IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 43, NO. 8, AUGUST 2007 687 Subwavelength Microdisk and Microring Terahertz Quantum-Cascade Lasers Gernot Fasching, Vincas Tamoˇ si¯ unas, Alexander Benz, Aaron Maxwell Andrews, Karl Unterrainer, Reinhard Zobl, Tomas Roch, Werner Schrenk, and Gottfried Strasser Abstract—We report on the emission characteristics of micro- cavity quantum-cascade lasers emitting in the terahertz frequency range based on circular-shaped microresonators. Strong mode confinement in the growth and in-plane directions are provided by a double-plasmon waveguide and due to the strong impedance mismatch between the gain material and air. This allows laser emission from devices with overall dimensions much smaller than the free-air emission wavelength 100 m . Hence, for the smallest microdisks we achieved a threshold current as low as 13.5 mA (350 A/cm ) in pulsed-mode operation at 5 K and stable single-mode emission up to 95 K in continuous-wave mode operation. We have observed dynamical frequency pulling of the resonator mode on the gigahertz scale, as a consequence of the gain shift due to the quantum-confined Stark effect. Thus, we were able to estimate the peak gain of the material to 27 cm . The smallest microcavities exhibited a strong dependence on the exact placement of the bond wire which resulted in single- as well as double-mode emission. Finite-difference time-domain simulations were performed in order to identify the modes of the recorded spectra. They confirm that most of the observed spectral features can be attributed to the lasing emission of whispering-gallery modes. Index Terms—Laser, microcavity, quantum-cascade, subwave- length, terahertz. I. INTRODUCTION T HE performance and frequency range of quantum-cascade lasers (QCLs) operating in the terahertz (THz) frequency region has tremendously increased since the first working laser structures were published [1]–[3]. The advances are based mainly on band structure engineering to optimize the carrier transport, e.g., the scattering times while maintaining a long lifetime of the upper and a small lifetime of the lower lasing transition. This has enabled a large spectral emission Manuscript received February 15, 2007; revised April 2, 2007. This work was supported in part by the Austrian Science Fund (SFB-ADLIS), in part by the EC Programs “TERANOVA” (IST) and “POISE” (TRM), and in part the Society for Microelectronics (GME, Austria). G. Fasching, A. Benz, A. M. Andrews, and K. Unterrainer are with the Pho- tonics Institute and the Centre for Micro- and Nanostructures, Vienna University of Technology, 1040 Wien, Austria (e-mail: gernot.fasching@tuwien.ac.at; Alexander.Benz@tuwien.ac.at; Aaron.Maxwell.Andrews@tuwien.ac.at; Karl.Unterrainer@tuwien.ac.at). V. Tamoˇ si¯ unas is with the Semiconductor Physics Institute, 01108 Vilnius, Lithuania and with the Vilnius Gediminas Technical University, 10223 Vilnius, Lithuania (e-mail: vincas@pfi.lt). R. Zobl, W. Schrenk, and G. Strasser are with the Institute of Solid-State Electronics and the Centre for Micro- and Nanostructures, Vienna University of Technology, 1040 Wien, Austria (e-mail: Reinhard.Zobl@tuwien.ac.at; Werner. Schrenk@tuwien.ac.at; Gottfried.Strasser@tuwien.ac.at). T. Roch is with the Faculty of Mathematics, Physics and Informatics, Come- nius University, 842 48 Bratislava, Slovakia (e-mail: roch@fmph.uniba.sk). Digital Object Identifier 10.1109/JQE.2007.900254 range between 1.39 and 4.9 THz [4], [5] and high temperature continuous-wave (CW) operation up to 117 K [6]. Besides a proper band structure design, a high quality double-metal resonator was required to achieve a high modal confinement and low loss. For all sensing applications, like industrial or environmental monitoring [7], imaging [8], [9] or for hetero- dyne detection [10], a stable single-mode operation with a low power consumption together with the possibility of frequency tunability is highly desirable. We have recently demonstrated the first double-metal microdisk and microring QCLs operating in the THz frequency region between 3.0 and 3.7 THz [11] and also very recently the first single-mode emitting microdisk lasers [12]. The realization of these small mode volume lasers is based on the combination of double-plasmon waveguiding and a circular symmetric resonator, which lase on the so called whispering-gallery modes (WGMs) [13] allowing for high- optical cavities. QCLs are well suited for the circular-shaped resonators due to their inherently in-plane, transverse magnetic mode emission. Hence, this type of resonator strongly reduces the threshold current and increases the temperature perfor- mance by decreasing the cavity size. The metallic waveguide is superior to the pure dielectric waveguides with respect to heat transfer and loss for increasing wavelengths. Therefore, circular-shaped microcavities give the opportunity to create ultrasmall devices with extremely low operational current and therefore low electrical power dissipation which enhances the CW performance. Apart from the pure size effect, microcavity lasers are of strong interest concerning cavity quantum elec- trodynamics (CQED) [14], [15]. The THz range offers the opportunity to fabricate high- semiconductor microcavities to study cavity effects. The control of the spontaneous emission can yield enhanced or suppressed spontaneous emission (Pur- cell effect) [15], [16] and thresholdless lasing [17]. CQED in the weak coupling [18], [19] as well as in the strong coupling regime [20] with WGMs has already been studied. To enter the strong coupling regime in CQED, the light-matter interaction must exceed the field loss rates, resulting in the well known Rabi splitting in the frequency domain [21]. The coupling efficiency is inversely proportional to the square root of the mode volume [22], thus strong confinement provided by WGM resonators or photonic crystals is a prerequisite. In this paper, we present a detailed study of the emission characteristics of subwavelength microdisk and microring THz QCLs exhibiting multi- as well as single-mode emission. We can assign the cor- responding WGMs by comparing the experimental results with our finite-difference time-domain (FDTD) calculations. The reduction of the cavity volume leads to single-mode emission and improved temperature performance compared to larger 0018-9197/$25.00 © 2007 IEEE