Generation of arbitrarily patterned polarizers using 2-photon polymerization.

Patterned polarizers are prepared using liquid crystals (LC) doped with a black dichroic dye and in combination with a linear polarizer. The pattern is achieved with a nanostructured LC alignment surface, that is generated using a two-photon polymerization direct laser write (2PP-DLW). This technique creates a pattern of high-resolution grooves in the photoresist at any arbitrary angle. The angle governs the LC orientation at any substrate surface point, determining the transmitted light linear polarization angle. This paper presents the first use of a 2PP-DLW cured positive tone photoresist for dichroic dye-doped LC alignment. Two complementary photoresists have been employed: conventional negative tone SU-8 photoresist and, in this context novel, positive tone S1805 photoresist. The alignment quality of the polarizers has been assessed by analyzing the transmission using an additional polarizer. For SU-8, the resulting grayscale pattern and a contrast ratio (CR) of 14 has measured. The uniformity of the alignment has been measured to be 65% using normalized Shannon entropy (H). For S1805, a CR of 37 was measured, and a uniformity of 63% was obtained. 2PP-DLW allows for shaping complex patterns in submicron dimensions and for the fabrication of arbitrarily patterned polarizers and other LC devices.

Flat variable liquid crystal diffractive spiral axicon enabling perfect vortex beams generation.

A transparent variable diffractive spiral axicon (DSA) based on a single LC cell is presented. The manufactured DSA can be switched between 24 different configurations, 12 convergent and 12 divergent, where the output angle is varied as a function of the applied topological charge. The active area of the device is created using a direct laser writing technique in indium-tin oxide coated glass substrates. Liquid crystal is used to modulate the phase of the incoming beam generating the different DSA configurations. The DSA consists in 24 individually driven transparent spiral shaped electrodes, each introducing a specific phase retardation. In this article, the manufacture and characterization of the tunable DSA is presented and the performance of the DSA is experimentally demonstrated and compared to the corresponding simulations.

Design and manufacture of an all-polymeric integrated multimode interferometer for visible photonics.

This work demonstrates an integrated multimode interferometer (MMI) based on a fully polymeric platform and optimized for visible range operation. The dimensions of a 2×2 MMI are first calculated analytically and simulated using finite elements method. The devices are manufactured using two layers of negative tone photoresists. The top layer is patterned by e-beam lithography demonstrating the adaptability of this material, naturally designed to respond to UV radiation. Fabrication tolerance was smaller than 100 nm. Devices were optically characterized with a 635 nm input source and the best performance for a 3 dB power splitter was found at an interferometric cavity dimension of 10.5 × 190.68 µm. Other interferometric lengths were characterized to establish a process design kit that allows future use of this platform in more complex photonic integrated circuits architectures.

High resolution 2D beam steerer made from cascaded 1D liquid crystal phase gratings.

Optical beam steering (BS) has multiple applications in fields like target seeking and tracking, optical tweezers, billboard displays and many others. In this work, a two-dimensional beam deflector based on blaze gratings is presented. Phase-only 1D blaze gratings have been prepared using maskless Direct Laser Writing (DLW) resulting in high-resolution structures in indium-tin oxide (ITO) coated glass wafers. The device is composed of two identical 1D liquid crystal (LC) cells cascaded orthogonally back-to-back, with a resultant active area of 1.1 × 1.1 mm2. The 1D cells have been prepared with 144 pixels each with a 7.5 µm pitch. The total 288 pixels are driven by a custom made 12-bit Pulse Width Modulation (PWM) electronic driver, allowing for an arbitrarily high resolution. The system performance is documented, and the efficiency of the system has been tested. A maximum diagonal steering angle of ± 3.42° was achieved.

Near-infrared, visible, and ultraviolet lidar echo emulator.

A compact and cost effective NIR-VIS-UV lidar echo emulator (LEE) was designed and manufactured as a compact breadboard. The LEE is an application specific optical pulse shaper delivering a short lidar return (echo) in the ns range overlapped with a long lidar return in the µs range with a repetition rate from 100 Hz - 500 Hz. The short echo power levels are in the range from 0.2-200 nW, whereas the long echo powers from 0.1-25 pW. The coarse power tuning between the two kind of echoes is done using variable attenuators. LEE provides three operation modes: short echo, long echo, or both echoes overlapped. In the overlapping mode, the power difference between the echoes exceeds 60 dB.

