This PDF file contains the front matter associated with SPIE Proceedings Volume 7224, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Epitaxial self-assembled quantum dots (SAQDs) result from Stranski-Krastanow growth whereby epitaxial 3D islands form spontaneously on a planar thin film. Common systems are Ge_xSi_1-x/Si and In_xGa_1-xAs/GaAs. SAQDs are typically grown on a (001) surface. The formation and evolution of SAQDs is governed in large part by the interaction of surface energy and elastic strain; however, the surface energy density is quite complicated and not well understood. Many growth processes take place at high temperature where stress and entropy effects can have a profound effect on the surface free energy. There are three competing theories of the nature of the planar (001) surface: I. It is a stable crystal facet. II. It is a stable non-faceted surface. III. It is an unstable crystal antifacet. Each leads to a different theory of the SAQD formation process. The first theory appears most often in modeling literature, but the second two theories take explicit account of the discrete nature of a crystal surface. Existing observational and theoretical evidence in support of and against these theories is reviewed. Then a simple statistical mechanics model is presented that yields a phase-diagram depicting when each of the three theories is valid. Finally, the Solid-on-Solid model of crystal surfaces is used to validate the proposed phase diagram and to calculate the orientation and height dependence of the surface free energy that is expressed as a wetting chemical potential, a wetting modulus and surface tilt moduli.

Three-dimensional (3D) InAs/GaAs quantum dots (QDs) island size and density evolution under different coverage and temperature is studied by using our 3D kinetic Monte Carlo (KMC) model. Our KMC model is based on the solid-on-solid one with bond counting and Ehrlich-Schwoebel barrier being incorporated. It is found that there is a QD island size limit for the growth coverage. Below this limit existing QD islands can adsorb new-coming adatoms; however, beyond the limit new QD islands will form and adopt new coming adatoms. It is also observed that with increasing temperature, the QD islands size will be increased while their density will be reduced.

The conventional approach to fabricate semiconductor based QDs is based on the Stranski-Krastnow (SK) growth mode, which has enjoyed considerable success in device applications. However, the SK QD approach is complicated by the randomness of the QD size distribution and inherent presence of the wetting layer. Carrier leakage to the wetting layer has been identified as one of the underlying causes for low optical gain and high temperature sensitivity in diode lasers. To fully exploit the potential advantages of ideal Quantum Dots (i.e. full 3D carrier confinement), elimination of the wetting layer and a uniform mono-modal QD size distribution is needed. Nanopatterning with selective MOCVD QD growth has potential for achieving a higher degree of control over the QD formation, compared with the SK process. Furthermore, the problematic wetting layer states are eliminated and improved optical gain is expected. The QD patterning is prepared by dense nanoscale (20-30 nm diameter) diblock copolymer lithography, which consists of perpendicularly ordered cylindrical domains of polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) matrix. For selective MOCVD growth, a dielectric template mask was utilized and the polymer patterning is transferred on it. The resulting GaAs QD densities are larger than 5×10¹⁰/cm², comparable to SK growth mode, with a nearly monomodal QD size distribution. Variable temperature PL has been used to characterize the optical properties of capped InGaAs QDs on GaAs (λ ~ 1.1 μm) and InP (λ ~ 1.5 μm) substrates.

The unique properties of quantum dots should allow enhanced or novel devices to be fabricated. However, the typical method of formation is to self-assemble quantum dots. This process causes quantum dots to have a distribution in properties such as size and to form at near random location. Since many of these possible devices require near exact positioning of the quantum dots with given sizes, most of these potential devices have been unrealized or exist in far from optimum conditions. In this work, we present a new method which is being examined for its potential to form uniform quantum dot structures. This technique is surface tension driven restructuring of a nano-patterned surface. In particular, we have formed a planar 5nm thick InAs film under metal rich conditions. The sample pattern was formed using a 3mg load measured with a Hysitron nano-indentor and maintained using STM scan electronics. The pattern consisted of a grid of 150 lines in x and y directions in nominal 9μm x 9μm square area. AFM analysis showed a series of lines which are spaced ~180 nm lines apart in the y direction and lines spaced ~60 nm and 120 nm in the x-direction. The patterned sample was annealed under a high As flux, near 5 x 10^-6 torr, after removal of the surface oxides. The resulting structure clearly shows the reorganization of the InAs in regions defined by the original patterning in AFM images. AFM analysis indicates large features with 80nm base width were formed.

