Christopher Hinz

Coupling of Excitons and Discrete Acoustic Phonons in Vibrationally Isolated Quantum Emitters

The photoluminescence emission by mesoscopic condensed matter is ultimately dictated by the fine-structure splitting of the fundamental exciton into optically allowed and dipole-forbidden states. In epitaxially grown semiconductor quantum dots, nonradiative equilibration between the fine-structure levels is mediated by bulk acoustic phonons, resulting in asymmetric spectral broadening of the excitonic luminescence. In isolated colloidal quantum dots, spatial confinement of the vibrational motion is expected to give rise to an interplay between the quantized electronic and phononic degrees of freedom. In most cases, however, zero-dimensional colloidal nanocrystals are strongly coupled to the substrate such that the charge relaxation processes are still effectively governed by the bulk properties. Here we show that encapsulation of single colloidal CdSe/CdS nanocrystals into individual organic polymer shells allows for systematic vibrational decoupling of the semiconductor nanospheres from the surroundings. In contrast to epitaxially grown quantum dots, simultaneous quantization of both electronic and vibrational degrees of freedom results in a series of strong and narrow acoustic phonon sidebands observed in the photoluminescence. Furthermore, an individual analysis of more than 200 compound particles reveals that enhancement or suppression of the radiative properties of the fundamental exciton is controlled by the interaction between fine-structure states via the discrete vibrational modes. For the first time, pronounced resonances in the scattering rate between the fine-structure states are directly observed, in good agreement with a quantum mechanical model. The unambiguous assignment of mediating acoustic modes to the observed scattering resonances complements the experimental findings. Thus, our results form an attractive basis for future studies on subterahertz quantum opto-mechanics and efficient laser cooling at the nanoscale.

Efficient Emission Enhancement of Single CdSe/CdS/PMMA Quantum Dots through Controlled Near-Field Coupling to Plasmonic Bullseye Resonators

A strong increase of spontaneous radiative emission from colloidally synthesized CdSe/CdS/PMMA hybrid particles is achieved when manipulated into plasmonic bullseye resonators with the tip of an atomic force microscope (AFM). This type of antenna provides a broadband resonance, which may be precisely matched to the exciton ground state energy in the inorganic cores. Statistically analyzing the spectral photoluminescence (PL) of a large number of individual coupled and uncoupled CdSe/CdS/PMMA quantum dots, we find an order of magnitude of intensity enhancement due to the Purcell effect. Time-resolved PL shows a commensurate increase of the spontaneous emission rate with radiative lifetimes below 230 ps for the bright exciton transition. The combination of AFM and PL imaging allows for sub-200 nm localization of the particle position inside the plasmonic antenna. This capability unveils a different coupling behavior of dark excitonic states: even stronger PL enhancement occurs at positions with maximum spatial gradient of the nearfield, effectively adding a dipolar component to original quadrupole transitions. The broadband maximization of light-matter interaction provided by our nanoengineered compound systems enables an attractive class of future experiments in ultrafast quantum optics.

Purcell effect and photoluminescence emission enhancement of individual CdSe/CdS/PMMA nano particles coupled to metallic bullseye resonators

Polymer-capped colloidal semiconductor quantum dots [1] offer a robust material system for time-resolved analysis and individual control of ultrafast charge carrier dynamics [2]. This goal necessitates a strong enhancement of light-matter interaction. In this work we demonstrate an efficient coupling of individual CdSe/CdS/PMMA quantum dots (QDs) to plasmonic resonators formed out of multiple concentric rings patterned in a 280-nm thick gold layer. Such a “bullseye” design allows for efficient manipulation of single QDs into the center of the antenna while imposing no polarization selectivity [3]. As a result, we achieve a dramatic increase of the photoluminescence (PL) emission intensity and the radiative decay rates due to the Purcell effect.

Sub-picosecond gain and magnetic field control of few-fermion dynamics in single CdSe/ZnSe quantum dots

II-VI semiconductor quantum dots (QDs) are endowed with large Coulomb correlation energies and high confinement potentials, rendering these systems ideal candidates for future sources of ultrafast quantum light [1, 2]. We employ a two-color pump-probe technique and an external magnetic field to selectively control single-photon gain from individual negatively-charged CdSe/ZnSe QDs on sub-picosecond timescales and with strong spin selectivity.

Confined acoustic phonon modes and exciton-phonon coupling in single CdSe/CdS/PMMA hybrid particles

The role of discrete acoustic phonons in radiative emission from single CdSe/CdS/PMMA nanoparticles is analyzed via photoluminescence and lifetime measurements. Controlled coupling between uncharged exciton fine-structure states dramatically enhances the brightness of the lowest-energy transition.

Femtosecond hole relaxation and biexcitonic transient absorption in single CdSe/ZnSe quantum dots

Femtosecond few-fermion dynamics in single CdSe/ZnSe quantum dots is studied by two-color pump-probe measurements. We observe sub-picosecond hole relaxation and induced absorption into biexciton states when pumping p-p and d-s transitions.

Ultrafast Hole Relaxation and Induced Biexciton Response in Single CdSe/ZnSe Quantum Dots

Two-color pump-probe studies reveal ultrafast dynamics of few-fermions in single epitaxial CdSe/ZnSe quantum dots. Induced biexciton absorption, sub-picosecond hole relaxation and deterministic single photon gain are observed under appropriate pumping conditions.

Photon antibunching from diamond nitrogen-vacancy centers inside a dielectric micropillar cavity

Diamond nanocrystals with single nitrogen-vacancy color centers are incorporated into dielectric micropillar resonators. We observe three-dimensional light confinement and antibunching in the photon emission.