Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals

Bloch Surface Wave-Coupled Emission from Quantum Dots by Ensemble and Single Molecule Spectroscopy

We report the spectral properties and spatial distribution of quantum dot (QD575 ) emission on a one-dimensional photonic crystal (1DPC). Our 1DPC substrate consists of multiple layers of dielectrics with a photonic band gap (PBG) near the QD575 emission maximum. The 1DPC was designed to display a surface-trapped electromagnetic state known as a Bloch surface wave (BSW) at the 1DPC-air (sample) interface. Ensemble angle-dependent emission intensities revealed a sharp angular emission peak near 41° from the normal which is consistent with the BSW resonance at 575 nm. We further examined the emission from single QDs on the 1DPC. A notable increase in fluorescence intensity from QD575 particles was observed on BSW substrate compared to the glass substrate from the scanning confocal fluorescence images and from the intensity-time trajectories of single QD575 particles. The intensity-decays showed substantially faster decay (4-fold decrease in emission lifetime) from the single QD575 particles on 1DPC substrate (∼4.8 nsec) as compared to the glass substrate (∼18 nsec). We observed the spectral characteristics of the individual QD575 particles on 1DPC and glass substrates, by recording the single particle emission spectra through the 1DPC. The emission spectra of the single QD575 particles are similar (with emission maxima around 575 nm) on both substrates except a substantial increase in intensity (over 10-fold) on the BSW substrate. Our results demonstrate that quantum dots can interact with Bloch Surface Waves (BSW) on a 1DPC. To the best of our knowledge, this is the first report on the single particle fluorescence studies on 1DPC substrate. The 10-fold increase in intensity in combination with 4-fold reduction in emission lifetime suggest 1DPCs with BSW modes have potential use in sensing and single molecule spectroscopy.

Fine Tuning the Emission Properties of Nanoemitters in Multilayered Structures by Deterministic Control of their Local Photonic Environment

Deterministic control on the dynamics of organic nanoemitters is achieved through precise control of its photonic environment. Resonators are fabricated by a combination of spin- and dip-coating techniques, which allows placement of the emitters at different positions within the sample, thus acting as a probe of the local density of states.

Efficient single photon emission and collection based on excitation of gap surface plasmons

Combining the advantages of ultrahigh photon emission rates achievable in the gap surface plasmon polaritons with high extraction decay rates into low-loss nanofibers, we demonstrate theoretically the efficient photon emission of a single dipole emitter and one-dimensional nanoscale guiding in metallic nanorod-coupled nanofilm structures coupled to dielectric nanofibers. We find that total decay rates and surface plasmon polariton channel decay rates orders of magnitude larger than those characteristic of metallic nanofilms alone can be achieved in ultrastrong hot spots of gap plasmons. For the requirement of practical applications, propagating single photons with decay rates of 290γ_{0}-770γ_{0} are guided into the phase-matched low-loss nanofibers. The proposed mechanism promises to have an important impact on metal-based optical cavities, on-chip bright single photon sources and plasmon-based nanolasers.

Single-molecule super-resolution microscopy reveals how light couples to a plasmonic nanoantenna on the nanometer scale

The greatly enhanced fields near metal nanoparticles have demonstrated remarkable optical properties and are promising for applications from solar energy to biosensing. However, direct experimental study of these light-matter interactions at the nanoscale has remained difficult due to the limitations of optical microscopy. Here, we use single-molecule fluorescence imaging to probe how a plasmonic nanoantenna modifies the fluorescence emission from a dipole emitter. We show that the apparent fluorophore emission position is strongly shifted upon coupling to an antenna and that the emission of dyes located up to 90 nm away is affected by this coupling. To predict this long-ranged effect, we present a framework based on a distance-dependent partial coupling of the dye emission to the antenna. Our direct interpretation of these light-matter interactions will enable more predictably optimized, designed, and controlled plasmonic devices and will permit reliable plasmon-enhanced single-molecule nanoscopy.

