Trapping and Deposition of Dye-Molecule Nanoparticles in the Nanogap of a Plasmonic Antenna

Optical force and torque in near-field excitation of C3H6: A first-principles study using RT-TDDFT

Optical trapping is an effective tool for manipulating micrometer-sized particles, although its application to nanometer-sized particles remains difficult. The field of optical trapping has advanced significantly, incorporating more advanced techniques such as plasmonic structures. However, single-molecule trapping remains a challenge. To achieve a deeper understanding of optical forces acting on molecular systems, a first-principles approach to analyze the optical force on molecules interacting with a plasmonic field is crucial. In our study, the optical force and torque induced by the near-field excitation of C3H6 were investigated using real-time time-dependent density functional theory calculations on real-space grids. The near field from the scanning tunneling probe was adopted as the excitation source for the molecule. The optical force was calculated using the polarization charges induced in the molecule based on Lorentz force. While the optical force and torque calculated as functions of the light energy were in moderate agreement with the oscillator strengths obtained from the far-field excitation of C3H6, a closer correspondence was achieved with the power spectrum of the induced dipole moment using near-field excitation. Time-domain analysis of the optical force suggests that the simultaneous excitation of multiple excited states generally weakens the force because of mismatches between the directions of the induced polarization and the electric field. This study revealed a subtle damping mechanism for the optical force arising from intrinsic electronic states and the influence of beating.

Enantioselective optical trapping of single chiral molecules in the superchiral field vicinity of metal nanostructures

In this study, we theoretically analyzed the optical force acting on single chiral molecules in the plasmon field induced by metallic nanostructures. Using the extended discrete dipole approximation, we quantitatively examined the optical response of single chiral molecules in the localized plasmon by numerically analyzing the internal polarization structure of the molecules obtained from quantum chemical calculations, without phenomenological treatment. We evaluated the chiral gradient force due to the optical chirality gradient of the superchiral field near the metallic nanostructures for chiral molecules. Our calculation method can be used to evaluate the molecular-orientation dependence and rotational torque by considering the chiral spatial structure inside the molecules. We theoretically showed that the superchiral field induced by chiral plasmonic nanostructures can be used to selectively optically capture the enantiomers of a single chiral molecule.

Room-Temperature Molecular Manipulation via Plasmonic Trapping at Electrified Interfaces

For the motion control of individual molecules at room temperature, optical tweezers could be one of the best approaches to realize desirable selectivity with high resolution in time and space. Because of physical limitations due to the thermal fluctuation, optical manipulation of small molecules at room temperature is still a challenging subject. The difficulty of the manipulation also emerged from the variation of molecular polarizability depending on the choice of molecules as well as the molecular orientation to the optical field. In this article, we have demonstrated plasmonic optical trapping of small size molecules with less than 1 nm at the gap of a single metal nanodimer immersed in an electrolyte solution. In situ electrochemical surface-enhanced Raman scattering measurements prove that a plasmonic structure under electrochemical potential control realizes not only the selective molecular condensation but also the formation of unique mixed molecular phases which is distinct from those under a thermodynamic equilibrium. Through detailed analyses of optical trapping behavior, we established the methodology of plasmonic optical trapping to create the novel adsorption isotherm under applying an optical force at electrified interfaces.

Nonlinearity-modulated single molecule trapping and Raman scattering analysis

Single molecule detection and analysis play important roles in many current biomedical researches. The deep-nanoscale hotspots, being excited and confined in a plasmonic nanocavity, make it possible to simultaneously enhance the nonlinear light-matter interactions and molecular Raman scattering for label-free detections. Here, we theoretically show that a nanocavity formed in a tip-enhanced Raman scattering (TERS) system can also achieve valid optical trapping as well as TERS signal detection for a single molecule. In addition, the nonlinear responses of metallic tip and substrate film can change their intrinsic physical properties, leading to the modulation of the optical trapping force and the TERS signal. The results demonstrate a new degree of freedom brought by the nonlinearity for effectively modulating the optical trapping and Raman detection in single molecule level. This proposed platform also shows a great potential in various fields of research that need high-precision surface imaging.

