Laser fabrication of crystalline silicon nanoresonators from an amorphous film for low-loss all-dielectric nanophotonics

FeAu mixing for high-temperature control of light scattering at the nanometer scale

Control of the optical properties of a nanoparticle (NP) through its structural changes underlies optical data processing, dynamic coloring, and smart sensing at the nanometer scale. Here, we report on the concept of controlling the light scattering by a NP through mixing of weakly miscible chemical elements (Fe and Au), supporting a thermal-induced phase transformation. The transformation corresponds to the transition from a homogeneous metastable solid solution phase of the (Fe,Au) NP towards an equilibrium biphasic Janus-type NP. We demonstrate that the phase transformation is thermally activated by laser heating up to a threshold of 800 °C (for NPs with a size of hundreds of nm), leading to the associated changes in the light scattering and color of the NP. The results thereby pave the way for the implementation of optical sensors triggered by a high temperature at the nanometer scale via NPs based on metal alloys.

Tuning dipolar and multipolar resonances of chiral silicon nanostructures for control of near field superchirality

Chiral materials display a property called optical activity, which is the capability to interact differentially with left and right circularly polarised light. This leads to the ability to manipulate the polarisation state of light, which has a broad range of applications spanning from energy efficient displays to quantum technologies. Both synthesised and engineered chiral nanomaterials are exploited in such devices. The design strategy for optimising the optical activity of a chiral material is typically based on maximising a single parameter, the electric dipole-magnetic dipole response. Here we demonstrate an alternative approach of controlling optical activity by manipulating both the dipole and multipolar response of a nanomaterial. This provides an additional parameter for material design, affording greater flexibility. The exemplar systems used to illustrate the strategy are nanofabricated chiral silicon structures. The multipolar response of the structures, and hence their optical activity, can be controlled simply by varying their height. This phenomenon allows optical activity and the creation of so called superchiral fields, with enhanced asymmetries, to be controlled over a broader wavelength range, than is achievable with just the electric dipole-magnetic dipole response. This work adds to the material design toolbox providing a route to novel nanomaterials for optoelectronics and sensing applications.

3D and Inkjet Printing by Colored Mie-Resonant Silicon Nanoparticles Produced by Laser Ablation in Liquid

Optically resonant silicon nanoparticles have emerged as a prospective platform for the structural coloration of surfaces because of their strong and spectrally selective light scattering. In this work, we developed colorful inks based on polymer mixed with monodisperse Mie-resonant silicon nanoparticles for 3D and inkjet printing. We applied a laser ablation method in a flow cell for the mass production of silicon nanoparticles in water and separated the resulting nanoparticles with different sizes by density-gradient centrifugation. Mixing the colorful nanoparticles with the polymer allows for the printing of 3D objects with various shapes and colors, which are rigid against environmental conditions.

All-Dielectric Nanostructures with a Thermoresponsible Dynamic Polymer Shell

We suggest a new strategy for creating stimuli-responsive bio-integrated optical nanostructures based on Mie-resonant silicon nanoparticles covered by an ensemble of similarity negatively charged polyelectrolytes (heparin and sodium polystyrene sulfonate). The dynamic tuning of the nanostructures' optical response is due to light-induced heating of the nanoparticles and swelling of the polyelectrolyte shell. The resulting hydrophilic/hydrophobic transitions significantly change the shell thickness and reversible shift of the scattering spectra for individual nanoparticles up to 60 nm. Our findings bring novel opportunities for the application of smart nanomaterials in nanomedicine and bio-integrated nanophotonics.

Laser-printed hemispherical silicon Mie resonators

Subwavelength nanostructures made of high-index low-loss materials have revolutionized the fields of linear and nonlinear nanophotonics, stimulating growing demands for efficient and inexpensive fabrication technologies. Here, we demonstrate high-precision and reproducible printing of hemispherical Si nanoparticles (NPs) via controllable dewetting of glass-supported $\alpha$-Si films driven by a single femtosecond laser pulse. The diameter of the formed nanocrystalline NPs can be fully controlled by initial $\alpha$-Si film thickness as well as lateral size of the laser spot and can be predicted by a simple empirical model based on conservation of energy and mass. A resonant optical response associated with Mie-type resonances supported by hemispherical NPs was confirmed by combining numerical modeling with optical microspectroscopy. Inexpensive and high-performing direct laser printing of nanocrystalline Si Mie resonators with a user-defined arrangement opens a pathway for various applications in optical sensing and nonlinear nanophotonics.

