Single Particle Extinction Research Articles

Optical trap-cavity ringdown spectroscopy as a single-aerosol-particle-scope

We report a single-aerosol-particle-scope using an optical trapping-cavity ringdown spectroscopy technique. The scope can not only view physical parameters such as size, motion, and restoring force constant of a single aerosol particle trapped in air but also display time-, particle-, or wavelength-resolved chemical properties such as single aerosol particle extinction. We demonstrate the scope by trapping and walking single carbon-nanotube particles of ∼50 μm in size and viewing those properties via changes of ringdown time. This single-aerosol-particle-scope offers a powerful tool to study both physical and chemical properties as well as their evolving dynamics.

Synthesis, Separation, and Hypermethod Characterization of Gold Nanoparticle Dimers Connected by a Rigid Rod Linker

Bonding individual metallic nanoparticles at small separation distances to let them form dimers and making them available in large quantities is a key requirement for various applications that wish to exploit the tremendous enhancement of electromagnetic fields in plasmonic junctions. Although progress has been witnessed in the past concerning the fabrication of dimers mediated by rigid molecular linkers, the exact bonding mechanism remains unclear. Here, we describe the fabrication of a rigid linker molecule and demonstrate its feasibility to achieve dimers made from closely spaced metallic nanoparticles in large quantities. Although the topography of the dimers proves the success of the fabrication method, we use what we call a hypermethod characterization approach to study the optical properties of dimers from various perspectives. Measuring the surface-enhanced Raman scattering signal of the linker molecule enables direct tracing of the optical environment it perceives. By reaching a strong field enhancement in the gap of the dimers, we are able to investigate optical and geometrical properties of the linker. Moreover, upon isolation of the dimers, we use single-particle extinction spectroscopy to study the optical response of a fabricated dimer directly. Full wave numerical simulations corroborate the experimental results and provide insights into quantities which cannot be accessed directly in experiments. The ability to fabricate and to characterize rigidly linked nanoparticles will pave the way toward various plasmonic applications such as sensors, photocatalysis, and plexcitonics.

Direct measurements of the optical cross sections and refractive indices of individual volatile and hygroscopic aerosol particles.

We present measurements of the evolving extinction cross sections of individual aerosol particles (spanning 700-2500 nm in radius) during the evaporation of volatile components or hygroscopic growth using a combination of a single particle trap formed from a Bessel light beam and cavity ring-down spectroscopy. For single component organic aerosol droplets of 1,2,6-hexanetriol, polyethylene glycol 400, and glycerol, the slow evaporation of the organic component (over time scales of 1000 to 10,000 s) leads to a time-varying size and extinction cross section that can be used to estimate the refractive index of the droplet. Measurements on binary aqueous-inorganic aerosol droplets containing one of the inorganic solutes ammonium bisulfate, ammonium sulfate, sodium nitrate, or sodium chloride (over time scales of 1000 to 15,000 s) under conditions of changing relative humidity show that extinction cross-section measurements are consistent with expectations from accepted models for the variation in droplet refractive index with hygroscopic growth. In addition, we use these systems to establish an experimental protocol for future single particle extinction measurements. The advantages of mapping out the evolving light extinction cross-section of an individual particle over extended time frames accompanied by hygroscopic cycling or component evaporation are discussed.

Theoretical Estimation of Extinction Coefficient Due to Time Evolution of Particle Size Distribution

The extinction coefficient of atmospheric aerosol particles influences the earth's radiation balance directly or indirectly and it can be determined by the scattering and absorption characteristics of aerosol particles. The problem of estimating the change of extinction coefficient due to time evolution of particle size distribution is studied and an improved method for calculating the single particle extinction efficiency is proposed. Through this improved method, the single particle extinction efficiency can be expressed simply in powers of particle diameter based on the anomalous diffraction approximate for the particle size range from 0.1 to 10 µm in diameter first, and then the change of the overall extinction coefficient undergoing (individually and simultaneous) Brownian and gravitational coagulation as well as condensation can be estimated comprehensively with the moment method based on the general dynamic equation. The feasibility and reliability of this method are investigated and the effect of time evolution of particle size distribution on the extinction coefficient is also estimated systemically. Simulation experiments indicate that the extinction coefficient obtained with the improved method coincides fairly well with that using the Mie theory, which provides a simple, dependable, and efficient method to estimate the change of extinction coefficient during the particle dynamic processes.

