Nanoparticle In Vacuum Research Articles

Cavity-mediated long-range interactions in levitated optomechanics

AbstractThe ability to engineer cavity-mediated interactions has emerged as a powerful tool for the generation of non-local correlations and the investigation of non-equilibrium phenomena in many-body systems. Levitated optomechanical systems have recently entered the multiparticle regime, which promises the use of arrays of strongly coupled massive oscillators to explore complex interacting systems and sensing. Here we demonstrate programmable cavity-mediated interactions between nanoparticles in vacuum by combining advances in multiparticle optical levitation and cavity-based quantum control. The interaction is mediated by photons scattered by spatially separated particles in a cavity, resulting in strong coupling that is long-range in nature. We investigate the scaling of the interaction strength with cavity detuning and interparticle separation and demonstrate the tunability of interactions between different mechanical modes. Our work will enable the exploration of many-body effects in nanoparticle arrays with programmable cavity-mediated interactions, generating entanglement of motion, and the use of interacting particle arrays for optomechanical sensing.

Molecular Dynamics Study of the Devitrification of Amorphous Copper Nanoparticles in Vacuum and in a Silver Shell

The process of the devitrification of copper nanoparticles in vacuum and in a silver shell during heating was studied using a molecular dynamics simulation. The results show that there is an inverse relationship between the particle diameter and devitrification temperature. As the size of the particles decreases, the temperature at which devitrification occurs increases due to a higher fraction of atoms near the interface. The presence of a silver shell leads to a significant increase in the devitrification temperature of the copper nanoparticles. For the considered particle sizes, the difference between the devitrification temperatures without a shell and with a shell ranged from 130 K for copper particles with a diameter of 11 nm to 250 K for 3 nm particles. The mechanisms of the nucleation of a crystalline phase in particles in vacuum and in a silver shell are significantly different. In the first case, crystalline nuclei are predominantly formed near the surface, while in the second case, on the contrary, they are formed within the particle’s volume.

Levitated Optomechanics with Meta-Atoms.

We propose to introduce additional control in levitated optomechanics by trapping a meta-atom, i.e., a subwavelength and high-permittivity dielectric particle supporting Mie resonances. In particular, we theoretically demonstrate that optical levitation and center-of-mass ground-state cooling of silicon nanoparticles in vacuum is not only experimentally feasible but it offers enhanced performance over widely used silica particles in terms of trap frequency, trap depth, and optomechanical coupling rates. Moreover, we show that, by adjusting the detuning of the trapping laser with respect to the particle's resonance, the sign of the polarizability becomes negative, enabling levitation in the minimum of laser intensity, e.g., at the nodes of a standing wave. The latter opens the door to trapping nanoparticles in the optical near-field combining red and blue-detuned frequencies, in analogy to two-level atoms, which is of interest for generating strong coupling to photonic nanostructures and short-distance force sensing.

Interaction between an Optically Levitated Nanoparticle and Its Thermal Image: Internal Thermometry via Displacement Sensing.

We propose and theoretically analyze an experiment where displacement sensing of an optically levitated nanoparticle in front of a surface can be used to measure the induced dipole-dipole interaction between the nanoparticle and its thermal image. This is achieved by using a surface that is transparent to the trapping light but reflective to infrared radiation, with a reflectivity that can be time modulated. This dipole-dipole interaction relies on the thermal radiation emitted by a silica nanoparticle having sufficient temporal coherence to correlate the reflected radiation with the thermal fluctuations of the dipole. The resulting force is orders of magnitude stronger than the thermal gradient force, and it strongly depends on the internal temperature of the nanoparticle for a particle-to-surface distance greater than two micrometers. We argue that it is experimentally feasible to use displacement sensing of a levitated nanoparticle in front of a surface as an internal thermometer in ultrahigh vacuum. Experimental access to the internal physics of a levitated nanoparticle in vacuum is crucial to understanding the limitations that decoherence poses to current efforts devoted to preparing a nanoparticle in a macroscopic quantum superposition state.

Dry launching of silica nanoparticles in vacuum

Clean loading of silica nanoparticles with a radius as small as ∼50 nm is required for experiments in levitated optomechanics that operate in ultra-high vacuum. We present a cheap and simple experimental method for dry launching of silica nanoparticles by shaking from a polytetrafluoroethylene surface (PTFE). We report on the successful launching of single silica nanoparticles with a minimum radius of 43 nm, which is enabled by the low stiction to the launching surface. Nanoparticles with radii of 43 and 71.5 nm are launched with a high flux and small spread. The measured velocities are significantly smaller than 1 m/s. The demonstrated launching method allows for controlled loading of dry nanoparticles with radii as small as 43 nm into optical traps in (ultra-)high vacuum, although we anticipate that loading of smaller sizes is equally feasible.

