Designable Luminescence with Quantum Dot–Silver Plasmon Coupler

We explore a strongly interacting QDs/Ag plasmonic coupling structure that enables multiple approaches to manipulate light emission from QDs. Group II–VI semiconductor QDs with unique surface states (SSs) impressively modify the plasmonic character of the contiguous Ag nanostructures whereby the localized plasmons (LPs) in the Ag nanostructures can effectively extract the non‐radiative SSs of the QDs to radiatively emit via SS–LP resonance. The SS–LP coupling is demonstrated to be readily tunable through surface‐state engineering both during QD synthesis and in the post‐synthesis stage. The combination of surface‐state engineering and band‐tailoring engineering allows us to precisely control the luminescence color of the QDs and enables the realization of white‐light emission with single‐size QDs. Being a versatile metal, the Ag in our optical device functions in multiple ways: as a support for the LPs, for optical reflection, and for electrical conduction. Two application examples of the QDs/Ag plasmon coupler for optical devices are given, an Ag microcavity + plasmon‐coupling structure and a new QD light‐emitting diode. The new QDs/Ag plasmon coupler opens exciting possibilities in developing novel light sources and biomarker detectors.     

Designed biointerface using near-infrared quantum dots for ultrasensitive surface plasmon resonance imaging biosensors

The surface plasmon resonance imaging chip biointerface is fully designed using near-infrared (NIR) quantum dots (QDs) for the enhancement of surface plasmon resonance imaging (SPRi) signals in order to extend their application for medical diagnostics. The measured SPRi detection signal following the QD binding to the surface was amplified 25-fold for a 1 nM concentration of single-stranded DNA (ssDNA) and 50-fold for a 1 μg/mL concentration of prostate-specific antigen (PSA), a cancer biomarker, thus substantiating their wide potential to study interactions of a diverse set of small biomolecules. This significant enhancement is attributed to the QD's mass-loading effect and spontaneous emission coupling with propagating surface plasmons, which allowed the SPRi limit of detection to be reduced to 100 fM and 100 pg/mL for ssDNA and PSA, respectively. Furthermore, this study illustrates the potential of SPRi to be easily integrated with fluorescent imaging for advanced correlative surface-interaction analysis.     

Surface plasmon mediated interference phenomena in low-q silver nanowire cavities

Optical excitation of surface plasmons in wet-chemically grown monocrystalline silver nanowires ( approximately 100 nm diameter and up to a few tens of micrometers length) is studied by broadband imaging spectroscopy. Surface plasmons excited by an incident light beam in the so-called Kretschmann-Raether configuration give optical interference phenomena in the spectral domain. These spectral oscillations are interpreted in terms of Fabry-Perot cavity modes for surface plasmons in silver nanowires and allow for a direct experimental determination of the surface plasmon group velocity and cavity losses.     

Surface plasmon enhanced energy transfer between donor and acceptor CdTe nanocrystal quantum dot monolayers

Surface plasmon enhanced Fo?rster resonant energy transfer (FRET) between CdTe nanocrystal quantum dots (QDs) has been observed in a multilayer acceptor QD-gold nanoparticle-donor QD sandwich structure. Compared to a donor-acceptor QD bilayer structure without gold nanoparticles, the FRET rate is enhanced by a factor of 80 and the Fo?rster radius increases by 103%. Furthermore, a strong impact of the donor QD properties on the surface plasmon mediated FRET is reported.     

Detecting single quantum dot motion with nanometer resolution for applications in cell biology

Quantum dots (QDs), semiconductor particles of nanometer dimension, have emerged as excellent fluorescent analogs in tracer experiments with single molecule sensitivity for bioassays. Cell imaging greatly benefits from the remarkable optical and physical properties of these inorganic nanocrystals: QDs are much brighter and exhibit a higher resistance to photobleaching than traditional fluorophores, and their narrow emission spectrum and flexible surface chemistry make them particularly suitable for multiplex imaging. Here, we have demonstrated the achievement of a nanometer spatial resolution on the position of a single QD in a simple optomechanical instrument using a high-sensitivity low-noise detector, an intensified CCD camera. Furthermore, nanometer variations in the amplitude of a QD's sinusoidal oscillations could be quantitatively distinguished after fast Fourier transform (FFT) based data processing. As confirmed by experiments where QDs were attached to the surface of bovine aortic endothelial cells, this method can be exploited in biology to assess molecular and subcellular contributions to responses such as motility, intracellular trafficking, and mechanotransduction, with high resolution and minimal disturbance to cells     

