Curved fork-like grating structure, curved fork-like grating, and preparation method therefor

A method for preparing a curved fork-like grating, comprising:

• step (i) coaxially disposing a laser (5), a beam expander (6), a linear polarizer (7), and a depolarization beam splitter (8);
• disposing a reflector group (15-16), a pin-holed microscopic objective (17), and a first collimating lens (18) along a reflection light path of the depolarization beam splitter (8);
• and disposing a spatial light modulator (SLM) (9) along a transmission light path of the depolarization beam splitter (8), and connecting the SLM to a personal computer (PC) control end (10);
• step (ii) starting the laser (5) so that reflected light from the SLM enters and is reflected by the depolarization beam splitter (8) to form a second beam of reflected light;
• step (iii) disposing a Fourier lens (11), an adjustable aperture diaphragm (12), a third reflector (13) and a second collimating lens (14) along the second beam of reflected light;
• step (iv) controlling the SLM (9) by the PC control end (10) to obtain Bessel-Gaussian light with a target bending factor γ and a topological charge value l, and defining such a light field as an object light field;
• step (v) placing a reflector perpendicular to a selected base (20), and adjusting an angle of the rotating base so that an object light path returns in the same way;
• rotating the rotating base for an angle φ/2, and adjusting a position and angle of the reflector group (15-16) so that two light paths coincide, wherein at the moment, an included angle formed by the two light paths is φ, and an average period of the curved fork-like grating is expressed as:
• Λ
• =
• λ
• 2
• ⁢
• sin
• ⁡
• (
• ϕ
• /
• 2
• )
• ,
• step (vi) adjusting a position and angle of the Fourier lens (11) so that a front focal plane coincides with a screen of the SLM, adjusting the diaphragm (12) to be located on a focal plane of the lens (11), and adjusting an aperture to only allow first-order diffracted light to pass through the diaphragm;
• step (vii) rotating the polarizer (7), selecting a polarization direction in which the first-order diffracted light has the highest light intensity, adjusting a position of the collimating lens (14) so that a front focal plane of the lens (14) coincides with a plane where the diaphragm (12) is located, and adjusting an angle of the collimating lens (14) so that the first-order diffracted light perpendicularly passes through the center of the lens (14), wherein at the moment, the two light paths interfere to obtain an exposure light field;
• and
• step (viii) preparing a metal, dielectric, or metal and dielectric hybrid grating, film coating, exposing the metal, dielectric, or metal and dielectric hybrid grating in an exposure light field obtained from steps (i) to (viii), developing, and etching, wherein the steps of preparing, film coating, exposing, developing, and etching are not limited to a particular order, arrangement, and combination.