Optical Manipulation and Spectroscopy Of Silicon Nanoparticles Exhibiting Dielectric Resonances

Chemical Vapor Deposition of Spherical Amorphous Selenium Mie Resonators for Infrared Meta-Optics

Applying direct growth and deposition of optical surfaces holds great promise for the advancement of future nanophotonic technologies. Here, we report on a chemical vapor deposition (CVD) technique for depositing amorphous selenium (a-Se) spheres by desorption of selenium from Bi₂Se₃ and re-adsorption on the substrate. We utilize this process to grow scalable, large area Se spheres on several substrates and characterize their Mie-resonant response in the mid-infrared (MIR) spectral range. We demonstrate size-tunable Mie resonances spanning the 2-16 μm spectral range for single isolated resonators and large area ensembles. We further demonstrate strong absorption dips of up to 90% in ensembles of particles in a broad MIR range. Finally, we show that ultra-high-Q resonances arise in the case where Se Mie-resonators are coupled to low-loss epsilon-near-zero (ENZ) substrates. These findings demonstrate the enabling potential of amorphous Selenium as a versatile and tunable nanophotonic material that may open up avenues for on-chip MIR spectroscopy, chemical sensing, spectral imaging, and large area metasurface fabrication.

Synthesis of Germanium Nanospheres as High-Precision Optical Tweezers Probes

Force spectroscopy on single molecular machines generating piconewton forces is often performed using optical tweezers. Since trapping forces scale with the particle volume, piconewton-force measurements so far required micron-sized probes practically limiting the spatiotemporal resolution. Here, we have overcome this limit by developing high-refractive index germanium nanospheres as ultraresolution trapping probes. With a refractive index of 4.4, their trapping efficiency and maximum force per power is more than 10-fold higher compared to silica spheres of equal size. Therefore, the use of germanium allows piconewton-force measurements with nanometer sized probes. Using 70-nm-diameter germanium nanospheres as trappable optical probes (GeNTOPs), we could show that kinesin-1 walks with 4-nm-center-of-mass steps. In the long-term, the application of these novel high-precision GeNTOPs will provide new insight into the working mechanism of molecular machines and are promising candidates for other applications in microscopy, optoelectronics, and nanophotonics.

Dynamic Behavior of Charged Particles at the Nanopipette Orifice

Understanding the dynamic behavior of charged particles driven by flow and electric field in nanochannels/pores is highly important for both fundamental study and practical applications. While a great breakthrough has been made in understanding the translocation dynamics of charged particles within the nanochannels/pores, studies on the dynamics of particles at the orifice of nanochannels/pores are scarcely reported. Here, we study particle motion at a smaller-sized orifice of a nanopipette by combining experimentally observed current transients with simulated force conditions. The theoretical force analysis reveals that dielectrophoretic force plays an equally important role as electrophoretic force and electroosmotic force, although it has often been neglected in understanding the particle translocation dynamics within the nanopipette. Under the combined action of these forces, it thus becomes difficult for particles to physically collide with the orifice of the nanopipette, resulting in a relatively low decrease in the current transients, which coincides with experimental results. We then regulate the dynamic behavior by altering experimental conditions (i.e., bias potential, nanopipette surface charge, and particle size), and the results further validate the presence and influence of forces being considered. This study improves the understanding of the relationship between particle properties and observed current transients, providing more possibilities for accurate single-particle analysis and single-entity regulation.

Optical tweezers beyond refractive index mismatch using highly doped upconversion nanoparticles

