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We use polarized photocurrent spectroscopy in a nanowire device to investigate the band structure of hexagonal Wurtzite InAs. Signatures of optical transitions between four valence bands and two conduction bands are observed which are consistent with the symmetries expected from group theory. The ground state transition energy identified from photocurrent spectra is seen to be consistent with photoluminescence emitted from a cluster of nanowires from the same growth substrate. From the energies of the observed bands we determine the spin orbit and crystal field energies in Wurtzite InAs. This information is essential to the development of crystal phase engineering of this important III-V semiconductor., Comment: 25 pages, 5 figures

There is tremendous interest in measuring the strong electron–phonon interactions seen in topological Weyl semimetals. The semimetal NbIrTe4 has been proposed to be a Type-II Weyl semimetal with 8 pairs of opposite Chirality Weyl nodes which are very close to the Fermi energy. We show using polarized angular-resolved micro-Raman scattering at two excitation energies that we can extract the phonon mode dependence of the Raman tensor elements from the shape of the scattering efficiency versus angle. This van der Waals semimetal with broken inversion symmetry and 24 atoms per unit cell has 69 possible phonon modes of which we measure 19 modes with frequencies and symmetries consistent with Density Functional Theory calculations. We show that these tensor elements vary substantially in a small energy range which reflects a strong variation of the electron–phonon coupling for these modes.

Bi2Se3 is a prototypical topological insulator which has a small band gap (~0.3 eV) and topologically protected conducting surface states. This material exhibits quite strong thermoelectric effects. Here we show in a mechanically exfoliated thick (~100 nm) nanoflake device that we can measure the energy dependent optical absorption through the photothermoelectric effect. Spectral signatures are seen for a number of optical transitions between the valence and conduction bands, including a broad peak at 1.5 eV which is likely dominated by bulk band-to-band optical transitions but is at the same energy as the well known optical transition between the two topologically protected conducting surface states. We also observe a surprising linear polarization dependence in the response of the device that reflects the influence of the metal contacts., Comment: 17 pages, 4 figures

We use transient Rayleigh scattering to study the thermalization of hot photoexcited carriers in single GaAs0.7Sb0.3/InP nanowire heterostructures. By comparing the energy loss rate in single core-only GaAs0.7Sb0.3 nanowires which do not show substantial hot carrier effects with the core-shell nanowires, we show that the presence of an InP shell substantially suppresses the longitudinal optical phonon emission rate at low temperatures which then leads to strong hot carrier effects.

Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-split H4 and H5 and the degenerate H6 valence bands (VB) and the lowest degenerate H6 conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of the H6 CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriers decay. Using ab initio density functional theory (DFT) calculations, we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along the c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties., The complex band structure and carrier decay response upon photoexcitation of the chiral semiconductor tellurium remain to be unveiled. Here, the authors report unusual dynamic band modifications in Te near band-edge structure due to photoinduced symmetry breaking and strong anisotropy in carrier decay dynamics.

Bismuth selenide (Bi2Se3) is a prototypical three-dimensional topological insulator whose Dirac surface states have been extensively studied theoretically and experimentally. Surprisingly little, however, is known about the energetics and dynamics of electrons and holes within the bulk band structure of the semiconductor. We use mid-infrared femtosecond transient reflectance measurements in a thick exfoliated nanoflake to study the ultrafast thermalization and recombination dynamics of photoexcited electrons and holes within the extended bulk band structure over a wide energy range (0.3-1.2 eV). Theoretical modeling of the reflectivity spectral lineshapes at 10 K demonstrates that the electrons and holes are photoexcited within a dense and cold electron gas with a Fermi level positioned well above the bottom of the lowest conduction band. Direct optical transitions from the first and the second spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum are identified. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas. The minority carrier holes recombine with the dense electron gas within 150 ps at 10 K and 50 ps at 300 K. Such knowledge of interaction of electrons and holes within the bulk band structure provides a foundation for understanding of how such states interact dynamically with the topologically protected Dirac surface states., Comment: 30 pages, 4 figures (Suppl. Info. 8 pages, 3 Tables, 3 Figures)

