Crystal Quantum Dots Research Articles

A reversibly tunable photonic crystal nanocavity laser using photochromic thin film

We demonstrate a reversibly tunable photonic crystal quantum dot laser using a photochromic thin film. The laser is composed of a photonic crystal cavity with a bare cavity Q as high as 4500 coupled to a high density ensemble of indium arsenide quantum dots. By depositing a thin layer of photochromic material on the photonic crystal cavities, the laser can be optically tuned smoothly and reversibly over a wavelength range of 2.68 nm. Lasing is observed at temperatures as high as 80 K in the 900-1000 nm near-infrared wavelength range. The spontaneous emission coupling factor is measured to be as high as β=0.41, indicating that the laser operates in the high-β regime.

Optical properties of a carbon-zirconia quantum-dot photonic crystal

We report the optical properties of a new type of photonic crystal: a transparent fused silica matrix containing quantum dots—nanoparticles of another material. In this study, nanoparticles consist of graphite zones several nanometers in size, stabilized by zirconia. The photonic crystal is prepared by high-temperature annealing (1200°C) of synthetic opal infiltrated with zirconia and a small amount of carbon. We demonstrate selective reflection of visible light from the surface of the quantum-dot crystal under broadband illumination. Such crystals are potentially attractive as narrow-band selective filters that would reflect the exciting light in Raman measurements and might be used to convert short-wavelength broadband radiation to quasi-monochromatic light in the visible range.

3D SiGe QUANTUM DOT CRYSTALS: STRUCTURAL CHARACTERIZATION AND ELECTRONIC COUPLING

We report on the growth of SiGe quantum dot crystals which are realized by depositing Ge on a two-dimensionally pit-patterned Si substrate and subsequent growth of Si spacer and Ge island layers. Lateral periods of 100 nm are obtained by employing deep UV lithography using synchrotron radiation. The vertical period of the typically 10 period dot superlattices was of the order of 10 nm. Ordering of the islands was investigated by atomic force microscopy as well as by high resolution x-ray diffraction studies. From the quantitative evaluation of the x-ray diffraction data a mean Ge content of about 60% in the quantum dots was obtained and an rms. deviation from ideal lattice sites of about 3 nm was found. A simulation of the eigenenergies based on the nextnano3 simulation package was used to interpret the measured photoluminescence data.

Formation of stacked linear InAs quantum dot arrays on InGaAsP/InP(100) by self‐organized anisotropic strain engineering

AbstractWe have previously demonstrated the formation of linear InAs quantum dot (QD) arrays based on self‐organized anisotropic strain engineering of InAs/InGaAsP superlattice (SL) templates on InP(100) by chemical beam epitaxy (CBE). The important step forward is the vertical stacking of the QD arrays with identical emission wavelength, realizing a three‐dimensionally self‐ordered QD crystal. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray diffraction investigation of a three-dimensional Si/SiGe quantum dot crystal

High-resolution x-ray diffraction was employed to study the structural properties of a three-dimensional periodic arrangement of SiGe quantum dots in a Si matrix. Using extreme ultraviolet lithography at a synchrotron source a two-dimensional array of pits (period $90\ifmmode\times\else\texttimes\fi{}100\text{ }{\text{nm}}^{2}$) was defined and transferred into a (001) Si wafer by reactive ion etching. By molecular-beam epitaxy SiGe islands of about 30 nm diameter and 3 nm height were grown into the pits. Subsequent deposition of Si spacer layers of 10 nm thickness and SiGe island layers results in a three-dimensionally periodic arrangement of quantum dots, mediated by the strain fields of the buried dots. Their so far unmatched structural perfection is assessed by coplanar x-ray diffractometry using synchrotron radiation. Reciprocal-space maps around the (004) and (224) reciprocal-lattice maps were recorded and analyzed to get quantitative information on the disorder of the dot positions and to obtain the mean Ge content of the dots. In addition, information on the strain fields was deduced from the analysis of the diffraction data. Together with atomic force microscopy data on the island shape and size distribution, a complete structural characterization is achieved.