Plasmonic coupled-cavity system for enhancement of surface plasmon localization in plasmonic detectors

A plasmonic coupled-cavity system, which consists of a quarter-wave coupler cavity, a resonant Fabry-Pérot detector nanocavity, and an off-resonant reflector cavity, is used to enhance the localization of surface plasmons in a plasmonic detector. The coupler cavity is designed based on transmission line theory and wavelength scaling rules in the optical regime, while the reflector cavity is derived from off-resonant resonator structures to attenuate transmission of plasmonic waves. We observed strong coupling of the cavities in simulation results, with an 86% improvement of surface plasmon localization achieved. The plasmonic coupled-cavity system may find useful applications in areas of nanoscale photodetectors, sensors, and an assortment of plasmonic-circuit devices.     

Propagation and diffraction of locally excited surface plasmons.

With the use of optical near-field techniques, it is now possible to excite or observe surface plasmons with high lateral resolution. A theoretical study is presented of surface plasmon excitation by near-field optical probes and the influence of well-defined structures on surface plasmon propagation and surface plasmon detection in the far field. The generation and the diffraction of the surface plasmon is calculated by using a theoretical scheme founded upon a first-order perturbation expansion of the Rayleigh-Fano method. A very good agreement is obtained between numerical and experimental results. The theoretical tools used should prove a useful guideline for future experiments of nanooptics with surface plasmons.     

Site-controlled self-organization of InAs quantum dots

In situ site-control techniques for self-organized InAs quantum dots (QDs) have been developed using an electron beam (EB) and a scanning tunneling microscope (STM) probe combined with molecular beam epitaxy. In the in situ EB-assisted process, InAs dots are preferentially formed in shallow, sub-μm-size GaAs holes with the InAs supply. We find that the specific slope of a hole acts as a favorable site for dot formation. In the in situ STM probe-assisted process, the size and pitch of the holes are considerably reduced into nanoscale. InAs QDs are then self-organized only at the hole sites due to strain-induced selective nucleation. Using this process, two- and three-dimensional QD arrays are fabricated with nanometer precision.     

Superlocalization spectral imaging microscopy of a multicolor quantum dot complex

The key factor of realizing super-resolution optical microscopy at the single-molecule level is to separately position two adjacent molecules. An opportunity to independently localize target molecules is provided by the intermittency (blinking) in fluorescence of a quantum dot (QD) under the condition that the blinking of each emitter can be recorded and identified. Herein we develop a spectral imaging based color nanoscopy which is capable of determining which QD is blinking in the multicolor QD complex through tracking the first-order spectrum, and thus, the distance at tens of nanometers between two QDs is measured. Three complementary oligonucleotides with lengths of 15, 30, and 45 bp are constructed as calibration rulers. QD585 and QD655 are each linked at one end. The measured average distances are in good agreement with the calculated lengths with a precision of 6 nm, and the intracellular dual-color QDs within a diffraction-limited spot are distinguished.     

Cryogenic STM/STS observation on oxide superconductors

The topographic and electronic properties of the surfaces of (001) and (110) oriented YBa₂Cu₃O_y, epitaxial films have been probed by atomic resolution STM/STS at 4.2 K. The STM image on the (001) surface clearly revealed the atomic corrugation of the tetragonal lattice with an average spacing of 0.4 nm. while on the (110) surface the orthorhombic atomic lattice, corresponding to the Cu atoms of both CuO₂ and CuO chain planes, was observed. The STS result on the (001) surface indicated the semiconducting nature of the terminating layer. As the tunneling tip came closer to the surface, however, the shape of the tunneling spectrum became more metallic and showed a superconducting energy gap, which seems to arise from the underlying superconducting layer. On the other hand, the tunneling spectra on the (110) surface indicated superconducting gap structures, independent of the tip-sample distance.     

