Form birefringent polymeric structures realized by 3D laser printing.

The 3D laser printing of form birefringent structures promises fast prototyping of polarization-sensitive photonic elements. However, achieving the quarter- and half-wave phase retardation levels needed in applications still remains a challenge, especially at visible wavelengths. Thickness of the birefringent region, usually consisting of simple 1D gratings, must be sufficiently large to ensure the required retardance, making the 3D laser-printed gratings prone to mechanical collapse. Here we demonstrate 3D laser-printed mechanically robust form birefringent 3D structures whose thickness and phase retardation can be increased without loss of mechanical stability, and report on the realization of compact self-supporting structures exhibiting quarter- and half-wave phase retardation at visible wavelengths.

Optically-Thin Broadband Graphene-Membrane Photodetector.

A broadband graphene-on-Si3N4-membrane photodetector for the visible-IR spectral range is realised by simple lithography and deposition techniques. Photo-current is produced upon illumination due to presence of the build-in potential between dissimilar metal electrodes on graphene as a result of charge transfer. The sensitivity of the photo-detector is ∼ 1 . 1 µ A/W when irradiated with 515 and 1030 nm wavelengths; a smaller separation between the metal contacts favors gradient formation of the built-in electric field and increases the efficiency of charge separation. This optically-thin graphene-on-membrane photodetector and its interdigitated counterpart has the potential to be used within 3D optical elements, such as photonic crystals, sensors, and wearable electronics applications where there is a need to minimise optical losses introduced by the detector.

Optical damage thresholds of microstructures made by laser three-dimensional nanolithography: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 13 (2020).OPLEDP0146-959210.1364/OL.45.000013.

Passive fluidic micro-sensor with all-optical readout realized using a direct laser writing technique.

In this Letter, we report on design and realization of solvent-sensitive microstructures based on three-dimensional periodic lattices fabricated in a polymeric photoresist. Sensing is based on reversible size change in polymeric microstructures upon immersion in wetting and non-wetting solvents. Its readout is achieved purely optically by observing modification of a Moiré pattern formed by grating-like deformable and rigid polymeric structures. A compact micro-sensor using these principles was realized using a direct laser writing technique in the photoresist. High sensitivity and easy optical readout of the sensor were demonstrated. In the future, sensors based on similar principles may find applications in microfluidic systems, such as lab-on-a-chip.

Nanoscale optical and structural characterisation of silk.

The nanoscale composition of silk defining its unique properties via a hierarchial structural anisotropy needs to be analysed at the highest spatial resolution of tens of nanometers corresponding to the size of fibrils made of ß-sheets, which are the crystalline building blocks of silk. Nanoscale optical and structural properties of silk have been measured from 100 nm thick longitudinal slices of silk fibers with ca. 10 nm resolution, the highest so far. Optical sub-wavelength resolution in hyperspectral mapping of absorbance and molecular orientation were carried out for comparison at IR wavelengths of 2-10 µm using synchrotron radiation. A reliable distinction of transmission changes by only 1-2% as the anisotropy of amide bands was obtained from nanometer-thin slices of silk.

Infrared Polariscopy Imaging of Linear Polymeric Patterns with a Focal Plane Array.

Polariscopy is demonstrated using hyperspectral imaging with a focal plane array (FPA) detector in the infrared (IR) spectral region under illumination by thermal and synchrotron light sources. FPA Fourier-transform IR (FTIR) imaging microspectroscopy is useful for monitoring real time changes at specific absorption bands when combined with a high brightness synchrotron source. In this study, several types of samples with unique structural motifs were selected and used for assessing the capability of polariscopy under this FPA-FTIR imaging technique. It was shown that the time required for polariscopy at IR wavelengths can be substantially reduced by the FPA-FTIR imaging approach. By using natural and laser fabricated polymers with sub-wavelength features, alignment of absorbing molecular dipoles and higher order patterns (laser fabricated structures) were revealed. Spectral polariscopy at the absorption peaks can reveal the orientation of sub-wavelength patterns (even when they are not spatially resolved) or the orientation of the absorbing dipoles.

Simple multi-wavelength imaging of birefringence:case study of silk.

Polarised light imaging microscopy, with the addition of a liquid crystal (LC) phase retarder, was used to determine the birefringence of silk fibres with high (∼1 µm) spatial resolution. The measurement was carried out with the silk fibres (the optical slow axis) and the slow axis of the LC-retarder set at parallel angles. The direct fit of the transmission data allowed for high fidelity determination of the birefringence Δn ≈ 1.63 × 10-2 (with ∼2% uncertainty) of the brown silk fibre, (Antheraea pernyi) averaged over the wavelength range λ = (425-625) nm. By measuring retardance at four separate wavelengths, it was possible to determine the true value of the birefringence of a thicker sample when an optical path may include a large number of wavelengths. The numerical procedures and required hardware are described for the do-it-yourself assembly of the imaging polariscope at a fractional budget compared to commercial units.

Ultrafast laser processing of materials: from science to industry.

