A photon sieve with 10(7) holes has been constructed for operation at optical wavelengths. Details of the design, fabrication, and performance of this device are presented. The 1 m focal-length, 0.1 m diameter element is diffraction limited over a significant bandwidth and has a moderate field of view.

Large optical photon sieve

Freeform optics aims to expand the toolkit of optical elements by allowing for more complex phase geometries beyond rotational symmetry. Complex, asymmetric curvatures are employed to enhance the performance of optical components while minimizing their size. Unfortunately, these high curvatures and complex forms are often difficult to manufacture with current technologies, especially at the micron scale. Metasurfaces are planar sub-wavelength structures that can control the phase, amplitude, and polarization of incident light, and can thereby mimic complex geometric curvatures on a flat, wavelength-scale thick surface. We present a methodology for designing analogues of freeform optics using a silicon nitride based metasurface platform for operation at visible wavelengths. We demonstrate a cubic phase plate with a point spread function exhibiting enhanced depth of field over 300 micron along the optical axis with potential for performing metasurface-based white light imaging, and an Alvarez lens with a tunable focal length range of over 2.5 mm corresponding to a change in optical power of ~1600 diopters with 100 micron of total mechanical displacement. The adaptation of freeform optics to a sub-wavelength metasurface platform allows for further miniaturization of optical components and offers a scalable route toward implementing near-arbitrary geometric curvatures in nanophotonics.

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Metasurface Freeform Nanophotonics

Alvarez and Lohmann lenses are variable focus optical devices based on lateral shifts of two lenses with cubic-type surfaces. I analyzed the optical performance of these types of lenses computing the first order optical properties (applying wavefront refraction and propagation) without the restriction of the thin lens approximation, and the spot diagram using a ray tracing algorithm. I proposed an analytic and numerical method to select the most optimum coefficients and the specific configuration of these lenses. The results show that Lohmann composite lens is slightly superior to Alvarez one because the overall thickness and optical aberrations are smaller.

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The Alvarez and Lohmann refractive lenses revisited

After an introduction illustrating the relevance and motivation of laser ignition of engines, the physical background is reviewed in a first technical section. This mainly comprises the mechanisms of plasma formation, which are the generation of first free electrons and their avalanche-like multiplication, being illustrated by emission and Schlieren diagnostics. Thereafter, combustion fundamentals are discussed leading to an understanding of flammability and minimum ignition laser energies. An important section is represented by the description of the components of laser ignition. This covers the setup and function of a diode-pumped passively Q-switched solid-state laser system, which preferentially is based on Nd:YAG. Other laser sources are mentioned for comparison. Furthermore, multiplexing schemes are discussed that are motivated by cost-saving issues. It is explained why present-day optical fibers are not capable of transporting nanosecond ignition pulses. As a last section, the concepts for an incoupling window and focusing optics are elucidated. Finally, after a summary, in an outlook the chances for commercial realization are analyzed.

Laser‐initiated ignition

An adaptive-focus lens is a device that is capable of tuning its focal length by means of an external stimulus. Numerous techniques for the demonstration of such devices have been reported thus far. Moving beyond traditional solutions, several new approaches have been proposed in recent years based on the use of liquid crystals, which can have a great impact in emerging applications. This work focuses on the recent advances in liquid crystal lenses with diameters larger than 1 mm. Recent demonstrations and their performance characteristics are reviewed, discussing the advantages and disadvantages of the reported technologies and identifying the challenges and future prospects in the active research field of adaptive-focus liquid crystal (LC) lenses.

Recent Advances in Adaptive Liquid Crystal Lenses

A diffractive Alvarez lens is demonstrated that consists of two separate phase plates, each having complementary 16-level surface-relief profiles that contain cubic phase delays. Translation of these two components in the plane of the phase plates is shown to produce a variable astigmatic focus. Both spherical and cylindrical phase profiles are demonstrated with good accuracy, and the discrete surface-relief features are shown to cause less than λ/10 wave-front aberration in the transmitted wave front over a 40 mm×80 mm region.

Diffractive Alvarez lens

We describe the advantages of using diffractive (Fresnel) lenses on thin membranes over conventional optics for, among others, future space telescope projects. Fabrication methods are presented for lenses on two types of freestanding membrane up to 50 cm in size. The first is a Fresnel lens etched into a thin (380-microm) glass sheet, and the second is an approximately 50-microm-thick polymer membrane containing a Fresnel lens made by replication process from a specially made fused-silica master. We show optical performance analysis of all the lenses that are fabricated, including a diffraction-limited Airy spot from a 20-m- focal-length membrane lens in a diffractive telescope system.

Fabrication of large-aperture lightweight diffractive lenses for use in space

We have fabricated demonstration diffractive optic plates at full scale for the NIF laser. These include an off-axis focusing beam sampling grating, a color separation grating, and a kinoform phase plate for spatial beam smoothing. Fabrication methods and optical performance of these DOPs are discussed. IT was discovered that the sol-gel antireflective coating normally applied to high-power transmissive optics partially planarizes the diffractive structures, particularly on the color separation grating used for color management at target, to the extent that optical performance and laser damage threshold are negatively impacted. The effect of sol-gel coatings on grating performance, the feasibility of placing all diffractive structures on a single surface, and future work in this area are discussed.

Manufacture, optical performance, and laser damage characteristics of diffractive optics for the National Ignition Facility

We have produced large aperture (up to 50 cm diameter) diffractive Fresnel lenses in thin (380 µm) glass sheets by photolithography and also in thin (60 µm) freestanding polymer membranes by replication.

Large-aperture, lightweight diffractive lenses for space applications

We demonstrate the fully color-corrected, near-diffraction limited operation of a large aperture (SO cm) diffractive telescope and present captured images of solar flares, sun spots, lunar surface and planets in the solar system using this telescope.

Ian M. Barton

Papers

Diffractive Alvarez lens

Fabrication of large-aperture lightweight diffractive lenses for use in space

Manufacture, optical performance, and laser damage characteristics of diffractive optics for the National Ignition Facility

Large-aperture, lightweight diffractive lenses for space applications

Eyeglass: A color-corrected, large-aperture diffractive telescope