We report an innovative label-free biosensor based on statistical analysis of several whispering gallery modes spectral shifts in polystyrene fluorescent microspheres using a custom microflow cytometer. Whispering gallery modes analysis enables detection of nanometer-sized analytes showing promising possibilities for virus, bacteria and molecular detection. To demonstrate this, fluorophore-doped microspheres of the appropriate size parameter are mixed in an aqueous solution. Then, a syringe pump pushes the solution through a fiber optic flow cell where a laser beam illuminates the analysis area to excite the microspheres and their fluorescence is collected. This device provides a low-cost and user friendly solution that could enhance spectrum acquisition rates up to 5 spectra per second thanks to the considerable amount of microspheres flowing through the excitation area per unit time. Finally, the fluorescence spectra are statistically investigated using an instantaneous measurement of apparent refractive index algorithm to determine a reliable value for the refractive index of the environment since the exact radius of the microsphere scanned is unknown. This refractive index becomes an effective value for the local perturbation caused by inhomogeneities on the microsphere surface and hence, determines whether or not inhomogeneities, such as bacteria, are adsorbed by comparing to a control sample. Combining a flow cell with our detection algorithm, we reduce the period of a 50 microspheres experiment from 161 minutes to 14 minutes when the flow rate is 2000 µl/h and the microsphere concentration is 5 µsphere/µl.
Semiconductor quantum dot nanocrystals (QDs) have unique optical properties such as size tunable photoluminescence (PL) wavelength and a chemically functionalized sufrace. Our CdSe/ZnS quantum dot nanocrystals have been made water-soluble by encapsulation in a micelle of positively charged amphiphilic copolymers. Layer-by-layer deposition of these QDs was done on sub-micrometer silica beads as well as magnetic and polymeric micro-sized beads. The negative surface charge of these various beads allowed successive stacking of cationic polyethylnimine, anionic polyacrylic acid sodium salt and the cationic encapsulated QDs. Multiple QD layers can be added by repeating the stacking process. The PL spectral of green QDs is moduoated by whispering galley mode resonances when the QDs arecoating a singe 3 μm bead. Depending on the quality factor of this microsphere, it can also be possible to detect perturbations caused by sufficient adsorption of biomolecules or even living microorganisms on a bead's surface by observing spectral shifts of the resonances. If different colorsof QDs are used to coat smaller beads where the modes are not spectrally resolved, an optical coating system can be devised base on the relative emission intensity for each color. The uniformity of a bead ensemble coded with 2 QD colors has been invesetigated, revealing a ~20% relataive standard deviation for various intensity levels. Better control of photobleaching through QD passivation reduced this number to ~8%, which would allow us to differentiate up to 2.6x1010 optical codes on our setup. Labelling large amount of molecules in solution, e.g. DNA sequences, then becomes possible with an appropriate biofunctionalization of bead surfaces.
Self-assembled quantum dot (QD) Semiconductor Optical Amplifiers (SOAs) are believed to have faster carrier recovery times than conventional multiple quantum well, or bulk SOAs. It is therefore of interest to study the carrier dynamics of QD SOAs to assess their potential as ultrafast nonlinear devices for switching and signal processing. In this work we report experimental characterization of the ultrafast carrier dynamics of a novel InAs/InGaAsP self-assembled QD SOA with its peak gain in the important 1.55 μm telecommunications wavelength range. The temporal dynamics are measured with a heterodyne pump-probe technique with 150 fs resolution. The measurements show carrier heating dynamics with lifetimes of 0.5-2.5 ps, and a 13.2 ps gain recovery, making the device a promising candidate for ultrafast switching applications. The results are compared to previous reports on QD amplifiers operating in the 1.3 μm and 1.1 μm spectral regions. This report represents the first study of the temporal dynamics of a QD SOA operating at 1.55 μm.
