Session K9: Superlattices & Nanostructures (Wires, Dots, etc): Optical Phenomenona

Spin polarization and spatial texture in quantum dots

Spins in semiconductor quantum dots (QD) are promising qubits. Zeeman-split states form two-level systems with pseudo spin 1/2. Rotations of these qubits typically use magnetic fields B. However, this pseudo spin is not the physical spin of the state. Due to confinement, strain and strong spin-orbit coupling, the physical spin can be strongly mixed and spatially varying in the QD. We use atomistic tight-binding theory for strained InAs/GaAs and strain-free GaAs/AlAs QDs to investigate the influence of strain, QD geometry and magnetic field orientation on spin polarization and spatial texturing. For electrons, with weak spin-orbit coupling, spin is almost fully polarized and nearly aligned with B. For holes, with strong spin-orbit coupling, there can be incomplete spin polarization and spin locked to the QD axis, rather than B, even for B far off the QD axis. Spatial spin texturing occurs on the atomic scale with spin flipping between nearby atoms. In the Voigt configuration, hole spin remains nearly locked locally to the QD axis, but with opposite orientation on opposite sides of the dot, creating a spin-dipole. The influence of this spin polarization and spatial texturing on spin manipulation, exchange interaction and decoherence will be discussed.   

Photon Statistics of Quantum Dot Resonance Fluorescence under the Influence of an Above Band-Gap Laser

We study the statistical behavior of resonance fluorescence from self-assembled InAs quantum dots (QDs) as a function of the density of free charge carriers introduced by an above band-gap laser. Second-order correlation measurements show bunching behavior that changes with above-band laser power and is absent in purely above-band excited emission. Resonant photoluminescence excitation spectra indicate that the QD experiences discrete spectral shifts and continuous drift due to changes in the local charge environment. These spectral changes, combined with tunneling of charges from the environment to the QD, provide an explanation of the bunching observed in the correlations.   

Hole spins in quantum dot molecules: novel tuning by GaBiAs barriers

Hole spins in semiconductor quantum dots (QD) are promising qubits. Tunneling in vertical quantum dot molecules (QDM) provides additional freedom to use fields to manipulate hole g-factors and induce spin mixing. Interdot barriers made from GaBiAs should provide novel opportunities to further engineer these hole spin properties, because heavy- and light-holes in GaBiAs are modified by the Bi concentration without affecting conduction electrons or split off bands. For low Bi concentrations, GaBiAs provides a lower barrier for hole tunneling, allowing hole tunneling more comparable to electron tunneling and enhancing opportunities for g-factor modification. We use atomistic tight-binding theory for InAs QDMs with GaBiAs in the interdot barrier to assess the utility of this barrier material. We model the alloy barrier regions both with the virtual crystal approximation and with random realizations of atomic configurations for the alloy region in the barrier. Results are presented for electron and hole energies in QDMs with GaBiAs barriers as a function of applied electric and magnetic fields. These results allow us to quantify g-factor modification and hole-spin mixing in asymmetric structures to show how different GaBiAs barrier configurations modify hole spin physics in QDMs.   

Tunable emission from InAs quantum dots gated with graphene

We demonstrate Stark shifted photo-luminescence from InAs quantum dots (QD) using an n-i-Schottky diode where graphene has been used as the Schottky barrier material. This hybrid photonic device is motivated by the need for tunable single photon sources with high flux and storage capabilities. Photonic crystal nanocavities decorated with a single QD provide a rich environment for coupling spins and photons, in addition to accessing cavity quantum electrodynamic physics. Methods currently used for electrically tuning the QD inside the cavity suffer from a loss of the cavity quality factor, or high leakage currents in the diode which impacts the spin-photon coupling of the device. Our measurements are a first step towards using a graphene flake to electrically tune the emission of a strongly coupled QD-cavity system.   

Electronic Structure and Optical Properties of Spectrally Uniform Nanotemplate-Directed InGaAs/GaAs Quantum Dots in Regular Arrays.

