Coherent Control of Optical Properties of Semiconductors

In a quantum system the presence of two or more interactions allows for quantum interference when the system is driven from an initial state to a final state. This is to say that the transition rate from a particular initial state to a final state is not simply the sum of rates from the individual interactions but contains interference terms. If one has control over the relative phase between the dominating interactions one can switch the interference term between constructive and descructive interference. Hence, transition rates can be manipulated "coherently". Semiconductor systems are of high technological importance and miniaturization in an effort to increase device speed naturally leads to quantum effects in these systems. Hence the question arises to what extent material properties can be designed by control of quantum interference between competing pathways. As many-body effects usually have a contribution which tends to destroy coherence one is usually limited to a short (picosecond) time regime to beat phase breaking. Here we propose a theoretical study of coherent control of optical properties of semiconductors and semiconductor microstructors. Our approach is based on a non-equilibrium Green`s function method which captures both coherent dynamics and many-body effects in a non-phenomenological way. Specifically we study the possibility to control light absorption, optical gain, the refractive index, and the generation of radiation in the microwave and terahertz regime. The slowly-varying Maxwell equations are employed to obtain the induced electromagnetic field. This allows a self-consistent calculation of the net electromagnetic field. This work is motivated by the quest for a better understanding of the border region between coherent and incoherent electron dynamics in semiconductors, the feasibility of coherent control of optical properties of semiconductors , the role of many-body effects, and fundamentally new principles of operation for future optoelectronic devices.

Miniaturization of conventional electronic and opto-electronic semiconductor devices is limited by an inevitable increase in electric fields present in the device, the power loss, and the occurrence of quantum effects, such as unwanted tunneling through potential barriers. Once can either try to reduce these effects by using clever design which combat these effects, as has been done in the past and is envisioned to be successful for another 10 years or so, or try radically different designs which, rather than to fight quantum effects make them ones friend by using them as their operational principle. The latter strategy applied to semiconductor physics has shown some promising success and been adopted in the present study. Moreover, it is the mandatory strategy when implementing ideas of quantum information processing into real, physical systems. In this work we have addressed this issue in two ways. Firstly, we have followed the ideas of the original proposal and have studied quantum interference in semiconductor heterostructures induced by two or more externally applied electromagnetic fields. The basic idea is that a solid exposed to an electromagnetic (pump) field, when probed by a second field, shows optical properties different from the "bare solid". The effects are particularly striking, when the pump pulse induces "coherence" in the system to produce a coherent state of matter termed "Phaseonium". Then the phase of the probe pulse relative to the state of the system has influence on the response of the system. In this work we have shown theoretically, that semiconductor heterostructures can act as phase- dependent absorbers or gain material, i. e. the heterostructure can either amplify or weaken Incoming light, depending on its phase. Optimization of coherent control has lead to a second direction of research. This second direction of work was also inspired by the current interest in the physical realization of quantum gates. One of the main problems here is that real systems are dissipative which ultimately is detrimental for quantum information processing. We have developed strategies to control dissipative single qubits by optimal coherent control, both by analytic and numerical procedures and have shown that the concept of quantum interference can be used to externally control the effective system-bath interaction. For quantum information processing, a reduction is desirable. In other instances, such as laser cooling of a quantum system, a temporary increase in the effective coupling strength is of advantage.