High forward thrust metasurface beam-riding sail.

The radiation pressure force and torque on a one-dimensional bi-grating composed of a Si-SiO2 high contrast binary metagrating is analyzed for the purpose of stable beam riding whereupon a high power laser having an expanding Gaussian irradiance distribution propels the grating in outer space, free from gravitational forces. The binary metagrating structure has been simultaneously optimized to afford high forward thrust, and corrective restoring forces and torques in the event of small linear and angular disturbances. We demonstrate that stability may be enhanced at the expense of forward thrust. The validity of our metamaterial findings is reinforced owing to good agreements between finite-difference time-domain and finite element numerical methods. To reduce mass and enhance forward acceleration this laser-driven sail was designed to be free of a stabilizing boom.

Optical vortex coronagraphy for high-contrast imaging of femtosecond laser filaments.

A high-contrast imaging technique for phase objects based on the optical vortex coronagraph (OVC) is proposed. This method offers the unique advantage of background-free imaging due to the introduction of azimuthal phase in the Fourier plane. We employed the OVC method to detect femtosecond laser-induced air plasma and compared it with the classic diffractometry and fluorescent imaging methods. We achieved a phase sensitivity of ∼0.035 waves that surpassed the capabilities of the other two methods. The combination of this highly sensitive imaging technique with the pump-probe method holds promise for applications in ultrafast imaging of laser-material interactions.

Broadband radiation pressure on a small period diffractive film.

The p-polarization component of radiation pressure force from an unpolarized blackbody light source is predicted by the use of a Maxwell equation solver for a right triangular prism grating of period 2 µm and refractive index 3.5. The transmitted and reflected angular scattering distributions are found to qualitatively agree with diffraction theory: At relatively short wavelengths the transmitted light is concentrated near the refraction angle, and reflected light is concentrated near the reflection angle. Owing to diffraction and multiple internal reflections, however, the spectral irradiance of transmitted and reflected light was found to significantly vary with wavelength. We found that the high value of the refractive index produced a large fraction of reflected light, thereby reducing the net transverse component of radiation pressure force. These results suggest that low index transmission gratings, anti-reflection coatings, optimized metasurface films, or reflection gratings should be explored for future solar sailing missions.

Parametric control of a diffractive axicon beam rider.

A laser beam rider is a large-scale optical structure designed so that it is attracted toward the optical axis, while also affording forward propulsion via radiation pressure along the beam path. Such structures form the basis of laser-driven light sails. Experimental measurements are described whereby a thin diffractive axicon film is shown to exhibit a natural restoring force when its axis is displaced from the optical axis. This effect is attributed to the optical momentum change of diffracted light. Whereas continuous illumination supports harmonic motion, modulated illumination is shown to support both parametric gain and parametric damping. The 12.7µm period photopolymer axicon grating was suspended in a vacuum torsion oscillator and irradiated with a 1.5 W near-infrared laser modulated at a period of 38 s.

Half-ring point spread functions.

We applied point spread function engineering to design an optimized diffuser producing half-ring irradiance patterns satisfying two objectives: reduced peak irradiance in the focal plane, and high fidelity of the reconstructed image. Applications of these optical elements may include sensor protection or reversible diffusers. Experimental and numerical techniques were used to demonstrate three orders of magnitude of suppression of the peak irradiance. Finally, we found a general power-law trend between the Strehl ratio and the light suppression factor.

Stable diffractive beam rider.

A passively stable laser-driven lightsail is described, which makes use of a spatially variant "bi-grating" sail with a mass (payload) attached to a boom (patent pending). A two-dimensional analytical model of the system, a linear stability analysis, and numerical solutions of the equations of motion are reported. The acceleration of the light sail along the beam path is found to depend on the grating parameters. We identify stability conditions that depend on the grating design, beam parameters, and boom length. The inclusion of damping is found to enhance the dynamic range of stability.

Experimental Verification of a Bigrating Beam Rider.

An optical beam rider making use of a light sail comprising two opposing diffraction gratings is experimentally demonstrated for the first time. We verify that the illuminated space-variant grating structure provides an optical restoring force, exhibiting stable oscillations when the bigrating is displaced from equilibrium. We further demonstrate parametric cooling by illuminating the sail with synchronized light pulses. This experiment enhances the technical feasibility of a laser-driven light sail based on diffractive radiation pressure.

Measurements of Radiation Pressure Owing to the Grating Momentum.

The radiation pressure force on a nearly single-order diffraction grating was measured for a transmission grating near the Littrow angles at wavelengths of 808 and 447 nm. The component of force parallel to the grating agreed well with our prediction, being proportional to the product of the grating order and the ratio of the wavelength and grating period. The normal component of force varied with the incident angle, vanishing near the Littrow angle as expected. The measurements verify a correspondence between the Fourier grating momentum and the mechanical momentum. This Letter provides opportunities for in-space fly-by-light sailcraft as well as terrestrial applications.

Optimized pupil-plane phase masks for high-contrast imaging.

A differential evolution algorithm (DEA) is shown to improve the design of phase-only pupil plane masks for imaging in the presence of a high-intensity point source. Strehl and suppression ratio metrics for various phase basis functions were experimentally and numerically calculated. Experiments performed with a spatial light modulator demonstrated 100 times suppression of the peak intensity. The Strehl ratio was found to be higher for the DEA masks compared to the individual phase basis function.

