Highly efficient multiple exciton generation in colloidal PbSe and PbS quantum dots

We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.     

Mixing of states in quantum wells for terahertz polariton emitters

Two possible InGaAs/GaAs quantum-well structures ensuring the presence of radiative transitions between the polariton states in a microresonator with a quantum well, which are accompanied by generation of terahertz photons, are discussed in this work. For the first structure, symmetry breakdown that is required for the emission of a terahertz photon is conducted in a quantum well with refractive index gradient profile, which results in mixing of the states of a polariton and a dark exciton. Parameters of the quantum well, in which the energy of the second exciton level corresponds to the upper polariton energy, are determined. A double quantum well with exciton states split due to quantum-mechanical tunneling through a barrier is used in the second structure. Symmetry breakdown, which allows one to mix an exciton with a “dark” exciton, is ensured by adjusting the energy of electron levels in a double quantum well by applying an electric field to the structure. A hole remains localized in one of the wells.     

A Nanophotonic Structure Containing Living Photosynthetic Bacteria

Photosynthetic organisms rely on a series of self‐assembled nanostructures with tuned electronic energy levels in order to transport energy from where it is collected by photon absorption, to reaction centers where the energy is used to drive chemical reactions. In the photosynthetic bacteria Chlorobaculum tepidum, a member of the green sulfur bacteria family, light is absorbed by large antenna complexes called chlorosomes to create an exciton. The exciton is transferred to a protein baseplate attached to the chlorosome, before migrating through the Fenna–Matthews–Olson complex to the reaction center. Here, it is shown that by placing living Chlorobaculum tepidum bacteria within a photonic microcavity, the strong exciton–photon coupling regime between a confined cavity mode and exciton states of the chlorosome can be accessed, whereby a coherent exchange of energy between the bacteria and cavity mode results in the formation of polariton states. The polaritons have energy distinct from that of the exciton which can be tuned by modifying the energy of the optical modes of the microcavity. It is believed that this is the first demonstration of the modification of energy levels within living biological systems using a photonic structure.     

Seven excitons at a cost of one: redefining the limits for conversion efficiency of photons into charge carriers

The performance of photovoltaic and photochemical devices is directly linked to the efficiency with which absorbed photons are converted into electron-hole pairs (excitons). A usual assumption is that one photon produces a single exciton, while the photon energy in the excess of the material's energy gap (the gap that separates the conduction from the valence band) is wasted as heat. Here we experimentally demonstrate that using semiconductor nanocrystals we can reduce this energy loss to a nearly absolute minimum allowed by energy conservation by producing multiple excitons per single photon. Specifically, we generate seven excitons from a photon with an energy of 7.8 energy gaps, which corresponds to only approximately 10% energy loss, while in the normal scenario (one photon produces one exciton) approximately 90% of the photon energy would be dissipated as heat. Such large yields of charge carriers (photon-to-exciton conversion efficiency up to 700%) has the potential to dramatically increase the performance of photovoltaic cells and can greatly advance solar fuel producing technologies.     

Dynamics and the statistics of three-photon cascade emissions from single semiconductor quantum dots with pulse excitation

We analyse the population dynamics and the photon emission statistics of a single semiconductor quantum dot with an exciton–biexciton–triexciton multilevel driven by a pulse field, in which the quantum regression theorem and optical Bloch equations are employed. The populations of the three states are comparable under resonant excitation with the input pulse area around 1.5π. Three second-order cross-correlation functions demonstrate the correlated three-photon emissions at a given order from triexciton, biexciton and exciton states driven by single pulses, while the probability of the bunched three-photon cascaded emissions decreases to the minimum as the input pulse area increases to 2π in every period.     

Spin-resolved Purcell effect in a quantum dot microcavity system

We demonstrate the spin selective coupling of the exciton state with cavity mode in a single quantum dot (QD)-micropillar cavity system. By tuning an external magnetic field, each spin polarized exciton state can be selectively coupled with the cavity mode due to the Zeeman effect. A significant enhancement of spontaneous emission rate of each spin state is achieved, giving rise to a tunable circular polarization degree from -90% to 93%. A four-level rate equation model is developed, and it agrees well with our experimental data. In addition, the coupling between photon mode and each exciton spin state is also achieved by varying temperature, demonstrating the full manipulation over the spin states in the QD-cavity system. Our results pave the way for the realization of future quantum light sources and the quantum information processing applications.     

All-optical switching based on the electromagnetically induced transparency effect of an active photonic crystal microcavity

We report a new all-optical switching in a two-dimensional photonic crystal microcavity made of semiconductor multiple quantum wells and realized based on the electromagnetically induced transparency effect with exciton and two-exciton energy levels. We use the quantum coherence effects to achieve small absorption of the probe field, and the absorption of the probe field can be adjusted by controlling the pump field and decay rate. We turn the control field into pulses of light field so that we can regulate the efficiency of the switch. Through selecting the appropriate control light field intensity, we can obtain a switching efficiency of 85% and a switching time is 10 ps. This result can be used for the development of new types of nanoelectronic devices for realizing the switching process.     

