Name: measurement of scattering nonlinearities from a single plasmonic nanoparticle
Uploaded: 2016-01-03T14:00:00
Description: Watch this Scientific Journal Video about Measurement of Scattering Nonlinearities from a Single Plasmonic Nanoparticle at JoVE.com

This work is supported by Ministry of Science and Technology under NSC-101-2923-M-002-001-MY3 and NSC-102-2112-M-002-018-MY3. This research is also supported by the Japan Society for the Promotion of Science (JSPS) through the &#8220;Funding Program for Next Generation World-Leading Researchers (NEXT Program),&#8221; initiated by the Council for Science and Technology Policy (CSTP) and JSPS Asian CORE Program.

1. GNS Sample Preparation
<ol>
	<li>Before preparing the sample, sonicate 1 ml GNS colloid solution for at least 15 min at about 40 kHz to prevent particle aggregation, which may cause the LSPR peak to shift.</li>
	<li>Drop 100-200 &#181;l of GNS colloid onto a slide glass with commercial magnesium aluminum silicate (MAS) coating to fix the GNSs.</li>
	<li>After at least 1 min, remove the extra colloid by flushing with distilled water. The waiting time depends on the required distribution density of the GNSs. Typically...

<ol>
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In the protocol, there are several critical steps. First, when preparing the samples, the density of nanoparticles should not be too high, to avoid plasmonic coupling among particles. If two or more particles are very close to each other, the coupling results in the LSPR wavelength shifting toward longer wavelengths, thus significantly reducing the nonlinearity. However, this imaging technique actually maps the distribution of plasmonic modes, instead of the particles themselves. Therefore, it is expected that with an ap...

The study of plasmonics has attracted great interest due to its applications in many different fields1-4. One of the most investigated fields in plasmonics is surface plasmonics, in which the collective oscillation of conduction electrons couples with an external electromagnetic wave at an interface between a metal and dielectric. Surface plasmonics has been explored for its potential applications in subwavelength optics, biophotonics, and microscopy5,6. The strong field enhancement in the ultra-small volume of metallic nanoparticles due to localized surface plasmon resonance (LSPR) has attracted extensive attention, not only because of its exceptional sensitivity to particle sizes, particle shapes, and the dielectric properties of the surrounding medium7-10, but also because of its ability to boost inherently weak nonlinear optical effects11. The exceptional sensitivity of LSPR is valuable for bio-sensing and near-field imaging techniques12,13. On the other hand, the enhanced nonlinearity of plasmonic structures can be utilized in photonic integrated circuits in applications such as optical switching and all-optical signal processing14,15. It is well known that the plasmonic absorption is linearly proportional to the excitation intensity at low intensity levels. When the excitation is strong enough, the absorption reaches saturation. Intriguingly, at higher intensities, the absorption increases again. These nonlinear effects are called saturable absorption (SA)15-17 and reverse saturable absorption (RSA)18, respectively.
It is known that due to the LSPR, scattering is particularly strong in plasmonic structures. Based on fundamental electromagnetics, the response of scattering versus incident intensity should be linear. However, in nanoparticles, scattering and absorption are closely linked via the Mie theory, and both can be expressed in terms of real and imaginary parts of the dielectric constant. Under the assumption that a single GNS behaves as a dipole under light illumination, the scattering coefficient (Qsca) and absorption coefficient (Qabs) from a single plasmonic nanoparticle according to the Mie theory can be expressed as19
<img alt="Equation 1" src="/files/ftp_upload/53338/53338eq1.jpg" />
where x is 2&#960;a/&#955;, a is the radius of the sphere, and m2 is &#949;m/&#949;d. Here, &#949;m and &#949;d correspond to the dielectric constants of the metal and of the surrounding dielectrics, respectively. Since the form of the scattering coefficient is similar to that of the absorption coefficient, it is therefore expected to observe saturable scattering in a single plasmonic nanoparticle20.
Recently, nonlinear saturable scattering in an isolated plasmonic particle was demonstrated for the first time21. It is remarkable that at deep saturation, the scattering intensity in fact decreased slightly when the excitation intensity increased. Even more remarkably, when the excitation intensity continued increasing after the scattering became saturated, the scattering intensity rose again, showing the effect of reverse saturable scattering20. Wavelength- and size-dependent studies have shown a strong relationship between LSPR and nonlinear scattering21. The intensity and wavelength dependences of plasmonic scattering are very similar to those of absorption, suggesting a common mechanism underlying these nonlinear behaviors.
In terms of applications, it is well known that nonlinearity helps to improve optical microscopy resolution. In 2007, saturated excitation (SAX) microscopy was proposed, which can enhance resolution by extracting the saturated signal via a temporal sinusoidal modulation of the excitation beam22. SAX microscopy is based on the concept that, for a laser focal spot, the intensity is stronger at the center than at the periphery. If the signal (either fluorescence or scattering) exhibits saturation behavior, the saturation must start from the center, while the linear response remains at the periphery. Therefore, if there is a method to extract only the saturated part, it will leave only the central part while rejecting the peripheral part, thus effectively enhancing the spatial resolution. In principle, there is no lower resolution limit in SAX microscopy, as long as deep saturation is reached and there is no sample damage due to the intense illumination.
It has been shown that the resolution of fluorescence imaging can be significantly enhanced by utilizing the SAX technique. However, fluorescence suffers from the photobleaching effect. Combining the discovery of scattering nonlinearity and the concept of SAX, super-resolution microscopy based on scattering can be realized21. Compared to conventional super-resolution microscopies, the scattering-based technique provides a novel non-bleaching contrast method. In this paper, a step-by-step description is given to outline the procedures required to obtain and extract the nonlinearity of plasmonic scattering. Methods of identifying scattering nonlinearities introduced by changing the incident intensity are described. More details will be provided to unravel how these nonlinearities affect images of single nanoparticles and how spatial resolution can be enhanced accordingly by the SAX technique.