InAs quantum dots embedded in InGaAs quantum well (DWELL) structures grown by metal-organic chemical-vapor deposition on nano-patterned GaAs pyramids and planar GaAs (001) substrate are comparatively investigated. Photoluminescence (PL), PL excitation, and time-resolved PL measurements demonstrate that the DWELL grown on the GaAs pyramids has a broad QW PL band (FWHM ~ 90 meV) and a better QD emission efficiency than the DWELL structure grown on the planar GaAs (001) substrate. These properties are attributed to the InGaAs QW with distributed thickness profile on the faceted GaAs pyramid, which introduces tapered energy band structure and assists the carrier capture into the QDs. This research provides useful data for further improving the performance of DWELL structures for device applications.

We report a study of InSb quantum dots and quantum rings grown on InAs(100) substrate by LPE-MOVPE combine method. Characterization of InSb/InAs(Sb,P) quantum dots was performed using atomic force microscopy and transmission electron microscopy. The bimodal growth of uncapped InSb quantum dots was observed in the temperature range T=420-450 °C. The low-density (5×10⁸ cm^-2) large quantum dots with dimensions of 12-14 nm in height and 45-50 nm in diameter are appeared at 445 °C, whereas high-density (1×10¹⁰ cm^-2) dislocation-free small quantum dots with dimensions of 3-5 nm in height and 11-13 nm in diameter were obtained at 430 °C. Capping of the InSb quantum dots by binary InAs or InAsSbP epilayers lattice-matched with InAs substrate was performed using MOVPE method. Tunnel-related behavior in a forward curve of I-V characteristics was observed in heterostructures with buried InSb quantum dots inserted in InAs p-n junction. Evolution of electroluminescence spectra on driving current at negative bias and suppression of negative luminescence from buried InSb/InAs quantum dots were found out in the spectral range 3-4 μm at 300 K. Deposition from the InSb melt over the InAsSb_0.05P_0.10 capping layer resulted in the formation of InSb quantum rings with outer and inner diameters about 20-30 nm and 15-18 nm respectively. Surface density of the quantum rings of 2.6×10¹⁰ cm^-2 was reached at 430 °C.

Si and Ge nanowires have potential applications in a wide variety of areas including thermoelectrics, optoelectronics, and sensors. Nanowires are most commonly grown via the vapor-liquid-solid (VLS) process. In this method, a vapor phase containing the material of interest preferentially dissociates at a liquid catalyst and is incorporated as a solid at the solid-liquid interface. However, despite 40 years of research in this area, several aspects of nanowire growth remain unclear, even for relatively simple elemental Si and Ge wires. Here, we will review our in situ transmission electron microscopy (TEM) investigations of Si and Ge nanowire growth kinetics. The observations are carried out in an ultra-high vacuum TEM (the IBM UHV-TEM) equipped with facilities for deposition during observation. Using Au as the catalyst, we study the VLS growth of Si and Ge nanowires as a function of disilane or digermane pressure and substrate temperature. We find surprisingly different growth mechanisms for the two materials. The insights gained from in situ results may help devise methods for large-scale fabrication of wires with controlled architecture.

Vertical III-V compound semiconductor nanowires grown on Si(111) surfaces have been attracting interest for application to opto-electronic integrated circuits (OEICs). In nanowire growth, heterostructures in the axial and radial direction can be obtained by combining different materials with different growth conditions. These effects should make it possible to fabricate complicated and functional three-dimensional structures in a bottom-up manner. These advances should lead to new types of nanodevices. We describe the formation of several heterostructures using GaP-based nanowires on Si(111). The catalysts used were Au particles obtained from Au colloids. We obtained GaP/GaAs/GaP nanowires bent at thinned GaAs nodes, InP egg-like structures in GaP nanowires, core-multishell Ga(In)P/GaAs(or air-gap)/ GaP nanowires with flat tops, and GaAs/AlInAs capped GaInAs nanowires for long-wavelength photon emission. These structures were successively grown on vertical GaP nanowires on Si(111) substrates.