Processing of materials by ultrashort laser pulses has evolved significantly over the last decade and is starting to reveal its scientific, technological and industrial potential. In ultrafast laser manufacturing, optical energy of tightly focused femtosecond or picosecond laser pulses can be delivered to precisely defined positions in the bulk of materials via two-/multi-photon excitation on a timescale much faster than thermal energy exchange between photoexcited electrons and lattice ions. Control of photo-ionization and thermal processes with the highest precision, inducing local photomodification in sub-100-nm-sized regions has been achieved. State-of-the-art ultrashort laser processing techniques exploit high 0.1-1 µm spatial resolution and almost unrestricted three-dimensional structuring capability. Adjustable pulse duration, spatiotemporal chirp, phase front tilt and polarization allow control of photomodification via uniquely wide parameter space. Mature opto-electrical/mechanical technologies have enabled laser processing speeds approaching meters-per-second, leading to a fast lab-to-fab transfer. The key aspects and latest achievements are reviewed with an emphasis on the fundamental relation between spatial resolution and total fabrication throughput. Emerging biomedical applications implementing micrometer feature precision over centimeter-scale scaffolds and photonic wire bonding in telecommunications are highlighted.

Evidence of superdense aluminium synthesized by ultrafast microexplosion.

At extreme pressures and temperatures, such as those inside planets and stars, common materials form new dense phases with compacted atomic arrangements and unusual physical properties. The synthesis and study of new phases of matter at pressures above 100 GPa and temperatures above 10(4) K--warm dense matter--may reveal the functional details of planet and star interiors, and may lead to materials with extraordinary properties. Many phases have been predicted theoretically that may be realized once appropriate formation conditions are found. Here we report the synthesis of a superdense stable phase of body-centred-cubic aluminium, predicted by first-principles theories to exist at pressures above 380 GPa. The superdense Al phase was synthesized in the non-equilibrium conditions of an ultrafast laser-induced microexplosion confined inside sapphire (α-Al(2)O(3)). Confined microexplosions offer a strategy to create and recover high-density polymorphs, and a simple method for tabletop study of warm dense matter.

Numerical analysis on the optical role of nano-randomness on the Morpho butterfly's scale.

The blue coloration of Morpho butterflies has anomalously low angular dependence despite the production of color with a selected wavelength based on an interference effect. A key to the mechanism of the specific Morpho-color was suggested to be the randomness of its scale. Using finite-difference time-domain (FDTD) analysis, the role of different kinds of randomness in the structure of the Morpho butterfly's scale was investigated, which was impossible by conventional analytical calculations. The results revealed that incoherence in the incident light plays an essential role, which cannot be realized only by structural randomness. On the other hand, the lateral and vertical randomness, and the number of random components were found each to have an independent role to realize the specific Morpho-color preventing the sharp reflective angular dependence. The direction obtained by the numerical simulations to analyze optically complex random structures will serve not only to understand the scientific principles, but also to design the optical properties of artificial materials.

Resonant localization, enhancement, and polarization of optical fields in nano-scale interface regions for photo-catalytic applications.

We report results of theoretical simulations of optical field enhancement in a system consisting of spherical and hemispherical noble metal nanoparticles on a smooth titania surface, which is a model system relevant to applications in photo-catalysis and solar energy harvesting. Simulations conducted using Finite-Difference Time-Domain (FDTD) technique reveal presence of resonant optical extinction bands at visible wavelengths, whose optical scattering is weak, but the associated localization and intensity enhancement of optical near-field are significant. For hemispheres, the field is strongly localized at the metal-substrate interface, where intensity enhancement of up to 10(4) times is reached. Moreover, the field is predominantly polarized along the normal to the substrate. These findings indicate potential of the hemisphere-substrate system for applications relying on optically promoted charge transport through the metal-substrate interface, such as photochemical reactions and light-to-current conversion. The results of theoretical analysis are compared with reported experimental data on photo-catalytic reactions.

Structural characterization of femtosecond laser modified regions inside sapphire.

We report on structural characterization of sapphire photomodified by voids of sub-wavelength diameter surrounded by amorphised regions formed after exposure by tightly-focused femtosecond laser pulses of 800 nm wavelength and 150 fs duration at the single and double-pulse irradiation inside crystalline sapphire. Regrowth of a crystalline phase near the edge between the amorphous and crystalline phases was observed by transmission electron microscopy (TEM) in the case of double-pulse-irradiated locations. Regions patterned by single-pulse-induced voids inside sapphire were characterized by synchrotron X-ray diffraction (XRD) technique. The XRD patterns indicate presence of an expanded phase of the host crystal. The origin of structural changes observed in TEM and XRD is discussed and is consistent with fast thermal quenching.

On-chip fraction collection for multiple selected ssDNA fragments using isolated extraction channels.

