WO2022185096A1

WO2022185096A1 - Laser beam transforming element

Info

Publication number: WO2022185096A1
Application number: PCT/IB2021/051763
Authority: WO
Inventors: Antanas URBAS; Vytautas JUKNA
Original assignee: Uab Altechna R&B; Valstybinis Moksliniu Tyrimu Institutas Fiziniu Ir Technologijos Mokslu Centras
Priority date: 2021-03-03
Filing date: 2021-03-03
Publication date: 2022-09-09

Abstract

The invention relates to optical elements intended to modify a linearly polarized laser radiation and can be used for material processing. Said element comprises a substrate of transparent material, with the entry and exit planes. The nanostructures inscribed in or on the said optical element are geometric nanostructures of Pancharatnam-Berry phase with a continuous variation of their orientation angle, which defines polarization angle in each point of the beam cross-section as a function ϑ =f(ϕ, ψ), where ϕ - the azimuth angle changing from -π to π and ψ is a parameter describing the shape of the beam. Said element is configured so that the homogeneously polarized laser beam impinging onto said element exits the element such that at least two points of the beam transversal cross section in X-Y plane present opposite X or Y components of polarization vector.

Description

LASER BEAM TRANSFORMING ELEMENT

Technical field to which invention relates

The invention relates to optical elements intended to modify a homogeneously polarized cylindrically symmetric laser radiation beam incident on it by forming a distribution of the energy having rotational symmetry order 2 at the exit of the optical element, due to the effect of the geometric phase (Pancharatnam-Berry) metasurface created in the plane transversal to beam propagation and can be used in the field of material processing, for example, for the processing of transparent media, including glasses, related to use of a laser radiation beam for the modification of material in a workpiece.

Indication of the background art

Most known methods of laser cutting, dicing, or scribing of transparent media are based on the formation of a crack contour followed by the separation of parts of the workpiece along that crack contour. That contour is formed by beams, in which the distribution of energy is non-cylindrically symmetric and has a pronounced direction in which the dimension of the distribution is larger than one along the direction perpendicular to it. Such beams can be formed by various optical elements placed in the path of the laser radiation beam having cylindrical symmetry.

In descriptions of patents KR100991720B1 (03-11-2010) and EP2965853B1 (21-09- 2016) asymmetric distributions in the focal area are obtained by using aspheric

(cylindrical) optics, which elongates the distribution along a direction perpendicular to the axis of the cylindrical lens. Optical schemes with such elements are easily built and the distributions they create are accurately described in theory. However, when focusing the beam with such elements, the dimensions of the focal area along the direction of light propagation and perpendicularly to it are of the same order of magnitude. Therefore, when focusing light in the volume of material, the area of damage is small in size, usually several micrometers in all directions. Therefore, several passages are required at different depths in order to form the separation contour, which increases the processing time several times and reduces the smoothness of the cut along the separation contour. The patent applications WO2016/079257A1 (26-05-2016), US2017/0157700A1 (08- 08-2017), US2018/0093914A1 (05-04-2018) describe the formation of a focal area elongated in the direction of light propagation by using non-diffractive beams, the distribution of which is elongated in a chosen direction in a plane perpendicular to the direction of propagation. By focusing such beams in the volume of the workpiece, the above-mentioned need to pass the separation contour at several depths is eliminated, thus the speed of the process is significantly increased. Patent applications WO2016/079257A1 , US2017/0157700A1 , and US2018/0093914A1 create an elongated focal area by using several different optical elements such as conical lenses (axicons), aspheric optics, spatial filters, phase masks, phase plates, and sectorized phase plates. Such optical schemes require extremely precise alignment because they are very sensitive to the accuracy of element positioning. Furthermore, the quality of each element has a significant impact on the result, thus it is very important to minimize the number of elements required to form the desired distribution of energy.

