P. Andreakou

Single-mode tunable laser emission in the single-exciton regime from colloidal nanocrystals

Whispering-gallery-mode resonators have been extensively used in conjunction with different materials for the development of a variety of photonic devices. Among the latter, hybrid structures, consisting of dielectric microspheres and colloidal core/shell semiconductor nanocrystals as gain media, have attracted interest for the development of microlasers and studies of cavity quantum electrodynamic effects. Here we demonstrate single-exciton, single-mode, spectrally tuned lasing from ensembles of optical antenna-designed, colloidal core/shell CdSe/CdS quantum rods deposited on silica microspheres. We obtain single-exciton emission by capitalizing on the band structure of the specific core/shell architecture that strongly localizes holes in the core, and the two-dimensional quantum confinement of electrons across the elongated shell. This creates a type-II conduction band alignment driven by coulombic repulsion that eliminates non-radiative multi-exciton Auger recombination processes, thereby inducing a large exciton–bi-exciton energy shift. Their ultra-low thresholds and single-mode, single-exciton emission make these hybrid lasers appealing for various applications, including quantum information processing.

Resonance energy transfer from PbS colloidal quantum dots to bulk silicon: the road to hybrid photovoltaics

Semiconductor Quantum Dots (QDs) are promising materials for photovoltaic applications because they can be engineered to absorb light from visible to near infrared and single absorbed photons can generate multiple excitons. However, these materials suffer from low carrier mobility, which severely limits the prospects of efficient charge extraction and carrier transport. We take advantage of the optical properties of QDs and overcome their drawback by using a hybrid photovoltaic device. This photovoltaic configuration exploits the absorption of solar photons in the QDs and the transfer of excitons from the QDs to a silicon p-n junction. We study the Resonance Energy Transfer (RET) mechanism to inject excitons from the QDs into the depletion layer of a silicon p-n junction. Lead sulphide (PbS) nanocrystals are deposited onto the silicon substrate and the efficiency of Resonance Energy Transfer (RET) from the PbS nanoparticles to bulk silicon is investigated. We study the efficiency of this transfer channel between the PbS nanocrystals and silicon by varying their separation distance. These results demonstrate RET from colloidal quantum dots to bulk silicon. Temperature measurements are also presented and show that the RET efficiency is as high as 44% at room temperature. Such a hybrid photovoltaic device makes a potentially inexpensive scheme for achieving highefficiency and low-cost solar-cell platforms.

Time-resolved spectroscopic study of resonant energy transfer between lead-sulphide quantum dots and bulk silicon

The brightness, large absorption cross-section and flexibility of colloidal nanocrystal quantum-dots (QDs) make these materials promising candidates for light harvesting applications. The difficulty of efficiently extracting photogenerated carriers from the QDs however drastically limits the power conversion efficiency of NQD solar cells. A possible way to circumvent these issues is to engineer hybrid devices that utilize alternative energy transfer schemes to effectively separate light harvesting and charge extraction in different materials. In this context, hybrid bulk semiconductor/QDs devices coupled through near-field resonant energy transfer offer a promising route towards low cost ultra-thin photovoltaics. In this work, we demonstrate non-radiative resonance energy transfer between lead sulphide (PbS) QDs and bulk silicon using time-resolved spectroscopy.

Hybrid lasers based on CdSe/CdS core/shell colloidal quantum rods on silica microspheres

Single-mode lasing at ~628 nm above an absorbed pump power threshold of 67.5 μW, tunable within a 2.1-nm range (30% of the free-spectral-range) was obtained from colloidal CdSe/CdS core/shell nanorods on whispering-gallery-mode silica microspheres.

Colloidal nanoprobes for the optical detection of ethanol

Gold nanoparticles (GPs) are employed as versatile labels for the optical detection of chemical or bio-recognition events by monitoring local and surrounding Refractive Index (RI)[1,2]. The change of RI affects GPs localized surface plasmon resonance spectra (LSPR), depending on the size and shape of the individual particles [3].

Tunable near field dipole-dipole coupling between CdSe/CdS nanorods

Colloidal semiconductor quantum rods (nanorods) exhibit size tunable absorption and emission spectra with a high oscillator strength in combination with the emission of linearly polarised light [1]. The emission of light polarised along the long axis of the nanorod has been observed on the single particle level, where an individual nanorod can serve as a single photon source [2]. The optical properties of nanorods are desirable for application in optical devices such as white LEDs, solar cells or as fluorescent markers in the life sciences. The ability to predict the polarisation of the light emitted from a nanorod favours them for the use in polarisation selective microscopy.