Session T9: Focus Session: Optics of Nanostructures - Near Field, Single Molecule, and Plasmonics

Near-field optical microscopy of plasmonic metamaterials

We have studied the effect of aperture-sample interactions on the near-field optical imaging and its application for plasmonic metamaterials. Specifically, periodic arrays of paired and single gold nanorods were studied at the near-field using reflection and transmission modes of a near-field scanning optical microscope (NSOM) at various wavelengths and polarizations of light in the visible range. The paired nanorods act like nanoantennae in resonant coupling. In non-resonant coupling, enhanced reverse contrast in reflection is observed with strong polarization dependence and the average near-field transmission exhibits an opposite sign of anisotropy relative to the far-field case. The results demonstrate that the broad angular spectra of small-aperture sources play a crucial role and also show that angular redistributions of these spectra after transmission or reflection from the nanorod array are likely due to excitation of localized and propagating plasmons. We quantify this probe-nanorods system using Finite Difference Time Domain (FDTD) simulations. By varying the NSOM tip geometry and the wavelength we determine and tune the resonance wavelengths of the probe-sample system where the near-field interaction is enhanced. The near-field maps of the electric and magnetic fields in the metamaterial structure are also obtained.   

Single Molecule Studies of Energy Transfer in Semiconductor Nanocrystal Clusters

Enhanced fluorescence intermittency has been reported in single molecule fluorescence experiments on small clusters of semiconductor nanocrystals$^{1}$ (NCs).~ We report here on studies of small clusters of NCs by single molecule time-correlated single photon counting. According to this analysis, clusters typically blink on a microsecond to millisecond time scale; whereas, isolated NCs blink on a much longer millisecond to second time scale. A fast-decay component in the cluster fluorescence lifetime, not present in single NCs, is correlated with low fluorescence intensity.~ A model based on nonradiative energy transfer to NCs with smaller bandgap, combined with independent blinking for the NCs in the cluster, accounts for the main experimental features.~ In this model the smallest-gap NC dominates the emission properties, in particular the ``off'' time distribution of the cluster, which experimentally resembles that for a single NC. [1] Yu, M. and A. Van Orden, \textit{Enhanced Fluorescence Intermittency of CdSe-ZnS Quantum-Dot Cluster}, Physical Review Letters, 2006 \textbf{237402-4}   

Plasmonic Dicke Effect in Molecular Fluorescence near a Metal Nanoparticle

We study theoretically the role of dipole-dipole interactions in surface-plasmon-mediated cooperative emission of light by an ensemble of molecules near a metal nanoparticle (plasmonic Dicke effect). In a typical experimental situation, fluorescing molecules are attached to nanoparticle surface via DNA linkers with controllable lengths, and the inter-molecule interactions lead to the ordering of molecules into a periodic structure. We calculated radiative and non-radiative decay rates as well as quantum efficiency of such systems by taking into account both dipole-dipole interactions and energy shifts due to plasmon exchange between molecules in the ensemble. We found that, in contrast to the usual (photonic) Dicke effect, the interactions merely shift the energy of sub-radiant mode without significantly affecting super-radiant modes, thus preserving the cooperative nature of the emission. Deviations from periodicity also do not significantly alter the structure of collective states.   

Novel concepts in infrared imaging at nanoscale resolution

Within the recent years, various novel optical concepts have been invented to improve the diffraction-limited resolution of optical microscopy. The first approach of scanning near-field optical microscopy (SNOM) employed a small, subwavelength-sized aperture that is scanned close to the object of interest, capable of a resolution of about 50 nm. More advanced concepts rely on the light scattering of a sharp tip probing the sample, allowing for higher resolution (10-30 nm) and the use of longer wavelengths. Another exciting new imaging device, a planar slab of a material with negative permittivity called a superlens, allows for subwavelength resolved imaging over large areas. I will focus on the latter two systems that operate with \textit{infrared light} and offer the capability of chemical sensing by directly probing molecular vibrations. Particularly, I will present the latest results on superlensing that became accessible by \textit{phase-sensitive} infrared near-field microscopy and thus provide new insight into the imaging process of a such a device [1]. I will also explain the basics of scattering-type near-field optical microscopy (s-SNOM) and present various examples of unambiguous nanoscale material characterization from various areas such as semiconductor analysis, materials science, chemistry, and biology [2-4]. In these examples, the use of infrared spectroscopy allows to sense molecular vibrations as well as collective excitation of lattice vibrations (``phonons'') in polar crystals [5]. Currently, the main limitation of this technique comprises of the low signals that demand tunable laser sources and restrict the spectral range of operation. Consequently, I will introduce new concepts for increasing the sensitivity of infrared near-field spectroscopy to ultimately allow for a broadband operation. \\[4pt] [1] T. Taubner, D. Korobkin, Y. Urzhumov, G. Shvets, R. Hillenbrand, \textit{Science} \textbf{313}, 1595 (2006). \\[0pt] [2] T. Taubner, R. Hillenbrand, F. Keilmann, \textit{Applied Physics Letters} \textbf{85}, 5064 (2004). \\[0pt] [3] A. Huber, D. Kazantsev, F. Keilmann, J. Wittborn, R. Hillenbrand, \textit{Advanced Materials} \textbf{19}, 2209 (2007). \\[0pt] [4] M. Brehm, T. Taubner, R. Hillenbrand, F. Keilmann, \textit{Nano Letters} \textbf{6}, 1307 (2006). \\[0pt] [5] R. Hillenbrand, T. Taubner, F. Keilmann, \textit{Nature} \textbf{418}, 159 (2002).   