Spectrally uniform single photon emitters in spatially regular arrays are highly sought for their potential use in quantum information processing systems. We have utilized nanotemplate-directed on-site growth of quantum dots (NTQDs) approach[1] that exploits engineered surface stress to provide preferred direction for adatom migration during growth to synthesize regular arrays of single InGaAs/GaAs QDs of controlled flat-top pyramidal shape residing on GaAs(001) nanomesa arrays[2].The GaAs/In0.5Ga0.5As/GaAs NTQDs reported here are spectrally uniform within 5nm over 1000um2, order of magnitude better than island and colloidal quantum dots. Photoluminescence (PL) and PL excitation studies of individual NTQDs shows that first excited electron state and dense hole states are, respectively, \textasciitilde 40meV and \textasciitilde 10meV higher than ground state. Electrons escape out of QDs through thermally activated tunneling to first excited electron state, which is also manifest in the temperature-dependent behavior of the QD PL decay time. Suitability of such arrays of NTQDs as single photon emitter array will be discussed. [1] A. Konkar, et. al., Jour. Cryst. Growth, 150, 311 (1995) [2] J. Zhang et. al., Jour. Vac. Sc. Tech. B32, 02C106 (2014)   

Localized magnetoplasmons in quantum dots: Scrutinizing the eligibility of FIR, Raman, and electron energy-loss spectroscopies

We report on a one-component, quasi-zero dimensional, quantum plasma exposed to a parabolic potential and an applied magnetic field in the symmetric gauge. If the size of such a system as can be realized in the semiconducting quantum dots is on the order of the de-Broglie wavelength, the electronic and optical properties become highly tunable. Then the quantum size effects challenge the observation of many-particle phenomena such as the magneto-optical absorption, Raman intensity, and electron-energy-loss spectrum. An exact analytical solution of the problem leads us to infer that these many-particle phenomena are, in fact, dictated by the generalized Kohn's theorem in the long-wavelength limit. Maneuvering the confinement and/or the magnetic field furnishes the resonance energy capable of being explored with the FIR, Raman, or electron-energy-loss spectroscopy. This implies that either of these probes should be competent in observing the localized magnetoplasmons in the system. A deeper insight into the physics of quantum dots is paving the way for their implementation in such diverse fields as quantum computing and medical imaging$^{1}$. 1. M.S. Kushwaha, Unpublished.   

Influence of Indium Segregation on InGaN/GaN QD Band Alignment

InGaN/GaN QD systems are promising for optoelectronic devices, such as photovoltaics, light emitters, and lasers due to their high mobility, high absorption coefficient, and direct wide bandgap. However, indium segregation within InGaN quantum structures can lead to inefficiencies in device performance and has not been investigated in InGaN/GaN QD systems. Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we have investigated the influence of indium nanostructure on the band structure of single or multi-layered InGaN/GaN QDs. We observe a mixture of indium mounds and QDs in the single layered InGaN/GaN QD system, where local STS measurements suggest a gradient in indium concentration across the indium mound. Furthermore, STM imaging suggests a higher density of InGaN/GaN QDs for multi-layered InGaN/GaN QDs compared to that of a single layered InGaN/GaN QDs, where STS measurements suggest indium clustering within InGaN QDs. We discuss the comparison of the band structure of InGaN/GaN mounds vs. QD systems.   

Droplet heteroepitaxy of zinc-blende vs. wurtzite GaN quantum dots

We have developed a GaN droplet heteroepitaxy process based upon plasma-assisted molecular-beam epitaxy. Using various surface treatments and Ga deposition parameters, we have demonstrated polycrystalline, zinc-blende (ZB), and wurtzite (WZ) GaN quantum dots (QDs) on Si(001), r-Al$_{\mathrm{2}}$O$_{\mathrm{3}}$, Si(111), and c-GaN substrates. For the polar substrates (i.e. Si(111) and c-GaN), high-resolution transmission electron microscopy and coherent Bragg rod analysis reveals the formation of coherent WZ GaN QDs with nitridation-temperature-dependent sizes and densities. For the non-polar substrates (i.e. Si(001) and r-Al$_{\mathrm{2}}$O$_{\mathrm{3}})$, QDs with strong near-band photoluminescence emission are observed and ZB GaN QD growth on Si(001) is demonstrated for the first time.   

