Quantum-well devices

An important concern is two-photon absorption which can also become a significant problem at high power densities, especially in the guided wave geometry. These excitations are even more of a problem in multiple quantum well devices and quantum confined structures because direct two-photon absorption can create free carriers which decay very slowly, giving rise to a slow nonlinear response. Molecular and polymeric materials offer additional flexibility to shift the two-photon resonances by chemical modifications. 

It should be noted that the salient difference between the energy-momentum relation between bulk semiconductor and quantum well material is that the k vector associated with Eq takes on discrete, well-separated values. In the quantum well device, the density of states is obtained fi om the magnitude of the two-dimensional k vector associated with the y-z plane, as compared to the three-dimensional wavevector for the bulk semiconductor. As a result, the final density of states for the quantum well structure is given by... 

To realize an automatic evaluation system, it would be desirable to also suppress geometrically caused signals as well, so that only the actual defect signals are obtained. Several approaches have already been made which are also to be implemented as part of a SQUID research project (SQUID = Super Conducting Quantum Interference Device). 

GaAs, GaAlAs, and GaP based laser diodes are manufactured using the LPE, MOCVD, and molecular beam epitaxy (MBE) technologies (51). The short wavelength devices are used for compact disc (CD) players, whereas the long wavelength devices, mostly processed by MBE, are used in the communication field and in quantum well stmctures. 

The purpose of this work is to demonstrate that the techniques of quantum control, which were developed originally to study atoms and molecules, can be applied to the solid state. Previous work considered a simple example, the asymmetric double quantum well (ADQW). Results for this system showed that both the wave paeket dynamics and the THz emission can be controlled with simple, experimentally feasible laser pulses. This work extends the previous results to superlattices and chirped superlattices. These systems are considerably more complicated, because their dynamic phase space is much larger. They also have potential applications as solid-state devices, such as ultrafast switches or detectors. 

Stoichiometric reaction of 5 with phenylsilane produced the iron(O) bis(silane) c-complex 18, which was confirmed by the single-crystal X-ray analysis as well as SQUID (Superconducting QUantum Interference Device) magnetometry (Scheme 19). Complex 18 as a precatalyst showed high activity for the hydrosilylation of 1-hexene. 

The creation of nanoscale sandwiches of compound semiconductor heterostructures, with gradients of chemical composition that are precisely sculpted, could produce quantum wells with appropriate properties. One can eventually think of a combined device that incorporates logic, storage, and communication for computing—based on a combination of electronic, spintronic, photonic, and optical technologies. Precise production and integrated use of many different materials will be a hallmark of future advanced device technology. 

R. Cingolani, Optical Properties of Excitons in ZnSe-Based Quantum Well Heterostructures A. Ishihashi and A. V. Nurmikko, II-VI Diode Lasers A Current View of Device Performance... 

Sivco, and Alfred Y. Cho, Quantum Interference Effects in Intersubband Transitions H. C. Liu, Quantum Well Infrared Photodetector Physics and Novel Devices S. D. Gunapala and S. V. Bandara, Quantum Well Infrared Photodetector (QWIP) Focal Plane Arrays... 

Production of single crystals with carefully controlled varying composition is vital to make semiconductor devices. The production of crystals for quantum well lasers illustrates how carefully such syntheses can be controlled. 

Blue lasers allow higher resolution, and hence higher density of optical storage of information, on devices such as DVDs than the infrared GaAs lasers allow. The earliest blue lasers were based on ZnSe but their lifetime proved too short for commercial applications. Lasers based on gallium nitride (GaN), first demonstrated in 1995, have proved to have greater lifetimes. In these lasers, the photons are produced not in a bulk semiconductor but in quantum wells. 

The active region containing the quantum wells is sandwiched between layers of n-and p-doped GaN and aluminum-doped GaN, Al ai j,N, which provide the electrons entering the quantum well and keep them confined to the active region. All these layers are built up on a substrate, for example, sapphire. The ends of the whole device are etched or cleaned to form a partial mirror that reflects the emitted photons allowing a coherent beam to build up. 

The electronic and magnetic properties of nanolayers are important in devices formed from electronic materials that are more conventional. We have already discussed quantum well lasers (see Chapter 8) and giant magnetoresistance (GMR) devices used for hard disk read heads (see Chapter 9). Quantum well lasers may be an important component of light-based computers. Other possibilities include magnets with unusual properties (Section 11.2). 

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