<table>
	<tbody>
		<tr>
			<td>microscope body</td>
			<td>Olympus, Japan</td>
			<td>BX-51</td>
			<td></td>
		</tr>
		<tr>
			<td>objective lens</td>
			<td>Olympus, Japan</td>
			<td>UPlanSapo, 100X, NA 1.4</td>
			<td></td>
		</tr>
		<tr>
			<td>80-nm gold colloid</td>
			<td>BBI Solutions, UK</td>
			<td>EM.GC80</td>
			<td></td>
		</tr>
		<tr>
			<td>supercontinuum laser</td>
			<td>Fianium, United Kingdom</td>
			<td>SC400-2-PP</td>
			<td></td>
		</tr>
		<tr>
			<td>broadband dielectric mirrors</td>
			<td>Thorlabs, USA</td>
			<td>BB1-E02</td>
			<td></td>
		</tr>
		<tr>
			<td>field emission SEM</td>
			<td>JEOL, Japan</td>
			<td>JSM-6330F</td>
			<td>optional</td>
		</tr>
		<tr>
			<td>spectrometer</td>
			<td>Andor Technology, UK</td>
			<td>Shamrock 163</td>
			<td></td>
		</tr>
		<tr>
			<td>&#160;charge-coupled device</td>
			<td>Andor Technology, UK</td>
			<td>iDus DV420A-OE</td>
			<td></td>
		</tr>
		<tr>
			<td>acousto-optic modulators</td>
			<td>&#160;IntraAction Corp., USA</td>
			<td>AOM-402AF1</td>
			<td></td>
		</tr>
		<tr>
			<td>lock-in amplifier</td>
			<td>Stanford Research Systems, USA</td>
			<td>SR-830</td>
			<td></td>
		</tr>
		<tr>
			<td>MAS-coated slide glass</td>
			<td>Matsunami Glass, Japan,</td>
			<td>S9215</td>
			<td></td>
		</tr>
	</tbody>
</table>

