Optical sectioning by multiexcitonic ladder climbing in colloidal quantum dots

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Depth resolved multiphoton microscopy is performed by collecting the fluorescent emission of two-exciton states in colloidal quantum dots. This process involves two consecutive resonant absorption events, thus requiring unprecedented low excitation energy and peak power.

In nondegenerate FWM (NDFWM) experiments employing spectrally narrower pulses, we can limit the FWM processes to specific ones. Thus, NDFWM measurements will give new insights on the excitonic dynamics. In our previous study using spectrally-resolved DFWM techniques on a self-organized quantum-well material, (C/sub 6/H/sub 13/NH/sub 3/)/sub 2/PbI/sub 4/, it has been shown that two exciton states, including biexciton and weakly interacting two-exciton state, play an important role in FWM processes. However, the signals arising from various two-exciton states overlapped spectrally with each other in DFWM experiments. In this study, we have employed NDFWM techniques on (C/sub 6/H/sub 13/NH/sub 3/)/sub 2/PbI/sub 4/ to observe the contributions of two-exciton states separately.

The engineering and manipulation of delocalized molecular exciton states is a key component for artificial biomimetic light harvesting complexes as well as alternative circuitry platforms based on exciton propagation. Here we examine the consequences of strong electronic coupling in cyanine homodimers on DNA duplex scaffolds. The most closely spaced dyes, attached to positions directly across the double-helix from one another, exhibit pronounced Davydov splitting due to strong electronic coupling. We demonstrate that the DNA scaffold is sufficiently robust to support observation of the transition from the lowest energy (J-like) one-exciton state to the nonlocal two-exciton state, where each cyanine dye is in the excited state. This transition proceeds via sequential photon absorption and persists for the lifetime of the exciton, establishing this as a controlled method for creating two-exciton states. Our observations suggest that DNA-organized dye networks have potential as platforms for molecular logic gates and entangled photon emission based on delocalized two-exciton states.

The authors report on the photoluminescence spectroscopy of a single GaSb∕GaAs type II quantum dot (QD) at 8K. A sharp exciton emission with a linewidth of less than 250μeV was observed. Two-exciton emission at the higher energy side of the exciton emission indicates that the two excitons in a type II QD do not form a bound biexciton. The energies of the exciton and two-exciton states were calculated using an atomic pseudopotential model, which provides a quantitative description of the antibound nature of the two-exciton state in type II QDs.

Two-exciton states with lattice relaxation in one-dimensional semiconductors

We theoretically analyze the optical response from an ultrathin film built up of oriented molecular aggregates, the operating states of which are represented by Frenkel exciton states. A four-level model, involving transitions between the ground, one-exciton and two-exciton states, exciton-exciton annihilation from the two-exciton state as well as relaxation from the annihilation level back to the one-exciton and ground states, is used for describing the film optical response. It is proved that the exciton-exciton annihilation may act not as a destructive but, on the contrary, as a constructive factor tending towards the occurrence of bistability. In particular, the effect of inhomogeneous broadening of the exciton optical transition, preventing the bistable behavior, may be suppressed considerably due to a fast exciton-exciton annihilation.

The energetics and dynamics of multiply excited states in single material colloidal quantum dots have already been shown to exhibit universal trends. Here we attempt to identify similar trends in exciton-exciton interactions within compound colloidal quantum dots. For this end, we thoroughly review previously available data and also present experimental data on several newly synthesized systems, focusing on core/shell nanocrystals with a type-II band alignment. A universal condition for the transition from binding to repulsion of the biexciton (type-I-type-II transition) is established in terms of the change in the exciton radiative lifetime. A scaling rule is also presented for the magnitude of exciton-exciton repulsion. In contrast, we do not identify a clear universal scaling of the non-radiative Auger recombination lifetime of the biexciton state. Finally, a perspective on future applications of engineered multiexcitonic states is presented.

