Garnett W. Bryant

Plasmons in nearly touching metallic nanoparticles: singular response in the limit of touching dimers.

The response of gold nanoparticle dimers is studied theoretically near and beyond the limit where the particles are touching. As the particles approach each other, a dominant dipole feature is observed that is pushed into the infrared due to interparticle coupling and that is associated with a large pileup of induced charge in the interparticle gap. The redshift becomes singular as the particle separation decreases. The response weakens for very small separation when the coupling across the interparticle gap becomes so strong that dipolar oscillations across the pair are inhibited. Lowerwavelength, higher-order modes show a similar separation dependence in nearly touching dimers. After touching, singular behavior is observed through the emergence of a new infrared absorption peak, also accompanied by huge charge pileup at the interparticle junction, if initial interparticle-contact is made at a single point. This new mode is distinctly different from the lowest mode of the separated dimer. When the junction is made by contact between flat surfaces, charge at the junction is neutralized and mode evolution is continuous through contact. The calculated singular response explains recent experiments on metallic nanoparticle dimers and is relevant in the design of nanoparticle-based sensors and plasmon circuits.

Semiconductor-Metal Nanoparticle Molecules: Hybrid Excitons and the Nonlinear Fano Effect

Modern nanotechnology opens the possibility of combining nanocrystals of various materials with very different characteristics in one superstructure. Here we study theoretically the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly coupled and demonstrate novel properties. At low incident light intensity, the exciton peak in the absorption spectrum is broadened and shifted due to incoherent and coherent interactions between metal and semiconductor nanoparticles. At high light intensity, the absorption spectrum demonstrates a surprising, strongly asymmetric shape. This shape originates from the coherent internanoparticle Coulomb interaction and can be viewed as a nonlinear Fano effect which is quite different from the usual linear Fano resonance.

Mapping the Plasmon Resonances of Metallic Nanoantennas

We study the light scattering and surface plasmon resonances of Au nanorods that are commonly used as optical nanoantennas in analogy to dipole radio antennas for chemical and biodetection field-enhanced spectroscopies and scanned-probe microscopies. With the use of the boundary element method, we calculate the nanorod near-field and far-field response to show how the nanorod shape and dimensions determine its optical response. A full mapping of the size (length and radius) dependence for Au nanorods is obtained. The dipolar plasmon resonance wavelength lambda shows a nearly linear dependence on total rod length L out to the largest lengths that we study. However, L is always substantially less than lambda/2, indicating the difference between optical nanoantennas and long-wavelength traditional lambda/2 antennas. Although it is often assumed that the plasmon wavelength scales with the nanorod aspect ratio, we find that this scaling does not apply except in the extreme limit of very small, spherical nanoparticles. The plasmon response depends critically on both the rod length and radius. Large (500 nm) differences in resonance wavelength are found for structures with different sizes but with the same aspect ratio. In addition, the plasmon resonance deduced from the near-field enhancement can be significantly red-shifted due to retardation from the resonance in far-field scattering. Large differences in near-field and far-field response, together with the breakdown of the simple scaling law must be accounted for in the choice and design of metallic lambda/2 nanoantennas. We provide a general, practical map of the resonances for use in locating the desired response for gold nanoantennas.

Optical response of strongly coupled quantum dot-metal nanoparticle systems: double peaked Fano structure and bistability.

In this communication, we study the optical response of a semiconductor quantum dot (SQD) coupled with a metal nanoparticle (MNP). In particular, we explore the relationship between the size of the constituents and the response of the system. We identify, here, three distinct regimes of behavior in the strong field limit that each exhibit novel properties. In the first regime, we find that the energy absorption spectrum displays an asymmetrical Fano shape (as previously predicted). It occurs when there is interference between the applied field and the induced field produced by the SQD at the MNP. When the coupling is increased by increasing the size of the SQD, we find a double peaked Fano structure in the response. This second peak occurs when the induced field becomes stronger than the external field. As the coupling is further increased by increasing the sizes of both the SQD and the MNP, we find a regime of bistability. This originates when the self-interaction of the SQD becomes significant. We explore these three regimes in detail and set bounds on each.

