Bimodal exciton-plasmon light sources controlled by local charge carrier injection

Electrical charges can generate photon emission in nanoscale quantum systems by two independent mechanisms. First, radiative recombination of pairs of oppositely charged carriers generates sharp excitonic lines. Second, coupling between currents and collective charge oscillations results in broad plasmonic bands. Both luminescence modes can be simultaneously generated upon charge carrier injection into thin C-60 crystallites placed in the plasmonic nanocavity of a scanning tunneling microscope (STM). Using the sharp tip of the STM as a subnanometer-precise local electrode, we show that the two types of electroluminescence are induced by two separate charge transport channels. Holes injected into the valence band promote exciton generation, whereas electrons extracted from the conduction band cause plasmonic luminescence. The different dynamics of the two mechanisms permit controlling their relative contribution in the combined bimodal emission. Exciton recombination prevails for low charge injection rates, whereas plasmon decay outshines for high tunneling currents. The continuous transition between both regimes is described by a rate model characterizing emission dynamics on the nanoscale. Our work provides the basis for developing blended exciton-plasmon light sources with advanced functionalities.

Dynamics of quantum dot lasers

Pablo Moreno

The exceptional performance of self-assembled Quantum Dot (QD) materials renders them extremely appealing for their use as optical communications devices. As lasers, they feature reduced and temperature independent threshold current and proper emission wavelength at the fiber telecommunication windows. These characteristics, together with the low linewidth enhancement factor and broad spectrum, make QD materials extremely attractive for application as light emitters or amplifiers. There exist, nevertheless, several unclear issues which prevent QDs from conquering the new generation of optoelectronic devices. Their differential efficiency is lower than expected. The output power of QD lasers is lower than that of their quantum well counterpart. Still, it is their dynamics which has incited the majority of studies. The modulation bandwidth of these devices seems to be limited by the relaxation of carriers from the upper energetic layers to the low levels within the dot. Besides, the electron-hole interaction is widely unknown, the extent of the electron-hole Coulombic attraction is not yet established. Throughout this thesis I present a theoretical and experimental study of the gain and phase dynamics of quantum dot lasers. I explain the appearance of different decay times observed in pump and probe experiments in QD amplifiers as a result of the different electron and hole relaxation times, by means of an electron-hole rate-equation model. The ultrafast hole relaxation first leads to an ultrafast recovery of the gain, which is then followed by electron relaxation and, on the nanosecond timescale, radiative and non-radiative recombinations. The phase dynamics is slower and is affected by thermal redistribution of carriers within the dot. Our results corroborate with spectral measurements of the dephasing and gain in QD amplifiers. Additionally, our work is compared with existing pump and probe results. Exploiting the capacity of QD lasers to emit at two different wavelengths corresponding to the ground state (GS) and excited state (ES), I present a theoretical study of the QD dynamics, based on a linearization of the QD rate-equations. The results predict the existence of single oscillation frequency of GS and ES, meaning that both states are highly coupled. In order to verify our theory, we perform two kinds of experiments. By modulating these lasers at high frequency, we measure separately the dynamics of GS and ES. However, in contradiction to our theory, two different modulation frequencies are found. Additional temporally-resolved measurements of the laser dynamics reveal a surprising effect. By injecting a sub-bandgap pump in an InAs/InGaAs QD laser, the emitted photons are depleted. Through additional transmission and photocurrent measurements, we relate this observation with carrier photoexcitation, which was so far only theoretically addressed. The role of carrier photoexcitation in our experimental laser dynamics is further supported by a rate-equation model. Impelled by this finding, we study the effect of carrier photoexcitation in the static and dynamic characteristics. We find that carrier photoexcitation reduces the efficiency of QD lasers, which is one of the major QD handicaps, and depletes the GS lasing after the ES threshold, as observed experimentally. Moreover, by adding carrier photoexcitation to our linearization of the rate-equations, we find that the theory predicts the appearance of two lasing resonance frequencies, in agreement with our previous experimental results. Additionally, we deal with the improvement of carrier relaxation. In tunnel injection devices, carriers are given an additional path towards the ground state of the dot by growing a quantum well layer close to the QD active plane. Through the quantum-mechanical tunneling effect, carriers relax from the nearby quantum well layer to the QDs, which speeds up relaxation. We aim at the increase of the modulation bandwidth while keeping the good performances quantum dot lasers have exhibited, such as low and temperature insensitive threshold current and proper emission wavelength. In the final part of this work, we present dynamical measurements of 1.5 µm InAs/InP tunnel injection and non-tunnel injection QD lasers, which display remarkable static characteristics. After proving with static measurements that tunnel injection is actually taking place in these structures, we show several dynamic measurements. Pump and probe measurements on QD devices show that the tunnel injection samples exhibit a slightly faster relaxation time than the non-tunnel injection samples used as reference, meaning that relaxation time is improved with tunnel injection. However, by probing the device with an ultrafast pump no improvement of the dynamic characteristics is observed. These results confirm that the laser dynamic properties of InP QD lasers, both standard and tunnel-injection designs, are actually not limited by relaxation of carriers. We point towards the size distribution of these quantum dash-like structures as the limiting factor of the modulation frequency.