Nanophotonic control of circular dipole emission

Controlling photon emission by single emitters with nanostructures is crucial for scalable on-chip information processing. Nowadays, nanoresonators can affect the lifetime of linear dipole emitters, while nanoantennas can steer the emission direction. Expanding this control to the emission of orbital angular momentum-changing transitions would enable a future coupling between solid state and photonic qubits. As these transitions are associated with circular dipoles, such control requires knowledge of the interaction of a complex dipole with optical eigenstates containing local helicity. We experimentally map the coupling of classical, circular dipoles to photonic modes in a photonic crystal waveguide. We show that, depending on the combination of the local helicity of the mode and the dipole helicity, circular dipoles can couple to left- or rightwards propagating modes with a near-unity directionality. The experimental maps are in excellent agreement with calculations. Our measurements, therefore, demonstrate the possibility of coupling the spin to photonic pathway.

Photonic effects on the radiative decay rate and luminescence quantum yield of doped nanocrystals

Nanocrystals (NCs) doped with luminescent ions form an emerging class of materials. In contrast to excitonic transitions in semiconductor NCs, the optical transitions are localized and not affected by quantum confinement. The radiative decay rates of the dopant emission in NCs are nevertheless different from their bulk analogues due to photonic effects, and also the luminescence quantum yield (QY, important for applications) is affected. In the past, different theoretical models have been proposed to describe the photonic effects for dopant emission in NCs, with little experimental validation. In this work we investigate the photonic effects on the radiative decay rate of luminescent doped NCs using 4 nm LaPO4 NCs doped with Ce(3+) or Tb(3+) ions in different refractive index solvents and bulk crystals. We demonstrate that the measured influence of the refractive index on the radiative decay rate of the Ce(3+) emission, having near unity QY, is in excellent agreement with the theoretical nanocrystal-cavity model. Furthermore, we show how the nanocrystal-cavity model can be used to quantify the nonunity QY of Tb(3+)-doped LaPO4 NCs and demonstrate that, as a general rule, the QY is higher in media with higher refractive index.

Noninvasive theranostic imaging of HSV-TK/GCV suicide gene therapy in liver cancer by folate-targeted quantum dot-based liposomes

We have successfully developed folate-targeting liposomes for effective and safe HSV-TK suicide gene theranostics.

Electronic structures of two types of TiO2 electrodes: inverse opal and nanoparticulate cases

We present a comparison between the electronic structures of inverse opal (IO) and nanoparticulate (NP)-TiO₂ electrodes.

One-pot preparation of cationic charged Pt nanoparticles by the autocatalytic hydrolysis of acetylthiocholine

Autocatalytic synthesis of cationic charged platinum nanoparticles.

Percolating plasmonic networks for light emission control

Optical nanoantennas have revolutionised the way we manipulate single photons emitted by individual light sources in a nanostructured photonic environment. Complex plasmonic architectures allow for multiscale light control by shortening or stretching the light wavelength for a fixed operating frequency, meeting the size of the emitter and that of propagating modes. Here, we study self-assembled semi-continuous gold films and lithographic gold networks characterised by large local density of optical state (LDOS) fluctuations around the electrical percolation threshold, a regime where the surface is characterised by large metal clusters with fractal topology. We study the formation of plasmonic networks and their effect on light emission from embedded fluorescent probes in these systems. Through fluorescence dynamics experiments we discuss the role of global long-range interactions linked to the degree of percolation and to the network fractality, as well as the local near-field contributions coming from the local electro-magnetic fields and the topology. Our experiments indicate that local properties dominate the fluorescence modification.

Statistical measurements of quantum emitters coupled to Anderson-localized modes in disordered photonic-crystal waveguides

We present a statistical study of the Purcell enhancement of the light emission from quantum dots coupled to Anderson-localized cavities formed in disordered photonic-crystal waveguides. We measure the time-resolved light emission from both single quantum emitters coupled to Anderson-localized cavities and directly from the cavities that are fed by multiple quantum dots. Strongly inhibited and enhanced decay rates are observed relative to the rate of spontaneous emission in a homogeneous medium. From a statistical analysis, we report an average Purcell factor of 4.5 ± 0.4 without applying any spectral tuning. By spectrally tuning individual quantum dots into resonance with Anderson-localized modes, a maximum Purcell factor of 23.8 ± 1.5 is recorded, which is at the onset of the strong-coupling regime. Our data quantify the potential of Anderson-localized cavities for controlling and enhancing the light-matter interaction strength in a photonic-crystal waveguide, which is of relevance for cavity quantum-electrodynamics experiments, efficient energy harvesting and random lasing.

