A Fresnel lens or zone plate is a type of compact lens divided into a set of concentric annular sections. In each section, the overall thickness is decreased to effectively divide the continuous surface of a standard lens into a set of surfaces of the same curvature, with stepwise discontinuities between them. This arrangement allows for the construction of lenses without the mass and volumes of material that would be required by lenses of conventional design.
Fresnel zone plates with their flat surfaces and small volumes are attractive for focusing, collimating and bending light in miniaturized optical systems and integrated optics. Unlike curved lenses or mirrors, Fresnel zone plates typically use diffraction of the incident light instead of the refraction or reflection thereof. More specifically, a Fresnel zone plate includes a set of radially symmetric rings, known as Fresnel zones, which alternate between opaque and transparent. Incident light encountering the Fresnel zone plate will diffract around the opaque zones. The opaque zones are spaced such that the diffracted light constructively interferes at a desired focus spot, thereby creating an image. While functional for their intended purpose, conventional Fresnel zone plates do have certain problems associated therewith. For example, the focal length of a conventional Fresnel zone plate is fixed. It can be appreciated that modifying the convention Fresnel zone plate to have an adjustable focal length would be highly desirable.
Recently, a reflective Fresnel zone plate incorporating carbon nanotube forests was proposed. As is known, carbon nanotube forests exhibit near-perfect optical absorption due to low reflectance and nanoscale surface roughness. It is contemplated for the opaque Fresnel zones of the Fresnel zone plate to be defined by dark nanotube forests which effectively prevent light reflection from the opaque region. This arrangement allows for the Fresnel zone plate to exhibit a high contrast focusing of light as a result of the near-perfect optical absorption of the carbon nanotubes, thereby providing efficient focusing performance at optical wavelengths. As a result, the reflective Fresnel lens may be used for efficiently focusing and collimating light in optical data transfer and communication systems. In addition, two-dimensional source arrays for neural network architectures can be realized using reflective Fresnel lens arrays. However, given that fabrication of the carbon nanotube forests on prior, reflective Fresnel zone plates requires chemical vapor deposition of the carbon at extremely, high temperatures, e.g. 540° C., integration of the reflective Fresnel zone plates with silicon-based electronic circuits is difficult.
Therefore, it is a primary object and feature of the present invention to provide a Fresnel zone plate having an adjustable focal length.
It is a further object and feature of the present invention to provide a Fresnel zone plate which is simple and inexpensive to manufacture.
It is a still further object and feature of the present invention is to provide a Fresnel zone plate which may be simply and easily integrated with electronic circuits.
In accordance with the present invention, a Fresnel zone plate is provided for encountering incident light having a wavelength. The Fresnel zone plate includes a first set of rings radially spaced about a central axis. The first set of rings is transparent. A second set of rings is also radially spaced about the central axis. Each ring of the second set of rings includes a surface lying in a plane perpendicular to the central axis and is opaque. A plurality of silicon nanowires extend into at least one of the surfaces of the second set of rings.
Each of the plurality of silicon nanowires is spaced from an adjacent one of the plurality of silicon nanowires by a distance. The distance is less than the wavelength of the incident light. A mounting surface is spaced from and generally parallel to the surfaces of the second set of rings and a tuning structure is mounted to the mounting surface. The tuning structure includes a plate wherein mechanically stretching of the plate adjusts a focal length of the Fresnel zone plate. The plate is fabricated from an elastomer and is transparent. The plurality of silicon nanowires have lengths. The lengths of the plurality of silicon nanowires are in the range of 1 micrometer to 6 micrometers.
In accordance with a further aspect of the present invention, a Fresnel zone plate is provided for encountering incident light having a wavelength. The Fresnel zone plate has a focal length and includes a wafer having alternating transparent and opaque zones, and a mounting surface. A tuning structure mounted to the mounting surface such that actuation of the tuning structure adjusts the focal length.
The mounting surface is on a first side of the wafer and the second side of the wafer includes first and second sets of rings. The first set of rings is radially spaced about a central axis. The first set of rings is transparent. The second set of rings is radially spaced about the central axis. Each ring of the second set of rings includes a surface lying in a plane perpendicular to the central axis and is opaque. A plurality of silicon nanowires extend into at least one of the surfaces of the second set of rings. Each of the plurality of silicon nanowires is spaced from an adjacent one of the plurality of silicon nanowires by a distance. The distance is less than the wavelength of the incident light. The plurality of silicon nanowires have lengths. The lengths of the plurality of silicon nanowires are in the range of 1 micrometer to 6 micrometers. The tuning structure is actuated by mechanical stretching and includes a plate. The plate is fabricated from an elastomer and is transparent.
In accordance with a still further aspect of the present invention, a Fresnel zone plate is provided for encountering incident light having a wavelength. The Fresnel zone plate has a focal length and a wafer including alternating transparent and opaque zones, and a mounting surface. A plurality of silicon nanowires extend into opaque zone of the wafer. A mechanically stretchable tuning structure is mounted to the mounting surface such that stretching of the tuning structure, varies the focal length of the Fresnel zone plate.
The mounting surface is on a first side of the wafer and the second side of the wafer includes first and second sets of rings. The first set of rings is radially spaced about a central axis. The first set of rings is transparent. The second set of rings is radially spaced about the central axis. Each ring of the second set of rings includes a surface lying in a plane perpendicular to the central axis and is opaque. Each of the plurality of silicon nanowires is spaced from an adjacent one of the plurality of silicon nanowires by a distance. The distance is less than the wavelength of the incident light. The plurality of silicon nanowires have lengths. The lengths of the plurality of silicon nanowires are in the range of 1 micrometer to 6 micrometers. The tuning structure includes a plate that is fabricated from an elastomer and is transparent.