Position and mode dependent coupling of terahertz quantum cascade laser fields to an integrated diode

A Schottky diode integrated into a terahertz quantum cascade laser waveguide couples directly to the internal laser fields. In a multimode laser, the diode response is correlated with both the instantaneous power and the coupling strength to the diode of each lasing mode. Measurements of the rectified response of diodes integrated in two quantum cascade laser cavities at different locations indicate that the relative diode position strongly influences the laser-diode coupling. ∗ Now at Soraa, Freemont, California, 94555 USA † mcwanke@sandia.gov 1 ar X iv :1 60 5. 03 11 8v 1 [ co nd -m at .m es -h al l] 1 0 M ay 2 01 6 Quantum cascade lasers (QCLs) may be considered one of the most remarkable achievements in quantum engineering due to both the intensity and the broad tailorability of their emission. Since the operating range of these unipolar, intersubband lasers was extended to the terahertz (THz) band of the spectrum, a variety of applications requiring a compact high-power (>mW) source between 1-5 THz have become accessible. Of particular interest is the use of a THz QCL as a local oscillator (LO) for heterodyne mixing. THz QCLs provide ample power for mixing; however, it is non-trivial to efficiently couple the THz LO power from a QCL to a mixer such as a planar Schottky diode. One possible solution is to directly integrate a Schottky diode mixer into the core of a THz QCL to create a THz transceiver. We previously observed the direct coupling of the internal QCL fields to an integrated diode, however, several questions concerning the precise nature of this coupling remain open. For practical applications, the response of a Schottky diode mixer should be linear in both the LO and signal field amplitudes. However, prior measurements suggested that both the mode structure and the instantaneous power of the laser may affect the laserdiode coupling and lead to a non-linear response to the QCL (LO) power. In this letter we examine how the rectified response of Schottky diodes embedded into the core of THz QCLs depends upon diode position and QCL bias current. To determine the effect of diode position upon the diode’s coupling with the laser fields, we compare the rectified response of diodes with different relative positions in the laser waveguide to the emission spectra of two otherwise identical 2.8 THz QCL transceivers. The studied THz QCLs have a Schottky diode embedded into the core of the 3 mm long by 170 μm wide waveguide, as illustrated in Fig. 1. Both transceivers were cleaved from the same row of the processed die, and thus have identical cavity lengths. Sample A has the diode located by design at the center of the QCL waveguide relative to the laser facets, 1.5 mm from both facets. Sample B has the diode shifted +4 μm from that of the diode in Sample A. Given the slight uncertainty of the cleave planes relative to the diode position, the exact locations of the diodes in Samples A and B may differ from design. But the relative positions of the two diodes are fixed by the device layouts. Rectified and intermediate frequency (IF) signals result from the coupling of THz laser fields to a Schottky diode. If only nearest-neighbor modes in a Fabry-Perot laser (FP) cavity separated by the angular frequency ωFP are considered, the rectified and IF signals,