All-Optical Rapid Formation, Transport, and Sustenance of a Sessile Droplet in a Two-Dimensional Slit with Few-Micrometer Separation.

We present a sessile droplet manipulation platform that enables the formation and transport of a droplet on a light-absorbing surface via local laser-beam irradiation. The mechanism relies on solutocapillary Marangoni flow arising from a concentration gradient in a binary mixture liquid. Because the mixture is strongly confined in a two-dimensional slit with a spacing of a few micrometers, the wetting film is stably sustained, enabling the rapid formation, deformation, and transport of a sessile droplet. In addition, to sustain the droplet in the absence of laser irradiation, we developed a method to bridge the droplet between the top and bottom walls of the slit. The bridge is stably sustained because of the hydrophilicity of the slit wall. Splitting and merging of the droplet bridges are also demonstrated.

Tunable pheromone interactions among microswimmers.

Indirect interactions via shared memory deposited on the field ("field memory") play an essential role in collective motions. Some motile species, such as ants and bacteria, use attractive pheromones to complete many tasks. Mimicking these kinds of collective behavior at the laboratory scale, we present a pheromone-based autonomous agent system with tunable interactions. In this system, colloidal particles leave phase-change trails reminiscent of the process of pheromone deposition by individual ants, and the trails attract other particles and themselves. To implement this, we combine two physical phenomena: the phase change of a Ge2Sb2Te5 (GST) substrate by self-propelled Janus particles (pheromone deposition) and the AC (alternating current) electroosmotic (ACEO) flow generated by this phase change (pheromone attraction). Laser irradiation causes the GST layer to crystalize locally beneath the Janus particles, owing to the lens heating effect. Under AC field application, the high conductivity of the crystalline trail causes a field concentration and generates ACEO flow, and we introduce this flow as an attractive interaction between the Janus particles and the crystalline trail. By changing the AC frequency and voltage, we can tune the attractive flow, i.e., the sensitivity of the Janus particles to the trail, and the isolated particles undergo diverse states of motion, from self-caging to directional motion. A swarm of Janus particles also shows different states of collective motion, including colony formation and line formation. This tunability enables a reconfigurable system driven by a pheromone-like memory field.

Theoretical Design of a Janus-Nanoparticle-Based Sandwich Assay for Nucleic Acids.

Nanoparticles exhibit diverse self-assembly attributes and are expected to be applicable under unique settings. For instance, biomolecules can be sandwiched between dimer nanoparticles and detected by surface-enhanced Raman scattering. Controlling the gap between extremely close dimers and stably capturing the target molecule in the gap are crucial aspects of this strategy. Therefore, polymer-tethered nanoparticles (PTNPs), which show promise as high-performance materials that exhibit the attractive features of both NPs and polymers, were targeted in this study to achieve stable biomolecule sensing. Using coarse-grained molecular dynamics simulations, the dependence of the PTNP interactions on the length of the grafted polymer, graft density, and coverage ratio of a hydrophobic tether were examined. The results indicated that the smaller the tether length and graft density, the smaller was the distance between the PTNP surfaces (Rsurf). In contrast, Rsurf decreased as the coverage ratio of the hydrophobic surface (ϕ) increased. The sandwiching probability of the sensing target increased in proportion to the coverage ratio. At high ϕ values, the PTNPs aggregated into three or more particles, which hindered their sensing attributes. These results provide fundamental insight into the sensing applications of NPs and demonstrate the usefulness of PTNPs in sensing biomolecules.

Self-adaptive control of infrared emissivity in a solution-processed plasmonic structure.

Active control of optical properties, particularly in the infrared (IR) regime, is critical for the regulation of thermal emission. However, most photonic structures and devices are based on a sophisticated design, making the dynamic control of their IR properties challenging. Here, we demonstrate self-adaptive control of IR absorptivity/emissivity in a simple stacked structure that consists of an oxide plasmonic nanocrystal layer and a phase change material (VO2) layer, both fabricated via a solution process. The resonance wavelength and emission intensity for this structure depend on the phase of the VO2. This has potential applications for thermal emission structures (e.g., self-adaptive radiative cooling and IR camouflage). The proposed structure is a candidate low-cost and scalable active photonic platform.