Single GaSb Nanowire Field Effect Transistors (NWFETs) were fabricated and their electrical transport measurements were conducted at the temperatures ranging from 298 K to 503 K. The current on/off ratios as large as 3 orders of magnitude were observed. The Raman spectra and EDAX were performed on single wires to verify the GaSb property before and after the transport study. The temperature dependent current-voltage characteristic shows asymmetric current through the device due to asymmetric back-to-back Schottky contacts at the two ends of the wire. Arrhenius plots revealed effective Schottky barrier heights around Ø_Beff =0.53eV. Measurement conducted on back-gated nanowire transistors shows the polarity of nanowire to be n-type.

Because of their unique chemical and physical properties, nanometer-sized semiconductor materials have attracted broad attention in the fields of physics, chemistry, and biology. These capabilities are mainly due to the unusual dependence of the electronic and optical properties on quantum confinement, which for semiconductor materials restricts the particle size in the 1 to 10 nm range. We have fabricated and measured novel nanostructures of semiconductor quantum dots capped with surfactants and embedded in organic thin films that exhibit several orders of magnitude increase in their multiphoton cross-sections of absorption relative to semiconductor quantum dots capped with surfactants in solution or in other matrix materials. Large values of optical nonlinearity have been measured for CdS and CdSe quantum dots in these engineered nanostructures. We will discuss these findings within the context of theoretically proposed hybrid excitons in organic-inorganic nanostructures.

Presently VCSELs covering a significant spectral range (840-1300 nm) can be produced based on quantum dot (QD) active elements. Herein we report progress on selected QD based vertical-cavity surface-emitting lasers (VCSELs) suitable for high-speed operation. An open eye diagram at 20 Gb/s with error-free transmission (a bit-error-rate < 10^-15) is achieved at 850 nm. The 850 nm QD VCSELs also achieve error-free 20 Gb/s single mode transmission operation through multimode fiber without the use of optical isolation. Our 980 nm-range QD VCSELs achieve error free transmission at 25 Gb/s at up to 150°C. These 980 nm devices operate in a temperature range of 25-85°C without current or modulation voltage adjustment. We anticipate that the primary application areas of QD VCSELs are those that require degradation-robust operation under extremely high current densities. Temperature stability at ultrahigh current densities, a forte of QDs, is needed for ultrahigh-speed (> 40 Gb/s) current-modulated VCSELs for a new generation of local and storage area networks. Finally we discuss aspects of QD vertical extended-cavity surface emitting lasers with ultra high power density per emitting surface for high power (material processing) and frequency conversion (display) applications.

We discuss our work on light emitters and photonic circuits realized using colloidal quantum dot composites. Specifically we will report our recent work on flexible microcavity laser, microdisk emitters and integrated active - passive waveguides. The entire microcavity laser structure was realized using spin coating and consisted of an all-polymer distributed Bragg reflector with a poly-vinyl carbazole cavity layer embedded with InGaP/ZnS colloidal quantum dots. These microcavities can be peeled off the substrate yielding a flexible structure that can conform to any shape and whose emission spectra can be mechanically tuned. The microdisk emitters and the integrated waveguide structures were realized using soft lithography and photo-lithography, respectively and were fabricated using a composite consisting of quantum dots embedded in SU8 matrix. Finally, we will discuss the effect of the host matrix on the optical properties of the quantum dots using results of steady-state and time-resolved luminescence measurements. In addition to their specific functionalities, these novel device demonstrations and their development present a low cost alternative to the traditional photonic device fabrication techniques.

Emission of higher-order modes caused by spatial hole burning in a quantum dot (QD) laser is studied. The critical tolerable values of the structure parameters are discussed beyond which higher-order longitudinal modes can not oscillate. The higher the mode order, the narrower the range of allowed QD-size scatter for lasing of the mode. The higher the mode order, the denser the QD-ensemble and the longer the cavity should be for lasing of the mode. The output powers of the lasing modes are calculated versus the injection-current density from the solution of the rate equations.