Hydrothermal Synthesis and Photoluminescence of Boron Nitride Quantum Dots

Boron nitride quantum dots (BNQDs), as a new member of heavy metal-free quantum dots, have attracted great interest owing to its unique structure as well as fascinating physical/chemical properties. However, it is still a challenge to controllably synthesize high quality BNQDs with high quantum yield (QY), uniform size and strong luminescence. Here we present a facile and effective approach to controllablly fabricate BNQDs by snoication-solvothermal technique. Encouragingly, the as-prepared BNQDs possess strong blue luminescence with high QY of up to 19.5{\%}, which can be attributed to the synergic effect of size, surface chemistry and edge defects. In addition, the size of the BNQDs could be controlled with a narrow size distribution of 1.32 nm and the smallest average size achieved is 2.62 nm with an average thickness of \textasciitilde 3 atomic layers. Furthermore, the as-prepared BNQDs are non-toxic to cells and show nanosecond-scaled lifetimes and little photobleaching effect. Therefore, it is believed that BNQDs are promising as one of the novel heavy metal-free QDs for multi-purpose applications in a range of fields. Moreover, this synthesis concept is expected to open a new window to controllably prepare other heavy metal-free QDs, as well as to understand their luminescence mechanism.   

Insight into Factors Affecting the Presence, Degree, and Temporal Stability of Fluorescence Intensification on ZnO Nanorod Ends

We present a combined experimental and simulation study identifying the key physical and optical parameters affecting the presence and degree of fluorescence intensification measured on zinc oxide nanorod (ZnO NR) ends. We aim to provide an insight into the unique optical phenomenon of fluorescence intensification on NR ends (\textit{FINE}) through experimental and simulation approaches and to elucidate the key factors affecting the occurrence, degree, and temporal stability of \textit{FINE}. Specifically, we examined the effect of the length, width, and growth orientation of single ZnO NRs on the NR-enhanced biomolecular emission profile after decorating the NR surfaces with different amounts and types of fluorophore-coupled protein molecules. We quantitatively and qualitatively profiled the biomolecular fluorescence signal from individual ZnO NRs as a function of both position along the NR long axis and time. Additionally, we employed finite-difference time-domain methods to examine both near- and far-field emission characteristics when considering various scenarios of fluorophore locations, polarizations, spectroscopic characteristics, and NR dimensions. Our efforts may provide a deeper insight into the unique optical phenomenon of \textit{FINE} and further be beneficial to highly miniaturized biodetection favoring the use of single ZnO NRs.   

Scattering Intensity and Directionality Probed Along Individual Semiconducting Oxide Nanorods with Precisely Controlled Light Polarization and Nanorod Orientation

We elucidate the light-matter interaction properties of individual semiconducting oxide nanorods (NRs) with a monochromatic beam of linearly polarized light that scatters elastically from the NRs by performing forward scattering in a dark-field setting. Specifically, individual NRs of ZnO, SnO$_{2}$, ITO, and ZTO are probed. We precisely control the electric field vector of the incident light and the NR orientation within the plane of light interaction, and spatially resolve the scattering response from different interaction points along the NR long axis. We then discern the effects of light polarization, analyzer angle, and NR orientation on the intensity and directionality of the optical responses both qualitatively and quantitatively along the length of the single NRs. We identify distinctive, forward scattering profiles from individual NRs subject to various incident light polarizations and NR orientations. Fundamental light interaction behavior of the NRs is likely to govern their functional outcomes in photonics, optoelectronics, and sensor devices. Hence, our efforts providing much needed insight into unique optical responses from individual 1D semiconducting oxide nanomaterials can be highly beneficial in developing next-generation optoelectronic systems and optical biodetectors with improved device efficiency and sensitivity.   

