D. S. Chemla

The quantum well self-electrooptic effect device: Optoelectronic bistability and oscillation, and self-linearized modulation

We report extended experimental and theoretical results for the quantum well self-electrooptic effect devices. Four modes of operation are demonstrated: 1) optical bistability, 2) electrical bistability, 3) simultaneous optical and electronic self-oscillation, and 4) self-linearized modulation and optical level shifting. All of these can be observed at room-temperature with a CW laser diode as the light source. Bistability can be observed with 18 nW of incident power, or with 30 ns switching time at 1.6 mW with a reciprocal relation between switching power and speed. We also now report bistability with low electrical bias voltages (e.g., 2 V) using a constant current load. Negative resistance self-oscillation is observed with an inductive load; this imposes a self-modulation on the transmitted optical beam. With current bias, self-linearized modulation is obtained, with absorbed optical power linearly proportional to current. This is extended to demonstrate light-by-light modulation and incoherent-to-incoherent conversion using a separate photodiode. The nature of the optoelectronic feedback underlying the operation of the devices is discussed, and the physical mechanisms which give rise to the very low optical switching energy (∼4 fJ/ μm2) are discussed.

Electric-field dependence of linear optical properties in quantum well structures: Waveguide electroabsorption and sum rules

We summarize the electric-field dependence of absorption and luminescence in quantum wells for fields perpendicular to the layers, present extended discussion of electroabsorption spectra and devices in waveguide samples, and derive sum rules for electroabsorption. Optical bistability, self-linearized modulation, and optical level shifting are demonstrated in self-electrooptic effect device configurations, with good modulation contrast and polarization-dependent properties. The electroabsorption spectra enable quantitative comparison of theory and experiment for absorption strengths in quantum wells with field. The sum rules enable excitonic effects to be included in the comparison, and good agreement is seen. One sum rule is also more generally applicable to electroabsorption in semiconductors.

Multiple quantum well reflection modulator

We demonstrated a quantum‐confined Stark effect electroabsorption modulator consisting of quantum wells of AlGaAs and GaAs on an epitaxial multilayer dielectric mirror, all grown by molecular beam epitaxy. The resulting reflection modulator avoids problems of substrate absorption, and has relatively high contrast ratio (up to ∼8:1 with peak reflectivity of 25% at 853 nm) because the light passes twice through the quantum wells. Reflection modulators are of interest for bidirectional communication systems, in parallel arrays of optical switching and processing devices and for optical interconnects. For the latter there exists the possibility of this device grown on the same substrate alongside a GaAs integrated circuit or even on Si substrates.

131 ps optical modulation in semiconductor multiple quantum wells (MQW's)

A new optical modulator has been fabricated which uses the recently discovered electroabsorption effect in MQWs. Optical pulses 131 ps long were generated when the device was driven with 122 ps electrical pulses. The input-output characteristics of the device show that it has low insertion loss with reasonable modulation depth and drive voltage.

Femtosecond excitonic optoelectronics

The authors discuss a novel approach to femtosecond optoelectronics which uses the excitonic response to electric fields as a detector and the excitonic nonlinear response to optical fields as a generator. The sensitivity of the quantum-well exciton to applied electric fields is used to measure electrical transients with femtosecond time resolution. The authors examine several mechanisms for femtosecond electrical pulse generation, including exciton ionization and two-photon absorption, and present measurements of the propagation properties of coplanar striplines on ultrathin semiconductor substrates in the 1-100-THz frequency range. The generation and detection of an electrical pulse with a 180-fs risetime propagating on a coplanar stripline on GaAs/AlGaAs quantum wells are demonstrated. >

Optical Properties of Quasi-Zero-Dimensional Magneto-Excitons

We discuss the evolution of optical properties of semiconductor quantum wells, as the quasi-two-dimensional electronic states are further confined into quasi-zero dimensions by a perpendicular magnetic field. We show that confinement in all three directions strongly modifies both linear and nonlinear optical response. In particular, quasi-zero-dimensionality makes an ensemble of magneto-excitons a unique many-body system, distinct from higher-dimensional excitons and the one-component Coulomb system in the fractional quantum Hall regime or Wigner crystal.