Femtosecond Laser Fabrication of Hybrid Metal-Dielectric Structures with Nonlinear Photoluminescence

Fabrication of hybrid micro- and nanostructures with a strong nonlinear response is challenging and represents a great interest due to a wide range of photonic applications. Usually, such structures are produced by quite complicated and time-consuming techniques. This work demonstrates laser-induced hybrid metal-dielectric structures with strong nonlinear properties obtained by a single-step fabrication process. We determine the influence of several incident femtosecond pulses on the Au/Si bi-layer film on produced structure morphology. The created hybrid systems represent isolated nanoparticles with a height of 250–500 nm exceeding the total thickness of the Au-Si bi-layer. It is shown that fabricated hybrid nanostructures demonstrate enhancement of the SHG signal (up to two orders of magnitude) compared to the initial planar sample and a broadband photoluminescence signal (more than 200 nm in width) in the visible spectral region. We establish the correlation between nonlinear signal and phase composition provided by Raman scattering measurements. Such laser-induced structures have significant potential in optical sensing applications and can be used as components for different nanophotonic devices.

Directional light emission by electric and magnetic dipoles near a nanosphere: an analytical approach based on the generalized Mie theory

We present a theoretical study of directional light emission by dipole emitters near a spherical nanoparticle. Our analysis is extended from an exact electrodynamical approach for solving the coupling between a dipole and a sphere, providing a full picture of the directional emission for a complete set of combinations of variable emitters, particles, and their orientations. In particular, we show that the Mie resonances of a dielectric sphere are strongly influenced by the coupled dipole emitter, leading to the scattering properties that are different from the prediction by the standard Mie theory. Moreover, we demonstrate that the dielectric spheres have opposite effects on the emission direction and a decay rate of electric and magnetic dipoles. Our approach enriches the analytical toolbox for designing optical antennas and understanding dipole-sphere coupling.

Non-isolated sources of electromagnetic radiation by multipole decomposition for photonic quantum technologies on a chip with nanoscale apertures

The creation of single photon sources on a chip is a mid-term milestone on the road to chip-scale quantum computing. An in-depth understanding of the extended multipole decomposition of non-isolated sources of electromagnetic radiation is not only relevant for a microscopic description of fundamental phenomena, such as light propagation in a medium, but also for emerging applications such as single-photon sources. To design single photon emitters on a chip, we consider a ridge dielectric waveguide perturbed with a cylindrical inclusion. For this, we expanded classical multipole decomposition that allows simplifying and interpreting complex optical interactions in an intuitive manner to make it suitable for analyzing light-matter interactions with non-isolated objects that are parts of a larger network, e.g. individual components such as a single photon source of an optical chip. It is shown that our formalism can be used to design single photon sources on a chip.

Fabrication of Ge2Sb2Te5 crystal micro/nanostructures through single-shot Gaussian-shape femtosecond laser pulse irradiation

Femtosecond (fs) laser-thin film interaction is one of the most practical methods for fabricating functional nanostructures. However, the details of the interaction mechanism remain unclear. In this study, we demonstrate an abnormal ablation effect on nanofilms by using a tightly focused single fs laser pulse. After the irradiation of a single Gaussian-shaped femtosecond laser pulse, a molten micro/nanopatch at the irradiated central high-power zone is isolated from the surrounding film. The confined localized threshold effect is proposed as the main mechanism for the phase isolation. With this effect, the high refractive index dielectric Ge₂Sb₂Te₅ crystal nanostructures can be fabricated by directed dewetting of the isolated molten micro/nanopatch on Si substrates. After the laser irradiation, the central isolated liquid through an amorphous GST film is transformed into a crystalline state after resolidification. The isolated central micro/nanopatch size can be controlled by the focused spot size and pulse energy, so that the morphologies (size, geometrical morphology, and distribution) of GST nanostructures can be flexibly modulated. Furthermore, separated solid and liquid phase states detected using spatial-temporal-resolved microscopy validates the crucial role of the confined-localized threshold effect in the dewetting effect based on the separated liquid phase.

