I. J. Blewett

Fastscan z-scan system for determining optical non-linearities in semiconductors

Abstract Traditional z -scan techniques have been used to determine the non-linear optical coefficients of semiconductors. This is usually carried out by scanning a sample along the optical z -axis using a manual or computer-controlled linear translation stage. We report upon a novel z -scan method involving a fastscanning sampleholder: This gives real-time z -scan traces and greatly improves experimental efficiency and applicability. The technique is used here to study the two-photon absorption coefficient of ZnSe.

Comparison of the biexciton and exciton coherent polarization lifetimes in ZnSe/(Zn,Cd)Se multiple quantum wells

We report the relative coherence lifetimes for the biexciton polarization and the exciton polarization following ultrafast resonant excitation in ZnSe/(Zn,Cd)Se multiple quantum wells. The excitonic and biexcitonic nature of the observed third-order polarization is demonstrated by selection rules and comparison with a theoretical treatment. We show the polarization coherence lifetimes to be related by a factor of two and discuss the origin of this relationship.

Polarization-state beating in four-wave-mixing experiments on ZnSe epilayers

The polarization angle of the time-integrated four-wave-mixing signal from a ZnSe epilayer is found to oscillate as a function of the delay time between the two incident pulses, the modulation depth being dependent on the polarization angle between the incident pulses. The oscillation is shown to be a feature of heavy-hole–light-hole quantum beating, and is described by a solution of the optical Bloch equations.

Red-shift of stimulated emission in ZnSe-based semiconductors

Abstract A red-shift in the stimulated emission wavelength with increasing intensity is observed using picosecond laser excitation of molecular-beam-epitaxial ZnSe epilayers and ZnSe-based quantum-well structures. This red-shift occurs across a wide temperature range (4–300 K) and for a wide range of epilayer and quantum-well samples. We show that sample heating is not the origin of this shift. A variety of emission mechanisms are discussed but none provides a complete description of the observed experimental trends.

Plasma-expansion induced absorption and refraction changes in ZnSe epilayers

Abstract A novel transient-grating technique with a well-defined and moveable optical probe is used to investigate the spatial extent of any optical changes arising from optically-driven electron-hole plasma expansion in molecular-beam-epitaxially grown ZnSe. We present experimental data demonstrating that optically-driven plasma expansion does not extend significantly beyond the originally excited area when pumped to carrier densities appropriate to lasing action in these materials (1018cm−3).

Spin relaxation and spin-dependent exciton interactions in ZnSe quantum wells

Abstract We demonstrate the mutual influence of the dynamics in the real-space and the spin parts of the exciton wavefunction evoked by the exchange interaction. In the regime of high exciton density, the exciton–exciton interaction in ZnSe/ZnMgSSe quantum wells is found to be strongly affected by the exciton spin orientation and the longitudinal spin relaxation via single-fermion spin flips. The excitons homogeneous linewidth as deduced from degenerate four-wave mixing with prepulse is significantly enhanced in spin-pure exciton populations by scattering including fermionic exchange. The transverse spin relaxation in the low-density limit is deduced from the decay of spin quantum-beats in photoluminescence experiments. We find an anti-proportionality between the spin relaxation and the optical dephasing times. The temperature dependence of both relaxation times is determined by the exciton–phonon interaction. The latter gradually reduces the electron–hole exchange coupling which is the dominating term in the excitons transverse spin relaxation.

Coherent exciton-biexciton interaction studied by femtosecond spectroscopy

We report the observation of oscillations with delay time in the time integrated four-wave-mixing signal emitted following sub-picosecond two-beam excitation. The oscillations are shown to originate from exciton-biexciton interference in the photon-echo signal. The results are modelled within the density-operator framework and good agreement is found. The dependence of biexciton binding energy on well width is obtained from the variation in oscillation period.

Femtosecond waveguide fabrication in bulk niobate (LiNbO/sub 3/)

We report fabrication of embedded waveguides supporting a well confined 1550 nm propagation mode in a z-cut lithium niobate crystal using focussed femtosecond pulses.

Ultrafast nonlinear refraction in an integrated Fabry-Perot etalon containing polydiacetylene

Summary form only given. Organic compounds with /spl pi/-conjugated polymers are considered to be candidates for practical third-order nonlinear optical (NLO) materials. Among them, polydiacetylenes (PDA) are an important class of conjugated polymer. PDAs, which can be obtained as single crystals, have been extensively studied since 1976. PDAs show large third-order nonlinear susceptibilities and ultrafast optical response, therefore they have been considered to be very promising materials for future photonic devices. However it is difficult to prepare large PDA crystals with high optical quality necessary for NLO device applications. In order to fabricate uniform thin films of PDA we have prepared nanometer size micro-crystals of poly-1,6-dicarbazoly 1-2,4-hexadyne (DCHC).

Exciton-acoustic-phonon scattering in (Zn,Cd)Se/ZnSe quantum wells-the influence of quantum confinement

To study the influence of quantum confinement on exciton-acoustic-phonon scattering, we have grown seven (Zn,Cd)Se/ZnSe samples by MBE onto GaAs substrates. Exciton-resonant, femtosecond time-integrated four-wave mixing (TI-FWM) was carried out on each sample at five temperatures between 4 and 70 K and five excitation densities. Acquisition of this large data set of 175 TI-FWM delay scans is made possible by the use of fast-scan time-delay techniques. Exponential fits to the data, assuming a photon-echo configuration, yield measured values of /spl gamma/ (homogeneous exciton linewidth). Linear fits to these /spl gamma/ values at low temperature (<50 K) for a given excitation density provide the exciton-acoustic-phonon scattering parameter, /spl alpha/, for a particular sample and confirmed /spl alpha/ to be independent of excitation density. We have observed that the maximum of both the exciton-acoustic-phonon scattering parameter and the exciton binding energy occur at a similar well width indicating that the exciton-acoustic-phonon scattering rate is also directly correlated to the degree of quantum confinement.