Session B22: Nano Particles, Wires, and Cavities

Optical properties and circular dichroism of chiral metal nanoparticles

In nature, biological systems are built up by homochiral building blocks, such as a sugar and protein. Circular dichroism (CD) is an effective tool of resolving molecular conformations. It utilizes circularly polarized light to detect differential absorption of chiral materials. In medicine, it will help us to develop new drugs and therapies, if we understand the connection between the physical or chemical properties of drug molecules and their conformations. With the rapid development of nanotechnologies, chiral nanomaterials attract lots of attention nowadays. CD signals of chiral molecules can be enhanced or shifted to the visible band in the presence of plasmonic nanocrystals. Here we present a plasmonic CD mechanism from a single chiral metal nanocrystal[1]. The mechanism is essentially different from the dipolar plasmon-plasmon interaction in a chiral NP assembly[2], which mimics the CD mechanism of chiral molecules. Chiral metal nanocrystals are expected to have promising applications in biosensing. Recently a few experimental papers reported successful realizations of chiral nanocrystals in a macroscopic ensemble in solution. Particularly the paper[3] described silver nanoparticles grown on chiral template molecules and demonstrating characteristic CD signals at a plasmonic wavelength. The plasmonic CD signals in Ref.[3] can come from a dipolar plasmon-molecule interaction or from a chiral shape of nanocrystals. [1] Z.Fan, et al. Nano Lett.,12, 3283 (2012). [2] A. Kuzyk, et al., Nature 483, 311 (2012). [3] B.Maoz, et al. J. Am. Chem. Soc.134, 17807 (2012).   

Transport measurements across single nanoparticles

During this last decade, numerous progresses have been obtained in the chemical synthesis of nanoparticle. Various materials (oxides, chalcogenides) known for their peculiar electronic or magnetic properties -- superconductivity, Mott localization, topological protection -- can now be obtained as nanoparticles through chemical synthesis. These new nano-materials are offering a unique opportunity to study the effect of quantum confinement on unconventional electronic orders. To improve the preparation of samples with single nanoparticles trapped within a nanogap, we developed a new method where nanoparticles are projected in-vacuum on chip circuits covered by nanogap spaced electrodes. Continuous current measurements during the projection allow identifying the trapping of a single nanoparticle within the nanogap. We apply the method for trapping single gold nanoparticles, which led to the observation of Coulomb blockade. We also applied the method to magnetite (Fe3O4) nanoparticles, which allows to study the electric field induced insulator to metal transition in only a few nanoparticles.   

Characterization of TbAs nanoparticles embedded in GaAs using pump-probe measurements of carrier relaxation dynamics

Rare-earth-monopnictide nanoparticles epitaxially deposited within III-V semiconductors have been shown to improve the performance of devices for applications ranging from thermoelectrics to THz pulse generation. However, the electronic structure of small (approximately 1.5 nm diameter) TbAs nanoparticles remains poorly understood. We use ultrafast pump-probe spectroscopy to investigate the electronic structure of the TbAs nanoparticles. The samples studied were grown by co-deposition of Tb, Ga, and As on a GaAs substrate, resulting in TbAs nanoparticles embedded within a GaAs host. We study the dynamics of carrier relaxation into the TbAs states, which essentially act as traps, using both optical-pump terahertz-probe and optical-pump optical-probe techniques. By analyzing how the carrier relaxation rates depend on both pump fluence and sample temperature we conclude that the TbAs states are saturable, which suggests the existence of a bandgap for TbAs nanoparticles.   

Optical and electronic properties of self-assembled nanoparticle-ligand metasurfaces

The optical and electronic properties of inorganic nanoparticles organized into two-dimensional lattices sensitively depend on the properties of the organic ligand shell coating the nanoparticles. We study the optical and electronic properties of these two-dimensional metasurfaces consisting of gold nanoparticles functionalized with ligands and self-assembled into macroscopic monolayers on non-templated substrates. Using these metasurfaces we demonstrate an average surface-enhanced Raman scattering (SERS) enhancement factor on the order of 10$^{\mathrm{8}}$ for benzenethiol ligands and study the mechanisms that influence the enhancement. These metasurfaces may provide a platform for the development of low-power, low-cost next-generation chem/bio-sensors and new insights into the organic-inorganic interface at the nanoscale.   