Compact and blinking-suppressed quantum dots for single-particle tracking in live cells

Quantum dots (QDs) offer distinct advantages over organic dyes and fluorescent proteins for biological imaging applications because of their brightness, photostability, and tunability. However, a major limitation is that single QDs emit fluorescent light in an intermittent on-and-off fashion called "blinking". Here we report the development of blinking-suppressed, relatively compact QDs that are able to maintain their favorable optical properties in aqueous solution. Specifically, we show that a linearly graded alloy shell can be grown on a small CdSe core via a precisely controlled layer-by-layer process, and that this graded shell leads to a dramatic suppression of QD blinking in both organic solvents and water. A substantial portion (>25%) of the resulting QDs does not blink (more than 99% of the time in the bright or "on" state). Theoretical modeling studies indicate that this type of linearly graded shell not only can minimize charge carrier access to surface traps but also can reduce lattice defects, both of which are believed to be responsible for carrier trapping and QD blinking. Further, we have evaluated the biological utility of blinking-suppressed QDs coated with polyethylene glycol (PEG)-based ligands and multidentate ligands. The results demonstrate that their optical properties are largely independent of surface coatings and solvating media, and that the blinking-suppressed QDs can provide continuous trajectories in live-cell receptor tracking studies.

Quantum yield and excitation rate of single molecules close to metallic nanostructures

The interaction of dyes and metallic nanostructures strongly affects the fluorescence and can lead to significant fluorescence enhancement at plasmonic hot spots, but also to quenching. Here we present a method to distinguish the individual contributions to the changes of the excitation, radiative and non-radiative rate and use this information to determine the quantum yields for single molecules. The method is validated by precisely placing single fluorescent dyes with respect to gold nanoparticles as well as with respect to the excitation polarization using DNA origami nanostructures. Following validation, measurements in zeromode waveguides reveal that suppression of the radiative rate and enhancement of the non-radiative rate lead to a reduced quantum yield. Because the method exploits the intrinsic blinking of dyes, it can generally be applied to fluorescence measurements in arbitrary nanophotonic environments.

Inhibition and enhancement of the spontaneous emission of quantum dots in micropillar cavities with radial-distributed Bragg reflectors

We present a micropillar cavity where nondesired radial emission is inhibited. The photonic confinement in such a structure is improved by implementation of an additional concentric radial-distributed Bragg reflector. Such a reflector increases the reflectivity in all directions perpendicular to the micropillar axis from a typical value of 15-31% to above 98%. An inhibition of the spontaneous emission of off-resonant excitonic states of quantum dots embedded in the microcavity is revealed by time-resolved experiments. It proves a decreased density of photonic states related to unwanted radial leakage of photons out of the micropillar. For on-resonance conditions, we find that the dot emission rate is increased, evidencing the Purcell enhancement of spontaneous emission. The proposed design can increase the efficiency of single-photon sources and bring to micropillar cavities the functionalities based on lengthened decay times.

Photonic metal-organic framework composite spheres: a new kind of optical material with self-reporting molecular recognition

Exploiting metal-organic framework (MOF) materials as novel building blocks to construct superstructures with extended and enhanced functions represents a big challenge. In biological systems, the ordering of many components is not achieved by interaction of the components with each other, but by interaction of each component with the host protein which provides a matrix to support the entire assembly. Inspired by biological systems, in this work, a general strategy for efficient spatial arrangement of MOF materials was developed by using spherical colloidal crystals as host matrices, affording a new class of highly tunable MOF composite spheres with a series of distinctive properties. It was found that the synergetic combination of the unique features of both MOF and photonic colloidal crystal imparted these hierarchically structured spheres intrinsic optical properties, specific molecular recognition with self-reporting signalling, derivatization capability, and anisotropy. More importantly, the unique photonic band-gap structure integrated in these composite spheres provides a more convenient means to manipulate the photophysical and photochemical behaviour of the trapped guest molecules in MOF nanocavities.

