Silicon microprotrusions with tailored chirality enabled by direct femtosecond laser ablation.

Here, we report on formation of nanoprotrusions on the surface of a bulk crystalline silicon wafer under femtosecond-laser ablation with a donut-shaped laser beam. By breaking circular symmetry of the irradiating donut-shaped fs-pulse beam, a switch in geometry of the formed surface nanoprotrusions from regular to chiral was demonstrated. The chirality of the obtained Si nanostructures was promoted with an asymmetry degree of the laser beam. An uneven helical flow of laser-melted Si caused by asymmetry of the initial intensity and temperature pattern on the laser-irradiated Si surface explains this phenomenon. Chirality of the formed protrusions was confirmed by visualizing cross-sectional cuts produced by focused ion beam milling as well as Raman activity of these structures probed by circularly polarized light with opposite handedness. Our results open a pathway towards easy-to-implement inexpensive fabrication of chiral all-dielectric nanostructures for advanced nanophotonic applications and sensing of chiral molecules.

Influence of the dielectric substrate on the effective optical constants of silver plasmonic films.

The effective optical properties of plasmonic thin films can be used to model the far-field response of nanostructured materials to an incident electromagnetic field. In the present work, optically thin nanostructured silver (Ag) plasmonic films were fabricated on transparent dielectric substrates of soda-lime glass, sapphire, and fused silica using oblique angle deposition. The influence of the underlying dielectric substrate on the effective optical properties of the nanostructured layer was investigated by an ellipsometric-optical model based on Mueller matrix ellipsometry. The wavelength-dependent uniaxial optical responses of the nanostructured Ag films fabricated on sapphire were modeled with three Gaussian and one Tanguy oscillator, representing key optical phenomena over the range from 300 to 1000 nm. In comparison with the same Ag films on glass, the results confirm that the effective optical properties cannot be considered in isolation from the substrate. As expected, the extinction peak associated with the localized surface plasmon resonance was redshifted by approximately 220 nm per unit of the substrate refractive index. Importantly, it was found that the direction of incidence also influences the film behavior, with a substantial redshift in the extinction peak for light directed through the dielectric compared to free-space illumination. This property can have a significant effect on the far-field performance of these films.

Photoluminescence in hexagonal silicon carbide by direct femtosecond laser writing.

Direct femtosecond laser writing has been used to produce localized regions of photo-luminescent emission in 4H- and 6H-silicon carbide (SiC). Arrays of active color centers were fabricated by different pulse laser energies in the sites of square grids at various depths (from surface level to 10 µm below surface). We optically characterized the fabricated color centers using confocal imaging with 532 and 780 nm excitation, photo-luminescence spectroscopy, and lifetime decay at room temperature. We show that the technique can produce specifically the silicon vacancy color center emitting in the range 850-950 nm and other emitters in the 700 nm. This method can be adopted to engineer color centers in (SiC) at different depths in the material for single-photon generation, sensing, display fabrication, and light emitting diodes.

Effective optical constants of anisotropic silver nanoparticle films with plasmonic properties: erratum.

This erratum reports a correction to Fig. 5 in the original manuscript, Opt. Lett.41, 5495 (2016)OPLEDP0146-959210.1364/OL.41.005495.

Fluorescent color centers in laser ablated 4H-SiC nanoparticles.

Nanostructured and bulk silicon carbide (SiC) has recently emerged as a novel platform for quantum nanophotonics due to its harboring of paramagnetic color centers, having immediate applications as a single photon source and spin optical probes. Here, using ultra-short pulsed laser ablation, we fabricated from electron irradiated bulk 4H-SiC, 40-50 nm diameter SiC nanoparticles, fluorescent at 850-950 nm. This photoluminescence is attributed to the silicon vacancy color centers. We demonstrate that the original silicon vacancy color centers from the target sample were retained in the final nanoparticles solution, exhibiting excellent colloidal stability in water over several months. Our work is relevant for quantum nanophotonics, magnetic sensing, and biomedical imaging applications.

Hybrid curved nano-structured micro-optical elements.

Tailoring the spatial degree of freedom of light is an essential step towards the realization of advanced optical manipulation tools. A topical challenge consists of device miniaturization for improved performance and enhanced functionality at the micron scale. We demonstrate a novel approach that combines the additive three-dimensional (3D) structuring capability of laser polymerization and the subtractive subwavelength resolution patterning of focused ion beam lithography. As a case in point hybrid (dielectric/metallic) micro-optical elements that deliver a well-defined topological shaping of light are produced. Here we report on hybrid 3D binary spiral zone plates with unit and double topological charge. Their optical performances are compared to corresponding 2D counterparts both numerically and experimentally. Cooperative refractive capabilities without compromising topological beam shaping are shown. Realization of advanced designs where the dielectric architecture itself is endowed with singular properties is also discussed.