Recently, nanowires (NWs) have come into the spotlight for future integrated optoelectronic devices. III-V compound semiconductor NWs are unique, in that they can be grown epitaxially on Si substrates, which allow the integration of III-V-based optoelectronics with established Si microelectronics technologies [1-3]. That is to say, well-aligned NW morphology with vertical growth is a rudimentary necessity in the applications of III-V compound semiconductor NWs on Si substrates. However, several problems must be overcome to obtain the successful integration of III-V compound semiconductors with Si [4,5], including lattice and thermal expansion mismatch, and the formation of antiphase domains. For the case of Si, in particular, the existence of a native oxide layer at the interface between NWs and substrates is a major factor preventing the epitaxial growth of NWs [1]. To achieve nanoscale optoelectronic devices, it is also crucial to fabricate NWs with a high-quality crystal structure and controllable composition. For optoelectronic device applications, NW defects such as twin defects, phase polytypism, and stacking faults may give rise to the deterioration of optical and carrier transport properties and are therefore undesirable [6]. Overcoming these issues, namely, the interface defects between NWs and substrates and crystal 604structure defects, is the first priority to obtain perfect III-V compound semiconductor NWs on Si substrates for nanoelectronic device applications.

The development of earth abundant materials for optoelectronics and photovoltaics promises improvements in sustainability and scalability. Recent studies have further demonstrated enhanced material efficiency through the superior light management of novel nanoscale geometries such as the nanowire. Here we show that an industry standard epitaxy technique can be used to fabricate high quality II-V nanowires (1D) and nanoplatelets (2D) of the earth abundant semiconductor Zn3As2. We go on to establish the optoelectronic potential of this material by demonstrating efficient photoemission and detection at 1.0 eV, an energy which is significant to the fields of both photovoltaics and optical telecommunications. Through dynamical spectroscopy this superior performance is found to arise from a low rate of surface recombination combined with a high rate of radiative recombination. These results introduce nanostructured Zn3As2 as a high quality optoelectronic material ready for device exploration.

Recently, Fickenscher et al. [1] have shown that, in a core-multi-shell structure where a GaAs quantum well is embedded into an AlGaAs shell wrapped around a [111] oriented GaAs nanowire, the electron and hole ground states are strongly confined to the corners of the hexagonally symmetric quantum well. Thus this confinement defines quantum wires which run along the length of the nanowires along its corners. Here we review single nanowire photoluminescence measurements which show the significant confinement energy of the excitons. For well widths larger than 5 nm, optical transitions between electron and hole excited states can be seen in excitation spectra, while for widths less than 5 nm only the ground state optical transitions are observed. For well widths smaller than 5 nm, high resolution spatially resolved photoluminescence measurements show directly the appearance of localized states. Single nanowire spectra from the 4 nm QWT sample display ultranarrow emission lines on the high energy side of the luminescence band. Spatially-resolved PL images show that these quantum dots are localized randomly along the length of the wire.

We use Raman scattering to study the spatially-resolved strain and stress in a complex zinc blende GaAs/GaP heterostructured nanowire which contains both axial and radial interfaces. The nanowires are grown by metal-organic chemical vapor deposition in the [111] direction with Au nano particles as catalysts, High spatial resolution Raman scans along the nanowires show the GaAs/GaP interface is clearly identifiable. We interpret the phonon energy shifts in each material as one approaches the interface.

We demonstrate how growth parameters may be adopted to produce morphologically controlled high-quality indium phosphide (InP) nanowires suitable for optoelectronic device applications. Growth temperature, V/III ratio, and catalyst particle size have a significant effect on the morphology, crystallographic quality, and optical properties of the resulting nanowires. Significantly, we find that higher growth temperatures or higher V/III ratios promote the formation of wurtzite (WZ) nanowires while zinc-blende (ZB) nanowires are favourable at lower growth temperatures and lower V/III ratios. Results also show that InP nanowires grow preferably in the WZ crystal structure than the ZB crystal structure with increasing V/III ratio or decreasing diameter. This causes a blue-shift in the bandgap as growth temperature increases. These results show that careful control of growth temperature, V/III ratio and catalyst size are crucial for obtaining InP nanowires of a specific crystal structure needed for device applications.

