April 9, 2008
3pm in Howey Physics Lecture Room 5

 

Mikhail Belkin
School of Engineering and Applied Sciences
Harvard University

 

"Quantum cascade lasers - bridging the THz gap with semiconductor lasers"

 

The frequency range 1-100 THz (l=3-300µm) has long been devoid of a convenient semiconductor source; a breakthrough in this area occurred in 1994 with the demonstration of the quantum cascade laser (QCL). Currently, QCLs can operate at room temperature in mid-infrared (l=3-15µm) and at cryogenic temperatures in terahertz spectral range. There is a growing interest to utilize QCLs for various applications that include environmental and chem/bio sensing, free-space communications, terahertz security screening, and spectroscopy.

QCLs are unipolar devices in which the optical transitions occur between the electronic states confined in coupled semiconductor quantum wells. As a result, the energy levels, transition dipole moments, and lifetimes of the electronic states can be controlled by changing the width of wells and barriers in a semiconductor superlattice. Thus, one can design virtually any configuration of electronic states using coupled quantum wells as building blocks.

I will give an introduction to the principles of QCLs, provide an example of a QCL-based system for chem/bio sensing, developed in our group, and then describe our progress towards developing a room-temperature terahertz QCL source.  In particular, I will talk about our “traditional” THz QCLs, which currently operate at a record temperature of 178K, and a novel type of THz source, operable at room temperature, based on THz difference-frequency generation in a mid-infrared QCL engineered to emit two wavelengths and possess giant second-order nonlinear susceptibility for efficient intra-cavity frequency mixing.