Photonic Systems Brown Bag Seminar Series

Thursday, April 13, 2006, at 12 noon           RLE Haus  Conf. room 36-428

Terahertz quantum-cascade lasers with distributed feedback in metal-metal waveguides

Sushil Kumar

Terahertz quantum cascade lasers (QCLs) have now been demonstrated at frequencies ranging from 1.9-5.0 THz (wavelengths of 60 - 160 um), a spectral region that has long lacked convenient and efficient sources. These lasers are expected to find applications as local oscillators for heterodyne receivers or as sources for imaging. Stable continuous-wave single-mode emission is desired for local oscillators and tens of milliwatt of optical power in a narrow beam is desired to obtain large dynamic range and high spatial resolution in an imaging system. While conventional dielectric waveguides are not practical in the terahertz, waveguides based on surface-plasmon modes bound to metal and/or heavily-doped semiconductor regions have provided the best solution. The highest temperature of operation for terahertz QCLs has been demonstrated in the so called 'metal-metal' waveguides, which provide low waveguide loss and tight confinement for the terahertz field. However, they suffer from low out-coupling efficiency and poor beam shape due to sub-wavelength dimensions of the emitting aperture.

After introducing the metal-metal waveguides, I will describe the development of distributed feedback (DFB) structures in the metal-metal waveguides to address the aforementioned applications. First-order edge emitting DFBs have been demonstrated to provide single-mode continous-wave operation of terahertz QCLs over a wide temperature range. The development of second-order surface-emitting DFBs, which are essential for obtaining a good beam shape and higher optical power for imaging applications, is ongoing. I will describe the challenges we have faced while designing such structures and possible solutions to overcome those challenges.