Quantitative optical extinction-based parametric method for sizing a single core–shell Ag–Ag2O nanoparticle

This paper develops a parametric method for determining the core radius and shell thickness in small silver–silver-oxide core–shell nanoparticles (Nps) based on single particle optical extinction spectroscopy.The method is based on the study of the relationship between plasmon peak wavelength, full width at half maximum (FWHM) and contrast of the extinction spectra as a function of core radius and shell thickness. This study reveals that plasmon peak wavelength is strongly dependent on shell thickness, whereas FWHM and contrast depend on both variables. These characteristics may be used for establishing an easy and fast stepwise procedure to size core–shell NPs from single particle absorption spectrum. The importance of the method lies in the possibility of monitoring the growth of the silver-oxide layer around small spherical silver Nps in real time.Using the electrostatic approximation of Mie theory, core–shell single particle extinction spectra were calculated for a silver particle's core size smaller than about 20 nm and different thicknesses of silver oxide around it. Analysis of the obtained curves shows a very particular characteristic of the plasmon peak of small silver–silver-oxide Nps, expressed in the fact that its position is strongly dependent on oxide thickness and weakly dependent on the core radius. Even a very thin oxide layer shifts the plasmon peak noticeably, enabling plasmon tuning with appropriate shell thickness.This characteristic, together with the behaviour of FWHM and contrast of the extinction spectra can be combined into a parametric method for sizing both core and shell of single silver Nps in a medium using only optical information. In turn, shell thickness can be related to oxygen content in the Np's surrounding media.The method proposed is applied to size silver Nps from single particle extinction spectrum. The results are compared with full optical spectrum fitting using the electrostatic approximation in Mie theory. The method may be the basis for developing a plasmonic sensor for O2 concentration based on Ag single NP spectroscopy.

Extinction and the optical theorem Part I Single particles

We study the extinction caused by a single particle and present a conceptual phase-based explanation for the related optical theorem. Simulations of the energy flow caused by a particle's presence in a collimated beam of light demonstrate how the extinction process occurs. It is shown that extinction does not necessarily cause a reduction of the energy flow along the exact forward direction. Implications regarding the measurement of the single-particle extinction cross section are discussed. This work is extended to noninteracting and interacting multiparticle groups in Part II [J. Opt. Soc. Am. A25, pp. 1514 (2008)].

Technical Note: Particle Extinction Coefficient for Polydispersed Aerosol Using a Harmonic Mean Type General Approximated Solution

In the present study, an approximated expression for the single particle extinction efficiency (Q ext ) has been obtained, which was valid for a wide range of particle sizes. The approximate single particle extinction efficiency was obtained using the harmonic mean type approximation between the particle extinction efficiencies from the Rayleigh scattering and geometric scattering regions. The results were compared with the numerically calculated Mie's results, which showed that both the approximated and original Mie's results were comparable. The approximated polydispersity of the overall extinction coefficient (b ext ) obtained using the moment method was then tested and compared with the numerically integrated results. The approximated solution obtained showed how the b ext effectively varied with the size distribution and greatly reduced computational time. Consequently, a simple and straightforward method for estimating the extinction coefficient for polydispersed aerosols, which can be widely applie...

Cavity ring-down spectroscopy measurement of single aerosol particle extinction. II. Extinction of light by an aerosol particle in an optical cavity excited by a cw laser

The authors present an analytical derivation of the scattered power from a spherical, homogeneous, nonabsorbing particle in a plane standing wave. The scattered power changes significantly with the position of the particle with respect to the peaks and nodes of the standing wave, even for particles whose diameters are many times the wavelength of the light. The analysis is applicable to continuous-wave cavity ring-down spectroscopy on aerosol particles, and the structure of the standing wave is expected to affect both the measured ring-down time and the shape of the ring-down trace. The dependence of the extinction on the phase of the standing wave at the location of the particle is captured in a parameter zeta which connects the current treatment to standard Mie scattering theory. Methods for calculating zeta are presented.