We use polarized photoluminescence excitation spectroscopy to observe the energy and symmetry of the predicted second conduction band in 130 nm diameter wurtzite InP nanowires. We find direct spectroscopic signatures for optical transitions among the A, B, and C hole bands and both the first and the second conduction bands. We determine that the splitting between the first and second conduction bands is 228 ± 7 meV in excellent agreement with theory. From these energies we show that the spin-orbit energy changes substantially between zinc blende and wurtzite InP. We discuss the two quite different solutions within the quasi-cubic approximation and the implications for these measurements. Finally, the observation of well-defined optical transitions between the B- and C-hole bands and the second conduction band suggests that either the theoretical description of the second conduction band as possessing Γ8 symmetry is incomplete, or other interactions are enabling these forbidden transitions.

We investigate how growth parameters may be chosen to obtain high quality GaAs nanowires suitable for optoelectronic device applications. Growth temperature and precursor flows have a significant effect on the morphology, crystallographic quality, intrinsic doping and optical properties of the resulting nanowires. Significantly, we find that low growth temperature and high arsine flow rate improve nanowire optical properties, reduce carbon impurity incorporation and drastically reduce planar crystallographic defects. Additionally, cladding the GaAs nanowire cores in an AlGaAs shell enhances emission efficiency. These high quality nanowires should create new opportunities for optoelectronic devices. © 2008 IEEE.

We report straight and vertically aligned defect-free GaAs nanowires grown on Si(111) substrates by metal-organic chemical vapor deposition. By deposition of thin GaAs buffer layers on Si substrates, these nanowires could be grown on the buffer layers with much less stringent conditions as otherwise imposed by epitaxy of III-V compounds on Si. Also, crystal-defect-free GaAs nanowires were grown by using either a two-temperature growth mode consisting of a short initial nucleation step under higher temperature followed by subsequent growth under lower temperature or a rapid growth rate mode with high source flow rate. These two growth modes not only eliminated planar crystallographic defects but also significantly reduced tapering. Core-shell GaAs-AlGaAs nanowires grown by the two-temperature growth mode showed improved optical properties with strong photoluminescence and long carrier life times. © 2011 American Chemical Society.

GaAs nanowires were grown on Si (111) substrates. By coating a thin GaAs buffer layer on Si surface and using a two-temperature growth, the morphology and crystal structure of GaAs nanowires were dramatically improved. The strained GaAs/GaP core-shell nanowires, based on the improved GaAs nanowires with a shell thickness of 25 nm, showed a significant shift in emission energy of 260 meV from the unstrained GaAs nanowires. © 2010 IEEE.

GaAs and InP based nanowires were grown epitaxially on GaAs or InP (111)B substrates by MOCVD via VLS mechanism. In this paper, I will give an overview of nanowire research activities in our group. © 2009 IEEE.

CW and time-resolved photoluminescence measurements are used to investigate exciton recombination dynamics in GaAsAlGaAs heterostructure nanowires grown with a recently developed technique which minimizes twinning. A thin capping layer is deposited to eliminate the possibility of oxidation of the AlGaAs shell as a source of oxygen defects in the GaAs core. We observe exciton lifetimes of ∼1 ns, comparable to high quality two-dimensional double heterostructures. These GaAs nanowires allow one to observe state filling and many-body effects resulting from the increased carrier densities accessible with pulsed laser excitation. © 2008 American Institute of Physics.

Temperature-dependent polarized microphotoluminescence measurements of single GaAsAlGaAs core-shell nanowires are used to probe their electronic states. The low-temperature emission from these wires is strongly enhanced compared with that observed in bare GaAs nanowires and is strongly polarized, reflecting the dielectric mismatch between the nanowire and the surrounding air. The temperature-dependent band gap of the nanowires is seen to be somewhat different from that observed in bulk GaAs, and the PL rapidly quenches above 120 K, with an activation energy of 17 meV reflecting the presence of nonradiative defects. © 2006 American Institute of Physics.

Semiconductor nanowires have recently emerged as a new class of materials with significant potential to reveal new fundamental physics and to propel new applications in quantum electronic and optoelectronic devices. Semiconductor nanowires show exceptional promise as nanostructured materials for exploring physics in reduced dimensions and in complex geometries, as well as in one-dimensional nanowire devices. They are compatible with existing semiconductor technologies and can be tailored into unique axial and radial heterostructures. In this contribution we review the recent efforts of our international collaboration which have resulted in significant advances in the growth of exceptionally high quality IIIV nanowires and nanowire heterostructures, and major developments in understanding the electronic energy landscapes of these nanowires and the dynamics of carriers in these nanowires using photoluminescence, time-resolved photoluminescence and terahertz conductivity spectroscopy. © 2011 Elsevier Ltd. All rights reserved.