Effective optical constants of anisotropic silver nanoparticle films with plasmonic properties.

Mueller matrix ellipsometry has been used to determine the effective optical constants of island-like Ag films deposited by thermal evaporation. These films depart substantially from bulk silver with a prominent localized surface plasmon resonance. Moreover, despite the isotropic appearance, they exhibit uniaxial optical properties with the optical axis inclined by 83.4° from the substrate normal toward the direction of the incoming vapor flux. The uniaxial model supports the plasmon resonance peaks revealed by in-plane absorbance measurements of the films. The uniaxial behavior suggests that the resonances along the ordinary axes are weakly coupled between neighboring particles, whereas the extraordinary resonance is relatively strongly coupled. Therefore, the anisotropy should be considered in the practical applications of these plasmonic films.

Photoluminescence from voids created by femtosecond-laser pulses inside cubic-BN.

Photoluminescence (PL) from femtosecond-laser-modified regions inside cubic-boron nitride (c-BN) was measured under UV and visible light excitation. Bright PL at the red spectral range was observed, with a typical excited state lifetime of ∼4 ns. Sharp emission lines are consistent with PL of intrinsic vibronic defects linked to the nitrogen vacancy formation (via Frenkel pair) observed earlier in high-energy electron-irradiated and ion-implanted c-BN. These, formerly known as the radiation centers, RC1, RC2, and RC3, have been identified at the locus of the voids formed by a single femtosecond-laser pulse. The method is promising to engineer color centers in c-BN for photonic applications.

Hierarchical anti-reflective laser-induced periodic surface structures (LIPSSs) on amorphous Si films for sensing applications.

Here, we applied direct laser-induced periodic surface structuring to drive the phase transition of amorphous silicon (a-Si) into nanocrystalline (nc) Si imprinted as regular arrangement of Si nanopillars passivated with a SiO2 layer. By varying the laser beam scanning speed at a fixed pulse energy, we successfully tailored the resulting unique surface morphology of the formed LIPSSs that change from ordered arrangement of conical protrusions to highly uniform surface gratings, where sub-wavelength scale ripples decorate the valleys between near-wavelength scale ridges. Along with the surface morphology, the nc-Si/SiO2 volume ratio can also be controlled via laser processing parameters allowing the tailoring of the optical properties of the produced textured surfaces to achieve anti-reflection performance or partial transmission in the visible spectral range. Diverse hierarchical LIPSSs can be fabricated and replicated over large-scale areas opening a pathway for various applications including optical sensors, nanoscale temperature management, and solar light harvesting. By taking advantage of good wettability, enlarged surface area and remarkable light-trapping characteristics of the produced hierarchical morphologies, we demonstrated the first LIPSS-based surface enhanced fluorescent sensor that allowed the identification of metal cations providing a sub-nM detection limit unachievable by conventional fluorescence measurements in solutions.

Tailoring spontaneous infrared emission of HgTe quantum dots with laser-printed plasmonic arrays.

Chemically synthesized near-infrared to mid-infrared (IR) colloidal quantum dots (QDs) offer a promising platform for the realization of devices including emitters, detectors, security, and sensor systems. However, at longer wavelengths, the quantum yield of such QDs decreases as the radiative emission rate drops following Fermi's golden rule, while non-radiative recombination channels compete with light emission. Control over the radiative and non-radiative channels of the IR-emitting QDs is crucially important to improve the performance of IR-range devices. Here, we demonstrate strong enhancement of the spontaneous emission rate of near- to mid-IR HgTe QDs coupled to periodically arranged plasmonic nanoantennas, in the form of nanobumps, produced on the surface of glass-supported Au films via ablation-free direct femtosecond laser printing. The enhancement is achieved by simultaneous radiative coupling of the emission that spectrally matches the first-order lattice resonance of the arrays, as well as more efficient photoluminescence excitation provided by coupling of the pump radiation to the local surface plasmon resonances of the isolated nanoantennas. Moreover, coupling of the HgTe QDs to the lattice plasmons reduces the influence of non-radiative decay losses mediated by the formation of polarons formed between QD surface-trapped carriers and the IR absorption bands of dodecanethiol used as a ligand on the QDs, allowing us to improve the shape of the emission spectrum through a reduction in the spectral dip related to this ligand coupling. Considering the ease of the chemical synthesis and processing of the HgTe QDs combined with the scalability of the direct laser fabrication of nanoantennas with tailored plasmonic responses, our results provide an important step towards the design of IR-range devices for various applications.

