Control of Plasmonic Superradiance in Metallic Nanoparticle Assembly by Light-Induced Force and Fluctuations

Quasi-band structure of quantum-confined nanocrystals

We discuss the electronic properties of quantum-confined nanocrystals. In particular, we show how, starting from the discrete molecular states of small nanocrystals, an approximate band structure (quasi-band structure) emerges with increasing particle size. Finite temperature is found to broaden the discrete states in energy space forming even for nanocrystals in the quantum-confinement regime quasi-continuous bands in k-space. This bands can be, to a certain extend, interpreted along the lines of standard band structure theory, while taking also finite size and surface effects into account. We discuss this on various prototypical nanocrystal systems.

Light-Induced Condensation of Biofunctional Molecules around Targeted Living Cells to Accelerate Cytosolic Delivery

The light-induced force and convection can be enhanced by the collective effect of electrons (superradiance and red shift) in high-density metallic nanoparticles, leading to macroscopic assembly of target molecules. We here demonstrate application of the light-induced assembly for drug delivery system with enhancement of cell membrane accumulation and penetration of biofunctional molecules including cell-penetrating peptides (CPPs) with superradiance-mediated photothermal convection. For induction of photothermal assembly around targeted living cells in cell culture medium, infrared continuous-wave laser light was focused onto high-density gold-particle-bound glass bottom dishes exhibiting plasmonic superradiance or thin gold-film-coated glass bottom dishes. In this system, the biofunctional molecules can be concentrated around the targeted living cells and internalized into them only by 100 s laser irradiation. Using this simple approach, we successfully achieved enhanced cytosolic release of the CPPs and apoptosis induction using a pro-apoptotic domain with a very low peptide concentration (nM level) by light-induced condensation.

Tailoring Cooperative Emission in Molecules: Superradiance and Subradiance from First-Principles Simulations

Cooperative optical effects provide a pathway to both the amplification (superradiance) and the suppression (subradiance) of photon emission from electronically excited states. These captivating phenomena offer a rich variety of possibilities for photonic technologies aimed at electromagnetic energy manipulation, including lasers and high-speed emitting devices in the case of superradiance or optical energy storage in that of subradiance. The employment of molecules as the building pieces in these developments requires a precise understanding of the roles of separation, orientation, spatial distribution, and applied fields, which remains challenging for theory and experiments. These questions are addressed here through ab initio quantum dynamics simulations of collective emission on the basis of a novel semiclassical formalism and time-dependent density functional theory. By establishing the configurations leading to decoherence and how the fine-tuning of a pulse can accumulate or release optical energy in H₂ arrays, this report provides fundamental insight toward the design of real superradiant and subradiant devices.

Attogram-level light-induced antigen-antibody binding confined in microflow

The analysis of trace amounts of proteins based on immunoassays and other methods is essential for the early diagnosis of various diseases such as cancer, dementia, and microbial infections. Here, we propose a light-induced acceleration of antigen-antibody reaction of attogram-level proteins at the solid-liquid interface by tuning the laser irradiation area comparable to the microscale confinement geometry for enhancing the collisional probability of target molecules and probe particles with optical force and fluidic pressure. This principle was applied to achieve a 102-fold higher sensitivity and ultrafast specific detection in comparison with conventional protein detection methods (a few hours) by omitting any pretreatment procedures; 47-750 ag of target proteins were detected in 300 nL of sample after 3 minutes of laser irradiation. Our findings can promote the development of proteomics and innovative platforms for high-throughput bio-analyses under the control of a variety of biochemical reactions.