Photonic polymeric structures and electrodynamics simulation method based on a coupled oscillator finite-difference time-domain (O-FDTD) approach.

We use femtosecond laser-based two-photon polymerization (TPP) to fabricate a 2.5D micropillar array. Using an angular detection setup, we characterize the structure's scattering properties and compare the results against simulation results obtained from a novel electrodynamics simulation method. The algorithm employs a modified formulation of the Lorentz Oscillator Model and a leapfrog time differentiation to define a 2D coupled Oscillator Finite-Difference Time-Domain (O-FDTD). We validate the model by presenting several simulation examples that cover a wide range of photonic components, such as multi-mode interference splitters, photonic crystals, ring resonators, and Mach-Zehnder interferometers.

Liquid crystal tunable claddings for polymer integrated optical waveguides.

Optical waveguides in photonic integrated circuits are traditionally passive elements merely carrying optical signals from one point to another. These elements could contribute to the integrated circuit functionality if they were modulated either by variations of the core optical properties, or by using tunable claddings. In this work, the use of liquid crystals as electro-optically active claddings for driving integrated waveguides has been explored. Tunable waveguides have been modeled and fabricated using polymers. Optical functions such as variable coupling and optical switching have been demonstrated.

Evaluation of replicas manufactured in a 3D-printed nanoimprint unit.

Nanoimprint lithography has become a useful tool to prepare elements containing nanoscale features at quite reasonable cost, especially if the fabrication elements are created in the own laboratory. We have designed and fabricated a whole nanoimprint manufacturing system and analyzed the resulting surfaces using ad hoc packages developed on an open-software AFM image analysis suite. To complete the work, a number of polymers have been thoroughly studied in order to select the best material for this implementation. It turned out that the best alternative was not always the same, but depended on the application. A comparative study of the polymers, which takes into account the values and dispersion of numerous sample parameters, has been carried out. As a large number of samples was prepared, an automatized procedure for characterization of nanoimprint surfaces had to be set up. The procedure includes figures of merit for comparative purposes. Materials without the requirement of a solvent were found to be superior for most nanoimprint applications. A large dispersion of the samples was found.

All-organic switching polarizer based on polymer waveguides and liquid crystals.

This paper reports on the design, fabrication and characterization of an all-organic photonic integrated circuit working as a switching polarizer for visible light (630nm), combining organic waveguides and liquid crystals that can be electrically driven. The device was made in commercially available epoxy by laser direct writing lithography. A device with a 2dB loss and a 20dB extinction ratio for both polarizations, was simulated; the manufactured devices proved the working principle of the design. The results have led to the design of a switching polarization splitter, in which a careful choice of waveguide material and liquid crystal can lead to devices working on a wide range of wavelengths.

A 3D-printed device for polymer nanoimprint lithography.

Nanoimprint lithography (NIL) is an imprinting technique which has experienced an increasing popularity due to its versatility in fabrication processes. Commercial NIL machines are readily available achieving high quality results; however, these machines involve a relatively high investment. Hence, small laboratories often choose to perform NIL copies in a more rudimentary and cheaper way. A new simple system is presented in this document. It is based on two devices which can be made in-house in plastic by using a 3D printer or in aluminum. Thus, the overall manufacturing complexity is vastly reduced. The presented system includes pressure control and potentially temperature control. Replicas have been made using a sawtooth grating master with a pitch around half micrometre. High quality patterns with low density of imperfections have been achieved in 2.25 cm2 surfaces. The material chosen for the negative intermediary mould is PDMS. Tests of the imprint have been performed using the commercial hybrid polymer Ormostamp®.

Thermally tunable polarization by nanoparticle plasmonic resonance in photonic crystal fibers.

A photonic crystal fiber selectively filled with silver nanoparticles dispersed in polydimethylsiloxane has been numerically studied via finite elements analysis. These nanoparticles possess a localized surface plasmon resonance in the visible region which depends on the refractive index of the surrounding medium. The refractive index of polydimethylsiloxane can be thermally tuned leading to the design of polarization tunable filters. Filters found with this setup show anisotropic attenuation of the x-polarization fundamental mode around α(x) = 1200dB/cm remarkably higher than the y-polarization mode. Moreover, high fiber birefringence and birefringence reversal is observed in the spectral region of the plasmon.