Large spontaneous emission rate enhancement in grating coupled hyperbolic metamaterials

Hyperbolic metamaterial (HMM), a sub-wavelength periodic artificial structure with hyperbolic dispersion, can enhance the spontaneous emission of quantum emitters. Here, we demonstrate the large spontaneous emission rate enhancement of an organic dye placed in a grating coupled hyperbolic metamaterial (GCHMM). A two-dimensional (2D) silver diffraction grating coupled with an Ag/Al₂O₃ HMM shows 18-fold spontaneous emission decay rate enhancement of dye molecules with respect to the same HMM without grating. The experimental results are compared with analytical models and numerical simulations, which confirm that the observed enhancement of GCHMM is due to the outcoupling of non-radiative plasmonic modes as well as strong plasmon-exciton coupling in HMM via diffracting grating.

Mapping and quantifying electric and magnetic dipole luminescence at the nanoscale

We report on an experimental technique to quantify the relative importance of electric and magnetic dipole luminescence from a single nanosource in structured environments. By attaching a Eu^{3+}-doped nanocrystal to a near-field scanning optical microscope tip, we map the branching ratios associated with two electric dipole and one magnetic dipole transitions in three dimensions on a gold stripe. The relative weights of the electric and magnetic radiative local density of states can be recovered quantitatively, based on a multilevel model. This paves the way towards the full electric and magnetic characterization of nanostructures for the control of single emitter luminescence.

Nanophotonic enhancement of the Förster resonance energy-transfer rate with single nanoapertures

Tailoring the light-matter interaction and the local density of optical states (LDOS) with nanophotonics provides accurate control over the luminescence properties of a single quantum emitter. This paradigm is also highly attractive to enhance the near-field Förster resonance energy transfer (FRET) between two fluorescent emitters. Despite the wide applications of FRET in nanosciences, using nanophotonics to enhance FRET has remained a debated and complex challenge. Here we demonstrate enhanced energy transfer within single donor-acceptor fluorophore pairs confined in single gold nanoapertures. Experiments monitoring both the donor and the acceptor emission photodynamics clearly establish a linear dependence of the FRET rate on the LDOS in nanoapertures, demonstrating that nanophotonics can be used to intensify the near-field energy transfer. Strikingly, we observe a significant six-fold increase in the FRET rate for large donor-acceptor separations exceeding 13 nm. Exciting opportunities are opened to investigate biochemical structures with donor-acceptor distances much beyond the classical Förster radius. Importantly, our approach is fully compatible with the detection of single biomolecules freely diffusing in water solution under physiological conditions.

Real-time visualizing and tracing of HSV-TK/GCV suicide gene therapy by near-infrared fluorescent quantum dots

Exploring intracellular behavior of suicide gene is significant for improving the efficacy and safety of herpes simplex virus thymidine kinase gene/ganciclovir (HSV-TK/GCV) system in cancer therapy. Molecular imaging represents a powerful tool to understand gene transportation and function dynamics. In this work, we reported a quantum-dot-based technique for revealing the procedure of HSV-TK/GCV suicide gene therapy by constructing covalent linkage between near-infrared fluorescent quantum dots (QDs) and TK gene. This stable QD labeling did not influence either the QDs fluorescence or the biological activity of TK gene. Furthermore, we visualized and dynamically traced the intracellular behavior antitumor effect of TK gene in vitro and in vivo. It is demonstrated that TK gene was shuttled to the nucleus after a-24 h treatment; at that time the single dose of GCV administration exerts the gradually increasing lethal effect until to 72 h. Real-time tracing the formation of hepatocellular carcinoma treated with HSV-TK/GCV suicide gene system in vivo by QD-based NIR fluorescence imaging provides useful insight toward QD-based theranostics in future cancer therapy.

Efficient energy transfer from inserted CdTe quantum dots to YVO₄:Eu³⁺ inverse opals: a novel strategy to improve and expand visible excitation of rare earth ions

