Switching nanoscale temperature fields with high-order plasmonic modes in transition metal nanorods.

Depending on the photoirradiation conditions, metal nanostructures exhibit various plasmonic modes, including dipolar, quadrupolar, and hexapolar modes. This work demonstrates numerically that these high-order plasmonic modes can be used to switch nanoscale temperature distributions during the plasmonic heating of a manganese (Mn) nanorod. The key feature of Mn is its low thermal conductivity. Generally, when noble metal nanostructures are used for plasmonic heating, the nanostructure surface will be almost isothermal regardless of the order of the excited plasmonic modes because of the high thermal conductivity of noble metals, e.g., the thermal conductivity of gold is 314 W m-1 K-1. However, unlike noble metals, Mn has a significantly lower thermal conductivity of 7.8 W m-1 K-1. Due to this lower thermal conductivity, the distinct spatial characteristics of the high-order plasmonic modes can be transcribed clearly into nanoscale temperature fields, which are achieved by generating polarization currents by high-order plasmons within the nanorod. These findings strongly suggest that high-order plasmonic modes hold significant potential for the advanced and precise manipulation of heat generation at the nanometer scale in thermoplasmonics.

Fluctuation-mediated orbital rotation of microparticles in non-coaxially counter-propagating optical tweezers.

We have demonstrated in the present report that dielectric microparticles exhibited orbital rotation in the light field of non-coaxially configured two counter-propagating laser beams both in numerical simulations and experiments. A series of computational simulations indicated that when irradiated with two non-coaxially counter-propagating parallel laser beams with the same intensity distributions in the absence of thermal (Brownian) motion, a microparticle did not exhibit orbital rotation due to the symmetry of the optical field. However, the computations predicted that a microparticle exhibited one directional orbital rotation in the presence of thermal motion because of the symmetry breaking of the optical force acting on the particle. This spontaneous orbital rotation was experimentally demonstrated for 1-µm dielectric particles in water at room temperature.

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 (kTiN = 29 [W m-1 K-1]) compared with ordinary plasmonic metals such as Au (kAu = 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.

Opto-thermophoretic separation and trapping of plasmonic nanoparticles.

Optical tweezers are powerful tools to trap, transport, and analyse individual nano-objects at dilute concentrations. However, it is still challenging to manipulate isolated single nano-objects in dense target environments with various kinds of materials, such as in living cells and mixtures of nanocolloids. In the present work, we have succeeded in the selective trapping of a few gold nanoshells with specific sizes and sweeping others out completely, only by irradiating the dense colloidal suspension of gold nanoshells with a focused near infrared continuous-wave (CW) laser. This was achieved by an interplay between optical gradient- and thermophoretic forces: while the gradient force traps the targets at the focal spot, the thermophoretic force pushes others out from the focal spot. The distance between the trapped targets and the separated others was longer than 20 µm, allowing us to measure plasmonic scattering spectra of the trapped targets at a single-nanoparticle level. The present method paves a way for manipulating and analysing single nano-objects in dense mixtures of targets and various kinds of materials.

Mesoscopic Motion of Optically Trapped Particle Synchronized with Photochromic Reactions of Diarylethene Derivatives.

Not only the energy but also the momentum of photons transfers to material via photoabsorption; this momentum transfer, known as radiation pressure, can induce motions of small particles. It can therefore be expected to induce mechanical motions of mesoscopic materials synchronized with the reversible change of their absorption coefficient by external stimuli. We demonstrated quantitative photomechanical motions in mesoscopic regions by combining optical tweezer and photochromic reactions of diarylethene (DAE). A microparticle including DAE was optically trapped with 532 nm laser and the absorption band of the DAE was photoswitched with UV laser, resulting in the modulation of the radiation force through the change in the complex dielectric constant of the particle. In this process, mesoscopic mechanical motions were successfully induced by the photochromic reaction. The present approach is potentially applicable in a wide variety of nano/micromechanical devices and also paves the way for monitoring the absorption of photons by molecules via photomechanical response.

Plasmonic Control and Stabilization of Asymmetric Light Scattering from Ag Nanocubes on TiO2.