Randomized apertures: high resolution imaging in far field.

We explore opportunities afforded by an extremely large telescope design comprised of ill-figured randomly varying subapertures. The veracity of this approach is demonstrated with a laboratory scaled system whereby we reconstruct a white light binary point source separated by 2.5 times the diffraction limit. With an inherently unknown varying random point spread function, the measured speckle images require a restoration framework that combine support vector machine based lucky imaging and non-negative matrix factorization based multiframe blind deconvolution. To further validate the approach, we model the experimental system to explore sub-diffraction-limited performance, and an object comprised of multiple point sources.

Randomized apertures: high resolution imaging in far field: erratum.

We note a correction to Eq. (6) for [ Opt. Express25(15), 18296 (2017)].

Experimental observations of a laser suppression imaging system using pupil-plane phase elements.

To help diminish the undesirable effects of laser irradiation on an imaging sensor, a pupil-plane phase element was introduced to broaden the point spread function, thereby reducing the focused laser irradiance. A sharpened image was subsequently restored via Wiener deconvolution. Successful experimental demonstrations employing a spatial light modulator in the pupil plane are reported for the vortex, axicon, and cubic phase. Furthermore, to circumvent information loss owing to zero values in the modulation transfer function, we demonstrate how images with different phase elements, combined with a joint deconvolution operation, provide an improved image.

Low-angle optical vortex coronagraphic scatterometer.

The important, but difficult-to-measure zero and low-angle scattering spectrum, as well as the broader angular spectrum, was obtained by use of an optical vortex coronagraphic scatterometer (patent pending). The experimental measurements agreed well with the predictions from the Mie scattering theory. High contrast discrimination allowed us to remove the unscattered coherent illumination, revealing a low-angle superimposed scattered signal.

Optical gradient force assist maneuver.

We describe an energy transfer process whereby a moving particle loses (or gains) kinetic energy upon interacting with the moving optical potential of a swept beam of light. This approach is akin to a gravitational assist maneuver for interplanetary satellite propulsion. Special consideration is given to the stopping condition. For analytical convenience, we examine the Rayleigh scattering regime, providing examples at small and large scattering angles. A 5% uncertainty in the initial particle speed and position has negligible effect on the slowing/speeding ability when the beam size is much larger than the particle.

Digital generation of partially coherent vortex beams.

We present an experimental technique to generate partially coherent vortex beams with an arbitrary azimuthal index using only a spatial light modulator. Our approach is based on digitally simulating the intrinsic randomness of broadband light passing through a spiral phase plate. We illustrate the versatility of the technique by generating partially coherent beams with different coherence lengths and orbital angular momentum content, without any moving optical device. Consequently, we study its cross-correlation function in a wavefront folding interferometer. The comparison with theoretical predictions yields excellent agreement.

Reducing the risk of laser damage in a focal plane array using linear pupil-plane phase elements.

A compact imaging system with reduced risk of damage owing to intense laser radiation is presented. We find that a pupil phase element may reduce the peak image plane irradiance from an undesirable laser source by two orders of magnitude, thereby protecting the detector from damage. The desired scene is reconstructed in postprocessing. The general image quality equation (GIQE) [Appl. Opt.36, 8322 (1997)] is used to estimate the interpretability of the resulting images. A localized loss of information caused by laser light is also described. This system may be advantageous over other radiation protection approaches because accurate pointing and nonlinear materials are not required.

Vortex-phase filtering technique for extracting spatial information from unresolved sources.

A white light vortex coronagraph was used to experimentally achieve sub-resolution detection. The angular location of the centroid γ, and the angular extent of circular pinhole sources Θ, were measured to within errors of δγ=±0.015λ/D and δΘ=±0.026λ/D. This technique has two advantages over conventional imaging: simple power measurements are made and shorter exposure times may be required to achieve a sufficient signal-to-noise ratio.

Rocking motion of an optical wing: theory.

The intensity-dependent rocking frequency of an illuminated semicylindrical refractive rod (or "optical wing") on a flat, nonslip surface is investigated. Both longitudinal and transverse radiation pressure forces (scatter and lift forces), as well as radiation pressure torque, transform the mechanical system into one having a bistable potential energy above a critical intensity. The equation of motion may be written as a parametrically driven nonlinear bistable harmonic oscillator, resulting in complex rocking dynamics. The effects of linear and sinusoidal intensity modulation schemes are explored, and experimental conditions to verify these results are discussed.

Refractive optical wing oscillators with one reflective surface.

An optical wing is a cambered rod that experiences a force and torque owing to the reflection and transmission of light from the surface. Here we address how such a wing may be designed to maintain an efficient thrust from radiation pressure (RP) while also providing a torque that returns the wing to a source facing orientation. The torsional stiffness of two different wing cross-sections is determined from numerical ray-tracing analyses. These results demonstrate the potential to construct a passive sun-tracking, space flight system or a microscopic surface measurement device based on RP force and torque.

Optical vortex coronagraphy with an elliptical aperture.

An optical vortex coronagraph that makes efficient use of a larger fraction of the clear aperture of a Cassegrain-type telescope is described. This design incorporates an elliptical subaperture rather than the conventional circular subaperture. We derive a new vortex phase mask that maintains the same theoretical contrast of a circularly symmetric vortex coronagraph.