We demonstrate the widened broadband emission of self-assembled quantum dash laser using impurity-free vacancy induced disordering (IFVD) technique. The 100 nm blueshifted lasers exhibit higher internal quantum efficiency and lower threshold current densities than the as-grown devices. The laser emission from multiple groups of quantum-dash (Qdash) families convoluted with multiple orders of subband energy levels within a single Qdash ensemble is experimentally observed. However, the suppression of laser emission in short wavelength and the progressive redshift of peak emission with injection current from devices with short cavity length occur. These effects have been attributed to the nonequilibrium carrier distribution and energy exchange among different sizes of Qdash ensembles. In addition, we perform the far-field lateral mode measurements from the fabricated as-grown Qdash laser. The analysis of mode patterns indicate that Qdash lasers exhibit gradual broadening of beam divergence (FWHM of 3.4° to 10.8°) with increasing injection current. However, these beam divergence angles are still narrower than the quantum well (QW) laser (FWHM ~13°) at an injection up to 2.5 x J_th. Qdash laser exhibits an improved output beam quality, therefore reduced filamentation, as compared to the QW laser, owing to the inherent characteristics from quantum-dot (Qdot) laser, where injected carriers are confined by the lateral energy barriers as Qdots are disconnected laterally and are cladded by larger bandgap materials. Our results imply a highly attractive wavelength trimming method, well suited for improved performance, and monolithic Qdash integration of optoelectronics components.

Structures with tunnel-coupled pairs consisting of InGaAs quantum wells (QWs) grown on top of self-assembled InAs quantum dots (QDs) were used previously as a gain medium for vertical cavity surface emitting lasers (VCSELs) to eliminate problems with QD-limited maximum saturated gain. Conventional molecular beam epitaxy of tunnel-coupled QDs with slow InAs growth rate and InGaAs solid solution QW injector with high InAs growth rate required a long delay in growth process for changing indium source temperature/flux. This leads to non-intentional doping of tunnel barrier and reproducibility issues. To overcome these problems, structures of tunnel-coupled QDs-QW pairs consisting of InAs/InGaAs short period superlattice (SPSL) QW injector with compatible slow InAs growth rate (QDs-SPSL) were developed and compared with traditional InAs-InGaAs (QDs-InGaAs). Photoluminescence (PL) and electroluminescence were used to study the properties of the "well-on-dots" active medium with InAs/InGaAs SPSL QW and with InGaAs QW. The optimized tunnel triple pair QDs-SPSL structure with 2x reduction of growth time has demonstrated a 2x enhanced PL efficiency as compared with traditional QDs-InGaAs structures. A novel tunnel-coupled triple QDs InAs-SPSL was successfully employed as a gain medium of VCSELs with doped all-epitaxial distributed Bragg reflectors (DBRs). Room temperature CW lasing wavelengths in the range from 1100 nm to 1150 nm were measured in VCSELs with attuned DBRs. These QDs-SPSL VCSELs demonstrated minimum threshold current value I_th = 0.85 mA and maximum differential efficiency of 0.16 W/A.

We present a high-temperature single-photon source based on a CdSe quantum dot in a ZnSe nanowire. The nanowires have been grown by Molecular Beam Epitaxy in the Vapour-Liquid-Solid growth mode. We utilized a two-step growth process, where a thin, defect free ZnSe nanowire on a top of a nanoneedle is grown. Quantum dots are formed by incorporating a narrow zone of CdSe into the nanowire. We observe an intense and highly polarized photoluminescence. Efficient photon anti-bunching was observed up to 220 K, while conserving a normalized anti-bunching dip of at most 36%.

InAs quantum dots embedded in InGaAs quantum wells with InAlAs barriers on InP substrate grown by metalorganic chemical vapor deposition are utilized for high operating temperature detectors and focal plane arrays in the middle wavelength infrared. This dot-well combination is unique because the small band offset between the InAs dots and the InGaAs well leads to weak dot confinement of carriers. As a result, the device behavior differs significantly from that in the more common dot systems that have stronger confinement. Here, we present energy level modeling of our QD-QW system and apply these results to interpret the detector behavior. Detectors showed high performance with D* over 10¹⁰ cmHz^1/2/W at 150 K operating temperature and with high quantum efficiency over 50%. Focal plane arrays have been demonstrated operating at high temperature due to the low dark current observed in these devices.