Two photon excitation fluorescence from Ag nanotriangles and nanohexagons

We report on measurements of two photon excitation fluorescence (TPEF) from arrays of silver nanotriangles and nanohexagons fabricated by nanosphere lithography. The silver nanoparticles exhibit localized surface plasmon resonances (LSPRs) that depend on the size, shape and aspect ratio of the particles. When the particles are excited by femtosecond pulsed laser light resonant with the LSPRs, they emit TPEF with significantly higher intensity than when excited off resonance. Moreover, if the light intensity is turned up sufficiently to cause some of the particles to melt into spherical particles, we observed an increase in the TPEF from the spheres by as much as an order of magnitude, even though their LSPRs are no longer resonant with the laser. Finally, we note that the silver particles also generate light at the second harmonic of the laser frequency, although the efficiency of this process depends strongly on the dielectric environment of the silver particles, which is not the case for the TPEF.   

Investigation of the electronic transport in polarization-induced nanowires using conductive atomic force microscopy (AFM)

In the search to improve short wavelength light emitting diodes (LED's), where the dislocations limit their performance and hole doping (Mg) is a fundamental challenge, the III-Nitride polarization-induced nanowire LED provides a promising system to address these problems. The new type of pn diode, polarization-induced nanowire LED (PINLED), was developed by linearly grading AlGaN composition of the nanowires (from GaN to AlN and back to GaN) from 0{\%} to 100{\%} and back to 0{\%} Al (Carnevale et al, \textit{Nano Lett.}, \textbf{12}, 915 (2012)). In III-Nitrides (Ga,Al/N), the effects of polarization are commonly observed at the surfaces and interfaces. Thus, in the case of the polarization-induced nanowire LEDs, taking advantage of the bound polarization charge, due to the grading of the AlGaN, the pn diodes are formed. The polarity of the nanowires determines the carrier type in each graded region, and therefore the diode orientation (n/p vs p/n). We used conductive AFM to investigate polarity of the PINLED's as well as hole conductivity in PINLED's made of AlGaN with and without acceptor doping. The results reveal that most of the wires are n-top/p-bottom (N-face), but some are p-top/n-bottom (Ga-face). Also, we found that the current density is 3 orders of magnitude larger in the case of the doped nanowires than the nanowires with no impurity doping.   

Critical Role of Modal Spatial Overlap in Nanoscale Nonlinear Optics

We unambiguously demonstrate the critical role of modal spatial overlap in \textit{nonlinear} optics for nanoscale structures. Our experimental and theoretical investigations show that, within a sub-wavelength metallic hole, spatial overlap between the linear and nonlinear modes strongly correlates to the conversion efficiency. Our results provide an accurate explanation for the long-emphasized but elusive shape effect. Moreover, our investigation stimulates new angles for and deeper insights into general nonlinear optics at nanoscale.   

Nanocluster effects on magneto-resistance and optical second-harmonic generation in Au-Co composite films

Magnetic nanomaterials typically exhibit significant differences in their magnetic and magnetic-optical properties compared to bulk. A viable nanoscale platform to investigate the magnetic and magneto-optical properties of magnetic nanomaterials is in composite thin films to have magnetic clusters embedded on a different matrix material which size can be tailored. The Au-Co binary system is a typical phase-separation system in bulk phase diagram . The nanocomposite geometry allows tailoring the actual composition and microstructure of the composite by exploiting different temperature during deposition. In our previous studies, Au/Co/Au trilayers as well as Au-Co nanocomposite thin films exhibit strong enhancement of the magneto-optical activities due to surface plasmon polariton excitation in the noble metal. In this study, we investigate other non-linear optical properties such as second harmonic generation (SHG) in Au-Co nanocomposite thin films and understand its correlation with the magneto-transport properties of the composite. Optical SHG is a sensitive probe of surface and buried interfaces due to inversion symmetry breaking at the interfaces of centrosymmetric materials which allows probing of the structural and morphological properties near interfaces.   

