Spectroscopy Of Single Quantum Dots Research Articles

Research progress of single quantum-dot spectroscopy and exciton dynamics

Colloidal semiconductor quantum dots (QDs) have strong light absorption, continuously adjustable narrowband emission, and high photoluminescence quantum yields, thereby making them promising materials for light-emitting diodes, solar cells, detectors, and lasers. Single-QD photoluminescence spectroscopy can remove the ensemble average to reveal the structure information and exciton dynamics of QD materials at a single-particle level. The study of single-QD spectroscopy can provide guidelines for rationally designing the QDs and giving the mechanism basis for QD-based applications. We can also carry out the research of the interaction between light and single QDs on a nanoscale, and prepare QD-based single-photon sources and entangled photon sources. Here, we review the recent research progress of single-QD photoluminescence spectroscopy and exciton dynamics, mainly including photoluminescence blinking dynamics, and exciton and multi-exciton dynamics of single colloidal CdSe-based QDs and perovskite QDs. Finally, we briefly discuss the possible future development trends of single-QD spectroscopy and exciton dynamics.

Unraveling the Emission Pathways in Copper Indium Sulfide Quantum Dots.

Semiconductor copper indium sulfide quantum dots are emerging as promising alternatives to cadmium- and lead-based chalcogenides in solar cells, luminescent solar concentrators, and deep-tissue bioimaging due to their inherently lower toxicity and outstanding photoluminescence properties. However, the nature of their emission pathways remains a subject of debate. Using low-temperature single quantum dot spectroscopy on core-shell copper indium sulfide nanocrystals, we observe two subpopulations of particles with distinct spectral features. The first class shows sharp resolution-limited emission lines that are attributed to zero-phonon recombination lines of a long-lived band-edge exciton. Such emission results from the perfect passivation of the copper indium sulfide core by the zinc sulfide shell and points to an inversion in the band-edge hole levels. The second class exhibits ultrabroad spectra regardless of the temperature, which is a signature of the extrinsic self-trapping of the hole assisted by defects in imperfectly passivated quantum dots.

Combining in-situ lithography with 3D printed solid immersion lenses for single quantum dot spectroscopy

In the current study, we report on the deterministic fabrication of solid immersion lenses (SILs) on lithographically pre-selected semiconductor quantum dots (QDs). We demonstrate the combination of state-of-the-art low-temperature in-situ photolithography and femtosecond 3D direct laser writing. Several QDs are pre-selected with a localization accuracy of less than 2 nm with low-temperature lithography and three-dimensional laser writing is then used to deterministically fabricate hemispherical lenses on top of the quantum emitter with a submicrometric precision. Due to the printed lenses, the QD light extraction efficiency is enhanced by a factor of 2, the pumping laser is focused more, and the signal-to-noise ratio is increased, leading to an improved localization accuracy of the QD to well below 1 nm. Furthermore, modifications of the QD properties, i.e. strain and variation of internal quantum efficiency induced by the printed lenses, are also reported.

Deconstructing the photon stream from single nanocrystals: from binning to correlation

Prior to the advent of single-molecule fluorescence spectroscopy, many of the fundamental optical properties of colloidal semiconductor nanocrystal quantum dots were obscured by ensemble averaging over their inherent inhomogeneities. Single quantum dot spectroscopy has become a leading technique for the unambiguous determination of the governing excitonic physics of these quantum-confined systems. The analysis and interpretation of the timing and energies of photons emitted from individual nanocrystals have uncovered unexpected and fundamental electronic processes at the nanoscale. We review several different paradigms for deconstructing the photon stream from single nanocrystals, ranging from intensity "binning" techniques to more sophisticated methods based on single-photon counting. In particular, we highlight photon correlation - a powerful developing paradigm in single-nanocrystal studies. The application of photon-correlation techniques to single nanocrystals is changing the study of multiexcitonic recombination dynamics, uncovering the basic processes governing spectral linewidths and spectral diffusion, and enabling the extraction of single-nanocrystal properties directly from an ensemble with high statistical significance. These single-molecule techniques have proven invaluable for understanding the physics of nanocrystals and can provide unique insight into other heterogeneous and dynamical systems.

