Due to the high emission powers and narrow line widths in continuous-wave (cw) operation, THz QCLs are promising sources for high-resolution spectroscopy. As examples, the transition between rotational states of the hydroxyl radical (OH) at 3.55 THz and the fine-structure transition of neutral atomic oxygen (OI) at 4.75 THz are of current interest in astronomy and planetary science. These spectral lines can be measured by a heterodyne spectrometer using a THz QCL in a local oscillator. Furthermore, THz QCLs are developed as suitable radiation sources for the detection of Si, Al, N+, and O atoms/ions in plasma processes by high-resolution absorption spectroscopy of the respective fine-structure transitions in the THz spectral region.
The relative change in the growth rate and thickness of the QCL across the wafer were evaluated by ex-situ spectral reflectivity scans for three wafers A, B, and C. Figure 1(a) depicts the relative change in the growth rate (QCL thickness) with respect to the growth rate (QCL thickness) at the center of the wafer A. The emission frequencies of a number of QCLs fabricated from wafer A (B and C) based on a design for 3.36 THz (3.92 THz), which are located in different regions of the wafer, exhibit a blue shift with a decreasing growth rate from the center to the edge of the wafer. Using Fourier transform-based simulations, we calculated the frequency as a function of the relative thickness of the QCL. By utilizing the dependence shown in Fig. 1(a), the frequency is calculated as a function of the location, which is well reproduced by a power law. This power law describes the correlation between the emission frequency of the QCLs and the location on the wafer as indicated by the dashed lines in Figs. 1(b) and 1(c). The simulated frequencies agree well with the experimental results. Using this universal power law, the covered frequency range for a new wafer can now be predicted, and the suitable position on the wafer from which a THz QCL with an application-defined emission frequency is fabricated can be efficiently determined. Rather than growing several homogeneous wafers with a wafer-to-wafer variation of the nominal active-region thickness, THz QCLs with customized emission frequencies can be efficiently fabricated by making use of this well-defined frequency dependence across the wafer.