Incoherent Optical Tweezers on Black Titanium

Optical tweezers enable the manipulation of micro- and nanodielectric particles through entrapment using a tightly focused laser. Generally, optical trapping of submicron size particles requires high-intensity light in the order of MW/cm². Here, we demonstrate a technique of stable optical trapping of submicron polymeric beads on nanostructured titanium surfaces (black-Ti) without the use of lasers. Fluorescent polystyrene beads with a diameter d = 20-500 nm were successfully trapped on black-Ti by low-intensity focused illumination of incoherent light at λ = 370 m from a Hg lamp. Light intensity was 5.5 W/cm², corresponding to a reduced light intensity of 6 orders of magnitude. Upon switching off illumination, trapped particles were released from the illuminated area, indicating that trapping was optically driven and reversible. Such trapping behavior was not observed on nonstructured Ti surfaces or on nanostructured silicon surfaces. Thus, the Ti nanostructures were demonstrated to play a key role.

Coupled plasmonic systems: controlling the plasmon dynamics and spectral modulations for molecular detection

This review describes recent studies on coupled plasmonic systems for controlling plasmon dynamics and molecular detection using spectral modulations. The plasmon dephasing time can be controlled by weak and strong coupling regimes between the plasmonic nanostructures or localized surface plasmon resonances (LSPRs) and the other optical modes such as microcavities. The modal coupling induces near-field enhancement by extending the plasmon dephasing time to increase the near-field enhancement at certain wavelengths resulting in the enhancement of molecular detection. On the other hand, the interaction between LSPR and molecular excited or vibrational states also modulates the resonance spectrum, which can also be used for detecting a small number of molecules with a subtle change in the spectrum. The spectral modulation is induced by weak and strong couplings between LSPRs and the electronic or vibrational states of molecules, and this method is sensitive enough to measure a single molecule.

Plasmonic tweezers: for nanoscale optical trapping and beyond

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science.

Dynamic electrical measurement of biomolecule behavior via plasmonically-excited nanogap fabricated by electromigration

The dynamic motion of DNA oligomers at the nanoscale gap between nanoelectrodes is measured under plasmonic excitation using laser irradiation. The use of a nanogap enables highly sensitive detection of individual molecules using an electrical readout or an optical readout such as Raman spectroscopy. However, the target molecule must reach the nanogap in order to be detected. This study focuses on the use of plasmonic excitation to trap molecules at the nanogap surface. The nanogap electrode is fabricated by electromigration and is, therefore, a much smaller nanogap than the top-down fabrication in the conventional plasmonic trapping studies. To demonstrate the individual molecule detection and to investigate the molecular behavior, the molecules are monitored using an electrical readout under a bias voltage instead of an optical readout used in the conventional studies. The conductance change due to DNA oligomer penetration to the nanogap is observed with the irradiated light intensity of over 1.23 mW. The single-molecule detection is confirmed irradiating the laser to the nanogap. The results suggest that DNA oligomers are spontaneously attracted and concentrated to the nanogap corresponding to the detection point, resulting in high detection probability and sensitivity.

Quantifying the Role of the Surfactant and the Thermophoretic Force in Plasmonic Nano-optical Trapping

Plasmonic nanotweezers use intense electric field gradients to generate optical forces able to trap nano-objects in liquids. However, part of the incident light is absorbed into the metal, and a supplementary thermophoretic force acting on the nano-object arises from the resulting temperature gradient. Plasmonic nanotweezers thus face the challenge of disentangling the intricate contributions of the optical and thermophoretic forces. Here, we show that commonly added surfactants can unexpectedly impact the trap performance by acting on the thermophilic or thermophobic response of the nano-object. Using different surfactants in double nanohole plasmonic trapping experiments, we measure and compare the contributions of the thermophoretic and the optical forces, evidencing a trap stiffness 20× higher using sodium dodecyl sulfate (SDS) as compared to Triton X-100. This work uncovers an important mechanism in plasmonic nanotweezers and provides guidelines to control and optimize the trap performance for different plasmonic designs.

Theoretical analysis of optically selective imaging in photoinduced force microscopy

We present a theoretical study on the measurement of photoinduced force microscopy (PiFM) for composite molecular systems. Using discrete dipole approximation, we calculate the self-consistent response electric field of the entire system, including the PiFM tip, substrate, and composite molecules. We demonstrate a higher sensitivity for PiFM measurement on resonant molecules than the previously obtained tip-sample distance dependency, z^-4, owing to multifold enhancement of the localized electric field induced at the tip-substrate nanogap and molecular polarization. The enhanced localized electric field in PiFM allows high-resolution observation of forbidden optical electronic transitions in dimer molecules. We investigate the wavelength dependence of PiFM for dimer molecules, obtaining images at incident light wavelengths corresponding to the allowed and forbidden transitions. We reveal that these PiFM images drastically change with the frequency-dependent spatial structures of the localized electric field vectors and resolve different types of nanoparticles beyond the resolution for the optically allowed transitions. This study demonstrates that PiFM yields multifaceted information based on microscopic interactions between nanomaterials and light.

