T. S. Sosnowski

In(Ga)As/GaAs self-organized quantum dot lasers: DC and small-signal modulation properties

Self-organized growth of InGaAs/GaAs strained epitaxial layers gives rise to an ordered array of islands via the Stranski-Krastanow growth mode, for misfits >1.8%. These islands are pyramidal in shape with a base diagonal of /spl sim/20 nm and height of /spl sim/6-7 nm, depending of growth parameters. They therefore exhibit electronic properties of zero-dimensional systems, or quantum dots. One or more layers of such quantum dots can be stacked and vertically coupled to form the gain region of lasers. We have investigated the properties of such single-layer quantum dot (SLQD) and multilayer quantum dot (MLQD) lasers with a variety of measurements, including some at cryogenic temperatures. The experiments have been complemented with theoretical calculations of the electronic properties and carrier scattering phenomena in the dots. Our objective has been to elucidate the intrinsic behavior of these devices. The lasers exhibit temperature independent threshold currents up to 85 K, with T/sub 0//spl les/670 K. Typical threshold currents of 200-/spl mu/m long room temperature lasers vary from 6 to 20 mA. The small-signal modulation bandwidths of ridge waveguide lasers are 5-7.5 GHz at 300 K and increased to >20 GHz at 80 K. These bandwidths agree well with electron capture times of /spl sim/30 ps determined from high-frequency laser impedance measurements at 300 K and relaxation times of /spl sim/8 ps measured at 18 K by differential transmission pump-probe experiments. From the calculated results we believe that electron-hole scattering intrinsically limits the high-speed performance of these devices, in spite of differential gains as high as /spl sim/7/spl times/10/sup -14/ cm/sup 2/ at room temperature.

Real-time dispersion analyzer of femtosecond laser pulses with use of a spectrally and temporally resolved upconversion technique

We demonstrate a real-time femtosecond-laser-pulse analyzer by using a spectrally and temporally resolved upconversion technique (STRUT) for characterization of the phase and the intensity. The STRUT provides simple but reliable analysis of femtosecond pulses by employing a narrow-bandpass dielectric filter in one arm of a conventional single-shot upconversion autocorrelator and analyzing the spatiotemporal upconversion signal with a monochromator. The resulting spatiotemporal and spatiospectral image presents clear and complete information about femtosecond pulses produced by either oscillators or amplifiers. Characterization of 2-nJ, 60-fs Ti:sapphire oscillator pulses is achieved with 0.5 s data acquisition time and 0.2-s computational time.

High-carrier-density electron dynamics in low-temperature-grown GaAs

Pump-probe differential transmission measurements examine high-carrier-density phenomena in as-grown and annealed GaAs samples grown at temperatures from 210 to 270 °C. We observe trap saturation and Auger recombination, and accurately model the measurements on annealed samples with a simple two level rate equation, allowing us to extract the trapped-electron lifetimes.

Photoluminescence and time-resolved photoluminescence characteristics of InxGa(1−x)As/GaAs self-organized single- and multiple-layer quantum dot laser structures

The characteristics of ground and excited state luminescent transitions in In0.4Ga0.6As/GaAs and In0.35Ga0.65As/GaAs self-organized single- and multiple-layer quantum dots forming the active regions of lasers have been studied as a function of incident excitation intensity, temperature and number of dot layers. The results have been correlated with molecular beam epitaxial growth conditions. The threshold excitation density for the saturation of the ground state increases with the number of dot layers and no saturation is observed in samples with more than six dot layers up to an excitation power density of 2 kW/cm2. The luminescent decay times for the ground and excited states are around 700 and 250 ps, respectively, almost independent of the number of dot layers.

Chirped-pulse amplification of 85-fs pulses at 250 kHz with third-order dispersion compensation by use of holographic transmission gratings

We demonstrate pulse stretching and compression in a high-repetition-rate chirped-pulse Ti:sapphire regenerative amplifier, using high-efficiency holographic transmission gratings. A quantitative dispersion measurement technique is developed to characterize dispersion of the system to the third order. After recompression with third-order dispersion compensation, 3.1-microJ 85-fs, nearly transform-limited pulses are obtained.

PRODUCTION OF 30-FS PULSES TUNABLE THROUGHOUT THE VISIBLE SPECTRAL REGION BY A NEW TECHNIQUE IN OPTICAL PARAMETRIC AMPLIFICATION

A multipass optical parametric amplif ier is pumped by a 250-kHz microjoule-level Ti:sapphire regenerative amplif ier system. Tuning each pass of the amplif ier to provide gain for slightly different spectral regions of a white-light continuum source produces broadband pulses that may be compressed to less than 30 fs.

Double-pulse fluorescence lifetime measurements

It is demonstrated that fluorescence lifetimes in the nanosecond and picosecond time‐scale range can be observed with the recently proposed double‐pulse fluorescence lifetime imaging technique (Müller et al., 1995, Double‐pulse fluorescence lifetime imaging in confocal microscopy. J. Microsc177, 171–179).

Chirped-pulse amplification of ultrashort pulses with a multimode Tm:ZBLAN fiber upconversion amplifier

Microjoule pulse energies are achieved from a single-stage upconversion fiber amplifier for the first time, to our knowledge, in this demonstration of chirped-pulse amplification with a multimode Tm:ZBLAN fiber. A Ti:sapphire laser system provides the seed pulse for the fiber upconversion amplifier that produces picosecond pulse trains with energies as great as 16 μJ at a repetition rate of 4.4 kHz.

Injection of ultrafast regenerative amplifiers with low energy femtosecond pulses from an Er-doped fiber laser

Abstract A very compact, low-noise and stable frequency-doubled Er-fiber laser system has been used to injection-seed a 10 Hz and a 250 kHz Ti:sapphire regenerative amplifier systems.

Upconversion chirped-pulse amplification of ultrashort pulses using a multimode Tm:ZBLAN fiber

Microjoule pulse energies are achieved from a single stage upconversion fiber amplifier for the first time in this demonstration of chirped pulse amplification using a multimode Tm:ZBLAN fiber. A Ti:sapphire laser system provides the seed pulse for the upconversion fiber amplifier which produces subpicosecond pulse trains with energies as great as 16 (mu) J at repetition rate of 4.4 kHz. The compressed pulse peak power is more than 1 MW, and the pulse is characterized both temporally and spatially.