Size Effects on the Thermal Properties of Self-assembled Ge Quantum Dots in Single-crystal Silicon

AbstractSuperlattices with an ultra-low thermal conductivity were extensively studied to design thermoelectric materials. However, since they are made up of superposed materials showing lattice mismatches, they often show cracks and dislocations. Therefore, it is challenging to fabricate superlattices with a thermoelectric figure of merit ZT higher than unity. Moreover, like nanowires, they decrease heat transport in only one main direction. Self-assembly from epitaxial layers on a Si substrate is a major bottom-up technology to fabricate 3D Ge quantum-dot (QD) arrays in Si, which have been used for 3D quantum-device applications. Using the model of the atomic-scale 3D phononic crystal, we showed that 3D high-density arrays of self-assembled Ge QDs in Si can also show an ultra-low thermal conductivity in 3D, which can be several orders of magnitude lower than that of bulk Si. As a result, they can be considered to design novel 3D thermoelectric devices showing CMOS compatibility. In an example QD crystal, the thermal conductivity can be decreased below only 0.2 W/m/K. The main objective of this paper is to show the size dependence of the thermal conductivity versus the supercell lattice parameter d. For a constant QD-crystal filling ratio x = 12.5 at%, a decrease of the thermal conductivity is observed for an increasing d. This analysis enables us to predict that the optimal d-value will be of the order of 11 nm for the given filling ratio. At this optimum, the thermal conductivity decreases to the global minimum value of 0.9 W/m/K. The presented results are a first step towards the optimal design of thermoelectric devices with a maximal ZT obtained by global optimization of the size parameters.

Advanced quantum dot and photonic crystal technologies for integrated nanophotonic circuits

Two types of nanophotonic technologies—two-dimensional photonic crystal (2D PC) slab waveguides (WGs) and quantum dots (QDs)—were developed for key photonic device structures in the future. For an ultrafast digital photonic network, an ultrasmall and ultrafast symmetrical Mach–Zehnder (SMZ)-type all-optical switch (PC-SMZ) and an optical flip–flop device (PC-FF) have been developed. To realize these devices, one method is to develop a selective-area molecular beam epitaxial growth QD technique by employing a metal mask method. Another method is to establish a new design method, i.e., topology optimization of the 2DPC WG with a wide and flat bandwidth, high transmittance, and low reflectivity. We also fabricated an optical microcavity in a photonic crystal slab embedded with GaAs QDs by droplet epitaxy. The Purcell effect on the exciton emission of GaAs QDs was confirmed by microphotoluminescence and lifetime measurements.

Quantum-dot crystal defects

The authors investigate the formation of quantum-dot crystal (QDC) defects during the growth of two-dimensional QDCs on flat surfaces. The flat surface is modulated by the periodic strain fields of a buried quantum-dot (QD) seed layer, which in turn is grown onto a patterned substrate surface. Two types of QDC defects, QD vacancies, and QD interstitials are observed whenever the QD density controlled by the growth conditions does not match the areal density of the patterned sites. Our kinetic Monte Carlo growth simulations take into account the realistic strain profile of buried QDs and are able to describe the occurrence of both types of QDC defects during the growth of seeded QDCs.

Coupling of quantum-dot light emission with a three-dimensional photonic-crystal nanocavity

We report on the first demonstration of the coupling of fully confined electrons and photons using a combination of three-dimensional photonic crystal nanocavities and quantum dots. The three dimensional photonic crystals were assembled by stacking planar components using a sophisticated micromanipulation technique. Point defects, containing embedded quantum dots, were introduced into the photonic crystals as active sites. By measuring the photoluminescence spectra of the assembly, the process by which light emitted from the quantum dots is coupled to the defect modes of a three dimensional photonic crystal was demonstrated for the first time. The characteristics of the sharp emission peaks agreed well with numerical simulations, and these were confirmed to be resonant modes by polarization measurements. The highest quality factor (Q-factor) for three dimensional photonic crystals (2,300) was achieved.