Aspects of emission variation in CdSeTe/ZnS quantum dots conjugated to antibodies

CdSeTe/ZnS quantum dots (QDs) with the emission peak at 705 nm have been studied comparatively in the non-conjugated state and after bioconjugation to anti-pseudo rabies virus antibodies (ABs) by means of photoluminescence (PL) and Raman scattering methods. It is revealed that PL spectra of QDs vary significantly after conjugating to ABs. In PL spectra of non-conjugated QDs only one PL band of Gaussian shape peaked at 1.76–1.78 eV and related to exciton emission in the CdSeTe core has been detected. The PL spectra of bio-conjugated QDs demonstrate the high energy spectral shift and asymmetric shape of PL bands. The study of Raman scattering spectra permits to estimate the CdSeTe alloy composition and to detect the surface enhanced Raman scattering (SERS) effect for bioconjugated QDs. The last fact testifies on the interaction of excitation light electromagnetic field with the electric dipoles excited in ABs. The optical band gap in CdSeTe core has been calculated numerically versus core radius on the base of the effective mass approximation model. Then the energy band diagrams for non-conjugated and bio-conjugated states of CdSeTe/ZnS QDs have been designed. It is revealed the type II quantum well in CdSeTe core that explains the optical transition at 705 nm in the wide band gap CdSeTe alloy. The analysis has shown that AB dipoles excited in bio-conjugated QDs stimulate changing the profile of QD energy band diagram that manifests itself in the mentioned PL spectrum transformations. Actually, the study of PL spectrum varying in CdSeTe/ZnS QDs conjugated to specific antibodies can be an informative tool in biology and medicine for early medical diagnostics.     

Tailoring of morphology and emission wavelength of AlGaInAs quantum dots

We present a study of the growth, morphology and optical properties of Al(x)Ga(1-x-y)In(y)As quantum dots (QDs) for a wide range of Al and In concentrations (0≤x≤0.34 and 0.43≤y≤0.60). Short emission wavelengths between 660 and 940?nm and QD surface densities up to 1.1 × 10(11)?cm(-2) have been achieved. Our results show that by varying both the Al concentration and the In concentration an independent adjustment of strain and QD band gap is possible. This additional degree of freedom can be employed for tailoring AlGaInAs QDs with the desired emission wavelength, surface density and average size. AlGaInAs QDs thus offer new possibilities for future QD device design.     

Tunable subradiant lattice plasmons by out-of-plane dipolar interactions

Plasmonic nanostructures concentrate optical fields into nanoscale volumes, which is useful for plasmonic nanolasers, surface enhanced Raman spectroscopy and white-light generation. However, the short lifetimes of the emissive plasmons correspond to a rapid depletion of the plasmon energy, preventing further enhancement of local optical fields. Dark (subradiant) plasmons have longer lifetimes, but their resonant wavelengths cannot be tuned over a broad wavelength range without changing the overall geometry of the nanostructures. Also, fabrication of the nanostructures cannot be readily scaled because their complex shapes have subwavelength dimensions. Here, we report a new type of subradiant plasmon with a narrow (～5 nm) resonant linewidth that can be easily tuned by changing the height of large (>100 nm) gold nanoparticles arranged in a two-dimensional array. At resonance, strong coupling between out-of-plane nanoparticle dipolar moments suppresses radiative decay, trapping light in the plane of the array and strongly localizing optical fields on each nanoparticle. This new mechanism can open up applications for subradiant plasmons because height-controlled nanoparticle arrays can be manufactured over wafer-scale areas on a variety of substrates.     

Surface Plasmon Resonance Magneto-Optical Kerr Effect in Au/Co/Au Magneto-Plasmonic Multilayer

Surface plasmon resonance magneto-optical Kerr effect is studied in magneto-plasmonic multilayer as Au (11 nm)/Co (11 nm)/Au (11 nm). Our experimental setup is consists of spectral magneto-optical rotation in Kretschmann-based attenuated total reflection condition as surface plasmon resonance magneto-optical Kerr effect. Based on this new experimental setup, the sample exposed under external magnetic filed at surface plasmon resonance angle. Our results show sufficient surface plasmon resonance magneto-optical Kerr effect in visible region, thanks to the resonant excitation of surface plasmons which is very suitable for miniaturized and controllable magneto-optical imaging systems, memory, and also magneto-optical isolators.     

