In this work we reveal an influence of polarization of the laser beam on polymerization in direct laser writing. It was experimentally found that the width of suspended lines fabricated in SZ2080, OrmoComp and PETA (pentaerythritol triacrylate) pre-polymers directly depends on the incident polarization and is largest when the angle between the electric field vector and the sample translation direction is α = 90° and the smallest when α = 0°. The size of polymerized structures is consistent with theoretical simulations based on vectorial Debye theory. Experiments were performed by using average laser power corresponding to the middle value of the fabrication window. Polarization was found to be affecting feature sizes while structuring various widespread photoresists, the observed variation was material dependent and measured from 5 to 22% in the line-width. The performed study proves that polarization can be used as a variable parameter for fine tuning of the voxel's aspect ratio.
Three dimensional (3D) fast (< 0.5 hour) printing of micro-optical elements down to sub-wavelength resolution over 100 μm footprint areas using femtosecond (fs-)laser oscillator is presented. Using sub-1 nJ pulse energies, optical vortex generators made of polymerised grating segments with an azimuthally changing orientation have been fabricated in SZ2080 resist; width of polymerised rods was ~ 150 nm and period 0.6-1 μm. Detailed phase retardance analysis was carried out manually with Berek compensator (under a white light illumination) and using an equivalent principle by an automated Abrio implementation at 546 nm. Direct experimental measurements of retardance was required since the period of the grating was comparable (or larger) than the wavelength of visible light. By gold sputtering, transmissive optical vortex generators were turned into reflective ones with augmented retardance, Δn × h defined by the form birefringence, Δn, and the height h = 2d where d is the thickness of the polymerised structure. Retardance reached 315 nm as measured with Berek compensator at visible wavelengths. Birefringent phase delays of π (or λ/2 in wavelength) required for high purity vortex generators can be made based on the proposed approach. Optical vortex generators for telecom wavelengths with sub-wavelength patterns of azimuthally oriented gratings are amenable by direct laser polymerisation.
Interaction between the polarization and spatial degrees of freedom of a light field has become a powerful tool to tailor the amplitude and phase of light beams. This usually implies the use of space-variant photonic elements involving sophisticated fabrication technologies. Here we report on the optical spin–orbit engineering of the intensity, phase, and polarization structure of Bessel light beams using a homogeneous birefringent axicon. Various kinds of spatially modulated free-space light fields are predicted depending on the nature of the incident light field impinging on the birefringent axicon. In particular, we present the generation of bottle beam arrays, hollow beams with periodic modulation of the core size, and hollow needle beams with periodic modulation of the orbital angular momentum. An experimental attempt is also reported. The proposed structured light fields may find applications in long-distance optical manipulation endowed with self-healing features, periodic atomic waveguides, contactless handling of high aspect ratio micro-objects, and optical shearing of matter.
Controlling the angular spectrum content of light fields is a basic beam shaping strategy implying the control of the wavevector distribution. Here we propose a novel class of refractive optical elements generated by folding the conical surface of a usual (conical) axicon. In particular, we explore folding processes where the continuous deformation of the circular cross-section of an axicon lead to the appearance of geometrical singularities (cusps). This is illustrated considering two prototypical families derived from hypocycloidal and epicycloidal geometries. Such topological axicons have been fabricated at the micron scale by using photopolymerization technique and characterized both experimentally and theoretically.
In 1931, French astronomer Bernard Lyot suggested that placing a beam stop in the center of the Fourier plane of a telescope allows observing faint objects nearby on-axis bright sources. This opened a new chapter in astronomical imaging called coronagraphy. Since then various techniques have been proposed and implemented experimentally. In particular, it was shown that pure phase masks, instead of amplitude ones, is an efficient way to reject on-axis light. Since one decade, there is a growing interest in spiraling phase (optical vortex) masks that may create of a nodal area at the exit pupil plane of the telescope outside which on-axis light is rejected. Such optical vortex coronagraphy implies the development of singular phase masks endowed with well defined topological properties. To this aim, nowadays nanofabrication tools are a prime choice, which includes liquid crystal photo-alignment technology. Here we show that spontaneously occurring liquid crystal topological defects offer a smart alternative to optical vortex phase masks. Our first experimental smart coronagraphy observations will be presented and discussed.
Liquid crystalline materials are well-known to exhibit various kinds of structural defects, whose space-variant optical properties are actually useful for a number of applications, including recently developed integrated optical vortex generators. In addition, since liquid crystal defects display well-defined topological features, their very existence also appears attractive in terms of information storage at the microscopic scale. An illustrative example is provided by frustrated chiral liquid crystals films, in which different localized metastable states can be written by either structured or unstructured light beams. We will present recent results regarding the controlled generation of multiple chiral topological states in liquid crystals.
To exploit the angular momentum degree of freedom of the light to control the mechanical effects that results from its linear momentum is an intriguing challenge that may open several new routes towards enhanced optical trapping, manipulation and sorting of microscopic entities. This issue can be addressed by exploiting the interplay between the chirality of matter and the chirality of optical fields. Here we will report on our recent progresses on helicity-dependent optomechanics of chiral microparticles.
