P. Cusumano

Suppression of quantum well intermixing in GaAs/AlGaAs laser structures using phosphorus-doped SiO2 encapsulant layer

A phosphorus-doped silica (SiO2:P) cap containing 5 wt% P has been demonstrated to inhibit the bandgap shifts of p-i-n and n-i-p GaAs/AlGaAs quantum well laser structures after rapid thermal processing. The intermixing suppression has been attributed to the fact that SiO2:P is more dense and void free compared with standard SiO2 together with a strain relaxation effect of the cap layer during annealing. Band gap shift differences as large as 100 meV have been observed from samples capped with SiO2 and with SiO2:P. The n-i-p structure showed a higher degree of intermixing compared to p-i-n structure. This behaviour has been attributed to the rise of Fermi level in the n doped structure, through which the formation energy of Ga vacancies is reduced compared to the p doped structure.

Improvements in mode-locked semiconductor diode lasers using monolithically integrated passive waveguides made by quantum-well intermixing

By using the technique of quantum-well intermixing (QWI), monolithically integrated passive, and active waveguides can be fabricated. It is shown that mode-locked extended cavity semiconductor lasers with integrated low-loss passive waveguides display superior performance to devices in which the entire waveguide is active: the threshold current is a factor of 3-5 lower, the pulsewidth is reduced from 10.2 ps in the all active laser to 3.5 ps in the extended cavity device and there is a decrease in the free-running jitter level from 15 to 6 ps (10 kHz-10 MHz).

High-power diode-pumped AlGaAs surface-emitting laser.

We report the development and characterization of an efficient diode-pumped surface-emitting semiconductor laser operating at approximately 870 nm. By using a semiconductor Bragg reflector stack/multiple GaAs quantum well structure, mounted within a conventional laser cavity, we achieved single transverse mode laser output powers of 153 mW. Self-tuning over a 15-nm spectral range has been obtained.

Effect of driving method on the degradation of organic light emitting diodes

Abstract Lifetime testing results are reported for organic light emitting diodes (OLEDs) having the structure ITO (anode)/ N , N ′-diphenyl- N , N ′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD)/tris-(8-hydroxyquinoline)aluminum (AlQ)/Al (cathode) and operated using dc and pulsed waveforms for comparison. In ambient atmosphere non-encapsulated devices show a lifetime of about 70xa0h in pulsed operation at an initial luminance of 500xa0cd/m 2 , almost four times longer than in dc operation. A fast initial decay of luminance is observed for dc operation. It is most probably due to a combination of Joule heating and mobile ionic impurities migration within the OLED structure under the continuous forward electric field. We study also the transient electroluminescence (EL) and report the observation of a short duration spike in the leading edge of the EL response. Its origin is attributed to the temporal shift of the recombination zone in the AlQ layer from the bulk to the interface with TPD subsequent the application of a voltage step.

High-quality extended cavity ridge lasers fabricated by impurity-free vacancy diffusion with a novel masking technique

By using phosphorous doped (5% wt P) silica as masking material and standard silica capping to promote quantum well interdiffusion, GaAs-AlGaAs ridge waveguide QW lasers with integrated transparent waveguides were fabricated. With a selective differential blue-shift of 30 nm in the absorption edge, devices with 400-/spl mu/m/2.73-mm-long active/passive sections exhibited threshold currents of 8 mA in CW operation, only 1 mA higher than that for normal lasers of the same active length and from the same chip. This 14% increase in threshold current was accompanied by a slope efficiency decrease of 40%. Losses of 3.2 cm/sup -1/ were measured in the passive waveguides at the lasing wavelength using the Fabry-Perot resonance method. This value is among the lowest reported so far using an impurity-free disordering technique.

Mode-locked operation of a diode-pumped, external-cavity GaAs/AlGaAs surface emitting laser

Summary form only given. Optically-pumped, external-cavity, surface-emitting semiconductor lasers show great potential for overcoming a number of limitations in conventional semiconductor laser sources. Such devices offer wavelength flexibility, reduced criticality of design, are largely free of post growth processing, and in principle offer power scaling in a circularly-symmetric transverse mode to 0.1-1.0 W output levels. Furthermore, various optical elements can be introduced into the external cavity to control the nature of the laser output to give, for example, tunable single frequency or mode-locked operation. Here, we report on a diode-pumped GaAs/AlGaAs external-cavity surface emitting laser, concentrating particularly on power scaling and mode-locked performance.

GaAs/AlGaAs photonic integrated circuits fabricated using impurity-free vacancy disordering

Impurity free vacancy disordering (IFVD) using dielectric caps to induce intermixing in the GaAs/AlGaAs system is described. Silica is used to promote intermixing whilst strontium fluoride is used as a mask against intermixing. Selective bandgap-widening of GaAs/AlGaAs double quantum well laser material has been used to fabricate monolithic extended cavity strip- loaded waveguide lasers. With a differential shift of 21 nm in the wavelength of the photoluminescence peak, overall losses in the extended cavities were less than 6 cm-1 and a red-shift of the lasing spectrum with increasing passive section length is reported. Electroabsorption optical modulators integrated with passive waveguides have been fabricated using an epitaxial structure identical to that of the laser. At a wavelength of 861.6 nm, devices with a 400 micrometers long modulator section showed ON/OFF ratios greater than 35 dB for a reverse bias voltage of 3 V. A variation of the IFVD technique uses partial area coverage by a strontium fluoride mask under a silica cap to determine the amount of quantum well intermixing. The bandgap can then be varied at will across a wafer. Bandgap tuned lasers were fabricated using this technique. Five distinguishable lasing wavelengths were observed from five selected intermixed regions on a single chip. These lasers showed no significant change in transparency current, internal quantum efficiency or internal propagation loss, which indicates that the material quality was not degraded after intermixing.

Graded reflectivity micromirror arrays.

A technique for fabricating arrays of graded reflectivity micromirrors with diameters as small as 25 microm is reported. It is based on laser-induced physical vapor deposition through microholes on a thin free-standing noncontact mask, and it is suitable for applications in micro-optics and solid-state laser technology.

Effects of temperature on radiative transitions of erbium in lead-tellurium-germanate glass

Summary form only given. Er/sup 3+/ has been shown to be a good rare-earth dopant of crystals and glasses for practical applications as laser-active media and infrared-to-visible signal converters. In the present work we used low-temperature photoluminescence measurements in the 4 to 300 K range to study the effects of temperature on some radiative transitions of Er/sup 3+/ in lead-tellurium-germanate glass being developed for fiber laser applications. The transitions studied include the green emission lines /sup 4/S/sub 5/2//spl rarr//sup 4/I/sub 15/2/ (550 nm) and /sup 2/H/sub 11/2//spl rarr//sup 4/I/sub 15/2/ (530 nm), the red emission band /sup 4/F/sub 9/2//spl rarr//sup 4/I/sub 15/2/ (670 nm), and the near-infrared band /sup 4//sub 3/2//spl rarr//sup 4/I/sub 15/2/ (850 nm).

Measurement of drift mobilities in amorphous organic films using the Time of Flight method

We apply the Time of Flight (TOF) technique to study carrier mobility in N, N’-diphenyl-N,N’-bis(3-methylphenyl) -1,1-biphenyl-4,4’-diamine (TPD) and tris(8-hydroxyquinolato) aluminium (Alq3). These materials are two examples of, respectively, hole and electron transporting molecular materials. Measurements are performed in free air or under vacuum varying the experimental parameters such as laser pulse intensity and single shot irradiation. We observe a transition from dispersive to non dispersive transport changing the experimental conditions.