Plasmonic Control of the Shape of the Raman Spectrum of a Single Molecule in a Silver Nanoparticle Dimer

We study surface-enhanced Raman scattering (SERS) of individual organic molecules embedded in dimers of two metal nanoparticles. The good control of the dimer preparation process, based on the usage of bifunctional molecules, enables us to study quantitatively the effect of the nanoparticle size on the SERS intensity and spectrum at the single molecule level. We find that as the nanoparticle size increases the total Raman intensity increases and the lower energy Raman modes become dominant. We perform an electromagnetic calculation of the Raman enhancement and show that this behavior can be understood in terms of the overlap between the plasmonic modes of the dimer structure and the Raman spectrum. As the nanoparticle size increases, the plasmonic dipolar mode shifts to longer wavelength and thereby its overlap with the Raman spectrum changes. This suggests that the dimer structure can provide an external control of the emission properties of a single molecule. Indeed, clear and systematic differences are observed between Raman spectra of individual molecules adsorbed on small versus large particles.

Formation of quantum-dot quantum-well heteronanostructures with large lattice mismatch: ZnS/CdS/ZnS

Two-dimensional heterostructures have been exploited extensively in the synthesis of optoelectronic devices. Structures with small lattice mismatch can be synthesized readily. Large lattice mismatch in II–VI film heterostructures makes synthesis of devices with these materials more difficult. However, these large mismatch heterostructures usually have useful optical properties. One such heterostructure is the ZnS/CdS system with a large exciton binding energy and a large band gap useful for blue–green emitting devices. In this work, small II–VI nanoparticles are studied. We show that II–VI heterostructures can be made in quantum dots, despite the large bulk lattice mismatch. Two well-known techniques are combined to synthesize first very small ZnS and CdS seed nanoparticles and then do nanoepitaxy on them to produce ZnS/CdS core/shell quantum-dot quantum-well heteronanostructures. These structures are characterized by UV visible absorbance. Measured spectra are compared with electronic level structures ca...

Size-dependent electronic level structure of InAs nanocrystal quantum dots: Test of multiband effective mass theory

The size dependence of the electronic spectrum of InAs nanocrystals ranging in radius from 10–35 A has been studied by size-selective spectroscopy. An eight-band effective mass theory of the quantum size levels has been developed which describes the observed absorption level structure and transition intensities very well down to smallest crystal size using bulk band parameters. This model generalizes the six-band model which works well in CdSe nanocrystals and should adequately describe most direct semiconductor nanocrystals with band edge at the Γ-point of the Brillouin zone.

Exciton–Plasmon Interactions in Quantum Dot–Gold Nanoparticle Structures

We present a self-assembly method to construct CdSe/ZnS quantum dot-gold nanoparticle complexes. This method allows us to form complexes with relatively good control of the composition and structure that can be used for detailed study of the exciton-plasmon interactions. We determine the contribution of the polarization-dependent near-field enhancement, which may enhance the absorption by nearly two orders of magnitude and that of the exciton coupling to plasmon modes, which modifies the exciton decay rate.

First-principles calculations of structural, electronic, vibrational, and magnetic properties of C-60 and C48N12: A comparative study

The structural, electronic, vibrational, and magnetic properties of the C48N12 azafullerene and C60 are comparatively studied from the first-principles calculations. Full geometrical optimization and Mulliken charge analysis are performed. Electronic structure calculations of C48N12 show that the highest occupied molecular orbital (HOMO) is a doubly degenerate level of ag symmetry and the lowest unoccupied molecular orbital (LUMO) is a nondegenerate level of au symmetry. The calculated binding energy per atom and HOMO-LUMO energy gap of C48N12 are about 1 eV smaller than those of C60. Because of electron correlations, the HOMO-LUMO gap decreases about 5 eV and the binding energy per atom increases about 2 eV. The average second-order hyperpolarizability of C48N12 is about 55% larger than that of C60. Our vibrational frequency analysis predicts that C48N12 has 58 infrared-active and 58 Raman-active vibrational modes. Two different methods for calculating nuclear magnetic shielding tensors of C60 and C48N12...

Observation of optical Shockley-like surface states in photonic superlattices

We provide what we believe to be the first experimental demonstration of linear Shockley-like surface states in an optically induced semi-infinite photonic superlattice. Such surface states appear only when the induced superlattice consisting of alternating strong and weak bonds is terminated properly at the surface. Our experimental results are in good agreement with our theoretical analysis.