Far-infrared transmission through periodic arrays of cross-shaped holes

The far-infrared transmission of light incident on a free-standing metal film perforated with periodic cross-shaped holes is investigated. These metal-mesh filters show enhanced ``extraordinary'' infrared transmission at particular wavelengths. A number of filter samples having different periodicities and geometries have been measured over frequencies from 20-650 cm$^{-1}$/0.6-19.5 THz. The results will be compared with calculations from surface plasmon polariton (SPP) theory. It is shown that for certain periodicity and geometry, the SPP mode and the localized surface plasmon (LSP) mode may have their resonance peaks nearly superimposed on each other. The bandwidth of this transmission peak is related to the ratio of the width and length of the cross-shaped holes. The correlation between transmission properties and the incident angle of the far-infrared light has also been measured for both polarization conditions. As the incident angle is increased, the transmission peak shows a blue shift when illuminated by s-polarized light, while for p-polarized light it splits into two parts which shift in opposite directions.   

Time-resolved nonlinear dynamics of quantum dots coupled to a photonic crystal cavity in the Purcell regime

Recently, there has been great interest in studying the optical nonlinearities of light confined in a solid-state nano-cavity interacting with a quantum emitter for on-chip applications. The nonlinearity in the strong coupling regime has enabled ultrafast all-optical switching at low incident power using exciton-photon coupled systems. In this report, we show that nonlinear optical properties can also be observed in the Purcell regime using a cavity with a moderate quality factor (Q), which arises from the saturation of a single quantum dot and describes the time-resolved dynamics of two transitions (exciton and biexciton) exhibiting different nonlinearities. In order to conduct these investigations, we used the luminescence intensity autocorrelation method and measured the variation of nonlinear emission dynamics while varying the incident power over nearly three orders of magnitude and found excellent agreement with a numerical simulation. We expect the method and the theoretical model will be applicable for understanding other nonlinear effects such as lasing and cavity-QED.   

Entangled photons from the polariton vacuum in a switchable optical cavity

We study theoretically the entanglement of two-photon states in the ground state of the intersubband (ISB) cavity system, called polariton vacuum. The system is formed by a sequence of quantum wells (QWs) located inside a microcavity and the interaction of cavity photons with ISB excitations inside the QWs leads to the formation of polariton states. In the ultrastrong coupling regime, the polariton vacuum already contains a finite number of photons, of which pairs with opposite in-plane wave vectors are correlated. In an explicit solution for the polariton vacuum, we only consider certain two-photon states by post-selection and analyze them for mode entanglement, i.e.~in the momentum degree of freedom. We find an analytical expression for the entanglement using the concurrence [1], which depends on the absolute values of the in-plane wave vectors of the photons. In the limit of large cavities and for photon energies around the ISB resonance in the mid infrared regime, the photons are almost maximally entangled, which is fundamentally important for their possible use in quantum information processing. Furthermore, there exists a continuous set of mode pairs, for which the photons are maximally entangled.\\[4pt] [1] A. Auer and G. Burkard, Phys. Rev. B \textbf{85}, 235140 (2012).   

Coherent flow and Bose-Einstein Condensation of Long Lifetime Polaritons

Exciton-polaritons with lifetimes of the order of 100ps are created in semiconductor microcavity of extremely high quality factor (? 106). Due to this long lifetime and very few defects in the sample, the polaritons can travel ballistically over macroscopic distances up to millimeter. The properties of the system changes dramatically with the particle density. At moderate density, the polaritons behave like a superfluid, maintaining phase coherence after propagating over hundreds of microns. This indicates the existence of long range spatial coherence in the system. As the density increases above a threshold value, the polaritons condense into the lowest-energy state of the effective trap produced by the repulsive interaction between the polaritons and excitons within the excitation region and the cavity gradient across the sample. The coherence time of this polariton BEC is measured to be at least 280ps. By creating a exciton barrier at the center of a stress trap, we are able to obtain a ring shape polariton BEC which provides the opportunity for studying the constant flow of a superfluid in the polariton system.