Rare earth (RE)-based phosphors demonstrate sharp emission lines, long lifetimes and high luminescence quantum yields; thus, they have been employed in various photoelectric devices, such as light-emitting diodes (LEDs) and solar spectral converters. However, their applications are largely confined by their narrow excitation bands and small absorption cross sections of 4f-4f transitions. In this paper, we demonstrate a novel strategy to improve and expand the visible excitation bands of Eu(3+) ions through the interface energy transfer (ET) from CdTe quantum dots (QDs) to YVO₄:Eu(3+) inverse opal photonic crystals (IOPCs). The significant effects observed in the CdTe QDs/YVO₄:Eu(3+) IOPCs composites were that the excitation of Eu(3+) ions was continuously extended from 450 to 590 nm and that the emission intensity of the (5)D₀-(7)FJ transitions was enhanced ∼20-fold, corresponding to the intrinsic (7)F₁-(5)D₁ excitation at 538 nm. Furthermore, in the IOPC network, the ET efficiency from the QDs to YVO₄:Eu(3+) was greatly improved because of the suppression of energy migration among the CdTe QDs, which gave an optimum ET efficiency as high as 47%. Besides, the modulation of photonic stop bands (PSBs) on the radiative transition rates of the QDs and Eu(3+) ions was studied, which showed that the decay lifetime constants for Eu(3+) ions were independent of PSBs, while those of QDs demonstrated a suppression in the PSBs. Their physical nature was explained theoretically.

Three-dimensional photonic crystals created by single-step multi-directional plasma etching

We fabricate 3D photonic nanostructures by simultaneous multi-directional plasma etching. This simple and flexible method is enabled by controlling the ion-sheath in reactive-ion-etching equipment. We realize 3D photonic crystals on single-crystalline silicon wafers and show high reflectance (>95%) and low transmittance (<-15dB) at optical communication wavelengths, suggesting the formation of a complete photonic bandgap. Moreover, our method simply demonstrates Si-based 3D photonic crystals that show the photonic bandgap effect in a shorter wavelength range around 0.6 μm, where further fine structures are required.

Quantifying local density of optical states of nanorods by fluorescence lifetime imaging

In this letter, we demonstrate a facile far-field approach to quantify the near-field local density of optical states (LDOS) of a nanorod using CdTe quantum dots (QDs) emitters tethered to the surface of nanorods as beacons for optical read-outs. Radiative decay rate was extracted to quantify the LDOS; our analysis indicates that the LDOS of the nanorod enhance both the radiative and nonradiative decay of QD, particularly radiative decay of QDs at the end of nanorod is enhanced by 1.17 times greater than that at the waist, while the nonradiative decay was uniformly enhanced over the nanorod. To the best of our knowledge, our effort constitutes the first to map the LDOS of a nanostructure via far-field method, to provide clarity on the interaction mechanism between emitters and the nanostructure, and to be potentially employed in the LDOS mapping of high-throughput nanostructures.

Spontaneous emission inhibition of telecom-band quantum disks inside single nanowire on different substrates

We investigate the inhibited spontaneous emission of telecom-band InAs quantum disks (Qdisks) in InP nanowires (NWs). We have evaluated how the inhibition is affected by different disk diameter and thickness. We also compared the inhibition in standing InP NWs and those NWs laying on silica (SiO(2)), and silicon (Si) substrates. We found that the inhibition is altered when we put the NW on the high-refractive-index materials of Si. Experimentally, the inhibition factor ζ of the Qdisk emission at 1,500 nm decreases from 4.6 to 2.5 for NW on SiO(2) and Si substrates, respectively. Those inhibitions are even much smaller than that of 6.4 of the standing NW. The inhibition factors well agree with those calculated from the coupling of the Qdisk to the fundamental guided mode and the continuum of radiative modes. Our observation can be useful for the integration of the NW as light sources in the photonic nanodevices.

Homogeneous coating of photonic macroporous oxides with inorganic nanocrystals

A simple method to obtain homogeneous sub-monolayer coverage of metal oxide and chalcogenide nanocrystals onto porous oxide supports is described. Quantitative nanoparticle coverage was probed using photonic macroporous oxide supports. Composites of nanocrystals of TiO₂, Fe₃O₄ or CdS dispersed onto macroporous SiO₂ or ZrO₂ all show a predictable linear shift in the photonic stop band position.

Real-time light scattering tracking of gold nanoparticles- bioconjugated respiratory syncytial virus infecting HEp-2 cells

Real-time tracking of virus invasion is crucial for understanding viral infection mechanism, which, however, needs simple and efficient labeling chemistry with improved signal-to-noise ratio. For that purpose, herein we investigated the invasion dynamics of respiratory syncytial virus (RSV) through dark-field microscopic imaging (iDFM) technique by using Au nanoparticles (AuNPs) as light scattering labels. RSV, a ubiquitous, non-segmented, pleiomorphic and negative-sense RNA virus, is an important human pathogen in infants, the elderly, and the immunocompromised. In order to label the enveloped virus of paramyxoviridae family, an efficient streptavidin (SA)-biotin binding chemistry was employed, wherein AuNPs and RSV particles modified with SA and biotin, respectively, allowing the AuNP-modified RSVs to maintain their virulence without affecting the native activities of RSV, making the long dynamic visualization successful for the RSV infections into human epidermis larynx carcinoma cells.

