Bright and Dark Field Imaging of PlasmonResonance in Nanoparticles

Armand Rundquist, Chad Ropp, Professor Edo Waks

An important aspect of the field of nanophotonics is the characterization of the optical properties of nanoparticles. On the nanometer scale, the interaction of light and matter takes on a different character than that observed in either bulk materials or individual atoms, and new effects can be produced. One such phenomenon, occurring in conductive nanoparticles, is known as surface plasmon resonance. An incident photon will cause the charge carriers in the conduction band to oscillate, and depending on the size, shape, and free carrier density of the nanoparticle, different frequencies of light will be scattered at different intensities.

Thus, by observing the light scattered from the surface of a conductive nanoparticle, it is possible to deduce the plasmon resonance frequencies, which in turn provide useful information about the structure of the particle.

Introduction

Optical Setup

Dark Field Imaging

Acknowledgments

In order to isolate single nanoparticles and examine the light they scatter, the following microscope setup was constructed.

Here, the objective serves not only to provide illumination, but also to collect the scattered light (a setup known as epi-illumination). A green laser is combined parallel with the white light source using a system of adjustable mirrors and beam splitters, and is used for both alignment into the spectrometer and excitation of some types of nanoparticles (such as quantum dots). The collected light is then focused onto a camera, which allows the light from individual particles to be isolated using the iris, and eventually sent to the spectrometer.

Silver particles were investigated first, due to the fact that they exhibit a strong plasmon resonance in the visible range. A sample1 was spun onto a microscope slide in order to spread the particles evenly, and the image shown below was taken using an ordinary optical microscope. The fact that many of the particles appear as different colors indicates that they are of a variety of shapes and sizes, and hence have different plasmon resonance frequencies.

One of the problems with the setup as described so far is that the particle appears in a “bright field” of reflected light from the stage, which can easily mask the plasmon resonance. In order to examine the light from the particle alone, an axicon (conical lens) is used to focus the white light source into a ring, which is then scattered onto the sample by the periphery of the objective2. This eliminates directly reflected light, allowing for dark field microscopy.

Schematic of Epi-illumination Optical Setup

Silver Particles Under Optical Microscope (150x)

Bright Field Image Dark Field Image

Spectra of Isolated Silver Particles

After recording the spectrum of light from an individual silver particle, it is also necessary to take the spectrum of light reflecting from an adjacent blank spot on the slide for comparison. The spectra on the left were produced by taking the difference between the two and normalizing by the intensity of the white light source.

As can be seen from the camera images inset into each graph, these spectra correspond to the color of light each particle scatters, although significant noise is present.

As can be seen from the normalized spectra to the right, the dark field signal is dimmer, but also exhibits less noise than the bright field signal for the same particle, which allows for a more precise analysis of the frequency peaks.

So far, this setup has proven useful for the detection of light scattered from individual particles. In addition to the analysis of metal particles, it has also been used to collect the photoluminescence of quantum dots (shown below) by exciting them with the green laser. This excitation pumps

[1] Thanks to Ali Faghih, Prof. Oded Rabin for preparing the silver particles.[2] The dark field setup is based on that presented by A. Curry, W. Hwang,

and A. Wax, "Epi-illumination through the microscope objective applied to darkfield imaging and microspectroscopy of nanoparticle interaction with cells in culture," Opt. Express 14, 6535-6542 (2006).

In addition, I would like to thank my faculty advisor Prof. Edo Waks and his entire group for their constant support.

charge carriers into the conduction band, which suggests the possibility of investigating the plasmon resonance (and hence, free charge carrier density) of excited quantum dots through the same method described for silver particles.

Comparison of Silver Spectra

Single CdSe QD Luminescence

Conclusions and Future Work

Detection of Plasmon Resonance inSilver Nanoparticles