We report on the development of a portable fluorescence detection system. By combining a CMOS sensor and crosspolarization
scheme, we achieved multiplexed detection with a single white emission LED excitation. We demonstrated
fluorescence detection of Fluorescein and Rhodamine B in PDMS channels and achieved 1μM limit of detection (LOD).
Microparticles with green and red fluorescence were detected simultaneously without changing the light sources or
filters. We were able to clearly resolve microparticles, even if aggregated. The compact microfluorescence approach
offers high spatial and spectral resolution, and is suitable for multiplexed detection in point-of-care applications.
In this paper, we present a system for fluorescent monitoring of multiple gas concentrations using a simple and robust
single detector setup. Two gas-sensitive fluorescent films are illuminated by two separate excitation sources modulated
at different frequencies. Cross-polarization is used to shield the excitation light from the detector, allowing fluorescent
signals from both films to be simultaneously monitored and quantified using a microprocessor and lock-in detection.
Simultaneous detection of O2 and CO2 in a mixture of gases is done as a proof-of-concept of this frequency
discrimination technique. The detection of oxygen is based on the fluorescence quenching of platinum octaethylporphine
(PtOEP) lumiphore in presence of O2. The detection of CO2 is based on fluorescence quenching of hydroxypyrene
trisulfonic acid trisodium salt (HPTS) in presence of CO2. A single microprocessor is used to drive the excitation source
(different color LEDs), and sample and analyze the detector response at the two different frequencies. The device
demonstrated minimal crosstalk between the O2 and CO2 signals. The O2 concentration was measured in the useful
range between 20 and 0%, and CO2 demonstrated a useful range between 5% and 0%. The polarization filtering is
color-independent and can be readily extended to systems with more than two colors; due to the frequency
discrimination, it is immune to cross-talk in which one dye excites another. The whole arrangement is a compact, lowcost,
simultaneous multi-color fluorescent sensor system suitable for many biological, chemical, and gas-monitoring
Quantum dot (QD) lasers have many attractive features including low-threshold current density, high gain, low chirp and
superior temperature stability. In this paper, design, fabrication and characteristics of wafer-level index coupled
holographically fabricated 1.3μm QD distributed feedback (DFB) lasers are reported. Previously, 1.3 μm QD-DFB lasers
were fabricated with metal surface gratings, which are lossy and (being typically written by e-beam lithography) are
difficult to fabricate. In this paper, devices are fabricated using molecular beam epitaxy (MBE) for QD growth,
metalorganic chemical vapor deposition (MOCVD) for grating overgrowth, and wafer level interference lithography for
grating fabrication. Design and fabrication methods for these devices are reported. Analysis of broad area devices gives
a material transparency current density of ~150A/cm2. Single mode ridge waveguide devices with cavity length of 500
μm were tested. Device characteristics were fairly uniform, with typical DC characteristics of the devices of threshold
currents of ~35mA and slope efficiencies of ~0.11W/A. Measured bandwidths at room temperature were around 1.5
GHz, with very flat responses. Further analysis and design revision of the laser is ongoing.
Modulating retroreflectors (MRRs) couple passive optical retroreflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing-acquisition-tracking system required on only one end of the link. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large-area MQW placed in front of the aperture of a corner cube. For a MQW modulator, the maximum modulation rate can range into the gigahertz, limited only by the RC time constant of the device. Most MRR systems have used corner-cube retroreflectors with apertures of about 1 cm, which require large, and hence high-capacitance, modulators. Thus data rates exceeding a few megabits per second are not possible. We describe a new kind of MQW MRR that uses a cat's-eye retroreflector with the MQW in the focal plane of the cat's-eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 45-Mbit/s free space link over a range of 7 km is demonstrated.