Manipulation Technique for Precise Transfer of Single Perovskite Nanoparticles

In this article, we present the pick-and-place technique for the manipulation of single nanoparticles on non-conductive substrates using a tungsten tip irradiated by a focused electron beam from a scanning electron microscope. The developed technique allowed us to perform the precise transfer of single BaTiO₃ nanoparticles from one substrate to another in order to carry out measurements of elastic light scattering as well as second harmonic generation. Also, we demonstrate a fabricated structure made by finely tuning the position of a BaTiO₃ nanoparticle on top of a dielectric nanowaveguide deposited on a glass substrate. The presented technique is based on the electrostatic interaction between the sharp tungsten tip charged by the electron beam and the nanoscale object. A mechanism for nanoparticle transfer to a non-conductive substrate is proposed and the forces involved in the manipulation process are evaluated. The presented technique can be widely utilized for the fabrication of nanoscale structures on optically transparent non-conductive substrates, which presents a wide range of applications for nanophotonics.

Direct and High-Throughput Fabrication of Mie-Resonant Metasurfaces via Single-Pulse Laser Interference

High-index dielectric metasurfaces featuring Mie-type electric and magnetic resonances have been of great interest in a variety of applications such as imaging, sensing, photovoltaics, and others, which led to the necessity of an efficient large-scale fabrication technique. To address this, here we demonstrate the use of single-pulse laser interference for direct patterning of an amorphous silicon film into an array of Mie resonators a few hundred nanometers in diameter. The proposed technique is based on laser-interference-induced dewetting. A precise control of the laser pulse energy enables the fabrication of ordered dielectric metasurfaces in areas spanning tens of micrometers and consisting of thousands of hemispherical nanoparticles with a single laser shot. The fabricated nanoparticles exhibit a wavelength-dependent optical response with a strong electric dipole signature. Variation of the predeposited silicon film thickness allows tailoring of the resonances in the targeted visible and infrared spectral ranges. Such direct and high-throughput fabrication is a step toward a simple realization of spatially invariant metasurface-based devices.

Plasmonic nanosponges filled with silicon for enhanced white light emission

Plasmonic nanosponges are a powerful platform for various nanophotonic applications owing to extremely high local field enhancement in metallic nanopores. The filling of the nanopores with high-refractive index semiconductors (e.g. Si, Ge, GaP, etc.) opens up opportunities for the enhancement of nonlinear effects in these materials. However, this task remains challenging due to the lack of knowledge on the integration process of metal and high-index semiconductor components in such nanoobjects. Here, we investigate metal-dielectric nanoparticles fabricated from bilayer Si/Au films by the laser printing technique via a combination of theoretical and experimental methods. We reveal that these hybrid nanoparticles represent the Au sponge-like nanostructure filled with Si nanocrystallites. We also demonstrate that the Au net provides strong near-field enhancement in the Si grains increasing the white light photoluminescence in the hybrid nanostructures compared to uniform Si nanoparticles. These results pave the way for engineering the internal structure of the sponge-like hybrid nanoparticles possessing white light luminescence and control of their optical properties on demand.

Hollow-structure engineering of a silicon–carbon anode for ultra-stable lithium-ion batteries

Hollow silicon nanotubes were successfully synthesized in situ on a carbon substrate, which effectively accommodate the volume expansion of silicon and exhibit ultra-stable performance as the anode of lithium-ion batteries.

Probing optical resonances of silicon nanostructures using tunable-excitation Raman spectroscopy

Optical materials with a high refractive index enable effective manipulation of light at the nanoscale through strong light confinement. However, the optical near field, which is mainly confined inside such high-index nanostructures, is difficult to probe with existing measurement techniques. Here, we exploit the connection between Raman scattering and the stored electric energy to detect resonance-induced near-field enhancements in silicon nanostructures. We introduce a Raman setup with a wavelength-tunable laser, which allows us to tune the Raman excitation wavelength and thereby identify Fabry-Pérot and Mie type resonances in silicon thin films and nanodisk arrays, respectively. We measure the optical near-field enhancement by comparing the Raman response on and off resonance. Our results show that tunable-excitation Raman spectroscopy can be used as a complimentary far-field technique to reflection measurements for nanoscale characterization and quality control. As proof-of-principle for the latter, we demonstrate that Raman spectroscopy captures fabrication imperfections in the silicon nanodisk arrays, enabling an all-optical quality control of metasurfaces.