Cavity ring-down spectroscopy measurements of single aerosol particle extinction. I. The effect of position of a particle within the laser beam on extinction

A continuous wave distributed feedback diode laser operating in the near infrared at wavelengths close to 1650 nm has been used to measure the extinction of light by single aerosol particles. The technique of optical feedback cavity ring-down spectroscopy (CRDS) was used for measurement of CRDS events at a repetition rate of 1.25 kHz. This very high repetition rate enabled multiple measurements of the extinction of light by single aerosol particles for the first time and demonstrated the dependence of light scattering on the position of a particle within the laser beam. A model is proposed to explain quantitatively this phenomenon. The minimum detectable dimensionless extinction coefficient epsilonmin was determined to be 3x10(-6). Extinction values obtained for single spherical polymer beads from a monodisperse sample of particles of diameter of 4 microm are in near-quantitative agreement with the values calculated by the Mie scattering theory. The deviations from the Mie theory expected for measurement of extinction by CRDS using a continuous wave laser are discussed in the companion paper.

Retrieval of particle size distribution in the dependent model using the moment method

The problem of determining particle size distribution using the moment method in the spectral extinction technique is studied. The feasibility and reliability of the retrieval of spherical particle size distribution using the moment method are investigated. The single spherical particle extinction efficiency, which is derived theoretically using the Mie's solution to Maxwell's equation, is approximated with a higher order polynomial in order to apply the moment method. Simulation and experimental results indicate that a fairly reasonable representation of the particle size distribution can be obtained using the moment method in the dependent model algorithm. The method has advantages of simplicity, rapidity, and suitability for in-line particle size measurement.

Numerical estimation of the effects of condensation and coagulation on visibility using the moment method

An algorithm in which both aerosol dynamics and overall particle extinction coefficient are calculated is developed. The change of overall extinction coefficient and resultant visibility due to coagulation and condensation processes in the atmospheric aerosol is simulated for the aerosol size range of 0.005– 2.5 μ m in diameter. The moment method is used under the assumption that aerosol retains log-normal size distributions with varying geometric mean diameters and geometric standard deviations. The single particle extinction efficiency is approximated with a six order polynomial for the nuclei and accumulation mode particles, respectively. Three refractive indices which represent typical ambient aerosol conditions, i.e., hazy, urban, and clean area (Garcia Níeto, 2002, Aerosol Science and Technology, 36, 814–827) are considered. Subsequently, overall particle extinction coefficient and visibility from Koschmieder (1925, Atmosphere, 12, 171–181) for polydisperse aerosol are obtained. The simulation results show that coagulation enhances the visibility and condensation can enhance or reduce the visibility depending on the particle size and refractive index of accumulation mode particles for the simulated particles. The degree of the change is the highest for the hazy condition.

Optimization of extinction from surface plasmon resonances of gold nanoparticles

We present a method to determine the total extinction of laser light due toexcitations of the surface plasmon modes in gold nanoparticles, even for very largeparticle concentrations. The method uses the Mie theory efficiency parameter forsingle-particle scattering and absorption, a single-particle extinction parameter.We have previously shown that, by properly screening the scattered light, wecan determine the extinction of laser light when the beam transverses asmany as ten scattering mean free paths in a dense medium, i.e. in thepresence of significant multiple scattering. We present the application of themethod to the total extinction due mainly to the excitations of the surfaceplasmon resonance in gold nanoparticles. Although the results obtained frommeasurements in this multiple-particle medium are explained in terms of thesingle-particle Mie extinction parameter, we find that the optimum particle sizefor maximum multiple-scattering extinction is smaller than that for themaximum single-particle extinction parameter predicted by the Mie theory.

Coherent extinction and self-homodyning by small particles

Single-particle extinction using a coherent light source and where the Van Cittert-Zernike theorem applies to the forward-scattered light requires the angular summation of scattered fields instead of scattered intensities as done in the usual incoherent calculations. In spite of this approach, the resulting signals are calculated to be indistinguishable for single particles that are held stationary in the beam. When a flowing stream of particles moves perpendicularly through the beam one at a time, a modulated signal results which is described by an optical homodyning process and is called self-homodyning. The detection limit is a factor of 10 lower than that for incoherent extinction, allowing the detection of single submicrometer particles by simple extinction measurements. Theoretical calculations predicted the minimum detectable diameter for polystyrene latex (PSL) spheres to be 0.22 microm, and experimental measurements were able to detect PSL spheres of 0.31 microm.