Nanoscale rotational optical manipulation

Light has momentum, and hence, it can move small particles. The optical tweezer, invented by Ashkin et al. [Opt. Lett. 11, 288 (1986)] is a representative application. It traps and manipulates microparticles and has led to great successes in the biosciences. Currently, optical manipulation of "nano-objects" is attracting growing attention, and new techniques have been proposed and realized. For flexible manipulation, push-pull switching [Phys. Rev. Lett. 109, 087402 (2012)] and super-resolution trapping by using the electronic resonance of nano-objects have been proposed [ACS Photonics 5, 318 (2017)]. However, regarding the "rotational operation" of nano-objects, the full potential of optical manipulation remains unknown. This study proposes mechanisms to realize rotation and direction switching of nano-objects in macroscopic and nanoscopic areas. By controlling the balance between the dissipative force and the gradient force by using optical nonlinearity, the direction of the macroscopic rotational motion of nano-objects is switched. Further, conversion between the spin angular momentum and orbital angular momentum by light scattering through localized surface plasmon resonance in metallic nano-complexes induces optical force for rotational motion in the nanoscale area. This study pieces out fundamental operations of the nanoscale optical manipulation of nanoparticles.

Plasmonic Manipulation of DNA using a Combination of Optical and Thermophoretic Forces: Separation of Different-Sized DNA from Mixture Solution

We demonstrate the size-dependent separation and permanent immobilization of DNA on plasmonic substrates by means of plasmonic optical tweezers. We found that a gold nanopyramidal dimer array enhanced the optical force exerted on the DNA, leading to permanent immobilization of the DNA on the plasmonic substrate. The immobilization was realized by a combination of the plasmon-enhanced optical force and the thermophoretic force induced by a photothermal effect of the plasmons. In this study, we applied this phenomenon to the separation and fixation of size-different DNA. During plasmon excitation, DNA strands of different sizes became permanently immobilized on the plasmonic substrate forming micro-rings of DNA. The diameter of the ring was larger for longer DNA (in base pairs). When we used plasmonic optical tweezers to trap DNA of two different lengths dissolved in solution (φx DNA (5.4 kbp) and λ-DNA (48.5 kbp), or φx DNA and T4 DNA (166 kbp)), the DNA were immobilized, creating a double micro-ring pattern. The DNA were optically separated and immobilized in the double ring, with the shorter sized DNA and the larger one forming the smaller and larger rings, respectively. This phenomenon can be quantitatively explained as being due to a combination of the plasmon-enhanced optical force and the thermophoretic force. Our plasmonic optical tweezers open up a new avenue for the separation and immobilization of DNA, foreshadowing the emergence of optical separation and fixation of biomolecules such as proteins and other ncuelic acids.

Localized ZnO Growth on a Gold Nanoantenna by Plasmon-Assisted Hydrothermal Synthesis

The excitation of localized surface plasmon resonances (LSPRs) in metal nanostructures enables subwavelength photon localization and large electric field enhancement, which can be advantageously used to strongly enhance light-matter interactions at the nanoscale. For this purpose, efficient methods for deterministically handling and arranging nanomaterials at the exact position of the localized electric field are required. In this Letter, we propose a novel method based on a hydrothermal synthesis reaction to locally and selectively synthesize zinc oxide in a plasmonic nanoantenna. We first make evident the role of LSPR for achieving efficient heating of gold nanostructures. Then, by selectively addressing one of the LSPRs of a gold antenna, we demonstrate that localized zinc oxide formation at the targeted location of the antenna can be achieved due to the nanoscale confinement of the heat production.

Plasmonic tweezers for optical manipulation and biomedical applications

This comprehensive minireview highlights the recent research on the subtypes, optical manipulation, and biomedical applications of plasmonic tweezers.

Plasmonic Tweezers towards Biomolecular and Biomedical Applications

With the capability of confining light into subwavelength scale, plasmonic tweezers have been used to trap and manipulate nanoscale particles. It has huge potential to be utilized in biomolecular research and practical biomedical applications. In this short review, plasmonic tweezers based on nano-aperture designs are discussed. A few challenges should be overcome for these plasmonic tweezers to reach a similar level of significance as the conventional optical tweezers.