EPFL2008

Dynamics of nanoscale systems probed by time-resolved STM-induced luminescence

Anna Maria Roslawska

The efficiency and peculiarities of processes such as surface adsorption or electron-to-photon energy conversion in organic and inorganic structures are determined by the dynamics at the scale of individual molecules, atoms and charges. The timescales of such effects reach down to the ps regime, which can be routinely probed using ultrafast optics methods. However, these far-field techniques suffer from ensemble-averaging that obscures the information encoded at the atomic level. These limits can be overcome by combining scanning tunnelling microscopy (STM) with time-resolved light detection so that pm and ps scales can be reached simultaneously. STM-induced luminescence (STML) methodologies are employed in this thesis to explore the nanoscale dynamics of different systems: light emitting defects in C60 thin films, H2 molecules weakly physisorbed on Au(111), metal-metal tunnel junctions and single atom contacts. First, we study the dynamics associated with the radiative decay of single excitons at defect-related emission centres localized in the C60 molecular film. Such emission centres serve as the smallest possible model of an organic light emitting diode where a single electron and hole recombine producing a photon. Using time-resolved STML (TR-STML) excited by voltage pulses, we follow the formation of an individual exciton and map this process in real space with ns resolution. Besides the excitonic emission, on C60 thin films we observe luminescence due to the radiative decay of plasmons. We study the interplay of these two processes and create a nanoscale bimodal source whose emission can be dynamically tuned by controlling the charge injection rate. A kinetic model is employed to thoroughly describe the charge and exciton dynamics in this system. The developed kinetic model can be also used to analyse the motion of hydrogen molecules adsorbed on an Au(111) crystal. The weak interaction with the metal and the continuous charge injection by the tip leads to recurrent excursions of the molecule inside and outside from the tunnelling junction, which chop the plasmonic luminescence intensity. These dynamical changes in the light intensity can be followed by correlation spectroscopy and allow extraction of the molecule's residence and excursion times. In the next step, we probe the dynamics of a generic metal-metal tunnel junction. Interestingly, when a high bias is applied, the emission from the junction exhibits photon bunching on a time scale below 50 ps as evidenced by Hanbury Brown-Twiss interferometry. Such a process is a result of a distribution of the energy of one electron between two photons, which may possibly be entangled. The opposite process may occur as well, under conditions where two electrons interact supplying energy to one photon. This overbias emission is a feature of single atom contacts and is used here to probe the fluctuations of plasmonic enhancement driven by the motion of single atoms in the vicinity of such a contact.

EPFL2019

Single molecule fluorescence and phosphorescence studies using a scanning tunneling microscope

Abhishek Grewal

This thesis investigates novel single-molecule luminescence phenomena at their inherent, sub-molecular length scale. The microscopic understanding of luminescence processes will be crucial for the continued improvement of organic optoelectronic and semiconductor devices.In this work, a scanning tunneling microscope (STM) is combined with optical spectroscopy and Hanbury Brown and Twiss interferometry to image topography, electronically characterize, and analyze the vibrational mode excitations and emission statistics, all at the atomic scale. Moreover, the precise STM tip positioning allows tuning the coupling between the molecular exciton and the tip-induced plasmon with sub-molecular spatial control, which is key to the decay rate enhancement of molecular phosphors.A prototypical platinum phthalocyanine (PtPc) molecule, decoupled from a metal substrate by a thin film of NaCl, provides a model platform for examining STM-induced bipolar fluorescence, phosphorescence, and energy upconversion electroluminescence (UCEL). First, an STM investigation of only the ionic insulator NaCl demonstrates that both valence and conduction bands are of anionic Cl- character. This is relevant for the coupling between the molecule and the metal substrate. Theoretical considerations reveal a dominant contribution from low-lying molecular orbitals under certain conditions.Voltage and polarity of fluorescence onsets can be controlled using different metal substrates because their different work functions tune the energy alignment of the molecular orbitals with respect to the substrate Fermi level. For PtPc atop an Au(111) substrate, fluorescence is observed at both voltage polarities, even under tunneling conditions where UCEL occurs. This remarkable behavior allows the testing of different emission mechanisms and reveals new general guidelines for designing future optoelectronic devices. An analysis of photon emission statistics from a single molecule under UCEL conditions refutes previously proposed excitation mechanisms. It guides theory towards a novel mechanism, briefly introduced in the thesis.PtPc atop NaCl on Ag(111) exhibits fluorescence, and in this thesis, its phosphorescence is also observed by local excitation in the STM. Electronic characterization reveals a simultaneous voltage onset of both types of emission. Phosphorescence is also detected using tip-enhanced resonant photoexcitation of the singlet state. Since direct optical excitation of the triplet state is forbidden, this observation is the first demonstration of intersystem crossing in a tip-controlled single-molecule experiment. Exciton-plasmon coupling can be varied by fine positioning the tip in proximity to the molecule. This allows tuning the triplet emission intensity through control of the singlet lifetime. These findings are directly transferrable to highly topical phosphorescent OLEDs that employ plasmonic enhancement.The phenomenon of plasmonic antibunching is discovered for electron tunneling from a few monolayer thick C60 films to the STM tip. The layer thickness and the applied voltage control the sequential tunneling of charges by a Coulomb blockade mechanism. This mechanism eventually causes the emission of one photon at a time. Such a plasmonic source allows for a higher single-photon emission rate than an individual molecule. The design principles for other semiconducting films are discussed, suggesting a quest for similar nanostructured single-photon emitters.

EPFL2022