In the patent US10730783B2 (case with a segmented waveplate) and patent application W02020230064A1 , there are non-diffractive beams with a distribution of rotational symmetry order 2 formed. In US10730783B2, the beam with an elliptical cross section in the transversal plane is created by using a quarter-wave phase plate, an axicon, a 4f optical scheme, with a segmented quarter-wave plate placed in its Fourier plane. Along with above said drawbacks, this scheme also forms two separate beams with different polarizations by using quarter-wave plates. These cause pulse delays, different in different parts of the beam, and as stated in the description of the invention, eliminate the coherence of those two parts of the beam. These parts of the beams are then put back together, and the radiation phases of the beams no longer correlate with each other, i. e. the light in the distribution of the total beam becomes incoherent. This incoherence becomes a drawback in cases when further manipulations with radiation, e.g., beam vectorization, are required, and when ultrashort pulses of hundreds of femtoseconds are used. Also, converting a part of the beam with a quarter wavelength plate causes diffractive losses due to the edge of the plate, which brings additional distortions of the beam, as well as phase irregularities, which later affect the result of micro-processing. The closest to the proposed technical solution is the material processing method and device described in the patent application W02020230064A1 , priority date 15-05- 2019, utilizing an optical element intended to modify a linear polarization laser radiation beam incident on it by forming a non-diffractive laser beam with an energy distribution of non-cylindrical symmetry, at the exit of the optical element, due to the introduced geometrical phase, which rotates laser radiation polarization vector in each point of radiation cross-section by an angle characteristic to a particular point of that plane, where the optical element comprises: a substrate of transparent material, with the entry and exit surfaces comprised of entry and exit planes arranged one in opposite to the other in parallel; a structural modification resulting in the volume of the said transparent material optical element between the entry and exit planes or on its surface. This structural modification contains periodic nanostructures, angularly oriented on a plane, perpendicular to the light propagation direction, while function describing the angular orientation of said nanostructures with respect to neighbouring ones is uniform across an angular or annular segment of the element. The nanostructures inscribed in or on the said optical element introduce Pancharatnam-Berry phase variation in the passing light and consequentially, the pattern of the orientation of nanostructures defines the pattern of polarization vector directions in the beam exiting the element. The optical element described in this application creates a distribution with a zero-intensity area along the direction of light propagation. If there is a wish to obtain a distribution without said zero-intensity area, an additional element, such as a quarter-wave phase plate, must be placed in the optical scheme, which makes this scheme more complex. Furthermore, the coherence length of the laser pulse is important to this scheme, which limits the choice of lasers suitable for use with this element.

Technical problem to be solved

The invention aims to expand the possibilities of the element use when compared with the one described in the patent application W02020230064A1 and to simplify the optical scheme used by patent US10730783B2. The proposed element can be used in both optical schemes focusing Gaussian beam and ones generating non- diffractive beam. The use of this element makes it possible to simplify the optical scheme of beam formation and delivery by reducing the number of components in it, while at the same time allowing more flexibility when choosing the components of that scheme. Suggested element has no influence on coherence of the light downstream from it and therefore, eliminates prevention of further manipulations with the light where coherence is important like vectorization of the beam.

Disclosure of the essence of the invention

The essence of the problem solution according to the proposed invention is that in an laser beam transforming element, converting an input laser beam of cylindrical symmetry into a beam having rotational symmetry order 2, comprising:

- a substrate of transparent material, having entrance and exit surfaces comprised of entrance and exit planes arranged one opposite to the other in parallel

- a structural modification introduced in the volume of the substrate of transparent material between the entrance and exit planes or on surface on one of the said planes,

- said structural modification being periodic nanostructures that cause local birefringence and consequently Pancharatnam-Berry phase is introduced in the laser beam passing location of said nanostructure, while said nanostructures are oriented with continuously varying angle with respect to neighbouring ones

- a metasurface consisting of said nanostructures that is located in the plane perpendicular to the direction of the laser beam propagation and defines distribution of linear polarization of the laser beam exiting the element while this distribution is described by function of polarization angle variation ϋ = f{ f, y) where ϋ is direction of polarization vector of the light exiting the element, f is the azimuth angle that varies from - p to TT, and y is the maximum polarization rotation angle for particular pattern, wherein the element consisting of a Pancharatnam-Berry metasurface is configured so that the homogeneously polarized laser beam impinging onto said element exits the element such that at least two points of the beam transversal cross section in X-Y plane present opposite X or Y components of polarization vector.

