Near index matching enables solid diffractive optical element fabrication via additive manufacturing

Diffractive optical elements(DOEs)have a wide range of applications in optics and photonics,thanks to their capability to perform complex wavefront shaping in a compact form.However,widespread applicability of DOEs is still limited,because existing fabrication methods are cumbersome and expensive.Here,we present a simple and cost-effective fabrication approach for solid,high-performance DOEs.The method is based on conjugating two nearly refractive index-matched solidifiable transparent materials.The index matching allows for extreme scaling up of the elements in the axial dimension,which enables simple fabrication of a template using commercially available 3D printing at tens-of-micrometer resolution.We demonstrated the approach by fabricating and using DOEs serving as microlens arrays,vortex plates,including for highly sensitive applications such as vector beam generation and super-resolution microscopy using MINSTED,and phase-masks for three-dimensional single-molecule localization microscopy.Beyond the advantage of making DOEs widely accessible by drastically simplifying their production,the method also overcomes difficulties faced by existing methods in fabricating highly complex elements,such as high-order vortex plates,and spectrum-encoding phase masks for microscopy.

Nonlinear optical holograms for spatial and spectral shaping of light waves

Shaping either the spatial or the spectral output of a nonlinear interaction is accomplished by introducing basic concepts of computer-generated holography into the nonlinear optics regime. The possibilities of arbitrarily spatially shaping the result of a nonlinear interaction are presented for different phase-matching schemes allowing for both one- and two-dimensional shaping. Shaping the spectrum of a beam in nonlinear interaction is also possible by utilizing similar holographic techniques. The novel and complete control of the output of a nonlinear interaction opens exciting options in the fields of particle manipulation, optical communications, spectroscopy and quantum information.

Particle manipulation beyond the diffraction limit using structured super-oscillating light beams

The diffraction-limited resolution of light focused by a lens was derived in 1873 by Ernst Abbe.Later in 1952,a method to reach sub-diffraction light spots was proposed by modulating the wavefront of the focused beam.In a related development,super-oscillating functions,that is,band-limited functions that locally oscillate faster than their highest Fourier component,were introduced and experimentally applied for super-resolution microscopy.Up till now,only simple Gaussian-like sub-diffraction spots were used.Here we show that the amplitude and phase profile of these sub-diffraction spots can be arbitrarily controlled.In particular,we utilize Hermite–Gauss,Laguerre–Gauss and Airy functions to structure super-oscillating beams with subdiffraction lobes.These structured beams are then used for high-resolution trapping and manipulation of nanometer-sized particles.The trapping potential provides unprecedented localization accuracy and stiffness,significantly exceeding those provided by standard diffraction-limited beams.

The geometric phase in nonlinear frequency conversion

The geometric phase of light has been demonstrated in various platforms of the linear optical regime, raising interest both for fundamental science as well as applications, such as flat optical elements. Recently, the concept of geometric phases has been extended to nonlinear optics, following advances in engineering both bulk nonlinear photonic crystals and nonlinear metasurfaces. These new technologies offer a great promise of applications for nonlinear manipulation of light. In this review, we cover the recent theoretical and experimental advances in the field of geometric phases accompanying nonlinear frequency conversion. We first consider the case of bulk nonlinear photonic crystals, in which the interaction between propagating waves is quasi-phase-matched, with an engineerable geometric phase accumulated by the light. Nonlinear photonic crystals can offer efficient and robust frequency conversion in both the linearized and fully-nonlinear regimes of interaction, and allow for several applications including adiabatic mode conversion, electromagnetic nonreciprocity and novel topological effects for light. We then cover the rapidly-growing field of nonlinear Pancharatnam-Berry metasurfaces, which allow the simultaneous nonlinear generation and shaping of light by using ultrathin optical elements with subwavelength phase and amplitude resolution. We discuss the macroscopic selection rules that depend on the rotational symmetry of the constituent meta-atoms, the order of the harmonic generations, and the change in circular polarization. Continuous geometric phase gradients allow the steering of light beams and shaping of their spatial modes. More complex designs perform nonlinear imaging and multiplex nonlinear holograms, where the functionality is varied according to the generated harmonic order and polarization. Recent advancements in the fabrication of three dimensional nonlinear photonic crystals, as well as the pursuit of quantum light sources based on nonlinear metasurfaces, offer exciting new possibilities for novel nonlinear optical applications based on geometric phases.

Storing and retrieving multiple images in 3D nonlinear photonic crystals

A nonlinear hologram enables to record the amplitude and phase of a waveform by spatially modulating the second order nonlinear coefficient,so that when a pump laser illuminates it,this waveform is reconstructed at the second harmonic frequency.The concept was now extended to enable the generation of multiple waveforms from a single hologram,with potential applications in high density storage,quantum optics,and optical microscopy.