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Semiconductor quantum dots (QDs) are widely used in photovoltaic and optoelectronic devices due to their unique optical properties. Photoluminescence (PL) properties of QDs can be significantly improved by their electromagnetic coupling with plasmonic nanoparticles (PNPs). The excitation of resonant localized plasmon modes leads to the enhancement of the density of photon states and increase of electromagnetic field near the surface of PNPs, what boosts the acceleration of the exciton radiative decay, known as the Purcell effect. To study the dependence of the degree of acceleration of radiative decay rate (Purcell factor) on the distance between QDs and PNPs, we fabricated thin-film hybrid structures based on CdSe(core)/ZnS/CdS/ZnS(multishell) QDs and silver or gold PNPs with a controllable distance between these components. The change in the radiative decay rate of excitons was calculated from the PL intensities and lifetimes before and after the deposition of PNPs on top of the QD thin film covered by a poly(methyl methacrylate) (PMMA) spacer. For both PNP types, the PL lifetime of underlying QDs decreased, whereas the PL intensity of the latter decreased only slightly for gold PNPs and even increased for silver PNPs. This indicates the acceleration of QDs radiative decay (Purcell effect) mediated by exciton-plasmon interaction. The Purcell factor was higher for silver PNPs than that for gold PNPs, what can be explained by the better spectral overlap between the QDs PL band and silver PNPs absorbance and the absence of interband absorption in silver at the wavelength of QDs PL. The results of this study provide better understanding of the Purcell effects in hybrid materials based on QDs and PNPs.

Strong coupling of plasmons and excitons can form hybrid states, the so called “plexcitons”. Especially with molecular J-aggregated dye molecules strong interaction becomes observable even under ambient conditions. In our work we investigate the nature of plexcitonic states formed in gold nanorods coated with a cyanine dye by transient absorption spectroscopy. We demonstrate that plexcitons show tunable plasmonic and excitonic non-linear optical behaviour. Our experimental data can be explained on the basis of a simple Lorentz oscillator model. We find that both the quality factor and the coupling strength between plasmons and excitons can be optically manipulated on an ultrashort time scale. T. Simon, D. Melnikau, A. Sanchez-Iglesias, M. Grzelczak, L. Liz-Marzan, Y. Rakovich, J. Feldmann and A. Urban, Exploring the optical non-linearities of plasmon-exciton hybrid resonances in coupled colloidal nanostructures, submitted (2016)

We have studied the optical properties of a hybrid system consisting of cyanine dye J-aggregates (both PIC and TDBC) attached to a spherical microcavity. Instead of the commonly accepted chemical bonding of dye molecules to the surface of microspheres or deposition of dye-doped sol-gel film, in our experiments microspheres were coated with J-aggregate shell utilizing the layer-by-layer assembly of the ultrathin films. In this approach we aimed to take advantage of light confinement in the Whispering Gallery Modes (WGMs) microcavity by placing the emitter (shell of J-aggregates) just at the rim of the microsphere, where the resonant electromagnetic field reaches its maximum. A periodic structure of narrow peaks was observed in the photoluminescence spectrum of the J-aggregates, arising from the coupling between the emission of J-aggregates and the WGMs of the microcavity. The most striking result of our study is the observation of polarization sensitive mode damping caused by re-absorption of J-aggregate emission. This effect manifests itself in dominating emission from the transverse magnetic modes in the spectral region of J-aggregates absorption band where the transverse electric (TE) modes are strongly suppressed. Strong suppression of TE modes reflects preferential tangential orientation of transition dipole moment of J-aggregates in deposited microcavity shell. Observed polarization sensitive mode damping observed in the spectral region of high J-aggregate absorption can be used for suppression of unwanted modes in high Q optical resonators. We also demonstrate that the emission intensity can be further enhanced by depositing a hybrid layer of J-aggregates and Ag nanoparticles onto the spherical microcavity. Owing to the concerted action of WGMs and plasmonic hot spots in the Ag aggregates, we observe strongly enhanced Raman signal from the Jaggregates. Microcavities covered by J-aggregates and plasmonic nanoparticles could be thus useful for a variety of photonic applications in basic science and technology.