Enhanced excitation and emission from 2D transition metal dichalcogenides with all-dielectric nanoantennas

The recently emerged concept of all-dielectric nanophotonics based on optical Mie resonances in high-index dielectric nanoparticles has proven to be a promising pathway to boost light-matter interactions at the nanoscale. In this work, we discuss the opportunities enabled by the interaction of dielectric nanoresonators with 2D transition metal dichalcogenides (2D TMDCs), leading to weak and strong coupling regimes. We perform a comprehensive analysis of bright exciton photoluminescence (PL) enhancement from various 2D TMDCs, including WS₂, MoS₂, WSe₂, and MoSe₂ via their coupling to Mie resonances of a silicon nanoparticle. For each case, we find the system parameters corresponding to maximal PL enhancement taking into account excitation rate, Purcell factor and radiation efficiency. We demonstrate numerically that all-dielectric Si nanoantennas can significantly enhance the PL intensity from 2D TMDC by a factor of hundred through precise optimization of the geometrical and material parameters. Our results may be useful for high-efficiency 2D TMDC-based optoelectronic, nanophotonic, and quantum optical devices.

Laser printing of optically resonant hollow crystalline carbon nanostructures from 1D and 2D metal-organic frameworks

Using a hybrid approach involving a slow diffusion method to synthesize 1D and 2D MOFs followed by their treatment with femtosecond infrared laser radiation, we generated 100-600 nm well-defined hollow spheres and hemispheres of graphite. This ultra-fast technique extends the library of shapes of crystalline MOF derivatives appropriate for all-dielectric nanophotonics.

Tunable Hybrid Fano Resonances in Halide Perovskite Nanoparticles

Halide perovskites are known to support excitons at room temperatures with high quantum yield of luminescence that make them attractive for all-dielectric resonant nanophotonics and meta-optics. Here we report the observation of broadly tunable Fano resonances in halide perovskite nanoparticles originating from the coupling of excitons to the Mie resonances excited in the nanoparticles. Signatures of the photon-exciton (" hybrid") Fano resonances are observed in dark-field spectra of isolated nanoparticles, and also in the extinction spectra of aperiodic lattices of such nanoparticles. In the latter case, chemical tunability of the exciton resonance allows reversible tuning of the Fano resonance across the 100 nm bandwidth in the visible frequency range, providing a novel approach to control optical properties of perovskite nanostructures. The proposed method of chemical tuning paves the way to an efficient control of emission properties of on-chip-integrated light-emitting nanoantennas.

Enabling silicon-on-silicon photonics with pedestalled Mie resonators

High-refractive-index Mie resonators are regarded as promising building blocks for low-loss all-dielectric nanophotonic applications. To avoid the otherwise excessive damping and loss of symmetry such devices typically need to be implemented over a low-index substrate, which hampers their integration in many practical applications. In this paper we propose a new photonic structure consisting of silicon-on-silicon spheroidal-like resonators, each one supported by a slim silicon pedestal that makes the micro-cavities stand optically separated from the substrate while providing both mechanical stability and electrical contact with the substrate. These structures are produced in high-quality monocrystalline Si and their size and arrangement can be precisely controlled through standard lithography. We demonstrate that such structures present an optical performance similar to the one achieved with low-index substrates, opening new avenues for developing novel hybrid photonic/electronic devices.

The conformation of bovine serum albumin adsorbed to the surface of single all-dielectric nanoparticles following light-induced heating

Interaction between nanoparticles and biomolecules leads to the formation of biocompatible or bioadverse complexes. Despite the rapid development of nanotechnologies for biology and medicine, relatively little is known about the structure of such complexes. Here, we report on the changes in conformation of a blood protein (bovine serum albumin) adsorbed on the surface of single all-dielectric nanoparticles (silicon and germanium) following light-induced heating to 640 K. This protein is considerably more resistant to heat when adsorbed on the nanoparticle than when in solution or in the solid state. Intriguingly, with germanium nanoparticles this heat resistance is more pronounced than with silicon. These observations will facilitate biocompatible usage of all-dielectric nanoparticles.

Resonant silicon nanoparticles with controllable crystalline states and nonlinear optical responses

High-throughput laser printing of resonant silicon nanoparticles has emerged as a novel tool for the fabrication of deeply subwavelength objects with various functionalities. The applications of resonant silicon nanoparticles crucially depend on their crystalline state. However, the ways to control the crystalline structure during laser printing remain unstudied. Here we demonstrate, both experimentally and theoretically, how the crystalline structure of silicon nanoparticles fabricated by a laser printing technique can be varied from almost amorphous to a polycrystalline state. In particular, we propose a method of crystalline structure control via changing the distance between the irradiated silicon film and the receiving substrate. This study allows the most optimal conditions for second harmonic generation to be revealed. We believe that the proposed method opens the door to fully controllable laser printing of functional nanoparticles and nanostructures.

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.