Wavelength controlled multilayer-stacked linear InAs quantum dot arrays on InGaAsP/InP (100) by self-organized anisotropic strain engineering: A self-ordered quantum dot crystal

Multilayer-stacked linear InAs quantum dot (QD) arrays are created on InAs/InGaAsP superlattice templates formed by self-organized anisotropic strain engineering on InP (100) substrates in chemical beam epitaxy. Stacking of the QD arrays with identical emission wavelength in the 1.55 μm region at room temperature is achieved through the insertion of ultrathin GaAs interlayers beneath the QDs with increasing interlayer thickness in successive layers. The increment in the GaAs interlayer thickness compensates the QD size/wavelength increase during strain correlated stacking. This is the demonstration of a three-dimensionally self-ordered QD crystal with fully controlled structural and optical properties.

Formation of linear InAs/InGaAsP/InP (1 0 0) quantum dot arrays by self-organized anisotropic strain engineering in chemical beam epitaxy

The formation of laterally ordered linear InAs quantum dot (QD) arrays based on self-organized anisotropic strain engineering is demonstrated. An InAs/InGaAsP superlattice (SL) on InP (1 0 0) serves as a template for the QD arrays grown by chemical beam epitaxy. The InAs QD arrays exhibit excellent photoluminescence emission up to room temperature which is tuned into the 1.55-μm telecom wavelength region through the insertion of ultra-thin GaAs interlayers. Stacking of the QD arrays with identical emission wavelength upon adjusting the GaAs interlayer thickness produces a three-dimensionally self-ordered QD crystal.

Probing the interaction between a single quantum dot and a photonic crystal cavity

AbstractThis article reviews our recent work on the development of nanophotonic devices for quantum information processing. We have developed a temperature tuning technique, that allows us to independently tune photonic crystal cavities and quantum dots on the same chip. The high quality factor to mode volume ratio of such cavities leads to a strong interaction between the cavity and the quantum dot. The cavity modifies the quantum dot lifetime, which, in turn, modifies the transmission properties of the cavity.We observe that the resonant nonlinearity of the quantum dot is greatly enhanced by the recirculation of light, and leads to a giant optical nonlinearity, where single photons are able to saturate the quantum dot and modify the transmission function of the cavity. Along with these results, we review the experimental developments, which have led to the realization of the experiment. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Controlling cavity reflectivity with a single quantum dot

Solid-state cavity quantum electrodynamics (QED) systems offer a robust and scalable platform for quantum optics experiments and the development of quantum information processing devices. In particular, systems based on photonic crystal nanocavities and semiconductor quantum dots have seen rapid progress. Recent experiments have allowed the observation of weak and strong coupling regimes of interaction between the photonic crystal cavity and a single quantum dot in photoluminescence. In the weak coupling regime, the quantum dot radiative lifetime is modified; in the strong coupling regime, the coupled quantum dot also modifies the cavity spectrum. Several proposals for scalable quantum information networks and quantum computation rely on direct probing of the cavity-quantum dot coupling, by means of resonant light scattering from strongly or weakly coupled quantum dots. Such experiments have recently been performed in atomic systems and superconducting circuit QED systems, but not in solid-state quantum dot-cavity QED systems. Here we present experimental evidence that this interaction can be probed in solid-state systems, and show that, as expected from theory, the quantum dot strongly modifies the cavity transmission and reflection spectra. We show that when the quantum dot is coupled to the cavity, photons that are resonant with its transition are prohibited from entering the cavity. We observe this effect as the quantum dot is tuned through the cavity and the coupling strength between them changes. At high intensity of the probe beam, we observe rapid saturation of the transmission dip. These measurements provide both a method for probing the cavity-quantum dot system and a step towards the realization of quantum devices based on coherent light scattering and large optical nonlinearities from quantum dots in photonic crystal cavities.