Plasmonic nanoscale temperature shaping on a single titanium nitride nanostructure

Arbitrary shaping of temperature fields at the nanometre scale is an important goal in nanotechnology; however, this is challenging because of the diffusive nature of heat transfer. In the present work, we numerically demonstrated that spatial shaping of nanoscale temperature fields can be achieved by plasmonic heating of a single titanium nitride (TiN) nanostructure. A key feature of TiN is its low thermal conductivity (k_TiN = 29 [W m^-1 K^-1]) compared with ordinary plasmonic metals such as Au (k_Au = 314 [W m^-1 K^-1]). When the localised surface plasmon resonance of a metal nanostructure is excited, the light intensity is converted to heat power density in the nanostructure via the Joule heating effect. For a gold nanoparticle, non-uniform spatial distributions of the heat power density will disappear because of the high thermal conductivity of Au; the nanoparticle surface will be entirely isothermal. In contrast, the spatial distributions of the heat power density can be clearly transcribed into temperature fields on a TiN nanostructure because the heat dissipation is suppressed. In fact, we revealed that highly localised temperature distributions can be selectively controlled around the TiN nanostructure at a spatial resolution of several tens of nanometres depending on the excitation wavelength. The present results indicate that arbitrary temperature shaping at the nanometre scale can be achieved by designing the heat power density in TiN nanostructures for plasmonic heating, leading to unconventional thermofluidics and thermal chemical biology.

Formulation of resonant optical force based on the microscopic structure of chiral molecules

Optical manipulation, exemplified by Ashkin's optical tweezers, is a promising technique in the fields of bioscience and chemistry, as it enables the non-destructive and non-contact selective transport or manipulation of small particles. To realize the separation of chiral molecules, several researchers have reported on the use of light and discussed feasibility of selection. Although the separation of micrometer-sized chiral molecules has been experimentally demonstrated, the separation of nanometer-sized chiral molecules, which are considerably smaller than the wavelength of light, remains challenging. Therefore, we formulated an optical force under electronic resonance to enhance the optical force and enable selective manipulation. In particular, we incorporated the microscopic structures of molecular dipoles into the nonlocal optical response theory. The analytical expression of optical force could clarify the mechanism of selection exertion of the resonant optical force on chiral molecules. Furthermore, we quantitatively evaluated the light intensity and light exposure time required to separate a single molecule in a solvent. The results can facilitate the design of future schemes for the selective optical manipulation of chiral molecules.

Damage-free light-induced assembly of intestinal bacteria with a bubble-mimetic substrate

Rapid evaluation of functions in densely assembled bacteria is a crucial issue in the efficient study of symbiotic mechanisms. If the interaction between many living microbes can be controlled and accelerated via remote assembly, a cultivation process requiring a few days can be ommitted, thus leading to a reduction in the time needed to analyze the bacterial functions. Here, we show the rapid, damage-free, and extremely dense light-induced assembly of microbes over a submillimeter area with the "bubble-mimetic substrate (BMS)". In particular, we successfully assembled 104-105 cells of lactic acid bacteria (Lactobacillus casei), achieving a survival rate higher than 95% within a few minutes without cultivation process. This type of light-induced assembly on substrates like BMS, with the maintenance of the inherent functions of various biological samples, can pave the way for the development of innovative methods for rapid and highly efficient analysis of functions in a variety of microbes.

Light-induced assembly of living bacteria with honeycomb substrate

Some bacteria are recognized to produce useful substances and electric currents, offering a promising solution to environmental and energy problems. However, applications of high-performance microbial devices require a method to accumulate living bacteria into a higher-density condition in larger substrates. Here, we propose a method for the high-density assembly of bacteria (106 to 107 cells/cm2) with a high survival rate of 80 to 90% using laser-induced convection onto a self-organized honeycomb-like photothermal film. Furthermore, the electricity-producing bacteria can be optically assembled, and the electrical current can be increased by one to two orders of magnitude simply by increasing the number of laser irradiations. This concept can facilitate the development of high-density microbial energy conversion devices and provide new platforms for unconventional environmental technology.