Interference-based wide-range dynamic tuning of the plasmonic color of single gold nanoparticles.

Dynamic tuning of nanoscale coloration by exploiting localized surface plasmon resonance of gold nanoparticles (AuNPs) combined with an interference coloration mechanism is demonstrated experimentally. When interference between the scattering field from AuNPs and the reflected field from the substrate is observed under back-scattering white-light microscopy, the AuNPs exhibit various colors depending on their distance to the substrate. When the numerical aperture of the microscope objective is optimized, much greater coverage of the color space than was achieved with previously reported plasmon-based approaches is attained. Also, color tunability is examined by exploiting the temperature-induced volume change of a temperature-responsive hydrogel with embedded AuNPs to dynamically modify the distance to the substrate.

Multimodal Multiphoton Imaging of the Lipid Bilayer by Dye-Based Sum-Frequency Generation and Coherent Anti-Stokes Raman Scattering.

Coherent anti-Stokes Raman scattering (CARS) imaging is widely used for imaging molecular vibrations inside cells and tissues. Lipid bilayers are potential analytes for CARS imaging due to their abundant CH2 vibrational bonds. However, identifying the plasma membrane is challenging since it possesses a thin structure and is closely apposed to lipid structures inside the cells. Since the plasma membrane provides the most prominent asymmetric location within cells, orientation sensitive sum-frequency generation (SFG) imaging is a promising technique for selective visualization of the plasma membrane labeled by a nonfluorescent and SFG-specific dye, Ap3, when using a CARS microscope system. In this study, we closely compare the characteristics of lipid bilayer imaging by dye-based SFG and CARS using giant vesicles (GVs) and N27 rat dopaminergic neural cells. As a result, we show that CARS imaging can be exploited for the visualization of whole lipid structures inside GVs and cells but is insufficient for identification of the plasma membrane, which instead can be achieved using dye-based SFG imaging. In addition, we demonstrate that these unique properties can be combined and applied to the live-cell tracking of intracellular lipid structures such as lipid droplets beneath the plasma membrane. Thus, multimodal multiphoton imaging through a combination of dye-based SFG and CARS can serve as a powerful chemical imaging tool to investigate lipid bilayers in GVs and living cells.

Electro-osmotic trapping and compression of single DNA molecules while passing through a nanopore.

Complicated DNA molecular behaviors exist during translocation into a nanopore because their large and coiled structure needs to unwind. In this work, we investigated DNA translocation dynamics through a 200 nm pore using a fast photon counting system (FPCS). We found that the dwell time of the DNA molecules depends on the inverse of voltage (τ∝V-1.02) with a large constant term (∼1 ms). In other words, spherical fluorescence bead translocation involves electrophoresis as well as other additional factors. Our theoretical calculation suggested that one additional factor is electro-osmotic trapping associated with the instantaneous Brownian motion before and after translocation. Furthermore, compressed DNA molecular conformation was seen as a result of the increase of peak photon counts and the decrease of electrophoretic mobility with voltage. Our experiments showed that the polymers at the vicinity of a nanopore can be trapped and compressed, which is necessary to understand how to control the polymer translocation into a nanopore.

Rapid Assembly of Colloidal Crystals under Laser Illumination on a GeSbTe Substrate.