Photoluminescence (PL) was used to investigate the interdifflision of self-assembled InAs/GaAs quantum dots (QDs)
treated by rapid thermal annealing (RTA) and laser annealing. The observation ofintense and sharp shell structures confirmed
that the QDs retained their zero-dimensional density of states. In addition, three main effects of alloy intermixing were
demonstrated in QDs having different intersublevel spacings. The emission has been strongly blue-shifted, up to -200 meV for
RTA samples and 298 meV for the laser annealed ones. The intersublevel spacing was tuned between 6O meV to '-'25 meV
in the RTA case, but down to -12 meV in the case of laser-induced intermixing. Finally the inhomogeneous broadening
linearly decreased from a FWHM of-46 meV down to smaller than 15 meV for RTA and 8 meV in the most extreme case of
laser annealing. For samples annealed at the highest temperatures, the most energetic shells of QDs become unbound. Across
varying samples, the result ofthe intermixing was to increase the uniformity of their PL spectra. A onedimensional model of
Fickian diffusion for the growth direction was used to model their PL emission. Rapid thermal annealing and laser annealing
provide two additional ways of manipulating the energy levels of self-assembled QD ensembles by tuning the intersublevel
energy-spacing and the number ofconfined states.
From a recent study of the growth and optical properties of quantum dots (QD's), we demonstrated that artificial atoms with sharp electronic shells can be fabricated with good control, using self-assembled QD's grown by molecular beam epitaxy. Size and shape engineering of the QD's during growth permits the tailoring of their intersublevel energy spacings. We demonstrate a much improved uniformity of the macroscopic ensembles of QD's, with well-resolved electronic shells. In addition to size and shape engineering of the QDS's in the case of single-layer samples, we demonstrate significant improvements in the uniformity of the vertically self-aligned stacked QD's. State-filling spectroscopy of the zero-dimensional transitions between confined electrons and holes demonstrates that the energy levels are readily tunable. One to five confined levels, with an inter-level energy spacing between 25 and 90 meV, are obtained by adjusting the growth temperature or with post-growth annealings. Such QD's having well-defined excited-states have been grown in the active region of devices and results will be presented for lasers, detectors, or for structures displaying optical memory effects. For example, QD laser diodes with well-defined electronic shells are fabricated, and shape-engineered stacks of self-aligned QD's are used to increase the gain in the active region. Lasing is observed in the upper QD shells for small gain media, and progresses towards the QD ground states for longer cavity lengths. We obtained at 77K thresholds for Jth=15 A/cm2 for a 2 mm cavity lasing in the first excited state (p-shell), and at 300K for a 5 mm cavity, Jth is ~430A/cm2 with lasing in the d-shell. For an increased QD density, Jth is smaller than 100A/cm2 at room temperature. For inter- sublevel transitions, we demonstrated broadband normal incidence detection with responsivity approaching 1A/W at a detection wavelength of 5 microns. For interband detection, the photoluminescence decay time of p-i-n diode can be changed from ~3nsec to ~0.3nsec (3Ghz) with a reverse bias. For Qds capped with less than ~10 nm, remarkable charge transfers between QD and surface states lead to optical memory effects lasting over time-scales of several minutes.
Quantum Dot laser diodes have been made using InAs self- assembled quantum dots (QDs) in the active region of separate confinement heterostructures. The lasers grown by Molecular Beam Epitaxy (MBE) with stacked InAs QDs grown on GaAs gave record low thresholds of 13 A/cm2 at 77 K and 82 A/cm2 at 15 degree(s)C. On InP substrates, InAs QDs have been grown by Metal-Organic Chemical Vapor Deposition (MOCVD) with InP claddings, and by MBE in InGaAlAs separate confinement heterostructures. For the InAs/InP by MOCVD, the QD photoluminescence (PL) peaks between 1.51 micrometers and 1.57 micrometers at 77 K and close to 1.6 micrometers at 300 K. Transmission Electron Microscope analysis correlated with the PL results reveal that the QD density depends on the growth interrupt time which follows the InAs deposition. For the InAs/InGaAlAs by MBE, the QD electroluminescence peaks at approximately 1.42 micrometers at 300 K.