Near-infrared nano-spectroscopy and emission energy control of semiconductor quantum dots using a phase-change material

We have proposed a method to achieve near-field imaging spectroscopy of single semiconductor quantum dots with high sensitivity by using an optical mask layer of a phase-change material. Sequential formation and elimination of an amorphous aperture allows imaging spectroscopy with high spatial resolution and high collection efficiency. We present numerical simulation and experimental result that show the effectiveness of this technique. Inspired by this optical mask effect, a new approach which can precisely control the emission energy of semiconductor quantum dots has been proposed. This method uses the volume expansion of a phase change material upon amorphization, which allows reversible emission energy tuning of quantum dots. A photoluminescence spectroscopy of single quantum dots and simulation were conducted to demonstrate and further explore the feasibility of this method.

Excitation spectroscopy of single quantum dots at tunable positive, neutral, and negative charge states

We present a comprehensive study of the optical transitions and selection rules of variably charged single self-assembled InAs/GaAs quantum dots. We apply high resolution polarization sensitive photoluminescence excitation spectroscopy to the same quantum dot for three different charge states: neutral and negatively or positively charged by one additional electron or hole. From the detailed analysis of the excitation spectra, a full understanding of the single-carrier energy levels and the interactions between carriers in these levels is extracted for the first time.

Symmetries and the Polarized Optical Spectra of Exciton Complexes in Quantum Dots

A systematic and simple theoretical approach is proposed to analyze true degeneracies and polarized decay patterns of exciton complexes in semiconductor quantum dots. The results provide reliable spectral signatures for efficient symmetry characterization, and predict original features for low C(2v) and high C(3v) symmetries. Excellent agreement with single quantum dot spectroscopy of real pyramidal InGaAs/AlGaAs quantum dots grown along [111] is demonstrated. The high sensitivity of biexciton quantum states to exact high symmetry can be turned into an efficient uninvasive postgrowth selection procedure for quantum entanglement applications.

Room temperature excitation spectroscopy of single quantum dots

We report a single molecule detection scheme to investigate excitation spectra of single emitters at room temperature. We demonstrate the potential of single emitter photoluminescence excitation spectroscopy by recording excitation spectra of single CdSe nanocrystals over a wide spectral range of 100 nm. The spectra exhibit emission intermittency, characteristic of single emitters. We observe large variations in the spectra close to the band edge, which represent the individual heterogeneity of the observed quantum dots. We also find specific excitation wavelengths for which the single quantum dots analyzed show an increased propensity for a transition to a long-lived dark state. We expect that the additional capability of recording excitation spectra at room temperature from single emitters will enable insights into the photophysics of emitters that so far have remained inaccessible.

Two-photon photoluminescence excitation spectroscopy of single quantum dots

We present an experimental and theoretical study of single semiconductor quantum dots excited by two nondegenerate, resonantly tuned, variably polarized lasers. The first laser is tuned to excitonic resonances. Depending on its polarization it photogenerates a coherent single-exciton state. The second laser is tuned to biexciton resonances. By scanning the energy of the second laser for various polarizations of the two lasers, while monitoring the emission from the biexciton and exciton spectral lines, we map the biexciton photoluminescence excitation spectra. The resonance-rich spectra of the second photon absorption are analyzed and fully understood in terms of a many-carrier theoretical model which takes into account the direct and exchange Coulomb interactions between the quantum confined carriers.

Scanning Fabry-Pérot interferometer with largely tuneable free spectral range for high resolution spectroscopy of single quantum dots

We report on the implementation of a scanning Fabry-Pérot interferometer for photoluminescence spectroscopy investigation. We choose a conveniently small reflectivity of the two planar semitransparent mirrors which, in spite of a moderate cavity finesse, ensures a good mechanical stability over a long time. We also exploit the large tuneability of the cavity length (i.e., of the free spectral range) for changing the spectral resolution over two order of magnitude (from ~300 μeV to ~4 μeV in full width at half maximum). Such a characteristic easily allows to scan both sharp and broad luminescence bands. We test our Fabry-Pérot interferometer on sharp photoluminescence lines resulting from excitonic recombination in self-assembled GaAs quantum dots. We demonstrate the ability of our system to resolve linewidth as small as 4 μeV.