Optical techniques have been actively studied for manipulating nano- to microsized objects. However, long-range attraction and rapid transport of particles within thin quasi-two-dimensional systems are difficult because of the weak thermophoretic forces. Here, we introduce an experimental system that can rapidly generate quasi-two-dimensional colloidal crystals in deionized water, sandwiched between two hard plates. When a pulsed laser is irradiated on a chalcogenide phase-change material spattered on one side of the plates, the induced Marangoni-like flow causes a colloidal self-assembly in the order of tens of micrometers within the laser spot, with a transport velocity of a few tens of micrometers per second. This is due to the large thermal gradient induced by chalcogenide characteristics of high laser absorption and low thermal conductivity, and a strong hydrodynamic slip flow at the hydrophobic chalcogenide interface. Moreover, the colloidal crystals exhibit various lattice structures, depending on the laser intensity and chamber distance. For a certain range of the chamber distance, the colloidal crystal phases can be alternated by tuning the laser intensity in real time. Our system forms and deforms quasi-two-dimensional colloidal crystals at an on-demand location on a GeSbTe substrate.

DNA Hybridization Assay Using Gold Nanoparticles and Electrophoresis Separation Provides 1 pM Sensitivity.

The efficiency of gold nanoparticle (AuNP) dimerization mediated by hybridization between two probe DNA molecules and a complementary target DNA molecule was maximized by examining several possible hybridization combinations. The uniformity of the size of the AuNPs, the use of surface modification appropriate for high hybridization efficiency, together with efficient blocking of nonspecific binding, all contributed to achieving a 1 pM detection limit following conventional gel electrophoresis separation of the DNA-modified AuNP multimers. This practical homogeneous DNA hybridization assay methodology will provide a rapid, cost-effective, and field-portable tool for clinical diagnosis.

Implementation of pulse timing discriminator functionality into a GeSbTe/GeCuTe double layer structure.

The functionality of a pulse timing discriminator, which is commonly required in optical communication systems and artificial neuromorphic engineering, was implemented into chalcogenide phase-change materials. GeSbTe (GST) and GeCuTe (GCT), which exhibit opposite refractive index behavior in their respective crystalline and amorphous phases, were employed. A GST/GCT double layer enabled the order of arrival of two counter-propagating femtosecond pulses to be encoded as a difference in the degree of amorphization of the GCT layer, i.e., either a brighter or darker contrast of the amorphized area with respect to the crystalline background. Nonthermal ultrafast amorphization contributed to a picosecond time resolution in the discrimination of the pulse arrival order.

Ultrafast time-resolved electron diffraction revealing the nonthermal dynamics of near-UV photoexcitation-induced amorphization in Ge2Sb2Te5.

Because of their robust switching capability, chalcogenide glass materials have been used for a wide range of applications, including optical storages devices. These phase transitions are achieved by laser irradiation via thermal processes. Recent studies have suggested the potential of nonthermal phase transitions in the chalcogenide glass material Ge2Sb2Te5 triggered by ultrashort optical pulses; however, a detailed understanding of the amorphization and damage mechanisms governed by nonthermal processes is still lacking. Here we performed ultrafast time-resolved electron diffraction and single-shot optical pump-probe measurements followed by femtosecond near-ultraviolet pulse irradiation to study the structural dynamics of polycrystalline Ge2Sb2Te5. The experimental results present a nonthermal crystal-to-amorphous phase transition of Ge2Sb2Te5 initiated by the displacements of Ge atoms. Above the fluence threshold, we found that the permanent amorphization caused by multi-displacement effects is accompanied by a partial hexagonal crystallization.

Near-infrared nano-imaging spectroscopy using a phase change mask method.

