Influence of plasmonic array geometry on energy transfer from a quantum well to a quantum dot layer

Long-Range and High-Efficiency Plasmon-Assisted Förster Resonance Energy Transfer

The development of a long-range and efficient Förster resonance energy transfer (FRET) process is essential for its application in key enabling optoelectronic and sensing technologies. Via controlling the delocalization of the donor's electric field and Purcell enhancements, we experimentally demonstrate long-range and high-efficiency Förster resonance energy transfer using a plasmonic nanogap formed between a silver nanoparticle and an extended silver film. Our measurements show that the FRET range can be extended to over 200 nm while keeping the FRET efficiency over 0.38, achieving an efficiency enhancement factor of ∼108 with respect to a homogeneous environment. Reducing Purcell enhancements by removing the extended silver film increases the FRET efficiency to 0.55, at the expense of the FRET rate. We support our experimental findings with numerical calculations based on three-dimensional finite difference time-domain calculations and treat the donor and acceptor as classical dipoles. Our enhanced FRET range and efficiency structures provide a powerful strategy to develop novel optoelectronic devices and long-range FRET imaging and sensing systems.

Enhancing Up-Conversion Luminescence Using Dielectric Metasurfaces: Role of the Quality Factor of Resonance at a Pumping Wavelength

Photonic applications of up-conversion luminescence (UCL) suffer from poor external quantum yield owing to a low absorption cross-section of UCL nanoparticles (UCNPs) doped with lanthanide ions. In this regard, plasmonic nanostructures have been proposed for enhancing UCL intensity through strong electromagnetic local-field enhancement; however, their intrinsic ohmic loss opens additional nonradiative decay channels. Herein, we demonstrate that dielectric metasurfaces can overcome this disadvantage. A periodic array of amorphous-silicon nanodisks serves as a metasurface on which a layer of UCNPs is self-assembled. Sharp resonances supported by the metasurface overlap the absorption wavelength (λ = 980 nm) of UCNPs to excite them, resulting in the enhancement of UCL intensity. We further sharpen the resonances through rapid thermal annealing (RTA) of the metasurface, crystallizing silicon to reduce intrinsic optical losses. By optimizing the RTA condition (at 1000 °C for 20 min in N2/H2 (3 vol %) atmosphere), the resonance quality factor improves from 17.2 to 32.9, accompanied by an increase in the enhancement factor of the UCL intensity from 86- to over 600-fold. Moreover, a reduction in the intrinsic optical losses mitigates the UCL thermal quenching under a high excitation density. These findings provide a strategy for increasing light-matter interactions in nanophotonic composite systems and promote UCNP applications.

Plasmonic quenching and enhancement: metal-quantum dot nanohybrids for fluorescence biosensing

Plasmonic metal nanoparticles and semiconductor quantum dots (QDs) are two of the most widely applied nanomaterials for optical biosensing and bioimaging. While their combination for fluorescence quenching via nanosurface energy transfer (NSET) or Förster resonance energy transfer (FRET) offers powerful ways of tuning and amplifying optical signals and is relatively common, metal-QD nanohybrids for plasmon-enhanced fluorescence (PEF) have been much less prevalent. A major reason is the competition between fluorescence quenching and enhancement, which poses important challenges for optimizing distances, orientations, and spectral overlap toward maximum PEF. In this feature article, we discuss the interplay of the different quenching and enhancement mechanisms (a mixed distance dependence of quenching and enhancement - "quenchancement") to better understand the obstacles that must be overcome for the development of metal-QD nanohybrid-based PEF biosensors. The different nanomaterials, their combination within various surface and solution based design concepts, and their structural and photophysical characterization are reviewed and applications toward advanced optical biosensing and bioimaging are presented along with guidelines and future perspectives for sensitive, selective, and versatile bioanalytical research and biomolecular diagnostics with metal-QD nanohybrids.

Förster Resonance Energy Transfer and the Local Optical Density of States in Plasmonic Nanogaps

Förster resonance energy transfer (FRET) is a fundamental phenomenon in photosynthesis and is of increasing importance for the development and enhancement of a wide range of optoelectronic devices, including color-tuning LEDs and lasers, light harvesting, sensing systems, and quantum computing. Despite its importance, fundamental questions remain unanswered on the FRET rate dependency on the local density of optical states (LDOS). In this work, we investigate this directly, both theoretically and experimentally, using 30 nm plasmonic nanogaps formed between a silver nanoparticle and an extended silver film, in which the LDOS can be controlled using the size of the silver nanoparticle. Experimentally, uranin-rhodamine 6G donor-acceptor pairs coupled to such nanogaps yielded FRET rate enhancements of 3.6 times. This, combined with a 5-fold enhancement in the emission rate of the acceptor, resulted in an overall 14-fold enhancement in the acceptor's emission intensity. By tuning the nanoparticle size, we also show that the FRET rate in those systems is linearly dependent on the LDOS, a result which is directly supported by our finite difference time domain (FDTD) calculations. Our results provide a simple but powerful method to control FRET rate via a direct LDOS modification.