Time Resolved Nonlinear Optical Spectroscopy of Magnetically Confined Excitons

A perpendicular magnetic field confines the quasi-two-dimensional (2D) electronic states of a quantum well into quasi-zero-dimensional (0D) magneto-excitons. Short-pulse broadband optical studies demonstrate that at high field, a gas of magneto-excitons behaves like an ensemble of atomic two-level systems. Excitation of a nonthermal distribution of 2D carriers at zero field results in rapid relaxation to a thermal distribution; at high field, all low-energy scattering channels are eliminated, so that in 0D, relaxation rates decrease by several orders of magnitude relative to 2D. Magnetic confinement in quantum wells permits the study of the interactions and relaxation dynamics of electon-hole pairs, in materials of excellent optical quality, as their dimensionality is continuously tuned from 2D to 0D.

Femtosecond Spectroscopy of Quasi-Zero-Dimensional Magneto-Excitons

Quantum confinement of electronic states in quantum wells is achieved by layered thin film growth techniques. These materials demonstrate that linear and nonlinear optical properties depend critically on dimensionality[1]. A magnetic field applied perpendicularly to the quantum well confines the electronic states within the plane, producing the bound and discrete magneto-exciton eigenstates. By increasing the field strength, the magnetic confinement length can be made smaller than the excitonic Bohr radius. In this way, the dimensionality of the quantum well states can be tuned continuously from quasi-two to quasi-zero dimensions. We report the first observation of femtosecond time resolved optical nonlinearities in quasi-zero dimensional magneto-excitons. Our results demonstrate that confinement into quasi-zero dimensions strongly modifies both linear and nonlinear optical response, as well as the kinetic processes governing thermalization and energy relaxation.

Room-temperature operation of a GaAs/AlGaAs superlattice self-electrooptic-effect device

Summary form only given. The authors report the room-temperature operation of a blue-shift SEED (self-electrooptic-effect device) using Wannier-Stark localization in a GaAs/Al/sub 0.3/Ga/sub 0.7/As superlattice. They demonstrate that large modulation can be obtained over a wide spectral range and bias with contrast comparable to MQW (multiple quantum-well) SEEDs. Wannier-Stark localization associated with a blue shift of the superlattice (SL) absorption edge was clearly observed in the photocurrent measurements. Bistability of the SL SEED was obtained over a wide spectral range and was almost constant between 745 and 755 nm. The demonstration of large modulation bandwidth and optical switching behavior of the SL SEED makes this device an excellent candidate for optical signal processing and optical computing. >

IIIA-2 observation of large absorption modulation in a quantum-well field-effect device

Zone-melting-recrystallized (ZMR) silicon on insulator (SOX) typically employs graphite strip heaters or other low power density melting sources with relatively slow scanning speeds [l]. The results of thermal modeling simulations suggest that high quality SO1 can also be obtained by high power density, line-source rapid zone recrystallization 121. The advantage to rapid recrystallization is the shorter high temperature duration, with less damage to existing structures or impurity distributions in the sample and greater applicability to 3-D ICs. To date, only line-source electron-beams [3] have been used for rapid recrystallization. This paper present! electrical measurements from active devices fabricated in SO1 material processed in a rapid zone recrystallization system using ; pulsed arc lamp line source. The seeded polysilicon coated sample is rapidly preheated (above lOOOC, in room air ambient) and scanned by a linear translatior. table under the zone; speed is of the order of 35 cm . s-. (BJ comparison, strip heater ZMR systems typically operate at 100 1% . cm- with scan speeds of 1 mm * s-.) The lamp energy is C O L limated by a plate optic to a 0.5-mm line with 8-kW . cm- powel. density. A single lamp pulse melts the top film and the sample then cools by radiation. The sample is above room temperature for less: than 15 s. The experimental apparatus and materials properties art: reported elsewhere [4]. Three-inch Si wafers with 0.5-pm-thicl: seeded SO1 islands (50 pm by 2.5 cm) were prepared by thir; method. MOSFETs, resistors, capacitors, lateral junction diodes, Schottky diodes, and MESFETs were fabricated; standard CMOS LOCOS process technology was employed. Measurements reveal electron mobilities averaging 670 cm2 . 1 1 s in implanted channels (15 percent higher than in bulk monito : samples). Unpassivated backchannel leakage averaged 80 nA pm-. Minority-carrier lifetimes over 10 p s (comparable to line e beam recrystallized samples) exceed the values commonly seen in ZMR material. The electrical results confirm the material qualit!! suggested by SEM, TEM, and Nomarski observations. Reproducible results were obtained in FETs down to 1-pm effective chamne1 length. The process, with significantly higher throughput than one requiring a vacuum, has application to high-speed, dense, radiation-hard CMOS.