We propose a technique that employs an optical mask layer of a phase-change material, e.g. GeSbTe, which is widely used for rewritable optical recording media, for realizing highly sensitive near-field imaging spectroscopy of single semiconductor quantum constituents at optical telecommunication wavelengths. Semiconductor quantum dots (QDs) have shown great promise as efficient single photon emitters and entangled photon sources, making them attractive for quantum communication and quantum information processing applications. Self-assembled InAs QDs on InP substrate are promising as near-infrared (NIR) single photon and entangled photon emitters. In order to clarify and control the optical properties of QDs for telecommunication devices, photoluminescence (PL) spectroscopy studies of single QDs with high spatial resolution at NIR wavelength is necessary. The most useful technique to attain this is by using near-field scanning optical miscroscopy (NSOM). However, NSOM has a lower PL collection efficiency at NIR wavelength than at visible wavelength [1]. This problem inhibits NIR-PL spectroscopy based on NSOM to be practically realized. Therefore, we deveopled a method to overcome the low NIR-PL spectroscopy by using a nanoaperture on an optical mask layer of phase-change material (PCM) [2]. Due to the large optical contrast between the crystalline and amorphous phases of the phase-change material at visible wavelengths and its high transparency at NIR wavelengths, an amorphous nanoaperture can be used to realize imaging spectroscopy with a high spatial resolution and a high collection efficiency (Fig. 1). We demonstrate the effectiveness of the proposed method by performing numerical simulations and PL measurements of InAs/InP QDs.jmicro;63/suppl_1/i10-a/DFU089F1F1DFU089F1Fig. 1.Schematic illustration of phase change mask method PCM mask effect has also the potential to be applied in emission energy control of QDs. One of the main problems for realization of quantum communication applications is precise control of energy in QDs. We proposed a new approach to control the emission energy of QDs by applying a local strain using volume expansion of phase-change material [3-5]. We calculated the stress and energy shift distribution induced by volume expansion using finite element method. Simulation result reveals that redshift is obtained beneath the flat part of amorphous mark, while blueshift is obtained beneath the edge region of amorphous mark. Simulation result is accompanied by two experimental studies; two-dimensional PL intensity mapping of InAs/InP QD sample deposited by a layer of PCM, and an analysis on the relationship between PL intensity ratio and energy shift were performed.

Redshifted and blueshifted photoluminescence emission of InAs/InP quantum dots upon amorphization of phase change material.

We present the mechanisms underlying the redshifted and blueshifted photoluminescence (PL) of quantum dots (QDs) upon amorphization of phase change material (PCM). We calculated the stress and energy shift distribution induced by volume expansion using finite element method. Simulation result reveals that redshift is obtained beneath the flat part of amorphous mark, while blueshift is obtained beneath the edge region of amorphous mark. Simulation result is accompanied by two experimental studies; two-dimensional PL intensity mapping of InAs/InP QD sample deposited by a layer of PCM, and an analysis on the relationship between PL intensity ratio and energy shift were performed.

A fusion-spliced near-field optical fiber probe using photonic crystal fiber for nanoscale thermometry based on fluorescence-lifetime measurement of quantum dots.

We have developed a novel nanoscale temperature-measurement method using fluorescence in the near-field called fluorescence near-field optics thermal nanoscopy (Fluor-NOTN). Fluor-NOTN enables the temperature distributions of nanoscale materials to be measured in vivo/in situ. The proposed method measures temperature by detecting the temperature dependent fluorescence lifetimes of Cd/Se quantum dots (QDs). For a high-sensitivity temperature measurement, the auto-fluorescence generated from a fiber probe should be reduced. In order to decrease the noise, we have fabricated a novel near-field optical-fiber probe by fusion-splicing a photonic crystal fiber (PCF) and a conventional single-mode fiber (SMF). The validity of the novel fiber probe was assessed experimentally by evaluating the auto-fluorescence spectra of the PCF. Due to the decrease of auto-fluorescence, a six- to ten-fold increase of S/N in the near-field fluorescence lifetime detection was achieved with the newly fabricated fusion-spliced near-field optical fiber probe. Additionally, the near-field fluorescence lifetime of the quantum dots was successfully measured by the fabricated fusion-spliced near-field optical fiber probe at room temperature, and was estimated to be 10.0 ns.

Experimental study of near-field light collection efficiency of aperture fiber probe at near-infrared wavelengths.

We examined the near-field collection efficiency of near-infrared radiation for an aperture probe. We used InAs quantum dots as ideal point light sources with emission wavelengths ranging from 1.1 to 1.6 µm. We experimentally investigated the wavelength dependence of the collection efficiency and compared the results with computational simulations that modeled the actual probe structure. The observed degradation in the collection efficiency is attributed to the cutoff characteristics of the gold-clad tapered waveguide, which approaches an ideal conductor at near-infrared wavelengths.

