Ines Waldmueller

Nonequilibrium many-body theory of intersubband lasers

We present a theory for intersubband lasers, based on the solution of the Maxwell-semiconductor Bloch equations for the laser field and active medium. The collision contributions are treated within the relaxation rate approximation, where the relaxation rates are determined by microscopic scattering calculations. The theory is suitable for investigating steady-state as well as dynamical laser characteristics. As examples of applications of the theory, we examine the thermal dependence of the laser output versus current density curve and the response to modulation of the injection current, for a three-subband laser. The influence of the nonparabolicity of the conduction band and Hartree-Fock many-body effects are investigated.

Circumventing the manley-rowe quantum efficiency limit in an optically pumped terahertz quantum-cascade amplifier

Using a microscopic theory based on the Maxwell-semiconductor Bloch equations, we investigate the feasibility of an optically pumped electrically driven terahertz (THz) quantum-cascade laser as a pathway to room-temperature THz generation. In optical conversion schemes the power conversion efficiency is limited by the Manley-Rowe relation. We circumvent this constraint by incorporating an electrical bias in a four level intersubband scheme, thereby allowing coherent recovery of the optical pump energy. The observed THz radiation is generated through both stimulated emission and automatically phase-matched quantum coherence contributions--making the proposed approach both a promising source for THz radiation and a model system for quantum coherence effects such as lasing without inversion and electromagnetically induced transparency.

Inverse-Quantum-Engineering: A New Methodology for Designing Quantum Cascade Lasers

Bandstructure engineering has enabled a broad array of semiconductor heterostructure devices, such as quantum cascade lasers, whose performance is governed by a broad parameter space involving intertwined physical properties. Using present methods it is challenging if not impossible to design structures that isolate a specific physical property that directly correlates with experimental results. To overcome this problem, we developed a new methodology, inverse quantum engineering (IQE), which employs an evolutionary algorithm to design families of structures with everything identical except for a specific physical property of our choosing. We show that IQE allows creation of model families of designs that isolate targeted experimental effects, thus allowing direct investigation of specific physical mechanisms and their often complicated and counter-intuitive interplay.

Optically Pumped Electrically Driven THz Generation–-An Approach for an Efficient Room-Temperature THz Quantum Cascade Laser?

We investigate an optically pumped electrically driven laser scheme as an approach for terahertz (THz) quantum cascade lasers. The scheme keeps the advantages of optical conversion, and increases the efficiency by recycling the pump photons. Using a microscopic theory based on the Maxwell-semiconductor Bloch equations, we demonstrate that the approach offers a path to room-temperature THz generation. Furthermore, we show how successful recovery of the pump intensity can yield optical conversion efficiencies exceeding the Manley-Rowe limit.

Gain without Inversion: An Approach for THz Quantum Cascade Laser?

Using a microscopic theory based on the Maxwell-semiconductor Bloch equations, we investigate the possibilities of optically driven QCLs. We show that in a four level cascaded-emission scheme, quantum coherence effects can yield gain without inversion

High-temperature all-optical intersubband terahertz wave switch

We propose an intersubband all-optical THz switch. Adjusting the optical control switch beam even strong THz probe signals with intensities of up to 1 MW/cm2 can be modulated with extinction ratios of -80 dB/mm.

4-wave mixing for phase-matching free nonlinear optics in quantum cascade structures : LDRD 08-0346 final report.

Optical nonlinearities and quantum coherences have the potential to enable efficient, high-temperature generation of coherent THz radiation. This LDRD proposal involves the exploration of the underlying physics using intersubband transitions in a quantum cascade structure. Success in the device physics aspect will give Sandia the state-of-the-art technology for high-temperature THz quantum cascade lasers. These lasers are useful for imaging and spectroscopy in medicine and national defense. Success may have other far-reaching consequences. Results from the in-depth study of coherences, dephasing and dynamics will eventually impact the fields of quantum computing, optical communication and cryptology, especially if we are successful in demonstrating entangled photons or slow light. An even farther reaching development is if we can show that the QC nanostructure, with its discrete atom-like intersubband resonances, can replace the atom in quantum optics experiments. Having such an artificial atom will greatly improve flexibility and preciseness in experiments, thereby enhancing the discovery of new physics. This is because we will no longer be constrained by what natural can provide. Rather, one will be able to tailor transition energies and optical matrix elements to enhance the physics of interest. This report summarizes a 3-year LDRD program at Sandia National Laboratories exploring opticalmorexa0» nonlinearities in intersubband devices. Experimental and theoretical investigations were made to develop a fundamental understanding of light-matter interaction in a semiconductor system and to explore how this understanding can be used to develop mid-IR to THz emitters and nonclassical light sources.«xa0less

Inverse-quantum-engineering: A new methodology for designing THz QCLs for basic and applied research

We demonstrate the general capabilities of the developed methodology by tuning the emission frequency of a GaAs/AlxGa1-xAs THz QCL over a frequency range of 2.9 THz, and the Al fraction over a range of 0.17.

Optically-assisted electrically-driven THz generation - a new approach for efficient THz Quantum Cascade Lasers

The proposed optically-assisted electrically-driven laser keeps the advantages of optical conversion while overcoming its constraints by recycling the pump photons yielding conversion efficiencies exceeding the Manley-Rowe limit and a path to room temperature THz generation.

THz quantum cascade lasers for standoff molecule detection.

Remote optical detection of molecules, agents, and energetic materials has many applications to national security interests. Currently there is significant interest in determining under what circumstances THz frequency coverage will aid in a complete sensing package. Sources of coherent THz frequency (i.e. 0.1 to 10 THz) electromagnetic radiation with requisite power levels, frequency agility, compactness and reliability represent the single greatest obstacle in establishing a THz technology base, but recent advances in semiconductor-based quantum cascade lasers (QCLs) offer huge improvements towards the ultimate THz source goals. This project advanced the development of narrow-linewidth THz quantum cascade lasers. We developed theoretical tools to guide the improvement of standard THz quantum cascade lasers, the investigation of nonlinear optics employing infrared QCLs, and the exploration of quantum coherence to improve QCL performance. The latter was aimed especially towards achieving high temperature operation. In addition we developed a computer algorithm capable of shifting the frequencies of an existing THz QCL to a different frequency and invented a new type of laser that may enable room temperature THz generation in a electrically driven solid-state source.