N.J. Sauer

Free carrier and many-body effects in absorption spectra of modulation-doped quantum wells

The temperature-dependent optical absorption and luminescence spectra of GaAs/AlGaAs and InGaAs/InAlAs n-doped modulation-doped quantum wells is discussed with emphasis on the peak seen at the edge of the absorption spectra of these samples. A many-body calculation of the electron-hole correlation enhancement is presented, which identifies this peak with the Mahan exciton-the result of the Coulomb interaction between the photoexcited hole in the valence band and the sea of electrons in the conduction band. This calculation accounts for the strong dependence of the absorption edge peak on both the temperature and carrier concentration, in good qualitative agreement with experimental data and with previously published results. The changes induced by the carriers on the subband structure through self-consistent calculations are also analyzed, and it is concluded that in these symmetric structures, the changes are small for achievable carrier densities. >

Color center lasers passively mode locked by quantum wells

Using multiple-quantum-well (MQW) saturable absorbers, a NaCl color center was passively mode locked to produce 275-fs transform-limited, pedestal-free pulses with a peak power as high as 3.7 kW. The pulses are tunable from lambda =1.59 to 1.7 mu m by choosing MQWs with different bandgaps. The output pulses from the laser were shortened to 25 fs using the technique of soliton compression in a fiber. The steady-state operation of the laser requires the combination of a fast saturable absorber and gain saturation. >

Large refractive index changes in tunable-electron-density InGaAs/InAlAs quantum wells

Measurements of electrorefraction and electroabsorption in a multiple-quantum-well waveguide structure in which each InGaAs quantum well is provided with an individual electron reservoir are presented. External bias transfers electrons into the wells, thus quenching the absorption and producing a refractive index change at wavelengths below the bandedge which is linear in the applied voltage. It is shown that in this type of structure both the change in refractive index per applied field and the ratio of optical phase to intensity modulation can be significantly enhanced over those found in the quantum confined Stark effect.<<ETX>>

Polarization-independent strained InGaAs/InGaAlAs quantum-well phase modulators

Polarization-independent phase modulation in In/sub 1-x/Ga/sub x/As/InGaAlAs multiple-quantum-well waveguides is demonstrated for the first time. It is shown that by increasing the Ga fraction and hence the tensile strain in the quantum well the electric-field-induced refractive index change in the TM polarization Delta n/sub TM/ can be made to approach that in the TE polarization Delta n/sub TE/. At 1.523 mu m, the ratio Delta n/sub TM// Delta n/sub TE/=1 for x=0.7 with a phase shift coefficient of 17.4 degrees /V-mm was achieved. Polarization independence was maintained over the entire range of reverse bias voltage.<<ETX>>

Optical waveguide intensity modulators based on a tunable electron density multiple quantum well structure

With a recently developed semiconductor heterostructure it has become possible to tune continuously the electron density in multiple quantum wells. Here we demonstrate the first electro‐optic waveguide intensity modulators based on this concept. We achieve a 22 dB on/off ratio for 9 V applied at 1.54 μm wavelength in a rib waveguide electroabsorption modulator. Electrorefractive devices include a waveguide Mach–Zehnder interferometer with an active length 650 μm operating at 1.58 μm wavelength with 5.4 V half‐wave voltage. We show that the operating voltage can be further reduced by operating the Mach–Zehnder modulators in push‐pull configuration.

Weighted-coupling Y-branch optical switch in InGaAs/InGaAlAs quantum well electron transfer waveguides

We demonstrate the first weighted-coupling Y-branch switch in semiconductors. The active waveguide core contains an InGaAs/InGaAlAs chopped quantum well electron transfer structure which provides large voltage-controlled refractive index changes near 1.55 /spl mu/m with high speed capability. We obtain polarization-independent switching over a wide operating range, from 1.52 to 1.58 /spl mu/m wavelength. We show that shaping the Y-branch results in significant improvements in voltage-length product and crosstalk performance over conventional, non-shaped Y-branches. In push-pull configuration, the drive voltage requirement is only 3.5 V for a 550 /spl mu/m active length switch.<<ETX>>

Multi‐gigahertz‐bandwidth intensity modulators using tunable‐electron‐density multiple quantum well waveguides

We report the first measurement of modulation bandwidth in electron transfer quantum well modulators. A device with 1 pF capacitance provides ≳10 dB optical modulation depth at 1.537 μm wavelength with a 3 dB electrical bandwidth of 5.7 GHz. Optical pump‐probe measurements indicate that the fundamental response time is determined by the voltage‐dependent speed of carrier escape from the well.

InGaAs-InAlAs quantum well intersecting waveguide switch operating at 1.55 mu m

The first electrooptic waveguide switch based on voltage-controlled transfer of electrons into multiple quantum wells is demonstrated. This mechanism is named barrier, reservoir, and quantum well electron transfer (BRAQWET) In GaAs-InAlAs quantum wells it provides a large modulation of refractive index at 1.55 mu m with multigigahertz switching capability. The BRAQWET X-switch is free from heating and speed limitations associated with current injection. Beam propagation method (BPM) calculations suggest that the crosstalk performance of this preliminary device demonstration can be significantly enhanced by changes in waveguide geometry.<<ETX>>

Loss reduction in InGaAs/InGaAlAs quantum well electron transfer waveguides using ion implantation

We demonstrate that phosphorous ion implantation is an effective means of blue-shifting the absorption edge in InGaAs/InGaAlAs barrier, reservoir and quantum well electron transfer structures (BRAQWETS). The electroabsorption peak is wavelength-shifted by 100 nm after implant, with a complete recovery of the electroabsorption magnitude after an appropriate rapid thermal anneal cycle. Good electrical characteristics are also maintained after implant and anneal. We apply this technique to loss reduction for BRAQWETS waveguide devices. At 1.523 mm, the implant and anneal results in a decrease of rib waveguide propagation loss from 79.4 dB/mm to 6.2 dB/mm.<<ETX>>

Novel modulator structure permitting synchronous band filling of multiple quantum wells and extremely large phase shifts

The authors have demonstrated synchronous band filling of multiple quantum wells using a novel blockaded reservoir and quantum-well electron transfer structure (BRAQWETS). The resulting electroabsorptive response per quantum well is not only stronger than that produced by the quantum confined Stark effect (QCSE) but is also free of induced absorption below the bandgap. The measured electrorefractive response shows that the maximum phase shift that can be achieved in one absorption length is one order of magnitude larger than what is possible with QCSE. Furthermore, the dependence of induced refractive index change on the applied voltage is essentially linear. Design criteria for very-high-performance modulators with picosecond intrinsic speed are also discussed. It is concluded that BRAQWETS provides a novel basis for high-performance electroabsorption modulators as well as low-loss phase modulators and interferometric amplitude modulators and switches. With suitable engineering, it will also be possible to construct multi-quantum-well lasers and detectors using BRAQWETS.<<ETX>>