Vibrations of microspheres probed with ultrashort optical pulses.

We use ultrashort optical pulses to excite and detect vibrations of single silica spheres with a diameter of 5 microm placed at the surface of an acoustically mismatched substrate. In addition to the photoelastic detection of picosecond longitudinal acoustic pulses propagating inside the bulk, we detect gigahertz acoustic resonances of the sphere through probe beam defocusing. The mode frequencies are in close accord with those calculated from the elastic vibrations of a free sphere. We also record a resonant enhancement in the amplitude of specific modes of two touching spheres.

Si1-xGex nanoantennas with a tailored Raman response and light-to-heat conversion for advanced sensing applications.

Active light-emitting all-dielectric nanoantennas recently have demonstrated great potential as highly efficient nanoscale light sources owing to their strong luminescent and Raman responses. However, their large-scale fabrication faces a number of problems related to productivity limits of existing lithography techniques. Thus, high-throughput fabrication strategies allowing in a facile way to tailor of the nanoantenna emission and thermal properties in the process of their fabrication are highly desirable for various applications. Here, we propose a cost-effective approach to large-scale fabrication of Si1-xGex alloyed Mie nanoresonators possessing an enhanced inherent Raman response which can be simply tailored via tuning the Ge concentration. Moreover, by tailoring the relative Ge composition one can gradually change a complex refractive index of the produced Si1-xGex alloy, which affects the ratio between radiative and nonradiative losses in Si1-xGex nanoantennas, which is crucial for optimization of their optical heating efficiency. Composition-tunable Si1-xGex nanoantennas with an optimized size, light-to-heat conversion and Raman response are implemented for non-invasive sensing of 4-aminothiophenol molecules with a temperature feedback modality and high subwavelength spatial resolution. The results are important for advanced multichannel optical sensing, providing information on analyte's composition, analyte-nanoantenna temperature response and spatial position.

Chemically non-perturbing SERS detection of a catalytic reaction with black silicon.

All-dielectric resonant micro- and nano-structures made of high-index dielectrics have recently emerged as a promising surface-enhanced Raman scattering (SERS) platform which can complement or potentially replace the metal-based counterparts in routine sensing measurements. These unique structures combine the highly-tunable optical response and high field enhancement with the non-invasiveness, i.e. chemically non-perturbing the analyte, simple chemical modification and recyclability. Meanwhile, commercially competitive fabrication technologies for mass production of such structures are still missing. Here, we attest a chemically inert black silicon (b-Si) substrate consisting of randomly-arranged spiky Mie resonators for a true non-invasive (chemically non-perturbing) SERS identification of the molecular fingerprints at low concentrations. Based on the comparative in situ SERS tracking of the para-aminothiophenol (PATP)-to-4,4'-dimercaptoazobenzene (DMAB) catalytic conversion on the bare and metal-coated b-Si, we justify the applicability of the metal-free b-Si for ultra-sensitive non-invasive SERS detection at a concentration level as low as 10-6 M. We performed supporting finite-difference time-domain (FDTD) calculations to reveal the electromagnetic enhancement provided by an isolated spiky Si resonator in the visible spectral range. Additional comparative SERS studies of the PATP-to-DMAB conversion performed with a chemically active bare black copper oxide (b-CuO) substrate as well as SERS detection of the slow daylight-driven PATP-to-DMAB catalytic conversion in the aqueous methanol solution loaded with colloidal silver nanoparticles (Ag NPs) confirm the non-invasive SERS performance of the all-dielectric crystalline b-Si substrate. A proposed SERS substrate can be fabricated using the easy-to-implement scalable technology of plasma etching amenable on substrate areas over 10 × 10 cm2 making such inexpensive all-dielectric substrates promising for routine SERS applications, where the non-invasiveness is of high importance.

Inverse silica opal photonic crystals for optical sensing applications.

This work reports fabrication of inverse silica opal photonic crystal structures from direct polystyrene micro sphere opals using low-temperature sol-gel infiltration of silica, and examines performance of these photonic crystals as environmental refractive index sensors. Sensitivity of the spectral position and optical attenuation of photonic stop gaps is found to allow detection of the index changes by the amount of ~10(-3). The high value of sensitivity, which is comparable with those of other optical sensing techniques, along with simplicity of the optical detection setup required for sensing, and the low-temperature, energy-efficient fabrication process make inverse silica opals attractive systems for optical sensing applications.

Engineering 3D Nanoplasmonic Assemblies for High Performance Spectroscopic Sensing.