measurement of scattering nonlinearities from a single plasmonic nanoparticle

Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus have potential applications in nonlinear optics, which requires extraordinary optical intensity. One of the most studied nonlinearities in plasmonics is nonlinear absorption, including saturation and reverse saturation behaviors. Although scattering and absorption in nanoparticles are closely correlated by the Mie theory, there has been no report of nonlinearities in plasmonic scattering until very recently. 
Last year, not only saturation, but also reverse saturation of scattering in an isolated plasmonic particle was demonstrated for the first time. The results showed that saturable scattering exhibits clear wavelength dependence, which seems to be directly linked to the localized surface plasmon resonance (LSPR). Combined with the intensity-dependent measurements, the results suggest the possibility of a common mechanism underlying the nonlinear behaviors of scattering and absorption. These nonlinearities of scattering from a single gold nanosphere (GNS) are widely applicable, including in super-resolution microscopy and optical switches. 
In this paper, it is described in detail how to measure nonlinearity of scattering in a single GNP and how to employ the super-resolution technique to enhance the optical imaging resolution based on saturable scattering. This discovery features the first super-resolution microscopy based on nonlinear scattering, which is a novel non-bleaching contrast method that can achieve a resolution as low as l/8 and will potentially be useful in biomedicine and material studies.

Figure 6 shows the measured spectrum from an 80 nm GNS. A calculated curve based on the Mie theory is given in the same plot, showing excellent agreement. The LSPR peak is around 580 nm. In the following experiment, the laser wavelength was 532 nm, which was chosen as it is located inside the plasmonic band to enhance optical scattering with plasmonic effect and enable scattering saturation21.
Figure 7 presents scattering images of a single gold nan...

Watch this Scientific Journal Video about Measurement of Scattering Nonlinearities from a Single Plasmonic Nanoparticle at JoVE.com

Measurement of Scattering Nonlinearities from a Single Plasmonic Nanoparticle

Saturable and reverse saturable scattering were discovered in isolated plasmonic particles and adopted as a novel non-bleaching contrast method in super-resolution microscopy. Here the experimental procedures of detecting and extracting nonlinear scattering are explained in detail, as well as how to enhance resolution with the aid of saturated excitation microscopy.

Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus ...