Interference of the scattered vector light fields from two optically levitated nanoparticles.

We experimentally study the interference of dipole scattered light from two optically levitated nanoparticles in vacuum, which present an environment free of particle-substrate interactions. We illuminate the two trapped nanoparticles with a linearly polarized probe beam orthogonal to the propagation of the trapping laser beams. The scattered light from the nanoparticles are collected by a high numerical aperture (NA) objective lens and imaged. The interference fringes from the scattered vector light for the different dipole orientations in image and Fourier space are observed. Especially, the interference fringes of two scattered light fields with polarization vortex show the π shift of the interference fringes between inside and outside the center region of the two nanoparticles in the image space. As far as we know, this is the first experimental observation of the interference of scattered vector light fields from two dipoles in free space. This work also provides a simple and direct method to determine the spatial scales between optically levitated nanoparticles by the interference fringes.

A Chemical Nanoreactor Based on a Levitated Nanoparticle in Vacuum.

A single levitated nanoparticle is used as a nanoreactor for studying surface chemistry at the nanoscale. Optical levitation under controlled pressure, surrounding gas composition, and humidity provides extreme control over the nanoparticle, including dynamics, charge, and surface chemistry. Using a single nanoparticle avoids ensemble averages and allows studying how the presence of silanol groups at its surface affects the adsorption and desorption of water from the background gas with excellent spatial and temporal resolution. Herein, we demonstrate the potential of this versatile platform by studying the Zhuravlev model in silica particles. In contrast to standard methods, our system allowed the observation of an abrupt and irreversible change in scattering cross section, mass, and mechanical eigenfrequency during the dehydroxylation process, indicating changes in density, refractive index, and volume.

A generalized method for calculating plasmoelectric potential in non-Mie-resonant plasmonic systems

Abstract Since its first observation in 2014, plasmoelectric potential (PEP) has drawn a great deal of research interest in all-metal optoelectronics and photochemistry. As an optical thermodynamic phenomenon induced by the electron number dependent equilibrium temperature in plasmonic nanostructures, the early theoretical model developed for calculating PEP is only applicable to Mie-resonant nanostructures, such as a gold nanosphere on a conductive indium tin oxide (ITO) substrate, where the transfer efficiency of hot electrons from gold to ITO can be analytically determined. Without the presence of the substrate, the temperature increase on the gold nanosphere induced by plasmonic absorption was calculated on the basis of thermal radiation in vacuum, which probably over-estimates the actual temperature increase in comparison to realistic experimental conditions. Here, we propose an equilibrium-thermodynamics computational method to quantify the actual efficiency of plasmon-induced electron transfer between a non-Mie-resonant metallic nanostructure and a conductive substrate and hence determine the resultant plasmoelectric potential. With a less than 2.5% relative error in predicting the steady-state temperature of a Mie-resonant nanoparticle in vacuum, and a more strict evaluation of the plasmonic local heating induced temperature increase in a single plasmonic nanostructure or an array of such structures under continuous-wave illumination (CWI), our generalized method provides a robust and accurate approach for quantifying PEP in various plasmonic-particle (array)-on-film nanocavities.

On-chip optical levitation with a metalens in vacuum

Optical levitation of dielectric particles in vacuum is a powerful technique for precision measurements, testing fundamental physics, and quantum information science. Conventional optical tweezers require bulky optical components for trapping and detection. Here, we design and fabricate an ultrathin dielectric metalens with a high numerical aperture of 0.88 at 1064 nm in vacuum. It consists of 500-nm-thick silicon nano-antennas, which are compatible with an ultrahigh vacuum. We demonstrate optical levitation of nanoparticles in vacuum with a single metalens. The trapping frequency can be tuned by changing the laser power and polarization. We also transfer a levitated nanoparticle between two separated optical tweezers. Optical levitation with an ultrathin metalens in vacuum provides opportunities for a wide range of applications including on-chip sensing. Such metalenses will also be useful for trapping ultracold atoms and molecules.

Levitodynamics: Levitation and control of microscopic objects in vacuum.

The control of levitated nano- and micro-objects in vacuum—which capitalizes on scientific achievements in the fields of atomic physics, control theory, and optomechanics—is of considerable interest. The ability to couple the motion of levitated systems to internal degrees of freedom, as well as to external forces and systems, provides opportunities for science and technology. Attractive research directions, ranging from fundamental quantum physics to commercial sensors, have been unlocked by the many recent experimental achievements, including motional ground-state cooling of an optically levitated nanoparticle. Here we review the status, challenges, and prospects of levitodynamics, the multidisciplinary research area devoted to understanding, controlling, and using levitated nano- and micro-objects in vacuum.