Nanoassembled Peptide Biosensors for Rapid Detection of Matrilysin Cancer Biomarker

Early detection of cancer is likely to be one of the most effective means of reducing the cancer mortality rate. Hence, simple and ultra-quick methods for noninvasive detection of early-stage tumors are highly sought-after. In this study, a nanobiosensing platform with a rapid response time of nearly 30 s is introduced for the detection of matrilysin-the salivary gland cancer biomarker-with a limit of detection as low as 30 nm. This sensing platform is based on matrilysin-digestible peptides that bridge gold nanoparticle (AuNPs) cores (≈30-50 nm) and carbon quantum dot (CDs) satellites (≈9 nm). A stepwise synthesis procedure is used for self-assembly of AuNP-peptide-CDs, ensuring their long-term stability. The AuNP-peptide-CDs produce ideal optical signals, with noticeable fluorescence quenching effects. Upon peptide cleavage by matrilysin, CDs leave the surface of AuNPs, resulting in ultra-fast detectable violet and visible fluorescent signals.

Off-Resonance Control and All-Optical Switching: Expanded Dimensions in Nonlinear Optics

The theory of non-resonant optical processes with intrinsic optical nonlinearity, such as harmonic generation, has been widely understood since the advent of the laser. In general, such effects involve multiphoton interactions that change the population of each input optical mode or modes. However, nonlinear effects can also arise through the input of an off-resonant laser beam that itself emerges unchanged. Many such effects have been largely overlooked. Using a quantum electrodynamical framework, this review provides detail on such optically nonlinear mechanisms that allow for a controlled increase or decrease in the intensity of linear absorption and fluorescence and in the efficiency of resonance energy transfer. The rate modifications responsible for these effects were achieved by the simultaneous application of an off-resonant beam with a moderate intensity, acting in a sense as an optical catalyst, conferring a new dimension of optical nonlinearity upon photoactive materials. It is shown that, in certain configurations, these mechanisms provide the basis for all-optical switching, i.e., the control of light-by-light, including an optical transistor scheme. The conclusion outlines other recently proposed all-optical switching systems.

SERS- and luminescence-active Au-Au-UCNP trimers for attomolar detection of two cancer biomarkers

Alpha-fetoprotein (AFP) and mucoprotein1 (mucin-1) are two important disease biomarkers. Self-assembled gold nanoparticles (AuNPs) and upconversion nanoparticle (Au-Au-UCNP) trimers based on aptamers were developed for the ultrasensitive detection of AFP and mucin-1. The Au-Au-UCNP trimers produced ideal optical signals, with prominent Raman enhancement and fluorescence quenching effects. The surface-enhanced Raman scattering (SERS) intensity decreased in the presence of mucin-1 and the luminescence intensity increased in the presence of AFP. A limit of detection (LOD) of 4.1 aM and a wide linear range of 0.01-10 fM for the detection of mucin-1 were obtained with this SERS-encoded sensing system. Using the luminescence-encoded sensing system, a LOD of 0.059 aM and a wide linear range of 1-100 aM for the detection of AFP were obtained. These LODs are the lowest values reported so far. This approach has the advantage of detecting two disease biomarkers simultaneously.

Ag colloids and arrays for plasmonic non-radiative energy transfer from quantum dots to a quantum well

Non-radiative energy transfer (NRET) can be an efficient process of benefit to many applications including photovoltaics, sensors, light emitting diodes and photodetectors. Combining the remarkable optical properties of quantum dots (QDs) with the electrical properties of quantum wells (QWs) allows for the formation of hybrid devices which can utilize NRET as a means of transferring absorbed optical energy from the QDs to the QW. Here we report on plasmon-enhanced NRET from semiconductor nanocrystal QDs to a QW. Ag nanoparticles in the form of colloids and ordered arrays are used to demonstrate plasmon-mediated NRET from QDs to QWs with varying top barrier thicknesses. Plasmon-mediated energy transfer (ET) efficiencies of up to ∼25% are observed with the Ag colloids. The distance dependence of the plasmon-mediated ET is found to follow the same d ^-4 dependence as the direct QD to QW ET. There is also evidence for an increase in the characteristic distance of the interaction, thus indicating that it follows a Förster-like model with the Ag nanoparticle-QD acting as an enhanced donor dipole. Ordered Ag nanoparticle arrays display plasmon-mediated ET efficiencies up to ∼21%. To explore the tunability of the array system, two arrays with different geometries are presented. It is demonstrated that changing the geometry of the array allows a transition from overall quenching of the acceptor QW emission to enhancement, as well as control of the competition between the QD donor quenching and ET rates.