Exploring the Nature of Exciton Localization in Quasi One-Dimensional GaAs/AlGaAs Quantum Well Tube Nanowires

We explore the nature of exciton localization in single GaAs/AlGaAs nanowire quantum well tube (QWT) devices using photocurrent (PC) spectroscopy combined with simultaneous photoluminescence (PL) and photoluminescence excitation (PLE) measurements. Excitons confined to GaAs quantum well tubes of 8 and 4 nm widths embedded into an AlGaAs barrier are seen to ionize at high bias. Spectroscopic signatures of the ground and excited states confined to the QWT seen in PL, PLE and PC data are consistent with simple numerical calculations. The demonstration of good electrical contact with the QWTs enables the study of Stark effect shifts in the sharp emission lines of excitons localized to quantum dot-like states within the QWT. Atomic resolution cross-sectional TEM measurements, an analysis of the temperature dependence of PL and time-resolved PL as well as the quantum confined Stark effect of these dots provide insights into the nature of the exciton localization in these nanostructures.   

Surface sensitivity to dielectric environment of optical and magneto-optical properties in magnetoplasmonic nanodisks

The optical, ellipsometric and magneto-optical surface sensitivity to dielectric environment of magnetoplasmonic nanodisks is experimentally studied. Here the shift of the corresponding spectral structures as a function of the thickness of a coating SiO$_{2}$ layer is characterized. Our results reveal that the so called pseudo-Brewster Angle, easily identified in the ellipsometric phase ($\Delta$) spectrum, is up to four times more sensitive than the conventional features used in Surface Plasmon Resonance (SPR) based sensors. These results highlight the need of investigating the factual implementation of this technique to develop improved ellipsometric-phase based transducers for bio-chemical sensing purposes.   

Size and Morphology Dependent Raman Scattering

Through thermal evaporation, ripple-like CdS nanobelts (NBs) and ZnS:Al nanowires (NWs) were prepared. Room-temperature photoluminescence spectra showed two luminescence peaks at approximately 513 and 725 nm from the ripple-like CdS NBs, the two peaks can be ascribed to the near band gap transition and defect emissions, respectively. Raman spectra showed that the intensities of the longitudinal optical (LO) phonon and its replica peaks from the ripple-like CdS NBs are more than 4 times larger than those from the normal CdS NBs. The Huang$-$Rhys parameter S calculated from the intensity ratio of the 2LO to 1LO phonon increases from 3.21 to 3.56 for normal and ripple-like NBs, which is indicative of a strong exciton-phonon coupling interaction dominated mainly by a Fröhlich interaction through the charge transfer. The results from the ZnS:Al NWs exhibited that the morphology of ZnS:Al NWs greatly influences on the Raman scattering, while the Al-dopant concentration has a smaller effect on the Raman scattering. The Raman scattering intensity of the pine leaf -like morphological ZnS:Al NWs displayed more than eight times larger than the bulk one, which can be explained as a polarization dependent behavior and a multiple scattering.   

Multiple-pulse superradiance from an optically induced harmonic confinement in a semiconductor microcavity

We report the observation of macroscopic harmonic states in an optically induced confinement in a highly photoexcited semiconductor microcavity at room temperature. The spatially photomodulated refractive index changes result in the visualization of harmonic states in a micrometer-scale optical potential at quantized energies up to 4 meV even in the weak-coupling plasma limit. We characterize the time evolution of the harmonic states directly from the consequent pulse radiation and identify sequential multiple $\sim$10 ps pulse lasing with different emitting angles and frequencies. Such multiple-pulse coherent radiation is attributed to superradiance from correlated electron-hole pairs in a high-density plasma.