Abstract
Terahertz quantum cascade lasers (THz QCLs) were first demonstrated in 2002 [1] and have since undergone rapid development [2]. One of the most active directions of research has been the development of devices emitting in a single longitudinal mode through the use of distributed feedback (DFB) resonators. Because the optical mode in these devices arises through a surface plasmon, it is possible to implement them with periodically arranged slits in the top metallization. This has been demonstrated for both single- and double-metal waveguides [3-5]. More recently, such gratings have been applied to microdisk lasers [6], where they lead to a circular far-field in the vertical direction, whose angular spread depends on the ratio of wavelength and device diameter. To this end, we have modeled and fabricated THz QCLs in a circular double-metal DFB resonator (see Fig la) of much larger diameter, for which we compute the far-field shown in the inset. Devices are designed using a finite element solver. A second order grating with a double slit configuration was found to offer the best compromise between electrical and optical properties of the top electrode. Fig. lb) shows an L-I characteristics of a fabricated device. Even without collection optics employed, the devices show peak power up to 10 mW, with a slope efficiency of ~25mW/A, demonstrating the good radiative efficiency of this resonator and directionality of the emitted power. The inset of Fig. lb) shows the spectrum of the device. We observe single-mode emission with a side-mode suppression of more than 20 dB. Measured far-fields of the devices will also be discussed. We believe the current work will further simplify the integration of THz QCLs into possible applications.
© 2009 IEEE
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