Andrei Piryatinski

Single-exciton optical gain in semiconductor nanocrystals

Nanocrystal quantum dots have favourable light-emitting properties. They show photoluminescence with high quantum yields, and their emission colours depend on the nanocrystal size—owing to the quantum-confinement effect—and are therefore tunable. However, nanocrystals are difficult to use in optical amplification and lasing. Because of an almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron–hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. A complication associated with this multiexcitonic nature of light amplification is fast optical-gain decay induced by non-radiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. Here we demonstrate a practical approach for obtaining optical gain in the single-exciton regime that eliminates the problem of Auger decay. Specifically, we develop core/shell hetero-nanocrystals engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a giant (∼100 meV or greater) transient Stark shift of the absorption spectrum with respect to the luminescence line of singly excited nanocrystals. This effect breaks the exact balance between absorption and stimulated emission, and allows us to demonstrate optical amplification due to single excitons.

Suppressed blinking and auger recombination in near-infrared type-II InP/CdS nanocrystal quantum dots.

Nonblinking excitonic emission from near-infrared and type-II nanocrystal quantum dots (NQDs) is reported for the first time. To realize this unusual degree of stability at the single-dot level, novel InP/CdS core/shell NQDs were synthesized for a range of shell thicknesses (~1-11 monolayers of CdS). Ensemble spectroscopy measurements (photoluminescence peak position and radiative lifetimes) and electronic structure calculations established the transition from type-I to type-II band alignment in these heterostructured NQDs. More significantly, single-NQD studies revealed clear evidence for blinking suppression that was not strongly shell-thickness dependent, while photobleaching and biexciton lifetimes trended explicitly with extent of shelling. Specifically, very long biexciton lifetimes-up to >7 ns-were obtained for the thickest-shell structures, indicating dramatic suppression of nonradiative Auger recombination. This new system demonstrates that electronic structure and shell thickness can be employed together to effect control over key single-dot and ensemble NQD photophysical properties.

Absorption cross sections and Auger recombination lifetimes in inverted core-shell nanocrystals: Implications for lasing performance

We study inverted core-shell nanocrystals (NCs), in which a core of a wide-gap semiconductor (ZnSe) is overcoated with a shell of a narrower gap material (CdSe). Depending on the core radius and the shell thickness, these NCs can exhibit either type-I or type-II behavior. We show that these heterostructures can be used to significantly increase the absorption cross sections and simultaneously decrease the efficiency of Auger recombination compared to monocomponent CdSe NCs emitting at the same wavelength. These properties enhance the lasing performance of inverted core-shell structures and allow, in particular, efficient amplified spontaneous emission in the range of blue colors.

Simulations of two-dimensional femtosecond infrared photon echoes of glycine dipeptide

The multidimensional optical response of the amide I band of glycine dipeptide is calculated using a vibrational‐exciton model, treating each peptide bond as a localized anharmonic vibration. The 2D photon echo signal is obtained by solving the non-linear exciton equations. Comparison of different models of spectral broadening (homogeneous and diagonal and off-diagonal static disorder) shows completely different 2D signals even when the 1D infrared spectra are very similar. The phase of the 2D signal may be used to distinguish between overtone and collective types of two-exciton states. Vanishing of the 2D signal along certain directions can be attributed to the variation of the phase. Copyright ” 2000 John Wiley & Sons, Ltd.

Cross-polarized excitons in carbon nanotubes

Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E22 van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active “bright” excitonic band polarized parallel to the tube axis and several optically “weak” cross-polarized excitons. The rest are optically (near) forbidden “dark” transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E11 transition are more localized along the circumference of a tube, compared with others related to the E12 and E21 cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data.

Elucidation of Two Giants: Challenges to Thick-shell Synthesis in CdSe/ZnSe and ZnSe/CdS Core/Shell Quantum Dots

Core/thick-shell giant quantum dots (gQDs) possessing type II electronic structures exhibit suppressed blinking and diminished nonradiative Auger recombination. We investigate CdSe/ZnSe and ZnSe/CdS as potential new gQDs. We show theoretically and experimentally that both can exhibit partial or complete spatial separation of an excited-state electron-hole pair (i.e., type II behavior). However, we reveal that thick-shell growth is challenged by competing processes: alloying and cation exchange. We demonstrate that these can be largely avoided by choice of shelling conditions (e.g., time, temperature, and QD core identity). The resulting CdSe/ZnSe gQDs exhibit unusual single-QD properties, principally emitting from dim gray states but having high two-exciton (biexciton) emission efficiencies, whereas ZnSe/CdS gQDs show characteristic gQD blinking suppression, though only if shelling is accompanied by partial cation exchange.

Semiclassical simulations of multidimensional Raman echoes

A high-temperature and a weak-nonlinearity (low-temperature) semiclassical expansion are developed for computing two-dimensional vibrational Raman spectroscopies, and applied to an exactly solvable Brownian-oscillator model. The origin of photon echoes is discussed using phase-space-wave-packets. Impulsive and semi-impulsive echoes are shown to satisfy different phase-matching conditions, and are generated in different directions.

An exciton scattering model for carrier multiplication in semiconductor nanocrystals: Theory

The effect of carrier multiplication (CM) in semiconductor nanocrystals is systematically treated by employing an exciton scattering approach. Using projection operators, we reduce the Coulomb coupled multiexciton dynamics to scattering dynamics in the space spanning both single- and biexciton states. We derive a closed set of equations determining the scattering matrix elements. This allows us to interpret CM dynamics as a series of odd-order interband scattering events. Using the time-dependent density matrix formalism, we provide a rigorous description of the CM dynamics induced by a finite-time pump pulse. Within this approach, both processes of single- and biexciton photogeneration and the consequent population relaxation are treated on the same footing. This approach provides a framework for numerical calculations and for comparisons of the quantum efficiencies associated with each process. For applications, the limit of weak interband Coulomb coupling is considered. Finally, we demonstrate that three previously used theoretical models can be recovered as limiting cases of our exciton scattering model.

Vibrational-exciton relaxation probed by three-pulse echoes in polypeptides

Abstract Infrared photon echoes are simulated for an anharmonic vibrational dimer representing a dipeptide in the amide I spectral region by solving the nonlinear exciton equations for the vibrational-exciton model. New relaxation-induced resonances are predicted. Variations of the cross-peak intensities during the middle time interval show signatures of exciton population transfer and coherence dynamics. Enhanced resolution is achieved using phase sensitive detection.

Numerical Study of Carrier Multiplication Pathways in Photoexcited Nanocrystal and Bulk Forms of PbSe

Employing the interband exciton scattering model, we perform a numerical study of the direct photogeneration and population relaxation processes contributing to carrier multiplication (CM) in nanocrystalline and bulk PbSe. We argue that in both cases the impact ionization is the main mechanism of CM. This explains the weak contribution of the direct photogeneration to the total quantum efficiency (QE). An investigation of the size scaling of QE in nanocrystals and a comparison to the bulk limit provide microscopic insight into the experimentally observed trends.