Fast size estimation of single-levitated nanoparticles in a vacuum optomechanical system.

Optical trapping of single nanoparticles in vacuum has various applications in both precise measurements and fundamental physics. However, to date, the number and size of randomly loaded nanoparticles in an optical trap is difficult to determine unless in vacuum. In this Letter, an efficient method for nanoparticle size estimation in an optical tweezer system before the evacuation of air was proposed and demonstrated experimentally, using scattering light from levitated particles. The particle radii deduced from the scattering light power in our proposal and from the kinetic theory of particles in gas match well (with the differences of less than 10%). For sample particles with radii ranging within 50-100 nm, we also provide a preselection rule based on this method, where over half of the trapped particles are verified as single particles. Such a particle analysis method is applicable also for the size estimation of levitated diamond particles, gold particles, and other plasmonic particles and can be applied to discovering novel scattering effects.

Imaging the dipole scattering of an optically levitated dielectric nanoparticle

We experimentally observe the dipole scattering of a nanoparticle using a high numerical aperture (NA) imaging system. The optically levitated nanoparticle provides an environment free of a particle–substrate interaction. We illuminate the silica nanoparticle in vacuum with a 532 nm laser beam orthogonally to the propagation direction of the 1064 nm trapping laser beam strongly focused by the same high NA objective used to collect the scattering, which results in a dark background and high signal-noise ratio. The dipole orientations of the nanoparticle induced by the linear polarization of the incident laser are studied by measuring the scattering light distribution in the image and the Fourier space (k-space) as we rotate the illuminating light polarization. The polarization vortex (vector beam) is observed for the special case, when the dipole orientation of the nanoparticle is aligned along the optical axis of the microscope objective. Our work offers an important platform for studying the scattering anisotropy with Kerker conditions.

6 GHz hyperfast rotation of an optically levitated nanoparticle in vacuum

We report an experimental observation of a record-breaking ultrahigh rotation frequency about 6 GHz in an optically levitated nanoparticle system. We optically trap a nanoparticle in the gravity direction with a high numerical aperture (NA) objective lens, which shows significant advantages in compensating the influences of the scattering force and the photophoretic force on the trap, especially at intermediate pressure (about 100 Pa). This allows us to trap a nanoparticle from atmospheric to low pressure ( 10 − 3 Pa ) without using feedback cooling. We measure a highest rotation frequency about 4.3 GHz of the trapped nanoparticle without feedback cooling and a 6 GHz rotation with feedback cooling, which is the fastest mechanical rotation ever reported to date. Our work provides useful guides for efficiently observing hyperfast rotation in the optical levitation system and may find various applications such as in ultra-sensitive torque detection, probing vacuum friction, and testing unconventional decoherence theories.

Melting temperature of Ti and TiAl nanoparticles in vacuum and in Al matrix depending on their diameter: molecular dynamics study

The dependence of the melting temperature of Ti and TiAl nanoparticles on their diameter in vacuum and in Al matrix was studied by the method of molecular dynamics using EAM potentials of Zope and Mishin. Particles with a diameter of 2.5 to 12 nm were considered. The obtained values of the melting point are in good agreement with the approximation curves constructed on the basis that the decrease in the melting temperature is proportional to the ratio of the surface area of the particle to its volume. Wherein the values of the melting temperature of Ti and TiAl particles in the aluminum matrix turned out to be lower than those of particles in vacuum, which is explained by the smearing and disordering of the interface due to mutual diffusion. As the size of particles in vacuum and in aluminum increased, the values of their melting points tended to the same value, which is explained by the decrease in the role of the diffusion-blurred interface with an increase in the particle diameter. The particles began to melt from the surface. The velocity of movement of the melting front depended on temperature and increased with increasing temperature. In the case of particles in aluminum matrix, at temperatures close to the particle melting point, mutual diffusion was significantly accelerated due to melting of the particle boundary layer. Al atoms penetrating into the particle accelerated the movement of the melting front, rapidly occupying the next destroyed layer of the particle.

Optical levitation using broadband light

The ability to create dynamic, tailored optical potentials has become important across fields ranging from biology to quantum science. We demonstrate a method for the creation of arbitrary optical tweezer potentials using the broadband spectral profile of a superluminescent diode combined with the chromatic aberration of a lens. A tunable filter, typically used for ultrafast laser pulse shaping, allows us to manipulate the broad spectral profile and therefore the optical tweezer potentials formed by focusing of this light. We characterize these potentials by measuring the Brownian motion of levitated nanoparticles in vacuum and also demonstrate interferometric detection and feedback cooling of the particle’s motion. This simple and cost-effective technique will enable wide application and allow rapid modulation of the optical potential landscape in excess of megahertz frequencies.