Theoretical study of vibrations and Raman spectra in pristine and P-doped Si nanocrystals

Phonon confinement in Si nanostructures has been widely studied and size-dependent red-shifts as well as asymmetric broadening of Raman peaks have been commonly observed. However, a fully \textit{ab initio} study of the Raman spectra in Si nanostructures has yet to be performed. Here, we investigate the vibrational modes and non-resonant Raman spectra of Si nanoclusters using density functional theory. Vibrational modes were calculated using the force constant method and non-resonant Raman spectra were evaluated within the Placzek approximation. We have reproduced the experimental Raman peak shifts and demonstrated that Raman spectra in Si nanocrystals are highly sensitive to the introduction of dopants.   

Plasmonic resonances and hot spots in Ag octopods and octahedra assemblies

We study novel Ag plasmonic nanoparticles synthesized in the star shapes (octopods) as well as octahedra and their ensembles. The discrete dipole approximation shows a number of major resonances that can be tuned up to a large extent making them especially attractive to use in e.g. high-performance surface enhanced Raman (SERS) detectors. The excited resonant modes strongly depend on the geometrical parameters of the particles and their mutual arrangement. The field ``hot spots'' are mostly localized at the surface between the arms (in the stars) and between the octahedra (in the assemblies) and may be both ``electric'' and ``magnetic'' in character. The nanoparticles assemblies enable hot spot ``engineering.''   

Fabrication and SERS Characterization of Plasmonically Coupled Nanoparticles With Nanometer Separation

Electron beam lithography, in conjunction with an angle evaporation technique is used to produce arrays of nanoparticles separated by 1-2nm. High resolution transmission electron microscopy (HRTEM) enables us to image these nanometer-sized gaps when fabricated on thin silicon nitride membranes. These nearly touching nanoparticles produce exceptionally high electric field intensities when irradiated with light near the plasmon frequency, yielding surface-enhanced Raman spectroscopy (SERS) signals. We deposit a para-aminothiophenol (p-ATP) dye molecule on the nanoparticle structures and spatially map the Raman intensity using confocal micro-Raman spectroscopy to quantitatively study the SERS enhancement. We find a significant increase in Raman intensity with laser polarization oriented along the axis of the nanoparticle pairs and little enhancement for the perpendicular polarization. Finite difference time domain (FDTD) simulations based on the HRTEM images predict an enhancement in the electric field intensity of 44,000 at the center of the nanoparticle gap and a corresponding electromagnetic SERS enhancement factor on the order of 10$^{9}$.   

Designer Plasmonics Nanostructures Approaching Single Molecule Raman Scattering

Since the early reports of single molecule Raman scattering detection using randomly roughened metal substrates [Phys. Rev. Lett. 78, 1667 - 1670 (1997), Science 21, 275(5303), 1102 - 1106 (1997)], there has been considerable interest in achieving single molecule Raman spectroscopy from fabricated nanostructures that are not random. Such designer plasmonic nanostructures have the advantages of improved control over the near-field enhancement magnitude, deterministic placement of the local-field hot-spots, optimized collection efficiency and greater reproducibility. Previously, we have created a metal nanostructures with measured 20 molecule Raman signal limit of detection [J. Phys. Chem. C 112 (39), 15098-15101, (2008)]. To achieve the desired near-field electric field enhancements, those nanostructures contained familiar elements to the plasmonic community: concentric focusing rings and subwavelength focusing tapers. Here, we will describe improved designs that have enabled us to improve those results by a factor of 6. We will also show polarization dependent studies that clearly demonstrate the plasmonic nature of the subwavelength focusing structures, including experimental polarization-resolved Raman spectroscopy maps. We are beginning statistical analysis experiments to determine if single molecule Raman is present in these nanostructures.   