Sandwich approach toward inverse opals with linear and nonlinear optical functionalities

Three-dimensionally ordered macroporous materials have unique structural and optical properties, making them useful for numerous applications in catalysis, membrane science, and optics. Accessible and economic fabrication of these materials is essential to fully explore the many possibilities that these materials present. A new templating method to fabricate three-dimensionally ordered macroporous materials without overlayers is presented. The resulting structures are freestanding inverse opals with large-area uniformity. The versatility and power of our fabrication method is demonstrated by synthesizing inverse opals displaying fluorescence, chirality, upconversion, second harmonic generation, and third harmonic generation. This economical and versatile fabrication method will facilitate research on inverse opals in general and on linear and nonlinear optical effects in 3D photonic crystals specifically. The relative ease of synthesis and wide variety of resulting materials will help the characterization and improvement of existing anomalous dispersion effects in these structures, while providing a platform for the discovery and demonstration of novel effects.

Thermally tunable amplified spontaneous emission from Cu2ZnSnS4 quantum dot doped photonic cavity by a soft solvothermal route

In this article, non-toxic Cu₂ZnSnS₄ quantum dots (CZTSQDs) were synthesized by the solvothermal method and then embedded into a photonic cavity to tune the amplified spontaneous emission.

Syntheses, structures and properties of 5-azotetrazolyl salicylic acid and its dilanthanide complexes

Two hydrated 5-azotetrazolyl salicylic acids (H₃ASA) and five H₃ASA based dinuclear Ln³⁺ complexes have been synthesized and characterized by single crystal X-ray diffraction analysis.

Chiral nematic mesoporous films of ZrO2:Eu3+: new luminescent materials

Free-standing chiral nematic mesoporous films of ZrO₂:Eu³⁺ exhibit tunable optical properties and selective suppression of the spontaneous emission of Eu³⁺.

Spontaneous emission enhancement and saturable absorption of colloidal quantum dots coupled to photonic crystal cavity

We demonstrate spontaneous emission rate enhancement and saturable absorption of cadmium selenide colloidal quantum dots coupled to a nanobeam photonic crystal cavity. We perform time-resolved lifetime measurements and observe an average enhancement of 4.6 for the spontaneous emission rate of quantum dots located at the cavity as compared to those located on an unpatterned surface. We also demonstrate that the cavity linewidth narrows with increasing pump intensity due to quantum dot saturable absorption.

Enhanced trion emission from colloidal quantum dots with photonic crystals by two-photon excitation

For colloidal quantum dots, the ongoing biggest problem is their fluorescence blinking. Until now, there is no generally accepted model for this fluorescence blinking. Here, two-photon excited fluorescence from CdSe/ZnS nanocrystals on silicon nitride photonic crystals is studied using a femtosecond laser. From analysis of the spectra and decay processes, most of the relative trion efficiency is larger than 10%, and the largest relative trion efficiency reaches 46.7%. The photonic crystals enhance the trion emission of CdSe/ZnS nanocrystals, where the enhancement is due to the coupling of the trion emission to the leaky mode of the photonic crystal slab. Moreover, the photonic crystals enhance the Auger-assisted trapping efficiency of electrons/holes to surface states, and then enhance the efficiency of the generations of charge separation and DC electric field, which modifies the trion spectrum. Therefore, a model is present for explaining the mechanism of fluorescence blinking including the effect of the environment.

Controllable chrominance and highly improved luminescent quantum yield of YV(1-x)P(x)O4: Tm, Dy, Eu inverse opal white light phosphors

In this work, rare earth (RE) ions tri-doped YV(1-x)P(x)O(4): RE(3+) (RE = Tm, Dy, Eu) inverse opal photonic crystals (IOPCs) were fabricated by the PMMA template method, which demonstrated efficient white light emissions under ultraviolet excitation. It is significant to observe that the chrominance of the white light could be largely modulated by the photonic stop band of the IOPCs. And more, the photoluminescence quantum yield in the IOPCs was largely improved over the grinded reference (REF) because the undesired energy transfer (ET) process was effectively restrained.

