform, was compared with the calculated distance [10]: Δl ¼

4λf Dl

2

2f λ ; de

dspec ¼

ð4Þ

ð5Þ

The calculated value cross section of the homogenized beam is dp’ ¼480 μm. It coincides with experimental results. Due to the diffraction of the elementary beams on the square elements, high-frequency speckle took place in the focal plane of the collective lens. The diameter of the bright spots of high-frequency

2f λ ; Dl

ð6Þ

Calculated according to (6) (dspec’  30 μm) and experimentally determined (dspec ¼3575 μm) results are in a satisfaction agreement coincidence. From earlier work of RPP used as homogenizers of laser beams, it is known that the smaller size of the elements homogenizer provides a better level of homogenization. The number of elementary beams interfering in the focal plane of the collective lens could be defined as [11]: N

where Dl – diameter of the beam cross section in the plane of the RPP, Δl¼7.54 mm. The coincidence of the calculated and experimentally determined distance Δl is satisfactorily. Outside the value Δl in planes z1 and z2 the inhomogeneity of the intensity distribution across the beam (created by glass plate with thin wire (Fig. 8 «b») is clearly visible. The measured size in the focal plane of the cross section of the homogenized beam dp ¼445 μm (when we used RPP with size of square element de ¼500 μm) was calculated [11]: dp ¼

speckle (dspec) could be estimated from the expression [10,11]:

Dx Dy ; dx dy

ð7Þ

where Dx and Dy – cross section of the inhomogeneous laser beam in the x and y planes, respectively, dx, dy – dimensions of RPP element in the x and y planes, respectively. In our case, for RPP with de ¼500 μm, the number of elementary beams interfering in the focal plane of the collective lens was N¼256. Besides we change de ¼ 250 μm – N¼1024; for de ¼340 μm – N¼ 554; for de ¼680 μm – N¼138. The difference are shown in Fig. 9. From experiments with RPP on laser beam homogenizing, it was found out that the decreasing size of elements (de) led to the increasing number of illuminated spots in the homogenizing area and to their brightness degradation. It is connected with the interference in the focal plane of collective lens. In this experiment we could not identify the best result for the homogenization of the laser beam with artificial inhomogeneity. That's why we decided to test our RPPs for the homogenization of highly inhomogeneity beam with picosecond laser. The experiment on the homogenization of picosecond laser radiation was held on the experimental setup, which is shown in Fig. 10 «a». Experimental setup consisted of: picosecond laser (Ekspla PL 2143/SH/

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TH/FH) with λps ¼1.064 μm (1) with beam diameter of dps  8 mm. The RPP (2) was installed behind collective lens (3) (dl ¼ 60 mm, f¼ 300 mm) to create a π phase shift, with hRPP ¼ 1.1970.01 μm. The homogenizing quality of RPP (3) was determined by the processing of polished steel plate (4), which was placed in the focal plane of the collective lens (3). As a result of the experiment, all types of RPP, with different size of element de, rectified the ps laser beam inhomogeneity. The best result was achieved using the RPP with de ¼250 μm. This result is quite correlated with the results of work [10,11]. Fig. 11 shows the photographs of the stainless steel plate surface, which were made in a dark field. Fig. 10 shows “a” laser machining of steel plate surface without the RPP and Fig. 10 “b” – with the RPP (de

 250 μm). From the comparison of these two pictures it is evident that the usage of the RPP made the intensity of the beam distribution in the plane homogenization more homogeneous.

6. PDG testing for titanium films laser simultaneous processing Testing phase grating for processing of titanium film on a substrate of glass BK-7 was held on the experimental setup, which is shown in Fig. 10 «b». We used fiber laser radiation with λ¼1.07 μm (1), the laser beam was expanded by a telescopic system (2) with increasing 4x. The PDG (3) (p¼100 μm, ratio S/p¼0.5 and depth

Fig. 10. Experimental setup for RPP (a) and PDG (b) test.

Fig. 11. Picture of the modified surface of stainless steel treated without (a) and with RPP (b).

Fig. 12. Picture of three holes made on the Ti film by PDG laser simultaneous processing.

G.K. Kostyuk et al. / Optics and Lasers in Engineering 68 (2015) 16–24

h¼0.45 μm) was placed behind the telescopic system (2). Aperture (4) (circular or square shape) with a size (dapert ¼ 2 mm) was used to remove a high diffraction orders with a low intensity. Collective lens (5) was placed behind the diaphragm (4) its parameters were given before. The titanium film on the glass substrate (6) with film thinness hTi ¼ 60 nm was used as a target for laser micromachining. Fig. 12 shows three holes in the titanium film created by laser radiation having passed through the PDG in the focal plane of the collective lens. Laser processing parameters were the following: laser power was Pyt ¼ 1 W and exposure time was τ¼1 s. A smooth outline border of the holes in Fig. 12 shows the success of the experiment on simultaneous processing of the film. The usage of the PDG was reported in the work [13], where the advantages of PDG in comparison with an array of microlenses were marked. First of all, that microlens arrays had a spaced region between microlenses, what could create additional flash exposure on the machining plane. The second distinctive feature is that in most cases laser beam has a Gaussian intensity distribution profile, as a result incident radiation irregularly illuminates the microlens array. In such a manner, design and creation of MOEs, which can create equal and small spots, that is, suitable for simultaneous processing of materials – is a complex and difficult task. Therefore, we would like to emphasize that our PDGs only demonstrates the capabilities of the LIBBH technology of its creations and usage for simultaneous materials processing. Today, exist studies [69,70] where technology of direct laser writing on chromium films and amorphous silicon, allow forming structures with periods ranging from 0.6–1.5 μm. In some LIBWE technologies were reported about periodic structures with period p 1 μm. Small size and small period of these structures exists not only due to specials schemes of the technology, but the usage UV laser radiation. A significant reduction in the size and period of microstructures, which will be created by our technology, is possible, even if in a less degree, because of NIR laser radiation usage.

7. Conclusion We experimentally demonstrated LIBBH technology for MOEs formation on the glass surfaces. As a part of MOEs we created PDGs with p ¼ 30–100 μm, S/p ¼0.30–0.55 with depth hPDG ¼ 0.5– 1.25 μm and RPP with elements of a square shape with size de ¼0.2–0.7 μm with depth hPDG ¼ 0.45 μm. The time formation of these MOEs ranged from up 5 to 20 min depending on the elements square. The technology of MOEs optical characteristics measured was suggested. Our results were compared with the theoretical calculation and showed a satisfactory coincidence for both types of MOEs. Besides, the PDG was used for the simultaneous processing of a titanium film deposited on the BK-7 glass surface. The RPP was used to homogenize the picosecond laser beam, with strong heterogeneity distribution across the beam.

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