S. Ten

PbS quantum-dot doped glasses as saturable absorbers for mode locking of a Cr: forsterite laser

Quantum confined nanocrystals of PbS in glass were used as intracavity saturable absorbers to obtain passive continuous-wave mode locking in a Cr:forsterite laser. We obtained near transformed-limited 4.6 ps laser pulses at 110 MHz repetition rate, and a wide tunability range of 1207–1307 nm. The absorption saturation intensity of the quantum-dot PbS doped glasses was measured to be 0.2 MW/cm2.

Ultrafast electron and hole tunneling in (Ga,In)As/(Al,In)As asymmetric double quantum wells

We present unambiguous experimental evidence for the dramatic dependence of hole tunneling rates on in‐plane momentum in (Ga,In)As/(Al,In)As asymmetric double quantum wells. Holes generated near the band edge tunnel on hundred picosecond time scales, whereas holes excited with large excess energy tunnel on subpicosecond time scales. The mechanism responsible for this increase of more than three orders of magnitude in the hole tunneling rate is nonresonant delocalization of hole wave functions by band mixing in the valence band.

Temperature‐ and carrier‐density‐dependent electron tunneling kinetics in (Ga,In)As/(Al,In)As asymmetric double quantum wells

We present a detailed investigation of electron tunneling in (Ga,In)As/(Al,In)As asymmetric double quantum wells as a function of different excitation and temperature conditions. We show that tunneling dynamics depend strongly on the initial carrier temperature and momentum. For example, electron and hole tunneling out of the narrow well is complete at low temperature. However at room temperature carriers do not exhibit any tunneling kinetics. We propose a simple kinetic model which describes the observed population dynamics at different carrier densities, temperatures, and excitation conditions.

Ultrafast excitonic room temperature nonlinearity in neutron irradiated quantum wells

Sharp room temperature exciton features and complete recovery of the excitonic absorption with 21 ps time constant are demonstrated in neutron irradiated (Ga,Al)As/GaAs multiple quantum wells. Carrier lifetime reduction is consistent with the EL2 midgap defect which is efficiently generated by fast neutrons. Influence of gamma rays accompanying neutron irradiation is discussed. Neutron irradiation provides a straightforward way to control carrier lifetime in semiconductor heterostructures with minor deterioration of their excitonic properties.

Design of a novel semiconductor laser-based chemical sensor

We present concept and design considerations of a novel optical sensor based on the distributed Bragg reflector (DBR) laser. The operation of this sensor relies on the shift of the lasing wavelength upon exposure of the chemically sensitive element to the gas species to be detected. We discuss physics of the proposed device, show the compromises in its design, and estimate potential sensitivity. Compactness and simplicity of this DBR laser- based optical sensor may play a crucial role in its mass production for various applications.

In-situ chemical detection based on photonic devices

A novel scheme of a laser-based chemical sensor has been examined. The scheme is based on the lasing frequency shift of a DBR laser as a result of refractive index change of the sensitive coating in the presence of chemicals in question. The applicability and advantages of different schemes are discussed. The results of preliminary experiments related to the construction and stability of an external cavity DBR laser and interferometric measurements of refractive index change are presented.

Exciton tunneling in wide-band-gap semiconductors

We have studied exciton tunneling in (Zn,Cd)Se/ZnSe asymmetric double quantum wells using femtosecond time resolved transmission, photoluminescence and time-resolved photoluminescence measurements. The strong Coulomb correlation as well as Frohlich electron LO-phonon interaction in II-VI semiconductors make the tunneling process significantly different from that in III-VI structures. We observe fast (1 ps) exciton tunneling out of the narrow well, although LO-phonon scattering is forbidden for holes in a single- particle picture. However, our theoretical analysis shows that tunneling of the exciton as a whole entity with the emission of only one LO-phonon is very slow. Instead, the exciton tunnels via an indirect state in a two-step process whose efficiency is dramatically enhanced by Coulomb effects.

Electroabsorption and carrier dynamics in (Ga,In)As/(Al,In)As asymmetric double quantum wells

We report enhanced electroabsorption in selectively doped (Ga,In)As/(Al,In)As Asymmetric Double Quantum Wells (ADQWs) by the use of real space electron transfer. The electron concentration in both the wide and narrow wells is investigated by field dependent absorption and photoluminescence spectroscopy. Additionally, a study of carrier dynamics in these ADQW structures indicates that electrons tunnel between the coupled wells on picosecond time-scales.

Ultrafast hole tunneling in asymmetric double quantum wells

We present the results of an experimental study of tunneling in Asymmetric Double Quantum Well (ADQW) structures for which holes were found to tunnel from the narrow well to the wide well on sub-picosecond time-scales. These times are as fast, or faster than electron tunneling times despite the absence of resonances between hole states. Valence band structure calculations for our ADQW structures indicate that ultrafast hole tunneling can be attributed spin-dependent delocalization of the hole wavefunctions with a concomitant singularity (in principle) in the density of final wide well states.

Novel multiple quantum well modulators for optical interconnects

Novel multiple quantum well (MQW) optical modulators for use in time-division optical fiber interconnects are presented. A bit-error-rate analysis of a time-division receiver indicates high contrast ratio optical gates are required for high-speed interconnect applications. A high contrast MQW gate, consisting of a nonlinear asymmetric reflection modulator, suitable for use in optical time-division systems is presented which utilizes the GaAlInAs alloy lattice- matched to InP. This system is ideal for optical interconnect applications since MQW materials and devices are easily designed for operation in the optical fiber transmission windows of 1.3 and 1.5 micrometers . Utilizing asymmetric double quantum wells (ADQWs) as the nonlinear spacer for the asymmetric reflection modulator also is discussed. The recovery time of ADQWs can be tailored for interconnect applications by choosing the optimum width of the tunnel barrier. Electro-optic modulators which utilize real space transfer of electrons in ADQWs also are presented.