When plasmonic nanoparticles are placed on a highly refractive semiconductor substrate, we can expect three different effects: (i) resonance mode splitting, (ii) asymmetric light scattering based on the split modes, and (iii) site-selective nanoetching due to plasmon-induced charge separation (PICS) at the nanoparticle-semiconductor interface. Here, we develop novel photofunctional materials by taking advantage of those three effects. More specifically, we control the asymmetric scattering of Ag nanocubes on TiO2 by PICS, so as to develop materials for photodrawing of one-way visible translucent images and multicolor scattering images. The one-way visible translucent images, which are translucent scattering images visible only from the back side, are drawn by anaerobic bottom-selective etching of the Ag nanocubes. For drawing the multicolor scattering images, a scattering color of Ag nanocubes is changed from yellow to green by the anaerobic bottom-selective etching and from yellow to red by aerobic nonselective etching. We also theoretically and experimentally examined the contribution of a possible thermal effect to the nanoetching, and revealed that the contribution is negligible; Ag nanocubes on TiO2 are stable even at 473 K for 2 h in the dark, whereas the theoretically expected temperature increase under light is less than 1 K. In addition, we developed methods to stabilize the Ag nanocubes by inactivating PICS. When Ag nanocubes on TiO2 are coated with a thin polymer layer, PICS is decelerated and the stability is improved. Replacing TiO2 with diamond, which does not accept electrons from plasmonic nanoparticles, is also an effective method to stabilize the nanocubes.

Stationary bubble formation and Marangoni convection induced by CW laser heating of a single gold nanoparticle.

Gold nanoparticles (Au NPs) efficiently convert incident light into heat under the resonant conditions of localized surface plasmon. Controlling mass transfer through plasmonic heating of Au NPs has potential applications such as manipulation and fabrication within a small space. Here, we describe the formation of stationary microbubbles and subsequent fluid convection induced by CW laser heating of Au NPs in water. Stationary bubbles of about 1-20 µm in diameter were produced by irradiating individual Au NPs with a CW laser. Spatial profiles and velocity distribution of fluid convection around the microbubbles were visualized by the wide-field fluorescence imaging of tracer nanospheres. To evaluate the bubble-induced convection, numerical simulations were performed on the basis of general heat diffusion and Navier-Stokes equations. A comparison between the experimental and computational results revealed that a temperature derivative of surface tension at the bubble surface is a key factor to control the fluid convection. Temperature differences of a few Kelvin at the bubble surface resulted in convective velocities ranging from 102 to 103 µm s-1. The convective velocity gradually increased with increasing bubble diameter. This article covers both natural and Marangoni convection induced by plasmonic heating of Au NPs.

Stepwise Two-Photon-Induced Fast Photoswitching via Electron Transfer in Higher Excited States of Photochromic Imidazole Dimer.

Stepwise two-photon excitations have been attracting much interest because of their much lower power thresholds compared with simultaneous two-photon processes and because some stepwise two-photon processes can be initiated by a weak incoherent excitation light source. Here we apply stepwise two-photon optical processes to the photochromic bridged imidazole dimer, whose solution instantly changes color upon UV irradiation and quickly reverts to the initial color thermally at room temperature. We synthesized a zinc tetraphenylporphyrin (ZnTPP)-substituted bridged imidazole dimer, and wide ranges of time-resolved spectroscopic studies revealed that a ZnTPP-linked bridged imidazole dimer shows efficient visible stepwise two-photon-induced photochromic reactions upon excitation at the porphyrin moiety. The fast photoswitching property combined with stepwise two-photon processes is important not only for the potential for novel photochromic materials that are sensitive to the incident light intensity but also for fundamental photochemistry using higher excited states.

Laser-driven phase transitions in aqueous colloidal gold nanoparticles under high pressure: picosecond pump-probe study.