Photoluminescence quenching of dye molecules near a resonant silicon nanoparticle

Luminescent molecules attached to resonant colloidal particles are an important tool to study light-matter interaction. A traditional approach to enhance the photoluminescence intensity of the luminescent molecules in such conjugates is to incorporate spacer-coated plasmonic nanoantennas, where the spacer prevents intense non-radiative decay of the luminescent molecules. Here, we explore the capabilities of an alternative platform for photoluminescence enhancement, which is based on low-loss Mie-resonant colloidal silicon particles. We demonstrate that resonant silicon particles of spherical shape are more efficient for photoluminescence enhancement than their plasmonic counterparts in spacer-free configuration. Our theoretical calculations show that significant enhancement originates from larger quantum yields supported by silicon particles and their resonant features. Our results prove the potential of high-index dielectric particles for spacer-free enhancement of photoluminescence, which potentially could be a future platform for bioimaging and nanolasers.

Light-Emitting Halide Perovskite Nanoantennas

Nanoantennas made of high-index dielectrics with low losses in visible and infrared frequency ranges have emerged as a novel platform for advanced nanophotonic devices. On the other hand, halide perovskites are known to possess high refractive index, and they support excitons at room temperature with high binding energies and quantum yield of luminescence that makes them very attractive for all-dielectric resonant nanophotonics. Here we employ halide perovskites to create light-emitting nanoantennas with enhanced photoluminescence due to the coupling of their excitons to dipolar and multipolar Mie resonances. We demonstrate that the halide perovskite nanoantennas can emit light in the range of 530-770 nm depending on their composition. We employ a simple technique based on laser ablation of thin films prepared by wet-chemistry methods as a novel cost-effective approach for the fabrication of resonant perovskite nanostructures.

Resonant silicon nanoparticles for enhancement of light absorption and photoluminescence from hybrid perovskite films and metasurfaces

Recently, hybrid halide perovskites have emerged as one of the most promising types of materials for thin-film photovoltaic and light-emitting devices because of their low-cost and potential for high efficiency. Further boosting their performance without detrimentally increasing the complexity of the architecture is critically important for commercialization. Despite a number of plasmonic nanoparticle based designs having been proposed for solar cell improvement, inherent optical losses of the nanoparticles reduce photoluminescence from perovskites. Here we use low-loss high-refractive-index dielectric (silicon) nanoparticles for improving the optical properties of organo-metallic perovskite (MAPbI₃) films and metasurfaces to achieve strong enhancement of photoluminescence as well as useful light absorption. As a result, we observed experimentally a 50% enhancement of photoluminescence intensity from a perovskite layer with silicon nanoparticles and 200% enhancement for a nanoimprinted metasurface with silicon nanoparticles on top. Strong increase in light absorption is also demonstrated and described by theoretical calculations. Since both silicon nanoparticle fabrication/deposition and metasurface nanoimprinting techniques are low-cost, we believe that the developed all-dielectric approach paves the way to novel scalable and highly effective designs of perovskite based metadevices.

Efficient Second-Harmonic Generation in Nanocrystalline Silicon Nanoparticles

Recent trends to employ high-index dielectric particles in nanophotonics are motivated by their reduced dissipative losses and large resonant enhancement of nonlinear effects at the nanoscale. Because silicon is a centrosymmetric material, the studies of nonlinear optical properties of silicon nanoparticles have been targeting primarily the third-harmonic generation effects. Here we demonstrate, both experimentally and theoretically, that resonantly excited nanocrystalline silicon nanoparticles fabricated by an optimized laser printing technique can exhibit strong second-harmonic generation (SHG) effects. We attribute an unexpectedly high yield of the nonlinear conversion to a nanocrystalline structure of nanoparticles supporting the Mie resonances. The demonstrated efficient SHG at green light from a single silicon nanoparticle is 2 orders of magnitude higher than that from unstructured silicon films. This efficiency is significantly higher than that of many plasmonic nanostructures and small silicon nanoparticles in the visible range, and it can be useful for a design of nonlinear nanoantennas and silicon-based integrated light sources.