Coplanar stripline antenna design for optically detected magnetic resonance on semiconductor quantum dots

We report on the development and testing of a coplanar stripline antenna that is designed for integration in a magneto-photoluminescence experiment to allow coherent control of individual electron spins confined in single self-assembled semiconductor quantum dots. We discuss the design criteria for such a structure which is multi-functional in the sense that it serves not only as microwave delivery but also as electrical top gate and shadow mask for the single quantum dot spectroscopy. We present test measurements on hydrogenated amorphous silicon, demonstrating electrically detected magnetic resonance using the in-plane component of the oscillating magnetic field created by the coplanar stripline antenna necessary due to the particular geometry of the quantum dot spectroscopy. From reference measurements using a commercial electron spin resonance setup in combination with finite element calculations simulating the field distribution in the structure, we obtain a magnetic field of 0.12 mT at the position where the quantum dots would be integrated into the device. The corresponding π-pulse time of ≈0.5 μs meets the requirements set by the high sensitivity optical spin read-out scheme developed for the quantum dot.

Near-field Optical Wavefunction Mapping and Its Application to Single Quantum Dot Spectroscopy

Currently, near-field scanning optical microscopy offers a spatial resolution down to 10−30 nm. High resolution imaging spectroscopy enables us to visualize the exciton wavefunctions confined in semiconductor quantum structures. In this article, the principle of near-field optical wavefunction mapping is described. Then as an application, we performed mapping out of exciton and biexciton wavefunctions in single GaAs quantum dots. Significant displacement of the center of emission profiles was found, in contrast to the usual difference in the emission profiles of an exciton and a biexciton. By conducting a numerical calculation, such a displacement could be reproduced by introducing a shallow potential dip, which causes a significant difference in the penetration of the wavefunctions into the barrier. Precise mapping of exciton and biexciton wavefunctions of quantum-confined structures will provide a good probe for weakly localized states due to local strain and disorder.

Quantum Dot Spectroscopy Using Cavity Quantum Electrodynamics

We show how cavity quantum electrodynamics using a tunable photonic crystal nanocavity in the strong-coupling regime can be used for single quantum dot spectroscopy. From the distinctive avoided crossings observed in the strongly coupled system we can identify the neutral and single positively charged exciton as well as the biexciton transitions. Moreover we are able to investigate the fine structure of those transitions and to identify a novel cavity mediated mixing of bright and dark exciton states, where the hyperfine interactions with lattice nuclei presumably play a key role. These results are enabled by a deterministic coupling scheme which allowed us to achieve unprecedented coupling strengths in excess of 150 microeV.

Light‐Emission Performance of Silicon Nanocrystals Deduced from Single Quantum Dot Spectroscopy

AbstractSpectra of individual silicon nanocrystals within porous Si grains are studied by the wide‐field imaging microspectroscopy and their ON–OFF blinking is detected by the confocal single‐photon‐counting microscopy. Observed spectral and blinking properties comprise all features reported before in differently prepared single Si nanocrystals (SiNCs). Former apparently contradictory results are shown to be due to different experimental conditions. When the effect of dark periods (OFF switching) is removed the common ultimate photoluminescence properties of SiO2 passivated SiNCs are found, namely the quantum efficiency (QE) of about 10–20% up to the pumping rate corresponding to one exciton average excitation per quantum dot. At higher pump rates the QE is slowly decreasing as the 0.7th power of excitation. This is most likely due to Auger recombination which, however, seems to be weakened compared with measurements of nanocrystal assemblies. We conclude that SiNCs may be pumped above one exciton occupancy to yield a higher light emission, being advantageous for applications.

Semiconductor heterostructures for spintronics and quantum information

A selection of optical experiments is presented, demonstrating the utility of semiconductors in two novel areas of research: spintronics and quantum information. First we show examples of spin manipulation in semiconductor quantum wells. The light is used to generate a spin polarization and to detect it. Next we discuss application of optical methods in studies of carrier-induced ferromagnetism in quantum wells. Finally, we present examples of single quantum dot spectroscopy related to perspectives of application of quantum dots in quantum information, and, in particular, the use of photon correlation measurements as a tool to study the quantum dot excitation mechanisms. To cite this article: J.A. Gaj et al., C. R. Physique 8 (2007).