Semiconductor nanowire (NW) heterostructures are increasingly important building blocks for electronic and optoelectronic devices. III-V nanowires with attention to well-controlled growth parameters, high structural quality and high optical quality NWs with unusually long recombination lifetimes can be achieved. We probe these NWs experimentally with both CW and time-resolved photoluminescence spectroscopy. The large surface-to-volume ratio is an intrinsic characteristic of nanowires which highlights the critical importance of surface and interface quality.

In conventional planar growth of bulk III-V materials, a slow growth rate favors high crystallographic quality, optical quality, and purity of the resulting material. Surprisingly, we observe exactly the opposite effect for Au-assisted GaAs nanowire growth. By employing a rapid growth rate, the resulting nanowires are markedly less tapered, are free of planar crystallographic defects, and have very high purity with minimal intrinsic dopant incorporation. Importantly, carrier lifetimes are not adversely affected. These results reveal intriguing behavior in the growth of nanoscale materials, and represent a significant advance toward the rational growth of nanowires for device applications.

We use temperature-dependent photoluminescence (PL), photoluminescence imaging, and time-resolved photoluminescence measurements to gain insights into the localization of excitons in single 2 nm GaAs/AlGaAs quantum well tube nanowires. PL spectra reveal the coexistence of localized and delocalized states at low temperatures, with narrow quantum dot-like emission lines on the high energy side of a broad emission band, and delocalized states on the low energy side. We find that the high energy QD-like emissions are metastable, disappearing at higher temperatures with only delocalized states (quantum well tube ground states) surviving. By comparing temperature- and time-dependent PL, we develop a theoretical model which provides insights into the confinement potentials and relaxation dynamics which localize the excitons in these quantum well tube nanowires.

This chapter describes how optical experiments on single semiconductor nanowires and nanowire heterostructures have illuminated both new physics and influenced the development of new growth methods that have resulted in nanowires of high quality. New physics here means the understanding of fundamental properties including band structures, the roles of symmetry and of strain in core/shell structures, the dynamics of excited electron–hole plasmas, and the exploration of more complex nanowire heterostructures like quantum well tubes. We will concentrate in this chapter mainly on the III–V materials such as wurtzite and zincblende InP, GaAs and GaAs/AlGaAs core/shell, and core/multishell heterostructures, where incredible improvements in materials quality have enabled the development of many new applications in recent years, from high efficiency solar cells, single nanowire lasers at room temperature, and single photon emitters, to a variety of chemical and biological sensors. We end the chapter with a brief mention of very recent experiments on some very new nanowire materials like InGaAs and GaAsSb whose bandgaps move distinctly into infrared wavelength regime.

Using a new technique, transient Rayleigh scattering, we show that measurements from a single GaAs/AlGaAs core-shell semiconductor nanowire provide sensitive and detailed information on the time evolution of the density and temperature of the electrons and holes after photoexcitation by an intense laser pulse. Through band filling, band gap renormalization, and plasma screening, the presence of a dense and hot electron-hole plasma directly influences the real and imaginary parts of the complex index of refraction that in turn affects the spectral dependence of the Rayleigh scattering cross-section in well-defined ways. By measuring this spectral dependence as a function of time, we directly determine the thermodynamically independent density and temperature of the electrons and holes as a function of time after pulsed excitation as the carriers thermalize to the lattice temperature. We successfully model the results by including ambipolar transport, recombination, and cooling through optic and acoustic phonon emission that quantify the hole mobility at ∼68,000 cm(2)/V·s, linear decay constant at 380 ps, bimolecular recombination rate at 4.8 × 10(-9) cm(3)/s and the energy-loss rate of plasma due to optical and acoustic phonon emission.

Measuring the Energy Landscape in Single Semiconductor Nanowires L.M. Smith, H.E. Jackson, J. Yarrison-Rice and Ch. Jagadish Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221-0011, United States Department of Physics, Miami University, Oxford, Ohio 45056, United States Department of Electronic Materials Engineering, Research School of Physics and Engineering The Australian National University, Canberra, ACT 0200, Australia

We study the photocurrent from photoexcited charged carriers excited with lasers of energy both above and below the energy gap in CdS nanostructures. We observe non-linear photocurrents in CdS nanosheet devices in the metal-semiconductor-metal configuration with Schottky contacts for sub-band gap excitations. Analysis of two-photon absorption dominated photocurrents reveals a nonlinear coefficient of β = 2 cm/GW for these nanosheet devices, which is comparable to those of bulk CdS. We demonstrate the use of the photocurrent polarization measurements to determine the orientation of atoms in the nanosheet.