Concentration detection of quantum dots in the visible and near-infrared range based on surface plasmon resonance sensor

We employ an optical sensor based on surface plasmon resonance (SPR) operating in the near-infrared and in the visible range to determine the concentration of CdSe/ZnS core-shell quantum dots (QDs) which are embedded in the SU8 organic films. Attenuated total reflection (ATR) measurements show that the amplitude of the shift of the resonance dip is closely related to the concentration variation of QDs in the organic films and the incident laser. The sensitivity is enhanced by 1.5-time and the detect limitation is expanded to 10⁻⁵ μmol/L in the visible range as compared to that in the near-infrared. The sensitivity enhancement and the expansion of detect limitation of the visible SPR sensor may originate from the coupling of surface plasmons to luminescence from QDs.     

Electron emissions in InAs quantum dots containing a nitrogen incorporation induced defect state: the influence of thermal annealing

With the incorporation of nitrogen (N) into InAs quantum dots (QDs), the carrier distribution near the QD displays electron emissions from a localized N-induced defect state at 0.34?eV and a weak emission at 0.15?eV from the QD. This defect state causes drastic carrier depletion in the neighboring GaAs bottom layer near the QD, which can effectively suppress tunneling emission for the QD excited states. As a result, electrons escape from the QD ground state through thermal emission to near the GaAs conduction band, rather than through thermal emission to the QD first excited state and a subsequent tunneling to the GaAs conduction band, as observed in InAs QDs without N incorporation. Thermal annealing can weaken the defect emission and enhance the QD emission, suggesting a removal of the defect state and a recovery of carriers in the QD. Increasing annealing temperature can significantly decrease the emission time and energy of the QD emission, which is explained by a weakening of tunneling suppression due to the removal of the defect state.     

Polymer grafting from CdS quantum dots via AGET ATRP in miniemulsion

We report the synthesis of CdS quantum dot (QD)-poly(acrylate) nanocomposites using a recently developed catalytic system where activators are generated by electron transfer for atom-transfer radical polymerization (ATRP) in a miniemulsion. The QD surface was functionalized with a tris(alkyl)phosphine, previously modified with an ATRP chlorine initiator, and subsequent controlled polymerization was carried out from the functionalized surface of nanoparticles. The final material showed a high homogeneity and the QDs were evenly dispersed. The optical-absorption edge in the visible spectra of the nanocomposites attests the presence of the CdS QDs. Quantum confinement effects were assigned, though a blue shift in relation to the optical spectrum of the initial QDs has been observed.     

Optimization of Magneto-Optical Kerr Effect in Cu/Fe/Cu Nano-structure

We use an analytical approach to describe the optical response of a magnetoplasmonic structure upon surface plasmon polariton (SPP) excitation in glass/Cu/Fe/Cu multilayer. The proposed structure is based on Kretschmann prism couplers, Fe layer as magneto-optical (MO) medium, and Cu layer as plasmonic metal layer, to enhance the MO effects thanks to the resonant excitation of surface plasmons. The influence of constituent layer thicknesses and layer order is investigated to obtain the maximum MO Kerr signal and figure of merit (FOM) in polar geometry. In addition, we show a range of parameters to determine the maximum Kerr rotation accompanied by minimum Kerr ellipticity which is desirable for data storage applications. Results demonstrate the important role of film thicknesses and incidence angle distribution on resonant excitation of surface plasmons.     

Epitaxial self-alignment: A new route to hybrid active plasmonic nanostructures

Concepts of lateral ordering of epitaxial semiconductor quantum dots (QDs) are for the first time transferred to hybrid nanostructures for active plasmonics. We review our recent research on the self-alignment of epitaxial nanocrystals of In and Ag on ordered one-dimensional In(Ga)As QD arrays and isolated QDs by molecular beam epitaxy. By changing the growth conditions the size and density of the metal nanocrystals are easily controlled and the surface plasmon resonance wavelength is tuned over a wide range in order to match the emission wavelength of the QDs. Photoluminescence measurements reveal large enhancement of the emitted light intensity due to plasmon enhanced emission and absorption down to the single QD level.     

Surface plasmon interference nanolithography

A new nanophotolithography technique based on the interference of surface plasmon waves is proposed and demonstrated by using computer simulations. The wavelengths of the surface plasmon waves at metal and dielectric interfaces can reach the nanometer scale while their frequencies remain in the optical range. As a result, the resolution of this surface plasmon interference nanolithography (SPIN) can go far beyond the free-space diffraction limit of the light. Simulation results show that one-dimensional and two-dimensional periodical structures of 40-100 nm features can be patterned using interfering surface plasmons launched by 1D gratings. Detailed characteristics of SPIN such as field distribution and contrast are also investigated.