Said polarization angle variation function is ί(f, y) = ( tri ( 2 *f/p- 1)-tri(2 *f/p +1))*y, where tri is the function of the triangle which varies from -1 to 1 and has a maximum equal to 1 at 0 , f is the azimuth angle that varies from - p to TT, and y is the maximum polarization rotation angle.

Said polarization angle variation function is ί(f, y) =5ίh(f)*y, where f - the azimuth angle that varies from - p to TT, and y is the maximum polarization rotation angle.

Said polarization angle variation function is ί(f, y) =oo5(f)*y, where f - the azimuth angle that varies from - p to TT, and y is the maximum polarization rotation angle.

Retardance of Pancharatnam-Berry metasurface is homogeneous and equal to TT.

Retardance of Pancharatnam-Berry metasurface is homogeneous and equal to TT/2.

A laser material processing device employing an optical element designed to modify a laser radiation beam incident to it according to any one of claims 1-6.

Advantages of the invention

The proposed laser beam transforming element is suitable for forming both Gaussian and non-diffractive beams of different types, giving them elongation in a plane perpendicular to the direction of light propagation, it also expands the possibilities of using the proposed element and simplifies the optical scheme of the element. The proposed element can be used more efficiently than analogues to create areas of effect where micro-cracks form, facilitating the separation of workpiece parts, and where the workpiece material is affected by chemical reagents, used for laser- initiated chemical etching.

The use of the proposed optical element in the field of transparent material processing can improve the quality of transparent material processing when cutting transparent materials, while also improve the processing accuracy, and simplifying the optical schemes used, this way increasing their reliability.

Brief description of the drawings Fig.1 is an optical scheme in which a beam with the desired intensity distribution is formed from a cylindrically symmetric beam of the laser in accordance with a preferred embodiment of the present invention.

Fig. 2 is an arrangement of metasurface comprising Pancharatnam-Berry phase 5 nanostructures in the proposed element; Fig.2a - side view, Fig.2b - top view.

Fig. 3 shows an alignment of the structures to be inscribed in the beam-shaping element in polar coordinates.

Fig 4 shows three triangle functions corresponding to y=tt/2.35, y=tt/2.35 - tt/18, and y=tt/2.35 + tt/18. io Fig. 5 shows three harmonic (sine) functions corresponding to y=tt/2.9, y=tt/2.9 - tt/18, and y=tt/2.9 + tt/18.

Fig 6 shows one of the possible variants of arranging orientation of the fast axes of elements in the metasurface located in the beam-shaping element when their orientation is described as a triangle function with y value y = tt/2.35.

15 Fig. 7 shows the cross-section of the light intensity distribution along the direction of Y axis, when the polarization rotation is described by a triangle function with y = tt/2.35.

Fig.8 shows the section of the light intensity distribution along the direction of Y axis, when the polarization rotation is described by a triangle function with y=tt/2.35 - 20 tt/18.

Fig.9 shows the section of the light intensity distribution along the direction of Y axis, when the polarization rotation is described by a triangle function with y=tt/2.35 + tt/18.

Fig.10 shows the section of the light intensity distribution along the direction of Y 25 axis, when the polarization rotation is described by a sine function with y=tt/2.9 - tt/18. Fig.11 shows the section of the light intensity distribution along the direction of Y axis, when the polarization rotation is described by a sine function with y=tt/2.9 + tt/18.

Fig.12 shows measured distribution of the intensity in the beam shaped with the element containing metasurface described by a triangle function with y = tt/2.35 - tt/18.

Fig.13 shows measured distribution of the intensity in the beam shaped with the element containing metasurface described by a triangle function with y = tt/2.35.

