Tight focusing with a binary microaxicon

Interferometric measurement of arbitrary propagating vector beams that are tightly focused

In this work, we demonstrate a simple setup to generate and measure arbitrary vector beams that are tightly focused. The vector beams are created with a spatial light modulator and focused with a microscope objective with an effective numerical aperture of 1.2. The transverse polarization components (E_x, E_y) of the tightly focused vector beams are measured with three-step interferometry. The axial component E_z is reconstructed using the transverse fields with Gauss's law. We measure beams with the following polarization states: circular, radial, azimuthal, spiral, flower, and spider web.

Tight focus of light using micropolarizer and microlens

Using a binary microlens of diameter 14 μm and focal length 532 nm (NA=0.997) in resist, we focus a 633 nm laser beam into a near-circular focal spot with dimensions (0.35 ± 0.02)λ and (0.38 ± 0.02)λ (λ is incident wavelength) at full width half-maximum intensity. The area of the focal spot is 0.105λ(2). The incident light is a mixture of linearly and radially polarized beams generated by reflecting a linearly polarized Gaussian beam at a 100  μm × 100  μm four-sector subwavelength diffractive optical microelement with a gold coating. The focusing of a linearly polarized laser beam (the other conditions being the same) is found to produce an elliptical focal spot measuring (0.40 ± 0.02)λ and (0.50 ± 0.02)λ. To our knowledge, this is the first implementation of subwavelength focusing of light using a pair of micro-optic elements (a binary microlens and a micropolarizer).

Planar super-oscillatory lens for sub-diffraction optical needles at violet wavelengths

Planar optical lenses are fundamental elements of miniaturized photonic devices. However, conventional planar optical lenses are constrained by the diffraction limit in the optical far-field due to the band-limited wavevectors supported by free-space and loss of high-spatial-frequency evanescent components. As inspired by Einstein's radiation ‘needle stick', electromagnetic energy can be delivered into an arbitrarily small solid angle. Such sub-diffraction optical needles have been numerically investigated using diffractive optical elements (DOEs) together with specially polarized optical beams, but experimental demonstration is extremely difficult due to the bulky size of DOEs and the required alignment precision. Planar super-oscillatory lenses (SOLs) were proposed to overcome these constraints and demonstrated that sub-diffraction focal spots can actually be formed without any evanescent waves, making far-field, label-free super-resolution imaging possible. Here we extend the super-oscillation concept into the vectorial-field regime to work with circularly polarized light, and experimentally demonstrate, for the first time, a circularly polarized optical needle with sub-diffraction transverse spot size (0.45λ) and axial long depth of focus (DOF) of 15λ using a planar SOL at a violet wavelength of 405 nm. This sub-diffraction circularly polarized optical needle has potential applications in circular dichroism spectroscopy, super-resolution imaging, high-density optical storage, heat-assisted magnetic recording, nano-manufacturing and nano-metrology.

Axicon on a gradient index lens (AXIGRIN): integrated optical bench for Bessel beam generation from a point-like source

We propose and demonstrate a novel concept of a compact optical component aimed at transforming a point-like source into a Bessel beam. This component, called AXIGRIN, consists of an axicon fabricated at the end facet of a gradient index lens. It can be directly coupled to an optical fiber, a microscope objective, or vertical-external-cavity surface-emitting-laser to be used without preliminary adjustments, which is of practical interest for end users. This opens new avenues in domains, such as imaging, particle acceleration and manipulation, optical coherence tomography, data storage, laser cutting, etc. AXIGRIN also opens the perspective of using Bessel beams for endoscopy. The generation of linearly and radially polarized Bessel beams is demonstrated with a fiber AXIGRIN.

Bessel-like photonic nanojets from core-shell sub-wavelength spheres

It is accepted so far that the formation of photonic nanojets requires the use of large dielectric spheres (several wavelengths in diameter). Here we show both numerically and experimentally that similar effects can be obtained with properly engineered sub-wavelength core-shell colloids. The design of the spheres is strongly inspired by a far-field approach for the generation of Bessel beams.

High-quality fiber microaxicons fabricated by a modified chemical etching method for laser focusing and generation of Bessel-like beams

The fabrication method of the high-quality fiber microaxicons (FMAs) on the endface of the optical fiber was developed. Using several types of the commercially available optical fibers we experimentally demonstrated the fabrication of a high-quality FMA focusing a laser beam into a tiny spot with a FWHM≈0.6λ and Bessel-like field distribution. It was also demonstrated that choosing the appropriate chemical composition of the etching solution makes it possible to change the shape of the FMA tip from conical to hemispherical. This allows one to change the spatial distribution of the output laser beam, which can represent both the Bessel-like beam with a depth of focus of up to 49λ and a very tiny focal spot close to the diffraction limit size. Experimentally measured focusing characteristics of the fabricated FMAs obtained using a homemade collection-mode scanning near-field optical microscope setup demonstrate good agreement with numerical simulations based on the 3D finite-difference time-domain simulations.

Longitudinal polarized focusing of radially polarized sinh-Gaussian beam

We consider the focusing performance of a radially polarized sinh-Gaussian beam. The sinh-Gaussian beam can be considered as superposition of a series of eccentric Gaussian beam. Based on the Richards-Wolf formulas, high beam quality and subwavelength focusing are achieved for the radially polarized incident sinh-Gaussian beam. Therefore, sinh-Gaussian beam can be applied in the focusing system with high numerical aperture to achieve focusing with superresolution.

Analysis of the shape of a subwavelength focal spot for the linearly polarized light

By decomposing a linearly polarized light field in terms of plane waves, the elliptic intensity distribution across the focal spot is shown to be determined by the E-vector's longitudinal component. Considering that the Poynting vector's projection onto the optical axis (power flux) is independent of the E-vector's longitudinal component, the power flux cross section has a circular form. Using a near-field scanning optical microscope (NSOM) with a small-aperture metal tip, we show that a glass zone plate (ZP) having a focal length of one wavelength focuses a linearly polarized Gaussian beam into a weak ellipse with the Cartesian axis diameters FWHM(x)=(0.44±0.02)λ and FWHM(y)=(0.52±0.02)λ and the (depth of focus) DOF=(0.75±0.02)λ, where λ is the incident wavelength. The comparison of the experimental and simulation results suggests that NSOM with a hollow pyramidal aluminum-coated tip (with 70° apex and 100 nm diameter aperture) measures the transverse intensity, rather than the power flux or the total intensity. The conclusion that the small-aperture metal tip measures the transverse intensity can be inferred from the Bethe-Bouwkamp theory.

Intense Bessel-like beams arising from pyramid-shaped microtips

We show both numerically and experimentally that intense, narrow, and low-divergence beams of light are produced at the apex of dielectric pyramid-shaped microtips. These beams exhibit a Bessel transverse profile but are narrower than the usual Bessel beam, allowing for a significant enhancement of the light intensity inside the beam. They are generated by axicon-like structures with submicrometric height imprinted in glass by combining optical lithography and chemical etching. The resulting beams are experimentally imaged using fluorescence microscopy, in remarkable agreement with numerical computations.