Optical tweezers are widely used in materials assembly¹, characterization², biomechanical force sensing^3,4 and the in vivo manipulation of cells⁵ and organs⁶. The trapping force has primarily been generated through the refractive index mismatch between a trapped object and its surrounding medium. This poses a fundamental challenge for the optical trapping of low-refractive-index nanoscale objects, including nanoparticles and intracellular organelles. Here, we report a technology that employs a resonance effect to enhance the permittivity and polarizability of nanocrystals, leading to enhanced optical trapping forces by orders of magnitude. This effectively bypasses the requirement of refractive index mismatch at the nanoscale. We show that under resonance conditions, highly doping lanthanide ions in NaYF₄ nanocrystals makes the real part of the Clausius-Mossotti factor approach its asymptotic limit, thereby achieving a maximum optical trap stiffness of 0.086 pN μm^-1 mW^-1 for 23.3-nm-radius low-refractive-index (1.46) nanoparticles, that is, more than 30 times stronger than the reported value for gold nanoparticles of the same size. Our results suggest a new potential of lanthanide doping for the optical control of the refractive index of nanomaterials, developing the optical force tag for the intracellular manipulation of organelles and integrating optical tweezers with temperature sensing and laser cooling⁷ capabilities.

Single-walled carbon nanotube membranes as non-reflective substrates for nanophotonic applications

We demonstrate that single-walled carbon nanotube (SWCNT) membranes can be successfully utilized as nanometer-thick substrates for enhanced visualization and facilitated study of individual nanoparticles. As model objects, we transfer optically resonant 200 nm silicon nanoparticles onto pristine and ethanol-densified SWCNT membranes by the femtosecond laser printing method. We image nanoparticles by scanning electron and bright-field optical microscopy, and characterize by linear and Raman scattering spectroscopy. The use of a pristine SWCNT membrane allows to achieve an order-of-magnitude enhancement of the optical contrast of the nanoparticle bright field image over the results shown in the case of the glass substrate use. The observed optical contrast enhancement is in agreement with the spectrophotometric measurements showing an extremely low specular reflectance of the pristine membrane (≤0.1%). Owing to the high transparency, negligibly small reflectance and thickness, SWCNT membranes offer a variety of perspective applications in nanophotonics, bioimaging and synchrotron radiation studies.

Optical Rotation and Thermometry of Laser Tweezed Silicon Nanorods

Optical rotation of laser tweezed nanoparticles offers a convenient means for optical to mechanical force transduction and sensing at the nanoscale. Plasmonic nanoparticles are the benchmark system for such studies, but their rapid rotation comes at the price of high photoinduced heating due to Ohmic losses. We show that Mie resonant silicon nanorods with characteristic dimensions of ∼220 × 120 nm² can be optically trapped and rotated at frequencies up to 2 kHz in water using circularly polarized laser light. The temperature excess due to heating from the trapping laser was estimated by phonon Raman scattering and particle rotation analysis. We find that the silicon nanorods exhibit slightly improved thermal characteristics compared to Au nanorods with similar rotation performance and optical resonance anisotropy. Altogether, the results indicate that silicon nanoparticles have the potential to become the system of choice for a wide range of optomechanical applications at the nanoscale.

Directional scattering and multipolar contributions to optical forces on silicon nanoparticles in focused laser beams

Nanoparticles made of high index dielectric materials have seen a surge of interest and have been proposed for various applications, such as metalenses, light harvesting and directional scattering. With the advent of fabrication techniques enabling colloidal suspensions, the prospects of optical manipulation of such nanoparticles becomes paramount. High index nanoparticles support electric and magnetic multipolar responses in the visible regime and interference between such modes can give rise to highly directional scattering, in particular a cancellation of back-scattered radiation at the first Kerker condition. Here we present a study of the optical forces on silicon nanoparticles in the visible and near infrared calculated using the transfer matrix method. The zero-backscattering Kerker condition is investigated as an avenue to reduce radiation pressure in an optical trap. We find that while asymmetric scattering does reduce the radiation pressure, the main determining factor of trap stability is the increased particle response near the geometric resonances. The trap stability for non-spherical silicon nanoparticles is also investigated and we find that ellipsoidal deformation of spheres enables trapping of slightly larger particles.