Multiple exciton generation in colloidal silicon nanocrystals

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.     

Large blue shift of the biexciton state in tellurium doped CdSe colloidal quantum dots

The exciton-exciton interaction energy of tellurium doped CdSe colloidal quantum dots is experimentally investigated. The dots exhibit a strong Coulomb repulsion between the two excitons, which results in a huge measured biexciton blue shift of up to 300 meV. Such a strong Coulomb repulsion implies a very narrow hole wave function localized around the defect, which is manifested by a large Stokes shift. Moreover, we show that the biexciton blue shift increases linearly with the Stokes shift. This result is highly relevant for the use of colloidal QDs as optical gain media, where a large biexciton blue shift is required to obtain gain in the single exciton regime.     

Extrinsic and intrinsic effects on the excited-state kinetics of single-walled carbon nanotubes

We characterized the photoluminescence (PL) decay of 15 different, solubilized single-walled carbon nanotubes with tube diameters that ranged from 0.7 to 1.1 nm using time-correlated single photon counting. Each nanotube species was excited resonantly at the second excited state, E2, and PL was detected at the lowest energy exciton emission, E1. In a 10 ns window, the PL decays were described well by a biexponential fitting function with two characteristic time constants, suggesting that at least two kinetically distinct relaxation processes were observed. The dominant decay component increased from 60 to 200 ps with increasing tube diameter, while the lesser component, which contributed up to 8% of the total decay, increased from 200 ps to 4.8 ns. The observation of the second, longer decay time component is examined in terms of two possible models: an extrinsic behavior that implicates sample inhomogeneity and an intrinsic process associated with interconversion between kinetically distinct bright and dark exciton states. A common conclusion from both models is that nonradiative decay controls the PL decay by a process that is diameter dependent.     

Electro-elastic tuning of single particles in individual self-assembled quantum dots

We investigate the effect of uniaxial stress on InGaAs quantum dots in a charge tunable device. Using Coulomb blockade and photoluminescence, we observe that significant tuning of single particle energies (≈-0.22 meV/MPa) leads to variable tuning of exciton energies (+18 to -0.9 μeV/MPa) under tensile stress. Modest tuning of the permanent dipole, Coulomb interaction and fine-structure splitting energies is also measured. We exploit the variable exciton response to tune multiple quantum dots on the same chip into resonance.     

Impact ionization can explain carrier multiplication in PbSe quantum dots

The efficiency of conventional solar cells is limited because the excess energy of absorbed photons converts to heat instead of producing electron-hole pairs. Recently, efficient carrier multiplication has been observed in semiconductor quantum dots. In this process, a single, high-energy photon generates multiple electron-hole pairs. Rather exotic mechanisms have been proposed to explain the efficiency of carrier multiplication in PbSe quantum dots. Using atomistic pseudopotential calculations, we show here that the more conventional impact ionization mechanism, whereby a photogenerated electron-hole pair decays into a biexciton in a process driven by Coulomb interactions between the carriers, can explain both the rate (<1 ps) and the energy threshold ( approximately 2.2 times the band gap) of carrier multiplication, without the need to invoke alternative mechanisms.     

Photoluminescence study of low density InAs quantum clusters grown by molecular beam epitaxy

We report a systematic optical spectroscopy study of low density InAs quantum clusters (QCs) grown by molecular beam epitaxy. The photoluminescence (PL) spectra show emission features of a wetting layer (WL) which contains hybridized quantum well states. The low-energy tail of the QCs' PL profile is actually an ensemble of some sharp lines, originating from the emission of different exciton states (e.g. X, X*, XX*) in a single quasi-three-dimensional (Q3D) cluster as detailed in the micro-PL spectra. The temperature dependence of PL spectra indicates photocarrier distribution and transport in the QC-WL system. Furthermore, this small InAs Q3D cluster is integrated with a distributed Bragg reflector structure, and using optical excitation creates a single photon source with the second-order correlation function of g((2))(0) = 0.31 at 16 K.     

Electron energy distribution in nonideal Coulomb systems: Theory and results of a numerical experiment

The electron energy distribution function is calculated for the Coulomb model with a plateau under the conditions of significant Coulomb nonideality and compared with the results of numerical simulation. The theory and numerical experiment are found to be in satisfactory agreement. The performed analysis revealed that the density of states in the nonideal Coulomb model with a plateau is a continuous and monotonic function of energy and contains no anomalies. At negative energies, it corresponds to the classical bound states and continuously passes into the density of states of free particles through the region of quasibound states, which was theoretically predicted earlier.     

Electric field induced removal of the biexciton binding energy in a single quantum dot

We control the electrostatic environment of a single InAsP quantum dot in an InP nanowire with two contacts and two lateral gates positioned to an individual nanowire. We empty the quantum dot of excess charges and apply an electric field across its radial dimension. A large tuning range for the biexciton binding energy of 3 meV is obtained in a lateral electric field. At finite lateral electric field the exciton and biexciton emission overlap within their optical line width resulting in an enhancement of the observed photoluminescence intensity. The electric field dependence of the exciton and biexciton is compared to theoretical predictions and found to be in good qualitative agreement. This result is promising toward generating entangled photon pairs on demand without the requirement to remove the anisotropic exchange splitting from asymmetric quantum dots.     