Three-Dimensional Si/Ge Quantum Dot Crystals

Modern nanotechnology offers routes to create new artificial materials, widening the functionality of devices in physics, chemistry, and biology. Templated self-organization has been recognized as a possible route to achieve exact positioning of quantum dots to create quantum dot arrays, molecules, and crystals. Here we employ extreme ultraviolet interference lithography (EUV-IL) at a wavelength of lambda = 13.5 nm for fast, large-area exposure of templates with perfect periodicity. Si(001) substrates have been patterned with two-dimensional hole arrays using EUV-IL and reactive ion etching. On these substrates, three-dimensionally ordered SiGe quantum dot crystals with the so far smallest quantum dot sizes and periods both in lateral and vertical directions have been grown by molecular beam epitaxy. X-ray diffractometry from a sample volume corresponding to about 3.6 x 10(7) dots and atomic force microscopy (AFM) reveal an up to now unmatched structural perfection of the quantum dot crystal and a narrow quantum dot size distribution. Intense interband photoluminescence has been observed up to room temperature, indicating a low defect density in the three-dimensional (3D) SiGe quantum dot crystals. Using the Ge concentration and dot shapes determined by X-ray and AFM measurements as input parameters for 3D band structure calculations, an excellent quantitative agreement between measured and calculated PL energies is obtained. The calculations show that the band structure of the 3D ordered quantum dot crystal is significantly modified by the artificial periodicity. A calculation of the variation of the eigenenergies based on the statistical variation in the dot dimensions as determined experimentally (+/-10% in linear dimensions) shows that the calculated electronic coupling between neighboring dots is not destroyed due to the quantum dot size variations. Thus, not only from a structural point of view but also with respect to the band structure, the 3D ordered quantum dots can be regarded as artificial crystal.

Molecular beam epitaxy of IV–VI semiconductor hetero‐ and nano‐structures

AbstractIn this review, molecular beam epitaxy of IV–VI semiconductor hetero‐ and nanostructures is described. The properties of the IV–VI compounds differ in several respects from zinc‐blende III–V or II–VI semiconductors. This has significant consequences on the growth processes of molecular beam epitaxy. The main application of IV–VI or lead salt compounds is in mid‐infrared optoelectronic devices. As example, the growth of lead salt lasers is described with special emphasis on vertical cavity surface emitting lasers. Furthermore, it is shown that due to the large elastic anisotropy of IV–VI compounds, self‐organized quantum dot superlattices show remarkable ordering and vertical correlations. As a result, 3D ordered quantum dot crystals can be obtained. Thus, these materials are ideal model systems for the investigation of lateral, vertical and staggered dot ordering in multilayer structures. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Finding Fluorescent Needles in the Cardiac Haystack: Tracking Human Mesenchymal Stem Cells Labeled with Quantum Dots for Quantitative In Vivo Three-Dimensional Fluorescence Analysis

Stem cells show promise for repair of damaged cardiac tissue. Little is known with certainty, however, about the distribution of these cells once introduced in vivo. Previous attempts at tracking delivered stem cells have been hampered by the autofluorescence of host tissue and limitations of existing labeling techniques. We have developed a novel loading approach to stably label human mesenchymal stem cells with quantum dot (QD) nanoparticles. We report the optimization and validation of this long-term tracking technique and highlight several important biological applications by delivering labeled cells to the mammalian heart. The bright QD crystals illuminate exogenous stem cells in histologic sections for at least 8 weeks following delivery and permit, for the first time, the complete three-dimensional reconstruction of the locations of all stem cells following injection into the heart. Disclosure of potential conflicts of interest is found at the end of this article.