Real-Time Tunable Colors from Microfluidic Reconfigurable All-Dielectric Metasurfaces

Structural colors arising from all-dielectric nanostructures are very promising for high-resolution color nanoprinting and high-density optical storage. However, once the all-dielectric nanostructures are fabricated, their optical performances are usually static or change slowly, significantly limiting the practical applications in advanced displays. Herein, we experimentally demonstrate the real-time tunable colors with microfluidic reconfigurable all-dielectric metasurfaces. The metasurface is composed of an array of TiO₂ nanoblocks, which are embedded in a polymeric microfluidic channel. By injecting solutions with a different refractive index into the channel, the narrow band reflection peak and the corresponding distinct colors of a TiO₂ metasurface can be precisely controlled. The transition time is as small as 16 ms, which is orders of magnitude faster than the current techniques. By varying the lattice size of TiO₂ metasurfaces, the real-time tunable colors are able to span the entire visible spectrum. Meanwhile, the injection and ejection of solvent have also shown the capability of the erasion and the restoration of information encoded in TiO₂ metasurfaces. The combination of all-dielectric nanostructures with microfluidic channels shall boost their applications in functional color display, banknote security, anticounterfeiting, and point-of-care devices.

Optical Forces at the Nanoscale: Size and Electrostatic Effects

The reduced magnitude of the optical trapping forces exerted over sub-200 nm dielectric nanoparticles complicates their optical manipulation, hindering the development of techniques and studies based on it. Improvement of trapping capabilities for such tiny objects requires a deep understanding of the mechanisms beneath them. Traditionally, the optical forces acting on dielectric nanoparticles have been only correlated with their volume, and the size has been traditionally identified as a key parameter. However, the most recently published research results have shown that the electrostatic characteristics of a sub-100 nm dielectric particle could also play a significant role. Indeed, at present it is not clear what optical forces depend. In this work, we designed a set of experiments in order to elucidate the different mechanism and properties (i.e., size and/or electrostatic properties) that governs the magnitude of optical forces. The comparison between experimental data and numerical simulations have shown that the double layer induced at nanoparticle's surface, not considered in the classical description of nanoparticle's polarizability, plays a relevant role determining the magnitude of the optical forces. Here, the presented results constitute the first step toward the development of the dielectric nanoparticle over which enhanced optical forces could be exerted, enabling their optical manipulation for multiples purposes ranging from fundamental to applied studies.

Metasurfaces and Colloidal Suspensions Composed of 3D Chiral Si Nanoresonators

High-refractive-index silicon nanoresonators are promising low-loss alternatives to plasmonic particles in CMOS-compatible nanophotonics applications. However, complex 3D particle morphologies are challenging to realize in practice, thus limiting the range of achievable optical functionalities. Using 3D film structuring and a novel gradient mask transfer technique, the first intrinsically chiral dielectric metasurface is fabricated in the form of a monolayer of twisted silicon nanocrescents that can be easily detached and dissolved into colloidal suspension. The metasurfaces exhibit selective handedness and a circular dichroism as large as 160° µm^-1 due to pronounced differences in induced current loops for left-handed and right-handed polarization. The detailed morphology of the detached particles is analyzed using high-resolution transmission electron microscopy. Furthermore, it is shown that the particles can be manipulated in solution using optical tweezers. The fabrication and detachment method can be extended to different nanoparticle geometries and paves the way for a wide range of novel nanophotonic experiments and applications of high-index dielectrics.

Optical pulling and pushing forces exerted on silicon nanospheres with strong coherent interaction between electric and magnetic resonances

We investigated theoretically and numerically the optical pulling and pushing forces acting on silicon (Si) nanospheres (NSs) with strong coherent interaction between electric and magnetic resonances. We examined the optical pulling and pushing forces exerted on Si NSs by two interfering waves and revealed the underlying physical mechanism from the viewpoint of electric- and magnetic-dipole manipulation. As compared with a polystyrene (PS) NS, it was found that the optical pulling force for a Si NS with the same size is enlarged by nearly two orders of magnitude. In addition to the optical pulling force appearing at the long-wavelength side of the magnetic dipole resonance, very large optical pushing force is observed at the magnetic quadrupole resonance. The correlation between the optical pulling/pushing force and the directional scattering characterized by the ratio of the forward to backward scattering was revealed. More interestingly, it was found that the high-order electric and magnetic resonances in large Si NSs play an important role in producing optical pulling force which can be generated by not only s-polarized wave but also p-polarized one. Our finding indicates that the strong coherent interaction between the electric and magnetic resonances existing in nanoparticles with large refractive indices can be exploited to manipulate the optical force acting on them and the correlation between the optical force and the directional scattering can be used as guidance. The engineering and manipulation of optical forces will find potential applications in the trapping, transport and sorting of nanoparticles.