Effects of organic solvents on drug incorporation into polymeric carriers and morphological analyses of drug-incorporated polymeric micelles.

We incorporated an anticancer agent, camptothecin (CPT), into polymeric micelle carriers by using two different solvents (TFE and chloroform) in the solvent-evaporation drug incorporation process. We observed significant differences in the drug-incorporation behaviors, in the morphologies of the incorporated drug and the polymeric micelles, and in the pharmacokinetic behaviors between the two solvents' cases. In particular, the CPT-incorporated polymeric micelles prepared with TFE as the incorporation solvent exhibited more stable circulation in blood than those prepared with chloroform. This contrast indicates a novel technological perspective regarding the drug incorporation into polymeric micelle carriers. Morphological analyses of the inner core have revealed the presence of the directed alignment of the CPT molecules and CPT crystals in the micelle inner core. This is the first report of the morphologies of the drug incorporated into the polymeric micelle inner cores. We believe these analyses are very important for further pharmaceutical developments of polymeric micelle drug-carrier systems.

Ultrafast amorphization in Ge(10)Sb(2)Te(13) thin film induced by single femtosecond laser pulse.

We demonstrate amorphization in a Ge(10)Sb(2)Te(13) (GST) thin film through a nonthermal process by femtosecond electronic excitation. Amorphous recording marks were formed by irradiation with a single femtosecond pulse, and were confirmed to be recrystallized by laser thermal annealing. Scanning electron microscope observations revealed that amorphization occurred below the melting temperature. We performed femtosecond pump-probe measurements to investigate the amorphization dynamics of a GST thin film. We found that the reflectivity dropped abruptly within 500fs after excitation by a single pulse and that a small change in the reflectivity occurred within 5ps of this drop.

Writing and reading methodology for biochips with sub-100-nm chemical patterns based on near-field scanning optical microscopy.

This paper demonstrates a writing and reading methodology, which allows both to create and to detect sub-100-nm carboxyl-terminated patterns on light-transmissive quartz substrates by the same instrumental system. Such a technique, capable of creating carboxyl-terminated nanopatterns, offers several benefits for the miniaturization of biochips, since the carboxyl-terminated nanopatterns allow the easy immobilization of biomolecules by amide bond formation. As a consequence, increasingly miniaturized biochips require suitable analytical methods for the detection of nanopatterns. In our approach, carboxyl-terminated nanopatterns of down to 80 nm width were created using a photolabile silane coupling agent and a UV laser coupled to a near-field scanning optical microscope (NSOM). The same NSOM system was then used in a next step to detect the fabricated carboxyl-terminated nanopatterns after modification with a fluorescent label. Furthermore, as a first step towards biochip applications, the successful immobilization of several biomolecules, such as streptavidin, IgG and DNA on carboxyl-terminated nanopatterns was demonstrated. We have shown that our approach has the potential to lead to a new bioanalytical method, which enables one to write and to read biochips on a sub-100-nm scale by the same system.

Nano-optical probing of exciton wave-functions confined in a GaAs quantum dot.

We have enhanced the performance of near-field scanning optical microscopy (NSOM) in terms of the spatial resolution and the sensitivity in signal detection. A careful preparation of an aperture-NSOM probe provides us with a spatial resolution as high as 30 nm in fluorescence imaging spectroscopy. We have applied this technique to map out the center-of-mass wave functions of an exciton confined in a GaAs quantum dot (a monolayer-high island formed in a quantum well). The spatial profile of the exciton emission, which reflects the shape of the island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. A theoretical calculation of the spatial distribution of the polarization field quantitatively reproduced the experimental result. Furthermore, mapping of an excited state wave-function with a node structure is also demonstrated. The novel technique can be extensively applied to wave-function engineering in the design and fabrication of quantum devices.