**5. Conclusions**

High-speed operation with a 3-dB bandwidth over 20 GHz was realized in an uncooled QC detector with a 4.6-μm peak response. This was accomplished by reducing the parasitic capacitance and circuit inductance with a narrow 25 μm × 100 μm ridge-mesa structure, a thick active region of 90 cascade modules, and air-bridge wiring. A noise-equivalent power of 7.7 × <sup>10</sup>−<sup>11</sup> W/Hz1/2 was obtained with a peak responsivity of 5.7 mA/W, and a flat noise level of 4.4 × <sup>10</sup>−<sup>13</sup> A/Hz1/2. By using the high-speed QC detector, a broadband heterodyne absorption spectrum of N2O was obtained over the range of ~0.8 cm<sup>−</sup>1, with a resolution bandwidth of 5 MHz. The absorption strengths, spectral positions, and spectral widths of the absorption lines were in good agreement with the calculations. In future, a compact high-resolution broadband spectroscopic module could be realized with a heterodyne system incorporating QC lasers and detectors.

**Author Contributions:** Conceptualization, T.D. and N.A.; methodology, T.D. and N.A.; validation, T.D. and N.A.; formal analysis, T.D.; investigation, T.D.; resources, T.D.; data curation, T.D.; writing—original draft preparation, T.D.; writing—review and editing, N.A.; visualization, T.D. and N.A.; supervision, T.D. and N.A.; project administration, T.D. and N.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available from the corresponding author upon reasonable request.

**Acknowledgments:** The authors are grateful to M. Hitaka and A. Ito at Hamamatsu Photonics K.K. for the preparation of the device. The authors also express their gratitude to K. Fujita and M. Yamanishi at Hamamatsu Photonics K.K. for the valuable discussions with respect to the active region design and the physics of high-speed quantum cascade detectors. The authors are also grateful to T. Edamura at Hamamatsu Photonics K.K. for the fruitful discussions and his support throughout this work. We thank Alan Burns, PhD, from Edanz (https://jp.edanz.com/ac, accessed on 29 July 2021) for editing a draft of this manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