Ge quantum dot molecules and crystals: Preparation and properties

Templated self-organization has been used to prepare two-dimensional arrays as well as three-dimensional quantum dot crystals (QDC) containing Ge dots in a Si host crystal. Si(1 0 0) substrates have been patterned with two-dimensional hole gratings using extreme ultra-violet interference lithography (EUV-IL) and reactive ion etching. The EUV-IL was realized by multiple beam diffraction using Cr gratings on SiN x membranes fabricated by e-beam lithography. Si/Ge overgrowth was performed by molecular beam epitaxy. The impact of the microscopic shape and size of the prepattern using the mask design and the EUV-IL exposure dose as parameters on the Ge dot nucleation has been studied with atomic force microscopy, transmission electron microscopy and photoluminescence measurements. Adjusting the growth parameters in multiple layer deposition the initial two-dimensional configuration was transferred into three-dimensional QDC.

Morphology and photoluminescence of seeded three‐dimensional InAs/GaAs(001) quantum‐dot crystals

AbstractWe investigate the morphology and photoluminescence (PL) of quantum dots (QDs) in seeded three‐dimensional InAs/GaAs quantum‐dot crystals (QDCs). The QDs in the QDC show a dome shape for the second QD layer and they have a pyramid shape in all subsequent layers. The PL from the QDC with 11‐fold stacked QD layers shows an intense PL signal with narrow inhomogeneous linewidth. Both ground state PL peak energy and PL linewidth remain almost constant over the patterned area. A power‐dependent PL measurement is performed and compared with a simple quantum disk model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Complete band gaps in three-dimensional quantum dot photonic crystals

Because of the generation of polaritons, which are quasiparticles possessing the characteristics of both photonics and electronics, active photonic materials offer a possible solution to transfer electromagnetic energy below the diffraction limit and further increase the density of photonic integrated circuits. A theoretical investigation of these exciting materials is, therefore, very important for practical applications. Four different kinds of polaritons have been studied in this thesis, (1) surface polaritons of negative-index-material cylindric rods, (2) exciton polaritons of semiconductor quantum dots, (3) localized plasmon polaritons of metallic nanoshells, and (4) surface plasmon polaritons of subwavelength hole arrays in thin metal films. All these types of polaritons were found to strongly affect the optical properties of the studied active photonic materials. More specifically, (1) for two-dimensional photonic crystals composed of negative-index-material cylindric rods, the coupling among surface polaritons localized in the rods results in dispersionless anti-crossing bands; (2) for three-dimensional diamond-lattice quantum-dot photonic crystals, the exciton polariton resonances lead to the formation of complete band gaps in the dispersion relationships; (3) for metallic nanoshells, the thickness of the metal shell strongly modifies the localized plasmon polaritons, and therefore influences the degree of localization of the electromagnetic field inside the metallic nanoshells; (4) for subwavelength hole arrays in thin metal films, high-order surface-polariton Bloch waves contribute significantly to the efficient transmission. To numerically simulate these active photonic materials, we introduced three approaches, (1) an extended plane-wave-based transfer-matrix approach for negative- index-material media, (2) a plane-wave method for semiconductor quantum-dot photonic crystals, and (3) an auxiliary-differential-equation finite-difference time- domain approach for semiconductor quantum-dot arrays. A brief perspective is also given at the end of this thesis.

Photoluminescence from seeded three-dimensional InAs∕GaAs quantum-dot crystals

We investigate the photoluminescence (PL) properties of three-dimensional InAs∕GaAs quantum-dot (QD) crystals grown on shallow modulated periodic hole arrays patterned on GaAs(001). We find that the PL spectra become narrower and more intense with increasing number of QD layers. A deconvoluted PL linewidth of 14.9 meV is obtained from a defect-free QD crystal consisting of 11 stacked QD layers. The PL spectra obtained for QD crystals containing QD vacancies show significantly broader spectra. The PL peak energy and linewidth of the QDs across the whole pattern (100×100μm2) remain constant within 1.278±0.001eV and 21.0±1.7meV, respectively. From power-dependent PL measurement, we can resolve up to seven excited-state PL peaks confirming the remarkable size homogeneity of our QD crystals. This experimental result can be reasonably fitted by a calculation based on random population theory and on a simple model for the QD confinement potential.