The creation and manipulation of quantum superpositions is a fundamental goal for the development of materials with novel optoelectronic properties. In this letter, we report persistent (~80 fs lifetime) quantum coherence between the 1S and 1P excitonic states in zinc-blende colloidal CdSe quantum dots at room temperature, measured using Two-Dimensional Electronic Spectroscopy. We demonstrate that this quantum coherence manifests as an intradot phenomenon, the frequency of which depends on the size of the dot excited within the ensemble of QDs. We model the lifetime of the coherence and demonstrate that correlated interexcitonic fluctuations preserve relative phase between excitonic states. These observations suggest an avenue for engineering long-lived interexcitonic quantum coherence in colloidal quantum dots.

The establishment of the conditions for the formation of nanostructures with plasmon-exciton interaction based onquantum dots and plasmonic nanoparticles that provide unique nonlinear optical properties is an urgent task. The study demonstrates the formation of plasmon-exciton nanostructures based on hydrophilic colloidal Zn0.5Cd0.5S, Ag2S quantum dots and metal nanoparticles. Transmission electron microscopy and optical absorption and luminescence spectroscopy were used to substantiate the formation of plasmon-exciton hybrid nanostructures. The phase composition of the studied samples was determined by X-ray diffraction. The results obtained using ARLX’TRA diffractometer (Switzerland) indicated a cubic crystal structure (F43m) of synthesised Zn0.5Cd0.5S quantum dots and monoclinic (P21/C) crystal lattice of Ag2S. Transmission electron microscopy revealed that plasmonic nanoparticles are adsorption centres for quantum dots. The average sizes of the studied samples were determined: colloidal Ag2S quantum dots (2.6 nm), Zn0.5Cd0.5S(2.0 nm) and metal nanoparticles: silvernanospheres (10 nm) and gold nanorods (4x25 nm). The transformation of the extinction spectra of the light and the luminescence quenching of quantum dots have been established in mixtures of quantum dots and plasmonic nanoparticles. The nonlinear optical parameters of the studied samples were determined using the Z-scanning method at wavelengths of 355 and 532 nm in the field of nanosecond laser pulses. The conditions for the formation of hybrid nanostructures that provide an increase of the coefficient of nonlinear absorption of laser pulses (355 and 532 nm) up to 9 times with a duration of 10 ns due to the reverse saturable absorption occurring due to cascade two-quantum transitions in the intrinsic and local states of colloidal quantum dots and the suppression of nonlinear refraction, were determined. The observed changes were explained by the manifestation of the Purcell effect on the states of quantum dots in the presence of nanoresonators (gold nanorods and silver nanospheres). The results of these studies create new opportunities for the development of original systems for controlling the intensity of laser radiation, as well as quantum sensors of a new generation

Achieving low-threshold infrared stimulated emission in solution-processed quantum dots is critical to enable real-life applications including photonic integrated circuits (PICs), LIDAR application, and optical telecommunication. However, realization of low threshold infrared gain is fundamentally challenging due to high degeneracy of the first emissive state (e.g., 8-fold) and fast Auger recombination. In this Letter, we demonstrate ultra-low-threshold infrared stimulated emission with an onset of 110 μJ cm-2 employing cascade charge transfer (CT) in Pb-chalcogenide colloidal quantum dot (CQD) solids. In doing so, we investigate this idea in two different architectures including a mixture of multiband gap CQDs and a layer-by-layer (LBL) configuration. Using transient absorption spectroscopy, we show ultrafast cascade CT from large band gap PbS CQD to small band gap PbS/PbSSe core/shell CQDs in LBL (∼2 ps) and mixture (∼9 ps) configurations. These results indicate the feasibility of using cascade CT as an efficient method to reduce the optical gain threshold in CQD solid films.

Colloidal quantum dots (QDs) possess size/shape/surface-tunable optical and electronic properties, making them promising building blocks for optoelectronic applications. However, the fluorescence intermittency, also known as “blinking,” observed in individual QDs is a pervasive phenomenon. The dark state (trion state) in blinking experiences non-radiative recombination processes, such as trap-mediated recombination and Auger–Meitner recombination, which significantly diminish the quantum efficiency of the QDs. Despite efforts to mitigate blinking phenomena through chemical engineering of QDs structures and their environments, blinking continues to impede the application of single QDs, particularly in single photon sources. This study demonstrates that Förster resonance energy transfer (FRET) from green QDs (donor) to individual red QDs (acceptor) can effectively suppress fluorescence intermittency. The findings indicate that FRET facilitates the removal of excess charges from the charged state (dark state, trion state), allowing the QDs to transition from the lower quantum yield trion state to the higher quantum yield single-exciton state (bright state). Our research confirms that FRET can inhibit fluorescence intermittency by deactivating the charged state.