Large scale solution assembly of quantum dot-gold nanorod architectures with plasmon enhanced fluorescence

Tailoring the efficiency of fluorescent emission via plasmon-exciton coupling requires structure control on a nanometer length scale using a high-yield fabrication route not achievable with current lithographic techniques. These systems can be fabricated using a bottom-up approach if problems of colloidal stability and low yield can be addressed. We report progress on this pathway with the assembly of quantum dots (emitter) on gold nanorods (plasmonic units) with precisely controlled spacing, quantum dot/nanorod ratio, and long-term colloidal stability, which enables the purification and encapsulation of the assembled architecture in a protective silica shell. Overall, such controllability with nanometer precision allows one to synthesize stable, complex architectures at large volume in a rational and controllable manner. The assembled architectures demonstrate photoluminescent enhancement (5×) useful for applications ranging from biological sensing to advanced optical communication.

Plasmonic band structure controls single-molecule fluorescence

Plasmonics and photonic crystals are two complementary approaches to tailor single-emitter fluorescence, using strong local field enhancements near metals on one hand and spatially extended photonic band structure effects on the other hand. Here, we explore the emergence of spontaneous emission control by finite-sized hexagonal arrays of nanoapertures milled in gold film. We demonstrate that already small lattices enable highly directional and enhanced emission from single fluorescent molecules in the central aperture. Even for clusters just four unit cells across, the directionality is set by the plasmonic crystal band structure, as confirmed by full-wave numerical simulations. This realization of plasmonic phase array antennas driven by single quantum emitters opens a flexible toolbox to engineer fluorescence and its detection.

A new class of fluorescent-dots: long luminescent lifetime bio-dots self-assembled from DNA at low temperatures

Quantum-dots (QDs) have fuelled up intensive research efforts over the past two decades. Nevertheless, currently developed two classes of fluorescent QDs, colloidal semiconductor QDs and carbonaceous QDs suffer from either toxicity or short luminescence lifetime. Here, we report a new class of fluorescent bio-dots that are derived from DNA via self-assembly at low temperatures down to 80°C, which has an optical bandgap of 3.4 eV, and in particular possesses strong photoluminescence with a much longer luminescence lifetime (τ₁ = 10.44 ns) than the carbonaceous QDs (τ₁ < 0.5 ns). It is discovered that it is the interactions of base pair cytosines with each other to form sp² carbon–like centers as luminescence centers or chromophores for the photoluminescence. The use of bio-dots in cell imaging with strong photoluminescence signal and good biocompatibility demonstrates great potentials of broad biological and optoelectronic applications.

Long-lived quantum coherence of two-level spontaneous emission models within structured environments

We investigate the long-lived quantum coherence of two-level spontaneous emission models within structured environments. The population of the system under the asymptotic non-Markovian dynamics is linked to the spectral density of the reservoir through a general functional relation between them. We figure out explicitly the preservation of quantum coherence, via notions of entanglement and quantum discord, in connection with the spectral parameters of Ohmic class reservoirs, and then show how to achieve them optimally. We expect these results to contribute to reservoir engineering with the aim of enhancing stationary quantum coherence in noisy environments.

Enabling enhanced emission and low-threshold lasing of organic molecules using special Fano resonances of macroscopic photonic crystals

The nature of light interaction with matter can be dramatically altered in optical cavities, often inducing nonclassical behavior. In solid-state systems, excitons need to be spatially incorporated within nanostructured cavities to achieve such behavior. Although fascinating phenomena have been observed with inorganic nanostructures, the incorporation of organic molecules into the typically inorganic cavity is more challenging. Here, we present a unique optofluidic platform comprising organic molecules in solution suspended on a photonic crystal surface, which supports macroscopic Fano resonances and allows strong and tunable interactions with the molecules anywhere along the surface. We develop a theoretical framework of this system and present a rigorous comparison with experimental measurements, showing dramatic spectral and angular enhancement of emission. We then demonstrate that these enhancement mechanisms enable lasing of only a 100-nm thin layer of diluted solution of organic molecules with substantially reduced threshold intensity, which has important implications for organic light-emitting devices and molecular sensing.