Preparation, characterization, and protein-resistance of films derived from a series of α-oligo(ethylene glycol)-ω-alkenes on H–Si(111) surfaces

Films on Si(111) were prepared by photo-activated grafting of CH₂&#xe001;CH(CH₂)m(OCH₂CH₂)nOCH₃ (m = 8, 9; n = 3–7) by using different vacuum conditions. High vacuum produced a higher thickness (40 Å) and <0.8% fibrinogen adsorption (C₁₀EG₇). Films were stable even after 28 days.

Nanoscopic imaging of thick heterogeneous soft-matter structures in aqueous solution

Precise nanometre-scale imaging of soft structures at room temperature poses a major challenge to any type of microscopy because fast thermal fluctuations lead to significant motion blur if the position of the structure is measured with insufficient bandwidth. Moreover, precise localization is also affected by optical heterogeneities, which lead to deformations in the imaged local geometry, the severity depending on the sample and its thickness. Here we introduce quantitative thermal noise imaging, a three-dimensional scanning probe technique, as a method for imaging soft, optically heterogeneous and porous matter with submicroscopic spatial resolution in aqueous solution. By imaging both individual microtubules and collagen fibrils in a network, we demonstrate that structures can be localized with a precision of ∼10 nm and that their local dynamics can be quantified with 50 kHz bandwidth and subnanometre amplitudes. Furthermore, we show how image distortions caused by optically dense structures can be corrected for.

Integration time limits the capacity of super-resolution microscopy to study dynamics. Here, Bartsch et al. use the 3D scanning of a nanoparticle held in an optical trap, and its thermal noise motion within the trap, to image the local dynamics of soft-matter structures with a 50 kHz bandwidth.

All-Dielectric Colored Metasurfaces with Silicon Mie Resonators

The photonic resonances hosted by nanostructures provide vivid colors that can be used as color filters instead of organic colors and pigments in photodetectors and printing technology. Metallic nanostructures have been widely studied due to their ability to sustain surface plasmons that resonantly interact with light. Most of the metallic nanoparticles behave as point-like electric multipoles. However, the needs of an another degree of freedom to tune the color of the photonic nanostructure together with the use of a reliable and cost-effective material are growing. Here, we report a technique to imprint colored images based on silicon nanoparticles that host low-order electric and magnetic Mie resonances. The interplay between the electric and magnetic resonances leads to a large palette of colors. This all-dielectric fabrication technique offers the advantage to use cost-effective, reliable, and sustainable materials to provide vivid color spanning the whole visible spectrum. The interest and potential of this all-dielectric printing technique are highlighted by reproducing at a micrometer scale a Mondrian painting.

All-Dielectric Silicon Nanogap Antennas To Enhance the Fluorescence of Single Molecules

Plasmonic antennas have a profound impact on nanophotonics as they provide efficient means to manipulate light and enhance light-matter interactions at the nanoscale. However, the large absorption losses found in metals can severely limit the plasmonic applications in the visible spectral range. Here, we demonstrate the effectiveness of an alternative approach using all-dielectric nanoantennas based on silicon dimers to enhance the fluorescence detection of single molecules. The silicon antenna design is optimized to confine the near-field intensity in the 20 nm nanogap and reach a 270-fold fluorescence enhancement in a nanoscale volume of λ(3)/1800 with dielectric materials only. Our conclusions are assessed by combining polarization resolved optical spectroscopy of individual antennas, scanning electron microscopy, numerical simulations, fluorescence lifetime measurements, fluorescence burst analysis, and fluorescence correlation spectroscopy. This work demonstrates that all-silicon nanoantennas are a valid alternative to plasmonic devices for enhanced single molecule fluorescence sensing, with the additional key advantages of reduced nonradiative quenching, negligible heat generation, cost-efficiency, and complementary metal-oxide-semiconductor (CMOS) compatibility.