Highly luminescent semiconductor nanocrystals or quantum dots (QDs) possess a number of interesting and important properties that are tunable thanks to their size-dependent discrete electronic spectra. In this work we studied the optical properties of a novel type of hybrid structures that combine CdTe QDs with organic dye molecules (Pseudocyanine iodide) in a J-aggregate state. Due to the excitonic nature of electronic excitations, J-aggregates have the narrowest absorption and luminescence bands among organic materials, large oscillator strengths and giant third-order nonlinear susceptibility. In developed structures optical energy harvested by the quantum dots as artificial antennas then transferred to J-aggregates to enhance the photostability and efficiency of the carriers recombination. To fabricate CdTe/J-aggregates hybrid nanostructures we have used an approach based on electrostatic interaction between the positively charged dye and CdTe QDs capped with thioglycolic acid and, thus, carrying a negative charge. In order to develop an efficient hybrid material operating in the FRET regime, we carefully selected the PL colors (diameters) of the QD to be optically coupled with absorption of J-aggregates. We took advantage of extremely thin ligand shell (~0.5 nm) of CdTe QDs, which insures high efficiency of energy transfer. Formed QD/J-aggregate FRET system shows the broadband absorption in the visible and the ultraviolet part of the spectrum typical of QDs, along with the narrow emission linewidths characteristic of J-band emitters (~15 nm full width at half-maximum). We use absorption and photoluminescence spectroscopy and photoluminescence lifetime studies to conclude that efficiency of energy transfer is 95%. Also we report on development of active whispering-gallery microcavities integrated with with hybrid QDs/J-aggregate shell. Results of micro-PL spectroscopy and PL lifetime imaging confirm strong quenching of QDs emission and multifold shortening of their photoluminescence lifetime, which is consistent with highly efficient FRET in hybrid organic/inorganic semiconductor nanostructures coupled to microcavity modes.

Conical refraction produces the well-known ring profile when circularly polarised light is incident on a biaxial crystal. Conical diffraction of linearly polarised light in a biaxial crystal produces a beam with a crescent-shaped intensity profile. Rotation of the plane of polarisation of the incident light produces the unique effect of spatially moving the crescent-shaped beam around a ring. We use this effect to trap microspheres and white blood cells and to position them at any angular position on the ring. Continuous motion around the circle is also demonstrated by rotating the input linear polarisation. The ability to spatially locate a beam and an associated trapped object simply by varying the polarisation of light suggests that this optical process should find application in the manipulation and actuation of micro- and nano-scale physical and biological objects.

Optical techniques towards the realisation of sensitive and selective biosensing platforms have received a considerable amount of attention in recent times. Techniques based on interferometry, surface plasmon resonance, field-effect transistors and waveguides have all proved popular, and in particular, spectroscopy offers a large range of options. Raman spectroscopy has always been viewed as an information rich technique in which the vibrational frequencies reveal a lot about the structure of a compound. The issue with Raman spectroscopy has traditionally been that its rather low cross section leads to poor limits-of-detection. In response to this problem, Surface-enhanced Raman Scattering (SERS), which increases sensitivity by bringing the sample in contact with many types of enhanceing substrates, has been developed. Here we discuss a facile and rapid technique for the detection of pterins using colloidal silver suspensions. Pteridine compounds are a family of biochemicals, heterocyclic in structure, and employed in nature as components of colour pigmentation and also as facilitators for many metabolic pathways, particularly those relating to the amino acid hydroxylases. In this work, xanthopterin, isoxanthopterin and 7,8- dihydrobiopterin have been examined whilst absorbed to SERS-active silver colloids. SERS, while far more sensitive than regular Raman spectroscopy, has its own issues relating to the reproducibility of substrates. In order to obtain quantitative data for the pteridine compounds mentioned above, exploratory studies of methods for introducing an internal standard for normalisation of the signals have been carried out.e

We present a detailed study of the localized coupled-cavity modes in a photonic molecule formed from two dielectric spherical microcavities with CdTe nanocrystals. A layer-by-layer deposition technique provides controllable coating of the microspheres with a shell of close-packed nanocrystals of approximately 4 nm in diameter. The observed spectral structures originate from the coupling of the electronic transitions in nanocrystals and the photon states of interacting microspheres and, in analogy to the formation of molecular electronic orbits, can be assigned to bonding and antibonding photon localized states.

Coherent coupling of optical resonances in spherical microcavities results in the formation of so called photonic molecules. The name originates from the analogy to chemical molecules. The coupled optical modes are similar to the atomic orbitals of a chemical molecule. In this work, we studied the resonance spectrum of the symmetric photonic molecules. Furthermore, symmetric directional emission of light from the photonic molecules is experimentally shown. The photonic molecules are illuminated in the vertical direction with a defocused laser beam. The emission is attributed to photonic nanojets generated in the structure. Spectral analysis exhibit whispering gallery mode resonances of coupled and uncoupled modes. A benzene molecule-like structure consisting of a 7-microspheres cyclic photonic molecule shows a field emission pattern similar to the spatial distribution of the orbitals of the benzene molecule.