Radiative decay engineering 6: fluorescence on one-dimensional photonic crystals

During the past decade the interactions of fluorophores with metallic particles and surfaces has become an active area of research. These near-field interactions of fluorophores with surface plasmons have resulted in increased brightness and directional emission. However, using metals has some disadvantages such as quenching at short fluorophore-metal distances and increased rates of energy dissipation due to lossy metals. These unfavorable effects are not expected in dielectrics. In this article, we describe the interactions of fluorophores with one-dimensional (1D) photonic crystals (PCs), which have alternating layers of dielectrics with dimensions that create a photonic band gap (PBG). Freely propagating light at the PBG wavelength will be reflected. However, similar to metals, we show that fluorophores within near-field distances of the 1DPC interacts with the structure. Our results demonstrate that these fluorophores can interact with both internal modes and Bloch surface waves (BSWs) of the 1DPC. For fluorophores on the surface of the 1DPC, the emission dominantly occurs through the 1DPC and into the substrate. We refer to these two phenomena together as Bragg grating-coupled emission (BGCE). Here we describe our preliminary results on BGCE. 1DPCs are simple to fabricate and can be handled and reused without damage. We believe that BGCE provides opportunities for new formats for fluorescence detection and sensing.

Calibrating and controlling the quantum efficiency distribution of inhomogeneously broadened quantum rods by using a mirror ball

We demonstrate that a simple silver coated ball lens can be used to accurately measure the entire distribution of radiative transition rates of quantum dot nanocrystals. This simple and cost-effective implementation of Drexhage's method that uses nanometer-controlled optical mode density variations near a mirror, not only allows an extraction of calibrated ensemble-averaged rates, but for the first time also to quantify the full inhomogeneous dispersion of radiative and non radiative decay rates across thousands of nanocrystals. We apply the technique to novel ultrastable CdSe/CdS dot-in-rod emitters. The emitters are of large current interest due to their improved stability and reduced blinking. We retrieve a room-temperature ensemble average quantum efficiency of 0.87 ± 0.08 at a mean lifetime around 20 ns. We confirm a log-normal distribution of decay rates as often assumed in literature, and we show that the rate distribution-width, that amounts to about 30% of the mean decay rate, is strongly dependent on the local density of optical states.

Titania woodpiles with complete three-dimensional photonic bandgaps in the visible

Titania woodpile photonic crystals are fabricated by a combination of stimulated-emission depletion direct laser writing and a novel titania double-inversion procedure. The procedure relies on atomic-layer deposition, which is also used to fine-tune the template geometry to maximize the gapsize. Angle- and polarization-resolved transmittance spectroscopy and a comparison with theory provide evidence for the first complete photonic bandgap in the visible.

Numerical analysis on the feasibility of a multi-layered dielectric sphere as a three-dimensional photonic crystal

The radiative decay rate of a dipole emitter inside the core of a multi-layered dielectric sphere is theoretically investigated. It is shown that, when the thickness of each layer coincides with a quarter wavelength, the multi-layered sphere has a great potential to work as a three-dimensional photonic crystal with a high quality factor and a small mode volume. From the analysis of the dipole position dependence of a radiative decay rate, we show that a smaller core radius, a quarter wavelength at the smallest, is more suitable for real applications. The investigation on the tolerance for thickness nonuniformity reveals that the thickness variation of 10% is tolerable.

Nanocavities at the surface of three-dimensional photonic crystals

We investigate nanocavities at the surface of three-dimensional (3D) photonic crystals, where the polarization-independent surface-mode gap can be utilized. We consider the formation of various nanocavities by introducing artificial defects utilizing the 3D structures around the surface and discuss the possibilities for increasing the Q-factors of the surface nanocavities with TE-like polarization based on the advanced designs of donor-type defects. We also introduce the design of acceptor-type defects and show that TM-like nanocavities are obtained. We then fabricate the designed nanocavities and examine their resonant characteristics; we successfully demonstrate TE-like nanocavities with Q-factors of ~40,000, which is four-times higher than previous surface cavities and as high as that of the cavities embedded inside 3D photonic crystals. TM-like nanocavities with Q-factors of ~22,000 are also demonstrated for the first time.