A voltage-tunable multispectral 320×256 infrared imaging focal plane array (FPA) is reported. The multispectral FPA is based on InAs/GaAs quantum dots infrared photodetectors (QDIP) with different capping layers (i.e. GaAs and In_0.20Ga_0.80As), corresponding to the extended middle-wave infrared (EMIR, 5-8 μm) and long-wave infrared (LWIR, 8- 12 μm) detection bands, respectively. The FPA shows a sensitivity of 8.2 mV/C a noise equivalent temperature difference of 172 mK at the FPA operating temperature of 67 K. Thermal images were obtained at both the EMIR and the LWIR bands at a low FPA bias of -0.7V. Voltage-tunable multispectral imaging was also achieved. Since each of the detection spectral of the QD FPA can be individually tuned by engineering its QD capping layer, this approach offers greater flexibility in designing detection spectrum of a multispectral FPA.

A single-photon source capable of emitting indistinguishable photons is a key element in schemes for scalable quantum information processing with linear optics. Whilst several groups have reported such sources, up until now an electrically driven source capable of making these protocols technologically viable has yet to be reported. We present the first demonstration of an electrically driven single-photon source emitting indistinguishable photons. Our sample consists of a layer of InAs/GaAs quantum dots embedded in the intrinsic region of a p-i-n microcavity diode. We test the indistinguishability of consecutive photons by carrying out a Hong-Ou- Mandel-type two-photon interference experiment whereby two identical photons arriving simultaneously at two input ports of a 50:50 beamsplitter exit together. The device was operated under two modes, continuous and pulsed current injection. In the former case, we measured a coherence time of up to 400 ps at low pump current - the longest reported under these excitation conditions. A two-photon interference visibility was measured, limited only by the timing resolution of our detection system and further suggesting a 100% overlap of photon wavepackets at the output beamsplitter. In the case of pulsed injection, we employed a two-pulse voltage sequence which allowed us to carry out temporal filtering of photons which had undergone dephasing. The characteristic Hong-Ou-Mandel "dip" was measured resulting in a visibility of 64 ± 4%.

We report on a new sustainable approach to modulate the morphological and optical properties of aqueous colloids of gold nanorods by engineering of their overgrowth kinetics in the original pot. The overgrowth is realized by a fine control over the reduction of the gold ions which are left unreduced in the primal synthesis. We show the possibility to modify the intensity and position of the absorption and scattering peaks over several tens of nanometers.

We present calculations of one-photon absorption (OPA) spectra for (CdSe)_n (up to n = 34) quantum dots, carried out using time-dependent density functional theory. The effects of cluster size on OPA spectra are discussed and compared with experiment to provide preliminary insights into the formation of stable clusters.

Surface plasmon coupling effects of isolated and collective Au nanoparticle arrays are investigated through the optical transmission spectroscopy and near-field scanning optical microscope (NSOM). Nanoparticle arrays with different collective degrees are fabricated by the nanosphere lithography and self-assembling technique. The absorption wavelength of collective nanoparticle array at surface plasmon resonance orients to longer wavelengths due to the particularly strong localized electromagnetic field exists in the gap of nanoparticle pairs. Besides, results manifest that localized surface plasmon coupling not only depends on the inter-particle distance but also relates to the photon-plasmon resonance.

Interference patterns between photon excitation and plasmon mediated re-radiation are revealed on a metal nanoparticle basis. The plasmon-photon interaction is directly observed in the vicinity of silver nanoparticles through a near-field scanning optical microscope (NSOM). Isolated silver nanoparticles with different sizes on the pure quartz substrate are simultaneously fabricated through a high temperature annealing technique. Our results manifest that the correlation of phase-response and size-dependent optical enhancement can be further applied to control the spatial distribution of localized surface plasmon modes by means of framed nanostructures.

We have designed and fabricated a quantum dot (QD) gain medium which consists of InAs QDs in an InGaAsP matrix on an InP substrate. By using these InAs/InP QD layers, we have generated femtosecond (fs) pulses with pulse duration of 295 fs from a single-section monolithic Fabry-Perot (F-P) cavity at the repetition rate of 50 GHz around 1560 nm wavelength range without any external pulse compression. The average output power is 40.1 mW at the injection current of 200 mA. Optical signal-to-noise ratio (OSNR) of the proposed QD mode-locked laser (QD-MLL) is up to 50 dB. The lasing threshold current and the external differential quantum efficiency are 23 mA and 30 %, respectively. And the mode beating linewidth was measured to be less than 20 KHz. We have interpreted that several nonlinear optical effects related to interaction of QD excitons with intracavity laser fields could create nonlinear dispersion to compensate intracavity linear dispersion. So total dispersion is minimized and four-wave mixing (FWM) is dramatically enhanced within QD F-P cavity. If spectral bandwidth is broad enough, tens or hundreds of longitudinal modes would lase and their phases would be locked together through FWM process. Eventually a train of fs pulses with a repetition rate corresponding to cavity round-trip time is generated.