In this Letter, 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 levels. 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 and an analysis of the quantum confined Stark effect of these dots provide insights into the nature of the exciton localization in these nanostructures.

Semiconductors have played an important role in the development of information and communications technology, solar cells, solid state lighting. Nanowires are considered as building blocks for the next generation electronics and optoelectronics. In this talk, I will introduce the importance of nanowires and their potential applications and discuss about how these nanowires can be synthesized and how the shape, size and composition of the nanowires influence their structural and optical properties. I will present results on axial and radial heterostructures and how one can engineer the optical properties to obtain high performance lasers, THz detectors and solar cells. Use of nanowires for engineering of neuronal networks will be discussed. Future prospects of the semiconductor nanowires will be discussed.

Grating couplers with nanoscale periodicity have been fabricated on silicon-on-insulator (SOI) substrates by electron beam lithography and reactive ion etching. A versatile experimental apparatus has been implemented to measure the efficiency of these gratings in coupling free-space radiation into planar waveguides. This coupling efficiency has been measured as a function of grating depth and the angle and wavelength of incident radiation. Coupling efficiencies of at least 5% and as high as 20% are demonstrated for wavelengths in the vicinity of 1550 nm and incident angles around 45deg.

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Mid-infrared GaAs1-xSbx/InP core/shell nanowires are grown coherently with perfectly twin-free zinc blende crystal structure. An unusual triangular InP shell with predominantly {112}A facets instead of {112}B facets is reported. It is found that this polarity preference is due to the surfactant role of Sb, which inhibits InP shell growth rate in the 112-A directions. This behavior reveals a new degree of control and tunability allowed in manipulating nanowire facet geometry and polarity in radial heterostructures by a simple means. Tuning the Sb composition in the core yields controllable intense photoluminescence emission in both the 1.3 and 1.5 μm optical telecommunication windows, up to room temperature for single nanowires. The internal quantum efficiency of the core/shell nanowires is experimentally determined to be as high as 56% at room temperature. Transient Rayleigh scattering analysis brings complementary information, revealing the photoexcited carrier lifetime in the core/shell nanowire to be ≈100 ps at 300 K and ≈800 ps at 10 K. In comparison, the carrier lifetime of core-only nanowire is below the detection limit of the system (25 ps). The demonstrated superior optical quality of the core/shell nanowires and their ideal emission wavelength range makes them highly relevant candidates for near-infrared optoelectronic applications.

We investigate the optical and transport properties of single GaAs/AlGaAs core-multishell Quantum Well Tube (QWT) nanowire heterostructure. The QWT is defined by a thin (8 nm) GaAs layer embedded inside a thick Al 0.4 Ga 0.6 As shell which surrounds a 50 nm diameter GaAs NW core see Fig.1. The electrons and holes in the thin GaAs shell are radially confined and are free along the nanowire length. The radial confinement results in confined quantum states as shown in the schematics in Fig.1. These nanowires growth was achieved by two temperature MOCVD growth techniques [1, 2]. Optical studies of these nanowires show high quantum efficiency of the QWT ground state photoluminescence compared to the core luminescence. The shift to higher energies above the core corresponds to increasing quantum confinement of the ground state transition of the quantum well tube [1]. The energy eigenvalues depicted in Fig.1 are numerically calculated values of the energies of the electrons and holes confined to the quantum well tube where the hexagonal symmetry of the nanowires is approximated by cylindrical symmetry.

Unlike nanowires, GaAs nanosheets exhibit no twin defects, stacking faults, or dislocations even when grown on lattice mismatched substrates. As such, they are excellent candidates for optoelectronic applications, including LEDs and solar cells. We report substantial enhancements in the photoluminescence efficiency and the lifetime of passivated GaAs nanosheets produced using the selected area growth (SAG) method with metal organic chemical vapor deposition (MOCVD). Measurements are performed on individual GaAs nanosheets with and without an AlGaAs passivation layer. Both steady-state photoluminescence and time-resolved photoluminescence spectroscopy are performed to study the optoelectronic performance of these nanostructures. Our results show that AlGaAs passivation of GaAs nanosheets leads to a 30- to 40-fold enhancement in the photoluminescence intensity. The photoluminescence lifetime increases from less than 30 to 300 ps with passivation, indicating an order of magnitude improvement in the minority carrier lifetime. We attribute these enhancements to the reduction of nonradiative recombination due to the compensation of surface states after passivation. The surface recombination velocity decreases from an initial value of 2.5 × 10(5) to 2.7 × 10(4) cm/s with passivation.