In this paper we describe a novel technique for the fabrication of aluminosilicate microfibres and microtubes which are shown to act as optical cylindrical microresonators. The alumosilicate microfibres and microtubes were fabricated by using vacuum-assisted wetting and filtration of silica gel through a microchannel glass matrix. The microfibres and microtubes were studied using Scanning Electron Microscopy (SEM), micro-photoluminescence spectroscopy and fluorescence lifetime imaging confocal microscopy. In the emission spectra of the micro-resonators we find very narrow periodic peaks corresponding to the whispering gallery modes of two orthogonal polarizations with quality factors up to 3200. A strong enhancement in photoluminescence decay rates at high excitation power demonstrates the occurrence of amplified spontaneous emission from a single microtube. These microtubes show a large evanescent field extending many microns beyond the tube radius. Potential applications for these novel microresonators will be in the area of optical microsensors for a single molecule detection of biological and chemical species, including anti-terrorism and defense sectors.

Fdrster resonant energy transfer (FRET) between the CdTe quantum dot (QD) acting as donors and acceptors is investigated at nanoscale proximity to gold nanoparticles (Au NPs). Photoluminescence (PL) studies of the acceptor QD emission from a mixed monolayer showed a distance dependent enhancement of the acceptor emission compared with that achieved for a donor-acceptor mixed monolayer in the absence of the Au NP layer. Time-resolved photoluminescence measurements showing a reduction in the donor lifetime, accompanied by an increase in the acceptor PL lifetime, provide further evidence for surface plasmon enhanced FRET.

We present a detailed study of the localized coupled-cavity modes in a photonic molecule formed from two dielectric spherical microcavities with CdTe nanocrystals, which show a multi-peak narrowband modal structure resulting from lifting of the mode degeneracy with respect to the azimuthal quantum number. The feasibility of photonic molecules as the basis for a multi-channel, wavelength-tunable optical delay device is analysed.

We present details on the formation and optical peroperties of CdTe nanowires, which were found to grow in a standard phosphate-buffered solution, including micro-photoluminescence and fluorescence lifetime imaging.

A detailed study of the photonic modes in microtube cavity of ~ 7-8 μm outer diameter is presented. We demonstrate a new route to the fabrication of individual microtubes with the maximum length of 200 μm, using a vacuum assisted wetting and filtration through a microchannel glass matrix. The microtubes were studied using micro-photoluminescence spectroscopy and luminescence lifetime imaging confocal microscopy. In the emission spectra of the microresonators we find periodic very narrow peaks corresponding to the whispering gallery modes of two orthogonal polarizations with quality factors upto 3200 at room temperature. In order to identify the peaks in the observed mode structure, we have adopted the boundary-value solution to the problem of scattering of electromagnetic waves by a dielectric microcylinder. A strong enhancement in photoluminescence decay rates at high excitation power suggest the occurrence of amplified spontaneous emission from a single microtube. The evanescent field in these photonic structures extends a couple of micrometers into the surroundings providing the possibility for efficient coupling to an external photonic device.

We present a detailed study of the photonic modes in microtube cavity of ~ 7-8 μm outer diameter that can act as micron-scale optical cylindrical resonator. We demonstrate a new route to the fabrication of individual microtubes with the maximum length of 200 mm, using a vacuum assisted wetting and filtration through a microchannel glass matrix. The microtubes were studied using micro-photoluminescence spectroscopy and luminescence lifetime imaging confocal microscopy. In the emission spectra of the microresonators we find periodic very narrow peaks corresponding to the whispering gallery modes of two orthogonal polarizations with quality factors upto 3200 at room temperature. In order to identify the peaks in the observed mode structure, we have adopted the boundary-value solution to the problem of scattering of electromagnetic waves by a dielectric micro-cylinder. A strong enhancement in photoluminescence decay rates at high excitation power suggest the occurrence of amplified spontaneous emission from a single microtube. The evanescent field in these photonic structures extends a couple of micrometers into the surroundings providing the possibility for efficient coupling to an external photonic device.

We present the results of studies on small photonic microtube structures of less than 10 microns diameter. A new technique based on vacuum assisted filtration is used to produce the microtube resonantors. Whispering gallery modes are probed through the photoluminescence emission from the glass material forming the cavity. We observe resonances with polarized emission having quality factors upto 3000. These microresonator structures exhibit a large evanescent field which makes them interesting for potential photonic applications.

