Eye tracking based on polarization volume grating

1. An eye-tracker for determining a position of a pupil of an eye, the eye-tracker comprising:

a detector; and

a first optical element configured to:

receive first light reflected off the eye;

reflectively diffract a first portion of the first light toward the detector, the first portion of the first light having a first polarization; and

transmit second light, the second light including a second portion of the first light having a second polarization different from the first polarization.

2. The eye-tracker of claim 1 , wherein:

the detector is disposed off-axis with respect to an optical axis of the first optical element; and

the first optical element is further configured to receive the first light in a direction that is substantially along the optical axis of the first optical element.

3. The eye-tracker of claim 1 , further comprising:

a second optical element disposed adjacent to the first optical element, the second optical element being configured to:

receive the second portion of the first light and a third portion of the first light, wherein:

the third portion of the first light has the first polarization;

the first portion of the first light has wavelengths in a first wavelength range; and

the third portion of the first light has wavelengths in a second wavelength range that is different from the first wavelength range;

reflectively diffract the third portion of the first light toward the detector; and

transmit the second portion of the first light.

4. The eye-tracker of claim 1 , wherein the second portion of the first light includes light having the first polarization.

5. The eye-tracker of claim 1 , wherein the second light further includes third light from one or more light sources and the first optical element is configured to receive the third light and to transmit the third light toward the eye.

6. The eye-tracker of claim 5 , wherein:

the one or more light sources include a display system configured to project image light corresponding to one or more images toward the eye; and

the third light includes at least a portion of the image light.

7. The eye-tracker of claim 5 , wherein the first light includes a portion of the third light that is reflected off the eye.

8. The eye-tracker of claim 5 , wherein:

the one or more light sources includes an illumination source;

the third light includes fourth light from the illumination source; and

the eye-tracker further comprises a waveguide coupled to the first optical element, wherein:

the waveguide is configured to receive the fourth light and to reflectively diffract at least a portion of the fourth light.

9. The eye-tracker of claim 8 , wherein the waveguide includes a Bragg reflector configured to reflectively diffract the at least a portion of the fourth light.

10. The eye-tracker of claim 9 , wherein the Bragg reflector is a multiplexed Bragg reflector.

11. The eye-tracker of claim 8 , wherein the waveguide is disposed adjacent to the first optical element.

12. The eye-tracker of claim 1 , wherein:

the first light includes fifth light reflected from a first location of the eye and sixth light reflected from a second location of the eye; and

the detector is configured to:

detect a first intensity of a reflected portion of the fifth light and a second intensity of a reflected portion of the sixth light; and

determine, based on the first intensity and the second intensity, the position of the pupil of the eye.

13. The eye-tracker of claim 1 , wherein:

the first optical element includes photosensitive molecules arranged in a plurality of helical structures;

each helical structure of the plurality of helical structures has a helical axis;

the helical axes of the plurality of helical structures are parallel with each other; and

the helical structures correspond to the first polarization.

14. The eye-tracker of claim 13 , wherein:

the photosensitive molecules along each helical axis form a helical structure; and

the photosensitive molecules in the first optical element form refractive index planes each having a normal direction that is tilted at an angle with respect to an optical axis of the first optical element, enabling the first optical element to reflectively diffract the first portion of the first light toward the detector.

15. A head-mounted device configured to mount near an eye of a user, the head-mounted device comprising:

a display system configured to output image light corresponding to one or more images and to project the image light toward the eye; and

an eye-tracker, the eye-tracker including:

a detector; and

an optical element configured to:

receive first light reflected off the eye;

reflectively diffract a first portion of the first light toward the detector, the first portion of the first light having a first polarization;

transmit second light, the second light including a second portion of the first light having a second polarization different from the first polarization; and

transmit at least a portion of the image light toward the eye of the user.

16. The head-mounted device of claim 15 , wherein:

the eye-tracker is configured to determine one or more of a position of a pupil of the eye of the user and a gaze direction of the user; and

the display system is coupled to the eye-tracker and is configured to adjust the image light based on the position of the pupil determined by the eye-tracker.

17. The head-mounted device of claim 15 , wherein the eye-tracker further comprises a waveguide coupled to the optical element, the waveguide is configured to receive third light from a light source and to reflectively diffract at least a portion of the third light toward the eye of the user.

18. The head-mounted device of claim 17 , wherein:

the first light includes fourth light reflected from a first location of a surface of the eye and fifth light reflected from a second location of the surface of the eye; and

the detector is configured to:

detect a first intensity of the fourth light and a second intensity of the fifth light; and

determine, based on the first intensity and the second intensity, a position of a pupil of the eye.

19. The head-mounted device of claim 15 , wherein:

the optical element includes a first surface, a second surface, and photosensitive molecules between the first surface and the second surface;

the photosensitive molecules are arranged in a plurality of helical structures corresponding to the first polarization;

each helical structure of the plurality of helical structures has a helical axis of a plurality of helical axes;

each helical axis of the plurality of helical axes are aligned with each other;

the photosensitive molecules have a plurality of orientations that form a pattern with fringes on the first surface;

each of the fringes are formed by photosensitive molecules that have a same orientation; and

the fringes are shaped to enable the optical element to direct the at least a portion of the first light toward the detector.

20. A method for determining a position of a pupil of an eye, the method comprising:

receiving first light reflected off the eye;

reflectively diffract a first portion of the first light having a first polarization;

transmitting a second portion of light reflected off the eye as second light, the second light including a second portion of the first light having a second polarization different from the first polarization;

forming an image of the eye using the first light; and

determining the position of the pupil of the eye based on the image.