Electrically pumped photonic-crystal terahertz lasers controlled by boundary conditions

Semiconductor lasers based on two-dimensional photonic crystals(1,2) generally rely on an optically pumped central area, surrounded by un- pumped, and therefore absorbing, regions(3). This ideal configuration is lost when photonic- crystal lasers are electrically pumped, which is practically more attractive as an external laser source is not required. In this case, in order to avoid lateral spreading of the electrical current, the device active area must be physically defined by appropriate semiconductor processing. This creates an abrupt change in the complex dielectric constant at the device boundaries, especially in the case of lasers operating in the far- infrared, where the large emission wavelengths impose device thicknesses of several micrometres. Here we show that such abrupt boundary conditions can dramatically influence the operation of electrically pumped photonic- crystal lasers. By demonstrating a general technique to implement reflecting or absorbing boundaries, we produce evidence that whispering- gallery- like modes or true photonic- crystal states can be alternatively excited. We illustrate the power of this technique by fabricating photonic- crystal terahertz ( THz) semiconductor lasers, where the photonic crystal is implemented via the sole patterning of the device top metallization. Single- mode laser action is obtained in the 2.55-2.88 THz range, and the emission far field exhibits a small angular divergence, thus providing a solution for the quasi- total lack of directionality typical of THz semiconductor lasers based on metal - metal waveguides(4).