The overall goal of this experiment is to demonstrate resolution enhancement with non-linear scattering from plasmonic nanostructures. In the field of optical microscopy, contrast and the resolution are the most important factors. Our technique expand to the super resolution field with the new scattering based on contrast.The main advantage of this technique is that scattering from nanoparticles is strong, highly nonlinear, and non-bleaching. Demonstrating the procedure will be Hsuan Lee. A greater student from my lab.The experiment will be performed on an optical bench. Where there is a microscope and two laser sources. The first laser source is a 532 nanometer continuous wave laser used for imaging.The second is a super-continuum laser used for spectroscopy measurements. This is a schematic of the basic set up for both imaging and spectroscopy. Samples will be mounted on a piezoelectric positioning stage in the confocal microscope.Light from the 532 nanometer laser passes through a neutral density filter. The beam is directed by a beam splitter to galvano mirrors that provide raster scanning in the focal plane of the confocal microscope objective. The backward scattered light is collected by the objective and focused on the detector.Begin by aligning the white light illumination path of the microscope. Use the halogen light source under curler illumination. To verify the alignment of the internal iris, be certain to observe sharp edges on the octagon via the eyepiece or CCD.Next, use a piece of paper to check the external light path. Verify the beams are partially reflected by the beam splitter and propagate toward the laser by moving the paper along the light path. Now, have ready paper targets to aid in further alignment.The targets should be on thin paper with concentric rings. Place these targets along the beam path so that they are aligned with the halogen beam. Here, a target is in position on the back aperture of the objective lens.This is another target in position along the optical path. Next, turn on the 532 nanometer laser used for imaging in order to align it. Use the paper targets to aid in collimating the incident laser light opposite the halogen beam.Continue with finer alignment of the laser beam before removing the targets and proceeding. After aligning the laser, obtain a prepared gold nanosphere sample for the experiment. This sample has 80 nanometer gold nanospheres in oil.The mixture is sealed between two glass plates. Continue by mounting the sample on the microscope stage and adding objective oil. At this point, ready the detection system for imaging.Collect the scattered light with the photo multiplier tube at the end of the optical path. Back along the beam path, have a 20 micrometer diameter pinhole to block out of focus scattered light from the photo multiplier tube. Proceed with imaging by turning on the galvano mirrors and the photo multiplier.Observe the computer monitor to see the back scanner in signal of the gold nanoparticles. Maximize the back scanner in signal by adjusting the pinhole position and the height of the sample stage. After aligning the confocal microscope, characterize the scattering nonlinearity of the sample.To do this, first use a power meter to measure laser power after the objective lens. The reading should be less than 10 microwatts corresponding to an excitation intensity of less than 10 to the fourth watts per centimeter square. Then, acquire an image of the gold nanoparticles using the photo multiplier tube.Open the captured image in image analysis software to characterize the scattering intensity profile. Select a gold nanosphere among those in the image and draw a line across it. Then, follow the steps required in the software to retrieve the scattering profile.Fit the profile to a gaussian which allows a further check on the alignment of the imaging system. Now, begin to systematically scan through the excitation intensities. First, change the neutral density filter to increase the excitation intensity.Determine the new excitation intensity and record the back scattering image at the new intensity level. Use the image to extract a scattering profile of a nanosphere as before. Plot the scattering profile and identify its value at the center of the sphere as the scattering signal.Repeat these steps several times to build a plot of the scattering signal versus excitation intensity. In this figure, the blue points represent data. The red line is a polynomial fit.There is a linear relationship between the scattering signal and low values of the excitation energy. The drop below this linear relationship indicates that saturation has occurred. Next step is to perform spectroscopy on a single gold nanosphere.This schematic provides an overview of the setup. Use a super-continuum laser source with a wave length range of 450-1750 nanometers. Also, use a broadband 50/50 beamsplitter to ensure spectral coverage over the visible spectrum.Place a flipping mirror in front of the photo multiplier tube to direct the light toward the spectrometer. Which is equipped with a charge-coupled device. The super-continuum laser should be aligned using the same procedure as the 532 nanometer laser.At the output of the super-continuum laser, take some precautions to remove excess infrared power from the system. Place visible light reflecting mirrors in the optical path right after the laser output, before sending the beam to the microscope. In conjunction with the mirrors, use beam dumps to collect the infrared radiation that might damage the system.With the laser aligned, acquire and image of the gold nanospheres. View the image and identify a single nanosphere for study. Fix the focus of the incident broadband light on to the chosen nanosphere.Next, insure that the spectrometer is in the optical path. In this position, the flipping mirror directs the incident light to the photo multiplier tube. Reorient the mirror to direct the incident light to the spectrometer.Proceed to collect data on the scattering spectrum of the single gold nanoparticle. As in this example, the measured spectrum will be a mixture of nanosphere scattering and background due to reflections. After taking the spectrum, return the flipping mirror.Turn it to direct light to the photo multiplier tube. Take another image to confirm that the gold nanoparticle has not changed position. Next, shift the focus of the broadband light to a point where there is no particle.Change the mirror again to direct light to the spectrometer and proceed to make another spectrum measurement. The new spectrum is of the background. It will be subtracted from the spectrum of the gold nanosphere.Here is the clear back scattering spectrum produced by subtracting the background spectrum from the spectrum with the gold nanospheres. Saturated excitation microscopy of the sample requires a different setup. This is the schematic of the saturated excitation microscope.The laser source is a 532 nanometer laser. Use a 50/50 beam splitter to create two beams from the laser source. Pass each beam through a separate acoustal optic modulator.The different modulator frequencies generate a beat frequency that will serve as the modulation frequency of the saturated excitation signals. Combine the first order diffracted beams from the acoustal optic modulators with another 50/50 beam splitter. To monitor temporal modulation, use a glass slide to split a small portion of the laser light off to a photo detector.To check the system, connect the photo detector to an oscilloscope. Make sure no sample is in place. Assess the photo detector signal on the oscilloscope.The signal should be a sinusoid at the beat frequency if the modulation and beam overlapping are correct. For this experiment, the expected beat frequency is 10 kilohertz. Make efforts to achieve as perfect a sinusoid as possible with minimal nonlinearity.The next step, is to incorporate a lock-in amplifier in to the system. For saturated excitation microscopy, the lock-in amplifier has input from the photo detector and the photo multiplier tube. Disconnect the output of the photo detector from the oscilloscope and connect it to the reference input of the amplifier.Connect the output of the photo multiplier tube to the lock-in amplifier as the signal input. Send output from the amplifier to a data acquisiton card. At this point, mount the sample on the microscope stage.Use the same sample with 80 nanometer gold nanospheres in oil sealed between two glass plates. Return to the lock-in amplifier while taking measurements. Set the amplifier to export the absolute magnitude of the voltage signal.In this case, by selecting R"in the channel 1 display panel. Change the harmonic component setting at the reference channel to obtain the saturated excitation single amplitudes. These color images are formed using custom software to synchronize and combine the signals from the lock-in amplifier and the driving voltages of the galvano motors.The scanning electron microscope image is for comparison. The images demonstrate resolution enhancement with different harmonic components. The measured spectrum of an 80 nanometer gold nanosphere is in red.The solid curve calculated using Mie theory, shows excellent agreement. The localized surface plasmon resonance is at 580 nanometers. Scattering images, above, and line profiles, below have different features as a function of the excitation intensity.At low intensity, the point spread function is a standard gaussian profile. Higher intensities result in the function flattening and widening indicating saturation. At higher values, the central intensity is not the maximum, resulting in a donut-shaped point spread function.Eventually, the central intensity becomes the peak again during reverse saturation. The blue data points in this plot of the central intensity at different excitation intensities reveal both saturation and reverse saturation behavior. The data points are fit to a 5th order polynomial plotted in red.This data can be used to extract the harmonic frequency components. The harmonic components can also be found directly using the lock-in amplifier. This left plot is of experimental data.The right plot is the result of calculation using the 5th order polynomial fit. Both plots exhibit dips at specific intensities along the curves. For example, three dips in the second harmonic.Also in each plot, for each harmonic order, the slope increases after the first dip. Once mastered, this technique can be done in less than three hours if you perform it properly. While attempting this procedure, it is important to check the quality of the sinusoidal modulation, the linearity of the detection system, and the mechanical stability of the sample.After watching this video, you should have a good understanding of how to achieve resolution enhancement based on nonlinear scattering from plasmonic nanostructures. This technique provides an aspiring sample for researchers in the field of super-resolution microscopy to explore novel contrast in other fundamental light-matter interactions.