Fig.14 shows measured distribution of the intensity in the beam shaped with the element containing metasurface described by a triangle function with y = tt/2.35 + tt/18.

One of the examples of the embodiment of the invention

An optical scheme in which a beam with the desired intensity distribution is formed from a cylindrically symmetric beam of the laser in accordance with a preferred embodiment of the present invention is shown in Fig.1. The laser 1 generates laser beam 2 of ultrashort pulses. Cylindrically symmetric laser beam 2 with homogeneous polarization pattern of which is directed to a laser beam shaping element 3, made of a material transparent to laser radiation and with an entry and an exit surfaces comprised of entrance and exit planes (4, 5) arranged one opposite to the other in parallel. In the element 3, a metasurface 6 comprising Pancharatnam-Berry Phase controlling nanostructures is formed in a plane perpendicular to the direction of §ht the laser beam 2 propagation. The metasurface 6 can be formed in the volume 6a or on the entry surface 6b as well as on the exit surface 6c (Fig. 2). Because of these structures, the direction of the electric field vector of the laser beam 7 exiting the element 3, is continuously varying at each point of the cross-section in respect to neighbouring ones following the Pancharatnam-Berry phase differences introduced by the element 3.

The orientation of the individual nanostructures 8 defining the variation of the Pancharatnam-Berry phase in the metasurface 6 at each point of the plane is described by the angle of rotation q that is individual for each nanostructure and continuously varies between neighbouring nanostructures. The pattern of orientation said nanostructures is described by the function Q = f{ f, y), where f is the azimuth angle varying from -p to p and y is a maximum rotation angle for a particular pattern and this angle y defines the shape of beam cross section. In individual case said function can be Q (f, y) =(ί/7(2^*f/tt-1 )-ίG/(2^*f/ p +1 ))^*y, where tri it is the function of the triangle which varies in increments from -1 to 1 and has a maximum equal to 1 at 0, f is the azimuth angle that varies from - p to TT, and y is the maximum rotation angle of the polarization vector in the given element 9 Fig. 4. In other individual cases the rotation angle function is harmonic, Q (f, y) =5ίh(f)^*y, (10) Fig. 5 where f - the azimuth angle that varies from - p to TT, and y is the maximum polarization rotation angle in the given element. The peculiarity of the function Q = f (f, y) for the arrangement of nanostructures in the element is that fast axes of the Panchratnam- Berry phase nanostructures are completely or nearly opposite in at least one point of the cross-section 11 Fig. 6 of the element and consequentially, the phases of the polarization vector 12 are opposite, which creates a destructive interference to light passing through a said point.

The phases of light passing through the element 3 at each point of the beam cross- section correlate with each other, i. e. the beam remains coherent throughout the pulse in both space and time. Behind said element 3, a spatial distribution with retained coherence and a complex field of polarization vectors is formed across the beam. In a part of the polarization field, an area is created, where the radiation phases are opposite in at least one point of the element, i. e. where a point of destructive interference is created 11. This interference is characteristic to a light that passed the element 3 with the direction of a polarization vector oriented at an angle d - TT/2 (8).

By directing such a beam into a focusing element, which can be a lens, an axicon, or a Pancharatnam-Berry phase-altering element, a focal point or non-diffractive beam is formed that can be used to process materials. When such beam is directed to an axicon, the intensity of the beam 13 Fig. 7, generated in the polarized part of the distribution 12, in the cross-section is described in the first approach by even Mathieu functions, with m = 1 , q = 0 and this distribution has zero intensity at the centre of the distribution 14, while the intensity 15 of the part of the distribution with polarization perpendicular to the said part is described in the first approach by even Mathieu functions, with m = 0, q = 1. The total intensity of the entire beam 16 is circularly symmetric with order 2 with respect to the beam axis and has a transversal cross section elongated in the direction of a line connecting maxima of the curve 13. Additional maxima of intensity 17a - first order, 17b - second order, are observed around the principal maximum, consisting of the sum of both types of Mathieu functions 16, and the intensity of these maxima is significantly lower while their ratio with respect to the principal maximum depends on the maximal rotation angle y used when inscribing the structures of the metasurface. The dedicated direction 12 of the distribution’s long dimension /_,· 18 coincides with the line connecting the principal maxima 13 of the distribution of the light exiting the element, which has the same orientation as the axes of opposite structures inscribed in element at the point 11. Required retardance R to be introduced by Pancharatnam-Berry metasurface is depending on the polarization of the light impinging on the element. In the event when initial polarization is linear, retardance shall be R = p. Meanwhile, if impinging light is polarized circularly, retardance shall be R = TT/2.