Interparticle-Interaction-Mediated Anomalous Acceleration of Nanoparticles under Light-Field with Coupled Orbital and Spin Angular Momentum

Spin-orbit interaction is a crucial issue in the field of nanoscale physics and chemistry. Here, we theoretically demonstrate that the spin angular momentum (SAM) can accelerate and decelerate the orbital motion of nanoparticles (NPs) via light-induced interparticle interactions by a circularly polarized optical vortex. The Laguerre-Gaussian beam as a conventional optical vortex with orbital angular momentum (OAM) induces the orbital and spinning motion of a trapped object depending on the spatial configuration. On the contrary, it is not clear whether circularly polarized light induces the orbital motion for the particles trapped off-axis. The present study reveals that the interparticle light-induced force due to the SAM enhances or weakens the orbital torque and modulates rotational dynamics depending on the number of NPs, where the rotation speed of NPs in the optical field with both positive SAM and OAM can be 4 times faster than that in the optical field with negative SAM and positive OAM. The obtained results will not only clarify the principle for the control of NPs based on OAM-SAM coupling via light-matter interaction but also contribute to the unconventional laser processing technique for nanostructures with various chiral symmetries.

Mechanism in External Field-mediated Trapping of Bacteria Sensitive to Nanoscale Surface Chemical Structure

Molecular imprinting technique enables the selective binding of nanoscale target molecules to a polymer film, within which their chemical structure is transcribed. Here, we report the successful production of mixed bacterial imprinted film (BIF) from several food poisoning bacteria by the simultaneous imprinting of their nanoscale surface chemical structures (SCS), and provide highly selective trapping of original micron-scale bacteria used in the production process of mixed BIF even for multiple kinds of bacteria in real samples. Particularly, we reveal the rapid specific identification of E. coli group serotypes (O157:H7 and O26:H11) using an alternating electric field and a quartz crystal microbalance. Furthermore, we have performed the detailed physicochemical analysis of the specific binding of SCS and molecular recognition sites (MRS) based on the dynamic Monte Carlo method under taking into account the electromagnetic interaction. The dielectrophoretic selective trapping greatly depends on change in SCS of bacteria damaged by thermal treatment, ultraviolet irradiation, or antibiotic drugs, which can be well explained by the simulation results. Our results open the avenue for an innovative means of specific and rapid detection of unknown bacteria for food safety and medicine from a nanoscale viewpoint.

Optical Trap-Mediated High-Sensitivity Nanohole Array Biosensors with Random Nanospikes

We clarify an unconventional principle of the light-driven operation of a biosensor for enhanced sensitivity with the help of random nanospikes added to the surface of a nanohole array. Such a system is capable of optically guiding viruses and trapping them in the vicinity of a highly sensitive site by an anomalous light-induced force arising from random-nanospike-modulated extraordinary optical transmission and the plasmonic mirror image in a virus as a dielectric submicron object. In particular, after guiding the viruses near the apex of nanospikes, there are conditions where the spectral peak shift of extraordinary optical transmission can be greatly increased and reach several hundred nanometers in comparison with that of a conventional nanohole array without random nanospikes. These results will allow for the development of a simple, rapid, and highly sensitive virus detection method based on optical trapping with the help of random-nanospike-modulated extraordinary optical transmission, facilitating convenient medical diagnosis and food inspection.

Submillimetre Network Formation by Light-induced Hybridization of Zeptomole-level DNA

Macroscopic unique self-assembled structures are produced via double-stranded DNA formation (hybridization) as a specific binding essential in biological systems. However, a large amount of complementary DNA molecules are usually required to form an optically observable structure via natural hybridization, and the detection of small amounts of DNA less than femtomole requires complex and time-consuming procedures. Here, we demonstrate the laser-induced acceleration of hybridization between zeptomole-level DNA and DNA-modified nanoparticles (NPs), resulting in the assembly of a submillimetre network-like structure at the desired position with a dramatic spectral modulation within several minutes. The gradual enhancement of light-induced force and convection facilitated the two-dimensional network growth near the air-liquid interface with optical and fluidic symmetry breakdown. The simultaneous microscope observation and local spectroscopy revealed that the assembling process and spectral change are sensitive to the DNA sequence. Our findings establish innovative guiding principles for facile bottom-up production via various biomolecular recognition events.

