C. Coriasso

Loss analysis and interference effect in semiconductor integrated waveguide turning mirrors

Semiconductor integrated waveguide turning mirrors (IWTM) are investigated using the beam propagation method. A mirror is fabricated and modeled along with realistic geometries in the presence of offset and roughness of the mirror surface. An interference effect between waves reflected at different surfaces in the device is evidenced, that has strong impact on the device performance in the case of weakly-confining structures. A widely-accepted fabrication approach is criticized in view of this effect. A solution is outlined that minimizes the losses due to either technological limitations or interference.

Measurement of the refractive index of GaInAs/InP quantum wells by a grating coupling technique

Effective indices of modes propagating in multiple GaInAs wells on InP barriers (MQW) waveguides are accurately measured in the 1.4–1.75 μm wavelength range using the grating coupling technique. Quantum size effects cause a selective polarization absorption and high birefringence, strongly depending on wavelength. The temperature dependence of refractive index and absorption originating from the temperature‐dependent band gap is also observed. In the transparency region the refractive indices of GaInAs wells in InP barriers are deduced for two well widths using a multilayer slab waveguide model and an exciton model based on Lorentzian oscillator.

Wavelength Conversion Technologies

This paper reports a review of the different technologies used for wavelength conversion in wavelength-division multiplexed networks. The strengths and the weaknesses of the competing technologies, including optoelectronic wavelength conversion, four-wave mixing, difference frequency generation, cross-gain and cross-phase modulation in semiconductor optical amplifiers, are stressed with regard to conversion speed, transparency and regeneration of the converted signal.

BUTTERFLY BISTABILITY IN AN INGAAS/INP MULTIPLE-QUANTUM WELL WAVEGUIDE WITH DISTRIBUTED FEEDBACK

We report the experimental observation of butterfly bistability in an InGaAs/InP multiple‐quantum well waveguide with a distributed feedback grating, under cw operation and at sub‐milliwatt input power. Plasma effects on the excitonic optical properties in multiple‐quantum well structures are the basis of the observed bistability. Insight provided by a simple coupled‐mode description of the device and by many‐body theory of the field‐matter interaction is used both for device design and for interpretation of the results. In particular, the unconventional shape of the hysteresis is due to increased optical absorption at high injected optical power.

Nonlinear contradirectional coupler

Results of room-temperature experiments with a multiple-quantum-well, nonlinear, contradirectional coupler are reported. A power-dependent, contradirectional coupling condition is demonstrated, inducing an optically controlled switching of optical signals, with a switching energy of 1 pJ.

Observation of Superlattice Franz-Keldysh Oscillations

Using electroreflectance we report the first observation of the Franz-Keldysh oscillations originating from the band gap of a GaAs/Ga0.67Al0.33As short-period superlattice. The observed effect enables to measure the reduced effective mass in the direction of the growth axis.

High-resolution depth monitoring of reactive ion etching of InP/InGaAs(P) MQWs using reflectance measurements

Small-dimension photonic devices for telecommunications applications have recently attracted increasing interest along with the development of dry etching techniques. Extremely accurate control of etching depth is often needed for the fabrication of these devices containing multiple quantum well (MQW) structures. The excellent anisotropy, reasonable etching rates and excellent compatibility with resist masks of reactive ion etching (RIE) permit this technique to be used for fabrication of such devices. A drawback of the technique is that it is difficult to control etched depth with an accuracy of the order of the QW dimension. Laser interferometry (reflectometry) (LIR) is a powerful method for in situ monitoring of MQW structure etching. End-point detection (EPD) with very high depth resolution is obtained by appropriately combining the operating parameters of RIE and of LIR acquisition. In this paper we discuss the use of LIR at 670 nm to monitor the etching of InP/InGaAs(P) MQW structures by RIE and we report the real-time monitoring for etching of a 0.5 nm thick InGaAsP layer embedded between InP layers which is, to the best of our knowledge, the highest resolution ever reported for this technique. In the MQW etching process, the use of LIR combined with RIE successfully detects all stack layers with a resolution better than the well/barrier thicknesses. This capability permits etching to be monitored with well thickness as small as 3 nm in the case of InP/InGaAs layers. For structures having well/barrier layers with similar composition and hence a small refractive index difference, such as two layers with y = 0.57 and y = 0.84 respectively, monitoring of the etching of MQW with well thicknesses as small as 5 nm is demonstrated. The technique illustrated in the paper also proved to be useful as a simple method of analysing structure layers: the sequence and the uniformity of the QWs can be characterized with very high resolution.

InGaAs-InP superlattice electroabsorption waveguide modulator

An electroabsorption waveguide modulator with an extinction ratio for TE-polarized light at 1550 nm of more than 20 dB for a voltage swing of less than 3 V, and low insertion losses has been realized, which exploits the Wannier-Stark effect in an InGaAs-InP short period superlattice, grown by chemical beam epitaxy. Even for a fixed voltage swing of 2.6 V, a good transmission contrast has been obtained in a wide wavelength range.<<ETX>>

Study of exciton dynamics in InGaAs/InP quantum wells using a femtosecond optical parametric amplifier

We investigated the exciton dynamics in InGaAs/InP quantum wells using a novel infrared tunable femtosecond laser source, based on an optical parametric frequency converter pumped by a femtosecond Ti:sapphire laser. The wide tuning range of this source allowed to excite three different excitonic resonances in the absorption spectrum of the sample. The n=1 heavy‐hole excitons are approximately eight times more efficient than free electron‐hole plasma in the absorption saturation. This value is much higher than that measured in GaAs quantum wells. The exciton ionization time constant of the n=1 and n=2 heavy‐hole excitons has been measured to be 170 and 100 fs, respectively. No evidence of the n=1 light‐hole exciton dynamics was found in the measurements. The study of the dynamics at the n=2 heavy‐hole resonance allowed the measurement of a n=2 to n=1 intersubband relaxation time of 1 ps.

Relative efficiency of transient and stationary changes in excitonic optical properties of InGaAs/InP quantum wells

Using an infrared tunable femtosecond source, we investigate the time evolution of exciton absorption bleaching in InGaAs/InP quantum wells at energies slightly below the exciton absorption peak at room temperature. We find that the bleaching due to photogenerated excitons can be stronger than that due to a thermal electron/hole plasma by a factor of 40. Numerical modeling, based on many‐body theory, yields a qualitative explanation of this unusual behavior, along with its density and spectral dependence as well.