Semiconductor light-emitting diodes (LEDs) enable artificial lighting with an unprecedented level of efficiency. However, “efficiency droop” occurs in LEDs under high power injection density, practically limiting the feasible efficiency levels at high output powers. To address this problem, the concept of laser lighting has been proposed. Also, the current liquid crystal displays (LCDs) suffer the problems of low energy efficiency and small colour gamut, which can be addressed by employing the polarized white backlighting and saturated primary colours. Lasers with colloidal quantum dots (CQDs) as a gain medium can provide solutions to these limitations of current lighting and display technologies. Thus, the target of my Ph.D. thesis work is to develop and demonstrate low threshold colloidal quantum dot lasing with high linear polarization. In Chapter 2 of this thesis, unique properties of CQDs, including size dependent bandgap and discrete energy levels, which result from quantum confinement effect, are discussed. Here by adjusting the size, structure and chemical composition, CQDs that emit at various targeted wavelengths were synthesized. The resulting optical and structural characterizations are also presented. In Chapter 3, a brief review of the optical gain from CQDs is given, and the means that can be adopted to abate Auger recombination are discussed. In experimental part, CQD lasing of red, green, and blue CQDs was demonstrated. Moreover, a FRET-assisted indirect pumping scheme for CQD green lasing with standard pumping source was developed. In Chapter 4 and Chapter 5, highly polarized lasing from CQDs was demonstrated by utilization of the optical cavity effect and adoption of the polarized gain medium, respectively. The CQD DFB laser with mechanically flexible substrate was shown and analysed in Chapter 4. However, for a cylindrical optical cavity of a large diameter, which has low selectivity of TE and TM mode, the polarized gain medium that was fabricated by aligned nanorods was employed for realizing highly polarized Whispering Gallery mode lasing. These results indicate that highly polarized CQD lasing can find important uses in future lighting and displays.

Thanks to their tunability and versatility, the colloidal quantum dots (CQDs) made of II-VI semiconductor compound offer the potential to bridge the "green gap" in conventional semiconductors. However, when the CQDs are pumped to much higher initial excitonic states compared to their bandgap, multiexciton interaction is enhanced, leading to a much higher stimulated emission threshold. Here, to circumvent this drawback, for the first time, we show a fully colloidal gain in green enabled by a partially indirect pumping approach assisted by Förster resonance energy transfer process. By introducing the blue CQDs as exciton donors, the lasing threshold of the green CQDs, is reduced dramatically. The blue CQDs thus serve as an energy-transferring buffer medium to reduce excitation energy from pumping photons in a controlled way by injecting photoinduced excitons into green CQDs. Our newly developed colloidal pumping scheme could enable efficient CQD lasers of full visible colors by a single pump source and cascaded exciton transfer. This would potentially pave the way for an efficient multicolor laser for lighting and display applications.

All-optical approaches to change the wavelength of a data signal are considered more energy- and cost-effective than current wavelength conversion schemes that rely on back and forth switching between the electrical and optical domains. However, the lack of cost-effective materials with sufficiently adequate optoelectronic properties hampers the development of this so-called all-optical wavelength conversion. Here, we show that the interplay between intraband and band gap absorption in colloidal quantum dots leads to a very strong and ultrafast modulation of the light absorption after photoexcitation in which slow components linked to exciton recombination are eliminated. This approach enables all-optical wavelength conversion at rates matching state-of-the-art convertors in speed, yet with cost-effective solution-processable materials. Moreover, the stronger light-matter interaction allows for implementation in small-footprint devices with low switching energies. Being a generic property, the demonstrated effect opens a pathway toward low-power integrated photonics based on colloidal quantum dots as the enabling material.