We report on vortex-assisted femtosecond direct laser writing (DLW) in silver-containing phosphate glasses with
complex light fields endowed with optical phase singularities. This allows us to engrave complex patterns showing
sub-wavelength dimensions. The associated linear and nonlinear optical properties show remarkably correlated
but distinct spatial distributions. The creation of a perennial buried electric field leads to an efficient electric-
field induced second harmonic generation. The magnitude and distribution of such buried field is also considered
to actively drive the pattern topology of the fluorescent silver clusters. Using DLW with phase and amplitude
engineered beams, we demonstrate a promising approach to control both fluorescent and nonlinear responses
below the diffraction limit.
Femtosecond laser fabrication has been used to make hybrid refractive and di ractive micro-optical elements in photo-polymer SZ2080. For applications in micro- uidics, axicon lenses were fabricated (both single and arrays), for generation of light intensity patterns extending through the entire depth of a typically tens-of-micrometers deep channel. Further hybridisation of an axicon with a plasmonic slot is fabricated and demonstrated nu- merically. Spiralling chiral grooves were inscribed into a 100-nm-thick gold coating sputtered over polymerized micro-axicon lenses, using a focused ion beam. This demonstrates possibility of hybridisation between optical and plasmonic 3D micro-optical elements. Numerical modelling of optical performance by 3D-FDTD method is presented.
Chiral patterns are created by focused ion-beam milling nano-grooves with sub-15 nm resolution on thin metal films and arrays of nanoparticles, scattering and absorbing light selectively for left and right circularly polarized light, with high fidelity over fields up to 100 x 100 μm<sup>2</sup> without positioning errors. This allows to carry out numerical simulations to estimate light enhancement and circular dichroism both on ideal and realistic particles taken from SEM images, showing doubling of scattering cross-section and enhancement changes up to 5 times controlled by dichroism, with localized field enhancements up to 20000.
3D plasmonic structures extending out of a gold film plane are created by dry etching of the film in the openings of a resist mask defined by electron beam lithography. Conical vertical protrusions (nano-wells) are left, and their optical properties are numerically simulated, showing easily reachable out-of-plane trapping of both dielectric and metal plasmonic nano-spheres, with trapping forces up to 20 pN/W/μm<sup>2</sup>.
Wideband refractive index spectra in 3D-FDTD are correctly represented by overcoming the polynomial approximations to give accurate field and force/torque results for generalized artificial materials.
A combination of electron- and ion-beam lithographies has been applied to fabricate patterns of plasmonic nanoparticles having tailored optical functions: they create hot-spots at predefined locations on the nanoparticle at specific wavelengths and polarizations of the incident light field. Direct inscribing of complex chiral patterns into uniform nano-disks of sub-wavelength dimensions, over extensive 20-by-20 <i>μ</i>m<sup>2</sup> areas, is achieved with high fidelity and efficiency; typical groove widths are in 10-30 nm range. Such patterns can perform optical manipulation functions like nano-tweezing and chiral sorting. Fabrication procedures can be optimized to pattern thin 0.1-2.5 <i>μ</i>m-thick membranes with chiral nanoparticles having sub-15 nm grooves. Peculiarities of optical force and torque calculations using finite-difference time-domain method are presented.
Here we report on the use of liquid crystal topological defects for photonic applications that involve optical
singularities. The well-dened molecular organization around a liquid crystal defect enables the coupling between
the spin and the orbital angular momentum of light. Such an optical spin-orbit coupling is a general feature of
light propagation through inhomogeneous or anisotropic media, which makes liquid crystal topological defects
attractive micro-structures when orbital angular momentum of light is the key ingredient of an application.
The development of techniques of optical manipulation of matter attracts great attention. One of the most interesting mechanisms for such manipulation is the dielectric light- matter interaction. This point was used by R. Beth, in 1936, to demonstrate an angular manipulation of a birefringent uniform macroscopic object. We have proposed and developed a two-beam technique that is used for the manipulation of nematic liquid crystal materials. Pertinent external control parameters, such as the mutual polarization of the two beams, the ratio of the two beams' intensity, the total intensity and the interaction geometry, are pointed out. Effective optical control via the non-resonant angular momentum transfer to the cluster of illuminated molecules is predicted and realized experimentally. In particular, it is shown that quasi-uniform precession regimes may be light- controlled at 100 percent. Moreover it is shown that the circularly polarized light-induced breakdown of orientational symmetry in non-chiral liquid crystal may result in their macroscopic chiral organization. Stationary, precessing and oscillating chiral modes are observed which may be controlled via the above mentioned external parameters. Consequently, since the non-planar deformations are at the origin of the multi stability of the system, it is expected from the theory that some important features of the isothermal light-induced phase transition should be continuously light-driven, without breaking the circular symmetry of the excitation beams. We believe that our two- beam technique is not restricted to the field of liquid crystal and may be also applied in biophysics, where living entities should play the role of the non-absorbing and birefringent material.