High efficiency and high-purity fraction collection is highly sought in analysis of fragments-of-interest from selective polymerase chain reaction (PCR) products generated by High Coverage Gene Expression Profiling (HiCEP) methods. Here we demonstrate a new electrophoretic chip device enabling automatic high-efficient fractionation of multiple ssDNA target fragments during a run of separation. We used thoroughly isolated extraction channels for each selected target to reduce the risk of cross-contamination between targets due to cross-talk of extraction channels. Fragments of 35, 108 and 138 b, were successfully isolated, then the recovery was PCR-amplified and assessed by capillary electrophoresis (CE) analysis. Total impurity level of the targets due to unwanted fragments of 0.7%, 2% and 6% respectively, was estimated. Difficulties in collecting multiple target factions are due to band diffusion and DNA adsorption to the walls for the fragments in the separation channel, which is generated by transferring the DNA target fraction from the extraction section to the target reservoir. Therefore, we have carefully measured band broadening and analyzed its influence on the separation resolution due to the delay.

Optical transmission and laser structuring of silicon membranes.

The optical linear and nonlinear properties of ~ 340-nm thick Si membranes were investigated. The investigation included both experiments in which the reflection and transmission from the membranes were measured, and finite differences time domain simulations. The linear optical transmission of the Si membranes can be controlled by changing the thickness of a thermally grown oxide on the membrane. Illumination of the membranes with high levels of irradiation leads to optical modifications that are consistent with the formation of amorphous silicon and dielectric breakdown. When irradiated under conditions where dielectric breakdown occurs, the membranes can be ablated in a well-controlled manner. Laser micro-structuring of the membranes by ablation was carried out to make micrometer-sized holes by focused fs-pulses. Ns-pulses were also used to fabricate arrays of holes by proximity-ablation of a closely-packed pattern of colloidal particles.

Lasing with well-defined cavity modes in dye-infiltrated silica inverse opals.

Lasing in dye solution-embedded inverse silica opal structures was investigated. The opal films were prepared by sedimentation of polystyrene microspheres on a cover glass. The polystyrene structures were inverted using sol-gel infiltration of silica and subsequent removal of polystyrene. Photoluminescence of rhodamine (rhodamine B, 6G and sulfo-rhodamine 101) dye solutions embedded into the inverse silica opal structures exhibited clear signatures of the lasing via a distributed feedback (DFB) and gain modulation. The refractive index contrast between the dye and the inverse opal was small enough (approximately 0.03%) for the formation of refractive index coupling between the lasing modes. The lasing spectrum exhibited a highly regular periodic structure of modal peaks, rather than the chaotic superposition of peaks reported in previous studies. Lasing modes having a spectral width of about 0.25 nm and a free spectral range of about 0.75 nm appeared at the position of the maximum gain (the maximum fluorescence of the dye).

Tunable single-mode photonic lasing from zirconia inverse opal photonic crystals.

Lasing from zirconia inverse opal photonic crystal structures infiltrated by solutions of rhodamine dyes was found to exhibit single-mode lasing peaks with spectral width less than 1 nm and quality factor in excess of 4000. The lasing occurs within the approximate range of high-reflectance spectral region associated with photonic stop band along [111] crystallographic direction, but its wavelength is not fixed to the corresponding Bragg wavelength of the periodic structure, and depends on the spectral position of the gain band. This lasing regime can be useful for realizing tunable single-mode photonic crystal lasers.

Nanoparticle plasmon-assisted two-photon polymerization induced by incoherent excitation source.

We demonstrate the possibility to achieve optical triggering of photochemical reactions via two-photon absorption using incoherent light sources. This is accomplished by the use of arrays of gold nanoparticles, specially tailored with high precision to obtain high near-field intensity enhancement.

Fabrication and properties of metalo-dielectric photonic crystal structures for infrared spectral region.

We report structural and optical properties of three-dimensional periodic metallic woodpile structures obtained by direct laser writing in dielectric photoresist SU-8 and subsequent electroless coating by a thin Ni film. Signatures of photonic stop gaps were observed in optical reflection spectra of the structures at infrared wavelengths. This study demonstrates that the combination of DLW and chemical infiltration of metals is attractive as a simple and cost-efficient method for the fabrication of metalo-dielectric photonic crystals.

Inhibition of multipolar plasmon excitation in periodic chains of gold nanoblocks.

Periodically corrugated chains of gold nanoblocks, fabricated with high precision by electron-beam lithography and lift-off techniques, were found to exhibit optical signatures of particle plasmon states in which relative contribution of longitudinal multipolar plasmons is significantly lower than that in equivalent rectangular gold nanorods. Plasmonic response of periodic chains is dominated by dipolar plasmon modes, which in the absence of multipolar exciations are seen as background-free and spectrally well-resolved extinction peaks at infrared (IR) wavelengths. This observation may help improve spectral parameters of IR plasmonic sub-wavelength antennae. Comparative studies of plasmon damping and dephasing in corrugated chains of nanoblocks and smooth rectangular nanorods are also presented.

Spectrally-resolved atomic-scale length variations of gold nanorods.

Functionality of gold nanorod structures as ultra-sensitive optical rulers is demonstrated. Arrays of gold nanorods were fabricated by electron beam lithography and lift-off techniques with high accuracy and uniformity. Their longitudinal plasmon scattering spectra were found to exhibit extreme sensitivity to the length of the nanorods. This phenomenon enables optical detection of the nanorod length variations comparable to the thickness of a few atomic layers of gold.