Manipulation of a neutral and nonpolar nanoparticle in water using a nonuniform electric field

The manipulation of nanoparticles in water is of essential importance in chemical physics, nanotechnology, medical technology, and biotechnology applications. Generally, a particle with net charges or charge polarity can be driven by an electric field. However, many practical particles only have weak and even negligible charge and polarity, which hinders the electric field to exert a force large enough to drive these nanoparticles directly. Here, we use molecular dynamics simulations to show that a neutral and nonpolar nanoparticle in liquid water can be driven directionally by an external electric field. The directed motion benefits from a nonuniform water environment produced by a nonuniform external electric field, since lower water energies exist under a higher intensity electric field. The nanoparticle spontaneously moves toward locations with a weaker electric field intensity to minimize the energy of the whole system. Considering that the distance between adjacent regions of nonuniform field intensity can reach the micrometer scale, this finding provides a new mechanism of manipulating nanoparticles from the nanoscale to the microscale.

A high performance photothermal film with spherical shell-type metallic nanocomposites for solar thermoelectric conversion

A photothermal film (PTF) with densely assembled gold nanoparticle-fixed beads on a polymer substrate is fabricated. Remarkably, a temperature rise higher than 40 °C is achieved in the PTF with only 100 seconds of artificial solar irradiation, and the output power of the thermoelectric device was enhanced to be one order higher than that without PTF. These results will pioneer a rapid solar thermoelectric device.

Laser trapping of colloidal metal nanoparticles

Optical trapping using focused laser beams (laser tweezers) has been proven to be extremely useful for contactless manipulation of a variety of small objects, including biological cells, organelles within cells, and a wide range of other dielectric micro- and nano-objects. Colloidal metal nanoparticles have drawn increasing attention in the field of optical trapping because of their unique interactions with electromagnetic radiation, caused by surface plasmon resonance effects, enabling a large number of nano-optical applications of high current interest. Here we try to give a comprehensive overview of the field of laser trapping and manipulation of metal nanoparticles based on results reported in the recent literature. We also discuss and describe the fundamentals of optical forces in the context of plasmonic nanoparticles, including effects of polarization, optical angular momentum, and laser heating effects, as well as the various techniques that have been used to trap and manipulate metal nanoparticles. We conclude by suggesting possible directions for future research.

Selective optical assembly of highly uniform nanoparticles by doughnut-shaped beams

A highly efficient natural light-harvesting antenna has a ring-like structure consisting of dye molecules whose absorption band changes through selective evolutionary processes driven by external stimuli, i.e., sunlight depending on its territory and thermal fluctuations. Inspired by this fact, here, we experimentally and theoretically demonstrate the selective assembling of ring-like arrangements of many silver nanorods with particular shapes and orientations onto a substrate by the light-induced force of doughnut beams with different colours (wavelengths) and polarizations in conjunction with thermal fluctuations at room temperature. Furthermore, the majority of nanorods are electromagnetically coupled to form a prominent red-shifted collective mode of localized surface plasmons resonant with the wavelength of the irradiated light, where a spectral broadening also appears for the efficient broadband optical response. The discovered principle is a promising route for "bio-inspired selective optical assembly" of various nanomaterials that can be used in the wide field of nanotechnology.

Fluctuation-mediated optical screening of nanoparticles

Inspired by biological motors, we propose a guiding principle for selectively separating nanoparticles (NPs) by efficiently using the light-induced force (LIF) and thermal fluctuations. We demonstrate the possibility of transporting metallic NPs of different sizes with a size-selection accuracy of less than 10 nm even at room temperature by designing asymmetric spatiotemporal light fields. This technique will lead to unconventional nanoextraction processes based on light and fluctuations.