A 5-fold enhancement in the luminescence of CdTe nanocrystal quantum dots (QDs) is observed when they are placed in proximity to a nanostructured Au film deposited by pulsed laser deposition technique. No enhancement is observed with a nanostructured Ag film. The enhancement is due to the interaction of the QDs excitons with the localized surface plasmons (LSP). The Au surface plasmon (SP) frequency is closer to the QDs emission frequency than Ag LSP frequency and this accounts for the differences in observed behavior. As the SP-QD interaction strongly depends on the geometric structure and shape of the metal nanoparticles, a comparison with QDs deposited on a film of Au colloidal nanoparticles is presented. In the case of QDs placed directly on the Au colloids the luminescence quenching is much stronger and with a spacer layer a 3.5-fold enhancement over the bare QDs luminescence is observed.

ABSRACT We present a detailed study of the localized coupled-cavity modes in a photonic molecule formed from two dielectric spherical microcavities with CdTe nanocrystals, which show a multi-peak narrowband modal structure resulting from lifting of the mode degeneracy with respect to the azimuthal quantum number. The waveguiding through the coupled microcavities and wavelength switching effect is demonstrated. The feasibility of photonic molecules as the basis for a multi-channel, wavelength-tunable optical delay device is analysed.

In this paper, we experimentally studied both the bright-field and fluorescence images of microspheres by conventional and confocal scanning polarization microscopes. A qualitative analysis have been given to show a physical picture on the imaging of the microspheres. Emission spectra from melamine formaldehyde microspheres stained with Ethidium Bromide or covered by thin shell of CdTe nanocrystals have been experimentally studied. We adopted analytical expressions describing the resonance spacing in order to determine the size of the microspheres.

Spherical microcavities consisting of a dielectric material show unique optical characteristics as resonators in combination with semiconductor nanoparticles. A high quality factor results in a very narrow bandwidth of the resonant modes (whispering-gallery modes) inside the microcavity. The polystyrene microspheres are coated with one monolayer of CdTe nanocrystals which offer a high photostability and a high quantum yield at room temperature. Due to strong confinement of the electrons in all three dimensions, excitation from the quantum dots is highly size-dependent and tuneable over almost the whole visible spectrum. The deposition of the nanocrystals on the sphere surface allows efficient coupling of the light of the CdTe quantum dots into the microcavity. Photoluminescence and Raman spectra were taken with a Renishaw Raman system. The setup is equipped with an Ar+-laser and a HeNe-laser to excite the nanocrystals. Raman measurements show a series of very sharp resonant peaks instead of a continuous spectrum. Strong interaction between the electronic states of the nanocrystals and the resonant modes in the microsphere causes a considerable enhancement of the Raman scattering and luminescence from the CdTe quantum dots in Stokes and anti-Stokes region. Furthermore, a linear blue shift of the resonances in the photoluminescence spectrum was observed during continuous excitation for 18 minutes with a HeNe laser.

We present details on the CdTe nanowires formation, which were found to grow in a standard physiological phosphate-buffered solution, including in-situ observation of growth with a confocal microscope. The choice of proper nanocrystals concentration allowed reasonably slow growth rates and thus a controllable formation of nanowires. Once formed in solution, nanowires showed a significant degree of structural rigidity and resistance to externally applied mechanical stress. Luminescence and Raman spectroscopy data evidence on re-crystallization processes during the nanowire formation.

We report highly efficient Forster resonance energy transfer between CdTe nanocrystals and two different dyes, Rhodamine B and Oxazine, where the nanocrystals are mixed with the dyes on top of glass substrates. A faster NC decay curve is observed in the samples containing NCs mixed with dyes than in those containing NCs on their own. For the samples containing nanocrystals mixed with Rhodamine B, room temperature PL measurements are presented as a function of the ratio between the amount of acceptors and the amount of donors, CA/CD. This ratio is varied between 0.03 and 5. The strongest enhancement of the acceptor PL intensity relative to that of the donor PL intensity is reached for 0.2

Images of the microspheres are studied in three-dimensions by using the confocal and conventional scanning polarization microscopes. It is found that the polarization of the detected signals is mainly parallel to the initial polarization which is due to the high extinction coefficient of the confocal system. Arc pairs are observed at the edge of the microspheres with the conventional polarization microscope with a crossed analyzer. Theoretical analysis are given by using the vector field theory and the image formations of the two systems.

We present a photoluminescence and photoluminescence excitation study of CdTe quantum dots, prepared via a novel organometallic approach. The global photoluminescence (excited at the energy above the absorption edge) showed a red shift of 75 meV with respect to the first absorption peak. This band edge emission was found to be strongly dependent on the excitation photon. Resonant emission spectra showed a pronounced spectral shift and line narrowing with decreasing excitation energy. The resonant Stokes shifts were extracted from photoluminescence and photoluminescence excitation data. The minimum magnitude of the resonant Stokes shift of 14 meV was obtained at room temperature.