Organic solid thin films of PMMA and surfactant-treated salmon deoxyribonucleic acid (DNA) were used as
host materials to dope sulforhodamine (SRh) laser dye. Amplified Spontaneous Emission (ASE) was observed from the
dye-doped thin films pumped by frequency-doubled Nd:YAG laser, with DNA host showed a lower ASE pump
threshold. Distributed feedback (DFB) laser structures were fabricated on both dye-doped thin films for the 2nd order
emission of SRh at 650 nm. Stimulated Emission (Lasing) was obtained by pumping with a doubled Nd:YAG laser at
532 nm. The DNA DFB devices lasing threshold was 30&mgr;J/cm2 or 3.75kW/cm2. The emission linewidth decreased from
~ 30 nm in the ASE mode to < 0.4 nm in the lasing mode. The slope efficiency of the laser emission was ~ 1.2%. Similar
emission linewidth change was observed in PMMA DFB devices while the lasing threshold was 53 &mgr;J/cm2 or
6.63kW/cm2 with a slope efficiency of ~0.63%.
The trend in medical equipment is toward compact and integrated low cost medical test devices. Fluorescence-based assays are used to identify specific pathogens through the presence of dyes, but typically require specialized microscopes and narrow-band optical filters to extract information. We present a novel method of using polarizers in cross orientation with each other to filter out excitation light and allow detection of low signal levels of fluorescence with a simple intensity-based detector in the presence of high levels of excitation light. This concept is demonstrated using an inverted microscope fitted with a halogen lamp as the excitation source and an organic photovoltaic (PV) cell as the intensity detector. The excitation light is linearly polarized and used to illuminate a microfluidic device containing a 50µl volume of Rhodamine 6G dye dissolved in water. The detector (with a second polarizer orientated perpendicularly to the first) is placed over the microfluidic device. The resulting emission signal was detected by the organic PV cell down to a concentration of 100 nM This suggests that an integrated microfluidic device, with a PV detector and an organic light emitting excitation source and integrated polarizers, could be fabricated to realize a economical "lab on a chip" device for fluorescence assays.
Carrier dynamics in self-assembled quantum dots, grown by molecular beam epitaxy, have been studied. The temperature dependence of the relaxation times, measured by room temperature high frequency impedance response of quantum dot lasers and by low temperature (T=4K) differential transmission spectroscopy. strongly suggests that electronhole scattering is the dominant scattering mechanism in quantum dots. The favorable relaxation times can be exploited to realize far infrared emission and detection based on intersubband transitions in the dots.
The characteristics of high-speed quantum well and quantum dot lasers are described. It is seen that substantial improvements in small-signal modulation bandwidth are obtained in both 1 micrometers (48 GHz) and 1.55 micrometers (26 GHz) by tunneling electrons directly into the lasing subband. In quantum dots the small-signal modulation bandwidth is limited by electron-hole scattering to approximately 7 GHz at room temperature and 23 GHz at 80 K.
To circumvent the numerous performance-limiting effects of hot-carrier phenomena in semiconductor quantum-well lasers, we have demonstrated the innovative approach of directly injecting carriers into the lasing subband by tunneling. These lasers, made with a variety of material systems, have shown evidence of reduced hot-carrier effects. Specifically, measured small-signal modulation bandwidth of approximately 50 GHz and maximum intrinsic bandwidth of 110 GHz have been achieved with 0.98 micrometers lasers. These are the highest measured modulation bandwidths in any laser. Auger recombination has been virtually eliminated in 1.55 micrometers lasers and reduced chirp and temperature dependence are also demonstrated. Significant reduction of hot-carrier and carrier leakage effects have also been recently demonstrated in small-area vertical-cavity surface-emitting lasers. These experimental results are supported by recent simulations that identify gain suppression in high speed lasers to be caused by a coupling between the electron temperature and the quasi Fermi level. Lasers with quantum dots as gain media promise high differential gain, very low threshold current, temperature-insensitive operation and high modulation bandwidth. We have investigated room-temperature single-mode ridge-waveguide quantum box lasers in which the quantum box gain regions are realized by self-organized growth and carrier injection is achieved by conventional means over hetero-barriers and by tunneling. The tunneling injection quantum dot lasers show improvements in both differential gain (6 X 10-14 cm2) and modulation bandwidth. These results will be presented and discussed.