Using transient Rayleigh scattering (TRS) measurements, we obtain photoexcited carrier thermalization dynamics for both zincblende (ZB) and wurtzite (WZ) InP single nanowires (NW) with picosecond resolution. A phenomenological fitting model based on direct band to band transition theory is developed to extract the electron-hole-plasma density and temperature as a function of time from TRS measurements of single nanowires which have complex valence band structures. We find that the thermalization dynamics of hot carriers depends strongly on material (GaAs NW vs. InP NW) and less strongly on crystal structure (ZB vs. WZ). The thermalization dynamics of ZB and WZ InP NWs are similar. But a comparison of the thermalization dynamics in ZB and WZ InP NWs with ZB GaAs NW reveals more than an order of magnitude slower relaxation for the InP NWs. We interpret these results as reflecting their distinctive phonon band structures which lead to different hot phonon effects. Knowledge of hot carrier thermalization dynamics is an essential component for effective incorporation of nanowire materials into electronic devices.

The speed of a propagating pulse in polymer networks is strongly influenced by their microscopic structure. The measurement of pulse velocity can be used to determine network parameters, including ...

We have characterized a 10 μm square aperture selectively oxidized 850 nm vertical cavity surface emitting laser (VCSEL) utilizing a near-field scanning optical microscope coupled to a spectrometer. We have obtained both spatially and spectrally resolved images of the VCSEL’s emission. The spatially resolved intensity distributions were measured for the transverse modes of the VCSELs in the near field and far field at three different excitation currents, at threshold as well as above threshold. We identify the modes as being Hermite–Gaussian. Two-dimensional maps of the total integrated intensity of the spectra reveal an inhomogeneous gain distribution. The relative intensities of the various individual modes as well as their spatial orientation reflect how the available gain of the cavity is used. The wavelengths of the transverse modes were observed to increase with increasing current; calculations show that a temperature-induced change in refractive index of the cavity is the factor contributing to thi...

We have studied inelastic light scattering from acoustic phonons in end-linked poly(dimethylsiloxane) networks using a Brillouin scattering technique. A series of nearly monodisperse networks with molecular weights between cross links ranging from 800 g/mol to 30 000 g/mol are probed to study the effects of cross-linking and deformation on the phonon speed and attenuation. The Brillouin spectra are obtained using a combination of a three-pass tandem Fabry–Perot interferometer in conjunction with a CCD area detector. Longitudinal acoustic phonons of frequency 2–6 GHz propagating in the networks are studied at an excitation wavelength of 514.5 nm. The phonon frequencies and linewidths are measured as a function of the phonon wave vector by employing different scattering geometries. For highly cross-linked systems the phonon speed is shown to depend on the molecular weight. With decreasing cross-link density, the phonon velocity is observed to approach an asymptotic value. The dispersion of the acoustic phon...

Microfluorescence spectroscopy is used to measure the nature of stress transfer from matrix to fiber in a unidirectional fiber composite. A hybrid SiC fiber reinforced transparent glass matrix composite containing a few sapphire sensor fibers is developed. Sapphire fiber is used as a probe to generate fluorescence signal to measure stress in the fiber. The measured nature of load sharing by the fiber under flexural loading is compared with analytical predictions.

We present the fabrication and initial testing of novel optically interrogated pressure and temperature sensors fabricated directly on optical fibers using microelectromechanical systems (MEMS) technology. A new micromachining process for use on a flat fiber end face that includes photolithographic patterning, wet etching of a cavity and anodic bonding of a silicon diaphragm is utilized. Two prototype pressure sensors, fabricated on 400 μm diameter multimode fibers, have been tested displaying an approximately linear response to static pressure (14–80 psi). A prototype temperature sensor, fabricated by anodically bonding an ultra-thin crystalline silicon onto a fiber end face, has been tested in the range 25–300 °C. A minimum detectable temperature variation of 6 °C is observed. Since these sensors are significantly miniaturized, they will find application in situations where small size is advantageous and where dense arrays may be useful.