Synthesis of W-Cu composite nanoparticles by the electrical explosion of two wires and their consolidation by spark plasma sintering

This research shows the possibility of obtaining composite W-Cu nanoparticles with the core–shell structure and high content of copper by means of electrical explosion of intertwined W and Cu wires. The use of electric pulse plasma sintering on the obtained W-Cu composite nanoparticles in vacuum in the temperature range 900 °C–1030 °C over 10 min has allowed us to fabricate 3D materials with the relative density of 95.3 … 98.9% and a conductivity that is close to the theoretical one (63.5%–64.1%IACS). Also, despite a significant content of copper, the microhardness of the 3D material was (2.68–2.7 GPa). High microhardness of the two-phase composite is attributed to its fine-grain structure. The conductivity and microhardness of the 3D W-Cu composite make it a promising material for different electrical engineering applications.

Relationship between nanostructure-magnetic property induced by temperature for iron oxide nanoparticles in vacuum, Ar and O2 environments

It is known that superparamagnetic magnetite (Fe3O4) nanoparticles (NPs) become ferromagnetic under Si2+ ion irradiation due to particle size growth and microstructure evolution; it has been proposed that this feature could be used for in situ high temperature (up to 500 °C) radiation monitoring in the core of nuclear reactors. Herein, magnetite NPs synthesized by a nanocluster deposition system are heated to 800 °C in three different environments (argon, oxygen and vacuum), and nanostructure-magnetic property correlations are investigated by vibrating sample magnetometry, scanning electron microscopy, and X-ray diffraction. Magnetization of the NPs is increased due to the sintering and overall size growth by the agglomeration of the particles, while the morphology remains nearly unchanged up to 800 °C, with the one anomaly that zerovalent Fe appeared due to the reduction of the Fe3O4 at 800 °C in vacuum. In argon and oxygen at high-temperature, antiferromagnetic hematite is created, which causes a reduction of the magnetization, and abrupt growth of particle size above 500 °C.

On the mechanical stability conditions for nanoparticles in vacuum and under an external pressure

A condition of the mechanical stability of non-volatile nanoparticles in a chemically inert medium is derived from the non-negativity of the second variation of the Helmholtz free energy of the ‘small object - gaseous medium’ system. Two cases have been analyzed: (i) a non-volatile nanoparticle with the surface tension depending on its radius; (ii) a limiting case of bigger objects when the surface tension corresponds to its constant macroscopic value. It has been shown that the external pressure should increase stability of nanoparticles. However, the effect under consideration should be noticeable at high pressures only.

Cavity-Based 3D Cooling of a Levitated Nanoparticle via Coherent Scattering.

We experimentally realize cavity cooling of all three translational degrees of motion of a levitated nanoparticle in vacuum. The particle is trapped by a cavity-independent optical tweezer and coherently scatters tweezer light into the blue detuned cavity mode. For vacuum pressures around 10^{-5} mbar, minimal temperatures along the cavity axis in the millikelvin regime are observed. Simultaneously, the center-of-mass (c.m.) motion along the other two spatial directions is cooled to minimal temperatures of a few hundred millikelvin. Measuring temperatures and damping rates as the pressure is varied, we find that the cooling efficiencies depend on the particle position within the intracavity standing wave. This data and the behavior of the c.m. temperatures as functions of cavity detuning and tweezer power are consistent with a theoretical analysis of the experiment. Experimental limits and opportunities of our approach are outlined.

Influence of Atmospheres on the Initial Thermal Decomposition of 1,3,5-Trinitro-1,3,5-triazinane: Reactive Molecular Dynamics Simulation

Energetic materials (EMs) are usually surrounded in atmospheres during manufacture, storage, transportation, and application. Thus, an insight into the influence of atmospheres on the EM decay becomes of significance. In the present work, molecular dynamics simulations are separately performed on 10 systems of one 1,3,5-trinitro-1,3,5-triazinane (RDX) nanoparticle in vacuum and in nine atmospheres including CO2, CO, H2O, H2, N2, NH3, O2, NO, and NO2 to identify the influence. No evident reaction between RDX and any atmosphere is found at room temperature, suggesting such a negligible influence. Nevertheless, the influences of the nine atmospheres are variable at high temperatures and can be classified into three cases. NH3, CO, NO, and NO2, in particular NH3, promote the RDX decay by consuming RDX and the intermediates generated from it. Because NO2 that serves as a catalyzer to accelerate the RDX decay is considerably consumed by O2, the decay is significantly prohibited by O2. In addition, a few prohibi...