Since the dedicated direction of distribution 12, 18 is related to the composition of the beam shaping element 3, total distribution formed by said element rotates simultaneously with the rotation of the element around the axis perpendicular to the plane of inscribed metasurface 6.

Depending on the desired effect on the workpiece material, the structure orientation functions Q = f{ f, y) with different y values are used. Thus, when reducing the value of y in a triangle or harmonic function, the relative contribution of the even Mathieu function with m = 1, q = 0 decreases, resulting in a sharper peak of the total distribution 16 (Fig. 8, Fig.9, Fig.12). With increasing the value of y that contribution increases as well, the peak of the total distribution 16 spreads 13 in the direction of the line connecting the maxima of the distribution (Fig 13). As the value of y is further increased, the local minimum is observed at the centre of the total distribution (Fig. 10,Fig.11 ,Fig.14). The optical element described above, designed to modify a laser radiation beam aimed at it, can be used in laser processing equipment targeting various materials.

Claims

1. Laser beam transforming element (3) converting an input laser beam (2) of cylindrical symmetry into a beam having rotational symmetry order 2, comprising:

- a substrate of transparent material, having entrance and exit surfaces comprised of entrance and exit planes (4, 5) arranged one opposite to the other in parallel

- a structural modification introduced in the volume of the substrate of transparent material between the entrance and exit planes (4, 5) or on surface on one of the said planes (4, 5),

- a metasurface (6) consisting of said nanostructures that is located in the plane perpendicular to the direction of the laser beam (2) propagation and defines distribution of linear polarization of the laser beam (7) exiting the element (3) while this distribution is described by function of polarization angle variation ϋ = f{ f, y) where ϋ is direction of polarization vector of the light exiting the element, f is the azimuth angle that varies from - p to TT, and y is the maximum polarization rotation angle for particular pattern, characterized in that the element (3) consisting of a Pancharatnam-Berry metasurface is configured so that the homogeneously polarized laser beam (2) impinging onto said element (3) exits the element (3) such that at least two points of the laser beam (7) transversal cross section in X-Y plane present opposite X or Y components of polarization vector.

2. The element according to claim 1 , characterized in that said polarization angle variation function is ί(f, y) = ( tri ( 2 *f/p- 1)-tri(2 *f/p +1))*y, where tri is the function of the triangle which varies from -1 to 1 and has a maximum equal to 1 at 0 , f is the azimuth angle that varies from - p to TT, and y is the maximum polarization rotation angle.

3. The element according to claim 1 , characterized in that said polarization angle variation function is ί(f, y) =5ίh(f)*y, where f - the azimuth angle that varies from - p to TT, and y is the maximum polarization rotation angle.

4. The element according to claim 1 , characterized in that said polarization angle variation function is ί(f, y) =oo5(f)*y, where f - the azimuth angle that varies from - p to TT, and y is the maximum polarization rotation angle.

5. The element according to claim 1 characterized in that retardance of Pancharatnam-Berry metasurface is homogeneous and equal to TT.

6. The element according to claim 1 characterized in that retardance of Pancharatnam-Berry metasurface is homogeneous and equal to TT/2.

7. A laser material processing device employing an optical element designed to modify a laser radiation beam incident to it according to any one of claims 1-6.