The Cincinnati Engineering Enhanced Math and Science (CEEMS) Program, funded by the National Science Foundation, DUE #1102990, empowers secondary math and science teachers to integrate the engineering design process and challenge-based learning into the courses they teach. Some of the curricular materials developed by CEEMS participating teachers also incorporate the fine arts into the design process, thus increasing student engagement and demonstrating the importance of a multi-disciplinary approach to learning. This paper describes seven challenge-based learning endeavors developed for middle school and high school math classrooms that demonstrate the connection with math and fine arts. One curricular unit is described in detail to highlight the incorporation of engineering design and challenge-based learning, while the other six are summarized to provide other examples. Traditionally, math instruction features the teacher as the “sage on the stage” and students working individually to apply formulas in order to find the “right” answers. In contrast, CEEMS' pedagogical approaches result in student-directed instruction, which includes cooperative learning groups using creativity and critical thinking to develop unique solutions to design challenges.

We report cw and time-resolved photoluminescence, and Raman scattering from CdSe/ZnSe self-assembled quantum dots. We investigate excitons localized in CdSe dots, exciton relaxation, and the role of phonon–exciton coupling.

Residual stress in fibers of a sapphire-fiber-reinforced glass-matrix composite arising from thermal expansion mismatch is characterized by measuring the shift of the optical fluorescence lines of Cr+3 in sapphire. The stress is found to vary along the fiber length as well as across the fiber cross-section for an uncoated fiber embedded in a glass matrix. Some finite residual stress is also measured at the fiber ends. Analytical models to explain the experimentally measured stress variations are proposed. The high spatial resolution of the microprobe spectrometer is demonstrated for mapping stresses across a specific section of the composite.

Micro-Raman scattering experiments have been carried out to study a SiGe/Si photoelastic optical channel waveguide structure. A Si3N4 stressor stripe was fabricated on a SiGe/Si planar waveguide to create local stress variation beneath the stripe. To characterize the local stress distributions which create the channel waveguide structure, we obtained Si–Si Raman peak shifts in the SiGe layer by utilizing spatially resolved Raman spectra both from the top surface across the width of the Si3N4 stripe and from an edge (i.e., a waveguiding region) beneath the stripe. Longitudinal optical phonon shifts observed from the top surface show a significant increase in compressive stress within the stripe width and in tensile stress just outside of stripe edges for a sample annealed at 600 °C. To map local changes of stresses in the waveguiding region beneath the stripe, spectra from two transverse optical phonons were measured from the edge and interpreted using the Raman polarization selection rules. The Si–Si Rama...

The evolution of self-assembled CdSe quantum dots deposited on (and subsequently capped by) ZnSe was investigated on a series of samples grown by molecular beam epitaxy, with CdSe coverages from 0.5 to 2.6 monolayers. The samples were investigated by cross-sectional scanning transmission electron microscopy, as well as macro- and microphotoluminescence. The results clearly indicated a coexistence of 2D ZnCdSe platelets and 3D islands, showing clearly that the platelets act as precursors for the formation of the 3D islands.

We report Raman scattering from CdSe/ZnSe self-assembled quantum dot structures. A series of samples, each with a different growth-interruption time before ZnSe caping of the CdSe dots, is investigated. ZnSe longitudinal optical (LO) phonon shifts as a function of the interruption time show a complex pattern due to the evolution of CdSe dots. We observe an LO phonon from the CdSe dots at $\ensuremath{\sim}221{\mathrm{cm}}^{\ensuremath{-}1}$ as well as an interface phonon at $\ensuremath{\sim}245{\mathrm{cm}}^{\ensuremath{-}1}.$ We present evidence that the interface phonon is localized at the interface between the CdSe dots and the ZnSe cap layer.

The evolution of CdSe quantum dot (QD) formation on ZnSe was investigated on a series of samples grown by MBE, with nominal CdSe coverages ranging from 1.1 to 2.6 monolayers (ML). Micro-PL data suggest strongly confined zero-dimensional excitons even for coverages as low as 1.1 ML. PL and micro-PL data show emission from the QDs as well as from an accompanying 2 D layer; the temperature dependence of these emissions is notably different. Strong red-shifts of both emissions as the CdSe coverage increases are observed.