measurement-scattering-nonlinearities-from-single-plasmonic

Research

JoVE Journal

Engineering

Measuring Spatially- and Directionally-varying Light Scattering from Biological Material

Light interacts with an organism's integument on a variety of spatial scales. For example in an iridescent bird: nano-scale structures produce color; the milli-scale structure of barbs and barbules largely determines the directional pattern of reflected light; and through the macro-scale spatial structure of overlapping, curved feathers, these directional effects create the visual texture. Milli-scale and macro-scale effects determine where on the organism's body, and from what viewpoints and under what illumination, the iridescent colors are seen. Thus, the highly directional flash of brilliant color from the iridescent throat of a hummingbird is inadequately explained by its nano-scale structure alone and questions remain. From a given observation point, which milli-scale elements of the feather are oriented to reflect strongly? Do some species produce broader "windows" for observation of iridescence than others? These and similar questions may be asked about any organisms that have evolved a particular surface appearance for signaling, camouflage, or other reasons.
In order to study the directional patterns of light scattering from feathers, and their relationship to the bird's milli-scale morphology, we developed a protocol for measuring light scattered from biological materials using many high-resolution photographs taken with varying illumination and viewing directions. Since we measure scattered light as a function of direction, we can observe the characteristic features in the directional distribution of light scattered from that particular feather, and because barbs and barbules are resolved in our images, we can clearly attribute the directional features to these different milli-scale structures. Keeping the specimen intact preserves the gross-scale scattering behavior seen in nature. The method described here presents a generalized protocol for analyzing spatially- and directionally-varying light scattering from complex biological materials at multiple structural scales.

We present a non-destructive method for sampling spatial variation in the direction of light scattered from structurally complex materials. By keeping the material intact, we preserve gross-scale scattering behavior, while concurrently capturing fine-scale directional contributions with high-resolution imaging. Results are visualized in software at biologically-relevant positions and scales.

Light interacts with an organism's integument on a variety of spatial scales. For example in an iridescent bird: nano-scale structures produce color; the ...

