Huw D. Summers

Characterization of semiconductor laser gain media by the segmented contact method

In this paper, we describe methods for analysis of edge-emitted amplified spontaneous emission spectra measured as a function of the pumped stripe length. We show that both the modal gain and the unamplified spontaneous emission spectra can be extracted from the data, and we describe a means of calibrating the spontaneous emission in real units, without requiring the carrier populations to be described by Fermi functions. The gain and emission spectra can be determined for transverse electric and transverse magnetic polarizations and by summing the recombination currents for each polarization the total radiative current can be measured. This enables the overall internal radiative quantum efficiency to be calculated. Once the calibration factor is known the internal stimulated recombination rate at the facet can also be estimated. The experiment can be configured to give a measurement of the passive modal absorption of the gain medium. The internal optical mode loss can be determined from the long-wavelength region of the gain spectrum or the modal absorption spectrum. In summary, we show that measurements of amplified spontaneous emission spectra provide a full characterization of the gain medium.

Experimental investigation of the effect of wetting-layer states on the gain-current characteristic of quantum-dot lasers

Using experimental measurements of the gain–current characteristic as a function of temperature in InGaAs quantum-dot lasers, we demonstrate that it is the population of wetting-layer states that leads to a saturation of the population inversion in dot states and hence to the saturation of gain in a quantum-dot laser. At 300 K, the maximum modal gain for a three-layer structure is reduced from 53 to 14 cm−1.

Statistical analysis of nanoparticle dosing in a dynamic cellular system

The delivery of nanoparticles into cells is important in therapeutic applications and in nanotoxicology. Nanoparticles are generally targeted to receptors on the surfaces of cells and internalized into endosomes by endocytosis, but the kinetics of the process and the way in which cell division redistributes the particles remain unclear. Here we show that the chance of success or failure of nanoparticle uptake and inheritance is random. Statistical analysis of nanoparticle-loaded endosomes indicates that particle capture is described by an over-dispersed Poisson probability distribution that is consistent with heterogeneous adsorption and internalization. Partitioning of nanoparticles in cell division is random and asymmetric, following a binomial distribution with mean probability of 0.52-0.72. These results show that cellular targeting of nanoparticles is inherently imprecise due to the randomness of nature at the molecular scale, and the statistical framework offers a way to predict nanoparticle dosage for therapy and for the study of nanotoxins.

Determination of single-pass optical gain and internal loss using a multisection device

We describe a technique for the measurement of optical gain and loss in semiconductor lasers using a single, multisection device. The method provides a complete description of the gain spectrum in absolute units and over a wide current range. Comparison of the transverse electric and transverse magnetic polarized spectra also provides the quasi-Fermi-level energy separation. Measurements on AlGaInP quantum well laser structures with emission wavelengths close to 670 nm show an internal loss of 10 cm−1 and peak gain values up to 4000 cm−1 for current densities up to 4 kA cm−2.

Quantitative characterization of nanoparticle agglomeration within biological media

Quantitative analysis of nanoparticle dispersion state within biological media is essential to understanding cellular uptake and the roles of diffusion, sedimentation, and endocytosis in determining nanoparticle dose. The dispersion of polymer-coated CdTe/ZnS quantum dots in water and cell growth medium with and without fetal bovine serum was analyzed by transmission electron microscopy (TEM) and dynamic light scattering (DLS) techniques. Characterization by TEM of samples prepared by plunge freezing the blotted solutions into liquid ethane was sensitive to the dispersion state of the quantum dots and enabled measurement of agglomerate size distributions even in the presence of serum proteins where DLS failed. In addition, TEM showed a reduced packing fraction of quantum dots per agglomerate when dispersed in biological media and serum compared to just water, highlighting the effect of interactions between the media, serum proteins, and the quantum dots. The identification of a heterogeneous distribution of quantum dots and quantum dot agglomerates in cell growth medium and serum by TEM will enable correlation with the previously reported optical metrology of in vitro cellular uptake of this quantum dot dispersion. In this paper, we present a comparative study of TEM and DLS and show that plunge-freeze TEM provides a robust assessment of nanoparticle agglomeration state.

Development of FRET-based assays in the far-red using CdTe quantum dots.

Colloidal quantum dots (QDs) are now commercially available in a biofunctionalized form, and Förster resonance energy transfer (FRET) between bioconjugated dots and fluorophores within the visible range has been observed. We are particularly interested in the far-red region, as from a biological perspective there are benefits in pushing to ∼700 nm to minimize optical absorption (ABS) within tissue and to avoid cell autofluorescence. We report on FRET between streptavidin- (STV-) conjugated CdTe quantum dots, Qdot705-STV, with biotinylated DY731-Bio fluorophores in a donor-acceptor assay. We also highlight the changes in DY731-Bio absorptivity during the streptavidin-biotin binding process which can be attributed to the structural reorientation. For fluorescence beyond 700 nm, different alloy compositions are required for the QD core and these changes directly affect the fluorescence decay dynamics producing a marked biexponential decay with a long-lifetime component in excess of 100 nanoseconds. We compare the influence of the two QD relaxation routes upon FRET dynamics in the presence of DY731-Bio.

Thermodynamic balance in quantum dot lasers

The spontaneous emission and optical gain spectra from an InGaAs quantum dot laser have been independently measured under the same operating conditions. Using these spectra a combined probability-distribution function describing the electron occupancy in the conduction and valence bands has been experimentally determined. Comparison of this function with theoretical curves based on Fermi-Dirac statistics shows that for temperatures down to 100 K the carrier occupancy statistics are accurately described by thermal distributions. Measurements at 70 K show a breakdown of thermodynamic equilibrium indicated by non-thermal carrier distributions.

Optical mode loss and gain of multiple-layer quantum-dot lasers

Using an electrically pumped multisection technique, we have directly measured the internal optical mode loss of semiconductor-laser structures containing 1, 3, 5, and 7 layers of uncoupled InGaAs quantum dots. The optical loss does not increase with the number of dot layers so higher net modal gain can be achieved by using multiple layers. The maximum modal gain obtained from the ground state increases with dot layer number from 10±4 cm−1 for a single layer to 49±4 cm−1 for the 7 layer sample, which is typical of the threshold gain requirement of a 350 μm long device with uncoated facets.

Measurement of true spontaneous emission spectra from the facet of diode laser structures

Measurement of the spontaneous emission and gain spectra provides a complete characterization of a semiconductor gain medium, however, this requires the observation of emission in two directions to avoid amplification of the spontaneous emission spectrum. We show that both the gain spectrum and the true spontaneous emission spectrum can be obtained from amplified spontaneous emission (ASE) spectra measured from the end of a segmented-contact device. The spontaneous emission spectra agree with spectra measured through a top contact window. If the carrier populations are fully inverted at low photon energy, it is possible to convert the ASE-derived spontaneous emission into real units.

Label-free cell cycle analysis for high-throughput imaging flow cytometry

Imaging flow cytometry combines the high-throughput capabilities of conventional flow cytometry with single-cell imaging. Here we demonstrate label-free prediction of DNA content and quantification of the mitotic cell cycle phases by applying supervised machine learning to morphological features extracted from brightfield and the typically ignored darkfield images of cells from an imaging flow cytometer. This method facilitates non-destructive monitoring of cells avoiding potentially confounding effects of fluorescent stains while maximizing available fluorescence channels. The method is effective in cell cycle analysis for mammalian cells, both fixed and live, and accurately assesses the impact of a cell cycle mitotic phase blocking agent. As the same method is effective in predicting the DNA content of fission yeast, it is likely to have a broad application to other cell types.