Fabrication of a linear array of metallic nanodots and nanopillars for plasmonic waveguides is reported in this paper by two different processes - FIB milling of deposited thin films and electron beam-induced deposition of metallic nanostructures from a organometallic precursor gas introduced into the chamber. In the first FIB fabrication method, metallic nanorods and nanopillars were fabricated by depositing a 30-150 nm layer of a metallic (gold or silver) film on a planar substrate and subsequently employing FIB milling to pattern out the metallic nanopillars from the film. Employing FIB allowed formation of nanostructures such that the plasmon resonances associated with the nanostructures could be engineered and precisely controlled by controlling the nanostructure size and shape. Multistep FIB fabrication procedures were developed to form the nanostructures of complex geometries on planar substrates. The second fabrication processed used to create nanodots and nanopillars for plasmonic waveguides discussed in this paper is direct deposition of metal nanostructures, created when an electron beam (e-beam) is used to dissociate metal from an organometallic precursor gas in a predefined reaction region. Ionization energy required for decomposition of the Au precursor, i.e. Dimethyl Au (III) Fluoro Actylacetonate, is matched with that of the secondary electrons (between 5-50 eV) that are generated by exposing the substrate to a focused electron beam.

There are huge market demands for innovative, cheap and efficient light sources, including light emitting devices, such as LEDs and lasers. However, the light source development in the visible spectral range encounters significant difficulties these years. The available visible wavelength LEDs or lasers are few, large and expensive. The main challenge lies at the lack of efficient light media. Semiconductor nanocrystal quantum dots (QDs) have recently commanded considerable attention. As a result of quantum confinement effect, the emission color of these QDs covers the whole visible spectral range and can be modified dramatically by simply changing their size. Such spectral tunability, together with large photoluminescence quantum yield and photostability, make QDs attractive for potential applications in a variety of light emitting technologies. However, there are still several technical problems that hinder their application as light sources. One main issue is how to fabricate these QDs into a solid state device while still retaining their original optical emission properties. A vacuum assisted micro-fluidic fabrication of guided wave devices has demonstrated low waveguide propagation loss, lower crosstalk, and improved waveguide structures. We report herein the combination of the excellent emission properties of QDs and novel vacuum assisted micro-fluidic photonic structure fabrication technique to realize single-mode efficient light sources.

We present the enhancement of total internal reflection fluorescence microscopy by the excitation of localized surface plasmons using nanoisland embedded subwavelength grating. The presence of nanoislands may provide additional field enhancement even at moderate grating period. For fabrication of nanoisland embedded grating patterns, a silver film was first evaporated on a glass substrate. Next, silver grating was patterned by e-beam lithography. Subsequently, nanoisland shapes were chemically formed. Field enhancement was measured by fluorescent excitation of microbeads on periodic silver nanoislands. The performance is compared to the microbead excitation on a silver nanograting without nanoislands and nanoislands formed on a thin film without grating patterns as controls. The result confirms additional field enhancement by nanoisland embedded periodic patterns.

The optical properties of gold nanoclusters of size 2 - 20 have been investigated using time-dependent density functional theory (TDDFT) to simulate their linear absorption spectra. Relativistic effects have been included by using pseudopotentials, with the Douglas-Kroll (DK) approximation, and with the zero-order regular approximation (ZORA). The improved model core potential with scaled relativistic effects (iMCP-SR2) used in combination with either the BP86, BLYP, or B3LYP exchange-correlation density functional was found to fairly accurately model the spectra of clusters for which measured spectra are available, although the all-electron ZORA method was best both for accuracy and computational efficiency. The effects on the optical properties of organic chromophores from coordination with small gold clusters were preliminarily studied. The extent of enhancement of the absorption properties is seen to depend on the size and structure of the gold cluster.