Visible-light Induced Reduction of Graphene Oxide Using Plasmonic Nanoparticle

Present work demonstrates the simple, chemical free, fast, and energy efficient method to produce reduced graphene oxide (r-GO) solution at RT using visible light irradiation with plasmonic nanoparticles. The plasmonic nanoparticle is used to improve the reduction efficiency of GO. It only takes 30 min&#160;at RT by illuminating the solutions with Xe-lamp, the r-GO solutions can be obtained by completely removing gold nanoparticles through simple centrifugation step. The spherical gold nanoparticles (AuNPs) as compared to the other nanostructures is the most suitable plasmonic nanostructure for r-GO preparation. The reduced graphene oxide prepared using visible light and AuNPs was equally qualitative as chemically reduced graphene oxide, which was supported by various analytical techniques such as UV-Vis spectroscopy, Raman spectroscopy, powder XRD and XPS. The reduced graphene oxide prepared with visible light shows excellent quenching properties over the fluorescent molecules modified on ssDNA and excellent fluorescence recovery for target DNA detection. The r-GO prepared by recycled AuNPs is found to be of same quality with that of chemically reduced r-GO. The use of visible light with plasmonic nanoparticle demonstrates the good alternative method for r-GO synthesis.

A simple protocol for the preparation of reduced graphene oxide using visible light and plasmonic nanoparticle is described.

Present work demonstrates the simple, chemical free, fast, and energy efficient method to produce reduced graphene oxide (r-GO) solution at RT using ...

A System to Create Stable Nanoparticle Aerosols from Nanopowders

Nanoparticle aerosols released from nanopowders in workplaces are associated with human exposure and health risks. We developed a novel system, requiring minimal amounts of test materials (min. 200 mg), for studying powder aerosolization behavior and aerosol properties. The aerosolization procedure follows the concept of the fluidized-bed process, but occurs in the modified volume of a V-shaped aerosol generator. The airborne particle number concentration is adjustable by controlling the air flow rate. The system supplied stable aerosol generation rates and particle size distributions over long periods (0.5-2 hr and possibly longer), which are important, for example, to study aerosol behavior, but also for toxicological studies. Strict adherence to the operating procedures during the aerosolization experiments ensures the generation of reproducible test results. The critical steps in the standard protocol are the preparation of the material and setup, and the aerosolization operations themselves. The system can be used for experiments requiring stable aerosol concentrations and may also be an alternative method for testing dustiness. The controlled aerosolization made possible with this setup occurs using energy inputs (may be characterized by aerosolization air velocity) that are within the ranges commonly found in occupational environments where nanomaterial powders are handled. This setup and its operating protocol are thus helpful for human exposure and risk assessment.

We designed and developed an effective nanopowder aerosolization setup and operating protocol. The system generated nanoparticle aerosols with stable number concentrations and size distributions for long durations, requiring only small quantities of test material (min. 200 mg).

Nanoparticle aerosols released from nanopowders in workplaces are associated with human exposure and health risks. We developed a novel system, requiring ...

Testing of Nanoparticle Release from a Composite Containing Nanomaterial Using a Chamber System

With the rapid development of nanotechnology as one of the most important technologies in the 21st century, interest in the safety of consumer products containing nanomaterials is also increasing. Evaluating the nanomaterial release from products containing nanomaterials is a crucial step in assessing the safety of these products, and has resulted in several international efforts to develop consistent and reliable technologies for standardizing the evaluation of nanomaterial release. In this study, the release of nanomaterials from products containing nanomaterials is evaluated using a chamber system that includes a condensation particle counter, optical particle counter, and sampling ports to collect filter samples for electron microscopy analysis. The proposed chamber system is tested using an abrasor and disc-type nanocomposite material specimens to determine whether the nanomaterial release is repeatable and consistent within an acceptable range. The test results indicate that the total number of particles in each test is within 20% from the average after several trials. The release trends are similar and they show very good repeatability. Therefore, the proposed chamber system can be effectively used for nanomaterial release testing of products containing nanomaterials.

Nanoparticle release is tested using a chamber system that includes a condensation particle counter, an optical particle counter and sampling ports to collect filter samples for microscopy analysis. The proposed chamber system can be effectively used for nanomaterial release testing with a repeatable and consistent data range.

With the rapid development of nanotechnology as one of the most important technologies in the 21st century, interest in the safety of consumer products ...