We demonstrate the fabrication of plasmonic sensors that comprise gold nanopillar arrays exhibiting high surface areas, and narrow gaps, through self-assembly of amphiphilic diblock copolymer micelles on silicon substrates. Silicon nanopillars with high integrity over arbitrary large areas are obtained using copolymer micelles as lithographic templates. The gaps between metal features are controlled by varying the thickness of the evaporated gold. The resulting gold metal nanopillar arrays exhibit an engineered surface topography, together with uniform and controlled separations down to sub-10 nm suitable for highly sensitive detection of molecular analytes by Surface Enhanced Raman Spectroscopy (SERS). The significance of the approach is demonstrated through the control exercised at each step, including template preparation and pattern-transfer steps. The approach is a promising means to address trade-offs between resolutions, throughput, and performance in the fabrication of nanoplasmonic assemblies for sensing applications.

Laser manipulation of a smectic liquid-crystal droplet.

The laser trapping of a smectic-A liquid-crystal micro-droplet was spatially traced during its transient into the trapped position. The lateral and angular orientation of the droplet were determined and followed in time during the axial descent of the micro-droplet into the stationary trapped position using the analysis of polarization changes of the light passed through the droplet with temporal resolution of a video refresh rate of 30 ms. The spatial resolution of 0.1-1 microm has been achieved for typical laser trapping powers of 2-600 mW. The axial profile of a laser trapping force (an ellipticity of the focal spot) has been determined. The laser trapping mechanism of smectic micro-droplets is discussed in terms of minimization of a light-droplet interaction.

Laser-induced microexplosion confined in the bulk of a sapphire crystal: evidence of multimegabar pressures.

Extremely high pressures (approximately 10 TPa) and temperatures (5 x 10(5) K) have been produced using a single laser pulse (100 nJ, 800 nm, 200 fs) focused inside a sapphire crystal. The laser pulse creates an intensity over 10(14) W/cm2 converting material within the absorbing volume of approximately 0.2 microm3 into plasma in a few fs. A pressure of approximately 10 TPa, far exceeding the strength of any material, is created generating strong shock and rarefaction waves. This results in the formation of a nanovoid surrounded by a shell of shock-affected material inside undamaged crystal. Analysis of the size of the void and the shock-affected zone versus the deposited energy shows that the experimental results can be understood on the basis of conservation laws and be modeled by plasma hydrodynamics. Matter subjected to record heating and cooling rates of 10(18) K/s can, thus, be studied in a well-controlled laboratory environment.

Reversible phase transitions in polymer gels induced by radiation forces.

Many polymer gels undergo reversible, discontinuous volume changes in response to changes in the balance between repulsive intermolecular forces that act to expand the polymer network and attractive forces that act to shrink it. Repulsive forces are usually electrostatic or hydrophobic in nature, whereas attraction is mediated by hydrogen bonding or van der Waals interactions. The competition between these counteracting forces, and hence the gel volume, can thus be controlled by subtle changes in parameters such as pH (ref. 4), temperature, solvent composition or gel composition. Here we describe a more direct influence on this balance of forces, by showing that the radiation force generated by a focused laser beam induces reversible shrinkage in polymer gels. Control experiments confirm that the laser-induced volume phase transitions are due to radiation forces, rather than local heating, modifying the weak interactions in the gels, in agreement with previous observations of light-induced chain association in polymer solutions. We find that, owing to shear-relaxation processes, gel shrinkage occurs up to several tens of micrometres away from the irradiation spot, raising the prospect that the combination of stimuli-responsive polymer gels and laser light might lead to new gel-based systems for applications such as actuating or sensing.

Femtosecond laser-assisted three-dimensional microfabrication in silica.

We demonstrate direct three-dimensional (3-D) microfabrication inside a volume of silica glass. The whole fabrication process was carried out in two steps:(i) writing of the preprogrammed 3-D pattern inside silica glass by focused femtosecond (fs) laser pulses and (ii) etching of the written structure in a 5% aqueous solution of HF acid. This technique allows fabrication of 3-D channels as small as 10mum in diameter inside the volume with any angle of interconnection and a high aspect ratio (10mum -diameter channels in a 100mum -thick silica slab).

Arbitrary-lattice photonic crystals created by multiphoton microfabrication.

We used voxels of an intensely modified refractive index generated by multiphoton absorption at the focus of femtosecond laser pulses in Ge-doped silica as photonic atoms to build photonic lattices. The voxels were spatially organized in the same way as atoms arrayed in actual crystals, and a Bragg-like diffraction from the photonic atoms was evidenced by a photonic bandgap (PBG) effect. Postfabrication annealing was found to be essential for reducing random scattering and therefore enhancing PBG. This technique has an intrinsic capability of individually addressing single atoms. Therefore the introduction of defect structures was much facilitated, making the technique quite appealing for photonic research and applications.