Resonant Nonplasmonic Nanoparticles for Efficient Temperature-Feedback Optical Heating

We propose a novel photothermal approach based on resonant dielectric nanoparticles, which possess imaginary part of permittivity significantly smaller as compared to metal ones. We show both experimentally and theoretically that a spherical silicon nanoparticle with a magnetic quadrupolar Mie resonance converts light to heat up to 4 times more effectively than similar spherical gold nanoparticle at the same heating conditions. We observe photoinduced temperature raise up to 900 K with the silicon nanoparticle on a glass substrate at moderate intensities (<2 mW/μm²) and typical laser wavelength (633 nm). The advantage of using crystalline silicon is the simplicity of local temperature control by means of Raman spectroscopy working in a broad range of temperatures, that is, up to the melting point of silicon (1690 K) with submicrometer spatial resolution. Our CMOS-compatible heater-thermometer nanoplatform paves the way to novel nonplasmonic photothermal applications, extending the temperature range and simplifying the thermoimaging procedure.

Templated Solid-State Dewetting of Thin Silicon Films

Thin film dewetting can be efficiently exploited for the implementation of functionalized surfaces over very large scales. Although the formation of sub-micrometer sized crystals via solid-state dewetting represents a viable method for the fabrication of quantum dots and optical meta-surfaces, there are several limitations related to the intrinsic features of dewetting in a crystalline medium. Disordered spatial organization, size, and shape fluctuations are relevant issues not properly addressed so far. This study reports on the deterministic nucleation and precise positioning of Si- and SiGe-based nanocrystals by templated solid-state dewetting of thin silicon films. The dewetting dynamics is guided by pattern size and shape taking full control over number, size, shape, and relative position of the particles (islands dimensions and relative distances are in the hundreds nm range and fluctuate ≈11% for the volumes and ≈5% for the positioning).

Self-adjusted all-dielectric metasurfaces for deep ultraviolet femtosecond pulse generation

The advantage of metasurfaces and nanostructures with a high nonlinear response is that they do not require phase matching, and the generated pulses are short in the time domain without additional pulse compression. However, the fabrication of large-scale planar structures by lithography-based methods is expensive, time consuming, and requires complicated preliminary simulations to obtain the most optimized geometry. Here, we propose a novel strategy for the self-assembled fabrication of large-scale resonant metasurfaces, where incident femtosecond laser pulses adjust the initial silicon films via specific surface deformation to be as resonant as possible for a given wavelength. The self-adjusting approach eliminates the necessity of multistep lithography and designing, because interference between the incident and the scattered parts of each laser pulse "imprints" resonant field distribution within the film. The self-adjusted metasurfaces demonstrate a high damage threshold (≈10¹² W cm^-2) and efficient frequency conversion from near-IR to deep UV. The conversion efficiency is up to 30-fold higher compared with nonresonant smooth Si films. The resulting metasurfaces allow for the generation of UV femtosecond laser pulses at a wavelength of 270 nm with a high peak and average power (≈10⁵ W and ≈1.5 μW, respectively). The results pave the way to the creation of ultrathin nonlinear metadevices working at high laser intensities for efficient deep UV generation at the nanoscale.

Laser printing of resonant plasmonic nanovoids

Hollow reduced-symmetry resonant plasmonic nanostructures possess pronounced tunable optical resonances in the UV-vis-IR range, being a promising platform for advanced nanophotonic devices. However, the present fabrication approaches require several consecutive technological steps to produce such nanostructures, making their large-scale fabrication rather time-consuming and expensive. Here, we report on direct single-step fabrication of large-scale arrays of hollow parabolic- and cone-shaped nanovoids in silver and gold thin films, using single-pulse femtosecond nanoablation at high repetition rates. The lateral and vertical size of such nanovoids was found to be laser energy-tunable. Resonant light scattering from individual nanovoids was observed in the visible spectral range, using dark-field confocal microspectroscopy, with the size-dependent resonant peak positions. These colored geometric resonances in far-field scattering were related to excitation and interference of transverse surface plasmon modes in nanovoid shells. Plasmon-mediated electromagnetic field enhancement near the nanovoids was evaluated via finite-difference time-domain calculations for their model shapes simulated by three-dimensional molecular dynamics, and experimentally verified by means of photoluminescence microscopy and Raman spectroscopy.

Resonant Raman scattering from silicon nanoparticles enhanced by magnetic response

Enhancement of optical response with high-index dielectric nanoparticles is attributed to the excitation of their Mie-type magnetic and electric resonances. Here we study Raman scattering from crystalline silicon nanoparticles and reveal that magnetic dipole modes have a much stronger effect on the scattering than electric modes of the same order. We demonstrate experimentally a 140-fold enhancement of the Raman signal from individual silicon spherical nanoparticles at the magnetic dipole resonance. Our results confirm the importance of the optically-induced magnetic response of subwavelength dielectric nanoparticles for enhancing light-matter interactions.