Optical Trap Loading of Dielectric Microparticles In Air

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly scattered dielectric microparticles adhered to a glass substrate are momentarily detached using ultrasonic vibrations generated by a piezoelectric transducer (PZT). Then, the optical beam focused on a selected particle lifts it up to the optical trap while the vibrationally excited microparticles fall back to the substrate. A particle may be trapped at the nominal focus of the trapping beam or at a position above the focus (referred to here as the levitation position) where gravity provides the restoring force. After the measurement, the trapped particle can be placed at a desired position on the substrate in a controlled manner. 
In this protocol, an experimental procedure for selective optical trap loading in air is outlined. First, the experimental setup is briefly introduced. Second, the design and fabrication of a PZT holder and a sample enclosure are illustrated in detail. The optical trap loading of a selected microparticle is then demonstrated with step-by-step instructions including sample preparation, launching into the trap, and use of electrostatic force to excite particle motion in the trap and measure charge. Finally, we present recorded particle trajectories of Brownian and ballistic motions of a trapped microparticle in air. These trajectories can be used to measure stiffness or to verify optical alignment through time domain and frequency domain analysis. Selective trap loading enables optical tweezers to track a particle and its changes over repeated trap loadings in a reversible manner, thereby enabling studies of particle-surface interaction.

A protocol for launching and stably trapping selected dielectric microparticles in air is presented.

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly ...

Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few can observe immobilized particles. Moreover, a limited number of studies have experimentally measured the exertable forces on the particles. The designs can be classified as the protruding nanodisk type or the suppressed nanohole type. For the latter, microscopic observation is extremely challenging. In this paper, a new plasmonic tweezer system is introduced to monitor particles, both in directions parallel and orthogonal to the symmetric axis of a plasmonic nanohole structure. This feature enables us to observe the movement of each particle near the rim of the nanohole. Furthermore, we can quantitatively estimate the maximal trapping forces using a new fluidic channel.

A microchip fabrication process that incorporates plasmonic tweezers is presented here. The microchip enables the imaging of a trapped particle to measure maximal trapping forces.

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few ...

Scanning Light Scattering Profiler (SLPS) Based Methodology to Quantitatively Evaluate Forward and Backward Light Scattering from Intraocular Lenses

The scanning light scattering profiler (SLSP) methodology has been developed for the full-angle quantitative evaluation of forward and backward light scattering from intraocular lenses (IOLs) using goniophotometer principles. This protocol describes the SLSP platform and how it employs a 360&#176; rotational photodetector sensor that is scanned around an IOL sample while recording the intensity and location of scattered light as it passes through the IOL medium. The SLSP platform can be used to predict, non-clinically, the propensity for current and novel IOL designs and materials to induce light scatter. Non-clinical evaluation of light scattering properties of IOLs can significantly reduce the number of patient complaints related to unwanted glare, glistening, optical defects, poor image quality, and other phenomena associated with the unintended light scattering. Future studies should be conducted to correlate SLSP data with clinical results to help identify which measured light scatter is most problematic for patients that have undergone cataract surgery subsequent to IOL implantation.

Scanning Light Scattering Profiler SLPS Based Methodology to Quantitatively Evaluate Forward and Backward Light Scattering from Intraocular Lenses

This protocol describes the scanning light scattering profiler (SLSP) that enables the full-angle quantitative evaluation of forward and backward scattering of light from intraocular lenses (IOLs) using goniophotometer principles.

The scanning light scattering profiler (SLSP) methodology has been developed for the full-angle quantitative evaluation of forward and backward light ...

Standardized Method for Measuring Collection Efficiency from Wipe-sampling of Trace Explosives

One of the limiting steps to detecting traces of explosives at screening venues is effective collection of the sample. Wipe-sampling is the most common procedure for collecting traces of explosives, and standardized measurements of collection efficiency are needed to evaluate and optimize sampling protocols. The approach described here is designed to provide this measurement infrastructure, and controls most of the factors known to be relevant to wipe-sampling. Three critical factors (the applied force, travel distance, and travel speed) are controlled using an automated device. Test surfaces are chosen based on similarity to the screening environment, and the wipes can be made from any material considered for use in wipe-sampling. Particle samples of the explosive 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) are applied in a fixed location on the surface using a dry-transfer technique. The particle samples, recently developed to simulate residues made after handling explosives, are produced by inkjet printing of RDX solutions onto polytetrafluoroethylene (PTFE) substrates. Collection efficiency is measured by extracting collected explosive from the wipe, and then related to critical sampling factors and the selection of wipe material and test surface. These measurements are meant to guide the development of sampling protocols at screening venues, where speed and throughput are primary considerations.