TE- and TM-polarization-resolved spectroscopy on quantum wells under normal incidence

We present TE- and TM-polarization-resolved photocurrent measurements on quantum well pin diodes under normal incidence. Usually, optical experiments performed in such a geometry yield information only about transitions involving in-plane ( p x and p y ) components of the hole wave functions because of the in-plane (TE) polarization of the light. Information on transitions sensitive to the p z components can be obtained by focussing a radially polarized laser beam through a microscope objective with high numerical aperture ( NA = 0.9 ). With our setup, the electrical field vector at the focal tail has a significant component along the optical axis (TM-polarization!) which enables excitation of transitions sensitive to p z components also. Additionally, the existence of a degenerate (azimuthally polarized) optical mode enables switching these p z components on and off easily. Experimental evidence of these features has been achieved by exploiting the selection rules for e–hh and e–lh transitions in a quantum well structure. We present a comparison of our recorded spectra with theoretical predictions obtained from simple geometric optics assumptions. For our quantum wells the polarization effects are small because our measurement averages the intensity distribution of the whole focal plane. We plan to extend our measurements to polarization resolved single quantum dot spectroscopy. By restricting the detection region to the spatial extent of a single dot, one can exploit the almost pure TM-polarization on the optical axis for obtaining high contrast between heavy- and light-hole exciton absorption.

Space and time resolved coherent optical spectroscopy of single quantum dots

A novel experimental technique, combining near-field optics and femtosecond pump–probe spectroscopy, is demonstrated to analyse the coherent nonlinear optical response of single quantum dots on ultrafast time scales. The technique is used to study the effects of strong non-resonant light fields on the optical spectra of single excitons in interface quantum dots. Transient reflectivity spectra show dispersive line shapes reflecting the light-induced shift of the quantum dot resonance. The nonlinear spectra are governed by the phase shift of the coherent quantum dot polarization acquired during the interaction with the light field. The phase shift is measured and ultrafast control of the quantum dot polarization is demonstrated.

Solid immersion lens-enhanced nano-photoluminescence: Principle and applications

We demonstrate a far-field nano-photoluminescence setup based on the combination of a hemispherical solid immersion lens (SIL) with a confocal microscope. The spatial resolution is confirmed to be 0.4 times the wavelength in vacuum in terms of half width at half maximum. The collection efficiency is found to be about five times higher than the same microscope without SIL, which is consistent with our theoretical analysis. We investigate in detail the influence of an air gap between the SIL and the sample surface on the system performance, and prove both experimentally and theoretically the tolerance of this far-field system to an air gap of several micrometers. These features make the present setup an ideal system for spatially resolved spectroscopy of semiconductor nanostructures. In particular, we show two examples of such applications in which the present setup is clearly suitable: Studies of excitonic transport in quantum wells and spectroscopy of single quantum dots with emphasis on polarization dependence and weak-signal detection.

Optical transmission and reflection spectroscopy of single quantum dots

An analytical formulation of the interband optical transmission and reflectivity spectra of a single quantum dot embedded in a semiconductor is presented. We consider the effect of the sample surface as well as other reflecting surfaces on the shape of the spectra near the ground state exciton resonance. The saturation of the transmission and reflectivity spectra due to the quantum optical saturation of the transition at higher light power is presented.

Far-infrared spectroscopy of single quantum dots in high magnetic fields

Excitation spectra of single $\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum dots (QD's) are studied by using a technique of far-infrared single-photon detection. The spectra consist of a single resonance line, the resonance frequency of which is by several percent larger than the cyclotron-resonance frequency but is substantially smaller than the magnetoplasma frequencies of mesa-etched QD's earlier found through the standard far-infrared transmission spectroscopy. The resonance frequency is found to be in substantial agreement with the characteristic frequency of the parabolic bare confinement potential of the QD's, and is ascribed to the excitation of the upper branch of the Kohn-mode collective plasma oscillations.