Raman Measurement of Stress in High Aspect Ratio Double- and Single-Clamped Silicon Nanowires

Silicon nanowires (Si NWs) have been widely used in sensor applications due to their small size and the concomitant unique optical, electrical and mechanical properties. Understanding and determination of the characteristic properties therefore plays a crucial role in the sensor design. We employ non- destructive micro-Raman measurements to probe the optical and mechanical features of Si NWs with diameter of 100 nm and lengths between 10-20 $\mu$m. We observe downshifts of the Si Raman peak, as compared to bulk silicon, indicative of stress in the Si NWs. Both fixed/fixed- and fixed/free-ends Si NWs are studied to evaluate the contributions to the stress from fabrication and surface effects. In addition, enhanced scattering intensities for the nanowires are both wavelength and polarization dependent, and we attribute them to a combination of optical resonator and scattering effects. Determination of the stress in the Si NWs from Raman frequencies will be explored.   

Enhanced optical absorption and Raman scattering in carbon nanostructures

Hole-mask colloidal lithography and oxygen reactive ion etching is used to fabricate supported graphite and amorphous carbon nanostructures with well-defined diameters ranging from $\sim $100 to 350 nm and heights from $\sim $50 to 200 nm. Optical absorption/extinction spectra, as well as finite difference time domain (FDTD) calculations, reveal resonant absorption in the visible. The spectral maxima are correlated to nanostructures size and shape. While the nanostructures preserve the source material morphology, clear enhancement of the Raman scattering intensity, correlated to the resonant absorption is observed. Upon increasing the laser power, distinct peak-splits and --shifts, and increasing anti-Stokes signal intensity, suggest selective heating of the nanostructures. These correlations have been used to follow the oxidation of amorphous carbon nanostructures and to propose relevant model for investigations of soot oxidation.   

Wavelength engineered InAs quantum dots

Considerable efforts have been devoted in the wavelength tuning of InAs quantum dots for optical fiber based communication systems [1]. Experimentalists have tried to achieve 1.5$\mu $m range emissions through strain engineering or by insertion of nitrogen. In former method, covering the InAs quantum dots with an InGaAs capping layer, one can achieve considerable red shift in the emission spectra. Here, we use a full twenty band tight binding method [2] to quantitatively reproduce experimentally observed emission spectra. Detailed critical analysis of the red shift reveals new insights: QD changes its shape and In-As bimodal bond distribution is not valid. The quantitative simulation agreement with experiment without any material or geometry parameter adjustment in a general atomistic tool predicts that the era of nano Technology Computer Aided Design (nano-TCAD) is approaching.\\[4pt] [1] J. Crystal Growth (2008); J. Crystal Growth 275 pp. 415-421 (2005);J. Appl. Phys. Lett. 78, 3469 (2001); J. Crystal Growth 298 pp. 582-585 (2007)\\[0pt] [2] IEEE Trans. on Nanotechnology, Vol. 8, No. 3, May 2009, pp. 330-344   

An ``abnormal'' Raman features of phonons in nano-structures

The Raman features of nanostructures are dependent of their sizes due to the small size effect. This has been confirmed by many observations, for example, the observation on the correlation between the size of Si-, and C- nano-structure and their Raman frequencies. But we also observed strangely that Raman frequencies of single- and multiple- phonons are both independent of the size for the optical modes in polar nano-semiconductors, although all of other phonon frequencies are changed with sample size. The calculation of theoretical Raman spectra, except for exhibiting a similar result, explored that the above phonon feature is due to the long-range Coulomb (Fr\"ohlich) interaction existing in polar nano-semiconductors. We acknowledge the support from National Basic Research Program of China under grants No. 2009CB929403 and the NSF of China under grants Nos. 10774006,60876002.   

Plasmonic Enhancement of F\"orster Energy Transfer at a Metallic Nanoshell: nonlocal optical effects

The problem of F\"orster resonance energy transfer (FRET) between two molecules in the vicinity of a metallic nanoshell is studied within a phenomenological model which takes into account the nonlocal optical response of the metal. This model allows for arbitrary locations and orientations of the two molecular dipoles with respect to the nanoshell which can be of ultra-small sizes ( $<$ 10 nm) and for which nonlocal effects are of high significance. Numerical results show that the resonances in the enhanced FRET rate will be dominated by the multipolar bonding and antibonding cross-coupled plasmonic modes of the nanoshell; and that the nonlocal effects will generally lead to blue-shifted resonances, as well as diminution of the enhancement for the low-frequency portions of both modes.   

Plasmonics near a topological transition: from a hole to particle array

We investigate the optical response of nanostructures made by self-assembled sphere lithography. In particular, we study the evolution of the transmission spectra during the topological transition from an array of holes in a metallic film, to an array of disconnected, quasi-triangular metallic islands. We show that evolution of the spectra follows simple rules of an effective medium theory. The topological singularity between the two states of the system (continuous to discontinuous film transition) is shown to map specifically into the spectral response.