Fluorescence spectroscopy of electrochemically self-assembled ZnSe and Mn:ZnSe nanowires

We report room temperature fluorescence spectroscopy (FL) studies of ZnSe and Mn-doped ZnSe nanowires of different diameters (10, 25, 50?nm) produced by an electrochemical self-assembly technique. All samples exhibit increasing blue-shift in the band edge fluorescence with decreasing wire diameter because of quantum confinement. The 10?nm ZnSe nanowires show four distinct emission peaks due to band-to-band recombination, exciton recombination, recombination via surface states and via band gap (trap) states. The exciton binding energy in these nanowires exhibits a giant increase (～10-fold) over the bulk value due to quantum confinement, since the effective wire radius (taking into account side depletion) is smaller than the exciton Bohr radius in bulk ZnSe. The 25 and 50?nm diameter wires show only a single FL peak due to band-to-band electron-hole recombination. In the case of Mn-doped ZnSe nanowires, the band edge luminescence in 10?nm samples is significantly quenched by Mn doping but not the exciton luminescence, which remains relatively unaffected. We observe additional features due to Mn(2+) ions. The spectra also reveal that the emission from Mn(2+) states increases in intensity and is progressively red-shifted with increasing Mn concentration.     

Scanning Tunneling Luminescence of Individual CdSe Nanowires

The local luminescence properties of individual CdSe nanowires composed of segments of zinc blende and wurtzite crystal structures are investigated by low‐temperature scanning tunneling luminescence spectroscopy. Light emission from the wires is achieved by the direct injection of holes and electrons, without the need for coupling to tip‐induced plasmons in the underlying metal substrate. The photon energy is found to increase with decreasing wire diameter due to exciton confinement. The bulk bandgap extrapolated from the energy versus diameter dependence is consistent with photon emission from the zinc blende‐type CdSe sections.     

Two‐Photon Up‐Conversion Photoluminescence Realized through Spatially Extended Gap States in Quasi‐2D Perovskite Films

A new approach to generate a two‐photon up‐conversion photoluminescence (PL) by directly exciting the gap states with continuous‐wave (CW) infrared photoexcitation in solution‐processing quasi‐2D perovskite films [(PEA)₂(MA)₄Pb₅Br₁₆ with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two‐photon up‐conversion PL occurring in quasi‐2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two‐photon up‐conversion PL signal. This confirms that the gap states are indeed responsible for generating the two‐photon up‐conversion PL in quasi‐2D perovskites. Furthermore, mechanical scratching indicates that the different‐n‐value nanoplates are essentially uniformly formed in the quasi‐2D perovskite films toward generating multi‐photon up‐conversion light emission. More importantly, the two‐photon up‐conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi‐photon excitation. Polarization‐dependent up‐conversion PL studies reveal that the gap states experience the orbit–orbit interaction through Coulomb polarization to form spatially extended states toward developing multi‐photon up‐conversion light emission in quasi‐2D perovskites.     

Tunable graphene single electron transistor

We report electronic transport experiments on a graphene single electron transistor. The device consists of a graphene island connected to source and drain electrodes via two narrow graphene constrictions. It is electrostatically tunable by three lateral graphene gates and an additional back gate. The tunneling coupling is a strongly nonmonotonic function of gate voltage indicating the presence of localized states in the barriers. We investigate energy scales for the tunneling gap, the resonances in the constrictions, and for the Coulomb blockade resonances. From Coulomb diamond measurements in different device configurations (i.e., barrier configurations) we extract a charging energy of approximately 3.4 meV and estimate a characteristic energy scale for the constriction resonances of approximately 10 meV.     

Carrier multiplication in InAs nanocrystal quantum dots with an onset defined by the energy conservation limit

Carrier multiplication (CM) is a process in which absorption of a single photon produces not just one but multiple electron-hole pairs (excitons). This effect is a potential enabler of next-generation, high-efficiency photovoltaic and photocatalytic systems. On the basis of energy conservation, the minimal photon energy required to activate CM is two energy gaps (2Eg). Here, we analyze CM onsets for nanocrystal quantum dots (NQDs) based upon combined requirements imposed by optical selection rules and energy conservation and conclude that materials with a significant difference between electron and hole effective masses such as III-V semiconductors should exhibit a CM threshold near the apparent 2Eg limit. Further, we discuss the possibility of achieving sub-2Eg CM thresholds through strong exciton-exciton attraction, which is feasible in NQDs. We report experimental studies of exciton dynamics (Auger recombination, intraband relaxation, radiative recombination, multiexciton generation, and biexciton shift) in InAs NQDs and show that they exhibit a CM threshold near 2Eg.