Optimized sampling protocols and the development of new wipe materials can be facilitated by standardized measurements of collection efficiency from wipe-sampling. Our approach for sampling trace explosives uses an automated device to control speed, force, and distance during wipe-sampling followed by extraction of collected explosives.

One of the limiting steps to detecting traces of explosives at screening venues is effective collection of the sample. Wipe-sampling is the most common ...

Micro/Nano-scale Strain Distribution Measurement from Sampling Moir&#233; Fringes

This work describes the measurement procedure and principles of a sampling moir&#233; technique for full-field micro/nano-scale deformation measurements. The developed technique can be performed in two ways: using the reconstructed multiplication moir&#233; method or the spatial phase-shifting sampling moir&#233; method. When the specimen grid pitch is around 2 pixels, 2-pixel sampling moir&#233; fringes are generated to reconstruct a multiplication moir&#233; pattern for a deformation measurement. Both the displacement and strain sensitivities are twice as high as in the traditional scanning moir&#233; method in the same wide field of view. When the specimen grid pitch is around or greater than 3 pixels, multi-pixel sampling moir&#233; fringes are generated, and a spatial phase-shifting technique is combined for a full-field deformation measurement. The strain measurement accuracy is significantly improved, and automatic batch measurement is easily achievable. Both methods can measure the two-dimensional (2D) strain distributions from a single-shot grid image without rotating the specimen or scanning lines, as in traditional moir&#233; techniques. As examples, the 2D displacement and strain distributions, including the shear strains of two carbon fiber-reinforced plastic specimens, were measured in three-point bending tests. The proposed technique is expected to play an important role in the non-destructive quantitative evaluations of mechanical properties, crack occurrences, and residual stresses of a variety of materials.

Micro/Nano-scale Strain Distribution Measurement from Sampling Moir&amp;#233; Fringes

A sampling moir&#233; technique featuring 2-pixel and multi-pixel sampling methods for high-accuracy strain distribution measurements at the micro/nano-scale is presented here.

This work describes the measurement procedure and principles of a sampling moir&#233; technique for full-field micro/nano-scale deformation measurements. ...

Controllable Nucleation of Cavitation from Plasmonic Gold Nanoparticles for Enhancing High Intensity Focused Ultrasound Applications

In this study, plasmonic gold nanoparticles were simultaneously exposed to pulsed near-infrared laser light and high intensity focused ultrasound (HIFU) for the controllable nucleation of cavitation in tissue-mimicking gel phantoms. This in vitro protocol was developed to demonstrate the feasibility of this approach, for both enhancement of imaging and therapeutic applications for cancer. The same apparatus can be used for both imaging and therapeutic applications by varying the exposure duration of the HIFU system. For short duration exposures (10 &#181;s), broadband acoustic emissions were generated through the controlled nucleation of inertial cavitation around the gold nanoparticles. These emissions provide direct localization of nanoparticles. For future applications, these particles may be functionalized with molecular-targeting antibodies (e.g. anti-HER2 for breast cancer) and can provide precise localization of cancerous regions, complementing routine diagnostic ultrasound imaging. For continuous wave (CW) exposures, the cavitation activity was used to increase the localized heating from the HIFU exposures resulting in larger thermal damage in the gel phantoms. The acoustic emissions generated from inertial cavitation activity during these CW exposures was monitored using a passive cavitation detection (PCD) system to provide feedback of cavitation activity. Increased localized heating was only achieved through the unique combination of nanoparticles, laser light and HIFU. Further validation of this technique in pre-clinical models of cancer is necessary.

This protocol demonstrates the controllable nucleation of cavitation in gel phantoms, through simultaneous exposure to both near-infrared pulsed laser light and high intensity focused ultrasound (HIFU). The cavitation activity can then be used for enhancing imaging and/or therapeutic uses of HIFU.

In this study, plasmonic gold nanoparticles were simultaneously exposed to pulsed near-infrared laser light and high intensity focused ultrasound (HIFU) ...