Pump-probe transient extinction spectroscopy was used to analyze 355 nm picosecond laser heating-induced phenomena in 60 nm-diameter aqueous gold nanoparticles (AuNPs) under a high pressure of 60 MPa. Kinetic spectroscopy revealed that a supercritical layer surrounding the AuNP nucleated with a lifetime of approximately 1 ns during its dynamic expansion and decay for a fluence of 19.6 mJ cm(-2). Moreover, in the post-mortem transmission electron micrographs we observed a number of fragments, a small percentage of size-reduced cores, and erupted particles among the intact particles after 60 shots, suggesting that evaporation occurred under laser illumination. The particle temperature calculation indicated that evaporation begins with a liquid droplet AuNP surrounded by a supercritical layer at temperatures below the boiling point of gold. By applying high pressure, we obtained a clear picture of the evaporation event, which was not possible at ambient pressure because bubble formation caused particle temperatures to rise uncontrollably. In this study, we shed light on the critical role of the supercritical layer formed around the AuNP under high pressure during laser-induced evaporation.

CW-laser-induced morphological changes of a single gold nanoparticle on glass: observation of surface evaporation.

Pulsed-laser heating of colloidal noble-metal nanoparticles in an aqueous solution induces morphological changes such as size reduction. However, the technique suffers disadvantages through polydispersed products. Here, we show that continuous-wave (CW) laser heating of single gold nanoparticles is capable of generating particles of smaller diameters with superb control in terms of exposure time and intensity. We show, based on calculations of particle temperatures under illumination, that surface evaporation below the boiling point of bulk gold occurs, resulting in a gradual diameter decrease in air. In our experiment, a focused illumination of Au NPs through an objective lens of a microscope provided peak-power densities (10(6)-10(7) W cm(-2)) equivalent to that of a typical nanosecond laser. Nevertheless the heating rate under CW laser illumination is much lower than that under pulsed-laser illumination, resulting in better control over nanoparticle heating and related morphological changes. Furthermore, the single-particle study of such heating helps us to clarify the evolution of such changes to a given particle.

Picosecond-to-nanosecond dynamics of plasmonic nanobubbles from pump-probe spectral measurements of aqueous colloidal gold nanoparticles.

The photothermal generation of nanoscale vapor bubbles around noble metal nanoparticles is of significant interest, not only in understanding the underlying mechanisms responsible for photothermal effects, but also to optimize photothermal effects in applications such as photothermal cancer therapies. Here, we describe the dynamics in the 400-900 nm regime of the formation and evolution of nanobubbles around colloidal gold nanoparticles using picosecond pump-probe optical measurements. From excitations of 20-150 nm colloidal gold nanoparticles with a 355 nm, 15 ps laser, time-dependent optical extinction signals corresponding to nanobubble formation were recorded. The extinction spectra associated with nanobubbles of different diameters were simulated by considering a concentric spherical core-shell model within the Mie theory framework. In the simulations, we assumed an increase in particle temperature. From temporal changes in the experimental data of transient extinctions, we estimated the temporal evolution of the nanobubble diameter. Corrections to bubble-free temperature effects on the transient extinction decays were applied in these experiments by suppressing bubble formation using pressures as high as 60 MPa. The results of this study suggest that the nanobubbles generated around a 60 nm-diameter gold nanoparticle using a fluence of 5.2 mJ cm(-2) had a maximum diameter of 260 ± 40 nm, and a lifetime of approximately 10 ns. The combination of fast transient extinction spectral measurements and spectral simulations provides insights into plasmonic nanobubble dynamics.

Observation of nanoscale cooling effects by substrates and the surrounding media for single gold nanoparticles under CW-laser illumination.

Understanding the nanoscale heating-induced local thermal response is important but hampered by lack of information on temperatures at such small scales. This paper reports laser-induced heating and thermal equilibration of metal nanoparticles supported on different substrates and immersed in several media. We use single-particle spectroscopy to monitor nanoparticle temperature rises due to laser excitation. Because of changes in the refractive index of the surrounding medium, the scattering spectrum of the gold nanoparticles undergoes a shift that is related to the temperature of the system. We find that the temperature increase depends on both the surrounding medium and the supporting substrate. We furthermore model the nanoparticle temperature using a simplified 1-D heat conduction model with an effective thermal conductivity that takes both substrate and environment into account. The results from this model are also compared to a more detailed 2-D heat transfer analysis. The results presented here are quite new and important to many plasmonic nanoparticle applications where the strong absorption cross section of the nanoparticles leads to a significant temperature rise. In particular, the current work introduces an analysis that can be easily implemented to model the temperature of a nanoparticle supported on a substrate, as is the case in many single-particle measurements.