Method of forming birefringent structures in an optical element

Patent metadata

Field	Value
Publication number	US-12265245-B2
Application number	US-202017628998-A
Country	US
Kind code	B2
Filing date	Aug 27, 2020
Priority date	Aug 28, 2019
Publication date	Apr 1, 2025
Grant date	Apr 1, 2025

Abstract

A method of fabricating an optical element includes providing a substrate of a transparent material in which is to be formed a plurality of birefringent nanostructures spaced apart; generating from the output of a source of femtosecond laser pulses a laser beam group comprising a plurality of focused seeding beams having a circular polarisation; directing the laser beam group onto the surface of the substrate at a first position and applying one or more femtosecond laser pulses from each beam to corresponding volumes in the substrate; repeatedly translating the laser beam group relative to the substrate parallel to the line of seeding beams; translating the laser beam group relative to the substrate and repeating the repeated translation and application of the femtosecond laser pulses; wherein the relative translation of the laser beam group and the substrate aligns the writing beam with successive corresponding volumes.

First claim

The invention claimed is:

1. A method of fabricating an optical element comprising:
providing a substrate of a transparent material in which is to be formed a plurality of birefringent nanostructures spaced apart in plane substantially parallel to a surface of the substrate in a first direction by a distance X and in a second direction by a distance Y;
generating from the output of a source of femtosecond laser pulses a laser beam group comprising a plurality of focused seeding beams having a circular polarisation and spaced apart along a line by the distance X and a focused writing beam having a non-circular polarisation and spaced apart from one of the seeding beams by the distance Y or the distance X;
directing the laser beam group onto the surface of the substrate at a first position and applying one or more femtosecond laser pulses from each beam to corresponding volumes in the substrate;
repeatedly translating the laser beam group relative to the substrate parallel to the line of seeding beams and along the first direction by the distance X to a next position and applying one or more femtosecond laser pulses from each beam, in order to deliver a plurality of femtosecond laser pulses from the seeding beams to each corresponding volume to create substantially optically isotropic nanostructures in the corresponding volumes;
translating the laser beam group relative to the substrate along the second direction by the distance Y and repeating the repeated translation and application of the femtosecond laser pulses along the first direction; wherein
the relative translation of the laser beam group and the substrate aligns the writing beam with successive corresponding volumes in which substantially optically isotropic nanostructures have been created, so that the one or more femtosecond laser pulses applied from the writing beam transform the optically isotropic nanostructure in each corresponding volume into an optically anisotropic nanostructure with optical birefringence.

2. A method according to claim 1 , in which the writing beam is spaced apart from the line of seeding beams by the distance Y in a direction orthogonal to the line and aligned with one of the seeding beams, so that the relative translation applies pulses from the writing beam to corresponding volumes to which the seeding beams have been applied at a previous position along the second direction.

3. A method according to claim 2 , in which the plurality of focused seeding beams comprises an odd number of seeding beams, and the writing beam is aligned with a central seeding beam in the line of seeding beams.

4. A method according to claim 1 , in which the writing beam is spaced apart from a seeding beam at an end of the line of seeding beams by the distance X in a direction along the line of seeding beams, so that the relative translation applies pulses from the writing beam to corresponding volumes to which the seeding beams have been applied at previous positions of a same translation along the first direction.

5. A method according to claim 1 , in which the line of focused seeding beams comprises a straight line.

6. A method according to claim 1 , in which the plurality of focused seeding beams are generated by use of a hologram to produce spatial light modulation of the output of the femtosecond laser pulse source.

7. A method according to claim 1 , in which, at each position, a same number of femtosecond laser pulses is applied from each beam.

8. A method according to claim 7 , in which, at each position, one femtosecond laser pulse is applied from each beam.

9. A method according to claim 1 , in which, at least some positions, a same number of pulses is applied from each seeding beam and a different number of pulses is applied from the writing beam.

10. A method according to claim 1 , comprising delivering a plurality of femtosecond laser pulses from the seeding beams to each corresponding volume in the range of 5 to 25 pulses.

11. A method according to claim 1 , in which the output of the source of femtosecond laser pulses has a wavelength in the range of 500 nm to 530 nm.

12. A method according to claim 1 , in which the seeding beams and the writing beam are focused with a focusing arrangement having a numerical aperture in the range of 0.5 to 0.6.

13. A method according to claim 1 , further comprising selecting, for each corresponding volume, an orientation of the non-circular polarisation of the writing beam for the one or more femtosecond laser pulses applied from the writing beam to the corresponding volume in order to create optically anisotropic nanostructures with an intended orientation, so as to define a particular birefringence slow axis orientation for the optically anisotropic nanostructure.

14. A method according to claim 1 , further comprising selecting, for each corresponding volume, an ellipticity of the non-circular polarisation of the writing beam for the one or more femtosecond laser pulse applied from the writing beam to the corresponding volume in order to create optically anisotropic nanostructures with an intended birefringence retardance value.

15. A method according to claim 13 , in which the non-circular polarisation of the writing beam is a linear polarisation, with an ellipticity of substantially zero.

16. A method according to claim 13 , in which the non-circular polarisation of the writing beam is an elliptical polarisation, with an ellipticity greater than zero and less than 1.

17. A method according to claim 1 , further comprising selecting, for each corresponding volume, a number of femtosecond pulses to be applied from the writing beam to the corresponding volume in order to create optically anisotropic nanostructures with an intended birefringence retardance value.

18. A method according to claim 1 , in which each optically isotropic nanostructure comprise a plurality of substantially spherical nanopores, and each optically anisotropic nanostructure comprises a plurality of oblate spheroidal nanopores.

19. A method according to claim 1 , in which each optically isotropic nanostructure comprises a plurality of nanopores with a random distribution, and each optically anisotropic nanostructure comprises a plurality of nanopores assembled into a nanograting.

20. A method according to claim 1 , further comprising forming a plurality of the optically anisotropic nanostructures at each of a plurality of depths within the substrate, to form a three-dimensional array of spaced-apart optically anisotropic nanostructures.

21. A method according to claim 1 , comprising controlling the ellipticity and/or orientation of the non-circular polarisation of the writing beam for the at least one pulse applied from the writing beam to each corresponding volume in order to encode data in each optically anisotropic nanostructure, so that the optical element is an optical data storage element.

22. An optical element fabricated according to a method of claim 1 .

Assignees

• Univ Southampton

Inventors

• Sakakura Masaaki
• Wang Lei
• Kazansky Peter
• Yu Yanhao
• Lei Yuhao

Classifications

• G11B7/09

  with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following · CPC title

• G11B7/0045 Primary

  Recording (G11B7/006, G11B7/0065 take precedence) · CPC title

• G02B2207/101

  Nanooptics · CPC title

• G02B5/32

  Holograms used as optical elements · CPC title

• C03C23/0025

  by a laser beam · CPC title