A. Klimont, A. Ottomaniello, R. Degl’Innocenti, L. Masini, F. Bianco, Y. Wu1, Y. D. Shah, Y. Ren, D. S. Jessop, A. Tredicucci, H. E. Beere, D. A. Ritchie.
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| Scanning electron microscope (SEM) pictures of the line defect QCLs after reactive ion etching for three different line-defect waveguides. Figure (a) corresponds to D1, where the defects before BCB planarization are separated by one row of small pillars in the triangular PhC lattice. Figure (b) (D3) and (c) (D5) correspond to line-defect QCLs separated by three rows and five rows of small pillars, respectively. Figure (d) shows in more detail the defect area and identifies the main parameters of the PhC structure, such as the vectors on the direct triangular lattice, a1 and a2, the radius of the regular and defect pillars, r and R, respectively, as well as the line orientation. Figure (e) shows a typical line-defect QCL after the BCB planarization, leaving only the tops of ∼14 μm-tall pillars exposed. |
The terahertz (THz) quantum cascade laser (QCL) provides a versatile tool in a plethora of applications ranging from spectroscopy to astronomy and communications. In many of these fields, compactness, single mode frequency emission, and low threshold are highly desirable. The proposed approach, based on line defects in a photonic crystal (PhC) matrix, addresses all these features while offering unprecedented capabilities in terms of flexibility, light waveguiding, and emission directionality. Nine line-defect QCLs were realized in a triangular lattice of pillars fabricated in the laser active region (AR), centered around ∼2 THz by tuning the photonic design. A maximal 36% threshold reduction was recorded for these ultraflat dispersion line-defect QCLs in comparison to standard metal-metal QCL. The mode selectivity is an intrinsic property of the chosen fabrication design and has been achieved by lithographically scaling the dimension of the defect pillars and by acting on the PhC parameters in order to match the AR emission bandwidth. The measured line-defect QCLs emitted preferentially in the single frequency mode in the propagation direction throughout the entire dynamic range. An integrated active platform with multiple directional outputs was also fabricated as proof-of-principle to demonstrate the potential of this approach. The presented results pave the way for integrated circuitry operating in the THz regime and for fundamental studies on microcavity lasers.
