Boston Chapter of the IEEE Photonics Society

Wed
Nov 9, 2005
8:15 PM
 

MIT Lincoln Laboratory
 

Terahertz Quantum Cascade Lasers

Prof. Qing Hu, Massachusetts Institute of Technology, Cambridge, MA

Abstract:  Terahertz frequencies are among the most underdeveloped of the electromagnetic spectra, even though their potential usefulness can have a major impact on many areas of life today.  This underdevelopment is primarily due to the lack of coherent solid-state THz sources.  Unipolar lasers based on intersubband transitions of semiconductor quantum wells were proposed for long-wavelength sources and amplifiers as early as in the 1970s.  Electrically pumped intersubband-transition lasers (known as quantum-cascade lasers (QCLs)) at ~4-µm wavelength were first developed at Bell Laboratories in 1994.  However, the development of their THz counterparts turned out to be much more difficult, because of two unique challenges at THz frequencies.  First, the energy level separations that correspond to THz frequencies are quite narrow (~10 meV).  Thus, the selective depopulation mechanism based on energy-sensitive LO-phonon scattering, which has been widely used in mid-infrared QCLs, is not applicable.  Second, low-loss optical mode confinement is difficult to implement at THz frequencies.  In October 2001, almost eight years after the initial development of QCLs, the first QCL operating below 4.4 THz, was developed.   This laser was based on a chirped superlattice structure that had been successfully developed at mid-infrared frequencies.  Mode confinement in this THz QCL was achieved using a double-surface plasmon waveguide grown on a semi-insulating (SI) GaAs substrate.  Shortly afterward, THz QCLs based on a bound-to-continuum intersubband transition were also developed, which have yielded higher operating temperatures and greater output power levels than those based on chirped-superlattice structures.  Our group at MIT has pursued a different approach to achieve lasing at THz frequencies.  We have investigated possibilities of using fast LO-phonon scattering to depopulate the lower radiative level, and using double-sided metal-metal waveguides for THz mode confinement.  In November 2002, a 3.4-THz QCL was developed in which the depopulation of the lower radiative level was achieved through resonant LO-phonon scattering.  Following our initial success in developing the 3.4-THz laser, we demonstrated the first terahertz QCL that uses a double-sided metal-metal waveguide for mode confinement.  Based on this metal-metal waveguide structure and using improved gain media that reduced the lasing threshold current densities, we have achieved several records in the performance of THz QCLs.  Meanwhile, our collaborators in Europe have successfully pumped a superconducting heterodyne receiver using one of the our THz QCLs as the local oscillator, and they have achieved a record-low receiver noise temperature of 1400 K at 2.8 THz.  For sensing and local-oscillator applications, our collaborators at University of Colorado have demonstrated frequency/phase locking of our THz QCLs with a FWHM linewidth of 65 kHz, obtained over an indefinitely long period of time.  This rapid development is encouraging and we are optimistic that THz QCLs will find immediate applications in imaging experiments.

Biography:  Prof. Qing Hu received his Ph.D. in physics from Harvard University in 1987. After a three-year postdoctoral research experience at U. C. Berkeley, he joined the faculty of Electrical Engineering and Computer Science at MIT in 1990, and has been there ever since.