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 ...

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